add TraceAsString<T> -- sim traces it as a string rather than all its internal fields

also cleans up default debug formatting to use the struct/enum name
(or MaskType<StructName>) instead of the implementation detail type name.

.forgejo/workflows/test.yml

@@ -18,6 +18,7 @@ jobs:
           save-if: ${{ github.ref == 'refs/heads/master' }}
       - run: rustup override set 1.93.0
       - run: rustup component add rust-src
+      - run: make -C rocq-demo
       - run: cargo test
       - run: cargo build --tests --features=unstable-doc
       - run: cargo test --doc --features=unstable-doc

crates/fayalite-proc-macros-impl/src/fold.rs

@@ -257,5 +257,6 @@ no_op_fold!(syn::Token![let]);
 no_op_fold!(syn::Token![mut]);
 no_op_fold!(syn::Token![static]);
 no_op_fold!(syn::Token![struct]);
+no_op_fold!(syn::Token![type]);
 no_op_fold!(syn::Token![where]);
 no_op_fold!(usize);

crates/fayalite-proc-macros-impl/src/hdl_bundle.rs

@@ -3,8 +3,9 @@
 use crate::{
     Errors, HdlAttr, PairsIterExt,
     hdl_type_common::{
-        ItemOptions, MakeHdlTypeExpr, MaybeParsed, ParsedField, ParsedFieldsNamed, ParsedGenerics,
-        SplitForImpl, TypesParser, WrappedInConst, common_derives, get_target,
+        CustomDebugOptions, CustomDebugTrait, ItemOptions, MakeHdlTypeExpr, MaybeParsed,
+        ParsedField, ParsedFieldsNamed, ParsedGenerics, SplitForImpl, TypesParser, WrappedInConst,
+        common_derives, create_struct_debug_impl, get_target,
     },
     kw,
 };
@@ -30,6 +31,7 @@ pub(crate) struct ParsedBundle {
     pub(crate) fields: MaybeParsed<ParsedFieldsNamed, FieldsNamed>,
     pub(crate) field_flips: Vec<Option<HdlAttr<kw::flip, kw::hdl>>>,
     pub(crate) mask_type_ident: Ident,
+    pub(crate) mask_type_name: String,
     pub(crate) mask_type_match_variant_ident: Ident,
     pub(crate) mask_type_sim_value_ident: Ident,
     pub(crate) match_variant_ident: Ident,
@@ -88,6 +90,8 @@ impl ParsedBundle {
             no_runtime_generics: _,
             cmp_eq: _,
             ref get,
+            custom_debug: _,
+            custom_sim_display: _,
         } = options.body;
         if let Some((get, ..)) = get {
             errors.error(get, "#[hdl(get(...))] is not allowed on structs");
@@ -131,6 +135,7 @@ impl ParsedBundle {
             fields,
             field_flips,
             mask_type_ident: format_ident!("__{}__MaskType", ident),
+            mask_type_name: format!("MaskType<{}>", ident),
             mask_type_match_variant_ident: format_ident!("__{}__MaskType__MatchVariant", ident),
             mask_type_sim_value_ident: format_ident!("__{}__MaskType__SimValue", ident),
             match_variant_ident: format_ident!("__{}__MatchVariant", ident),
@@ -448,6 +453,7 @@ impl ToTokens for ParsedBundle {
             fields,
             field_flips,
             mask_type_ident,
+            mask_type_name,
             mask_type_match_variant_ident,
             mask_type_sim_value_ident,
             match_variant_ident,
@@ -464,11 +470,20 @@ impl ToTokens for ParsedBundle {
             no_runtime_generics,
             cmp_eq,
             get: _,
+            custom_debug: _,
+            custom_sim_display,
         } = &options.body;
+        let CustomDebugOptions {
+            type_: custom_debug_type,
+            sim: custom_debug_sim,
+            mask_type: custom_debug_mask_type,
+            mask_sim: custom_debug_mask_sim,
+        } = options.body.custom_debug();
         let target = get_target(target, ident);
+        let struct_name = ident.to_string();
         let mut item_attrs = attrs.clone();
-        item_attrs.push(common_derives(span));
-        ItemStruct {
+        item_attrs.push(common_derives(span, false));
+        let type_struct = ItemStruct {
             attrs: item_attrs,
             vis: vis.clone(),
             struct_token: *struct_token,
@@ -476,8 +491,8 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(fields.clone().into()),
             semi_token: None,
-        }
-        .to_tokens(tokens);
+        };
+        type_struct.to_tokens(tokens);
         let (impl_generics, type_generics, where_clause) = generics.split_for_impl();
         if let (MaybeParsed::Parsed(generics), MaybeParsed::Parsed(fields), None) =
             (generics, fields, no_runtime_generics)
@@ -503,6 +518,9 @@ impl ToTokens for ParsedBundle {
         }
         let mut wrapped_in_const = WrappedInConst::new(tokens, span);
         let tokens = wrapped_in_const.inner();
+        if custom_debug_type.is_none() {
+            create_struct_debug_impl(&type_struct, &struct_name, None).to_tokens(tokens);
+        }
         let builder = Builder {
             vis: vis.clone(),
             struct_token: *struct_token,
@@ -530,9 +548,9 @@ impl ToTokens for ParsedBundle {
         mask_type_builder.to_tokens(tokens);
         let unfilled_mask_type_builder_ty =
             mask_type_builder.builder_struct_ty(|_| BuilderFieldState::Unfilled);
-        ItemStruct {
+        let mask_type_struct = ItemStruct {
             attrs: vec![
-                common_derives(span),
+                common_derives(span, false),
                 parse_quote_spanned! {span=>
                     #[allow(non_camel_case_types, dead_code)]
                 },
@@ -543,17 +561,20 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(mask_type_fields.clone()),
             semi_token: None,
+        };
+        mask_type_struct.to_tokens(tokens);
+        if custom_debug_mask_type.is_none() {
+            create_struct_debug_impl(&mask_type_struct, mask_type_name, None).to_tokens(tokens);
         }
-        .to_tokens(tokens);
         let mut mask_type_match_variant_fields = mask_type_fields.clone();
         for Field { ty, .. } in &mut mask_type_match_variant_fields.named {
             *ty = parse_quote_spanned! {span=>
                 ::fayalite::expr::Expr<#ty>
             };
         }
-        ItemStruct {
+        let mask_type_match_variant_struct = ItemStruct {
             attrs: vec![
-                common_derives(span),
+                common_derives(span, false),
                 parse_quote_spanned! {span=>
                     #[allow(non_camel_case_types, dead_code)]
                 },
@@ -564,17 +585,19 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(mask_type_match_variant_fields),
             semi_token: None,
-        }
-        .to_tokens(tokens);
+        };
+        mask_type_match_variant_struct.to_tokens(tokens);
+        create_struct_debug_impl(&mask_type_match_variant_struct, mask_type_name, None)
+            .to_tokens(tokens);
         let mut match_variant_fields = FieldsNamed::from(fields.clone());
         for Field { ty, .. } in &mut match_variant_fields.named {
             *ty = parse_quote_spanned! {span=>
                 ::fayalite::expr::Expr<#ty>
             };
         }
-        ItemStruct {
+        let match_variant_struct = ItemStruct {
             attrs: vec![
-                common_derives(span),
+                common_derives(span, false),
                 parse_quote_spanned! {span=>
                     #[allow(non_camel_case_types, dead_code)]
                 },
@@ -585,19 +608,19 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(match_variant_fields),
             semi_token: None,
-        }
-        .to_tokens(tokens);
+        };
+        match_variant_struct.to_tokens(tokens);
+        create_struct_debug_impl(&match_variant_struct, &struct_name, None).to_tokens(tokens);
         let mut mask_type_sim_value_fields = mask_type_fields;
         for Field { ty, .. } in &mut mask_type_sim_value_fields.named {
             *ty = parse_quote_spanned! {span=>
                 ::fayalite::sim::value::SimValue<#ty>
             };
         }
-        ItemStruct {
+        let mask_type_sim_value_struct = ItemStruct {
             attrs: vec![
                 parse_quote_spanned! {span=>
                     #[::fayalite::__std::prelude::v1::derive(
-                        ::fayalite::__std::fmt::Debug,
                         ::fayalite::__std::clone::Clone,
                     )]
                 },
@@ -611,19 +634,34 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(mask_type_sim_value_fields),
             semi_token: None,
+        };
+        mask_type_sim_value_struct.to_tokens(tokens);
+        if custom_debug_mask_sim.is_none() {
+            create_struct_debug_impl(
+                &mask_type_struct,
+                mask_type_name,
+                Some(CustomDebugTrait {
+                    trait_path: &parse_quote_spanned! {span=>
+                        ::fayalite::ty::SimValueDebug
+                    },
+                    fn_name: &format_ident!("sim_value_debug", span = span),
+                    this_arg: &parse_quote_spanned! {span=>
+                        value: &<Self as ::fayalite::ty::Type>::SimValue
+                    },
+                }),
+            )
+            .to_tokens(tokens);
         }
-        .to_tokens(tokens);
         let mut sim_value_fields = FieldsNamed::from(fields.clone());
         for Field { ty, .. } in &mut sim_value_fields.named {
             *ty = parse_quote_spanned! {span=>
                 ::fayalite::sim::value::SimValue<#ty>
             };
         }
-        ItemStruct {
+        let sim_value_struct = ItemStruct {
             attrs: vec![
                 parse_quote_spanned! {span=>
                     #[::fayalite::__std::prelude::v1::derive(
-                        ::fayalite::__std::fmt::Debug,
                         ::fayalite::__std::clone::Clone,
                     )]
                 },
@@ -637,8 +675,36 @@ impl ToTokens for ParsedBundle {
             generics: generics.into(),
             fields: Fields::Named(sim_value_fields),
             semi_token: None,
+        };
+        sim_value_struct.to_tokens(tokens);
+        if custom_debug_sim.is_none() {
+            create_struct_debug_impl(
+                &type_struct,
+                &struct_name,
+                Some(CustomDebugTrait {
+                    trait_path: &parse_quote_spanned! {span=>
+                        ::fayalite::ty::SimValueDebug
+                    },
+                    fn_name: &format_ident!("sim_value_debug", span = span),
+                    this_arg: &parse_quote_spanned! {span=>
+                        value: &<Self as ::fayalite::ty::Type>::SimValue
+                    },
+                }),
+            )
+            .to_tokens(tokens);
+        }
+        if custom_sim_display.is_some() {
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics ::fayalite::__std::fmt::Display for #sim_value_ident #type_generics
+                #where_clause
+                {
+                    fn fmt(&self, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                        <#target #type_generics as ::fayalite::ty::SimValueDisplay>::sim_value_display(self, f)
+                    }
+                }
+            }.to_tokens(tokens);
         }
-        .to_tokens(tokens);
         let this_token = Ident::new("__this", span);
         let fields_token = Ident::new("__fields", span);
         let self_token = Token![self](span);
@@ -820,6 +886,14 @@ impl ToTokens for ParsedBundle {
                 }
             }
             #[automatically_derived]
+            impl #impl_generics ::fayalite::__std::fmt::Debug for #mask_type_sim_value_ident #type_generics
+            #where_clause
+            {
+                fn fmt(&self, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                    <#mask_type_ident #type_generics as ::fayalite::ty::SimValueDebug>::sim_value_debug(self, f)
+                }
+            }
+            #[automatically_derived]
             impl #impl_generics ::fayalite::expr::ValueType for #mask_type_sim_value_ident #type_generics
             #where_clause
             {
@@ -980,6 +1054,14 @@ impl ToTokens for ParsedBundle {
                 }
             }
             #[automatically_derived]
+            impl #impl_generics ::fayalite::__std::fmt::Debug for #sim_value_ident #type_generics
+            #where_clause
+            {
+                fn fmt(&self, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                    <#target #type_generics as ::fayalite::ty::SimValueDebug>::sim_value_debug(self, f)
+                }
+            }
+            #[automatically_derived]
             impl #impl_generics ::fayalite::expr::ValueType for #sim_value_ident #type_generics
             #where_clause
             {
@@ -1141,7 +1223,7 @@ impl ToTokens for ParsedBundle {
                 valueless_eq_body = quote_spanned! {span=>
                     let __lhs = ::fayalite::expr::ValueType::ty(&__lhs);
                     let __rhs = ::fayalite::expr::ValueType::ty(&__rhs);
-                    #(#fields_valueless_eq)|*
+                    #(#fields_valueless_eq)&*
                 };
                 valueless_ne_body = quote_spanned! {span=>
                     let __lhs = ::fayalite::expr::ValueType::ty(&__lhs);

crates/fayalite-proc-macros-impl/src/hdl_enum.rs

@@ -3,8 +3,9 @@
 use crate::{
     Errors, HdlAttr, PairsIterExt,
     hdl_type_common::{
-        ItemOptions, MakeHdlTypeExpr, MaybeParsed, ParsedGenerics, ParsedType, SplitForImpl,
-        TypesParser, WrappedInConst, common_derives, get_target,
+        CustomDebugOptions, ItemOptions, MakeHdlTypeExpr, MaybeParsed, ParsedGenerics, ParsedType,
+        SplitForImpl, TypesParser, WrappedInConst, common_derives, create_struct_debug_impl,
+        get_target,
     },
     kw,
 };
@@ -158,15 +159,32 @@ impl ParsedEnum {
             custom_bounds,
             no_static: _,
             no_runtime_generics: _,
-            cmp_eq,
+            cmp_eq: _,
             ref get,
+            custom_debug: _,
+            custom_sim_display: _,
         } = options.body;
-        if let Some((cmp_eq,)) = cmp_eq {
-            errors.error(cmp_eq, "#[hdl(cmp_eq)] is not yet implemented for enums");
-        }
         if let Some((get, ..)) = get {
             errors.error(get, "#[hdl(get(...))] is not allowed on enums");
         }
+        let CustomDebugOptions {
+            type_: _,
+            sim: _,
+            mask_type,
+            mask_sim,
+        } = options.body.custom_debug();
+        if let Some((mask_type,)) = mask_type {
+            errors.error(
+                mask_type,
+                "#[hdl(custom_debug(mask_type)] is not allowed on enums",
+            );
+        }
+        if let Some((mask_sim,)) = mask_sim {
+            errors.error(
+                mask_sim,
+                "#[hdl(custom_debug(mask_sim)] is not allowed on enums",
+            );
+        }
         attrs.retain(|attr| {
             if attr.path().is_ident("repr") {
                 errors.error(attr, "#[repr] is not supported on #[hdl] enums");
@@ -228,12 +246,21 @@ impl ToTokens for ParsedEnum {
             custom_bounds: _,
             no_static,
             no_runtime_generics,
-            cmp_eq: _, // TODO: implement cmp_eq for enums
+            cmp_eq,
             get: _,
+            custom_debug: _,
+            custom_sim_display,
         } = &options.body;
+        let CustomDebugOptions {
+            type_: custom_debug_type,
+            sim: custom_debug_sim,
+            mask_type: _,
+            mask_sim: _,
+        } = options.body.custom_debug();
         let target = get_target(target, ident);
+        let enum_name = ident.to_string();
         let mut struct_attrs = attrs.clone();
-        struct_attrs.push(common_derives(span));
+        struct_attrs.push(common_derives(span, false));
         struct_attrs.push(parse_quote_spanned! {span=>
             #[allow(non_snake_case)]
         });
@@ -273,7 +300,7 @@ impl ToTokens for ParsedEnum {
                 }
             },
         ));
-        ItemStruct {
+        let type_struct = ItemStruct {
             attrs: struct_attrs,
             vis: vis.clone(),
             struct_token: Token![struct](enum_token.span),
@@ -288,8 +315,8 @@ impl ToTokens for ParsedEnum {
                 })
             },
             semi_token: None,
-        }
-        .to_tokens(tokens);
+        };
+        type_struct.to_tokens(tokens);
         let (impl_generics, type_generics, where_clause) = generics.split_for_impl();
         if let (MaybeParsed::Parsed(generics), None) = (generics, no_runtime_generics) {
             generics.make_runtime_generics(tokens, vis, ident, &target, |context| {
@@ -373,6 +400,9 @@ impl ToTokens for ParsedEnum {
             }
             .to_tokens(tokens);
         }
+        if custom_debug_type.is_none() {
+            create_struct_debug_impl(&type_struct, &enum_name, None).to_tokens(tokens);
+        }
         let mut enum_attrs = attrs.clone();
         enum_attrs.push(parse_quote_spanned! {span=>
             #[allow(dead_code, non_camel_case_types)]
@@ -453,7 +483,6 @@ impl ToTokens for ParsedEnum {
         let mut enum_attrs = attrs.clone();
         enum_attrs.push(parse_quote_spanned! {span=>
             #[::fayalite::__std::prelude::v1::derive(
-                ::fayalite::__std::fmt::Debug,
                 ::fayalite::__std::clone::Clone,
             )]
         });
@@ -838,6 +867,240 @@ impl ToTokens for ParsedEnum {
                     },
                 )),
         );
+        if custom_debug_sim.is_none() {
+            let debug_match_arms = Vec::from_iter(
+                variants
+                    .iter()
+                    .map(
+                        |ParsedVariant {
+                             attrs: _,
+                             options: _,
+                             ident,
+                             field,
+                         }| {
+                            let variant_name = ident.to_string();
+                            if let Some(_) = field {
+                                quote_spanned! {span=>
+                                    #sim_value_ident::#ident(field, _) => {
+                                        f.debug_tuple(#variant_name).field(field).finish()
+                                    }
+                                }
+                            } else {
+                                quote_spanned! {span=>
+                                    #sim_value_ident::#ident(_) => {
+                                        f.write_str(#variant_name)
+                                    }
+                                }
+                            }
+                        },
+                    )
+                    .chain(sim_value_unknown_variant_name.as_ref().map(
+                        |sim_value_unknown_variant_name| {
+                            let sim_value_unknown_variant_name_str =
+                                sim_value_unknown_variant_name.to_string();
+                            quote_spanned! {span=>
+                                #sim_value_ident::#sim_value_unknown_variant_name(_) => {
+                                    f.write_str(#sim_value_unknown_variant_name_str)
+                                }
+                            }
+                        },
+                    )),
+            );
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics ::fayalite::ty::SimValueDebug for #target #type_generics
+                #where_clause
+                {
+                    fn sim_value_debug(
+                        value: &<Self as ::fayalite::ty::Type>::SimValue,
+                        f: &mut ::fayalite::__std::fmt::Formatter<'_>,
+                    ) -> ::fayalite::__std::fmt::Result {
+                        match value {
+                            #(#debug_match_arms)*
+                        }
+                    }
+                }
+            }
+            .to_tokens(tokens);
+        }
+        if custom_sim_display.is_some() {
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics ::fayalite::__std::fmt::Display for #sim_value_ident #type_generics
+                #where_clause
+                {
+                    fn fmt(&self, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                        <#target #type_generics as ::fayalite::ty::SimValueDisplay>::sim_value_display(self, f)
+                    }
+                }
+            }.to_tokens(tokens);
+        }
+        if let Some((cmp_eq,)) = cmp_eq {
+            let mut cmp_eq_where_clause =
+                Generics::from(generics)
+                    .where_clause
+                    .unwrap_or_else(|| syn::WhereClause {
+                        where_token: Token![where](span),
+                        predicates: Punctuated::new(),
+                    });
+            let mut variants_value_eq = vec![];
+            let mut variants_expr_eq = vec![];
+            let mut fields_valueless_eq = vec![];
+            for (
+                variant_index,
+                ParsedVariant {
+                    attrs: _,
+                    options: variant_options,
+                    ident: variant_ident,
+                    field,
+                },
+            ) in variants.iter().enumerate()
+            {
+                let VariantOptions {} = variant_options.body;
+                if let Some(ParsedVariantField {
+                    paren_token: _,
+                    attrs: _,
+                    options: field_options,
+                    ty: field_ty,
+                    comma_token: _,
+                }) = field
+                {
+                    let FieldOptions {} = field_options.body;
+                    cmp_eq_where_clause
+                        .predicates
+                        .push(parse_quote_spanned! {cmp_eq.span=>
+                            #field_ty: ::fayalite::expr::HdlPartialEqImpl<#field_ty>
+                        });
+                    variants_value_eq.push(quote_spanned! {span=>
+                        (#sim_value_ident::#variant_ident(__lhs_field, _), #sim_value_ident::#variant_ident(__rhs_field, _)) => {
+                            ::fayalite::expr::HdlPartialEqImpl::cmp_value_eq(
+                                __lhs.#variant_ident,
+                                ::fayalite::__std::borrow::Cow::Borrowed(__lhs_field),
+                                __rhs.#variant_ident,
+                                ::fayalite::__std::borrow::Cow::Borrowed(__rhs_field),
+                            )
+                        }
+                    });
+                    variants_expr_eq.push(quote_spanned! {span=>
+                        {
+                            let (#match_variant_ident::#variant_ident(__lhs), __scope) =
+                                ::fayalite::ty::MatchVariantAndInactiveScope::match_activate_scope(
+                                    ::fayalite::__std::iter::Iterator::next(&mut __lhs_match_variant_iter)
+                                        .expect("known to have enough variants"),
+                                )
+                            else {
+                                ::fayalite::__std::unreachable!();
+                            };
+                            let (#match_variant_ident::#variant_ident(__rhs), __scope) =
+                                ::fayalite::ty::MatchVariantAndInactiveScope::match_activate_scope(
+                                    ::fayalite::__std::iter::Iterator::nth(
+                                        &mut ::fayalite::module::match_(__rhs),
+                                        #variant_index,
+                                    )
+                                        .expect("known to have variant"),
+                                )
+                            else {
+                                ::fayalite::__std::unreachable!();
+                            };
+                            ::fayalite::module::connect(__retval, ::fayalite::expr::HdlPartialEqImpl::cmp_expr_eq(__lhs, __rhs));
+                        }
+                    });
+                    fields_valueless_eq.push(quote_spanned! {span=>
+                        ::fayalite::expr::HdlPartialEqImpl::cmp_valueless_eq(
+                            ::fayalite::expr::Valueless::new(__lhs.#variant_ident),
+                            ::fayalite::expr::Valueless::new(__rhs.#variant_ident),
+                        )
+                    });
+                } else {
+                    variants_value_eq.push(quote_spanned! {span=>
+                        (#sim_value_ident::#variant_ident(_), #sim_value_ident::#variant_ident(_)) => true,
+                    });
+                    variants_expr_eq.push(quote_spanned! {span=>
+                        {
+                            let (#match_variant_ident::#variant_ident, __scope) =
+                                ::fayalite::ty::MatchVariantAndInactiveScope::match_activate_scope(
+                                    ::fayalite::__std::iter::Iterator::next(&mut __lhs_match_variant_iter)
+                                        .expect("known to have enough variants"),
+                                )
+                            else {
+                                ::fayalite::__std::unreachable!();
+                            };
+                            let (#match_variant_ident::#variant_ident, __scope) =
+                                ::fayalite::ty::MatchVariantAndInactiveScope::match_activate_scope(
+                                    ::fayalite::__std::iter::Iterator::nth(
+                                        &mut ::fayalite::module::match_(__rhs),
+                                        #variant_index,
+                                    )
+                                        .expect("known to have variant"),
+                                )
+                            else {
+                                ::fayalite::__std::unreachable!();
+                            };
+                            ::fayalite::module::connect(__retval, true);
+                        }
+                    });
+                }
+            }
+            if let Some(sim_value_unknown_variant_name) = &sim_value_unknown_variant_name {
+                variants_value_eq.push(quote_spanned! {span=>
+                    (#sim_value_ident::#sim_value_unknown_variant_name(__lhs_unknown), #sim_value_ident::#sim_value_unknown_variant_name(__rhs_unknown)) => {
+                        __lhs_unknown == __rhs_unknown
+                    }
+                });
+            }
+            let valueless_eq_body = if fields_valueless_eq.is_empty() {
+                quote_spanned! {span=>
+                    ::fayalite::expr::Valueless::new(::fayalite::int::Bool)
+                }
+            } else {
+                quote_spanned! {span=>
+                    let __lhs = ::fayalite::expr::ValueType::ty(&__lhs);
+                    let __rhs = ::fayalite::expr::ValueType::ty(&__rhs);
+                    #(#fields_valueless_eq)&*
+                }
+            };
+            let cmp_expr_eq_wire_name = format!("{ident}_cmp_eq");
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics ::fayalite::expr::HdlPartialEqImpl<Self> for #target #type_generics
+                #cmp_eq_where_clause
+                {
+                    #[track_caller]
+                    fn cmp_value_eq(
+                        __lhs: Self,
+                        __lhs_value: ::fayalite::__std::borrow::Cow<'_, <Self as ::fayalite::ty::Type>::SimValue>,
+                        __rhs: Self,
+                        __rhs_value: ::fayalite::__std::borrow::Cow<'_, <Self as ::fayalite::ty::Type>::SimValue>,
+                    ) -> ::fayalite::__std::primitive::bool {
+                        match (&*__lhs_value, &*__rhs_value) {
+                            #(#variants_value_eq)*
+                            _ => false,
+                        }
+                    }
+
+                    #[track_caller]
+                    fn cmp_expr_eq(
+                        __lhs: ::fayalite::expr::Expr<Self>,
+                        __rhs: ::fayalite::expr::Expr<Self>,
+                    ) -> ::fayalite::expr::Expr<::fayalite::int::Bool> {
+                        let __retval = ::fayalite::module::wire(::fayalite::module::ImplicitName(#cmp_expr_eq_wire_name), ::fayalite::int::Bool);
+                        ::fayalite::module::connect(__retval, false);
+                        let mut __lhs_match_variant_iter = ::fayalite::module::match_(__lhs);
+                        #(#variants_expr_eq)*
+                        __retval
+                    }
+
+                    #[track_caller]
+                    fn cmp_valueless_eq(
+                        __lhs: ::fayalite::expr::Valueless<Self>,
+                        __rhs: ::fayalite::expr::Valueless<Self>,
+                    ) -> ::fayalite::expr::Valueless<::fayalite::int::Bool> {
+                        #valueless_eq_body
+                    }
+                }
+            }
+            .to_tokens(tokens);
+        }
         let variants_len = variants.len();
         quote_spanned! {span=>
             #[automatically_derived]
@@ -934,6 +1197,14 @@ impl ToTokens for ParsedEnum {
                 }
             }
             #[automatically_derived]
+            impl #impl_generics ::fayalite::__std::fmt::Debug for #sim_value_ident #type_generics
+            #where_clause
+            {
+                fn fmt(&self, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                    <#target #type_generics as ::fayalite::ty::SimValueDebug>::sim_value_debug(self, f)
+                }
+            }
+            #[automatically_derived]
             impl #impl_generics ::fayalite::sim::value::ToSimValueWithType<#target #type_generics>
             for #sim_value_ident #type_generics
             #where_clause

crates/fayalite-proc-macros-impl/src/hdl_type_alias.rs

@@ -215,6 +215,8 @@ impl ParsedTypeAlias {
             no_runtime_generics,
             cmp_eq,
             get: _,
+            ref custom_debug,
+            custom_sim_display,
         } = options.body;
         if let Some((no_static,)) = no_static {
             errors.error(no_static, "no_static is not valid on type aliases");
@@ -234,6 +236,15 @@ impl ParsedTypeAlias {
         if let Some((cmp_eq,)) = cmp_eq {
             errors.error(cmp_eq, "cmp_eq is not valid on type aliases");
         }
+        if let Some((custom_debug, _, _)) = custom_debug {
+            errors.error(custom_debug, "custom_debug is not valid on type aliases");
+        }
+        if let Some((custom_sim_display,)) = custom_sim_display {
+            errors.error(
+                custom_sim_display,
+                "custom_sim_display is not valid on type aliases",
+            );
+        }
         if let Some((custom_bounds,)) = custom_bounds {
             errors.error(
                 custom_bounds,
@@ -287,6 +298,8 @@ impl ParsedTypeAlias {
             no_runtime_generics: _,
             cmp_eq,
             ref mut get,
+            ref custom_debug,
+            custom_sim_display,
         } = options.body;
         if let Some(get) = get.take() {
             return Self::parse_phantom_const_accessor(
@@ -311,6 +324,15 @@ impl ParsedTypeAlias {
         if let Some((cmp_eq,)) = cmp_eq {
             errors.error(cmp_eq, "cmp_eq is not valid on type aliases");
         }
+        if let Some((custom_debug, _, _)) = custom_debug {
+            errors.error(custom_debug, "custom_debug is not valid on type aliases");
+        }
+        if let Some((custom_sim_display,)) = custom_sim_display {
+            errors.error(
+                custom_sim_display,
+                "custom_sim_display is not valid on type aliases",
+            );
+        }
         let generics = if custom_bounds.is_some() {
             MaybeParsed::Unrecognized(generics)
         } else if let Some(generics) = errors.ok(ParsedGenerics::parse(&mut generics)) {
@@ -356,6 +378,8 @@ impl ToTokens for ParsedTypeAlias {
                     no_runtime_generics,
                     cmp_eq: _,
                     get: _,
+                    custom_debug: _,
+                    custom_sim_display: _,
                 } = &options.body;
                 let target = get_target(target, ident);
                 let mut type_attrs = attrs.clone();
@@ -402,6 +426,8 @@ impl ToTokens for ParsedTypeAlias {
                     no_runtime_generics: _,
                     cmp_eq: _,
                     get: _,
+                    custom_debug: _,
+                    custom_sim_display: _,
                 } = &options.body;
                 let span = ident.span();
                 let mut type_attrs = attrs.clone();
@@ -427,7 +453,7 @@ impl ToTokens for ParsedTypeAlias {
                     format_ident!("__{}__GenericsAccumulation", ident);
                 ItemStruct {
                     attrs: vec![
-                        common_derives(span),
+                        common_derives(span, true),
                         parse_quote_spanned! {span=>
                             #[allow(non_camel_case_types)]
                         },

crates/fayalite-proc-macros-impl/src/hdl_type_common.rs

@@ -7,10 +7,10 @@ use std::{collections::HashMap, fmt, mem};
 use syn::{
     AngleBracketedGenericArguments, Attribute, Block, ConstParam, Expr, ExprBlock, ExprGroup,
     ExprIndex, ExprParen, ExprPath, ExprTuple, Field, FieldMutability, Fields, FieldsNamed,
-    FieldsUnnamed, GenericArgument, GenericParam, Generics, Ident, ImplGenerics, Index, ItemStruct,
-    Path, PathArguments, PathSegment, PredicateType, QSelf, Stmt, Token, TraitBound, Turbofish,
-    Type, TypeGenerics, TypeGroup, TypeParam, TypeParamBound, TypeParen, TypePath, TypeTuple,
-    Visibility, WhereClause, WherePredicate,
+    FieldsUnnamed, FnArg, GenericArgument, GenericParam, Generics, Ident, ImplGenerics, Index,
+    ItemStruct, Path, PathArguments, PathSegment, PredicateType, QSelf, Stmt, Token, TraitBound,
+    Turbofish, Type, TypeGenerics, TypeGroup, TypeParam, TypeParamBound, TypeParen, TypePath,
+    TypeTuple, Visibility, WhereClause, WherePredicate,
     parse::{Parse, ParseStream},
     parse_quote, parse_quote_spanned,
     punctuated::{Pair, Punctuated},
@@ -18,6 +18,17 @@ use syn::{
     token::{Brace, Bracket, Paren},
 };
 
+crate::options! {
+    #[options = CustomDebugOptions]
+    #[no_ident_fragment]
+    pub(crate) enum CustomDebugOption {
+        Type(type_),
+        Sim(sim),
+        MaskType(mask_type),
+        MaskSim(mask_sim),
+    }
+}
+
 crate::options! {
     #[options = ItemOptions]
     pub(crate) enum ItemOption {
@@ -28,6 +39,8 @@ crate::options! {
         NoRuntimeGenerics(no_runtime_generics),
         CmpEq(cmp_eq),
         Get(get, Expr),
+        CustomDebug(custom_debug, CustomDebugOptions),
+        CustomSimDisplay(custom_sim_display),
     }
 }
 
@@ -41,8 +54,36 @@ impl ItemOptions {
         {
             self.no_static = Some((kw::no_static(custom_bounds.span),));
         }
+        if let Some((kw, _, custom_debug)) = &mut self.custom_debug {
+            if let CustomDebugOptions {
+                type_: None,
+                sim: None,
+                mask_type: None,
+                mask_sim: None,
+            } = custom_debug
+            {
+                *custom_debug = CustomDebugOptions {
+                    type_: Some((kw::type_(kw.span),)),
+                    sim: Some((kw::sim(kw.span),)),
+                    mask_type: None,
+                    mask_sim: None,
+                };
+            }
+        }
         Ok(())
     }
+    pub(crate) fn custom_debug(&self) -> &CustomDebugOptions {
+        self.custom_debug.as_ref().map(|v| &v.2).unwrap_or(
+            const {
+                &CustomDebugOptions {
+                    type_: None,
+                    sim: None,
+                    mask_type: None,
+                    mask_sim: None,
+                }
+            },
+        )
+    }
 }
 
 pub(crate) struct WrappedInConst<'a> {
@@ -84,10 +125,17 @@ pub(crate) fn get_target(target: &Option<(kw::target, Paren, Path)>, item_ident:
     }
 }
 
-pub(crate) fn common_derives(span: Span) -> Attribute {
+pub(crate) fn common_derives(span: Span, include_debug: bool) -> Attribute {
+    let debug = include_debug
+        .then(|| {
+            quote_spanned! {span=>
+                ::fayalite::__std::fmt::Debug
+            }
+        })
+        .into_iter();
     parse_quote_spanned! {span=>
         #[::fayalite::__std::prelude::v1::derive(
-            ::fayalite::__std::fmt::Debug,
+            #(#debug,)*
             ::fayalite::__std::cmp::Eq,
             ::fayalite::__std::cmp::PartialEq,
             ::fayalite::__std::hash::Hash,
@@ -2975,7 +3023,7 @@ impl ParsedGenerics {
         let span = ident.span();
         ItemStruct {
             attrs: vec![
-                common_derives(span),
+                common_derives(span, true),
                 parse_quote_spanned! {span=>
                     #[allow(non_camel_case_types)]
                 },
@@ -4733,3 +4781,109 @@ impl ParsedVisibility {
             .map(|ord| if ord.is_lt() { self } else { other })
     }
 }
+
+pub(crate) struct CustomDebugTrait<'a> {
+    pub(crate) trait_path: &'a Path,
+    pub(crate) fn_name: &'a Ident,
+    pub(crate) this_arg: &'a FnArg,
+}
+
+#[must_use]
+pub(crate) fn create_struct_debug_impl(
+    item_struct: &ItemStruct,
+    debug_struct_name: &str,
+    custom_debug_trait: Option<CustomDebugTrait<'_>>,
+) -> TokenStream {
+    let ident = &item_struct.ident;
+    let span = ident.span();
+    let (impl_generics, type_generics, where_clause) = item_struct.generics.split_for_impl();
+    let trait_path;
+    let fn_name;
+    let this_arg;
+    let CustomDebugTrait {
+        trait_path,
+        fn_name,
+        this_arg,
+    } = match custom_debug_trait {
+        Some(v) => v,
+        None => {
+            trait_path = parse_quote_spanned! {span=>
+                ::fayalite::__std::fmt::Debug
+            };
+            fn_name = parse_quote_spanned! {span=>
+                fmt
+            };
+            this_arg = parse_quote_spanned! {span=>
+                &self
+            };
+            CustomDebugTrait {
+                trait_path: &trait_path,
+                fn_name: &fn_name,
+                this_arg: &this_arg,
+            }
+        }
+    };
+    let this_arg_name = match this_arg {
+        FnArg::Receiver(this_arg) => this_arg.self_token.to_token_stream(),
+        FnArg::Typed(this_arg) => match &*this_arg.pat {
+            syn::Pat::Ident(pat_ident) => pat_ident.ident.to_token_stream(),
+            _ => unreachable!(),
+        },
+    };
+    match &item_struct.fields {
+        Fields::Named(fields) => {
+            let field_idents = fields
+                .named
+                .iter()
+                .map(|v| v.ident.as_ref().expect("known to have field name"));
+            let field_names = field_idents.clone().map(|v| v.to_string());
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics #trait_path for #ident #type_generics
+                #where_clause
+                {
+                    fn #fn_name(#this_arg, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                        let _ = #this_arg_name;
+                        f.debug_struct(#debug_struct_name)
+                            #(.field(#field_names, &#this_arg_name.#field_idents))*
+                            .finish()
+                    }
+                }
+            }
+        }
+        Fields::Unnamed(fields) => {
+            let field_members = fields
+                .unnamed
+                .iter()
+                .enumerate()
+                .map(|(index, _)| syn::Index {
+                    index: index as _,
+                    span,
+                });
+            quote_spanned! {span=>
+                #[automatically_derived]
+                impl #impl_generics #trait_path for #ident #type_generics
+                #where_clause
+                {
+                    fn #fn_name(#this_arg, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                        let _ = #this_arg_name;
+                        f.debug_tuple(#debug_struct_name)
+                            #(.field(&#this_arg_name.#field_members))*
+                            .finish()
+                    }
+                }
+            }
+        }
+        Fields::Unit => quote_spanned! {ident.span()=>
+            #[automatically_derived]
+            impl #impl_generics #trait_path for #ident #type_generics
+            #where_clause
+            {
+                fn #fn_name(#this_arg, f: &mut ::fayalite::__std::fmt::Formatter<'_>) -> ::fayalite::__std::fmt::Result {
+                    let _ = #this_arg_name;
+                    f.write_str(#debug_struct_name)
+                }
+            }
+        },
+    }
+}

crates/fayalite-proc-macros-impl/src/lib.rs

@@ -42,6 +42,7 @@ pub(crate) trait CustomToken:
 
 mod kw {
     pub(crate) use syn::token::Extern as extern_;
+    pub(crate) use syn::token::Type as type_;
 
     macro_rules! custom_keyword {
         ($kw:ident) => {
@@ -75,6 +76,8 @@ mod kw {
     custom_keyword!(cmp_eq);
     custom_keyword!(connect_inexact);
     custom_keyword!(custom_bounds);
+    custom_keyword!(custom_debug);
+    custom_keyword!(custom_sim_display);
     custom_keyword!(flip);
     custom_keyword!(get);
     custom_keyword!(hdl);
@@ -83,6 +86,8 @@ mod kw {
     custom_keyword!(input);
     custom_keyword!(instance);
     custom_keyword!(m);
+    custom_keyword!(mask_sim);
+    custom_keyword!(mask_type);
     custom_keyword!(memory);
     custom_keyword!(memory_array);
     custom_keyword!(memory_with_init);

crates/fayalite-proc-macros-impl/src/module/transform_body/expand_match.rs

@@ -1096,11 +1096,9 @@ impl Visitor<'_> {
                 let (#(#bindings,)*) = {
                     type __MatchTy<T> = <T as ::fayalite::ty::Type>::SimValue;
                     let __match_value = #expr;
-                    let __match_value = {
-                        use ::fayalite::sim::value::match_sim_value::*;
-                        // use method syntax to deduce the correct trait to call
-                        ::fayalite::sim::value::match_sim_value::MatchSimValueHelper::new(__match_value).__fayalite_match_sim_value()
-                    };
+                    // use method syntax to deduce what type to convert to
+                    let __match_value = ::fayalite::sim::value::match_sim_value::MatchSimValueHelper::new(__match_value)
+                        .__fayalite_match_sim_value();
                     #let_token #pat #eq_token __match_value #semi_token
                     (#(#bindings_idents,)*)
                 };
@@ -1172,11 +1170,9 @@ impl Visitor<'_> {
                 {
                     type __MatchTy<T> = <T as ::fayalite::ty::Type>::SimValue;
                     let __match_value = #expr;
-                    let __match_value = {
-                        use ::fayalite::sim::value::match_sim_value::*;
-                        // use method syntax to deduce the correct trait to call
-                        ::fayalite::sim::value::match_sim_value::MatchSimValueHelper::new(__match_value).__fayalite_match_sim_value()
-                    };
+                    // use method syntax to deduce what type to convert to
+                    let __match_value = ::fayalite::sim::value::match_sim_value::MatchSimValueHelper::new(__match_value)
+                        .__fayalite_match_sim_value();
                     #match_token __match_value {
                         #(#arms)*
                     }

crates/fayalite/src/_docs/modules/module_bodies/hdl_let_statements/destructuring.rs

