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

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

@@ -1277,6 +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
+            }
         }
     };
 }

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/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)]
@@ -934,6 +952,10 @@ pub(crate) enum SimTraceKind {
     PhantomConst {
         ty: PhantomConst,
     },
+    TraceAsString {
+        layout: compiler::CompiledTypeLayout<TraceAsString>,
+        range: TypeIndexRange,
+    },
 }
 
 #[derive(PartialEq, Eq)]
@@ -941,6 +963,7 @@ pub(crate) enum SimTraceState {
     Bits(BitVec),
     SimOnly(DynSimOnlyValue),
     PhantomConst,
+    OpaqueSimValue(OpaqueSimValue),
 }
 
 impl Clone for SimTraceState {
@@ -949,6 +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) {
@@ -956,6 +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(),
         }
     }
@@ -990,6 +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 {
@@ -998,6 +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),
         }
     }
 }
@@ -1026,6 +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),
+                ))
+            }
         }
     }
 }
@@ -1180,6 +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) {
@@ -1230,7 +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"
                         ),
@@ -1273,7 +1350,8 @@ impl SimulationModuleState {
             | CanonicalType::Reset(_)
             | CanonicalType::Clock(_)
             | CanonicalType::PhantomConst(_)
-            | CanonicalType::DynSimOnly(_) => unreachable!(),
+            | CanonicalType::DynSimOnly(_)
+            | CanonicalType::TraceAsString(_) => unreachable!(),
             CanonicalType::AsyncReset(_) => true,
             CanonicalType::SyncReset(_) => false,
         }
@@ -1438,6 +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());
+            }
         }
     }
 }
@@ -1906,6 +2004,7 @@ struct SimulationImpl {
         ),
     >,
     waiting_sensitivity_sets_by_address: HashMap<*const SensitivitySet, Rc<SensitivitySet>>,
+    trace_as_string_buf: String,
 }
 
 impl fmt::Debug for SimulationImpl {
@@ -1995,6 +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)
@@ -2101,6 +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>(
@@ -2181,6 +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)
@@ -2233,6 +2343,9 @@ impl SimulationImpl {
                     .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 {
@@ -2286,6 +2399,20 @@ 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);
@@ -2818,6 +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 {
@@ -2833,6 +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(),
@@ -2909,14 +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,
@@ -2937,8 +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,
@@ -2970,6 +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(
@@ -2421,7 +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)
             }
         }
@@ -2591,6 +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()),
                 }
             }
         };
@@ -2823,6 +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(
@@ -2912,6 +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())
@@ -2963,6 +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,
@@ -2977,6 +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,
@@ -3722,6 +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,
@@ -3869,6 +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 {
@@ -4244,6 +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 {
@@ -4262,6 +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 {
@@ -4484,6 +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},
@@ -920,6 +920,12 @@ impl<K: StatePartKind> StatePart<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)
     }
@@ -1018,6 +1024,11 @@ 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();)*

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

@@ -260,6 +260,10 @@ pub(crate) trait StatePartKind:
         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>,
@@ -342,6 +346,12 @@ impl StatePartKind for StatePartKindMemories {
     ) -> 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>,
@@ -446,6 +456,14 @@ impl StatePartKind for StatePartKindSmallSlots {
     ) -> 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);
     }
@@ -530,6 +548,14 @@ impl StatePartKind for StatePartKindBigSlots {
     ) -> 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);
     }
@@ -614,6 +640,14 @@ impl StatePartKind for StatePartKindSimOnlySlots {
     ) -> 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);
     }

crates/fayalite/src/sim/value.rs

@@ -1098,7 +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(_)

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};
@@ -331,6 +332,7 @@ 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),
         }
     }
 }
@@ -726,6 +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 {
@@ -1093,6 +1123,7 @@ impl<W: io::Write> TraceWriterDecls for VcdWriterDecls<W> {
             finished_init: false,
             timescale,
             properties,
+            trace_as_string_buf: String::with_capacity(256),
         })
     }
 }
@@ -1100,6 +1131,7 @@ impl<W: io::Write> TraceWriterDecls for VcdWriterDecls<W> {
 enum MemoryElementPartBody {
     Scalar,
     EnumDiscriminant { ty: Enum },
+    TraceAsString { ty: TraceAsString },
 }
 
 struct MemoryElementPart {
@@ -1217,6 +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> {
@@ -1326,6 +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(())
@@ -1419,6 +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> {
@@ -1428,6 +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 +225,134 @@ 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.
+    ///
+    /// [`TraceAsString`]: struct@TraceAsString
+    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 +459,7 @@ 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 +468,7 @@ 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 {}
 
@@ -473,6 +606,7 @@ 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 {
@@ -757,13 +891,34 @@ 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 {
@@ -842,