@@ -95,7 +95,23 @@
 //! }
 //!
 //! #[hdl]
-//! fn destructure_to_sim_value<'a, T: Type>(v: impl ToSimValue<Type = MyStruct<T>>) {
+//! fn destructure_inner<T: Type>(v: <MyStruct<T> as Type>::SimValue) {
+//!     #[hdl(sim)]
+//!     let MyStruct::<T> {
+//!         a,
+//!         mut b,
+//!         c,
+//!     } = v;
+//!
+//!     // that gives these types:
+//!     let _: SimValue<UInt<8>> = a;
+//!     let _: SimValue<Bool> = b;
+//!     let _: SimValue<T> = c;
+//!     *b = false; // can modify b since mut was used
+//! }
+//!
+//! #[hdl]
+//! fn destructure_inner_ref<'a, T: Type>(v: &'a <MyStruct<T> as Type>::SimValue) {
 //!     #[hdl(sim)]
 //!     let MyStruct::<T> {
 //!         a,
@@ -104,8 +120,25 @@
 //!     } = v;
 //!
 //!     // that gives these types:
-//!     let _: SimValue<UInt<8>> = a;
-//!     let _: SimValue<Bool> = b;
-//!     let _: SimValue<T> = c;
+//!     let _: &'a SimValue<UInt<8>> = a;
+//!     let _: &'a SimValue<Bool> = b;
+//!     let _: &'a SimValue<T> = c;
+//! }
+//!
+//! #[hdl]
+//! fn destructure_inner_mut<'a, T: Type>(v: &'a mut <MyStruct<T> as Type>::SimValue) {
+//!     #[hdl(sim)]
+//!     let MyStruct::<T> {
+//!         a,
+//!         b,
+//!         c,
+//!     } = v;
+//!
+//!     **b = true; // you can modify v by modifying b which borrows from it
+//!
+//!     // that gives these types:
+//!     let _: &'a mut SimValue<UInt<8>> = a;
+//!     let _: &'a mut SimValue<Bool> = b;
+//!     let _: &'a mut SimValue<T> = c;
 //! }
 //! ```

crates/fayalite/src/_docs/modules/module_bodies/hdl_match_statements.rs

@@ -72,15 +72,47 @@
 //! }
 //!
 //! #[hdl]
-//! fn match_to_sim_value<'a, T: Type>(v: impl ToSimValue<Type = MyEnum<T>>) {
+//! fn match_inner_move<T: Type>(v: <MyEnum<T> as Type>::SimValue) -> String {
 //!     #[hdl(sim)]
 //!     match v {
-//!         MyEnum::<T>::A => println!("got A"),
-//!         MyEnum::<T>::B(b) => {
+//!         MyEnum::<T>::A => String::from("got A"),
+//!         MyEnum::<T>::B(mut b) => {
 //!             let _: SimValue<Bool> = b; // b has this type
-//!             println!("got B({b})");
+//!             let text = format!("got B({b})");
+//!             *b = true; // can modify b since mut was used
+//!             text
 //!         }
-//!         _ => println!("something else"),
+//!         _ => String::from("something else"),
+//!     }
+//! }
+//!
+//! #[hdl]
+//! fn match_inner_ref<'a, T: Type>(v: &'a <MyEnum<T> as Type>::SimValue) -> u32 {
+//!     #[hdl(sim)]
+//!     match v {
+//!         MyEnum::<T>::A => 1,
+//!         MyEnum::<T>::B(b) => {
+//!             let _: &'a SimValue<Bool> = b; // b has this type
+//!             println!("got B({b})");
+//!             5
+//!         }
+//!         _ => 42,
+//!     }
+//! }
+//!
+//! #[hdl]
+//! fn match_inner_mut<'a, T: Type>(v: &'a mut <MyEnum<T> as Type>::SimValue) -> Option<&'a mut SimValue<T>> {
+//!     #[hdl(sim)]
+//!     match v {
+//!         MyEnum::<T>::A => None,
+//!         MyEnum::<T>::B(b) => {
+//!             println!("got B({b})");
+//!             **b = true; // you can modify v by modifying b which borrows from it
+//!             let _: &'a mut SimValue<Bool> = b; // b has this type
+//!             None
+//!         }
+//!         MyEnum::<T>::C(v) => Some(v), // you can return matched values
+//!         _ => None, // HDL enums can have invalid discriminants, so we need this extra match arm
 //!     }
 //! }
 //! ```

crates/fayalite/src/array.rs

@@ -13,13 +13,13 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, MatchVariantWithoutScope, OpaqueSimValueSlice, OpaqueSimValueWriter,
-        OpaqueSimValueWritten, StaticType, Type, TypeProperties, TypeWithDeref,
+        OpaqueSimValueWritten, SimValueDebug, StaticType, Type, TypeProperties, TypeWithDeref,
         serde_impls::SerdeCanonicalType,
     },
     util::ConstUsize,
 };
 use serde::{Deserialize, Deserializer, Serialize, Serializer, de::Error};
-use std::{borrow::Cow, iter::FusedIterator, ops::Index};
+use std::{borrow::Cow, fmt, iter::FusedIterator, ops::Index};
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct ArrayType<T: Type = CanonicalType, Len: Size = DynSize> {
@@ -28,8 +28,8 @@ pub struct ArrayType<T: Type = CanonicalType, Len: Size = DynSize> {
     type_properties: TypeProperties,
 }
 
-impl<T: Type, Len: Size> std::fmt::Debug for ArrayType<T, Len> {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl<T: Type, Len: Size> fmt::Debug for ArrayType<T, Len> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(f, "Array<{:?}, {}>", self.element, self.len())
     }
 }
@@ -182,6 +182,15 @@ impl<T: Type + Visit<State>, Len: Size, State: Visitor + ?Sized> Visit<State>
     }
 }
 
+impl<T: Type, Len: Size> SimValueDebug for ArrayType<T, Len> {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl<T: Type, Len: Size> Type for ArrayType<T, Len> {
     type BaseType = Array;
     type MaskType = ArrayType<T::MaskType, Len>;

crates/fayalite/src/bundle.rs

@@ -14,8 +14,8 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, MatchVariantWithoutScope, OpaqueSimValue, OpaqueSimValueSize,
-        OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten, StaticType, Type,
-        TypeProperties, TypeWithDeref, impl_match_variant_as_self,
+        OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten, SimValueDebug,
+        StaticType, Type, TypeProperties, TypeWithDeref, impl_match_variant_as_self,
     },
     util::HashMap,
 };
@@ -271,6 +271,15 @@ impl Type for Bundle {
     }
 }
 
+impl SimValueDebug for Bundle {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 pub trait BundleType: Type<BaseType = Bundle> {
     type Builder: Default;
     fn fields(&self) -> Interned<[BundleField]>;
@@ -471,6 +480,14 @@ macro_rules! impl_tuples {
                 #[var($var)]
             })*]
         }
+        impl<$($T: Type,)*> SimValueDebug for ($($T,)*) {
+            fn sim_value_debug(
+                value: &<Self as Type>::SimValue,
+                f: &mut fmt::Formatter<'_>,
+            ) -> fmt::Result {
+                fmt::Debug::fmt(value, f)
+            }
+        }
         impl<$($T: Type,)*> Type for ($($T,)*) {
             type BaseType = Bundle;
             type MaskType = ($($T::MaskType,)*);
@@ -773,6 +790,15 @@ impl_tuples! {
     ]
 }
 
+impl<T: ?Sized + Send + Sync + 'static> SimValueDebug for PhantomData<T> {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl<T: ?Sized + Send + Sync + 'static> Type for PhantomData<T> {
     type BaseType = Bundle;
     type MaskType = ();

crates/fayalite/src/clock.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
+
 use crate::{
     expr::{Expr, ValueType},
     hdl,
@@ -9,10 +10,12 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, OpaqueSimValueSize, OpaqueSimValueSlice, OpaqueSimValueWriter,
-        OpaqueSimValueWritten, StaticType, Type, TypeProperties, impl_match_variant_as_self,
+        OpaqueSimValueWritten, SimValueDebug, StaticType, Type, TypeProperties,
+        impl_match_variant_as_self,
     },
 };
 use bitvec::{bits, order::Lsb0};
+use std::fmt;
 
 #[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Default)]
 pub struct Clock;
@@ -69,6 +72,15 @@ impl Type for Clock {
     }
 }
 
+impl SimValueDebug for Clock {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl Clock {
     pub fn type_properties(self) -> TypeProperties {
         Self::TYPE_PROPERTIES

crates/fayalite/src/enum_.rs

@@ -2,7 +2,7 @@
 // See Notices.txt for copyright information
 
 use crate::{
-    expr::{Expr, HdlPartialEq, HdlPartialEqImpl, ToExpr, ValueType, ops::VariantAccess},
+    expr::{Expr, ToExpr, ValueType, ops::VariantAccess},
     hdl,
     int::{Bool, UIntValue},
     intern::{Intern, Interned},
@@ -10,18 +10,18 @@ use crate::{
         EnumMatchVariantAndInactiveScopeImpl, EnumMatchVariantsIterImpl, Scope, connect,
         enum_match_variants_helper, incomplete_wire, wire,
     },
-    sim::value::SimValue,
+    sim::value::{SimValue, ToSimValue, ToSimValueWithType},
     source_location::SourceLocation,
     ty::{
         CanonicalType, MatchVariantAndInactiveScope, OpaqueSimValue, OpaqueSimValueSize,
-        OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten, StaticType, Type,
-        TypeProperties,
+        OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten, SimValueDebug,
+        StaticType, Type, TypeProperties,
     },
     util::HashMap,
 };
 use bitvec::{order::Lsb0, slice::BitSlice, view::BitView};
 use serde::{Deserialize, Serialize};
-use std::{borrow::Cow, convert::Infallible, fmt, iter::FusedIterator, sync::Arc};
+use std::{convert::Infallible, fmt, iter::FusedIterator, sync::Arc};
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Serialize, Deserialize)]
 pub struct EnumVariant {
@@ -410,6 +410,15 @@ impl Type for Enum {
     }
 }
 
+impl SimValueDebug for Enum {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, Hash, Debug, Default)]
 pub struct EnumPaddingSimValue {
     bits: Option<UIntValue>,
@@ -723,95 +732,12 @@ pub fn enum_type_to_sim_builder<T: EnumType>(v: T) -> T::SimBuilder {
     v.into()
 }
 
-#[hdl]
+#[hdl(cmp_eq)]
 pub enum HdlOption<T: Type> {
     HdlNone,
     HdlSome(T),
 }
 
-impl<Lhs: Type + HdlPartialEqImpl<Rhs>, Rhs: Type> HdlPartialEqImpl<HdlOption<Rhs>>
-    for HdlOption<Lhs>
-{
-    fn cmp_value_eq(
-        lhs: Self,
-        lhs_value: Cow<'_, Self::SimValue>,
-        rhs: HdlOption<Rhs>,
-        rhs_value: Cow<'_, <HdlOption<Rhs> as Type>::SimValue>,
-    ) -> bool {
-        type SimValueMatch<T> = <T as Type>::SimValue;
-        match (&*lhs_value, &*rhs_value) {
-            (SimValueMatch::<Self>::HdlNone(_), SimValueMatch::<HdlOption<Rhs>>::HdlNone(_)) => {
-                true
-            }
-            (SimValueMatch::<Self>::HdlSome(..), SimValueMatch::<HdlOption<Rhs>>::HdlNone(_))
-            | (SimValueMatch::<Self>::HdlNone(_), SimValueMatch::<HdlOption<Rhs>>::HdlSome(..)) => {
-                false
-            }
-            (
-                SimValueMatch::<Self>::HdlSome(l, _),
-                SimValueMatch::<HdlOption<Rhs>>::HdlSome(r, _),
-            ) => HdlPartialEqImpl::cmp_value_eq(
-                lhs.HdlSome,
-                Cow::Borrowed(&**l),
-                rhs.HdlSome,
-                Cow::Borrowed(&**r),
-            ),
-        }
-    }
-
-    #[hdl]
-    fn cmp_expr_eq(lhs: Expr<Self>, rhs: Expr<HdlOption<Rhs>>) -> Expr<Bool> {
-        #[hdl]
-        let cmp_eq = wire();
-        #[hdl]
-        match lhs {
-            HdlSome(lhs) =>
-            {
-                #[hdl]
-                match rhs {
-                    HdlSome(rhs) => connect(cmp_eq, lhs.cmp_eq(rhs)),
-                    HdlNone => connect(cmp_eq, false),
-                }
-            }
-            HdlNone =>
-            {
-                #[hdl]
-                match rhs {
-                    HdlSome(_) => connect(cmp_eq, false),
-                    HdlNone => connect(cmp_eq, true),
-                }
-            }
-        }
-        cmp_eq
-    }
-
-    #[hdl]
-    fn cmp_expr_ne(lhs: Expr<Self>, rhs: Expr<HdlOption<Rhs>>) -> Expr<Bool> {
-        #[hdl]
-        let cmp_ne = wire();
-        #[hdl]
-        match lhs {
-            HdlSome(lhs) =>
-            {
-                #[hdl]
-                match rhs {
-                    HdlSome(rhs) => connect(cmp_ne, lhs.cmp_ne(rhs)),
-                    HdlNone => connect(cmp_ne, true),
-                }
-            }
-            HdlNone =>
-            {
-                #[hdl]
-                match rhs {
-                    HdlSome(_) => connect(cmp_ne, true),
-                    HdlNone => connect(cmp_ne, false),
-                }
-            }
-        }
-        cmp_ne
-    }
-}
-
 #[allow(non_snake_case)]
 pub fn HdlNone<T: StaticType>() -> Expr<HdlOption<T>> {
     HdlOption[T::TYPE].HdlNone()
@@ -823,6 +749,123 @@ pub fn HdlSome<T: Type>(value: impl ToExpr<Type = T>) -> Expr<HdlOption<T>> {
     HdlOption[value.ty()].HdlSome(value)
 }
 
+impl<T: Type> From<SimValue<HdlOption<T>>> for Option<SimValue<T>> {
+    #[hdl]
+    fn from(value: SimValue<HdlOption<T>>) -> Self {
+        #[hdl(sim)]
+        match value {
+            HdlSome(v) => Some(v),
+            HdlNone => None,
+        }
+    }
+}
+
+impl<'a, T: Type> From<&'a SimValue<HdlOption<T>>> for Option<&'a SimValue<T>> {
+    #[hdl]
+    fn from(value: &'a SimValue<HdlOption<T>>) -> Self {
+        #[hdl(sim)]
+        match value {
+            HdlSome(v) => Some(v),
+            HdlNone => None,
+        }
+    }
+}
+
+impl<'a, T: Type> From<&'a mut SimValue<HdlOption<T>>> for Option<&'a mut SimValue<T>> {
+    #[hdl]
+    fn from(value: &'a mut SimValue<HdlOption<T>>) -> Self {
+        #[hdl(sim)]
+        match value {
+            HdlSome(v) => Some(v),
+            HdlNone => None,
+        }
+    }
+}
+
+impl<T: ValueType<Type: StaticType<MaskType: StaticType>>> ValueType for Option<T> {
+    type Type = HdlOption<T::Type>;
+    type ValueCategory = T::ValueCategory;
+
+    fn ty(&self) -> Self::Type {
+        StaticType::TYPE
+    }
+}
+
+impl<T: Type, V: ToSimValueWithType<T>> ToSimValueWithType<HdlOption<T>> for Option<V> {
+    #[hdl]
+    fn to_sim_value_with_type(&self, ty: HdlOption<T>) -> SimValue<HdlOption<T>> {
+        match self {
+            Some(v) =>
+            {
+                #[hdl(sim)]
+                ty.HdlSome(v)
+            }
+            None =>
+            {
+                #[hdl(sim)]
+                ty.HdlNone()
+            }
+        }
+    }
+    #[hdl]
+    fn into_sim_value_with_type(self, ty: HdlOption<T>) -> SimValue<HdlOption<T>> {
+        match self {
+            Some(v) =>
+            {
+                #[hdl(sim)]
+                ty.HdlSome(v)
+            }
+            None =>
+            {
+                #[hdl(sim)]
+                ty.HdlNone()
+            }
+        }
+    }
+}
+
+impl<T: ToSimValue<Type: StaticType<MaskType: StaticType>>> ToSimValue for Option<T> {
+    #[hdl]
+    fn to_sim_value(&self) -> SimValue<Self::Type> {
+        match self {
+            Some(v) =>
+            {
+                #[hdl(sim)]
+                HdlSome(v)
+            }
+            None =>
+            {
+                #[hdl(sim)]
+                HdlNone()
+            }
+        }
+    }
+    #[hdl]
+    fn into_sim_value(self) -> SimValue<Self::Type> {
+        match self {
+            Some(v) =>
+            {
+                #[hdl(sim)]
+                HdlSome(v)
+            }
+            None =>
+            {
+                #[hdl(sim)]
+                HdlNone()
+            }
+        }
+    }
+}
+
+impl<T: ToExpr<Type: StaticType<MaskType: StaticType>>> ToExpr for Option<T> {
+    fn to_expr(&self) -> Expr<Self::Type> {
+        match self {
+            Some(v) => HdlSome(v),
+            None => HdlNone(),
+        }
+    }
+}
+
 impl<T: Type> HdlOption<T> {
     #[track_caller]
     pub fn try_map<R: Type, E>(

crates/fayalite/src/expr.rs

@@ -17,7 +17,7 @@ use crate::{
     reg::Reg,
     reset::{AsyncReset, Reset, ResetType, ResetTypeDispatch, SyncReset},
     sim::value::{SimValue, ToSimValue, ToSimValueWithType},
-    ty::{CanonicalType, OpaqueSimValue, StaticType, Type, TypeWithDeref},
+    ty::{CanonicalType, OpaqueSimValue, StaticType, TraceAsString, Type, TypeWithDeref},
     util::{ConstBool, ConstUsize},
     wire::Wire,
 };
@@ -218,6 +218,8 @@ expr_enum! {
         SliceSInt(ops::SliceSInt),
         CastToBits(ops::CastToBits),
         CastBitsTo(ops::CastBitsTo),
+        AsTraceAsString(ops::AsTraceAsString),
+        TraceAsStringAsInner(ops::TraceAsStringAsInner),
         ModuleIO(ModuleIO<CanonicalType>),
         Instance(Instance<Bundle>),
         Wire(Wire<CanonicalType>),
@@ -389,6 +391,35 @@ impl<T: Type> Expr<T> {
             __flow: this.__flow,
         }
     }
+    #[track_caller]
+    pub fn as_trace_as_string(this: Self, ty: TraceAsString<T>) -> Expr<TraceAsString<T>> {
+        assert_eq!(this.ty(), ty.inner_ty());
+        ops::AsTraceAsString::new(Expr::canonical(this), ty).to_expr()
+    }
+}
+
+impl Expr<CanonicalType> {
+    pub fn unwrap_transparent_types(mut this: Self) -> Expr<CanonicalType> {
+        loop {
+            match this.ty() {
+                CanonicalType::UInt(_)
+                | CanonicalType::SInt(_)
+                | CanonicalType::Bool(_)
+                | CanonicalType::Array(_)
+                | CanonicalType::Enum(_)
+                | CanonicalType::Bundle(_)
+                | CanonicalType::AsyncReset(_)
+                | CanonicalType::SyncReset(_)
+                | CanonicalType::Reset(_)
+                | CanonicalType::Clock(_)
+                | CanonicalType::PhantomConst(_)
+                | CanonicalType::DynSimOnly(_) => return this,
+                CanonicalType::TraceAsString(_) => {
+                    this = *Expr::<TraceAsString>::from_canonical(this);
+                }
+            }
+        }
+    }
 }
 
 impl<T: Type> ToLiteralBits for Expr<T> {

crates/fayalite/src/expr/ops.rs

@@ -11,8 +11,9 @@ use crate::{
         HdlPartialEqImpl, HdlPartialOrd, HdlPartialOrdImpl, NotALiteralExpr, ReduceBitsImpl,
         ToExpr, ToLiteralBits, ToSimValueInner, ToValueless, ValueType, Valueless,
         target::{
-            GetTarget, Target, TargetPathArrayElement, TargetPathBundleField,
-            TargetPathDynArrayElement, TargetPathElement,
+            GetTarget, Target, TargetPathArrayElement, TargetPathAsTraceAsString,
+            TargetPathBundleField, TargetPathDynArrayElement, TargetPathElement,
+            TargetPathTraceAsStringInner,
         },
         value_category::ValueCategoryExpr,
     },
@@ -27,7 +28,7 @@ use crate::{
         ToSyncReset,
     },
     sim::value::{SimValue, ToSimValue},
-    ty::{CanonicalType, StaticType, Type},
+    ty::{CanonicalType, StaticType, TraceAsString, Type},
     util::ConstUsize,
 };
 use bitvec::{order::Lsb0, slice::BitSlice, vec::BitVec, view::BitView};
@@ -4694,3 +4695,185 @@ impl<This: ExprFromIterator<A>, A> FromIterator<A> for Expr<This> {
         This::expr_from_iter(iter)
     }
 }
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct AsTraceAsString<T: Type = CanonicalType> {
+    inner: Expr<CanonicalType>,
+    ty: TraceAsString<T>,
+    literal_bits: Result<Interned<BitSlice>, NotALiteralExpr>,
+    target: Option<Interned<Target>>,
+}
+
+impl<T: Type> fmt::Debug for AsTraceAsString<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            inner,
+            ty: _,
+            literal_bits: _,
+            target: _,
+        } = self;
+        f.debug_struct("AsTraceAsString")
+            .field("inner", inner)
+            .finish_non_exhaustive()
+    }
+}
+
+impl<T: Type> AsTraceAsString<T> {
+    pub fn new(inner: Expr<CanonicalType>, ty: TraceAsString<T>) -> Self {
+        assert_eq!(inner.ty(), ty.inner_ty().canonical());
+        let literal_bits = inner.to_literal_bits();
+        let target = inner.target().map(|base| {
+            Intern::intern_sized(
+                base.join(TargetPathElement::intern_sized(
+                    TargetPathAsTraceAsString {
+                        ty: ty.canonical_trace_as_string(),
+                    }
+                    .into(),
+                )),
+            )
+        });
+        Self {
+            inner,
+            ty,
+            literal_bits,
+            target,
+        }
+    }
+    pub fn inner(self) -> Expr<CanonicalType> {
+        self.inner
+    }
+}
+
+impl<T: Type> GetTarget for AsTraceAsString<T> {
+    fn target(&self) -> Option<Interned<Target>> {
+        self.target
+    }
+}
+
+impl<T: Type> ToLiteralBits for AsTraceAsString<T> {
+    fn to_literal_bits(&self) -> Result<Interned<BitSlice>, NotALiteralExpr> {
+        self.literal_bits
+    }
+}
+
+impl<T: Type> ValueType for AsTraceAsString<T> {
+    type Type = TraceAsString<T>;
+    type ValueCategory = ValueCategoryExpr;
+
+    fn ty(&self) -> Self::Type {
+        self.ty
+    }
+}
+
+impl<T: Type> ToExpr for AsTraceAsString<T> {
+    fn to_expr(&self) -> Expr<Self::Type> {
+        Expr {
+            __enum: ExprEnum::AsTraceAsString(AsTraceAsString {
+                inner: self.inner,
+                ty: self.ty.canonical_trace_as_string(),
+                literal_bits: self.literal_bits,
+                target: self.target,
+            })
+            .intern(),
+            __ty: self.ty,
+            __flow: Expr::flow(self.inner),
+        }
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct TraceAsStringAsInner<T: Type = CanonicalType> {
+    arg: Expr<TraceAsString<CanonicalType>>,
+    ty: T,
+    literal_bits: Result<Interned<BitSlice>, NotALiteralExpr>,
+    target: Option<Interned<Target>>,
+}
+
+impl<T: Type> fmt::Debug for TraceAsStringAsInner<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            arg,
+            ty: _,
+            literal_bits: _,
+            target: _,
+        } = self;
+        f.debug_struct("TraceAsStringAsInner")
+            .field("arg", arg)
+            .finish_non_exhaustive()
+    }
+}
+
+impl<T: Type> TraceAsStringAsInner<T> {
+    pub fn from_arg_and_ty(arg: Expr<TraceAsString<CanonicalType>>, ty: T) -> Self {
+        assert_eq!(arg.ty().inner_ty(), ty.canonical());
+        let literal_bits = arg.to_literal_bits();
+        let target = arg.target().map(|base| {
+            Intern::intern_sized(base.join(TargetPathElement::intern_sized(
+                TargetPathTraceAsStringInner {}.into(),
+            )))
+        });
+        Self {
+            arg,
+            ty,
+            literal_bits,
+            target,
+        }
+    }
+    pub fn new(arg: Expr<TraceAsString<T>>) -> Self {
+        Self::from_arg_and_ty(
+            Expr {
+                __enum: arg.__enum,
+                __ty: arg.__ty.canonical_trace_as_string(),
+                __flow: arg.__flow,
+            },
+            arg.ty().inner_ty(),
+        )
+    }
+    pub fn arg(self) -> Expr<TraceAsString<CanonicalType>> {
+        self.arg
+    }
+    pub fn arg_typed(self) -> Expr<TraceAsString<T>> {
+        Expr {
+            __enum: self.arg.__enum,
+            __ty: TraceAsString::from_canonical_trace_as_string(self.arg.__ty),
+            __flow: self.arg.__flow,
+        }
+    }
+}
+
+impl<T: Type> GetTarget for TraceAsStringAsInner<T> {
+    fn target(&self) -> Option<Interned<Target>> {
+        self.target
+    }
+}
+
+impl<T: Type> ToLiteralBits for TraceAsStringAsInner<T> {
+    fn to_literal_bits(&self) -> Result<Interned<BitSlice>, NotALiteralExpr> {
+        self.literal_bits
+    }
+}
+
+impl<T: Type> ValueType for TraceAsStringAsInner<T> {
+    type Type = T;
+    type ValueCategory = ValueCategoryExpr;
+
+    fn ty(&self) -> Self::Type {
+        self.ty
+    }
+}
+
+impl<T: Type> ToExpr for TraceAsStringAsInner<T> {
+    fn to_expr(&self) -> Expr<Self::Type> {
+        Expr {
+            __enum: ExprEnum::TraceAsStringAsInner(TraceAsStringAsInner {
+                arg: self.arg,
+                ty: self.ty.canonical(),
+                literal_bits: self.literal_bits,
+                target: self.target,
+            })
+            .intern(),
+            __ty: self.ty,
+            __flow: Expr::flow(self.arg),
+        }
+    }
+}

crates/fayalite/src/expr/target.rs

@@ -10,7 +10,7 @@ use crate::{
     reg::Reg,
     reset::{AsyncReset, Reset, ResetType, ResetTypeDispatch, SyncReset},
     source_location::SourceLocation,
-    ty::{CanonicalType, Type},
+    ty::{CanonicalType, TraceAsString, Type},
     wire::Wire,
 };
 use std::fmt;
@@ -46,11 +46,33 @@ impl fmt::Display for TargetPathDynArrayElement {
     }
 }
 
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub struct TargetPathTraceAsStringInner {}
+
+impl fmt::Display for TargetPathTraceAsStringInner {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, ".<inner>")
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub struct TargetPathAsTraceAsString {
+    pub ty: TraceAsString<CanonicalType>,
+}
+
+impl fmt::Display for TargetPathAsTraceAsString {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, ".as_trace_as_string(...)")
+    }
+}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub enum TargetPathElement {
     BundleField(TargetPathBundleField),
     ArrayElement(TargetPathArrayElement),
     DynArrayElement(TargetPathDynArrayElement),
+    TraceAsStringInner(TargetPathTraceAsStringInner),
+    AsTraceAsString(TargetPathAsTraceAsString),
 }
 
 impl From<TargetPathBundleField> for TargetPathElement {
@@ -71,12 +93,26 @@ impl From<TargetPathDynArrayElement> for TargetPathElement {
     }
 }
 
+impl From<TargetPathTraceAsStringInner> for TargetPathElement {
+    fn from(value: TargetPathTraceAsStringInner) -> Self {
+        Self::TraceAsStringInner(value)
+    }
+}
+
+impl From<TargetPathAsTraceAsString> for TargetPathElement {
+    fn from(value: TargetPathAsTraceAsString) -> Self {
+        Self::AsTraceAsString(value)
+    }
+}
+
 impl fmt::Display for TargetPathElement {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
             Self::BundleField(v) => v.fmt(f),
             Self::ArrayElement(v) => v.fmt(f),
             Self::DynArrayElement(v) => v.fmt(f),
+            Self::TraceAsStringInner(v) => v.fmt(f),
+            Self::AsTraceAsString(v) => v.fmt(f),
         }
     }
 }
@@ -100,6 +136,15 @@ impl TargetPathElement {
                 let parent_ty = Array::<CanonicalType>::from_canonical(parent.canonical_ty());
                 parent_ty.element()
             }
+            Self::TraceAsStringInner(_) => {
+                let parent_ty =
+                    TraceAsString::<CanonicalType>::from_canonical(parent.canonical_ty());
+                parent_ty.inner_ty()
+            }
+            &Self::AsTraceAsString(TargetPathAsTraceAsString { ty }) => {
+                assert_eq!(parent.canonical_ty(), ty.inner_ty());
+                ty.canonical()
+            }
         }
     }
     pub fn flow(&self, parent: Interned<Target>) -> Flow {
@@ -111,13 +156,18 @@ impl TargetPathElement {
                     .expect("field name is known to be a valid field of parent type");
                 parent.flow().flip_if(field.flipped)
             }
-            Self::ArrayElement(_) => parent.flow(),
-            Self::DynArrayElement(_) => parent.flow(),
+            Self::ArrayElement(_)
+            | Self::DynArrayElement(_)
+            | Self::TraceAsStringInner(_)
+            | Self::AsTraceAsString(_) => parent.flow(),
         }
     }
     pub fn is_static(&self) -> bool {
         match self {
-            Self::BundleField(_) | Self::ArrayElement(_) => true,
+            Self::BundleField(_)
+            | Self::ArrayElement(_)
+            | Self::TraceAsStringInner(_)
+            | Self::AsTraceAsString(_) => true,
             Self::DynArrayElement(_) => false,
         }
     }

crates/fayalite/src/firrtl.rs

@@ -16,6 +16,7 @@ use crate::{
         ops::{self, VariantAccess},
         target::{
             Target, TargetBase, TargetPathArrayElement, TargetPathBundleField, TargetPathElement,
+            TargetPathTraceAsStringInner,
         },
     },
     formal::FormalKind,
@@ -471,7 +472,7 @@ impl TypeState {
         Ok(self.enum_def(ty)?.1.variants.borrow_mut().get(name))
     }
     fn ty<T: Type>(&self, ty: T) -> Result<String, FirrtlError> {
-        Ok(match ty.canonical() {
+        Ok(match ty.canonical().unwrap_transparent_types() {
             CanonicalType::Bundle(ty) => self.bundle_ty(ty)?.to_string(),
             CanonicalType::Enum(ty) => self.enum_ty(ty)?.to_string(),
             CanonicalType::Array(ty) => {
@@ -490,6 +491,7 @@ impl TypeState {
             CanonicalType::DynSimOnly(_) => {
                 return Err(FirrtlError::SimOnlyValuesAreNotPermitted);
             }
+            CanonicalType::TraceAsString(_) => unreachable!("handled by unwrap_transparent_types"),
         })
     }
 }
@@ -1191,7 +1193,7 @@ impl<'a> Exporter<'a> {
         definitions: &RcDefinitions,
         extra_indent: Indent<'_>,
     ) -> Result<String> {
-        match ty {
+        match ty.unwrap_transparent_types() {
             CanonicalType::Bundle(ty) => {
                 self.expr_cast_bundle_to_bits(value_str, ty, definitions, extra_indent)
             }
@@ -1210,6 +1212,7 @@ impl<'a> Exporter<'a> {
             | CanonicalType::Reset(_) => Ok(format!("asUInt({value_str})")),
             CanonicalType::PhantomConst(_) => Ok("UInt<0>(0)".into()),
             CanonicalType::DynSimOnly(_) => Err(FirrtlError::SimOnlyValuesAreNotPermitted.into()),
+            CanonicalType::TraceAsString(_) => unreachable!("handled by unwrap_transparent_types"),
         }
     }
     fn expr_cast_bits_to_bundle(
@@ -1407,7 +1410,7 @@ impl<'a> Exporter<'a> {
         definitions: &RcDefinitions,
         extra_indent: Indent<'_>,
     ) -> Result<String> {
-        match ty {
+        match ty.unwrap_transparent_types() {
             CanonicalType::Bundle(ty) => {
                 self.expr_cast_bits_to_bundle(value_str, ty, definitions, extra_indent)
             }
@@ -1431,6 +1434,7 @@ impl<'a> Exporter<'a> {
                 return Ok(retval.to_string());
             }
             CanonicalType::DynSimOnly(_) => Err(FirrtlError::SimOnlyValuesAreNotPermitted.into()),
+            CanonicalType::TraceAsString(_) => unreachable!("handled by unwrap_transparent_types"),
         }
     }
     fn expr_unary<T: Type>(
@@ -1798,6 +1802,10 @@ impl<'a> Exporter<'a> {
                 write!(out, "[{index}]").unwrap();
                 Ok(out)
             }
+            ExprEnum::AsTraceAsString(expr) => self.expr(expr.inner(), definitions, const_ty),
+            ExprEnum::TraceAsStringAsInner(expr) => {
+                self.expr(Expr::canonical(expr.arg()), definitions, const_ty)
+            }
             ExprEnum::ModuleIO(expr) => Ok(self.module.ns.get(expr.name_id()).to_string()),
             ExprEnum::Instance(expr) => {
                 assert!(!const_ty, "not a constant");
@@ -1957,6 +1965,10 @@ impl<'a> Exporter<'a> {
                         .segments
                         .push(AnnotationTargetRefSegment::Index { index }),
                     TargetPathElement::DynArrayElement(_) => unreachable!(),
+                    TargetPathElement::AsTraceAsString(_)
+                    | TargetPathElement::TraceAsStringInner(_) => {
+                        // ignored
+                    }
                 }
                 Ok(retval)
             }
@@ -3211,6 +3223,8 @@ impl ScalarizeTreeNode {
                     TargetPathElement::DynArrayElement(_) => {
                         unreachable!("annotations are only on static targets");
                     }
+                    TargetPathElement::AsTraceAsString(_)
+                    | TargetPathElement::TraceAsStringInner(_) => parent,
                 }
             }
         }
@@ -3337,6 +3351,13 @@ impl ScalarizeTreeBuilder {
             CanonicalType::DynSimOnly(_) => {
                 return Err(ScalarizedModuleABIError::SimOnlyValuesAreNotPermitted);
             }
+            CanonicalType::TraceAsString(_) => self.build(
+                target
+                    .join(TargetPathElement::intern_sized(
+                        TargetPathTraceAsStringInner {}.into(),
+                    ))
+                    .intern_sized(),
+            )?,
         })
     }
 }

crates/fayalite/src/int.rs

@@ -15,8 +15,8 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, FillInDefaultedGenerics, OpaqueSimValueSize, OpaqueSimValueSlice,
-        OpaqueSimValueWriter, OpaqueSimValueWritten, StaticType, Type, TypeProperties,
-        impl_match_variant_as_self,
+        OpaqueSimValueWriter, OpaqueSimValueWritten, SimValueDebug, SimValueDisplay, StaticType,
+        Type, TypeProperties, impl_match_variant_as_self,
     },
     util::{ConstBool, ConstUsize, GenericConstBool, GenericConstUsize, interned_bit, slice_range},
 };
@@ -1019,6 +1019,24 @@ macro_rules! impl_int {
             }
         }
 
+        impl<Width: Size> SimValueDebug for $name<Width> {
+            fn sim_value_debug(
+                value: &<Self as Type>::SimValue,
+                f: &mut fmt::Formatter<'_>,
+            ) -> fmt::Result {
+                fmt::Debug::fmt(value, f)
+            }
+        }
+
+        impl<Width: Size> SimValueDisplay for $name<Width> {
+            fn sim_value_display(
+                value: &<Self as Type>::SimValue,
+                f: &mut fmt::Formatter<'_>,
+            ) -> fmt::Result {
+                fmt::Display::fmt(value, f)
+            }
+        }
+
         impl<Width: KnownSize> Default for $name<Width> {
             fn default() -> Self {
                 Self::TYPE
@@ -1259,6 +1277,9 @@ macro_rules! impl_int {
             pub fn bitvec_mut(&mut self) -> &mut BitVec {
                 Arc::make_mut(&mut self.bits)
             }
+            pub fn arc_bitvec_mut(&mut self) -> &mut Arc<BitVec> {
+                &mut self.bits
+            }
         }
     };
 }
@@ -1899,6 +1920,15 @@ impl Type for Bool {
     }
 }
 
+impl SimValueDebug for Bool {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl StaticType for Bool {
     const TYPE: Self = Bool;
     const MASK_TYPE: Self::MaskType = Bool;

crates/fayalite/src/int/uint_in_range.rs

@@ -14,7 +14,7 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten,
-        StaticType, Type, TypeProperties, impl_match_variant_as_self,
+        SimValueDebug, StaticType, Type, TypeProperties, impl_match_variant_as_self,
     },
 };
 use bitvec::{order::Lsb0, view::BitView};
@@ -94,6 +94,15 @@ impl Type for UIntInRangeMaskType {
     }
 }
 
+impl SimValueDebug for UIntInRangeMaskType {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl BundleType for UIntInRangeMaskType {
     type Builder = NoBuilder;
 
@@ -339,6 +348,15 @@ macro_rules! define_uint_in_range_type {
             }
         }
 
+        impl<Start: Size, End: Size> SimValueDebug for $UIntInRangeType<Start, End> {
+            fn sim_value_debug(
+                value: &<Self as Type>::SimValue,
+                f: &mut fmt::Formatter<'_>,
+            ) -> fmt::Result {
+                fmt::Debug::fmt(value, f)
+            }
+        }
+
         impl<Start: Size, End: Size> fmt::Debug for $UIntInRangeType<Start, End> {
             fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 let Self { value, range } = self;

crates/fayalite/src/memory.rs

@@ -1093,6 +1093,7 @@ pub fn splat_mask<T: Type>(ty: T, value: Expr<Bool>) -> Expr<AsMask<T>> {
             .to_expr(),
         )),
         CanonicalType::PhantomConst(_) => Expr::from_canonical(Expr::canonical(().to_expr())),
+        CanonicalType::TraceAsString(ty) => Expr::from_canonical(splat_mask(ty.inner_ty(), value)),
     }
 }
 

crates/fayalite/src/module.rs

@@ -12,7 +12,7 @@ use crate::{
         ops::VariantAccess,
         target::{
             GetTarget, Target, TargetBase, TargetPathArrayElement, TargetPathBundleField,
-            TargetPathElement,
+            TargetPathElement, TargetPathTraceAsStringInner,
         },
         value_category::ValueCategoryExpr,
     },
@@ -1111,7 +1111,10 @@ fn validate_clock_for_past<S: ModuleBuildingStatus>(
             let mut target = clock_for_past;
             while let Target::Child(child) = target {
                 match *child.path_element() {
-                    TargetPathElement::BundleField(_) | TargetPathElement::ArrayElement(_) => {}
+                    TargetPathElement::BundleField(_)
+                    | TargetPathElement::ArrayElement(_)
+                    | TargetPathElement::AsTraceAsString(_)
+                    | TargetPathElement::TraceAsStringInner(_) => {}
                     TargetPathElement::DynArrayElement(_) => {
                         panic!(
                             "clock_for_past: clock must be a static target (you can't use `Expr<UInt>` array indexes):\n{clock_for_past:?}"
@@ -1586,6 +1589,14 @@ impl TargetState {
                     declared_in_block,
                     written_in_blocks: RefCell::default(),
                 },
+                CanonicalType::TraceAsString(_) => {
+                    return Self::new(
+                        target
+                            .join(Intern::intern_sized(TargetPathTraceAsStringInner {}.into()))
+                            .intern_sized(),
+                        declared_in_block,
+                    );
+                }
             },
         }
     }
@@ -1605,44 +1616,58 @@ impl AssertValidityState {
     }
     fn get_target_states<'a>(
         &'a self,
-        target: &Target,
+        mut target: &Target,
         process_target_state: &dyn Fn(&'a TargetState, bool),
     ) -> Result<(), ()> {
-        match target {
-            Target::Base(target_base) => {
-                let target_state = self.get_base_state(*target_base)?;
-                process_target_state(target_state, false);
-                Ok(())
-            }
-            Target::Child(target_child) => self.get_target_states(
-                &target_child.parent(),
-                &|target_state, exact_target_unknown| {
-                    let TargetStateInner::Decomposed { subtargets } = &target_state.inner else {
-                        unreachable!(
-                            "TargetState::new makes TargetState tree match the Target type"
-                        );
-                    };
-                    match *target_child.path_element() {
-                        TargetPathElement::BundleField(_) => process_target_state(
-                            subtargets
-                                .get(&target_child.path_element())
-                                .expect("bundle fields filled in by TargetState::new"),
-                            exact_target_unknown,
-                        ),
-                        TargetPathElement::ArrayElement(_) => process_target_state(
-                            subtargets
-                                .get(&target_child.path_element())
-                                .expect("array elements filled in by TargetState::new"),
-                            exact_target_unknown,
-                        ),
-                        TargetPathElement::DynArrayElement(_) => {
-                            for target_state in subtargets.values() {
-                                process_target_state(target_state, true);
+        loop {
+            break match target {
+                Target::Base(target_base) => {
+                    let target_state = self.get_base_state(*target_base)?;
+                    process_target_state(target_state, false);
+                    Ok(())
+                }
+                Target::Child(target_child) => match *target_child.path_element() {
+                    TargetPathElement::BundleField(_)
+                    | TargetPathElement::ArrayElement(_)
+                    | TargetPathElement::DynArrayElement(_) => self.get_target_states(
+                        &target_child.parent(),
+                        &|target_state, exact_target_unknown| {
+                            let TargetStateInner::Decomposed { subtargets } = &target_state.inner
+                            else {
+                                unreachable!(
+                                    "TargetState::new makes TargetState tree match the Target type"
+                                );
+                            };
+                            match *target_child.path_element() {
+                                TargetPathElement::BundleField(_) => process_target_state(
+                                    subtargets
+                                        .get(&target_child.path_element())
+                                        .expect("bundle fields filled in by TargetState::new"),
+                                    exact_target_unknown,
+                                ),
+                                TargetPathElement::ArrayElement(_) => process_target_state(
+                                    subtargets
+                                        .get(&target_child.path_element())
+                                        .expect("array elements filled in by TargetState::new"),
+                                    exact_target_unknown,
+                                ),
+                                TargetPathElement::DynArrayElement(_) => {
+                                    for target_state in subtargets.values() {
+                                        process_target_state(target_state, true);
+                                    }
+                                }
+                                TargetPathElement::TraceAsStringInner(_)
+                                | TargetPathElement::AsTraceAsString(_) => unreachable!(),
                             }
-                        }
+                        },
+                    ),
+                    TargetPathElement::TraceAsStringInner(_)
+                    | TargetPathElement::AsTraceAsString(_) => {
+                        target = Interned::into_inner(target_child.parent());
+                        continue;
                     }
                 },
-            ),
+            };
         }
     }
     fn get_base_state(&self, target_base: Interned<TargetBase>) -> Result<&TargetState, ()> {
@@ -1716,6 +1741,8 @@ impl AssertValidityState {
                         TargetPathElement::DynArrayElement { .. } => {
                             Self::set_connect_target_written(sub_target_state, is_lhs, block, true);
                         }
+                        TargetPathElement::TraceAsStringInner(_)
+                        | TargetPathElement::AsTraceAsString(_) => unreachable!("never added"),
                     }
                 }
             }

crates/fayalite/src/module/transform/deduce_resets.rs

@@ -9,8 +9,9 @@ use crate::{
         ExprEnum, ValueType,
         ops::{self, ArrayLiteral},
         target::{
-            Target, TargetBase, TargetChild, TargetPathArrayElement, TargetPathBundleField,
-            TargetPathDynArrayElement, TargetPathElement,
+            Target, TargetBase, TargetChild, TargetPathArrayElement, TargetPathAsTraceAsString,
+            TargetPathBundleField, TargetPathDynArrayElement, TargetPathElement,
+            TargetPathTraceAsStringInner,
         },
     },
     formal::FormalKind,
@@ -26,6 +27,7 @@ use crate::{
     prelude::*,
     reset::{ResetType, ResetTypeDispatch},
     sim::ExternModuleSimulation,
+    ty::TraceAsString,
     util::{HashMap, HashSet},
 };
 use hashbrown::hash_map::Entry;
@@ -103,6 +105,10 @@ enum ResetsLayout {
         element: Interned<ResetsLayout>,
         reset_count: usize,
     },
+    Transparent {
+        inner: Interned<ResetsLayout>,
+        reset_count: usize,
+    },
 }
 
 impl ResetsLayout {
@@ -112,7 +118,8 @@ impl ResetsLayout {
             ResetsLayout::Reset | ResetsLayout::SyncReset | ResetsLayout::AsyncReset => 1,
             ResetsLayout::Bundle { reset_count, .. }
             | ResetsLayout::Enum { reset_count, .. }
-            | ResetsLayout::Array { reset_count, .. } => reset_count,
+            | ResetsLayout::Array { reset_count, .. }
+            | ResetsLayout::Transparent { reset_count, .. } => reset_count,
         }
     }
     fn new(ty: CanonicalType) -> Self {
@@ -166,6 +173,13 @@ impl ResetsLayout {
                     CanonicalType::Clock(_) => ResetsLayout::NoResets,
                     CanonicalType::PhantomConst(_) => ResetsLayout::NoResets,
                     CanonicalType::DynSimOnly(_) => ResetsLayout::NoResets,
+                    CanonicalType::TraceAsString(ty) => {
+                        let inner = ResetsLayout::new(ty.inner_ty()).intern_sized();
+                        ResetsLayout::Transparent {
+                            inner,
+                            reset_count: inner.reset_count(),
+                        }
+                    }
                 }
             }
         }
@@ -315,6 +329,12 @@ impl ResetGraph {
             } => {
                 self.append_new_nodes_for_layout(*element, node_indexes, source_location);
             }
+            ResetsLayout::Transparent {
+                inner,
+                reset_count: _,
+            } => {
+                self.append_new_nodes_for_layout(*inner, node_indexes, source_location);
+            }
         }
     }
 }
@@ -357,6 +377,21 @@ impl Resets {
             node_indexes: self.node_indexes,
         }
     }
+    fn trace_as_string_inner(self) -> Self {
+        let trace_as_string = TraceAsString::from_canonical(self.ty);
+        let ResetsLayout::Transparent {
+            inner,
+            reset_count: _,
+        } = self.layout
+        else {
+            unreachable!();
+        };
+        Self {
+            ty: trace_as_string.inner_ty(),
+            layout: *inner,
+            node_indexes: self.node_indexes,
+        }
+    }
     fn bundle_fields(self) -> impl Iterator<Item = Self> {
         let bundle = Bundle::from_canonical(self.ty);
         let ResetsLayout::Bundle {
@@ -480,6 +515,17 @@ impl Resets {
                     CanonicalType::SyncReset(SyncReset)
                 },
             ),
+            CanonicalType::TraceAsString(ty) => Ok(CanonicalType::TraceAsString(
+                ty.with_new_inner_ty(
+                    self.array_elements()
+                        .substituted_type(
+                            reset_graph,
+                            fallback_to_sync_reset,
+                            fallback_error_source_location,
+                        )?
+                        .intern_sized(),
+                ),
+            )),
         }
     }
 }
@@ -1013,7 +1059,8 @@ fn cast_bit_op<P: Pass, T: Type, A: Type>(
                             | CanonicalType::Bundle(_)
                             | CanonicalType::Reset(_)
                             | CanonicalType::PhantomConst(_)
-                            | CanonicalType::DynSimOnly(_) => unreachable!(),
+                            | CanonicalType::DynSimOnly(_)
+                            | CanonicalType::TraceAsString(_) => unreachable!(),
                             $(CanonicalType::$Variant(ty) => Expr::expr_enum($arg.cast_to(ty)),)*
                         }
                     };
@@ -1024,7 +1071,8 @@ fn cast_bit_op<P: Pass, T: Type, A: Type>(
                             CanonicalType::Array(_)
                             | CanonicalType::Enum(_)
                             | CanonicalType::Bundle(_)
-                            | CanonicalType::Reset(_) => unreachable!(),
+                            | CanonicalType::Reset(_)
+                            | CanonicalType::TraceAsString(_) => unreachable!(),
                             CanonicalType::PhantomConst(_) |
                             CanonicalType::DynSimOnly(_) => Expr::expr_enum(arg),
                             $(CanonicalType::$Variant(_) => {
@@ -1156,6 +1204,10 @@ impl<P: Pass> RunPass<P> for ExprEnum {
             ExprEnum::SliceSInt(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::CastToBits(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::CastBitsTo(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
+            ExprEnum::TraceAsStringAsInner(expr) => {
+                Ok(expr.run_pass(pass_args)?.map(ExprEnum::from))
+            }
+            ExprEnum::AsTraceAsString(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::ModuleIO(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::Instance(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::Wire(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
@@ -1536,6 +1588,67 @@ impl RunPassExpr for ops::CastBitsTo {
     }
 }
 
+impl RunPassExpr for ops::TraceAsStringAsInner {
+    type Args<'a> = [Expr<CanonicalType>; 1];
+
+    fn args<'a>(&'a self) -> Self::Args<'a> {
+        [Expr::canonical(self.arg())]
+    }
+
+    fn source_location(&self) -> Option<SourceLocation> {
+        None
+    }
+
+    fn union_parts(
+        &self,
+        resets: Resets,
+        args_resets: Vec<Resets>,
+        mut pass_args: PassArgs<'_, BuildResetGraph>,
+    ) -> Result<(), DeduceResetsError> {
+        pass_args.union(resets, args_resets[0].trace_as_string_inner(), None)
+    }
+
+    fn new(
+        &self,
+        _ty: CanonicalType,
+        new_args: Vec<Expr<CanonicalType>>,
+    ) -> Result<Self, DeduceResetsError> {
+        Ok(Self::new(Expr::from_canonical(new_args[0])))
+    }
+}
+
+impl RunPassExpr for ops::AsTraceAsString {
+    type Args<'a> = [Expr<CanonicalType>; 1];
+
+    fn args<'a>(&'a self) -> Self::Args<'a> {
+        [Expr::canonical(self.inner())]
+    }
+
+    fn source_location(&self) -> Option<SourceLocation> {
+        None
+    }
+
+    fn union_parts(
+        &self,
+        resets: Resets,
+        args_resets: Vec<Resets>,
+        mut pass_args: PassArgs<'_, BuildResetGraph>,
+    ) -> Result<(), DeduceResetsError> {
+        pass_args.union(resets.trace_as_string_inner(), args_resets[0], None)
+    }
+
+    fn new(
+        &self,
+        _ty: CanonicalType,
+        new_args: Vec<Expr<CanonicalType>>,
+    ) -> Result<Self, DeduceResetsError> {
+        Ok(Self::new(
+            new_args[0],
+            self.ty().with_new_inner_ty(new_args[0].ty().intern_sized()),
+        ))
+    }
+}
+
 impl RunPassExpr for ModuleIO<CanonicalType> {
     type Args<'a> = [Expr<CanonicalType>; 0];
 
@@ -1691,7 +1804,8 @@ impl RunPassDispatch for AnyReg {
                 | CanonicalType::Reset(_)
                 | CanonicalType::Clock(_)
                 | CanonicalType::PhantomConst(_)
-                | CanonicalType::DynSimOnly(_) => unreachable!(),
+                | CanonicalType::DynSimOnly(_)
+                | CanonicalType::TraceAsString(_) => unreachable!(),
             }
         })
     }
@@ -2173,30 +2287,6 @@ impl<P: Pass> RunPass<P> for StmtDeclaration {
     }
 }
 
-impl_run_pass_for_struct! {
-    impl[] RunPass for TargetPathBundleField {
-        name: _,
-    }
-}
-
-impl_run_pass_for_struct! {
-    impl[] RunPass for TargetPathArrayElement {
-        index: _,
-    }
-}
-
-impl_run_pass_for_struct! {
-    impl[] RunPass for TargetPathDynArrayElement {}
-}
-
-impl_run_pass_for_enum! {
-    impl[] RunPass for TargetPathElement {
-        BundleField(v),
-        ArrayElement(v),
-        DynArrayElement(v),
-    }
-}
-
 impl_run_pass_for_enum! {
     impl[] RunPass for Target {
         Base(v),
@@ -2204,11 +2294,28 @@ impl_run_pass_for_enum! {
     }
 }
 
-impl_run_pass_for_struct! {
-    #[constructor = TargetChild::new(parent, path_element)]
-    impl[] RunPass for TargetChild {
-        parent(): _,
-        path_element(): _,
+impl<P: Pass> RunPass<P> for TargetChild {
+    fn run_pass(
+        &self,
+        mut pass_args: PassArgs<'_, P>,
+    ) -> Result<PassOutput<Self, P>, DeduceResetsError> {
+        Ok(self.parent().run_pass(pass_args.as_mut())?.map(|parent| {
+            let path_element = match *self.path_element() {
+                TargetPathElement::BundleField(TargetPathBundleField { name: _ })
+                | TargetPathElement::ArrayElement(TargetPathArrayElement { index: _ })
+                | TargetPathElement::DynArrayElement(TargetPathDynArrayElement {})
+                | TargetPathElement::TraceAsStringInner(TargetPathTraceAsStringInner {}) => {
+                    self.path_element()
+                }
+                TargetPathElement::AsTraceAsString(TargetPathAsTraceAsString { ty }) => {
+                    TargetPathElement::from(TargetPathAsTraceAsString {
+                        ty: ty.with_new_inner_ty(parent.canonical_ty().intern_sized()),
+                    })
+                    .intern_sized()
+                }
+            };
+            TargetChild::new(parent, path_element)
+        }))
     }
 }
 

crates/fayalite/src/module/transform/simplify_enums.rs

@@ -17,7 +17,7 @@ use crate::{
         transform::visit::{Fold, Folder},
     },
     source_location::SourceLocation,
-    ty::{CanonicalType, Type},
+    ty::{CanonicalType, TraceAsString, Type},
     util::HashMap,
     wire::Wire,
 };
@@ -64,6 +64,7 @@ fn contains_any_enum_types(ty: CanonicalType) -> bool {
                     .fields()
                     .iter()
                     .any(|field| contains_any_enum_types(field.ty)),
+                CanonicalType::TraceAsString(ty) => contains_any_enum_types(ty.inner_ty()),
                 CanonicalType::UInt(_)
                 | CanonicalType::SInt(_)
                 | CanonicalType::Bool(_)
@@ -313,6 +314,24 @@ impl State {
         }
         Ok(())
     }
+    fn handle_stmt_connect_trace_as_string(
+        &mut self,
+        unfolded_lhs_ty: TraceAsString,
+        unfolded_rhs_ty: TraceAsString,
+        folded_lhs: Expr<TraceAsString>,
+        folded_rhs: Expr<TraceAsString>,
+        source_location: SourceLocation,
+        output_stmts: &mut Vec<Stmt>,
+    ) -> Result<(), SimplifyEnumsError> {
+        self.handle_stmt_connect(
+            unfolded_lhs_ty.inner_ty(),
+            unfolded_rhs_ty.inner_ty(),
+            ops::TraceAsStringAsInner::new(folded_lhs).to_expr(),
+            ops::TraceAsStringAsInner::new(folded_rhs).to_expr(),
+            source_location,
+            output_stmts,
+        )
+    }
     fn handle_stmt_connect_bundle(
         &mut self,
         unfolded_lhs_ty: Bundle,
@@ -509,6 +528,15 @@ impl State {
                 source_location,
                 output_stmts,
             ),
+            CanonicalType::TraceAsString(unfolded_lhs_ty) => self
+                .handle_stmt_connect_trace_as_string(
+                    unfolded_lhs_ty,
+                    TraceAsString::from_canonical(unfolded_rhs_ty),
+                    Expr::from_canonical(folded_lhs),
+                    Expr::from_canonical(folded_rhs),
+                    source_location,
+                    output_stmts,
+                ),
             CanonicalType::UInt(_)
             | CanonicalType::SInt(_)
             | CanonicalType::Bool(_)
@@ -528,6 +556,8 @@ fn connect_port(
     rhs: Expr<CanonicalType>,
     source_location: SourceLocation,
 ) {
+    let lhs = Expr::unwrap_transparent_types(lhs);
+    let rhs = Expr::unwrap_transparent_types(rhs);
     if lhs.ty() == rhs.ty() {
         stmts.push(
             StmtConnect {
@@ -573,6 +603,9 @@ fn connect_port(
                 connect_port(stmts, lhs[index], rhs[index], source_location);
             }
         }
+        (CanonicalType::TraceAsString(_), CanonicalType::TraceAsString(_)) => {
+            unreachable!("handled by unwrap_transparent_types")
+        }
         (CanonicalType::Bundle(_), _)
         | (CanonicalType::Enum(_), _)
         | (CanonicalType::Array(_), _)
@@ -584,7 +617,8 @@ fn connect_port(
         | (CanonicalType::SyncReset(_), _)
         | (CanonicalType::Reset(_), _)
         | (CanonicalType::PhantomConst(_), _)
-        | (CanonicalType::DynSimOnly(_), _) => unreachable!(
+        | (CanonicalType::DynSimOnly(_), _)
+        | (CanonicalType::TraceAsString(_), _) => unreachable!(
             "trying to connect memory ports:\n{:?}\n{:?}",
             lhs.ty(),
             rhs.ty(),
@@ -772,6 +806,8 @@ impl Folder for State {
             | ExprEnum::SliceSInt(_)
             | ExprEnum::CastToBits(_)
             | ExprEnum::CastBitsTo(_)
+            | ExprEnum::TraceAsStringAsInner(_)
+            | ExprEnum::AsTraceAsString(_)
             | ExprEnum::ModuleIO(_)
             | ExprEnum::Instance(_)
             | ExprEnum::Wire(_)
@@ -936,7 +972,8 @@ impl Folder for State {
             | CanonicalType::SyncReset(_)
             | CanonicalType::Reset(_)
             | CanonicalType::PhantomConst(_)
-            | CanonicalType::DynSimOnly(_) => canonical_type.default_fold(self),
+            | CanonicalType::DynSimOnly(_)
+            | CanonicalType::TraceAsString(_) => canonical_type.default_fold(self),
         }
     }
 

crates/fayalite/src/module/transform/simplify_memories.rs

@@ -90,7 +90,7 @@ impl MemSplit {
         }
     }
     fn new(element_type: CanonicalType) -> Self {
-        match element_type {
+        match element_type.unwrap_transparent_types() {
             CanonicalType::Bundle(bundle_ty) => MemSplit::Bundle {
                 fields: bundle_ty
                     .fields()
@@ -195,6 +195,7 @@ impl MemSplit {
             | CanonicalType::SyncReset(_)
             | CanonicalType::Reset(_) => unreachable!("memory element type is a storable type"),
             CanonicalType::DynSimOnly(_) => todo!("memory containing sim-only values"),
+            CanonicalType::TraceAsString(_) => unreachable!("handled by unwrap_transparent_types"),
         }
     }
 }
@@ -306,7 +307,9 @@ impl SplitMemState<'_, '_> {
         let outer_mem_name_path_len = self.mem_name_path.len();
         match self.split {
             MemSplit::Bundle { fields } => {
-                let CanonicalType::Bundle(bundle_type) = self.element_type else {
+                let CanonicalType::Bundle(bundle_type) =
+                    self.element_type.unwrap_transparent_types()
+                else {
                     unreachable!();
                 };
                 for ((field, field_offset), split) in bundle_type
@@ -321,7 +324,10 @@ impl SplitMemState<'_, '_> {
                     let field_ty_bit_width = field.ty.bit_width();
                     self.split_state_stack.push_map(
                         |e: Expr<CanonicalType>| {
-                            Expr::field(Expr::<Bundle>::from_canonical(e), &field.name)
+                            Expr::field(
+                                Expr::<Bundle>::from_canonical(Expr::unwrap_transparent_types(e)),
+                                &field.name,
+                            )
                         },
                         |initial_value_element| {
                             let Some(field_offset) = field_offset.only_bit_width() else {
@@ -377,8 +383,8 @@ impl SplitMemState<'_, '_> {
                         };
                         self.output_stmts.push(
                             StmtConnect {
-                                lhs: Expr::field(port_expr, name),
-                                rhs: Expr::field(wire_expr, name),
+                                lhs: Expr::unwrap_transparent_types(Expr::field(port_expr, name)),
+                                rhs: Expr::unwrap_transparent_types(Expr::field(wire_expr, name)),
                                 source_location: port.source_location(),
                             }
                             .into(),
@@ -389,7 +395,8 @@ impl SplitMemState<'_, '_> {
                 self.output_mems.push(new_mem);
             }
             MemSplit::Array { elements } => {
-                let CanonicalType::Array(array_type) = self.element_type else {
+                let CanonicalType::Array(array_type) = self.element_type.unwrap_transparent_types()
+                else {
                     unreachable!();
                 };
                 let element_type = array_type.element();
@@ -398,7 +405,7 @@ impl SplitMemState<'_, '_> {
                     self.mem_name_path.truncate(outer_mem_name_path_len);
                     write!(self.mem_name_path, "_{index}").unwrap();
                     self.split_state_stack.push_map(
-                        |e| Expr::<Array>::from_canonical(e)[index],
+                        |e| Expr::<Array>::from_canonical(Expr::unwrap_transparent_types(e))[index],
                         |initial_value_element| {
                             &initial_value_element[index * element_bit_width..][..element_bit_width]
                         },
@@ -464,7 +471,7 @@ impl ModuleState {
         assert_eq!(memory_element_array_range_len % input_array_type.len(), 0);
         let chunk_size = memory_element_array_range_len / input_array_type.len();
         for index in 0..input_array_type.len() {
-            let map = |e| Expr::<Array>::from_canonical(e)[index];
+            let map = |e| Expr::<Array>::from_canonical(Expr::unwrap_transparent_types(e))[index];
             let wire_rdata = wire_rdata.map(map);
             let wire_wdata = wire_wdata.map(map);
             let wire_wmask = wire_wmask.map(map);
@@ -505,8 +512,8 @@ impl ModuleState {
                             port_read: Expr<CanonicalType>| {
             output_stmts.push(
                 StmtConnect {
-                    lhs: wire_read,
-                    rhs: port_read,
+                    lhs: Expr::unwrap_transparent_types(wire_read),
+                    rhs: Expr::unwrap_transparent_types(port_read),
                     source_location,
                 }
                 .into(),
@@ -517,8 +524,8 @@ impl ModuleState {
                              port_write: Expr<CanonicalType>| {
             output_stmts.push(
                 StmtConnect {
-                    lhs: port_write,
-                    rhs: wire_write,
+                    lhs: Expr::unwrap_transparent_types(port_write),
+                    rhs: Expr::unwrap_transparent_types(wire_write),
                     source_location,
                 }
                 .into(),
@@ -530,7 +537,8 @@ impl ModuleState {
             connect_read(
                 output_stmts,
                 wire_read,
-                Expr::<UInt>::from_canonical(port_read).cast_bits_to(wire_read.ty()),
+                Expr::<UInt>::from_canonical(Expr::unwrap_transparent_types(port_read))
+                    .cast_bits_to(wire_read.ty()),
             );
         };
         let connect_write_enum =
@@ -544,7 +552,7 @@ impl ModuleState {
                 );
             };
         loop {
-            match input_element_type {
+            match input_element_type.unwrap_transparent_types() {
                 CanonicalType::Bundle(_) => {
                     unreachable!("bundle types are always split")
                 }
@@ -625,6 +633,9 @@ impl ModuleState {
                 | CanonicalType::SyncReset(_)
                 | CanonicalType::Reset(_) => unreachable!("memory element type is a storable type"),
                 CanonicalType::DynSimOnly(_) => todo!("memory containing sim-only values"),
+                CanonicalType::TraceAsString(_) => {
+                    unreachable!("handled by unwrap_transparent_types")
+                }
             }
             break;
         }

crates/fayalite/src/module/transform/visit.rs

@@ -13,8 +13,9 @@ use crate::{
     expr::{
         Expr, ExprEnum, ValueType, ops,
         target::{
-            Target, TargetBase, TargetChild, TargetPathArrayElement, TargetPathBundleField,
-            TargetPathDynArrayElement, TargetPathElement,
+            Target, TargetBase, TargetChild, TargetPathArrayElement, TargetPathAsTraceAsString,
+            TargetPathBundleField, TargetPathDynArrayElement, TargetPathElement,
+            TargetPathTraceAsStringInner,
         },
     },
     formal::FormalKind,
@@ -32,7 +33,7 @@ use crate::{
     reset::{AsyncReset, Reset, ResetType, SyncReset},
     sim::{ExternModuleSimulation, value::DynSimOnly},
     source_location::SourceLocation,
-    ty::{CanonicalType, Type},
+    ty::{CanonicalType, TraceAsString, Type},
     vendor::xilinx::{
         XdcCreateClockAnnotation, XdcIOStandardAnnotation, XdcLocationAnnotation, XilinxAnnotation,
     },

crates/fayalite/src/phantom_const.rs

@@ -9,7 +9,7 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, OpaqueSimValueSlice, OpaqueSimValueWriter, OpaqueSimValueWritten,
-        StaticType, Type, TypeProperties, impl_match_variant_as_self,
+        SimValueDebug, StaticType, Type, TypeProperties, impl_match_variant_as_self,
         serde_impls::{SerdeCanonicalType, SerdePhantomConst},
     },
 };
@@ -327,6 +327,15 @@ impl<T: ?Sized + PhantomConstValue> Type for PhantomConst<T> {
     }
 }
 
+impl<T: ?Sized + PhantomConstValue> SimValueDebug for PhantomConst<T> {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl<T: ?Sized + PhantomConstValue> Default for PhantomConst<T>
 where
     Interned<T>: Default,

crates/fayalite/src/reset.rs

@@ -1,5 +1,6 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
+
 use crate::{
     clock::Clock,
     expr::{CastToImpl, Expr, ValueType},
@@ -8,11 +9,13 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, OpaqueSimValueSize, OpaqueSimValueSlice, OpaqueSimValueWriter,
-        OpaqueSimValueWritten, StaticType, Type, TypeProperties, impl_match_variant_as_self,
+        OpaqueSimValueWritten, SimValueDebug, StaticType, Type, TypeProperties,
+        impl_match_variant_as_self,
     },
     util::ConstUsize,
 };
 use bitvec::{bits, order::Lsb0};
+use std::fmt;
 
 mod sealed {
     pub trait ResetTypeSealed {}
@@ -100,6 +103,15 @@ macro_rules! reset_type {
             }
         }
 
+        impl SimValueDebug for $name {
+            fn sim_value_debug(
+                value: &<Self as Type>::SimValue,
+                f: &mut fmt::Formatter<'_>,
+            ) -> fmt::Result {
+                fmt::Debug::fmt(value, f)
+            }
+        }
+
         impl $name {
             pub fn type_properties(self) -> TypeProperties {
                 Self::TYPE_PROPERTIES

crates/fayalite/src/sim.rs

@@ -9,6 +9,7 @@ use crate::{
         Flow,
         target::{
             GetTarget, Target, TargetPathArrayElement, TargetPathBundleField, TargetPathElement,
+            TargetPathTraceAsStringInner,
         },
     },
     int::BoolOrIntType,
@@ -38,7 +39,7 @@ use crate::{
     },
     ty::{
         OpaqueSimValue, OpaqueSimValueSize, OpaqueSimValueSizeRange, OpaqueSimValueSlice,
-        OpaqueSimValueWriter,
+        OpaqueSimValueWriter, TraceAsString,
     },
     util::{BitSliceWriteWithBase, DebugAsDisplay, HashMap, HashSet, copy_le_bytes_to_bitslice},
 };
@@ -432,6 +433,15 @@ impl_trace_decl! {
             ty: DynSimOnly,
             flow: Flow,
         }),
+        TraceAsString(TraceTraceAsString {
+            fn location(self) -> _ {
+                self.location
+            }
+            location: TraceLocation,
+            name: Interned<str>,
+            ty: TraceAsString,
+            flow: Flow,
+        }),
     }),
 }
 
@@ -543,6 +553,7 @@ pub trait TraceWriter: fmt::Debug + 'static {
         id: TraceScalarId,
         value: &DynSimOnlyValue,
     ) -> Result<(), Self::Error>;
+    fn set_signal_string(&mut self, id: TraceScalarId, value: &str) -> Result<(), Self::Error>;
 }
 
 pub struct DynTraceWriterDecls(Box<dyn TraceWriterDeclsDynTrait>);
@@ -607,6 +618,7 @@ trait TraceWriterDynTrait: fmt::Debug + 'static {
         id: TraceScalarId,
         value: &DynSimOnlyValue,
     ) -> std::io::Result<()>;
+    fn set_signal_string_dyn(&mut self, id: TraceScalarId, value: &str) -> std::io::Result<()>;
 }
 
 impl<T: TraceWriter> TraceWriterDynTrait for T {
@@ -680,6 +692,9 @@ impl<T: TraceWriter> TraceWriterDynTrait for T {
     ) -> std::io::Result<()> {
         Ok(TraceWriter::set_signal_sim_only_value(self, id, value).map_err(err_into_io)?)
     }
+    fn set_signal_string_dyn(&mut self, id: TraceScalarId, value: &str) -> std::io::Result<()> {
+        Ok(TraceWriter::set_signal_string(self, id, value).map_err(err_into_io)?)
+    }
 }
 
 pub struct DynTraceWriter(Box<dyn TraceWriterDynTrait>);
@@ -758,6 +773,9 @@ impl TraceWriter for DynTraceWriter {
     ) -> Result<(), Self::Error> {
         self.0.set_signal_sim_only_value_dyn(id, value)
     }
+    fn set_signal_string(&mut self, id: TraceScalarId, value: &str) -> Result<(), Self::Error> {
+        self.0.set_signal_string_dyn(id, value)
+    }
 }
 
 #[derive(Debug)]
@@ -828,6 +846,7 @@ where
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 pub(crate) struct SimTrace<K, S> {
     kind: K,
+    maybe_changed: bool,
     state: S,
     last_state: S,
 }
@@ -848,12 +867,14 @@ impl<K: fmt::Debug> SimTraceDebug<TraceScalarId> for SimTrace<K, ()> {
     fn fmt(&self, id: TraceScalarId, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self {
             kind,
+            maybe_changed,
             state,
             last_state,
         } = self;
         f.debug_struct("SimTrace")
             .field("id", &id)
             .field("kind", kind)
+            .field("maybe_changed", maybe_changed)
             .field("state", state)
             .field("last_state", last_state)
             .finish()
@@ -864,12 +885,14 @@ impl<K: fmt::Debug> SimTraceDebug<TraceScalarId> for SimTrace<K, SimTraceState>
     fn fmt(&self, id: TraceScalarId, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let Self {
             kind,
+            maybe_changed,
             state,
             last_state,
         } = self;
         f.debug_struct("SimTrace")
             .field("id", &id)
             .field("kind", kind)
+            .field("maybe_changed", maybe_changed)
             .field("state", state)
             .field("last_state", last_state)
             .finish()
@@ -929,6 +952,10 @@ pub(crate) enum SimTraceKind {
     PhantomConst {
         ty: PhantomConst,
     },
+    TraceAsString {
+        layout: compiler::CompiledTypeLayout<TraceAsString>,
+        range: TypeIndexRange,
+    },
 }
 
 #[derive(PartialEq, Eq)]
@@ -936,6 +963,7 @@ pub(crate) enum SimTraceState {
     Bits(BitVec),
     SimOnly(DynSimOnlyValue),
     PhantomConst,
+    OpaqueSimValue(OpaqueSimValue),
 }
 
 impl Clone for SimTraceState {
@@ -944,6 +972,7 @@ impl Clone for SimTraceState {
             Self::Bits(v) => Self::Bits(v.clone()),
             Self::SimOnly(v) => Self::SimOnly(v.clone()),
             Self::PhantomConst => Self::PhantomConst,
+            Self::OpaqueSimValue(v) => Self::OpaqueSimValue(v.clone()),
         }
     }
     fn clone_from(&mut self, source: &Self) {
@@ -951,6 +980,9 @@ impl Clone for SimTraceState {
             (SimTraceState::Bits(dest), SimTraceState::Bits(source)) => {
                 dest.clone_from_bitslice(source);
             }
+            (SimTraceState::OpaqueSimValue(dest), SimTraceState::OpaqueSimValue(source)) => {
+                dest.clone_from(source);
+            }
             _ => *self = source.clone(),
         }
     }
@@ -985,6 +1017,20 @@ impl SimTraceState {
             unreachable!()
         }
     }
+    fn unwrap_opaque_sim_value_ref(&self) -> &OpaqueSimValue {
+        if let SimTraceState::OpaqueSimValue(v) = self {
+            v
+        } else {
+            unreachable!()
+        }
+    }
+    fn unwrap_opaque_sim_value_mut(&mut self) -> &mut OpaqueSimValue {
+        if let SimTraceState::OpaqueSimValue(v) = self {
+            v
+        } else {
+            unreachable!()
+        }
+    }
 }
 
 impl fmt::Debug for SimTraceState {
@@ -993,6 +1039,7 @@ impl fmt::Debug for SimTraceState {
             SimTraceState::Bits(v) => BitSliceWriteWithBase(v).fmt(f),
             SimTraceState::SimOnly(v) => v.fmt(f),
             SimTraceState::PhantomConst => f.debug_tuple("PhantomConst").finish(),
+            SimTraceState::OpaqueSimValue(v) => v.fmt(f),
         }
     }
 }
@@ -1021,6 +1068,13 @@ impl SimTraceKind {
             }
             SimTraceKind::PhantomConst { .. } => SimTraceState::PhantomConst,
             SimTraceKind::SimOnly { index: _, ty } => SimTraceState::SimOnly(ty.default_value()),
+            SimTraceKind::TraceAsString { layout, range: _ } => {
+                let type_properties = layout.ty.type_properties();
+                SimTraceState::OpaqueSimValue(OpaqueSimValue::from_bits_and_sim_only_values(
+                    UIntValue::new_dyn(Arc::new(BitVec::repeat(false, type_properties.bit_width))),
+                    Vec::with_capacity(type_properties.sim_only_values_len),
+                ))
+            }
         }
     }
 }
@@ -1175,6 +1229,31 @@ impl SimulationModuleState {
                     true
                 }
             }
+            CompiledTypeLayoutBody::Transparent { .. } => {
+                let value = value.map_ty(|ty| match ty {
+                    CanonicalType::UInt(_)
+                    | CanonicalType::SInt(_)
+                    | CanonicalType::Bool(_)
+                    | CanonicalType::Array(_)
+                    | CanonicalType::Enum(_)
+                    | CanonicalType::Bundle(_)
+                    | CanonicalType::AsyncReset(_)
+                    | CanonicalType::SyncReset(_)
+                    | CanonicalType::Reset(_)
+                    | CanonicalType::Clock(_)
+                    | CanonicalType::PhantomConst(_)
+                    | CanonicalType::DynSimOnly(_) => unreachable!(),
+                    CanonicalType::TraceAsString(ty) => ty,
+                });
+                let sub_target = target
+                    .join(TargetPathElement::from(TargetPathTraceAsStringInner {}).intern_sized());
+                if self.parse_io(sub_target, value.inner()) {
+                    self.uninitialized_ios.insert(target, vec![sub_target]);
+                    true
+                } else {
+                    false
+                }
+            }
         }
     }
     fn mark_target_as_initialized(&mut self, mut target: Target) {
@@ -1225,7 +1304,10 @@ impl SimulationModuleState {
                 Target::Base(_) => break,
                 Target::Child(child) => {
                     match *child.path_element() {
-                        TargetPathElement::BundleField(_) | TargetPathElement::ArrayElement(_) => {}
+                        TargetPathElement::BundleField(_)
+                        | TargetPathElement::ArrayElement(_)
+                        | TargetPathElement::TraceAsStringInner(_)
+                        | TargetPathElement::AsTraceAsString(_) => {}
                         TargetPathElement::DynArrayElement(_) => panic!(
                             "simulator read/write expression must not have dynamic array indexes"
                         ),
@@ -1268,7 +1350,8 @@ impl SimulationModuleState {
             | CanonicalType::Reset(_)
             | CanonicalType::Clock(_)
             | CanonicalType::PhantomConst(_)
-            | CanonicalType::DynSimOnly(_) => unreachable!(),
+            | CanonicalType::DynSimOnly(_)
+            | CanonicalType::TraceAsString(_) => unreachable!(),
             CanonicalType::AsyncReset(_) => true,
             CanonicalType::SyncReset(_) => false,
         }
@@ -1433,6 +1516,26 @@ impl SimulationExternModuleClockForPast {
                     );
                 }
             }
+            CompiledTypeLayoutBody::Transparent { .. } => {
+                let map_ty_fn = |ty| match ty {
+                    CanonicalType::UInt(_)
+                    | CanonicalType::SInt(_)
+                    | CanonicalType::Bool(_)
+                    | CanonicalType::Array(_)
+                    | CanonicalType::Enum(_)
+                    | CanonicalType::Bundle(_)
+                    | CanonicalType::AsyncReset(_)
+                    | CanonicalType::SyncReset(_)
+                    | CanonicalType::Reset(_)
+                    | CanonicalType::Clock(_)
+                    | CanonicalType::PhantomConst(_)
+                    | CanonicalType::DynSimOnly(_) => unreachable!(),
+                    CanonicalType::TraceAsString(ty) => ty,
+                };
+                let current = current.map_ty(map_ty_fn);
+                let past = past.map_ty(map_ty_fn);
+                self.add_current_to_past_mapping(current.inner(), past.inner());
+            }
         }
     }
 }
@@ -1901,6 +2004,7 @@ struct SimulationImpl {
         ),
     >,
     waiting_sensitivity_sets_by_address: HashMap<*const SensitivitySet, Rc<SensitivitySet>>,
+    trace_as_string_buf: String,
 }
 
 impl fmt::Debug for SimulationImpl {
@@ -1990,6 +2094,7 @@ impl SimulationImpl {
             next_sensitivity_set_debug_id: _,
             waiting_sensitivity_sets_by_compiled_value,
             waiting_sensitivity_sets_by_address,
+            trace_as_string_buf: _,
         } = self;
         f.debug_struct("Simulation")
             .field("state", state)
@@ -2078,10 +2183,12 @@ impl SimulationImpl {
             traces: SimTraces(Box::from_iter(compiled.traces.0.iter().map(
                 |&SimTrace {
                      kind,
+                     maybe_changed: _,
                      state: _,
                      last_state: _,
                  }| SimTrace {
                     kind,
+                    maybe_changed: true,
                     state: kind.make_state(),
                     last_state: kind.make_state(),
                 },
@@ -2094,6 +2201,7 @@ impl SimulationImpl {
             next_sensitivity_set_debug_id: 0,
             waiting_sensitivity_sets_by_compiled_value: HashMap::default(),
             waiting_sensitivity_sets_by_address: HashMap::default(),
+            trace_as_string_buf: String::with_capacity(256),
         }
     }
     fn write_traces<const ONLY_IF_CHANGED: bool>(
@@ -2126,13 +2234,16 @@ impl SimulationImpl {
             id,
             &SimTrace {
                 kind,
+                maybe_changed,
                 ref state,
                 ref last_state,
             },
         ) in self.traces.0.iter().enumerate()
         {
-            if ONLY_IF_CHANGED && state == last_state {
-                continue;
+            if ONLY_IF_CHANGED {
+                if !(maybe_changed && state != last_state) {
+                    continue;
+                }
             }
             let id = TraceScalarId(id);
             match kind {
@@ -2171,6 +2282,15 @@ impl SimulationImpl {
                 SimTraceKind::SimOnly { .. } => {
                     trace_writer.set_signal_sim_only_value(id, state.unwrap_sim_only_ref())?
                 }
+                SimTraceKind::TraceAsString { layout, .. } => {
+                    self.trace_as_string_buf.clear();
+                    layout.ty.trace_fmt_append_to_string(
+                        &mut self.trace_as_string_buf,
+                        state.unwrap_opaque_sim_value_ref().as_slice(),
+                    );
+                    trace_writer.set_signal_string(id, &self.trace_as_string_buf)?;
+                    self.trace_as_string_buf.clear();
+                }
             }
         }
         Ok(trace_writer)
@@ -2193,10 +2313,48 @@ impl SimulationImpl {
     fn read_traces<const IS_INITIAL_STEP: bool>(&mut self) {
         for &mut SimTrace {
             kind,
+            ref mut maybe_changed,
             ref mut state,
             ref mut last_state,
         } in &mut self.traces.0
         {
+            let new_maybe_changed = match kind {
+                SimTraceKind::BigUInt { index, ty: _ }
+                | SimTraceKind::BigSInt { index, ty: _ }
+                | SimTraceKind::BigBool { index }
+                | SimTraceKind::BigAsyncReset { index }
+                | SimTraceKind::BigSyncReset { index }
+                | SimTraceKind::BigClock { index } => self
+                    .state
+                    .big_slots
+                    .state_index_fetch_maybe_modified_flag(index),
+                SimTraceKind::SmallUInt { index, ty: _ }
+                | SimTraceKind::SmallSInt { index, ty: _ }
+                | SimTraceKind::SmallBool { index }
+                | SimTraceKind::SmallAsyncReset { index }
+                | SimTraceKind::SmallSyncReset { index }
+                | SimTraceKind::SmallClock { index }
+                | SimTraceKind::EnumDiscriminant { index, ty: _ } => self
+                    .state
+                    .small_slots
+                    .state_index_fetch_maybe_modified_flag(index),
+                SimTraceKind::SimOnly { index, ty: _ } => self
+                    .state
+                    .sim_only_slots
+                    .state_index_fetch_maybe_modified_flag(index),
+                SimTraceKind::PhantomConst { ty: _ } => IS_INITIAL_STEP,
+                SimTraceKind::TraceAsString { layout: _, range } => self
+                    .state
+                    .type_index_range_fetch_maybe_modified_flags(range),
+            };
+            if !new_maybe_changed && !IS_INITIAL_STEP {
+                if *maybe_changed {
+                    last_state.clone_from(state);
+                }
+                *maybe_changed = false;
+                continue;
+            }
+            *maybe_changed = new_maybe_changed;
             if !IS_INITIAL_STEP {
                 mem::swap(state, last_state);
             }
@@ -2241,11 +2399,26 @@ impl SimulationImpl {
                         .unwrap_sim_only_mut()
                         .clone_from(&self.state.sim_only_slots[index]);
                 }
+                SimTraceKind::TraceAsString { layout, range } => {
+                    let CompiledTypeLayoutBody::Transparent { inner } = layout.body else {
+                        unreachable!()
+                    };
+                    Self::read_opaque_no_settle(
+                        &mut self.state,
+                        CompiledValue {
+                            layout: *inner,
+                            range,
+                            write: None,
+                        },
+                        state.unwrap_opaque_sim_value_mut(),
+                    );
+                }
             }
             if IS_INITIAL_STEP {
                 last_state.clone_from(state);
             }
         }
+        self.state.clear_all_maybe_modified_flags();
     }
     #[track_caller]
     fn advance_time(this_ref: &Rc<RefCell<Self>>, duration: SimDuration) {
@@ -2772,6 +2945,7 @@ impl SimulationImpl {
         + Copy,
         read_write_sim_only_scalar: impl Fn(usize, &mut Opaque, &mut DynSimOnlyValue) + Copy,
     ) {
+        let compiled_value = compiled_value.unwrap_transparent_types();
         match compiled_value.layout.body {
             CompiledTypeLayoutBody::Scalar => {
                 let signed = match compiled_value.layout.ty {
@@ -2787,6 +2961,7 @@ impl SimulationImpl {
                     CanonicalType::Clock(_) => false,
                     CanonicalType::PhantomConst(_) => unreachable!(),
                     CanonicalType::DynSimOnly(_) => false,
+                    CanonicalType::TraceAsString(_) => unreachable!(),
                 };
                 let indexes = OpaqueSimValueSizeRange::from(
                     start_index..start_index + compiled_value.layout.ty.size(),
@@ -2863,14 +3038,17 @@ impl SimulationImpl {
                     );
                 }
             }
+            CompiledTypeLayoutBody::Transparent { .. } => {
+                unreachable!("handled by unwrap_transparent_types")
+            }
         }
     }
     #[track_caller]
-    fn read_no_settle_helper(
+    fn read_opaque_no_settle(
         state: &mut interpreter::State,
-        io: Expr<CanonicalType>,
         compiled_value: CompiledValue<CanonicalType>,
-    ) -> SimValue<CanonicalType> {
+        opaque: &mut OpaqueSimValue,
+    ) {
         #[track_caller]
         fn read_write_sim_only_scalar(
             index: usize,
@@ -2891,8 +3069,7 @@ impl SimulationImpl {
                 },
             );
         }
-        let size = io.ty().size();
-        let mut opaque = OpaqueSimValue::with_capacity(size);
+        let size = compiled_value.layout.ty.size();
         opaque.rewrite_with(size, |mut writer| {
             SimulationImpl::read_write_sim_value_helper(
                 state,
@@ -2924,6 +3101,16 @@ impl SimulationImpl {
             );
             writer.fill_cloned_from_slice(OpaqueSimValueSlice::empty())
         });
+    }
+    #[track_caller]
+    fn read_no_settle_helper(
+        state: &mut interpreter::State,
+        io: Expr<CanonicalType>,
+        compiled_value: CompiledValue<CanonicalType>,
+    ) -> SimValue<CanonicalType> {
+        let size = io.ty().size();
+        let mut opaque = OpaqueSimValue::with_capacity(size);
+        Self::read_opaque_no_settle(state, compiled_value, &mut opaque);
         SimValue::from_opaque(io.ty(), opaque)
     }
     /// doesn't modify `opaque`

crates/fayalite/src/sim/compiler.rs

@@ -9,8 +9,8 @@ use crate::{
     expr::{
         ExprEnum, Flow, ValueType, ops,
         target::{
-            GetTarget, Target, TargetBase, TargetPathArrayElement, TargetPathBundleField,
-            TargetPathElement,
+            GetTarget, Target, TargetBase, TargetPathArrayElement, TargetPathAsTraceAsString,
+            TargetPathBundleField, TargetPathElement, TargetPathTraceAsStringInner,
         },
     },
     int::BoolOrIntType,
@@ -29,7 +29,8 @@ use crate::{
         TraceBool, TraceBundle, TraceClock, TraceDecl, TraceEnumDiscriminant, TraceEnumWithFields,
         TraceFieldlessEnum, TraceInstance, TraceLocation, TraceMem, TraceMemPort, TraceMemoryId,
         TraceMemoryLocation, TraceModule, TraceModuleIO, TracePhantomConst, TraceReg, TraceSInt,
-        TraceScalarId, TraceScope, TraceSimOnly, TraceSyncReset, TraceUInt, TraceWire,
+        TraceScalarId, TraceScope, TraceSimOnly, TraceSyncReset, TraceTraceAsString, TraceUInt,
+        TraceWire,
         interpreter::{
             self, Insn, InsnField, InsnFieldKind, InsnFieldType, InsnOrLabel, Insns, InsnsBuilding,
             InsnsBuildingDone, InsnsBuildingKind, Label, PrefixLinesWrapper, SmallUInt,
@@ -42,7 +43,7 @@ use crate::{
             },
         },
     },
-    ty::{OpaqueSimValueSize, StaticType},
+    ty::{OpaqueSimValueSize, StaticType, TraceAsString},
     util::{HashMap, chain},
 };
 use bitvec::vec::BitVec;
@@ -110,6 +111,9 @@ pub(crate) enum CompiledTypeLayoutBody {
     Bundle {
         fields: Interned<[CompiledBundleField]>,
     },
+    Transparent {
+        inner: Interned<CompiledTypeLayout<CanonicalType>>,
+    },
 }
 
 impl CompiledTypeLayoutBody {
@@ -128,6 +132,9 @@ impl CompiledTypeLayoutBody {
                     .map(|field| field.with_prefixed_debug_names(prefix))
                     .collect(),
             },
+            CompiledTypeLayoutBody::Transparent { inner } => CompiledTypeLayoutBody::Transparent {
+                inner: inner.with_prefixed_debug_names(prefix).intern_sized(),
+            },
         }
     }
     fn with_anonymized_debug_info(self) -> Self {
@@ -145,6 +152,9 @@ impl CompiledTypeLayoutBody {
                     .map(|field| field.with_anonymized_debug_info())
                     .collect(),
             },
+            CompiledTypeLayoutBody::Transparent { inner } => CompiledTypeLayoutBody::Transparent {
+                inner: inner.with_anonymized_debug_info().intern_sized(),
+            },
         }
     }
 }
@@ -179,7 +189,7 @@ impl<T: Type> CompiledTypeLayout<T> {
         impl Memoize for MyMemoize {
             type Input = CanonicalType;
             type InputOwned = CanonicalType;
-            type Output = CompiledTypeLayout<CanonicalType>;
+            type Output = (TypeLayout<InsnsBuildingDone>, CompiledTypeLayoutBody);
 
             fn inner(self, input: &Self::Input) -> Self::Output {
                 match input {
@@ -197,11 +207,7 @@ impl<T: Type> CompiledTypeLayout<T> {
                             ty: *input,
                         };
                         layout.big_slots = StatePartLayout::scalar(debug_data, ());
-                        CompiledTypeLayout {
-                            ty: *input,
-                            layout: layout.into(),
-                            body: CompiledTypeLayoutBody::Scalar,
-                        }
+                        (layout.into(), CompiledTypeLayoutBody::Scalar)
                     }
                     CanonicalType::Array(array) => {
                         let mut layout = TypeLayout::empty();
@@ -215,19 +221,16 @@ impl<T: Type> CompiledTypeLayout<T> {
                         if array.is_empty() {
                             elements_non_empty.push(element.with_prefixed_debug_names("[<none>]"));
                         }
-                        CompiledTypeLayout {
-                            ty: *input,
-                            layout: layout.into(),
-                            body: CompiledTypeLayoutBody::Array {
+                        (
+                            layout.into(),
+                            CompiledTypeLayoutBody::Array {
                                 elements_non_empty: elements_non_empty.intern_deref(),
                             },
-                        }
+                        )
+                    }
+                    CanonicalType::PhantomConst(_) => {
+                        (TypeLayout::empty(), CompiledTypeLayoutBody::PhantomConst)
                     }
-                    CanonicalType::PhantomConst(_) => CompiledTypeLayout {
-                        ty: *input,
-                        layout: TypeLayout::empty(),
-                        body: CompiledTypeLayoutBody::PhantomConst,
-                    },
                     CanonicalType::Bundle(bundle) => {
                         let mut layout = TypeLayout::empty();
                         let fields = bundle
@@ -246,11 +249,7 @@ impl<T: Type> CompiledTypeLayout<T> {
                                 },
                             )
                             .collect();
-                        CompiledTypeLayout {
-                            ty: *input,
-                            layout: layout.into(),
-                            body: CompiledTypeLayoutBody::Bundle { fields },
-                        }
+                        (layout.into(), CompiledTypeLayoutBody::Bundle { fields })
                     }
                     CanonicalType::DynSimOnly(ty) => {
                         let mut layout = TypeLayout::empty();
@@ -259,24 +258,30 @@ impl<T: Type> CompiledTypeLayout<T> {
                             ty: *input,
                         };
                         layout.sim_only_slots = StatePartLayout::scalar(debug_data, *ty);
-                        CompiledTypeLayout {
-                            ty: *input,
-                            layout: layout.into(),
-                            body: CompiledTypeLayoutBody::Scalar,
-                        }
+                        (layout.into(), CompiledTypeLayoutBody::Scalar)
+                    }
+                    CanonicalType::TraceAsString(ty) => {
+                        let inner = CompiledTypeLayout::get(ty.inner_ty()).intern_sized();
+                        (inner.layout, CompiledTypeLayoutBody::Transparent { inner })
                     }
                 }
             }
         }
-        let CompiledTypeLayout {
-            ty: _,
-            layout,
-            body,
-        } = MyMemoize.get_owned(ty.canonical());
+        let (layout, body) = MyMemoize.get_owned(ty.canonical());
         Self { ty, layout, body }
     }
 }
 
+impl CompiledTypeLayout<CanonicalType> {
+    #[must_use]
+    fn unwrap_transparent_types(mut self) -> Self {
+        while let CompiledTypeLayoutBody::Transparent { inner } = self.body {
+            self = *inner;
+        }
+        self
+    }
+}
+
 #[derive(Debug, PartialEq, Eq, Hash, Clone, Copy)]
 pub(crate) struct CompiledValue<T: Type> {
     pub(crate) layout: CompiledTypeLayout<T>,
@@ -324,6 +329,29 @@ impl<T: Type> CompiledValue<T> {
     }
 }
 
+impl CompiledValue<CanonicalType> {
+    #[must_use]
+    pub(crate) fn unwrap_transparent_types(self) -> Self {
+        let Self {
+            layout,
+            range,
+            write,
+        } = self;
+        Self {
+            layout: layout.unwrap_transparent_types(),
+            range,
+            write: write.map(|(layout, range)| (layout.unwrap_transparent_types(), range)),
+        }
+    }
+    #[must_use]
+    pub(crate) fn wrap_in_trace_as_string(self, ty: TraceAsString) -> CompiledValue<TraceAsString> {
+        self.map(|layout, range| {
+            assert_eq!(layout.ty, ty.inner_ty());
+            (CompiledTypeLayout::get(ty), range)
+        })
+    }
+}
+
 pub(crate) struct DebugCompiledValueStateAsMap<'a> {
     pub(crate) compiled_value: CompiledValue<CanonicalType>,
     pub(crate) state_layout: &'a interpreter::parts::StateLayout<InsnsBuildingDone>,
@@ -402,6 +430,17 @@ impl CompiledValue<Array> {
     }
 }
 
+impl CompiledValue<TraceAsString> {
+    pub(crate) fn inner(self) -> CompiledValue<CanonicalType> {
+        self.map(|layout, range| {
+            let CompiledTypeLayoutBody::Transparent { inner } = layout.body else {
+                unreachable!();
+            };
+            (*inner, range)
+        })
+    }
+}
+
 macro_rules! make_type_array_indexes {
     (
         type_plural_fields = [$($type_plural_field:ident,)*];
@@ -618,6 +657,16 @@ impl<T: Type> CompiledExpr<T> {
     }
 }
 
+impl CompiledExpr<CanonicalType> {
+    #[must_use]
+    pub(crate) fn wrap_in_trace_as_string(self, ty: TraceAsString) -> CompiledExpr<TraceAsString> {
+        CompiledExpr {
+            static_part: self.static_part.wrap_in_trace_as_string(ty),
+            indexes: self.indexes,
+        }
+    }
+}
+
 impl CompiledExpr<Bundle> {
     fn field_by_index(self, field_index: usize) -> CompiledExpr<CanonicalType> {
         CompiledExpr {
@@ -666,6 +715,15 @@ impl CompiledExpr<Array> {
     }
 }
 
+impl CompiledExpr<TraceAsString> {
+    pub(crate) fn inner(self) -> CompiledExpr<CanonicalType> {
+        CompiledExpr {
+            static_part: self.static_part.inner(),
+            indexes: self.indexes,
+        }
+    }
+}
+
 macro_rules! make_assignment_graph {
     (
         type_plural_fields = [$($type_plural_field:ident,)*];
@@ -1977,6 +2035,39 @@ macro_rules! impl_compiler {
                         flow,
                     }
                     .into(),
+                    CanonicalType::TraceAsString(ty) => {
+                        let location = match target {
+                            MakeTraceDeclTarget::Expr(target) => {
+                                let compiled_value = self.compile_expr(instantiated_module, target);
+                                let CompiledValue { layout, range, write: _ } =
+                                    self.compiled_expr_to_value(compiled_value, source_location).map_ty(Type::from_canonical);
+                                TraceLocation::Scalar(self.new_sim_trace(SimTraceKind::TraceAsString {
+                                    layout,
+                                    range,
+                                }))
+                            }
+                            MakeTraceDeclTarget::Memory {
+                                id,
+                                depth,
+                                stride,
+                                start,
+                                ty: _,
+                            } => TraceLocation::Memory(TraceMemoryLocation {
+                                id,
+                                depth,
+                                stride,
+                                start,
+                                len: ty.type_properties().bit_width,
+                            }),
+                        };
+                        TraceTraceAsString {
+                            location,
+                            name,
+                            ty,
+                            flow,
+                        }
+                        .into()
+                    }
                 }
             }
             fn compiled_expr_to_value(
@@ -2234,6 +2325,7 @@ impl Compiler {
         let id = TraceScalarId(self.traces.0.len());
         self.traces.0.push(SimTrace {
             kind,
+            maybe_changed: true,
             state: (),
             last_state: (),
         });
@@ -2420,7 +2512,8 @@ impl Compiler {
             | CanonicalType::Reset(_)
             | CanonicalType::Clock(_)
             | CanonicalType::DynSimOnly(_)
-            | CanonicalType::PhantomConst(_) => {
+            | CanonicalType::PhantomConst(_)
+            | CanonicalType::TraceAsString(_) => {
                 self.make_trace_scalar(instantiated_module, target, name, source_location)
             }
         }
@@ -2590,6 +2683,12 @@ impl Compiler {
                         parent.map_ty(Array::from_canonical).element(index)
                     }
                     TargetPathElement::DynArrayElement(_) => unreachable!(),
+                    TargetPathElement::TraceAsStringInner(TargetPathTraceAsStringInner {}) => {
+                        parent.map_ty(TraceAsString::from_canonical).inner()
+                    }
+                    TargetPathElement::AsTraceAsString(TargetPathAsTraceAsString { ty }) => parent
+                        .wrap_in_trace_as_string(ty)
+                        .map_ty(|ty| ty.canonical()),
                 }
             }
         };
@@ -2822,6 +2921,12 @@ impl Compiler {
             CanonicalType::PhantomConst(_) | CanonicalType::DynSimOnly(_) => {
                 self.compile_cast_aggregate_to_bits(instantiated_module, [])
             }
+            CanonicalType::TraceAsString(_) => self.compile_cast_to_bits(
+                instantiated_module,
+                ops::CastToBits::new(
+                    ops::TraceAsStringAsInner::new(Expr::from_canonical(expr.arg())).to_expr(),
+                ),
+            ),
         }
     }
     fn compile_cast_bits_to_or_uninit(
@@ -2911,6 +3016,16 @@ impl Compiler {
                     vec![]
                 });
             }
+            CanonicalType::TraceAsString(ty) => Expr::canonical(
+                ops::AsTraceAsString::new(
+                    match arg {
+                        Some(arg) => arg.cast_bits_to(ty.inner_ty()),
+                        None => ty.inner_ty().uninit(),
+                    },
+                    ty,
+                )
+                .to_expr(),
+            ),
         };
         let retval = self.compile_expr(instantiated_module, Expr::canonical(retval));
         self.compiled_expr_to_value(retval, instantiated_module.leaf_module().source_location())
@@ -2962,6 +3077,7 @@ impl Compiler {
                 CanonicalType::Clock(_) => false,
                 CanonicalType::PhantomConst(_) => unreachable!(),
                 CanonicalType::DynSimOnly(_) => unreachable!(),
+                CanonicalType::TraceAsString(_) => unreachable!(),
             };
             let dest_signed = match expr.ty() {
                 CanonicalType::UInt(_) => false,
@@ -2976,6 +3092,7 @@ impl Compiler {
                 CanonicalType::Clock(_) => false,
                 CanonicalType::PhantomConst(_) => unreachable!(),
                 CanonicalType::DynSimOnly(_) => unreachable!(),
+                CanonicalType::TraceAsString(_) => unreachable!(),
             };
             self.simple_nary_big_expr(instantiated_module, expr.ty(), [arg], |dest, [src]| match (
                 src_signed,
@@ -3721,6 +3838,14 @@ impl Compiler {
             ExprEnum::CastBitsTo(expr) => self
                 .compile_cast_bits_to_or_uninit(instantiated_module, Some(expr.arg()), expr.ty())
                 .into(),
+            ExprEnum::AsTraceAsString(expr) => self
+                .compile_expr(instantiated_module, expr.inner())
+                .wrap_in_trace_as_string(expr.ty())
+                .map_ty(|ty| ty.canonical()),
+            ExprEnum::TraceAsStringAsInner(expr) => self
+                .compile_expr(instantiated_module, Expr::canonical(expr.arg()))
+                .map_ty(TraceAsString::from_canonical)
+                .inner(),
             ExprEnum::ModuleIO(expr) => self
                 .compile_value(TargetInInstantiatedModule {
                     instantiated_module,
@@ -3868,6 +3993,21 @@ impl Compiler {
                 CanonicalType::DynSimOnly(_) => {
                     unreachable!("DynSimOnly mismatch");
                 }
+                CanonicalType::TraceAsString(_) => {
+                    let lhs = Expr::<TraceAsString>::from_canonical(lhs);
+                    let rhs = Expr::<TraceAsString>::from_canonical(rhs);
+                    let lhs_expr = ops::TraceAsStringAsInner::new(lhs).to_expr();
+                    let rhs_expr = ops::TraceAsStringAsInner::new(rhs).to_expr();
+                    return self.compile_connect(
+                        lhs_instantiated_module,
+                        lhs_conditions,
+                        lhs_expr,
+                        rhs_instantiated_module,
+                        rhs_conditions,
+                        rhs_expr,
+                        source_location,
+                    );
+                }
             }
         }
         let Some(target) = lhs.target() else {
@@ -4243,6 +4383,8 @@ impl Compiler {
         mut read: Option<MemoryPortReadInsns<'_>>,
         mut write: Option<MemoryPortWriteInsns<'_>>,
     ) {
+        let data_layout = data_layout.unwrap_transparent_types();
+        let mask_layout = mask_layout.unwrap_transparent_types();
         match data_layout.body {
             CompiledTypeLayoutBody::Scalar => {
                 let CompiledTypeLayoutBody::Scalar = mask_layout.body else {
@@ -4261,6 +4403,7 @@ impl Compiler {
                     CanonicalType::Clock(_) => false,
                     CanonicalType::PhantomConst(_) => unreachable!(),
                     CanonicalType::DynSimOnly(_) => false,
+                    CanonicalType::TraceAsString(_) => unreachable!(),
                 };
                 let width = data_layout.ty.bit_width();
                 if let Some(MemoryPortReadInsns {
@@ -4483,6 +4626,9 @@ impl Compiler {
                     start = start + field.ty.ty.bit_width();
                 }
             }
+            CompiledTypeLayoutBody::Transparent { .. } => {
+                unreachable!("handled by unwrap_transparent_types")
+            }
         }
     }
     fn compile_memory_port_rw(

crates/fayalite/src/sim/interpreter.rs

@@ -6,9 +6,9 @@ use crate::{
     int::{BoolOrIntType, SInt, UInt},
     intern::{Intern, Interned, Memoize},
     sim::interpreter::parts::{
-        StateLayout, StatePartIndex, StatePartKind, StatePartKindBigSlots, StatePartKindMemories,
-        StatePartKindSimOnlySlots, StatePartKindSmallSlots, StatePartLen, TypeIndexRange,
-        TypeLayout, get_state_part_kinds,
+        StateLayout, StatePartIndex, StatePartIndexRange, StatePartKind, StatePartKindBigSlots,
+        StatePartKindMemories, StatePartKindSimOnlySlots, StatePartKindSmallSlots, StatePartLen,
+        TypeIndexRange, TypeLayout, get_state_part_kinds,
     },
     source_location::SourceLocation,
     util::{HashMap, HashSet},
@@ -17,12 +17,11 @@ use bitvec::slice::BitSlice;
 use num_bigint::BigInt;
 use num_traits::{One, Signed, ToPrimitive, Zero};
 use std::{
-    borrow::BorrowMut,
     convert::Infallible,
     fmt::{self, Write},
     hash::Hash,
     marker::PhantomData,
-    ops::{ControlFlow, Deref, DerefMut, Index, IndexMut},
+    ops::{ControlFlow, Deref, Index, IndexMut},
 };
 use vec_map::VecMap;
 
@@ -915,6 +914,21 @@ impl<K: StatePartKind> StatePart<K> {
             value: K::borrow_state(&mut self.value),
         }
     }
+    pub(crate) fn state_index_fetch_maybe_modified_flag(
+        &self,
+        part_index: StatePartIndex<K>,
+    ) -> bool {
+        K::state_index_fetch_maybe_modified_flag(&self.value, part_index)
+    }
+    pub(crate) fn state_index_range_fetch_maybe_modified_flags(
+        &self,
+        part_index_range: StatePartIndexRange<K>,
+    ) -> bool {
+        K::state_index_range_fetch_maybe_modified_flags(&self.value, part_index_range)
+    }
+    pub(crate) fn clear_all_maybe_modified_flags(&mut self) {
+        K::clear_all_maybe_modified_flags(&mut self.value)
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
@@ -922,56 +936,38 @@ pub(crate) struct BorrowedStatePart<'a, K: StatePartKind> {
     pub(crate) value: K::BorrowedState<'a>,
 }
 
-impl<
-    'a,
-    K: StatePartKind<
-        BorrowedState<'a>: DerefMut<Target: IndexMut<usize, Output = T> + BorrowMut<[T]>>,
-    >,
-    T,
-> BorrowedStatePart<'a, K>
-{
+impl<K: StatePartKind> BorrowedStatePart<'_, K> {
     pub(crate) fn get_disjoint_mut<const N: usize>(
         &mut self,
         indexes: [StatePartIndex<K>; N],
-    ) -> [&mut T; N] {
-        (*self.value)
-            .borrow_mut()
-            .get_disjoint_mut(indexes.map(|v| v.value as usize))
-            .expect("indexes are disjoint")
+    ) -> [&mut K::StateElement; N] {
+        K::borrowed_state_get_disjoint_mut(&mut self.value, indexes)
     }
 }
 
-impl<K: StatePartKind<State: Deref<Target: Index<usize, Output = T>>>, T> Index<StatePartIndex<K>>
-    for StatePart<K>
-{
-    type Output = T;
+impl<K: StatePartKind> Index<StatePartIndex<K>> for StatePart<K> {
+    type Output = K::StateElement;
     fn index(&self, index: StatePartIndex<K>) -> &Self::Output {
-        &self.value[index.value as usize]
+        K::state_index(&self.value, index)
     }
 }
 
-impl<K: StatePartKind<State: DerefMut<Target: IndexMut<usize, Output = T>>>, T>
-    IndexMut<StatePartIndex<K>> for StatePart<K>
-{
+impl<K: StatePartKind> IndexMut<StatePartIndex<K>> for StatePart<K> {
     fn index_mut(&mut self, index: StatePartIndex<K>) -> &mut Self::Output {
-        &mut self.value[index.value as usize]
+        K::state_index_mut(&mut self.value, index)
     }
 }
 
-impl<'a, K: StatePartKind<BorrowedState<'a>: Deref<Target: Index<usize, Output = T>>>, T>
-    Index<StatePartIndex<K>> for BorrowedStatePart<'a, K>
-{
-    type Output = T;
+impl<K: StatePartKind> Index<StatePartIndex<K>> for BorrowedStatePart<'_, K> {
+    type Output = K::StateElement;
     fn index(&self, index: StatePartIndex<K>) -> &Self::Output {
-        &self.value[index.value as usize]
+        K::borrowed_state_index(&self.value, index)
     }
 }
 
-impl<'a, K: StatePartKind<BorrowedState<'a>: DerefMut<Target: IndexMut<usize, Output = T>>>, T>
-    IndexMut<StatePartIndex<K>> for BorrowedStatePart<'a, K>
-{
+impl<K: StatePartKind> IndexMut<StatePartIndex<K>> for BorrowedStatePart<'_, K> {
     fn index_mut(&mut self, index: StatePartIndex<K>) -> &mut Self::Output {
-        &mut self.value[index.value as usize]
+        K::borrowed_state_index_mut(&mut self.value, index)
     }
 }
 
@@ -1028,6 +1024,15 @@ macro_rules! make_state {
                     $($type_plural_field: self.$type_plural_field.borrow(),)*
                 }
             }
+            pub(crate) fn type_index_range_fetch_maybe_modified_flags(&self, range: TypeIndexRange) -> bool {
+                $(self.$type_plural_field.state_index_range_fetch_maybe_modified_flags(
+                    range.$type_plural_field,
+                ))||*
+            }
+            pub(crate) fn clear_all_maybe_modified_flags(&mut self) {
+                $(self.$state_plural_field.clear_all_maybe_modified_flags();)*
+                $(self.$type_plural_field.clear_all_maybe_modified_flags();)*
+            }
         }
 
         #[derive(Debug)]

crates/fayalite/src/sim/interpreter/parts.rs

@@ -236,6 +236,7 @@ pub(crate) trait StatePartKind:
     type LayoutData: Send + Sync + Eq + Hash + fmt::Debug + 'static + Copy;
     type State: fmt::Debug + 'static + Clone;
     type BorrowedState<'a>: 'a;
+    type StateElement;
     fn new_state(layout_data: &[Self::LayoutData]) -> Self::State;
     fn borrow_state<'a>(state: &'a mut Self::State) -> Self::BorrowedState<'a>;
     fn part_debug_data<BK: InsnsBuildingKind>(
@@ -247,6 +248,35 @@ pub(crate) trait StatePartKind:
         index: StatePartIndex<Self>,
         f: &mut impl fmt::Write,
     ) -> fmt::Result;
+    fn state_index<'a>(
+        state: &'a Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement;
+    fn state_index_mut<'a>(
+        state: &'a mut Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement;
+    fn state_index_fetch_maybe_modified_flag(
+        state: &Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> bool;
+    fn state_index_range_fetch_maybe_modified_flags(
+        state: &Self::State,
+        part_index_range: StatePartIndexRange<Self>,
+    ) -> bool;
+    fn clear_all_maybe_modified_flags(state: &mut Self::State);
+    fn borrowed_state_index<'a, 'b>(
+        state: &'a Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement;
+    fn borrowed_state_index_mut<'a, 'b>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement;
+    fn borrowed_state_get_disjoint_mut<'a, 'b, const N: usize>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_indexes: [StatePartIndex<Self>; N],
+    ) -> [&'a mut Self::StateElement; N];
 }
 
 macro_rules! make_state_part_kinds {
@@ -272,6 +302,7 @@ impl StatePartKind for StatePartKindMemories {
     type LayoutData = MemoryData<Interned<BitSlice>>;
     type State = Box<[MemoryData<BitBox>]>;
     type BorrowedState<'a> = &'a mut [MemoryData<BitBox>];
+    type StateElement = MemoryData<BitBox>;
     fn new_state(layout_data: &[Self::LayoutData]) -> Self::State {
         layout_data
             .iter()
@@ -297,19 +328,95 @@ impl StatePartKind for StatePartKindMemories {
     ) -> fmt::Result {
         write!(f, "{:#?}", &state.memories[index])
     }
+    fn state_index<'a>(
+        state: &'a Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state[part_index.as_usize()]
+    }
+    fn state_index_mut<'a>(
+        state: &'a mut Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        &mut state[part_index.as_usize()]
+    }
+    fn state_index_fetch_maybe_modified_flag(
+        _state: &Self::State,
+        _part_index: StatePartIndex<Self>,
+    ) -> bool {
+        true
+    }
+    fn state_index_range_fetch_maybe_modified_flags(
+        _state: &Self::State,
+        part_index_range: StatePartIndexRange<Self>,
+    ) -> bool {
+        part_index_range.len.value > 0
+    }
+    fn clear_all_maybe_modified_flags(_state: &mut Self::State) {}
+    fn borrowed_state_index<'a, 'b>(
+        state: &'a Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state[part_index.as_usize()]
+    }
+    fn borrowed_state_index_mut<'a, 'b>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        &mut state[part_index.as_usize()]
+    }
+    fn borrowed_state_get_disjoint_mut<'a, 'b, const N: usize>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_indexes: [StatePartIndex<Self>; N],
+    ) -> [&'a mut Self::StateElement; N] {
+        state
+            .get_disjoint_mut(part_indexes.map(StatePartIndex::as_usize))
+            .expect("indexes are disjoint")
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Default)]
+pub(crate) struct StateAndModified<T, M> {
+    pub(crate) state: T,
+    pub(crate) modified: M,
+}
+
+impl<T: Deref<Target = [E]>, M: Deref<Target = [bool]>, E: fmt::Debug> fmt::Debug
+    for StateAndModified<T, M>
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list()
+            .entries(self.state.iter().zip(self.modified.iter().copied()).map(
+                |(state, modified)| {
+                    fmt::from_fn(move |f| {
+                        state.fmt(f)?;
+                        if modified {
+                            f.write_str(" (modified)")?;
+                        }
+                        Ok(())
+                    })
+                },
+            ))
+            .finish()
+    }
 }
 
 impl StatePartKind for StatePartKindSmallSlots {
     const NAME: &'static str = "SmallSlots";
     type DebugData = SlotDebugData;
     type LayoutData = ();
-    type State = Box<[SmallUInt]>;
-    type BorrowedState<'a> = &'a mut [SmallUInt];
+    type State = StateAndModified<Box<[Self::StateElement]>, Box<[bool]>>;
+    type BorrowedState<'a> = StateAndModified<&'a mut [Self::StateElement], &'a mut [bool]>;
+    type StateElement = SmallUInt;
     fn new_state(layout_data: &[Self::LayoutData]) -> Self::State {
-        vec![0; layout_data.len()].into_boxed_slice()
+        StateAndModified {
+            state: vec![0; layout_data.len()].into_boxed_slice(),
+            modified: vec![false; layout_data.len()].into_boxed_slice(),
+        }
     }
     fn borrow_state<'a>(state: &'a mut Self::State) -> Self::BorrowedState<'a> {
-        state
+        let StateAndModified { state, modified } = state;
+        StateAndModified { state, modified }
     }
     fn part_debug_data<BK: InsnsBuildingKind>(
         state_layout: &StateLayout<BK>,
@@ -330,19 +437,80 @@ impl StatePartKind for StatePartKindSmallSlots {
         write!(f, "{value:#x} {}", value as SmallSInt)?;
         Ok(())
     }
+    fn state_index<'a>(
+        state: &'a Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn state_index_mut<'a>(
+        state: &'a mut Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn state_index_fetch_maybe_modified_flag(
+        state: &Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> bool {
+        state.modified[part_index.as_usize()]
+    }
+    fn state_index_range_fetch_maybe_modified_flags(
+        state: &Self::State,
+        part_index_range: StatePartIndexRange<Self>,
+    ) -> bool {
+        state.modified[part_index_range.start.as_usize()..]
+            [..part_index_range.len.as_index().as_usize()]
+            .contains(&true)
+    }
+    fn clear_all_maybe_modified_flags(state: &mut Self::State) {
+        state.modified.fill(false);
+    }
+    fn borrowed_state_index<'a, 'b>(
+        state: &'a Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_index_mut<'a, 'b>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_get_disjoint_mut<'a, 'b, const N: usize>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_indexes: [StatePartIndex<Self>; N],
+    ) -> [&'a mut Self::StateElement; N] {
+        for part_index in part_indexes {
+            state.modified[part_index.as_usize()] = true;
+        }
+        state
+            .state
+            .get_disjoint_mut(part_indexes.map(StatePartIndex::as_usize))
+            .expect("indexes are disjoint")
+    }
 }
 
 impl StatePartKind for StatePartKindBigSlots {
     const NAME: &'static str = "BigSlots";
     type DebugData = SlotDebugData;
     type LayoutData = ();
-    type State = Box<[BigInt]>;
-    type BorrowedState<'a> = &'a mut [BigInt];
+    type State = StateAndModified<Box<[Self::StateElement]>, Box<[bool]>>;
+    type BorrowedState<'a> = StateAndModified<&'a mut [Self::StateElement], &'a mut [bool]>;
+    type StateElement = BigInt;
     fn new_state(layout_data: &[Self::LayoutData]) -> Self::State {
-        layout_data.iter().map(|_| BigInt::default()).collect()
+        let state: Box<[_]> = layout_data.iter().map(|_| BigInt::default()).collect();
+        StateAndModified {
+            modified: vec![false; state.len()].into_boxed_slice(),
+            state,
+        }
     }
     fn borrow_state<'a>(state: &'a mut Self::State) -> Self::BorrowedState<'a> {
-        state
+        let StateAndModified { state, modified } = state;
+        StateAndModified { state, modified }
     }
     fn part_debug_data<BK: InsnsBuildingKind>(
         state_layout: &StateLayout<BK>,
@@ -361,19 +529,80 @@ impl StatePartKind for StatePartKindBigSlots {
     ) -> fmt::Result {
         write!(f, "{:#x}", state.big_slots[index])
     }
+    fn state_index<'a>(
+        state: &'a Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn state_index_mut<'a>(
+        state: &'a mut Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn state_index_fetch_maybe_modified_flag(
+        state: &Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> bool {
+        state.modified[part_index.as_usize()]
+    }
+    fn state_index_range_fetch_maybe_modified_flags(
+        state: &Self::State,
+        part_index_range: StatePartIndexRange<Self>,
+    ) -> bool {
+        state.modified[part_index_range.start.as_usize()..]
+            [..part_index_range.len.as_index().as_usize()]
+            .contains(&true)
+    }
+    fn clear_all_maybe_modified_flags(state: &mut Self::State) {
+        state.modified.fill(false);
+    }
+    fn borrowed_state_index<'a, 'b>(
+        state: &'a Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_index_mut<'a, 'b>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_get_disjoint_mut<'a, 'b, const N: usize>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_indexes: [StatePartIndex<Self>; N],
+    ) -> [&'a mut Self::StateElement; N] {
+        for part_index in part_indexes {
+            state.modified[part_index.as_usize()] = true;
+        }
+        state
+            .state
+            .get_disjoint_mut(part_indexes.map(StatePartIndex::as_usize))
+            .expect("indexes are disjoint")
+    }
 }
 
 impl StatePartKind for StatePartKindSimOnlySlots {
     const NAME: &'static str = "SimOnlySlots";
     type DebugData = SlotDebugData;
     type LayoutData = DynSimOnly;
-    type State = Box<[DynSimOnlyValue]>;
-    type BorrowedState<'a> = &'a mut [DynSimOnlyValue];
+    type State = StateAndModified<Box<[Self::StateElement]>, Box<[bool]>>;
+    type BorrowedState<'a> = StateAndModified<&'a mut [Self::StateElement], &'a mut [bool]>;
+    type StateElement = DynSimOnlyValue;
     fn new_state(layout_data: &[Self::LayoutData]) -> Self::State {
-        layout_data.iter().map(|ty| ty.default_value()).collect()
+        let state: Box<[_]> = layout_data.iter().map(|ty| ty.default_value()).collect();
+        StateAndModified {
+            modified: vec![false; state.len()].into_boxed_slice(),
+            state,
+        }
     }
     fn borrow_state<'a>(state: &'a mut Self::State) -> Self::BorrowedState<'a> {
-        state
+        let StateAndModified { state, modified } = state;
+        StateAndModified { state, modified }
     }
     fn part_debug_data<BK: InsnsBuildingKind>(
         state_layout: &StateLayout<BK>,
@@ -392,6 +621,61 @@ impl StatePartKind for StatePartKindSimOnlySlots {
     ) -> fmt::Result {
         write!(f, "{:?}", state.sim_only_slots[index])
     }
+    fn state_index<'a>(
+        state: &'a Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn state_index_mut<'a>(
+        state: &'a mut Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn state_index_fetch_maybe_modified_flag(
+        state: &Self::State,
+        part_index: StatePartIndex<Self>,
+    ) -> bool {
+        state.modified[part_index.as_usize()]
+    }
+    fn state_index_range_fetch_maybe_modified_flags(
+        state: &Self::State,
+        part_index_range: StatePartIndexRange<Self>,
+    ) -> bool {
+        state.modified[part_index_range.start.as_usize()..]
+            [..part_index_range.len.as_index().as_usize()]
+            .contains(&true)
+    }
+    fn clear_all_maybe_modified_flags(state: &mut Self::State) {
+        state.modified.fill(false);
+    }
+    fn borrowed_state_index<'a, 'b>(
+        state: &'a Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a Self::StateElement {
+        &state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_index_mut<'a, 'b>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_index: StatePartIndex<Self>,
+    ) -> &'a mut Self::StateElement {
+        state.modified[part_index.as_usize()] = true;
+        &mut state.state[part_index.as_usize()]
+    }
+    fn borrowed_state_get_disjoint_mut<'a, 'b, const N: usize>(
+        state: &'a mut Self::BorrowedState<'b>,
+        part_indexes: [StatePartIndex<Self>; N],
+    ) -> [&'a mut Self::StateElement; N] {
+        for part_index in part_indexes {
+            state.modified[part_index.as_usize()] = true;
+        }
+        state
+            .state
+            .get_disjoint_mut(part_indexes.map(StatePartIndex::as_usize))
+            .expect("indexes are disjoint")
+    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]

crates/fayalite/src/sim/value.rs

@@ -15,7 +15,8 @@ use crate::{
     source_location::SourceLocation,
     ty::{
         CanonicalType, OpaqueSimValue, OpaqueSimValueSize, OpaqueSimValueSlice,
-        OpaqueSimValueWriter, StaticType, Type, TypeProperties, impl_match_variant_as_self,
+        OpaqueSimValueWriter, SimValueDebug, StaticType, Type, TypeProperties,
+        impl_match_variant_as_self,
     },
     util::{
         ConstUsize, HashMap,
@@ -551,113 +552,119 @@ impl_sim_value_cmp_as_bool!(AsyncReset);
 
 #[doc(hidden)]
 pub mod match_sim_value {
-    use crate::{
-        sim::value::{SimValue, ToSimValue},
-        ty::Type,
-    };
+    use crate::{sim::value::SimValue, ty::Type};
+    use std::ops::{Deref, DerefMut};
+
+    macro_rules! wrapper {
+        (
+            $(pub struct $wrapper:ident<$T:ident>($inner:ty);)*
+        ) => {
+            $(#[doc(hidden)]
+            pub struct $wrapper<$T>($inner);
+
+            impl<$T> $wrapper<$T> {
+                #[inline(always)]
+                pub fn new(value: $T) -> Self {
+                    Self(<$inner>::new(value))
+                }
+            }
+
+            impl<$T> Deref for $wrapper<$T> {
+                type Target = $inner;
+
+                #[inline(always)]
+                fn deref(&self) -> &Self::Target {
+                    &self.0
+                }
+            }
+
+            impl<$T> DerefMut for $wrapper<$T> {
+                #[inline(always)]
+                fn deref_mut(&mut self) -> &mut Self::Target {
+                    &mut self.0
+                }
+            })*
+        };
+    }
+
+    wrapper! {
+        pub struct MatchSimValueHelperCheckSimValue<T>(MatchSimValueHelperCheckMutSimValue<T>);
+        pub struct MatchSimValueHelperCheckMutSimValue<T>(MatchSimValueHelperCheckRefSimValue<T>);
+        pub struct MatchSimValueHelperCheckRefSimValue<T>(MatchSimValueHelperCheckRefRefSimValue<T>);
+        pub struct MatchSimValueHelperCheckRefRefSimValue<T>(MatchSimValueHelperCheckRefMutSimValue<T>);
+        pub struct MatchSimValueHelperCheckRefMutSimValue<T>(MatchSimValueHelperCheckMutRefSimValue<T>);
+        pub struct MatchSimValueHelperCheckMutRefSimValue<T>(MatchSimValueHelperCheckMutMutSimValue<T>);
+        pub struct MatchSimValueHelperCheckMutMutSimValue<T>(MatchSimValueHelperIdentity<T>);
+    }
+
+    impl<T: Type> MatchSimValueHelperCheckSimValue<SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> T::SimValue {
+            SimValue::into_value(self.take())
+        }
+    }
+
+    impl<'a, T: Type> MatchSimValueHelperCheckMutSimValue<&'a mut SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'a mut T::SimValue {
+            self.take()
+        }
+    }
+
+    impl<'a, T: Type> MatchSimValueHelperCheckRefSimValue<&'a SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'a T::SimValue {
+            self.take()
+        }
+    }
+
+    impl<'a, 'b, T: Type> MatchSimValueHelperCheckRefRefSimValue<&'a &'b SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'b T::SimValue {
+            self.take()
+        }
+    }
+
+    impl<'a, 'b, T: Type> MatchSimValueHelperCheckRefMutSimValue<&'a &'b mut SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'a T::SimValue {
+            self.take()
+        }
+    }
+
+    impl<'a, 'b, T: Type> MatchSimValueHelperCheckMutRefSimValue<&'a mut &'b SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'b T::SimValue {
+            self.take()
+        }
+    }
+
+    impl<'a, 'b, T: Type> MatchSimValueHelperCheckMutMutSimValue<&'a mut &'b mut SimValue<T>> {
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> &'a mut T::SimValue {
+            self.take()
+        }
+    }
 
     #[doc(hidden)]
-    pub struct MatchSimValueHelper<T>(Option<T>);
+    pub struct MatchSimValueHelperIdentity<T>(Option<T>);
 
-    impl<T> MatchSimValueHelper<T> {
-        pub fn new(v: T) -> Self {
+    impl<T> MatchSimValueHelperIdentity<T> {
+        fn new(v: T) -> Self {
             Self(Some(v))
         }
-    }
-
-    #[doc(hidden)]
-    pub trait MatchSimValue {
-        type MatchValue;
-
-        /// use `self` so it comes first in the method resolution order
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue
-        where
-            Self: Sized;
-    }
-
-    impl<T: Type> MatchSimValue for MatchSimValueHelper<SimValue<T>> {
-        type MatchValue = T::SimValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            SimValue::into_value(self.0.expect("should be Some"))
+        #[inline(always)]
+        fn take(&mut self) -> T {
+            self.0.take().expect("known to be Some")
         }
-    }
-
-    impl<'a, T: Type> MatchSimValue for MatchSimValueHelper<&'a SimValue<T>> {
-        type MatchValue = &'a T::SimValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            SimValue::value(self.0.expect("should be Some"))
-        }
-    }
-
-    impl<'a, T: Type> MatchSimValue for MatchSimValueHelper<&'a mut SimValue<T>> {
-        type MatchValue = &'a mut T::SimValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            SimValue::value_mut(self.0.expect("should be Some"))
-        }
-    }
-
-    impl<'a, T> MatchSimValue for MatchSimValueHelper<&'_ &'a T>
-    where
-        MatchSimValueHelper<&'a T>: MatchSimValue,
-    {
-        type MatchValue = <MatchSimValueHelper<&'a T> as MatchSimValue>::MatchValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            MatchSimValue::__fayalite_match_sim_value(MatchSimValueHelper(self.0.map(|v| *v)))
-        }
-    }
-
-    impl<'a, T> MatchSimValue for MatchSimValueHelper<&'_ mut &'a T>
-    where
-        MatchSimValueHelper<&'a T>: MatchSimValue,
-    {
-        type MatchValue = <MatchSimValueHelper<&'a T> as MatchSimValue>::MatchValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            MatchSimValue::__fayalite_match_sim_value(MatchSimValueHelper(self.0.map(|v| *v)))
-        }
-    }
-
-    impl<'a, T> MatchSimValue for MatchSimValueHelper<&'a &'_ mut T>
-    where
-        MatchSimValueHelper<&'a T>: MatchSimValue,
-    {
-        type MatchValue = <MatchSimValueHelper<&'a T> as MatchSimValue>::MatchValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            MatchSimValue::__fayalite_match_sim_value(MatchSimValueHelper(self.0.map(|v| &**v)))
-        }
-    }
-
-    impl<'a, T> MatchSimValue for MatchSimValueHelper<&'a mut &'_ mut T>
-    where
-        MatchSimValueHelper<&'a mut T>: MatchSimValue,
-    {
-        type MatchValue = <MatchSimValueHelper<&'a mut T> as MatchSimValue>::MatchValue;
-
-        fn __fayalite_match_sim_value(self) -> Self::MatchValue {
-            MatchSimValue::__fayalite_match_sim_value(MatchSimValueHelper(self.0.map(|v| &mut **v)))
+        #[inline(always)]
+        pub fn __fayalite_match_sim_value(&mut self) -> T {
+            self.take()
         }
     }
 
     #[doc(hidden)]
-    pub trait MatchSimValueFallback {
-        type MatchValue;
-
-        /// use `&mut self` so it comes later in the method resolution order than MatchSimValue
-        fn __fayalite_match_sim_value(&mut self) -> Self::MatchValue;
-    }
-
-    impl<T: ToSimValue> MatchSimValueFallback for MatchSimValueHelper<T> {
-        type MatchValue = <T::Type as Type>::SimValue;
-
-        fn __fayalite_match_sim_value(&mut self) -> Self::MatchValue {
-            SimValue::into_value(self.0.take().expect("should be Some").into_sim_value())
-        }
-    }
+    pub type MatchSimValueHelper<T> = MatchSimValueHelperCheckSimValue<T>;
 }
 
 pub trait ToSimValue: ToSimValueWithType<<Self as ValueType>::Type> + ValueType {
@@ -1091,7 +1098,8 @@ impl ToSimValueWithType<CanonicalType> for bool {
             | CanonicalType::Enum(_)
             | CanonicalType::Bundle(_)
             | CanonicalType::PhantomConst(_)
-            | CanonicalType::DynSimOnly(_) => {
+            | CanonicalType::DynSimOnly(_)
+            | CanonicalType::TraceAsString(_) => {
                 panic!("can't create SimValue from bool: expected value of type: {ty:?}");
             }
             CanonicalType::Bool(_)
@@ -1394,6 +1402,15 @@ impl Type for DynSimOnly {
     }
 }
 
+impl SimValueDebug for DynSimOnly {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl<T: SimOnlyValueTrait> Type for SimOnly<T> {
     type BaseType = DynSimOnly;
     type MaskType = Bool;
@@ -1459,6 +1476,15 @@ impl<T: SimOnlyValueTrait> Type for SimOnly<T> {
     }
 }
 
+impl<T: SimOnlyValueTrait> SimValueDebug for SimOnly<T> {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 impl<T: SimOnlyValueTrait> StaticType for SimOnly<T> {
     const TYPE: Self = Self::new();
 

crates/fayalite/src/sim/vcd.rs

@@ -12,11 +12,12 @@ use crate::{
         TraceEnumDiscriminant, TraceEnumWithFields, TraceFieldlessEnum, TraceInstance,
         TraceLocation, TraceMem, TraceMemPort, TraceMemoryId, TraceMemoryLocation, TraceModule,
         TraceModuleIO, TracePhantomConst, TraceReg, TraceSInt, TraceScalar, TraceScalarId,
-        TraceScope, TraceSimOnly, TraceSyncReset, TraceUInt, TraceWire, TraceWriter,
-        TraceWriterDecls,
+        TraceScope, TraceSimOnly, TraceSyncReset, TraceTraceAsString, TraceUInt, TraceWire,
+        TraceWriter, TraceWriterDecls,
         time::{SimDuration, SimInstant},
         value::DynSimOnlyValue,
     },
+    ty::{OpaqueSimValueSlice, TraceAsString},
     util::HashMap,
 };
 use bitvec::{order::Lsb0, slice::BitSlice};
@@ -26,6 +27,7 @@ use std::{
     collections::BTreeMap,
     fmt::{self, Write as _},
     io, mem,
+    num::NonZeroU64,
 };
 
 #[derive(Default, Clone)]
@@ -186,6 +188,26 @@ impl<W: io::Write> fmt::Debug for VcdWriterDecls<W> {
     }
 }
 
+/// pass in scope to ensure it's not available in child scope
+fn try_write_vcd_scope<W: io::Write, R>(
+    writer: &mut W,
+    scope_type: &str,
+    scope_name: Interned<str>,
+    scope: Option<&mut Scope>,
+    f: impl FnOnce(&mut W, Option<&mut Scope>) -> io::Result<R>,
+) -> io::Result<R> {
+    let Some(scope) = scope else {
+        return f(writer, None);
+    };
+    write_vcd_scope(
+        writer,
+        scope_type,
+        scope_name,
+        scope,
+        move |writer, scope| f(writer, Some(scope)),
+    )
+}
+
 /// pass in scope to ensure it's not available in child scope
 fn write_vcd_scope<W: io::Write, R>(
     writer: &mut W,
@@ -237,6 +259,7 @@ trait_arg! {
 struct ArgModule<'a> {
     properties: &'a mut VcdWriterProperties,
     scope: &'a mut Scope,
+    instance_name: Option<Interned<str>>,
 }
 
 impl<'a> ArgModule<'a> {
@@ -244,6 +267,7 @@ impl<'a> ArgModule<'a> {
         ArgModule {
             properties: self.properties,
             scope: self.scope,
+            instance_name: self.instance_name,
         }
     }
 }
@@ -267,7 +291,7 @@ struct ArgInType<'a> {
     sink_var_type: &'static str,
     duplex_var_type: &'static str,
     properties: &'a mut VcdWriterProperties,
-    scope: &'a mut Scope,
+    scope: Option<&'a mut Scope>,
 }
 
 impl<'a> ArgInType<'a> {
@@ -277,7 +301,7 @@ impl<'a> ArgInType<'a> {
             sink_var_type: self.sink_var_type,
             duplex_var_type: self.duplex_var_type,
             properties: self.properties,
-            scope: self.scope,
+            scope: self.scope.as_deref_mut(),
         }
     }
 }
@@ -308,13 +332,14 @@ impl WriteTrace for TraceScalar {
             Self::AsyncReset(v) => v.write_trace(writer, arg),
             Self::PhantomConst(v) => v.write_trace(writer, arg),
             Self::SimOnly(v) => v.write_trace(writer, arg),
+            Self::TraceAsString(v) => v.write_trace(writer, arg),
         }
     }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
 #[repr(transparent)]
-struct VcdId(u64);
+struct VcdId(NonZeroU64);
 
 impl VcdId {
     const CHAR_RANGE: std::ops::RangeInclusive<u8> = b'!'..=b'~';
@@ -344,11 +369,14 @@ impl VcdId {
             };
             retval = v;
         }
+        let Some(retval) = NonZeroU64::new(retval) else {
+            return None;
+        };
         Some(Self(retval))
     }
     #[must_use]
     const fn write(self, out: &mut [u8]) -> usize {
-        let mut id = self.0;
+        let mut id = self.0.get();
         let mut len = 0;
         loop {
             let digit = (id % Self::BASE as u64) as u8 + *Self::CHAR_RANGE.start();
@@ -363,7 +391,7 @@ impl VcdId {
         }
         len
     }
-    const MAX_ID_LEN: usize = Self(u64::MAX).write(&mut []);
+    const MAX_ID_LEN: usize = Self(NonZeroU64::MAX).write(&mut []);
 }
 
 /// check that VcdId properly round-trips
@@ -423,7 +451,7 @@ impl<T: fmt::Display> fmt::Display for Escaped<T> {
 
 fn write_vcd_var<W: io::Write>(
     properties: &mut VcdWriterProperties,
-    scope: &mut Scope,
+    scope: Option<&mut Scope>,
     memory_element_part_body: MemoryElementPartBody,
     writer: &mut W,
     var_type: &str,
@@ -431,8 +459,6 @@ fn write_vcd_var<W: io::Write>(
     location: TraceLocation,
     name: Interned<str>,
 ) -> io::Result<()> {
-    let path_hash = scope.path_hash.clone().joined(name);
-    let name = scope.new_identifier(name);
     let id = match location {
         TraceLocation::Scalar(id) => id.as_usize(),
         TraceLocation::Memory(TraceMemoryLocation {
@@ -464,12 +490,21 @@ fn write_vcd_var<W: io::Write>(
             first_id + *element_index
         }
     };
-    let id = properties
-        .scalar_id_to_vcd_id_map
-        .builder_get_or_insert(id, &path_hash);
-    write!(writer, "$var {var_type} {size} ")?;
-    write_vcd_id(writer, id)?;
-    writeln!(writer, " {name} $end")
+    if let Some(scope) = scope {
+        let path_hash = scope.path_hash.clone().joined(name);
+        let name = scope.new_identifier(name);
+        let id = properties
+            .scalar_id_to_vcd_id_map
+            .builder_get_or_insert(id, &path_hash);
+        write!(writer, "$var {var_type} {size} ")?;
+        write_vcd_id(writer, id)?;
+        writeln!(writer, " {name} $end")
+    } else {
+        properties
+            .scalar_id_to_vcd_id_map
+            .builder_unused_scalar_id(id);
+        Ok(())
+    }
 }
 
 impl WriteTrace for TraceUInt {
@@ -693,6 +728,34 @@ impl WriteTrace for TraceSimOnly {
     }
 }
 
+impl WriteTrace for TraceTraceAsString {
+    fn write_trace<W: io::Write, A: Arg>(self, writer: &mut W, mut arg: A) -> io::Result<()> {
+        let ArgInType {
+            source_var_type: _,
+            sink_var_type: _,
+            duplex_var_type: _,
+            properties,
+            scope,
+        } = arg.in_type();
+        let Self {
+            location,
+            name,
+            ty,
+            flow: _,
+        } = self;
+        write_vcd_var(
+            properties,
+            scope,
+            MemoryElementPartBody::TraceAsString { ty },
+            writer,
+            "string",
+            1,
+            location,
+            name,
+        )
+    }
+}
+
 impl WriteTrace for TraceScope {
     fn write_trace<W: io::Write, A: Arg>(self, writer: &mut W, arg: A) -> io::Result<()> {
         match self {
@@ -712,14 +775,24 @@ impl WriteTrace for TraceScope {
 
 impl WriteTrace for TraceModule {
     fn write_trace<W: io::Write, A: Arg>(self, writer: &mut W, mut arg: A) -> io::Result<()> {
-        let ArgModule { properties, scope } = arg.module();
+        let ArgModule {
+            properties,
+            scope,
+            instance_name,
+        } = arg.module();
         let Self { name, children } = self;
-        write_vcd_scope(writer, "module", name, scope, |writer, scope| {
-            for child in children {
-                child.write_trace(writer, ArgModuleBody { properties, scope })?;
-            }
-            Ok(())
-        })
+        write_vcd_scope(
+            writer,
+            "module",
+            instance_name.unwrap_or(name),
+            scope,
+            |writer, scope| {
+                for child in children {
+                    child.write_trace(writer, ArgModuleBody { properties, scope })?;
+                }
+                Ok(())
+            },
+        )
     }
 }
 
@@ -727,7 +800,7 @@ impl WriteTrace for TraceInstance {
     fn write_trace<W: io::Write, A: Arg>(self, writer: &mut W, mut arg: A) -> io::Result<()> {
         let ArgModuleBody { properties, scope } = arg.module_body();
         let Self {
-            name: _,
+            name,
             instance_io,
             module,
             ty: _,
@@ -739,10 +812,17 @@ impl WriteTrace for TraceInstance {
                 sink_var_type: "wire",
                 duplex_var_type: "wire",
                 properties,
-                scope,
+                scope: None,
             },
         )?;
-        module.write_trace(writer, ArgModule { properties, scope })
+        module.write_trace(
+            writer,
+            ArgModule {
+                properties,
+                scope,
+                instance_name: Some(name),
+            },
+        )
     }
 }
 
@@ -781,7 +861,7 @@ impl WriteTrace for TraceMem {
                                         sink_var_type: "reg",
                                         duplex_var_type: "reg",
                                         properties,
-                                        scope,
+                                        scope: Some(scope),
                                     },
                                 )
                             },
@@ -813,7 +893,7 @@ impl WriteTrace for TraceMemPort {
                 sink_var_type: "wire",
                 duplex_var_type: "wire",
                 properties,
-                scope,
+                scope: Some(scope),
             },
         )
     }
@@ -834,7 +914,7 @@ impl WriteTrace for TraceWire {
                 sink_var_type: "wire",
                 duplex_var_type: "wire",
                 properties,
-                scope,
+                scope: Some(scope),
             },
         )
     }
@@ -855,7 +935,7 @@ impl WriteTrace for TraceReg {
                 sink_var_type: "reg",
                 duplex_var_type: "reg",
                 properties,
-                scope,
+                scope: Some(scope),
             },
         )
     }
@@ -877,7 +957,7 @@ impl WriteTrace for TraceModuleIO {
                 sink_var_type: "wire",
                 duplex_var_type: "wire",
                 properties,
-                scope,
+                scope: Some(scope),
             },
         )
     }
@@ -898,7 +978,7 @@ impl WriteTrace for TraceBundle {
             ty: _,
             flow: _,
         } = self;
-        write_vcd_scope(writer, "struct", name, scope, |writer, scope| {
+        try_write_vcd_scope(writer, "struct", name, scope, |writer, mut scope| {
             for field in fields {
                 field.write_trace(
                     writer,
@@ -907,7 +987,7 @@ impl WriteTrace for TraceBundle {
                         sink_var_type,
                         duplex_var_type,
                         properties,
-                        scope,
+                        scope: scope.as_deref_mut(),
                     },
                 )?;
             }
@@ -931,7 +1011,7 @@ impl WriteTrace for TraceArray {
             ty: _,
             flow: _,
         } = self;
-        write_vcd_scope(writer, "struct", name, scope, |writer, scope| {
+        try_write_vcd_scope(writer, "struct", name, scope, |writer, mut scope| {
             for element in elements {
                 element.write_trace(
                     writer,
@@ -940,7 +1020,7 @@ impl WriteTrace for TraceArray {
                         sink_var_type,
                         duplex_var_type,
                         properties,
-                        scope,
+                        scope: scope.as_deref_mut(),
                     },
                 )?;
             }
@@ -965,7 +1045,7 @@ impl WriteTrace for TraceEnumWithFields {
             ty: _,
             flow: _,
         } = self;
-        write_vcd_scope(writer, "struct", name, scope, |writer, scope| {
+        try_write_vcd_scope(writer, "struct", name, scope, |writer, mut scope| {
             discriminant.write_trace(
                 writer,
                 ArgInType {
@@ -973,7 +1053,7 @@ impl WriteTrace for TraceEnumWithFields {
                     sink_var_type,
                     duplex_var_type,
                     properties,
-                    scope,
+                    scope: scope.as_deref_mut(),
                 },
             )?;
             for field in non_empty_fields {
@@ -984,7 +1064,7 @@ impl WriteTrace for TraceEnumWithFields {
                         sink_var_type,
                         duplex_var_type,
                         properties,
-                        scope,
+                        scope: scope.as_deref_mut(),
                     },
                 )?;
             }
@@ -1026,6 +1106,7 @@ impl<W: io::Write> TraceWriterDecls for VcdWriterDecls<W> {
             ArgModule {
                 properties: &mut properties,
                 scope: &mut Scope::new(PathHash::default()),
+                instance_name: None,
             },
         )?;
         let ScalarIdToVcdIdMapOrBuilder::Builder(scalar_id_to_vcd_id_map_builder) =
@@ -1042,6 +1123,7 @@ impl<W: io::Write> TraceWriterDecls for VcdWriterDecls<W> {
             finished_init: false,
             timescale,
             properties,
+            trace_as_string_buf: String::with_capacity(256),
         })
     }
 }
@@ -1049,6 +1131,7 @@ impl<W: io::Write> TraceWriterDecls for VcdWriterDecls<W> {
 enum MemoryElementPartBody {
     Scalar,
     EnumDiscriminant { ty: Enum },
+    TraceAsString { ty: TraceAsString },
 }
 
 struct MemoryElementPart {
@@ -1065,23 +1148,29 @@ struct MemoryProperties {
 }
 
 struct ScalarIdToVcdIdMap {
-    scalar_id_to_vcd_id_map: Box<[VcdId]>,
+    scalar_id_to_vcd_id_map: Box<[Option<VcdId>]>,
 }
 
 #[derive(Default)]
 struct ScalarIdToVcdIdMapBuilder {
-    scalar_id_to_vcd_id_map: BTreeMap<usize, VcdId>,
+    scalar_id_to_vcd_id_map: BTreeMap<usize, Option<VcdId>>,
     lower_half_to_next_upper_half_map: HashMap<u64, u64>,
 }
 
 impl ScalarIdToVcdIdMapBuilder {
+    fn unused_scalar_id(&mut self, scalar_id: usize) {
+        self.scalar_id_to_vcd_id_map
+            .entry(scalar_id)
+            .or_insert(None);
+    }
     /// `VcdId`s are based off of `path_hash` (and not `scalar_id`) since the hash doesn't change
     /// when unrelated variables are added/removed, making the generated VCD more friendly for git diff.
     fn get_or_insert(&mut self, scalar_id: usize, path_hash: &PathHash) -> VcdId {
         *self
             .scalar_id_to_vcd_id_map
             .entry(scalar_id)
-            .or_insert_with(|| {
+            .or_insert(None)
+            .get_or_insert_with(|| {
                 let hash = u128::from_le_bytes(
                     *path_hash
                         .0
@@ -1094,7 +1183,7 @@ impl ScalarIdToVcdIdMapBuilder {
                 let next_upper_half = self
                     .lower_half_to_next_upper_half_map
                     .entry(lower_half)
-                    .or_insert(0);
+                    .or_insert(if lower_half == 0 { 1 } else { 0 });
                 let upper_half = *next_upper_half;
                 *next_upper_half += 1;
                 let Some(id) = upper_half
@@ -1103,7 +1192,7 @@ impl ScalarIdToVcdIdMapBuilder {
                 else {
                     panic!("too many VcdIds");
                 };
-                VcdId(id)
+                VcdId(NonZeroU64::new(id).expect("known to not be zero"))
             })
     }
     fn build(self) -> ScalarIdToVcdIdMap {
@@ -1129,7 +1218,7 @@ enum ScalarIdToVcdIdMapOrBuilder {
 }
 
 impl ScalarIdToVcdIdMapOrBuilder {
-    fn built_scalar_id_to_vcd_id(&self, scalar_id: usize) -> VcdId {
+    fn built_scalar_id_to_vcd_id(&self, scalar_id: usize) -> Option<VcdId> {
         let Self::Built(v) = self else {
             panic!("ScalarIdToVcdIdMap isn't built yet");
         };
@@ -1141,6 +1230,12 @@ impl ScalarIdToVcdIdMapOrBuilder {
         };
         v.get_or_insert(scalar_id, path_hash)
     }
+    fn builder_unused_scalar_id(&mut self, scalar_id: usize) {
+        let Self::Builder(v) = self else {
+            panic!("ScalarIdToVcdIdMap is already built");
+        };
+        v.unused_scalar_id(scalar_id)
+    }
 }
 
 struct VcdWriterProperties {
@@ -1154,6 +1249,7 @@ pub struct VcdWriter<W: io::Write + 'static> {
     finished_init: bool,
     timescale: SimDuration,
     properties: VcdWriterProperties,
+    trace_as_string_buf: String,
 }
 
 impl<W: io::Write + 'static> VcdWriter<W> {
@@ -1165,8 +1261,11 @@ impl<W: io::Write + 'static> VcdWriter<W> {
 fn write_string_value_change(
     writer: &mut impl io::Write,
     value: impl fmt::Display,
-    id: VcdId,
+    id: Option<VcdId>,
 ) -> io::Result<()> {
+    let Some(id) = id else {
+        return Ok(());
+    };
     write!(writer, "s{} ", Escaped(value))?;
     write_vcd_id(writer, id)?;
     writer.write_all(b"\n")
@@ -1175,8 +1274,11 @@ fn write_string_value_change(
 fn write_bits_value_change(
     writer: &mut impl io::Write,
     value: &BitSlice,
-    id: VcdId,
+    id: Option<VcdId>,
 ) -> io::Result<()> {
+    let Some(id) = id else {
+        return Ok(());
+    };
     match value.len() {
         0 => writer.write_all(b"s0 ")?,
         1 => writer.write_all(if value[0] { b"1" } else { b"0" })?,
@@ -1205,7 +1307,7 @@ fn write_enum_discriminant_value_change(
     writer: &mut impl io::Write,
     variant_index: usize,
     ty: Enum,
-    id: VcdId,
+    id: Option<VcdId>,
 ) -> io::Result<()> {
     write_string_value_change(
         writer,
@@ -1257,6 +1359,21 @@ impl<W: io::Write> TraceWriter for VcdWriter<W> {
                             .built_scalar_id_to_vcd_id(first_id + element_index),
                     )?
                 }
+                MemoryElementPartBody::TraceAsString { ty } => {
+                    self.trace_as_string_buf.clear();
+                    ty.trace_fmt_append_to_string(
+                        &mut self.trace_as_string_buf,
+                        OpaqueSimValueSlice::from_bitslice(&element_data[start..start + len]),
+                    );
+                    write_string_value_change(
+                        &mut self.writer,
+                        &self.trace_as_string_buf,
+                        self.properties
+                            .scalar_id_to_vcd_id_map
+                            .built_scalar_id_to_vcd_id(first_id + element_index),
+                    )?;
+                    self.trace_as_string_buf.clear();
+                }
             }
         }
         Ok(())
@@ -1350,6 +1467,16 @@ impl<W: io::Write> TraceWriter for VcdWriter<W> {
                 .built_scalar_id_to_vcd_id(id.as_usize()),
         )
     }
+
+    fn set_signal_string(&mut self, id: TraceScalarId, value: &str) -> Result<(), Self::Error> {
+        write_string_value_change(
+            &mut self.writer,
+            value,
+            self.properties
+                .scalar_id_to_vcd_id_map
+                .built_scalar_id_to_vcd_id(id.as_usize()),
+        )
+    }
 }
 
 impl<W: io::Write> fmt::Debug for VcdWriter<W> {
@@ -1359,6 +1486,7 @@ impl<W: io::Write> fmt::Debug for VcdWriter<W> {
             finished_init,
             timescale,
             properties: _,
+            trace_as_string_buf: _,
         } = self;
         f.debug_struct("VcdWriter")
             .field("finished_init", finished_init)

crates/fayalite/src/ty.rs

@@ -3,22 +3,27 @@
 
 use crate::{
     array::Array,
-    bundle::Bundle,
+    bundle::{Bundle, BundleField, BundleType},
     clock::Clock,
-    enum_::Enum,
-    expr::Expr,
+    enum_::{Enum, EnumType, EnumVariant},
+    expr::{Expr, ToExpr, ValueType, ops},
     int::{Bool, SInt, UInt, UIntValue},
-    intern::{Intern, Interned},
+    intern::{Intern, Interned, LazyInterned, Memoize, SupportsPtrEqWithTypeId},
+    module::transform::visit::{Fold, Folder, Visit, Visitor},
     phantom_const::PhantomConst,
     reset::{AsyncReset, Reset, SyncReset},
-    sim::value::{DynSimOnly, DynSimOnlyValue, SimValue, ToSimValueWithType},
+    sim::value::{DynSimOnly, DynSimOnlyValue, SimValue, ToSimValue, ToSimValueWithType},
     source_location::SourceLocation,
-    util::{ConstUsize, slice_range, try_slice_range},
+    util::{
+        ConstUsize, iter_eq_by,
+        serde_by_id::{SerdeByIdProperties, SerdeByIdTable, SerdeByIdTrait},
+        slice_range, try_slice_range,
+    },
 };
 use bitvec::{slice::BitSlice, vec::BitVec};
 use serde::{Deserialize, Deserializer, Serialize, Serializer, de::DeserializeOwned};
 use std::{
-    fmt,
+    fmt::{self, Write},
     hash::Hash,
     iter::{FusedIterator, Sum},
     marker::PhantomData,
@@ -69,6 +74,7 @@ pub enum CanonicalType {
     Clock(Clock),
     PhantomConst(PhantomConst),
     DynSimOnly(DynSimOnly),
+    TraceAsString(TraceAsString),
 }
 
 impl fmt::Debug for CanonicalType {
@@ -86,6 +92,7 @@ impl fmt::Debug for CanonicalType {
             Self::Clock(v) => v.fmt(f),
             Self::PhantomConst(v) => v.fmt(f),
             Self::DynSimOnly(v) => v.fmt(f),
+            Self::TraceAsString(v) => v.fmt(f),
         }
     }
 }
@@ -123,6 +130,7 @@ impl CanonicalType {
             CanonicalType::Clock(v) => v.type_properties(),
             CanonicalType::PhantomConst(v) => v.type_properties(),
             CanonicalType::DynSimOnly(v) => v.type_properties(),
+            CanonicalType::TraceAsString(v) => v.type_properties(),
         }
     }
     pub fn is_passive(self) -> bool {
@@ -217,11 +225,132 @@ impl CanonicalType {
                 };
                 lhs.can_connect(rhs)
             }
+            CanonicalType::TraceAsString(lhs) => {
+                let CanonicalType::TraceAsString(rhs) = rhs else {
+                    return false;
+                };
+                lhs.can_connect(rhs)
+            }
         }
     }
     pub(crate) fn as_serde_unexpected_str(self) -> &'static str {
         serde_impls::SerdeCanonicalType::from(self).as_serde_unexpected_str()
     }
+    /// Unwrap transparent types until reaching a non-transparent type. Currently [`TraceAsString`] is the only transparent type.
+    pub fn unwrap_transparent_types(mut self) -> Self {
+        loop {
+            self = match self {
+                Self::UInt(_)
+                | Self::SInt(_)
+                | Self::Bool(_)
+                | Self::Array(_)
+                | Self::Enum(_)
+                | Self::Bundle(_)
+                | Self::AsyncReset(_)
+                | Self::SyncReset(_)
+                | Self::Reset(_)
+                | Self::Clock(_)
+                | Self::PhantomConst(_)
+                | Self::DynSimOnly(_) => return self,
+                Self::TraceAsString(ty) => ty.inner_ty(),
+            };
+        }
+    }
+    pub fn is_layout_equivalent(self, other: Self) -> bool {
+        fn is_bit(ty: CanonicalType) -> bool {
+            match ty {
+                CanonicalType::UInt(ty) => ty.width() == 1,
+                CanonicalType::SInt(_) => false, // SInt<1> doesn't count since it would be -1/0 instead of 1/0
+                CanonicalType::Bool(_)
+                | CanonicalType::AsyncReset(_)
+                | CanonicalType::SyncReset(_)
+                | CanonicalType::Reset(_)
+                | CanonicalType::Clock(_) => true,
+                CanonicalType::Array(_)
+                | CanonicalType::Enum(_)
+                | CanonicalType::Bundle(_)
+                | CanonicalType::PhantomConst(_)
+                | CanonicalType::DynSimOnly(_) => false,
+                CanonicalType::TraceAsString(_) => {
+                    unreachable!("handled by unwrap_transparent_types")
+                }
+            }
+        }
+        #[derive(Copy, Clone, PartialEq, Eq, Hash)]
+        struct MyMemoize;
+        impl Memoize for MyMemoize {
+            type Input = (CanonicalType, CanonicalType);
+            type InputOwned = (CanonicalType, CanonicalType);
+            type Output = bool;
+
+            fn inner(self, input: &Self::Input) -> Self::Output {
+                let (this, other) = *input;
+                let this = this.unwrap_transparent_types();
+                let other = other.unwrap_transparent_types();
+                let this_is_bit = is_bit(this);
+                let other_is_bit = is_bit(other);
+                if this_is_bit || other_is_bit {
+                    return this_is_bit && other_is_bit;
+                }
+                let this_is_empty = this.size().is_empty();
+                let other_is_empty = other.size().is_empty();
+                if this_is_empty || other_is_empty {
+                    return this_is_empty && other_is_empty;
+                }
+                match this {
+                    CanonicalType::UInt(_)
+                    | CanonicalType::SInt(_)
+                    | CanonicalType::DynSimOnly(_) => this == other,
+                    CanonicalType::Array(this) => {
+                        let CanonicalType::Array(other) = other else {
+                            return false;
+                        };
+                        this.len() == other.len()
+                            && this.element().is_layout_equivalent(other.element())
+                    }
+                    CanonicalType::Enum(this) => {
+                        let CanonicalType::Enum(other) = other else {
+                            return false;
+                        };
+                        iter_eq_by(
+                            this.variants(),
+                            other.variants(),
+                            |EnumVariant { name, ty }, other_variant| {
+                                name == other_variant.name
+                                    && ty.unwrap_or_else(|| ().canonical()).is_layout_equivalent(
+                                        other_variant.ty.unwrap_or_else(|| ().canonical()),
+                                    )
+                            },
+                        )
+                    }
+                    CanonicalType::Bundle(this) => {
+                        let CanonicalType::Bundle(other) = other else {
+                            return false;
+                        };
+                        iter_eq_by(
+                            this.fields().iter().filter(|f| !f.ty.size().is_empty()),
+                            other.fields().iter().filter(|f| !f.ty.size().is_empty()),
+                            |&BundleField { name, flipped, ty }, other_field| {
+                                name == other_field.name
+                                    && flipped == other_field.flipped
+                                    && ty.is_layout_equivalent(other_field.ty)
+                            },
+                        )
+                    }
+                    CanonicalType::Bool(_)
+                    | CanonicalType::AsyncReset(_)
+                    | CanonicalType::SyncReset(_)
+                    | CanonicalType::Reset(_)
+                    | CanonicalType::Clock(_) => unreachable!("handled by is_bit"),
+                    CanonicalType::PhantomConst(_) => unreachable!("handled by is_empty"),
+                    CanonicalType::TraceAsString(_) => {
+                        unreachable!("handled by unwrap_transparent_types")
+                    }
+                }
+            }
+        }
+        MyMemoize.get_owned((self, other))
+    }
 }
 
 pub trait MatchVariantAndInactiveScope: Sized {
@@ -328,6 +457,7 @@ impl_base_type!(Reset);
 impl_base_type!(Clock);
 impl_base_type!(PhantomConst);
 impl_base_type!(DynSimOnly);
+impl_base_type!(TraceAsString);
 
 impl_base_type_serde!(Bool, "a Bool");
 impl_base_type_serde!(Enum, "an Enum");
@@ -336,6 +466,7 @@ impl_base_type_serde!(AsyncReset, "an AsyncReset");
 impl_base_type_serde!(SyncReset, "a SyncReset");
 impl_base_type_serde!(Reset, "a Reset");
 impl_base_type_serde!(Clock, "a Clock");
+impl_base_type_serde!(TraceAsString, "a TraceAsString");
 
 impl sealed::BaseTypeSealed for CanonicalType {}
 
@@ -367,7 +498,15 @@ impl<D: Type> TypeOrDefault<D> for crate::__ {
 }
 
 pub trait Type:
-    Copy + Hash + Eq + fmt::Debug + Send + Sync + 'static + FillInDefaultedGenerics<Type = Self>
+    Copy
+    + Hash
+    + Eq
+    + fmt::Debug
+    + Send
+    + Sync
+    + 'static
+    + FillInDefaultedGenerics<Type = Self>
+    + SimValueDebug
 {
     type BaseType: BaseType;
     type MaskType: Type<MaskType = Self::MaskType>;
@@ -402,6 +541,16 @@ pub trait Type:
     ) -> OpaqueSimValueWritten<'w>;
 }
 
+pub trait SimValueDebug {
+    fn sim_value_debug(value: &<Self as Type>::SimValue, f: &mut fmt::Formatter<'_>) -> fmt::Result
+    where
+        Self: Type;
+}
+
+pub trait SimValueDisplay: Type {
+    fn sim_value_display(value: &Self::SimValue, f: &mut fmt::Formatter<'_>) -> fmt::Result;
+}
+
 pub trait BaseType:
     Type<
         BaseType = Self,
@@ -455,6 +604,7 @@ impl Type for CanonicalType {
             CanonicalType::Clock(v) => v.mask_type().canonical(),
             CanonicalType::PhantomConst(v) => v.mask_type().canonical(),
             CanonicalType::DynSimOnly(v) => v.mask_type().canonical(),
+            CanonicalType::TraceAsString(v) => v.mask_type().canonical(),
         }
     }
     fn canonical(&self) -> CanonicalType {
@@ -490,6 +640,15 @@ impl Type for CanonicalType {
     }
 }
 
+impl SimValueDebug for CanonicalType {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(value, f)
+    }
+}
+
 #[derive(Clone, PartialEq, Eq, Hash, Debug, Serialize, Deserialize, Default)]
 #[non_exhaustive]
 pub struct OpaqueSimValueSizeRange {
@@ -730,13 +889,34 @@ impl Sum for OpaqueSimValueSize {
     }
 }
 
-#[derive(Clone, PartialEq, Eq, Hash, Debug, Serialize, Deserialize)]
+#[derive(PartialEq, Eq, Hash, Debug, Serialize, Deserialize)]
 pub struct OpaqueSimValue {
     bits: UIntValue,
     #[serde(skip_serializing_if = "Vec::is_empty", default)]
     sim_only_values: Vec<DynSimOnlyValue>,
 }
 
+impl Clone for OpaqueSimValue {
+    fn clone(&self) -> Self {
+        Self {
+            bits: self.bits.clone(),
+            sim_only_values: self.sim_only_values.clone(),
+        }
+    }
+    fn clone_from(&mut self, source: &Self) {
+        let Self {
+            bits,
+            sim_only_values,
+        } = self;
+        if let Some(bits) = Arc::get_mut(bits.arc_bitvec_mut()) {
+            bits.clone_from(source.bits.bits());
+        } else {
+            *bits = source.bits.clone();
+        }
+        sim_only_values.clone_from(&source.sim_only_values);
+    }
+}
+
 impl OpaqueSimValue {
     pub fn empty() -> Self {
         Self {
@@ -815,6 +995,9 @@ impl OpaqueSimValue {
     pub fn sim_only_values(&self) -> &[DynSimOnlyValue] {
         &self.sim_only_values
     }
+    pub(crate) fn sim_only_values_vec(&self) -> &Vec<DynSimOnlyValue> {
+        &self.sim_only_values
+    }
     pub fn sim_only_values_mut(&mut self) -> &mut Vec<DynSimOnlyValue> {
         &mut self.sim_only_values
     }
@@ -1116,3 +1299,413 @@ impl<T: Type> Index<T> for AsMaskWithoutGenerics {
         Interned::into_inner(Intern::intern_sized(ty.mask_type()))
     }
 }
+
+trait TraceAsStringTrait: fmt::Debug + 'static + Send + Sync + SupportsPtrEqWithTypeId {
+    fn trace_fmt(&self, opaque: OpaqueSimValueSlice<'_>, f: &mut fmt::Formatter<'_>)
+    -> fmt::Result;
+    fn serde_by_id_properties(&self) -> SerdeByIdProperties<Interned<dyn TraceAsStringTrait>> {
+        SerdeByIdProperties::of::<Self>()
+    }
+    fn can_substitute_type(&self, new_type: CanonicalType) -> bool;
+}
+
+impl<T: Type> TraceAsStringTrait for T {
+    fn trace_fmt(
+        &self,
+        opaque: OpaqueSimValueSlice<'_>,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        fmt::Debug::fmt(&Type::sim_value_from_opaque(self, opaque), f)
+    }
+    fn can_substitute_type(&self, new_type: CanonicalType) -> bool {
+        self.canonical().is_layout_equivalent(new_type)
+    }
+}
+
+impl crate::intern::InternedCompare for dyn TraceAsStringTrait {
+    type InternedCompareKey = crate::intern::PtrEqWithTypeId;
+
+    fn interned_compare_key_ref(this: &Self) -> Self::InternedCompareKey {
+        this.get_ptr_eq_with_type_id()
+    }
+}
+
+impl SerdeByIdTrait for Interned<dyn TraceAsStringTrait> {
+    fn serde_by_id_properties(&self) -> SerdeByIdProperties<Self> {
+        TraceAsStringTrait::serde_by_id_properties(&**self)
+    }
+
+    fn static_table() -> &'static SerdeByIdTable<Self> {
+        static TABLE: SerdeByIdTable<Interned<dyn TraceAsStringTrait>> = SerdeByIdTable::new();
+        &TABLE
+    }
+
+    const NAME: &'static str = "dyn TraceAsStringTrait";
+}
+
+/// When running the fayalite simulator, outputs a single string signal containing a formatted version of the inner value (uses [`fmt::Debug`] by default).
+/// This is a transparent type, meaning [`CanonicalType::unwrap_transparent_types`] will unwrap this type.
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct TraceAsString<T: Type = CanonicalType> {
+    inner_ty: LazyInterned<T>,
+    trace_as_string: LazyInterned<dyn TraceAsStringTrait>,
+}
+
+#[expect(non_upper_case_globals)]
+pub const TraceAsString: TraceAsStringWithoutGenerics = TraceAsStringWithoutGenerics;
+
+impl<T: Type> fmt::Debug for TraceAsString<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self {
+            inner_ty,
+            trace_as_string: _,
+        } = self;
+        f.debug_struct("TraceAsString")
+            .field("inner_ty", &inner_ty.interned())
+            .finish_non_exhaustive()
+    }
+}
+
+impl<T: Type> TraceAsString<T> {
+    pub fn new(inner_ty: T) -> Self {
+        Self::new_interned(inner_ty.intern_sized())
+    }
+    pub fn new_interned(inner_ty: Interned<T>) -> Self {
+        Self {
+            inner_ty: LazyInterned::Interned(inner_ty),
+            trace_as_string: LazyInterned::Interned(Interned::cast_unchecked(
+                inner_ty,
+                |v| -> &dyn TraceAsStringTrait { v },
+            )),
+        }
+    }
+    pub fn interned_inner_ty(self) -> Interned<T> {
+        self.inner_ty.interned()
+    }
+    pub fn inner_ty(self) -> T {
+        *self.interned_inner_ty()
+    }
+    /// create a new `TraceAsString` but try to keep the old formatting method
+    pub fn with_new_inner_ty<U: Type>(self, inner_ty: Interned<U>) -> TraceAsString<U> {
+        if self
+            .trace_as_string
+            .can_substitute_type(inner_ty.canonical())
+        {
+            TraceAsString {
+                inner_ty: LazyInterned::Interned(inner_ty),
+                trace_as_string: self.trace_as_string,
+            }
+        } else {
+            TraceAsString::new_interned(inner_ty)
+        }
+    }
+    pub fn canonical_trace_as_string(self) -> TraceAsString<CanonicalType> {
+        let Self {
+            inner_ty,
+            trace_as_string,
+        } = self;
+        TraceAsString {
+            inner_ty: LazyInterned::Interned(inner_ty.interned().canonical().intern_sized()),
+            trace_as_string,
+        }
+    }
+    pub fn from_canonical_trace_as_string(canonical: TraceAsString<CanonicalType>) -> Self {
+        let TraceAsString {
+            inner_ty,
+            trace_as_string,
+        } = canonical;
+        Self {
+            inner_ty: LazyInterned::Interned(
+                T::from_canonical(*inner_ty.interned()).intern_sized(),
+            ),
+            trace_as_string,
+        }
+    }
+    pub fn type_properties(self) -> TypeProperties {
+        self.interned_inner_ty().canonical().type_properties()
+    }
+    pub fn can_connect<T2: Type>(self, rhs: TraceAsString<T2>) -> bool {
+        self.interned_inner_ty()
+            .canonical()
+            .can_connect(rhs.interned_inner_ty().canonical())
+    }
+    pub fn trace_fmt(
+        self,
+        opaque: OpaqueSimValueSlice<'_>,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        self.trace_as_string.interned().trace_fmt(opaque, f)
+    }
+    pub fn trace_fmt_append_to_string(self, output: &mut String, opaque: OpaqueSimValueSlice<'_>) {
+        fn impl_fn(
+            trace_as_string: Interned<dyn TraceAsStringTrait>,
+            output: &mut String,
+            opaque: OpaqueSimValueSlice<'_>,
+        ) {
+            let initial_len = output.len();
+            if let Err(fmt::Error {}) = write!(
+                output,
+                "{}",
+                fmt::from_fn(|f| trace_as_string.trace_fmt(opaque, f))
+            ) {
+                output.truncate(initial_len);
+                output.push_str("<!!!failed to format!!!>");
+            }
+        }
+        impl_fn(self.trace_as_string.interned(), output, opaque)
+    }
+}
+
+impl<T: Type> SimValueDebug for TraceAsString<T> {
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        T::sim_value_debug(value, f)
+    }
+}
+
+impl<T: Type> Type for TraceAsString<T> {
+    type BaseType = TraceAsString<CanonicalType>;
+    type MaskType = T::MaskType;
+    type SimValue = TraceAsStringSimValue<T::SimValue>;
+    type MatchVariant = Expr<T>;
+    type MatchActiveScope = ();
+    type MatchVariantAndInactiveScope = MatchVariantWithoutScope<Self::MatchVariant>;
+    type MatchVariantsIter = std::iter::Once<Self::MatchVariantAndInactiveScope>;
+
+    fn match_variants(
+        this: Expr<Self>,
+        source_location: SourceLocation,
+    ) -> Self::MatchVariantsIter {
+        let _ = source_location;
+        std::iter::once(MatchVariantWithoutScope(
+            ops::TraceAsStringAsInner::new(this).to_expr(),
+        ))
+    }
+
+    fn mask_type(&self) -> Self::MaskType {
+        self.inner_ty.mask_type()
+    }
+
+    fn canonical(&self) -> CanonicalType {
+        CanonicalType::TraceAsString(self.canonical_trace_as_string())
+    }
+
+    fn from_canonical(canonical_type: CanonicalType) -> Self {
+        let CanonicalType::TraceAsString(canonical) = canonical_type else {
+            panic!("expected TraceAsString");
+        };
+        Self::from_canonical_trace_as_string(canonical)
+    }
+
+    fn source_location() -> SourceLocation {
+        SourceLocation::builtin()
+    }
+
+    fn sim_value_from_opaque(&self, opaque: OpaqueSimValueSlice<'_>) -> Self::SimValue {
+        TraceAsStringSimValue {
+            inner: self.inner_ty.sim_value_from_opaque(opaque),
+        }
+    }
+
+    fn sim_value_clone_from_opaque(
+        &self,
+        value: &mut Self::SimValue,
+        opaque: OpaqueSimValueSlice<'_>,
+    ) {
+        self.inner_ty
+            .sim_value_clone_from_opaque(&mut value.inner, opaque);
+    }
+
+    fn sim_value_to_opaque<'w>(
+        &self,
+        value: &Self::SimValue,
+        writer: OpaqueSimValueWriter<'w>,
+    ) -> OpaqueSimValueWritten<'w> {
+        self.inner_ty.sim_value_to_opaque(&value.inner, writer)
+    }
+}
+
+impl<T: Type> TypeWithDeref for TraceAsString<T> {
+    fn expr_deref(this: &Expr<Self>) -> &Self::MatchVariant {
+        Interned::into_inner(
+            ops::TraceAsStringAsInner::new(*this)
+                .to_expr()
+                .intern_sized(),
+        )
+    }
+}
+
+struct TraceAsStringStaticTypeHelper<T: StaticType>(PhantomData<T>);
+
+impl<T: StaticType> Default for TraceAsStringStaticTypeHelper<T> {
+    fn default() -> Self {
+        Self(PhantomData)
+    }
+}
+
+impl<T: StaticType> From<TraceAsStringStaticTypeHelper<T>> for Interned<dyn TraceAsStringTrait> {
+    fn from(_value: TraceAsStringStaticTypeHelper<T>) -> Self {
+        Interned::cast_unchecked(T::TYPE.intern_sized(), |v| -> &dyn TraceAsStringTrait { v })
+    }
+}
+
+impl<T: StaticType> Default for TraceAsString<T> {
+    fn default() -> Self {
+        Self::TYPE
+    }
+}
+
+struct MakeType<T: StaticType>(Interned<T>);
+
+impl<T: StaticType> From<MakeType<T>> for Interned<T> {
+    fn from(value: MakeType<T>) -> Self {
+        value.0
+    }
+}
+
+impl<T: StaticType> Default for MakeType<T> {
+    fn default() -> Self {
+        Self(T::TYPE.intern_sized())
+    }
+}
+
+impl<T: StaticType> StaticType for TraceAsString<T> {
+    const TYPE: Self = Self {
+        inner_ty: LazyInterned::new_const::<MakeType<T>>(),
+        trace_as_string: LazyInterned::new_const::<TraceAsStringStaticTypeHelper<T>>(),
+    };
+    const MASK_TYPE: Self::MaskType = T::MASK_TYPE;
+    const TYPE_PROPERTIES: TypeProperties = T::TYPE_PROPERTIES;
+    const MASK_TYPE_PROPERTIES: TypeProperties = T::MASK_TYPE_PROPERTIES;
+}
+
+#[doc(hidden)]
+pub struct TraceAsStringWithoutGenerics;
+
+impl<T: Type> Index<T> for TraceAsStringWithoutGenerics {
+    type Output = TraceAsString<T>;
+
+    fn index(&self, inner_ty: T) -> &Self::Output {
+        Interned::into_inner(TraceAsString::new(inner_ty).intern_sized())
+    }
+}
+
+#[derive(Copy, Clone, Eq, Ord, Hash, Default, Serialize, Deserialize)]
+pub struct TraceAsStringSimValue<T> {
+    inner: T,
+}
+
+impl<T> TraceAsStringSimValue<T> {
+    pub const fn new(inner: T) -> Self {
+        Self { inner }
+    }
+    pub fn into_inner(this: Self) -> T {
+        let Self { inner } = this;
+        inner
+    }
+}
+
+impl<T: ValueType> ValueType for TraceAsStringSimValue<T> {
+    type Type = TraceAsString<T::Type>;
+    type ValueCategory = T::ValueCategory;
+
+    fn ty(&self) -> Self::Type {
+        TraceAsString::new(self.inner.ty())
+    }
+}
+
+impl<T: ToExpr> ToExpr for TraceAsStringSimValue<T> {
+    fn to_expr(&self) -> Expr<Self::Type> {
+        let inner = self.inner.to_expr();
+        ops::AsTraceAsString::new(Expr::canonical(inner), TraceAsString::new(inner.ty())).to_expr()
+    }
+}
+
+impl<T: ToSimValueWithType<Ty>, Ty: Type> ToSimValueWithType<TraceAsString<Ty>>
+    for TraceAsStringSimValue<T>
+{
+    fn to_sim_value_with_type(&self, ty: TraceAsString<Ty>) -> SimValue<TraceAsString<Ty>> {
+        let inner = self.inner.to_sim_value_with_type(ty.inner_ty());
+        let inner = SimValue::into_value(inner);
+        SimValue::from_value(ty, TraceAsStringSimValue { inner })
+    }
+    fn into_sim_value_with_type(self, ty: TraceAsString<Ty>) -> SimValue<TraceAsString<Ty>> {
+        let inner = self.inner.into_sim_value_with_type(ty.inner_ty());
+        let inner = SimValue::into_value(inner);
+        SimValue::from_value(ty, TraceAsStringSimValue { inner })
+    }
+}
+
+impl<T: ToSimValue> ToSimValue for TraceAsStringSimValue<T> {
+    fn to_sim_value(&self) -> SimValue<Self::Type> {
+        self.to_sim_value_with_type(self.ty())
+    }
+    fn into_sim_value(self) -> SimValue<Self::Type> {
+        let ty = self.ty();
+        self.into_sim_value_with_type(ty)
+    }
+}
+
+impl<T> std::ops::Deref for TraceAsStringSimValue<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+impl<T> std::ops::DerefMut for TraceAsStringSimValue<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.inner
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for TraceAsStringSimValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { inner } = self;
+        fmt::Debug::fmt(inner, f)
+    }
+}
+
+impl<T: fmt::Display> fmt::Display for TraceAsStringSimValue<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { inner } = self;
+        fmt::Display::fmt(inner, f)
+    }
+}
+
+impl<T: PartialOrd<U>, U> PartialOrd<TraceAsStringSimValue<U>> for TraceAsStringSimValue<T> {
+    fn partial_cmp(&self, other: &TraceAsStringSimValue<U>) -> Option<std::cmp::Ordering> {
+        let Self { inner } = self;
+        inner.partial_cmp(&other.inner)
+    }
+}
+
+impl<T: PartialEq<U>, U> PartialEq<TraceAsStringSimValue<U>> for TraceAsStringSimValue<T> {
+    fn eq(&self, other: &TraceAsStringSimValue<U>) -> bool {
+        let Self { inner } = self;
+        *inner == other.inner
+    }
+}
+
+impl<T: Type + Fold<State>, State: ?Sized + Folder> Fold<State> for TraceAsString<T> {
+    fn fold(self, state: &mut State) -> Result<Self, State::Error> {
+        state.fold_trace_as_string(self)
+    }
+
+    fn default_fold(self, state: &mut State) -> Result<Self, State::Error> {
+        Ok(self.with_new_inner_ty(self.interned_inner_ty().fold(state)?))
+    }
+}
+
+impl<T: Type + Visit<State>, State: ?Sized + Visitor> Visit<State> for TraceAsString<T> {
+    fn visit(&self, state: &mut State) -> Result<(), <State as Visitor>::Error> {
+        state.visit_trace_as_string(self)
+    }
+
+    fn default_visit(&self, state: &mut State) -> Result<(), <State as Visitor>::Error> {
+        self.interned_inner_ty().visit(state)
+    }
+}

crates/fayalite/src/ty/serde_impls.rs

@@ -12,7 +12,8 @@ use crate::{
     prelude::PhantomConst,
     reset::{AsyncReset, Reset, SyncReset},
     sim::value::DynSimOnly,
-    ty::{BaseType, CanonicalType},
+    ty::{BaseType, CanonicalType, TraceAsString, TraceAsStringTrait},
+    util::serde_by_id::SerdeById,
 };
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
@@ -65,6 +66,10 @@ pub(crate) enum SerdeCanonicalType<
     Clock,
     PhantomConst(ThePhantomConst),
     DynSimOnly(DynSimOnly),
+    TraceAsString {
+        inner_ty: Interned<CanonicalType>,
+        trace_as_string: SerdeById<Interned<dyn TraceAsStringTrait>>,
+    },
 }
 
 impl<ArrayElement, PhantomConstInner> SerdeCanonicalType<ArrayElement, PhantomConstInner> {
@@ -82,6 +87,7 @@ impl<ArrayElement, PhantomConstInner> SerdeCanonicalType<ArrayElement, PhantomCo
             Self::Clock => "a Clock",
             Self::PhantomConst(_) => "a PhantomConst",
             Self::DynSimOnly(_) => "a SimOnlyValue",
+            Self::TraceAsString { .. } => "a TraceAsString",
         }
     }
 }
@@ -109,6 +115,15 @@ impl<T: BaseType> From<T> for SerdeCanonicalType {
             CanonicalType::Clock(Clock {}) => Self::Clock,
             CanonicalType::PhantomConst(ty) => Self::PhantomConst(SerdePhantomConst(ty.get())),
             CanonicalType::DynSimOnly(ty) => Self::DynSimOnly(ty),
+            CanonicalType::TraceAsString(TraceAsString {
+                inner_ty,
+                trace_as_string,
+            }) => Self::TraceAsString {
+                inner_ty: inner_ty.interned(),
+                trace_as_string: SerdeById {
+                    inner: trace_as_string.interned(),
+                },
+            },
         }
     }
 }
@@ -130,6 +145,13 @@ impl From<SerdeCanonicalType> for CanonicalType {
                 Self::PhantomConst(PhantomConst::new_interned(value.0))
             }
             SerdeCanonicalType::DynSimOnly(value) => Self::DynSimOnly(value),
+            SerdeCanonicalType::TraceAsString {
+                inner_ty,
+                trace_as_string,
+            } => Self::TraceAsString(TraceAsString {
+                inner_ty: crate::intern::LazyInterned::Interned(inner_ty),
+                trace_as_string: crate::intern::LazyInterned::Interned(trace_as_string.inner),
+            }),
         }
     }
 }

crates/fayalite/src/util.rs

@@ -46,3 +46,4 @@ pub(crate) use misc::{InternedStrCompareAsStr, chain, copy_le_bytes_to_bitslice}
 pub mod job_server;
 pub mod prefix_sum;
 pub mod ready_valid;
+pub(crate) mod serde_by_id;

crates/fayalite/src/util/serde_by_id.rs

@@ -0,0 +1,234 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use crate::util::HashMap;
+use hashbrown::hash_map::Entry;
+use serde::{Deserialize, Serialize, de::Error};
+use std::{
+    any::TypeId,
+    borrow::Cow,
+    fmt::Write,
+    hash::{BuildHasher, Hash, Hasher},
+    marker::PhantomData,
+    sync::Mutex,
+};
+
+pub(crate) struct SerdeByIdProperties<T: SerdeByIdTrait> {
+    type_id: TypeId,
+    type_name: &'static str,
+    _phantom: PhantomData<fn(T) -> T>,
+}
+
+impl<T: SerdeByIdTrait> Clone for SerdeByIdProperties<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: SerdeByIdTrait> Copy for SerdeByIdProperties<T> {}
+
+impl<T: SerdeByIdTrait> SerdeByIdProperties<T> {
+    pub fn of<U: ?Sized + 'static>() -> Self {
+        Self {
+            type_id: TypeId::of::<U>(),
+            type_name: std::any::type_name::<U>(),
+            _phantom: PhantomData,
+        }
+    }
+}
+
+pub(crate) trait SerdeByIdTrait: Hash + Eq + Clone + 'static + Send {
+    fn serde_by_id_properties(&self) -> SerdeByIdProperties<Self>;
+    fn static_table() -> &'static SerdeByIdTable<Self>;
+    const NAME: &'static str;
+}
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug, Serialize, Deserialize)]
+#[serde(transparent)]
+struct SerdeRandomId([u32; 4]);
+
+#[derive(Serialize, Deserialize)]
+pub(crate) struct SerdeId<'a, T: SerdeByIdTrait> {
+    random_id: SerdeRandomId,
+    #[serde(borrow)]
+    type_name: Cow<'a, str>,
+    #[serde(skip)]
+    _phantom: PhantomData<fn(T) -> T>,
+}
+
+impl<'a, T: SerdeByIdTrait> Clone for SerdeId<'a, T> {
+    fn clone(&self) -> Self {
+        Self {
+            random_id: self.random_id,
+            type_name: self.type_name.clone(),
+            _phantom: PhantomData,
+        }
+    }
+}
+
+impl<'a, T: SerdeByIdTrait> Eq for SerdeId<'a, T> {}
+
+impl<'a, 'b, T: SerdeByIdTrait> PartialEq<SerdeId<'b, T>> for SerdeId<'a, T> {
+    fn eq(&self, other: &SerdeId<'b, T>) -> bool {
+        let Self {
+            random_id,
+            type_name,
+            _phantom: _,
+        } = self;
+        *random_id == other.random_id && *type_name == other.type_name
+    }
+}
+
+impl<'a, T: SerdeByIdTrait> Hash for SerdeId<'a, T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        let Self {
+            random_id,
+            type_name: _,
+            _phantom: _,
+        } = self;
+        random_id.hash(state);
+    }
+}
+
+struct SerdeByIdTableRest<T: SerdeByIdTrait> {
+    from_serde: HashMap<SerdeId<'static, T>, T>,
+    serde_id_random_state: std::hash::RandomState,
+    buffer: String,
+}
+
+impl<T: SerdeByIdTrait> Default for SerdeByIdTableRest<T> {
+    fn default() -> Self {
+        Self {
+            from_serde: Default::default(),
+            serde_id_random_state: Default::default(),
+            buffer: Default::default(),
+        }
+    }
+}
+
+impl<T: SerdeByIdTrait> SerdeByIdTableRest<T> {
+    fn add_new(&mut self, value: T) -> SerdeId<'static, T> {
+        let properties = value.serde_by_id_properties();
+        let mut try_number = 0u64;
+        let mut hasher = self.serde_id_random_state.build_hasher();
+        // extract more bits of randomness from TypeId -- its Hash impl only hashes 64-bits
+        write!(self.buffer, "{:?}", properties.type_id).expect("shouldn't ever fail");
+        self.buffer.hash(&mut hasher);
+        loop {
+            let mut hasher = hasher.clone();
+            try_number.hash(&mut hasher);
+            try_number += 1;
+            let key = SerdeId {
+                random_id: SerdeRandomId(std::array::from_fn(|i| {
+                    let mut hasher = hasher.clone();
+                    i.hash(&mut hasher);
+                    hasher.finish() as u32
+                })),
+                type_name: Cow::Borrowed(properties.type_name),
+                _phantom: PhantomData,
+            };
+            match self.from_serde.entry(key) {
+                Entry::Occupied(_) => continue,
+                Entry::Vacant(e) => {
+                    let key = e.key().clone();
+                    e.insert(value);
+                    return key;
+                }
+            }
+        }
+    }
+}
+
+pub(crate) struct SerdeByIdTableMut<T: SerdeByIdTrait> {
+    to_serde: HashMap<T, SerdeId<'static, T>>,
+    rest: SerdeByIdTableRest<T>,
+}
+
+impl<T: SerdeByIdTrait> Default for SerdeByIdTableMut<T> {
+    fn default() -> Self {
+        Self {
+            to_serde: Default::default(),
+            rest: Default::default(),
+        }
+    }
+}
+
+impl<T: SerdeByIdTrait> SerdeByIdTableMut<T> {
+    pub(crate) fn to_serde(&mut self, value: &T) -> SerdeId<'static, T> {
+        if let Some(retval) = self.to_serde.get(value) {
+            return retval.clone();
+        }
+        self.to_serde_insert(value)
+    }
+    #[cold]
+    fn to_serde_insert(&mut self, value: &T) -> SerdeId<'static, T> {
+        let value = value.clone();
+        let retval = self.rest.add_new(value.clone());
+        self.to_serde.insert(value, retval.clone());
+        retval
+    }
+    pub(crate) fn from_serde(&self, id: &SerdeId<'_, T>) -> Option<T> {
+        self.rest.from_serde.get(id).cloned()
+    }
+}
+
+pub(crate) struct SerdeByIdTable<T: SerdeByIdTrait>(Mutex<Option<SerdeByIdTableMut<T>>>);
+
+impl<T: SerdeByIdTrait> SerdeByIdTable<T> {
+    pub(crate) const fn new() -> Self {
+        Self(Mutex::new(None))
+    }
+    pub(crate) fn to_serde(&self, value: &T) -> SerdeId<'static, T> {
+        self.0
+            .lock()
+            .expect("shouldn't be poison")
+            .get_or_insert_with(
+                #[cold]
+                || Default::default(),
+            )
+            .to_serde(value)
+    }
+    pub(crate) fn from_serde(&self, id: &SerdeId<'_, T>) -> Option<T> {
+        self.0
+            .lock()
+            .expect("shouldn't be poison")
+            .get_or_insert_with(
+                #[cold]
+                || Default::default(),
+            )
+            .from_serde(id)
+    }
+}
+
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, Default, Ord, PartialOrd)]
+pub(crate) struct SerdeById<T: SerdeByIdTrait> {
+    pub(crate) inner: T,
+}
+
+impl<'de, T: SerdeByIdTrait> Deserialize<'de> for SerdeById<T> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let id = SerdeId::deserialize(deserializer)?;
+        let inner = T::static_table().from_serde(&id).ok_or_else(|| {
+            D::Error::custom(format_args!(
+                "doesn't match any {} that was serialized this time this program was run: type_name={:?}",
+                T::NAME,
+                id.type_name,
+            ))
+        })?;
+        Ok(Self { inner })
+    }
+}
+
+impl<T: SerdeByIdTrait> Serialize for SerdeById<T> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        T::static_table()
+            .to_serde(&self.inner)
+            .serialize(serializer)
+    }
+}

crates/fayalite/src/vendor/xilinx/yosys_nextpnr_prjxray.rs

@@ -546,7 +546,7 @@ impl<W: fmt::Write> Visitor for XdcFileWriter<W> {
                                 base.source_location(),
                             )? {},
                         }
-                        match base.canonical_ty() {
+                        match base.canonical_ty().unwrap_transparent_types() {
                             CanonicalType::UInt(_)
                             | CanonicalType::SInt(_)
                             | CanonicalType::Bool(_)
@@ -563,6 +563,9 @@ impl<W: fmt::Write> Visitor for XdcFileWriter<W> {
                                 v,
                                 base.source_location(),
                             )? {},
+                            CanonicalType::TraceAsString(_) => {
+                                unreachable!("handled by unwrap_transparent_types")
+                            }
                         }
                         self.required_dont_touch_targets.insert(target);
                         match v {

crates/fayalite/tests/hdl_types_fmt.rs

@@ -0,0 +1,166 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+use fayalite::{prelude::*, ty::SimValueDebug};
+use std::fmt;
+
+#[hdl(outline_generated)]
+struct MyStruct0<T, S: Size> {
+    v: T,
+    a: ArrayType<UInt<8>, S>,
+}
+
+#[hdl]
+#[test]
+fn check_my_struct0() {
+    let ty = MyStruct0[UInt[8]][3];
+    assert_eq!(
+        format!("{ty:?}"),
+        "MyStruct0 { v: UInt<8>, a: Array<UInt<8>, 3> }",
+    );
+    assert_eq!(
+        format!("{:?}", ty.mask_type()),
+        "MaskType<MyStruct0> { v: Bool, a: Array<Bool, 3> }",
+    );
+    let v = #[hdl(sim)]
+    MyStruct0::<_, _> {
+        v: 0x23u8,
+        a: [1u8, 2, 3],
+    };
+    assert_eq!(
+        format!("{v:?}"),
+        "MyStruct0 { v: 0x23_u8, a: [0x1_u8, 0x2_u8, 0x3_u8] }",
+    );
+}
+
+#[hdl(outline_generated, custom_debug())]
+struct MyStruct1<T, S: Size> {
+    v: T,
+    a: ArrayType<UInt<8>, S>,
+}
+
+impl<T: Type, S: Size> fmt::Debug for MyStruct1<T, S> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { v, a } = self;
+        f.debug_struct("Custom<MyStruct1>")
+            .field("v", v)
+            .field("a", a)
+            .finish()
+    }
+}
+
+impl<T: Type, S: Size> SimValueDebug for MyStruct1<T, S> {
+    #[hdl]
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        #[hdl(sim)]
+        let Self { v, a } = value;
+        f.debug_struct("Custom<MyStruct1>")
+            .field("v", &v)
+            .field("a", &a)
+            .finish()
+    }
+}
+
+#[hdl]
+#[test]
+fn check_my_struct1() {
+    let ty = MyStruct1[UInt[8]][3];
+    assert_eq!(
+        format!("{ty:?}"),
+        "Custom<MyStruct1> { v: UInt<8>, a: Array<UInt<8>, 3> }",
+    );
+    assert_eq!(
+        format!("{:?}", ty.mask_type()),
+        "MaskType<MyStruct1> { v: Bool, a: Array<Bool, 3> }",
+    );
+    let v = #[hdl(sim)]
+    MyStruct1::<_, _> {
+        v: 0x23u8,
+        a: [1u8, 2, 3],
+    };
+    assert_eq!(
+        format!("{v:?}"),
+        "Custom<MyStruct1> { v: 0x23_u8, a: [0x1_u8, 0x2_u8, 0x3_u8] }",
+    );
+}
+
+#[hdl(outline_generated)]
+enum MyEnum0<T, S: Size> {
+    Unit,
+    V(T),
+    A(ArrayType<UInt<8>, S>),
+}
+
+#[hdl]
+#[test]
+fn check_my_enum0() {
+    let ty = MyEnum0[UInt[8]][3];
+    assert_eq!(
+        format!("{ty:?}"),
+        "MyEnum0 { Unit: (), V: UInt<8>, A: Array<UInt<8>, 3> }",
+    );
+    let v = #[hdl(sim)]
+    ty.Unit();
+    assert_eq!(format!("{v:?}"), "Unit");
+    let v = #[hdl(sim)]
+    ty.V(0x23u8);
+    assert_eq!(format!("{v:?}"), "V(0x23_u8)");
+    let v = #[hdl(sim)]
+    ty.A([1u8, 2, 3]);
+    assert_eq!(format!("{v:?}"), "A([0x1_u8, 0x2_u8, 0x3_u8])");
+}
+
+#[hdl(outline_generated, custom_debug())]
+enum MyEnum1<T, S: Size> {
+    Unit,
+    V(T),
+    A(ArrayType<UInt<8>, S>),
+}
+
+impl<T: Type, S: Size> fmt::Debug for MyEnum1<T, S> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { Unit, V, A } = self;
+        f.debug_struct("Custom<MyEnum1>")
+            .field("Unit", Unit)
+            .field("V", V)
+            .field("A", A)
+            .finish()
+    }
+}
+
+impl<T: Type, S: Size> SimValueDebug for MyEnum1<T, S> {
+    #[hdl]
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut fmt::Formatter<'_>,
+    ) -> fmt::Result {
+        type SimValueT<T> = <T as Type>::SimValue;
+        match value {
+            SimValueT::<Self>::Unit(_) => f.write_str("MyEnum1::Unit"),
+            SimValueT::<Self>::V(v, _) => f.debug_tuple("MyEnum1::V").field(v).finish(),
+            SimValueT::<Self>::A(a, _) => f.debug_tuple("MyEnum1::A").field(a).finish(),
+            SimValueT::<Self>::Unknown(_) => f.write_str("MyEnum1::Unknown"),
+        }
+    }
+}
+
+#[hdl]
+#[test]
+fn check_my_enum1() {
+    let ty = MyEnum1[UInt[8]][3];
+    assert_eq!(
+        format!("{ty:?}"),
+        "Custom<MyEnum1> { Unit: (), V: UInt<8>, A: Array<UInt<8>, 3> }",
+    );
+    let v = #[hdl(sim)]
+    ty.Unit();
+    assert_eq!(format!("{v:?}"), "MyEnum1::Unit");
+    let v = #[hdl(sim)]
+    ty.V(0x23u8);
+    assert_eq!(format!("{v:?}"), "MyEnum1::V(0x23_u8)");
+    let v = #[hdl(sim)]
+    ty.A([1u8, 2, 3]);
+    assert_eq!(format!("{v:?}"), "MyEnum1::A([0x1_u8, 0x2_u8, 0x3_u8])");
+}

crates/fayalite/tests/module.rs

@@ -13,7 +13,7 @@ use fayalite::{
 };
 use serde_json::json;
 
-#[hdl(outline_generated)]
+#[hdl(outline_generated, cmp_eq)]
 pub enum TestEnum {
     A,
     B(UInt<8>),
@@ -679,6 +679,366 @@ circuit check_enum_literals:
     };
 }
 
+#[hdl_module(outline_generated)]
+pub fn check_enum_cmp_eq() {
+    #[hdl]
+    let lhs: TestEnum = m.input();
+    #[hdl]
+    let rhs: TestEnum = m.input();
+    #[hdl]
+    let eq: Bool = m.output();
+    connect(eq, lhs.cmp_eq(rhs));
+}
+
+#[test]
+fn test_enum_cmp_eq() {
+    let _n = SourceLocation::normalize_files_for_tests();
+    let m = check_enum_cmp_eq();
+    dbg!(m);
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_enums: None,
+            ..ExportOptions::default()
+        },
+        "/test/check_enum_cmp_eq.fir": r"FIRRTL version 3.2.0
+circuit check_enum_cmp_eq:
+    type Ty0 = {|A, B: UInt<8>, C: UInt<1>[3]|}
+    module check_enum_cmp_eq: @[module-XXXXXXXXXX.rs 1:1]
+        input lhs: Ty0 @[module-XXXXXXXXXX.rs 2:1]
+        input rhs: Ty0 @[module-XXXXXXXXXX.rs 3:1]
+        output eq: UInt<1> @[module-XXXXXXXXXX.rs 4:1]
+        wire TestEnum_cmp_eq: UInt<1> @[module-XXXXXXXXXX.rs 5:1]
+        connect TestEnum_cmp_eq, UInt<1>(0h0) @[module-XXXXXXXXXX.rs 5:1]
+        match lhs: @[module-XXXXXXXXXX.rs 5:1]
+            A:
+                match rhs: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        connect TestEnum_cmp_eq, UInt<1>(0h1) @[module-XXXXXXXXXX.rs 5:1]
+                    B(_match_arm_value):
+                        skip
+                    C(_match_arm_value_1):
+                        skip
+            B(_match_arm_value_2):
+                match rhs: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        skip
+                    B(_match_arm_value_3):
+                        connect TestEnum_cmp_eq, eq(_match_arm_value_2, _match_arm_value_3) @[module-XXXXXXXXXX.rs 5:1]
+                    C(_match_arm_value_4):
+                        skip
+            C(_match_arm_value_5):
+                match rhs: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        skip
+                    B(_match_arm_value_6):
+                        skip
+                    C(_match_arm_value_7):
+                        wire _array_literal_expr: UInt<1>[3]
+                        connect _array_literal_expr[0], eq(_match_arm_value_5[0], _match_arm_value_7[0])
+                        connect _array_literal_expr[1], eq(_match_arm_value_5[1], _match_arm_value_7[1])
+                        connect _array_literal_expr[2], eq(_match_arm_value_5[2], _match_arm_value_7[2])
+                        wire _cast_array_to_bits_expr: UInt<1>[3]
+                        connect _cast_array_to_bits_expr[0], _array_literal_expr[0]
+                        connect _cast_array_to_bits_expr[1], _array_literal_expr[1]
+                        connect _cast_array_to_bits_expr[2], _array_literal_expr[2]
+                        wire _cast_to_bits_expr: UInt<3>
+                        connect _cast_to_bits_expr, cat(_cast_array_to_bits_expr[2], cat(_cast_array_to_bits_expr[1], _cast_array_to_bits_expr[0]))
+                        connect TestEnum_cmp_eq, andr(_cast_to_bits_expr) @[module-XXXXXXXXXX.rs 5:1]
+        connect eq, TestEnum_cmp_eq @[module-XXXXXXXXXX.rs 6:1]
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_enums: Some(SimplifyEnumsKind::SimplifyToEnumsWithNoBody),
+            ..ExportOptions::default()
+        },
+        "/test/check_enum_cmp_eq.fir": r"FIRRTL version 3.2.0
+circuit check_enum_cmp_eq:
+    type Ty0 = {|A, B, C|}
+    type Ty1 = {tag: Ty0, body: UInt<8>}
+    module check_enum_cmp_eq: @[module-XXXXXXXXXX.rs 1:1]
+        input lhs: Ty1 @[module-XXXXXXXXXX.rs 2:1]
+        input rhs: Ty1 @[module-XXXXXXXXXX.rs 3:1]
+        output eq: UInt<1> @[module-XXXXXXXXXX.rs 4:1]
+        wire TestEnum_cmp_eq: UInt<1> @[module-XXXXXXXXXX.rs 5:1]
+        connect TestEnum_cmp_eq, UInt<1>(0h0) @[module-XXXXXXXXXX.rs 5:1]
+        match lhs.tag: @[module-XXXXXXXXXX.rs 5:1]
+            A:
+                match rhs.tag: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        connect TestEnum_cmp_eq, UInt<1>(0h1) @[module-XXXXXXXXXX.rs 5:1]
+                    B:
+                        skip
+                    C:
+                        skip
+            B:
+                match rhs.tag: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        skip
+                    B:
+                        connect TestEnum_cmp_eq, eq(bits(lhs.body, 7, 0), bits(rhs.body, 7, 0)) @[module-XXXXXXXXXX.rs 5:1]
+                    C:
+                        skip
+            C:
+                match rhs.tag: @[module-XXXXXXXXXX.rs 5:1]
+                    A:
+                        skip
+                    B:
+                        skip
+                    C:
+                        wire _array_literal_expr: UInt<1>[3]
+                        wire _cast_bits_to_array_expr: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened[0], bits(bits(lhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr[0], _cast_bits_to_array_expr_flattened[0]
+                        connect _cast_bits_to_array_expr_flattened[1], bits(bits(lhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr[1], _cast_bits_to_array_expr_flattened[1]
+                        connect _cast_bits_to_array_expr_flattened[2], bits(bits(lhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr[2], _cast_bits_to_array_expr_flattened[2]
+                        wire _cast_bits_to_array_expr_1: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened_1: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened_1[0], bits(bits(rhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr_1[0], _cast_bits_to_array_expr_flattened_1[0]
+                        connect _cast_bits_to_array_expr_flattened_1[1], bits(bits(rhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr_1[1], _cast_bits_to_array_expr_flattened_1[1]
+                        connect _cast_bits_to_array_expr_flattened_1[2], bits(bits(rhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr_1[2], _cast_bits_to_array_expr_flattened_1[2]
+                        connect _array_literal_expr[0], eq(_cast_bits_to_array_expr[0], _cast_bits_to_array_expr_1[0])
+                        wire _cast_bits_to_array_expr_2: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened_2: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened_2[0], bits(bits(lhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr_2[0], _cast_bits_to_array_expr_flattened_2[0]
+                        connect _cast_bits_to_array_expr_flattened_2[1], bits(bits(lhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_flattened_2[1]
+                        connect _cast_bits_to_array_expr_flattened_2[2], bits(bits(lhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr_2[2], _cast_bits_to_array_expr_flattened_2[2]
+                        wire _cast_bits_to_array_expr_3: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened_3: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened_3[0], bits(bits(rhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr_3[0], _cast_bits_to_array_expr_flattened_3[0]
+                        connect _cast_bits_to_array_expr_flattened_3[1], bits(bits(rhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr_3[1], _cast_bits_to_array_expr_flattened_3[1]
+                        connect _cast_bits_to_array_expr_flattened_3[2], bits(bits(rhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr_3[2], _cast_bits_to_array_expr_flattened_3[2]
+                        connect _array_literal_expr[1], eq(_cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_3[1])
+                        wire _cast_bits_to_array_expr_4: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened_4: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened_4[0], bits(bits(lhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr_4[0], _cast_bits_to_array_expr_flattened_4[0]
+                        connect _cast_bits_to_array_expr_flattened_4[1], bits(bits(lhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr_4[1], _cast_bits_to_array_expr_flattened_4[1]
+                        connect _cast_bits_to_array_expr_flattened_4[2], bits(bits(lhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_flattened_4[2]
+                        wire _cast_bits_to_array_expr_5: UInt<1>[3]
+                        wire _cast_bits_to_array_expr_flattened_5: UInt<1>[3]
+                        connect _cast_bits_to_array_expr_flattened_5[0], bits(bits(rhs.body, 2, 0), 0, 0)
+                        connect _cast_bits_to_array_expr_5[0], _cast_bits_to_array_expr_flattened_5[0]
+                        connect _cast_bits_to_array_expr_flattened_5[1], bits(bits(rhs.body, 2, 0), 1, 1)
+                        connect _cast_bits_to_array_expr_5[1], _cast_bits_to_array_expr_flattened_5[1]
+                        connect _cast_bits_to_array_expr_flattened_5[2], bits(bits(rhs.body, 2, 0), 2, 2)
+                        connect _cast_bits_to_array_expr_5[2], _cast_bits_to_array_expr_flattened_5[2]
+                        connect _array_literal_expr[2], eq(_cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_5[2])
+                        wire _cast_array_to_bits_expr: UInt<1>[3]
+                        connect _cast_array_to_bits_expr[0], _array_literal_expr[0]
+                        connect _cast_array_to_bits_expr[1], _array_literal_expr[1]
+                        connect _cast_array_to_bits_expr[2], _array_literal_expr[2]
+                        wire _cast_to_bits_expr: UInt<3>
+                        connect _cast_to_bits_expr, cat(_cast_array_to_bits_expr[2], cat(_cast_array_to_bits_expr[1], _cast_array_to_bits_expr[0]))
+                        connect TestEnum_cmp_eq, andr(_cast_to_bits_expr) @[module-XXXXXXXXXX.rs 5:1]
+        connect eq, TestEnum_cmp_eq @[module-XXXXXXXXXX.rs 6:1]
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_enums: Some(SimplifyEnumsKind::ReplaceWithBundleOfUInts),
+            ..ExportOptions::default()
+        },
+        "/test/check_enum_cmp_eq.fir": r"FIRRTL version 3.2.0
+circuit check_enum_cmp_eq:
+    type Ty0 = {tag: UInt<2>, body: UInt<8>}
+    module check_enum_cmp_eq: @[module-XXXXXXXXXX.rs 1:1]
+        input lhs: Ty0 @[module-XXXXXXXXXX.rs 2:1]
+        input rhs: Ty0 @[module-XXXXXXXXXX.rs 3:1]
+        output eq: UInt<1> @[module-XXXXXXXXXX.rs 4:1]
+        wire TestEnum_cmp_eq: UInt<1> @[module-XXXXXXXXXX.rs 5:1]
+        connect TestEnum_cmp_eq, UInt<1>(0h0) @[module-XXXXXXXXXX.rs 5:1]
+        when eq(lhs.tag, UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+            when eq(rhs.tag, UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+                connect TestEnum_cmp_eq, UInt<1>(0h1) @[module-XXXXXXXXXX.rs 5:1]
+            else when eq(rhs.tag, UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+                skip
+        else when eq(lhs.tag, UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+            when eq(rhs.tag, UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+                skip
+            else when eq(rhs.tag, UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+                connect TestEnum_cmp_eq, eq(bits(lhs.body, 7, 0), bits(rhs.body, 7, 0)) @[module-XXXXXXXXXX.rs 5:1]
+        else when eq(rhs.tag, UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+            skip
+        else when eq(rhs.tag, UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+            skip
+        else:
+            wire _array_literal_expr: UInt<1>[3]
+            wire _cast_bits_to_array_expr: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened[0], bits(bits(lhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr[0], _cast_bits_to_array_expr_flattened[0]
+            connect _cast_bits_to_array_expr_flattened[1], bits(bits(lhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr[1], _cast_bits_to_array_expr_flattened[1]
+            connect _cast_bits_to_array_expr_flattened[2], bits(bits(lhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr[2], _cast_bits_to_array_expr_flattened[2]
+            wire _cast_bits_to_array_expr_1: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_1: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_1[0], bits(bits(rhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_1[0], _cast_bits_to_array_expr_flattened_1[0]
+            connect _cast_bits_to_array_expr_flattened_1[1], bits(bits(rhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_1[1], _cast_bits_to_array_expr_flattened_1[1]
+            connect _cast_bits_to_array_expr_flattened_1[2], bits(bits(rhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_1[2], _cast_bits_to_array_expr_flattened_1[2]
+            connect _array_literal_expr[0], eq(_cast_bits_to_array_expr[0], _cast_bits_to_array_expr_1[0])
+            wire _cast_bits_to_array_expr_2: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_2: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_2[0], bits(bits(lhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_2[0], _cast_bits_to_array_expr_flattened_2[0]
+            connect _cast_bits_to_array_expr_flattened_2[1], bits(bits(lhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_flattened_2[1]
+            connect _cast_bits_to_array_expr_flattened_2[2], bits(bits(lhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_2[2], _cast_bits_to_array_expr_flattened_2[2]
+            wire _cast_bits_to_array_expr_3: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_3: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_3[0], bits(bits(rhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_3[0], _cast_bits_to_array_expr_flattened_3[0]
+            connect _cast_bits_to_array_expr_flattened_3[1], bits(bits(rhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_3[1], _cast_bits_to_array_expr_flattened_3[1]
+            connect _cast_bits_to_array_expr_flattened_3[2], bits(bits(rhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_3[2], _cast_bits_to_array_expr_flattened_3[2]
+            connect _array_literal_expr[1], eq(_cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_3[1])
+            wire _cast_bits_to_array_expr_4: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_4: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_4[0], bits(bits(lhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_4[0], _cast_bits_to_array_expr_flattened_4[0]
+            connect _cast_bits_to_array_expr_flattened_4[1], bits(bits(lhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_4[1], _cast_bits_to_array_expr_flattened_4[1]
+            connect _cast_bits_to_array_expr_flattened_4[2], bits(bits(lhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_flattened_4[2]
+            wire _cast_bits_to_array_expr_5: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_5: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_5[0], bits(bits(rhs.body, 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_5[0], _cast_bits_to_array_expr_flattened_5[0]
+            connect _cast_bits_to_array_expr_flattened_5[1], bits(bits(rhs.body, 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_5[1], _cast_bits_to_array_expr_flattened_5[1]
+            connect _cast_bits_to_array_expr_flattened_5[2], bits(bits(rhs.body, 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_5[2], _cast_bits_to_array_expr_flattened_5[2]
+            connect _array_literal_expr[2], eq(_cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_5[2])
+            wire _cast_array_to_bits_expr: UInt<1>[3]
+            connect _cast_array_to_bits_expr[0], _array_literal_expr[0]
+            connect _cast_array_to_bits_expr[1], _array_literal_expr[1]
+            connect _cast_array_to_bits_expr[2], _array_literal_expr[2]
+            wire _cast_to_bits_expr: UInt<3>
+            connect _cast_to_bits_expr, cat(_cast_array_to_bits_expr[2], cat(_cast_array_to_bits_expr[1], _cast_array_to_bits_expr[0]))
+            connect TestEnum_cmp_eq, andr(_cast_to_bits_expr) @[module-XXXXXXXXXX.rs 5:1]
+        connect eq, TestEnum_cmp_eq @[module-XXXXXXXXXX.rs 6:1]
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_enums: Some(SimplifyEnumsKind::ReplaceWithUInt),
+            ..ExportOptions::default()
+        },
+        "/test/check_enum_cmp_eq.fir": r"FIRRTL version 3.2.0
+circuit check_enum_cmp_eq:
+    module check_enum_cmp_eq: @[module-XXXXXXXXXX.rs 1:1]
+        input lhs: UInt<10> @[module-XXXXXXXXXX.rs 2:1]
+        input rhs: UInt<10> @[module-XXXXXXXXXX.rs 3:1]
+        output eq: UInt<1> @[module-XXXXXXXXXX.rs 4:1]
+        wire TestEnum_cmp_eq: UInt<1> @[module-XXXXXXXXXX.rs 5:1]
+        connect TestEnum_cmp_eq, UInt<1>(0h0) @[module-XXXXXXXXXX.rs 5:1]
+        when eq(bits(lhs, 1, 0), UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+            when eq(bits(rhs, 1, 0), UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+                connect TestEnum_cmp_eq, UInt<1>(0h1) @[module-XXXXXXXXXX.rs 5:1]
+            else when eq(bits(rhs, 1, 0), UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+                skip
+        else when eq(bits(lhs, 1, 0), UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+            when eq(bits(rhs, 1, 0), UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+                skip
+            else when eq(bits(rhs, 1, 0), UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+                connect TestEnum_cmp_eq, eq(bits(bits(lhs, 9, 2), 7, 0), bits(bits(rhs, 9, 2), 7, 0)) @[module-XXXXXXXXXX.rs 5:1]
+        else when eq(bits(rhs, 1, 0), UInt<2>(0h0)): @[module-XXXXXXXXXX.rs 5:1]
+            skip
+        else when eq(bits(rhs, 1, 0), UInt<2>(0h1)): @[module-XXXXXXXXXX.rs 5:1]
+            skip
+        else:
+            wire _array_literal_expr: UInt<1>[3]
+            wire _cast_bits_to_array_expr: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened[0], bits(bits(bits(lhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr[0], _cast_bits_to_array_expr_flattened[0]
+            connect _cast_bits_to_array_expr_flattened[1], bits(bits(bits(lhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr[1], _cast_bits_to_array_expr_flattened[1]
+            connect _cast_bits_to_array_expr_flattened[2], bits(bits(bits(lhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr[2], _cast_bits_to_array_expr_flattened[2]
+            wire _cast_bits_to_array_expr_1: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_1: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_1[0], bits(bits(bits(rhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_1[0], _cast_bits_to_array_expr_flattened_1[0]
+            connect _cast_bits_to_array_expr_flattened_1[1], bits(bits(bits(rhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_1[1], _cast_bits_to_array_expr_flattened_1[1]
+            connect _cast_bits_to_array_expr_flattened_1[2], bits(bits(bits(rhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_1[2], _cast_bits_to_array_expr_flattened_1[2]
+            connect _array_literal_expr[0], eq(_cast_bits_to_array_expr[0], _cast_bits_to_array_expr_1[0])
+            wire _cast_bits_to_array_expr_2: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_2: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_2[0], bits(bits(bits(lhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_2[0], _cast_bits_to_array_expr_flattened_2[0]
+            connect _cast_bits_to_array_expr_flattened_2[1], bits(bits(bits(lhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_flattened_2[1]
+            connect _cast_bits_to_array_expr_flattened_2[2], bits(bits(bits(lhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_2[2], _cast_bits_to_array_expr_flattened_2[2]
+            wire _cast_bits_to_array_expr_3: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_3: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_3[0], bits(bits(bits(rhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_3[0], _cast_bits_to_array_expr_flattened_3[0]
+            connect _cast_bits_to_array_expr_flattened_3[1], bits(bits(bits(rhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_3[1], _cast_bits_to_array_expr_flattened_3[1]
+            connect _cast_bits_to_array_expr_flattened_3[2], bits(bits(bits(rhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_3[2], _cast_bits_to_array_expr_flattened_3[2]
+            connect _array_literal_expr[1], eq(_cast_bits_to_array_expr_2[1], _cast_bits_to_array_expr_3[1])
+            wire _cast_bits_to_array_expr_4: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_4: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_4[0], bits(bits(bits(lhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_4[0], _cast_bits_to_array_expr_flattened_4[0]
+            connect _cast_bits_to_array_expr_flattened_4[1], bits(bits(bits(lhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_4[1], _cast_bits_to_array_expr_flattened_4[1]
+            connect _cast_bits_to_array_expr_flattened_4[2], bits(bits(bits(lhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_flattened_4[2]
+            wire _cast_bits_to_array_expr_5: UInt<1>[3]
+            wire _cast_bits_to_array_expr_flattened_5: UInt<1>[3]
+            connect _cast_bits_to_array_expr_flattened_5[0], bits(bits(bits(rhs, 9, 2), 2, 0), 0, 0)
+            connect _cast_bits_to_array_expr_5[0], _cast_bits_to_array_expr_flattened_5[0]
+            connect _cast_bits_to_array_expr_flattened_5[1], bits(bits(bits(rhs, 9, 2), 2, 0), 1, 1)
+            connect _cast_bits_to_array_expr_5[1], _cast_bits_to_array_expr_flattened_5[1]
+            connect _cast_bits_to_array_expr_flattened_5[2], bits(bits(bits(rhs, 9, 2), 2, 0), 2, 2)
+            connect _cast_bits_to_array_expr_5[2], _cast_bits_to_array_expr_flattened_5[2]
+            connect _array_literal_expr[2], eq(_cast_bits_to_array_expr_4[2], _cast_bits_to_array_expr_5[2])
+            wire _cast_array_to_bits_expr: UInt<1>[3]
+            connect _cast_array_to_bits_expr[0], _array_literal_expr[0]
+            connect _cast_array_to_bits_expr[1], _array_literal_expr[1]
+            connect _cast_array_to_bits_expr[2], _array_literal_expr[2]
+            wire _cast_to_bits_expr: UInt<3>
+            connect _cast_to_bits_expr, cat(_cast_array_to_bits_expr[2], cat(_cast_array_to_bits_expr[1], _cast_array_to_bits_expr[0]))
+            connect TestEnum_cmp_eq, andr(_cast_to_bits_expr) @[module-XXXXXXXXXX.rs 5:1]
+        connect eq, TestEnum_cmp_eq @[module-XXXXXXXXXX.rs 6:1]
+",
+    };
+}
+
 #[hdl_module(outline_generated)]
 pub fn check_struct_enum_match() {
     #[hdl]

crates/fayalite/tests/sim/expected/array_rw.txt

@@ -1218,6 +1218,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xff,
             last_state: 0xff,
         },
@@ -1227,6 +1228,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(1),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x7f,
             last_state: 0x7f,
         },
@@ -1236,6 +1238,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x3f,
             last_state: 0x3f,
         },
@@ -1245,6 +1248,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x1f,
             last_state: 0x1f,
         },
@@ -1254,6 +1258,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x0f,
             last_state: 0x0f,
         },
@@ -1263,6 +1268,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x07,
             last_state: 0x07,
         },
@@ -1272,6 +1278,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(6),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x03,
             last_state: 0x03,
         },
@@ -1281,6 +1288,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(7),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x01,
             last_state: 0x01,
         },
@@ -1290,6 +1298,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(8),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -1299,6 +1308,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(9),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x80,
             last_state: 0x80,
         },
@@ -1308,6 +1318,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(10),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xc0,
             last_state: 0xc0,
         },
@@ -1317,6 +1328,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(11),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xe0,
             last_state: 0xe0,
         },
@@ -1326,6 +1338,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(12),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf0,
             last_state: 0xf0,
         },
@@ -1335,6 +1348,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(13),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf8,
             last_state: 0xf8,
         },
@@ -1344,6 +1358,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(14),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfc,
             last_state: 0xfc,
         },
@@ -1353,6 +1368,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(15),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfe,
             last_state: 0xfe,
         },
@@ -1362,6 +1378,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(16),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xff,
             last_state: 0xff,
         },
@@ -1371,6 +1388,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(17),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x7f,
             last_state: 0x7f,
         },
@@ -1380,6 +1398,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(18),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x3f,
             last_state: 0x3f,
         },
@@ -1389,6 +1408,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(19),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x1f,
             last_state: 0x1f,
         },
@@ -1398,6 +1418,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(20),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x0f,
             last_state: 0x0f,
         },
@@ -1407,6 +1428,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(21),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x07,
             last_state: 0x07,
         },
@@ -1416,6 +1438,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(22),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x03,
             last_state: 0x03,
         },
@@ -1425,6 +1448,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(23),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x01,
             last_state: 0x01,
         },
@@ -1434,6 +1458,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(24),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -1443,6 +1468,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(25),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x80,
             last_state: 0x80,
         },
@@ -1452,6 +1478,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(26),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xc0,
             last_state: 0xc0,
         },
@@ -1461,6 +1488,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(27),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xe0,
             last_state: 0xe0,
         },
@@ -1470,6 +1498,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(28),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf0,
             last_state: 0xf0,
         },
@@ -1479,6 +1508,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(29),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf8,
             last_state: 0xf8,
         },
@@ -1488,6 +1518,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(30),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfc,
             last_state: 0xfc,
         },
@@ -1497,6 +1528,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(31),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfe,
             last_state: 0xe1,
         },
@@ -1506,6 +1538,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(32),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -1515,6 +1548,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(33),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xff,
             last_state: 0xff,
         },
@@ -1524,6 +1558,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(34),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x10,
             last_state: 0x0f,
         },
@@ -1533,6 +1568,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(35),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0xe1,
         },
@@ -1541,6 +1577,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(36),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1550,6 +1587,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(37),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xff,
             last_state: 0xff,
         },
@@ -1559,6 +1597,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(38),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x7f,
             last_state: 0x7f,
         },
@@ -1568,6 +1607,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(39),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x3f,
             last_state: 0x3f,
         },
@@ -1577,6 +1617,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(40),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x1f,
             last_state: 0x1f,
         },
@@ -1586,6 +1627,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(41),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x0f,
             last_state: 0x0f,
         },
@@ -1595,6 +1637,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(42),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x07,
             last_state: 0x07,
         },
@@ -1604,6 +1647,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(43),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x03,
             last_state: 0x03,
         },
@@ -1613,6 +1657,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(44),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x01,
             last_state: 0x01,
         },
@@ -1622,6 +1667,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(45),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -1631,6 +1677,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(46),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x80,
             last_state: 0x80,
         },
@@ -1640,6 +1687,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(47),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xc0,
             last_state: 0xc0,
         },
@@ -1649,6 +1697,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(48),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xe0,
             last_state: 0xe0,
         },
@@ -1658,6 +1707,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(49),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf0,
             last_state: 0xf0,
         },
@@ -1667,6 +1717,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(50),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xf8,
             last_state: 0xf8,
         },
@@ -1676,6 +1727,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(51),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfc,
             last_state: 0xfc,
         },
@@ -1685,6 +1737,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(52),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xfe,
             last_state: 0xe1,
         },

crates/fayalite/tests/sim/expected/conditional_assignment_last.txt

@@ -155,6 +155,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -163,6 +164,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },

crates/fayalite/tests/sim/expected/connect_const.txt

@@ -124,6 +124,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x05,
             last_state: 0x05,
         },

crates/fayalite/tests/sim/expected/connect_const_reset.txt

@@ -175,6 +175,7 @@ Simulation {
             kind: BigAsyncReset {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -183,6 +184,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },

crates/fayalite/tests/sim/expected/counter_async.txt

@@ -332,6 +332,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -340,6 +341,7 @@ Simulation {
             kind: BigAsyncReset {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -349,6 +351,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x3,
             last_state: 0x2,
         },
@@ -358,6 +361,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x3,
             last_state: 0x2,
         },

crates/fayalite/tests/sim/expected/counter_sync.txt

@@ -313,6 +313,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -321,6 +322,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -330,6 +332,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x3,
             last_state: 0x2,
         },
@@ -339,6 +342,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x3,
             last_state: 0x2,
         },

crates/fayalite/tests/sim/expected/duplicate_names.txt

@@ -137,6 +137,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x05,
             last_state: 0x05,
         },
@@ -146,6 +147,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x06,
             last_state: 0x06,
         },

crates/fayalite/tests/sim/expected/enum_with_simple_body.txt

@@ -0,0 +1,749 @@
+Simulation {
+    state: State {
+        insns: Insns {
+            state_layout: StateLayout {
+                ty: TypeLayout {
+                    small_slots: StatePartLayout<SmallSlots> {
+                        len: 1,
+                        debug_data: [
+                            SlotDebugData {
+                                name: "",
+                                ty: Enum {
+                                    A,
+                                    B,
+                                    C,
+                                },
+                            },
+                        ],
+                        ..
+                    },
+                    big_slots: StatePartLayout<BigSlots> {
+                        len: 33,
+                        debug_data: [
+                            SlotDebugData {
+                                name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_in",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_in",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_out",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_out",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out",
+                                ty: Enum {
+                                    A(UInt<8>),
+                                    B(UInt<8>),
+                                    C(UInt<8>),
+                                },
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: Bool,
+                            },
+                            SlotDebugData {
+                                name: ".0",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: ".1",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: Enum {
+                                    A(UInt<8>),
+                                    B(UInt<8>),
+                                    C(UInt<8>),
+                                },
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: Bool,
+                            },
+                            SlotDebugData {
+                                name: ".0",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: ".1",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: Enum {
+                                    A(UInt<8>),
+                                    B(UInt<8>),
+                                    C(UInt<8>),
+                                },
+                            },
+                            SlotDebugData {
+                                name: ".0",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: ".1",
+                                ty: UInt<8>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<2>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<10>,
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: Enum {
+                                    A(UInt<8>),
+                                    B(UInt<8>),
+                                    C(UInt<8>),
+                                },
+                            },
+                            SlotDebugData {
+                                name: "",
+                                ty: UInt<8>,
+                            },
+                        ],
+                        ..
+                    },
+                    sim_only_slots: StatePartLayout<SimOnlySlots> {
+                        len: 0,
+                        debug_data: [],
+                        layout_data: [],
+                        ..
+                    },
+                },
+                memories: StatePartLayout<Memories> {
+                    len: 0,
+                    debug_data: [],
+                    layout_data: [],
+                    ..
+                },
+            },
+            insns: [
+                // at: module-XXXXXXXXXX.rs:1:1
+                0: Const {
+                    dest: StatePartIndex<BigSlots>(32), // (0x2) SlotDebugData { name: "", ty: UInt<8> },
+                    value: 0x2,
+                },
+                1: Const {
+                    dest: StatePartIndex<BigSlots>(27), // (0x2) SlotDebugData { name: "", ty: UInt<2> },
+                    value: 0x2,
+                },
+                2: Copy {
+                    dest: StatePartIndex<BigSlots>(25), // (0x2) SlotDebugData { name: ".0", ty: UInt<2> },
+                    src: StatePartIndex<BigSlots>(27), // (0x2) SlotDebugData { name: "", ty: UInt<2> },
+                },
+                3: Copy {
+                    dest: StatePartIndex<BigSlots>(26), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(1), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_in", ty: UInt<8> },
+                },
+                4: Shl {
+                    dest: StatePartIndex<BigSlots>(28), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(26), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    rhs: 2,
+                },
+                5: Or {
+                    dest: StatePartIndex<BigSlots>(29), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(25), // (0x2) SlotDebugData { name: ".0", ty: UInt<2> },
+                    rhs: StatePartIndex<BigSlots>(28), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                6: CastToUInt {
+                    dest: StatePartIndex<BigSlots>(30), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                    src: StatePartIndex<BigSlots>(29), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                    dest_width: 10,
+                },
+                7: Copy {
+                    dest: StatePartIndex<BigSlots>(31), // (0x386) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(30), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                8: Const {
+                    dest: StatePartIndex<BigSlots>(20), // (0x1) SlotDebugData { name: "", ty: UInt<2> },
+                    value: 0x1,
+                },
+                9: Copy {
+                    dest: StatePartIndex<BigSlots>(18), // (0x1) SlotDebugData { name: ".0", ty: UInt<2> },
+                    src: StatePartIndex<BigSlots>(20), // (0x1) SlotDebugData { name: "", ty: UInt<2> },
+                },
+                10: Copy {
+                    dest: StatePartIndex<BigSlots>(19), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(1), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_in", ty: UInt<8> },
+                },
+                11: Shl {
+                    dest: StatePartIndex<BigSlots>(21), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(19), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    rhs: 2,
+                },
+                12: Or {
+                    dest: StatePartIndex<BigSlots>(22), // (0x385) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(18), // (0x1) SlotDebugData { name: ".0", ty: UInt<2> },
+                    rhs: StatePartIndex<BigSlots>(21), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                13: CastToUInt {
+                    dest: StatePartIndex<BigSlots>(23), // (0x385) SlotDebugData { name: "", ty: UInt<10> },
+                    src: StatePartIndex<BigSlots>(22), // (0x385) SlotDebugData { name: "", ty: UInt<10> },
+                    dest_width: 10,
+                },
+                14: Copy {
+                    dest: StatePartIndex<BigSlots>(24), // (0x385) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(23), // (0x385) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                15: Const {
+                    dest: StatePartIndex<BigSlots>(16), // (0x1) SlotDebugData { name: "", ty: UInt<8> },
+                    value: 0x1,
+                },
+                16: CmpEq {
+                    dest: StatePartIndex<BigSlots>(17), // (0x0) SlotDebugData { name: "", ty: Bool },
+                    lhs: StatePartIndex<BigSlots>(0), // (0x2) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_in", ty: UInt<8> },
+                    rhs: StatePartIndex<BigSlots>(16), // (0x1) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                17: Const {
+                    dest: StatePartIndex<BigSlots>(11), // (0x0) SlotDebugData { name: "", ty: UInt<2> },
+                    value: 0x0,
+                },
+                18: Copy {
+                    dest: StatePartIndex<BigSlots>(9), // (0x0) SlotDebugData { name: ".0", ty: UInt<2> },
+                    src: StatePartIndex<BigSlots>(11), // (0x0) SlotDebugData { name: "", ty: UInt<2> },
+                },
+                19: Copy {
+                    dest: StatePartIndex<BigSlots>(10), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(1), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_in", ty: UInt<8> },
+                },
+                20: Shl {
+                    dest: StatePartIndex<BigSlots>(12), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(10), // (0xe1) SlotDebugData { name: ".1", ty: UInt<8> },
+                    rhs: 2,
+                },
+                21: Or {
+                    dest: StatePartIndex<BigSlots>(13), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    lhs: StatePartIndex<BigSlots>(9), // (0x0) SlotDebugData { name: ".0", ty: UInt<2> },
+                    rhs: StatePartIndex<BigSlots>(12), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                22: CastToUInt {
+                    dest: StatePartIndex<BigSlots>(14), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    src: StatePartIndex<BigSlots>(13), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                    dest_width: 10,
+                },
+                23: Copy {
+                    dest: StatePartIndex<BigSlots>(15), // (0x384) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(14), // (0x384) SlotDebugData { name: "", ty: UInt<10> },
+                },
+                24: Const {
+                    dest: StatePartIndex<BigSlots>(7), // (0x0) SlotDebugData { name: "", ty: UInt<8> },
+                    value: 0x0,
+                },
+                25: CmpEq {
+                    dest: StatePartIndex<BigSlots>(8), // (0x0) SlotDebugData { name: "", ty: Bool },
+                    lhs: StatePartIndex<BigSlots>(0), // (0x2) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_in", ty: UInt<8> },
+                    rhs: StatePartIndex<BigSlots>(7), // (0x0) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:7:1
+                26: BranchIfZero {
+                    target: 28,
+                    value: StatePartIndex<BigSlots>(8), // (0x0) SlotDebugData { name: "", ty: Bool },
+                },
+                // at: module-XXXXXXXXXX.rs:8:1
+                27: Copy {
+                    dest: StatePartIndex<BigSlots>(4), // (0x386) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(15), // (0x384) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                },
+                // at: module-XXXXXXXXXX.rs:7:1
+                28: BranchIfNonZero {
+                    target: 33,
+                    value: StatePartIndex<BigSlots>(8), // (0x0) SlotDebugData { name: "", ty: Bool },
+                },
+                // at: module-XXXXXXXXXX.rs:9:1
+                29: BranchIfZero {
+                    target: 31,
+                    value: StatePartIndex<BigSlots>(17), // (0x0) SlotDebugData { name: "", ty: Bool },
+                },
+                // at: module-XXXXXXXXXX.rs:10:1
+                30: Copy {
+                    dest: StatePartIndex<BigSlots>(4), // (0x386) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(24), // (0x385) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                },
+                // at: module-XXXXXXXXXX.rs:9:1
+                31: BranchIfNonZero {
+                    target: 33,
+                    value: StatePartIndex<BigSlots>(17), // (0x0) SlotDebugData { name: "", ty: Bool },
+                },
+                // at: module-XXXXXXXXXX.rs:11:1
+                32: Copy {
+                    dest: StatePartIndex<BigSlots>(4), // (0x386) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    src: StatePartIndex<BigSlots>(31), // (0x386) SlotDebugData { name: "", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                },
+                // at: module-XXXXXXXXXX.rs:1:1
+                33: Copy {
+                    dest: StatePartIndex<BigSlots>(5), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                    src: StatePartIndex<BigSlots>(4), // (0x386) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                },
+                34: SliceInt {
+                    dest: StatePartIndex<BigSlots>(6), // (0xe1) SlotDebugData { name: "", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(5), // (0x386) SlotDebugData { name: "", ty: UInt<10> },
+                    start: 2,
+                    len: 8,
+                },
+                // at: module-XXXXXXXXXX.rs:6:1
+                35: AndBigWithSmallImmediate {
+                    dest: StatePartIndex<SmallSlots>(0), // (0x2 2) SlotDebugData { name: "", ty: Enum {A, B, C} },
+                    lhs: StatePartIndex<BigSlots>(4), // (0x386) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::enum_out", ty: Enum {A(UInt<8>), B(UInt<8>), C(UInt<8>)} },
+                    rhs: 0x3,
+                },
+                // at: module-XXXXXXXXXX.rs:12:1
+                36: BranchIfSmallNeImmediate {
+                    target: 39,
+                    lhs: StatePartIndex<SmallSlots>(0), // (0x2 2) SlotDebugData { name: "", ty: Enum {A, B, C} },
+                    rhs: 0x0,
+                },
+                // at: module-XXXXXXXXXX.rs:13:1
+                37: Copy {
+                    dest: StatePartIndex<BigSlots>(2), // (0x2) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(7), // (0x0) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:14:1
+                38: Copy {
+                    dest: StatePartIndex<BigSlots>(3), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(6), // (0xe1) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:12:1
+                39: BranchIfSmallNeImmediate {
+                    target: 42,
+                    lhs: StatePartIndex<SmallSlots>(0), // (0x2 2) SlotDebugData { name: "", ty: Enum {A, B, C} },
+                    rhs: 0x1,
+                },
+                // at: module-XXXXXXXXXX.rs:15:1
+                40: Copy {
+                    dest: StatePartIndex<BigSlots>(2), // (0x2) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(16), // (0x1) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:16:1
+                41: Copy {
+                    dest: StatePartIndex<BigSlots>(3), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(6), // (0xe1) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:12:1
+                42: BranchIfSmallNeImmediate {
+                    target: 45,
+                    lhs: StatePartIndex<SmallSlots>(0), // (0x2 2) SlotDebugData { name: "", ty: Enum {A, B, C} },
+                    rhs: 0x2,
+                },
+                // at: module-XXXXXXXXXX.rs:17:1
+                43: Copy {
+                    dest: StatePartIndex<BigSlots>(2), // (0x2) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::which_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(32), // (0x2) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:18:1
+                44: Copy {
+                    dest: StatePartIndex<BigSlots>(3), // (0xe1) SlotDebugData { name: "InstantiatedModule(enum_with_simple_body: enum_with_simple_body).enum_with_simple_body::data_out", ty: UInt<8> },
+                    src: StatePartIndex<BigSlots>(6), // (0xe1) SlotDebugData { name: "", ty: UInt<8> },
+                },
+                // at: module-XXXXXXXXXX.rs:1:1
+                45: Return,
+            ],
+            ..
+        },
+        pc: 45,
+        memory_write_log: [],
+        memories: StatePart {
+            value: [],
+        },
+        small_slots: StatePart {
+            value: [
+                2,
+            ],
+        },
+        big_slots: StatePart {
+            value: [
+                2,
+                225,
+                2 (modified),
+                225 (modified),
+                902,
+                902,
+                225,
+                0,
+                0,
+                0,
+                225,
+                0,
+                900,
+                900,
+                900,
+                900,
+                1,
+                0,
+                1,
+                225,
+                1,
+                900,
+                901,
+                901,
+                901,
+                2,
+                225,
+                2,
+                900,
+                902,
+                902,
+                902,
+                2,
+            ],
+        },
+        sim_only_slots: StatePart {
+            value: [],
+        },
+    },
+    io: Instance {
+        name: <simulator>::enum_with_simple_body,
+        instantiated: Module {
+            name: enum_with_simple_body,
+            ..
+        },
+    },
+    main_module: SimulationModuleState {
+        base_targets: [
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.which_in,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.data_in,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.which_out,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.data_out,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.enum_out,
+        ],
+        uninitialized_ios: {},
+        io_targets: {
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.data_in,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.data_out,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.enum_out,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.which_in,
+            Instance {
+                name: <simulator>::enum_with_simple_body,
+                instantiated: Module {
+                    name: enum_with_simple_body,
+                    ..
+                },
+            }.which_out,
+        },
+        did_initial_settle: true,
+        clocks_for_past: {},
+    },
+    extern_modules: [],
+    trace_decls: TraceModule {
+        name: "enum_with_simple_body",
+        children: [
+            TraceModuleIO {
+                name: "which_in",
+                child: TraceUInt {
+                    location: TraceScalarId(0),
+                    name: "which_in",
+                    ty: UInt<8>,
+                    flow: Source,
+                },
+                ty: UInt<8>,
+                flow: Source,
+            },
+            TraceModuleIO {
+                name: "data_in",
+                child: TraceUInt {
+                    location: TraceScalarId(1),
+                    name: "data_in",
+                    ty: UInt<8>,
+                    flow: Source,
+                },
+                ty: UInt<8>,
+                flow: Source,
+            },
+            TraceModuleIO {
+                name: "which_out",
+                child: TraceUInt {
+                    location: TraceScalarId(2),
+                    name: "which_out",
+                    ty: UInt<8>,
+                    flow: Sink,
+                },
+                ty: UInt<8>,
+                flow: Sink,
+            },
+            TraceModuleIO {
+                name: "data_out",
+                child: TraceUInt {
+                    location: TraceScalarId(3),
+                    name: "data_out",
+                    ty: UInt<8>,
+                    flow: Sink,
+                },
+                ty: UInt<8>,
+                flow: Sink,
+            },
+            TraceModuleIO {
+                name: "enum_out",
+                child: TraceEnumWithFields {
+                    name: "enum_out",
+                    discriminant: TraceEnumDiscriminant {
+                        location: TraceScalarId(4),
+                        name: "$tag",
+                        ty: Enum {
+                            A(UInt<8>),
+                            B(UInt<8>),
+                            C(UInt<8>),
+                        },
+                        flow: Sink,
+                    },
+                    non_empty_fields: [
+                        TraceUInt {
+                            location: TraceScalarId(5),
+                            name: "A",
+                            ty: UInt<8>,
+                            flow: Source,
+                        },
+                        TraceUInt {
+                            location: TraceScalarId(6),
+                            name: "B",
+                            ty: UInt<8>,
+                            flow: Source,
+                        },
+                        TraceUInt {
+                            location: TraceScalarId(7),
+                            name: "C",
+                            ty: UInt<8>,
+                            flow: Source,
+                        },
+                    ],
+                    ty: Enum {
+                        A(UInt<8>),
+                        B(UInt<8>),
+                        C(UInt<8>),
+                    },
+                    flow: Sink,
+                },
+                ty: Enum {
+                    A(UInt<8>),
+                    B(UInt<8>),
+                    C(UInt<8>),
+                },
+                flow: Sink,
+            },
+        ],
+    },
+    traces: [
+        SimTrace {
+            id: TraceScalarId(0),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(0),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0x02,
+            last_state: 0x02,
+        },
+        SimTrace {
+            id: TraceScalarId(1),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(1),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0xe1,
+            last_state: 0xb4,
+        },
+        SimTrace {
+            id: TraceScalarId(2),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(2),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0x02,
+            last_state: 0x02,
+        },
+        SimTrace {
+            id: TraceScalarId(3),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(3),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0xe1,
+            last_state: 0xb4,
+        },
+        SimTrace {
+            id: TraceScalarId(4),
+            kind: EnumDiscriminant {
+                index: StatePartIndex<SmallSlots>(0),
+                ty: Enum {
+                    A(UInt<8>),
+                    B(UInt<8>),
+                    C(UInt<8>),
+                },
+            },
+            maybe_changed: true,
+            state: 0x2,
+            last_state: 0x2,
+        },
+        SimTrace {
+            id: TraceScalarId(5),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(6),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0xe1,
+            last_state: 0xb4,
+        },
+        SimTrace {
+            id: TraceScalarId(6),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(6),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0xe1,
+            last_state: 0xb4,
+        },
+        SimTrace {
+            id: TraceScalarId(7),
+            kind: BigUInt {
+                index: StatePartIndex<BigSlots>(6),
+                ty: UInt<8>,
+            },
+            maybe_changed: true,
+            state: 0xe1,
+            last_state: 0xb4,
+        },
+    ],
+    trace_memories: {},
+    trace_writers: [
+        Running(
+            VcdWriter {
+                finished_init: true,
+                timescale: 1 ps,
+                ..
+            },
+        ),
+    ],
+    clocks_triggered: [],
+    event_queue: EventQueue(EventQueueData {
+        instant: 18 μs,
+        events: {},
+    }),
+    waiting_sensitivity_sets_by_address: {},
+    waiting_sensitivity_sets_by_compiled_value: {},
+    ..
+}

crates/fayalite/tests/sim/expected/enum_with_simple_body.vcd

@@ -0,0 +1,133 @@
+$timescale 1 ps $end
+$scope module enum_with_simple_body $end
+$var wire 8 J&-ne which_in $end
+$var wire 8 \7mo/ data_in $end
+$var wire 8 ,`>ir which_out $end
+$var wire 8 0_gMP data_out $end
+$scope struct enum_out $end
+$var string 1 kFH/w \$tag $end
+$var wire 8 |EI_= A $end
+$var wire 8 !pRd4 B $end
+$var wire 8 &RAbd C $end
+$upscope $end
+$upscope $end
+$enddefinitions $end
+$dumpvars
+b0 J&-ne
+b0 \7mo/
+b0 ,`>ir
+b0 0_gMP
+sA\x20(0) kFH/w
+b0 |EI_=
+b0 !pRd4
+b0 &RAbd
+$end
+#1000000
+b101101 \7mo/
+b101101 0_gMP
+b101101 |EI_=
+b101101 !pRd4
+b101101 &RAbd
+#2000000
+b1011010 \7mo/
+b1011010 0_gMP
+b1011010 |EI_=
+b1011010 !pRd4
+b1011010 &RAbd
+#3000000
+b10000111 \7mo/
+b10000111 0_gMP
+b10000111 |EI_=
+b10000111 !pRd4
+b10000111 &RAbd
+#4000000
+b10110100 \7mo/
+b10110100 0_gMP
+b10110100 |EI_=
+b10110100 !pRd4
+b10110100 &RAbd
+#5000000
+b11100001 \7mo/
+b11100001 0_gMP
+b11100001 |EI_=
+b11100001 !pRd4
+b11100001 &RAbd
+#6000000
+b1 J&-ne
+b0 \7mo/
+b1 ,`>ir
+b0 0_gMP
+sB\x20(1) kFH/w
+b0 |EI_=
+b0 !pRd4
+b0 &RAbd
+#7000000
+b101101 \7mo/
+b101101 0_gMP
+b101101 |EI_=
+b101101 !pRd4
+b101101 &RAbd
+#8000000
+b1011010 \7mo/
+b1011010 0_gMP
+b1011010 |EI_=
+b1011010 !pRd4
+b1011010 &RAbd
+#9000000
+b10000111 \7mo/
+b10000111 0_gMP
+b10000111 |EI_=
+b10000111 !pRd4
+b10000111 &RAbd
+#10000000
+b10110100 \7mo/
+b10110100 0_gMP
+b10110100 |EI_=
+b10110100 !pRd4
+b10110100 &RAbd
+#11000000
+b11100001 \7mo/
+b11100001 0_gMP
+b11100001 |EI_=
+b11100001 !pRd4
+b11100001 &RAbd
+#12000000
+b10 J&-ne
+b0 \7mo/
+b10 ,`>ir
+b0 0_gMP
+sC\x20(2) kFH/w
+b0 |EI_=
+b0 !pRd4
+b0 &RAbd
+#13000000
+b101101 \7mo/
+b101101 0_gMP
+b101101 |EI_=
+b101101 !pRd4
+b101101 &RAbd
+#14000000
+b1011010 \7mo/
+b1011010 0_gMP
+b1011010 |EI_=
+b1011010 !pRd4
+b1011010 &RAbd
+#15000000
+b10000111 \7mo/
+b10000111 0_gMP
+b10000111 |EI_=
+b10000111 !pRd4
+b10000111 &RAbd
+#16000000
+b10110100 \7mo/
+b10110100 0_gMP
+b10110100 |EI_=
+b10110100 !pRd4
+b10110100 &RAbd
+#17000000
+b11100001 \7mo/
+b11100001 0_gMP
+b11100001 |EI_=
+b11100001 !pRd4
+b11100001 &RAbd
+#18000000

crates/fayalite/tests/sim/expected/enums.txt

@@ -1746,6 +1746,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -1754,6 +1755,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1762,6 +1764,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: false,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1771,6 +1774,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<2>,
             },
+            maybe_changed: false,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1780,6 +1784,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<4>,
             },
+            maybe_changed: false,
             state: 0xf,
             last_state: 0xf,
         },
@@ -1789,6 +1794,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1798,6 +1804,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(6),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xf,
             last_state: 0xf,
         },
@@ -1810,6 +1817,7 @@ Simulation {
                     HdlSome(Bundle {0: UInt<1>, 1: Bool}),
                 },
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1819,6 +1827,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(8),
                 ty: UInt<1>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1827,6 +1836,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(9),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1839,6 +1849,7 @@ Simulation {
                     HdlSome(Bundle {0: UInt<1>, 1: Bool}),
                 },
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1848,6 +1859,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(16),
                 ty: UInt<1>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1856,6 +1868,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(17),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1869,6 +1882,7 @@ Simulation {
                     C(Bundle {a: Array<UInt<1>, 2>, b: SInt<2>}),
                 },
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1878,6 +1892,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(27),
                 ty: UInt<1>,
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1886,6 +1901,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(28),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1895,6 +1911,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(34),
                 ty: UInt<1>,
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1904,6 +1921,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(35),
                 ty: UInt<1>,
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1913,6 +1931,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(36),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x3,
             last_state: 0x3,
         },

crates/fayalite/tests/sim/expected/extern_module.txt

@@ -221,6 +221,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: false,
             state: 0x1,
             last_state: 0x1,
         },
@@ -229,6 +230,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },

crates/fayalite/tests/sim/expected/extern_module2.txt

@@ -57,7 +57,7 @@ Simulation {
         big_slots: StatePart {
             value: [
                 0,
-                1,
+                1 (modified),
                 101,
             ],
         },
@@ -280,6 +280,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -288,6 +289,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -297,6 +299,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x65,
             last_state: 0x65,
         },

crates/fayalite/tests/sim/expected/last_connect.txt

@@ -433,7 +433,7 @@ Simulation {
                 1,
                 1,
                 1,
-                7,
+                7 (modified),
                 7,
                 3,
                 0,
@@ -614,6 +614,7 @@ Simulation {
                     HdlSome(Array<Bool, 4>),
                 },
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -622,6 +623,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -630,6 +632,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -638,6 +641,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(3),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -646,6 +650,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(4),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -658,6 +663,7 @@ Simulation {
                     HdlSome(UInt<8>),
                 },
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -667,6 +673,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(17),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x03,
             last_state: 0x02,
         },
@@ -676,6 +683,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(20),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x03,
             last_state: 0x02,
         },

crates/fayalite/tests/sim/expected/memories.txt

@@ -1168,6 +1168,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1176,6 +1177,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1184,6 +1186,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -1193,6 +1196,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xb0,
             last_state: 0xb0,
         },
@@ -1202,6 +1206,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: SInt<8>,
             },
+            maybe_changed: true,
             state: 0xc0,
             last_state: 0xc0,
         },
@@ -1211,6 +1216,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1219,6 +1225,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(6),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1227,6 +1234,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(7),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -1236,6 +1244,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(8),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xd0,
             last_state: 0xd0,
         },
@@ -1245,6 +1254,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(9),
                 ty: SInt<8>,
             },
+            maybe_changed: true,
             state: 0xe0,
             last_state: 0xe0,
         },
@@ -1253,6 +1263,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(10),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1261,6 +1272,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(11),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1270,6 +1282,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(12),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1278,6 +1291,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(13),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1286,6 +1300,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(14),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -1295,6 +1310,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(15),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xb0,
             last_state: 0xb0,
         },
@@ -1304,6 +1320,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(16),
                 ty: SInt<8>,
             },
+            maybe_changed: true,
             state: 0xc0,
             last_state: 0xc0,
         },
@@ -1313,6 +1330,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(17),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -1321,6 +1339,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(18),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1329,6 +1348,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(19),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -1338,6 +1358,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(20),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0xd0,
             last_state: 0xd0,
         },
@@ -1347,6 +1368,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(21),
                 ty: SInt<8>,
             },
+            maybe_changed: true,
             state: 0xe0,
             last_state: 0xe0,
         },
@@ -1355,6 +1377,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(22),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1363,6 +1386,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(23),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },

crates/fayalite/tests/sim/expected/memories2.txt

@@ -943,6 +943,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -951,6 +952,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -959,6 +961,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -968,6 +971,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<2>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -976,6 +980,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(4),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -985,6 +990,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<2>,
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -993,6 +999,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(6),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1002,6 +1009,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(7),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1010,6 +1018,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(8),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1018,6 +1027,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(9),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -1030,6 +1040,7 @@ Simulation {
                     HdlSome(Bool),
                 },
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1038,6 +1049,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(16),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1046,6 +1058,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(11),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1058,6 +1071,7 @@ Simulation {
                     HdlSome(Bool),
                 },
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1066,6 +1080,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(19),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1074,6 +1089,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(13),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },

crates/fayalite/tests/sim/expected/memories3.txt

@@ -2391,6 +2391,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2399,6 +2400,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2407,6 +2409,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -2416,6 +2419,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2425,6 +2429,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2434,6 +2439,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2443,6 +2449,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(6),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2452,6 +2459,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(7),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2461,6 +2469,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(8),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2470,6 +2479,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(9),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2479,6 +2489,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(10),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2488,6 +2499,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(11),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2496,6 +2508,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(12),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2504,6 +2517,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(13),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -2513,6 +2527,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(14),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2522,6 +2537,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(15),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2531,6 +2547,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(16),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2540,6 +2557,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(17),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2549,6 +2567,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(18),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2558,6 +2577,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(19),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2567,6 +2587,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(20),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2576,6 +2597,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(21),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2584,6 +2606,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(22),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2592,6 +2615,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(23),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2600,6 +2624,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(24),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2608,6 +2633,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(25),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2616,6 +2642,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(26),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2624,6 +2651,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(27),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2632,6 +2660,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(28),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2640,6 +2669,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(29),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2649,6 +2679,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(30),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2657,6 +2688,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(31),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2665,6 +2697,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(32),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -2674,6 +2707,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(33),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2683,6 +2717,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(34),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2692,6 +2727,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(35),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2701,6 +2737,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(36),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2710,6 +2747,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(37),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2719,6 +2757,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(38),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2728,6 +2767,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(39),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2737,6 +2777,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(40),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2746,6 +2787,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(57),
                 ty: UInt<3>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2754,6 +2796,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(58),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2762,6 +2805,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(59),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -2771,6 +2815,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(60),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2780,6 +2825,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(61),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2789,6 +2835,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(62),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2798,6 +2845,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(63),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2807,6 +2855,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(64),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2816,6 +2865,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(65),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2825,6 +2875,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(66),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2834,6 +2885,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(67),
                 ty: UInt<8>,
             },
+            maybe_changed: true,
             state: 0x00,
             last_state: 0x00,
         },
@@ -2842,6 +2894,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(68),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2850,6 +2903,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(69),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2858,6 +2912,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(70),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2866,6 +2921,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(71),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2874,6 +2930,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(72),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2882,6 +2939,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(73),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2890,6 +2948,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(74),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -2898,6 +2957,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(75),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },

crates/fayalite/tests/sim/expected/mod1.txt

@@ -445,6 +445,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xa,
             last_state: 0x3,
         },
@@ -454,6 +455,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(1),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x3,
         },
@@ -463,6 +465,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(2),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -472,6 +475,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xf,
             last_state: 0xe,
         },
@@ -481,6 +485,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(8),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xa,
             last_state: 0x3,
         },
@@ -490,6 +495,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(9),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x3,
         },
@@ -499,6 +505,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(10),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -508,6 +515,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(11),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xf,
             last_state: 0xe,
         },
@@ -517,6 +525,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xa,
             last_state: 0x3,
         },
@@ -526,6 +535,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x3,
         },
@@ -535,6 +545,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(6),
                 ty: SInt<2>,
             },
+            maybe_changed: true,
             state: 0x2,
             last_state: 0x2,
         },
@@ -544,6 +555,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(7),
                 ty: UInt<4>,
             },
+            maybe_changed: true,
             state: 0xf,
             last_state: 0xe,
         },

crates/fayalite/tests/sim/expected/mod1.vcd

@@ -6,18 +6,12 @@ $var wire 2 Q2~aG o $end
 $var wire 2 DXK'| i2 $end
 $var wire 4 cPuix o2 $end
 $upscope $end
-$scope struct child $end
+$scope module child $end
 $var wire 4 ($5K7 i $end
 $var wire 2 %6Wv" o $end
 $var wire 2 +|-AU i2 $end
 $var wire 4 Hw?%j o2 $end
 $upscope $end
-$scope module mod1_child $end
-$var wire 4 4}s%= i $end
-$var wire 2 }IY?g o $end
-$var wire 2 of42K i2 $end
-$var wire 4 D9]&= o2 $end
-$upscope $end
 $upscope $end
 $enddefinitions $end
 $dumpvars
@@ -25,10 +19,6 @@ b11 avK(^
 b11 Q2~aG
 b10 DXK'|
 b1110 cPuix
-b11 4}s%=
-b11 }IY?g
-b10 of42K
-b1110 D9]&=
 b11 ($5K7
 b11 %6Wv"
 b10 +|-AU
@@ -38,9 +28,6 @@ $end
 b1010 avK(^
 b10 Q2~aG
 b1111 cPuix
-b1010 4}s%=
-b10 }IY?g
-b1111 D9]&=
 b1010 ($5K7
 b10 %6Wv"
 b1111 Hw?%j

crates/fayalite/tests/sim/expected/phantom_const.txt

@@ -373,6 +373,7 @@ Simulation {
                     ["a","b"],
                 ),
             },
+            maybe_changed: true,
             state: PhantomConst,
             last_state: PhantomConst,
         },
@@ -383,6 +384,7 @@ Simulation {
                     ["a","b"],
                 ),
             },
+            maybe_changed: true,
             state: PhantomConst,
             last_state: PhantomConst,
         },
@@ -392,6 +394,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(0),
                 ty: UInt<0>,
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -400,6 +403,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -408,6 +412,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -418,6 +423,7 @@ Simulation {
                     "mem_element",
                 ),
             },
+            maybe_changed: true,
             state: PhantomConst,
             last_state: PhantomConst,
         },

crates/fayalite/tests/sim/expected/ripple_counter.txt

@@ -687,18 +687,7 @@ Simulation {
                 1,
                 0,
                 0,
-                0,
-                0,
-                0,
-                0,
-                1,
-                0,
-                0,
-                0,
-                1,
-                0,
-                0,
-                0,
+                0 (modified),
                 0,
                 0,
                 0,
@@ -709,9 +698,20 @@ Simulation {
                 1,
                 0,
                 0,
+                0 (modified),
                 0,
                 0,
                 0,
+                1,
+                0,
+                0,
+                0,
+                1,
+                0,
+                0,
+                0 (modified),
+                0,
+                0,
             ],
         },
         sim_only_slots: StatePart {
@@ -1284,6 +1284,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: false,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1293,6 +1294,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(1),
                 ty: UInt<6>,
             },
+            maybe_changed: false,
             state: 0x00,
             last_state: 0x00,
         },
@@ -1301,6 +1303,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1309,6 +1312,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(3),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1317,6 +1321,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(4),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1325,6 +1330,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(5),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1333,6 +1339,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(6),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1341,6 +1348,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(7),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1349,6 +1357,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(24),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1357,6 +1366,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(33),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1365,6 +1375,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(34),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1373,6 +1384,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(31),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1381,6 +1393,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(32),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1389,6 +1402,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(36),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1397,6 +1411,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(44),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1405,6 +1420,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(45),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1413,6 +1429,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(42),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1421,6 +1438,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(43),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1429,6 +1447,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(47),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1437,6 +1456,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(55),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1445,6 +1465,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(56),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1453,6 +1474,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(53),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1461,6 +1483,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(54),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },

crates/fayalite/tests/sim/expected/shift_register.txt

@@ -458,6 +458,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x0,
         },
@@ -466,6 +467,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -474,6 +476,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -482,6 +485,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(3),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -490,6 +494,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(4),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -498,6 +503,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(7),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -506,6 +512,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(9),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -514,6 +521,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(11),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },

crates/fayalite/tests/sim/expected/sim_fork_join.txt

@@ -68,12 +68,12 @@ Simulation {
         },
         big_slots: StatePart {
             value: [
+                0 (modified),
                 0,
                 0,
-                0,
-                49,
-                50,
-                50,
+                49 (modified),
+                50 (modified),
+                50 (modified),
             ],
         },
         sim_only_slots: StatePart {
@@ -356,6 +356,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -364,6 +365,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -372,6 +374,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -381,6 +384,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x31,
             last_state: 0x31,
         },
@@ -390,6 +394,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x32,
             last_state: 0x32,
         },
@@ -399,6 +404,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x32,
             last_state: 0x32,
         },

crates/fayalite/tests/sim/expected/sim_fork_join_scope.txt

@@ -68,12 +68,12 @@ Simulation {
         },
         big_slots: StatePart {
             value: [
+                0 (modified),
                 0,
                 0,
-                0,
-                49,
-                50,
-                50,
+                49 (modified),
+                50 (modified),
+                50 (modified),
             ],
         },
         sim_only_slots: StatePart {
@@ -356,6 +356,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -364,6 +365,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: false,
             state: 0x0,
             last_state: 0x0,
         },
@@ -372,6 +374,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x1,
         },
@@ -381,6 +384,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(3),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x31,
             last_state: 0x31,
         },
@@ -390,6 +394,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(4),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x32,
             last_state: 0x32,
         },
@@ -399,6 +404,7 @@ Simulation {
                 index: StatePartIndex<BigSlots>(5),
                 ty: UInt<8>,
             },
+            maybe_changed: false,
             state: 0x32,
             last_state: 0x32,
         },

crates/fayalite/tests/sim/expected/sim_only_connects.txt

@@ -392,7 +392,7 @@ Simulation {
                 0,
                 1,
                 0,
-                1,
+                1 (modified),
                 0,
                 0,
                 0,
@@ -400,7 +400,7 @@ Simulation {
                 0,
                 1,
                 0,
-                1,
+                1 (modified),
                 0,
             ],
         },
@@ -1252,6 +1252,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(0),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1260,6 +1261,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(1),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1269,6 +1271,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(0),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "extra": "value",
             },
@@ -1282,6 +1285,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(1),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "extra": "value",
             },
@@ -1295,6 +1299,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(2),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "",
                 "extra": "value",
@@ -1312,6 +1317,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(3),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "baz",
                 "extra": "value",
@@ -1328,6 +1334,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(4),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1336,6 +1343,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(5),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1345,6 +1353,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(6),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "extra": "value",
             },
@@ -1358,6 +1367,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(7),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "",
                 "extra": "value",
@@ -1374,6 +1384,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(2),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1382,6 +1393,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(3),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1391,6 +1403,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(4),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "extra": "value",
             },
@@ -1404,6 +1417,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(5),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "",
                 "extra": "value",
@@ -1421,6 +1435,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(8),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "extra": "value",
             },
@@ -1433,6 +1448,7 @@ Simulation {
             kind: BigBool {
                 index: StatePartIndex<BigSlots>(6),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1441,6 +1457,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(12),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1449,6 +1466,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(13),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1458,6 +1476,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(13),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "",
                 "extra": "value",
@@ -1475,6 +1494,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(14),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "baz",
                 "extra": "value",
@@ -1491,6 +1511,7 @@ Simulation {
             kind: BigClock {
                 index: StatePartIndex<BigSlots>(10),
             },
+            maybe_changed: true,
             state: 0x1,
             last_state: 0x1,
         },
@@ -1499,6 +1520,7 @@ Simulation {
             kind: BigSyncReset {
                 index: StatePartIndex<BigSlots>(11),
             },
+            maybe_changed: true,
             state: 0x0,
             last_state: 0x0,
         },
@@ -1508,6 +1530,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(11),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "",
                 "extra": "value",
@@ -1525,6 +1548,7 @@ Simulation {
                 index: StatePartIndex<SimOnlySlots>(12),
                 ty: SimOnly<alloc::collections::btree::map::BTreeMap<alloc::string::String, alloc::rc::Rc<str>>>,
             },
+            maybe_changed: true,
             state: {
                 "bar": "baz",
                 "extra": "value",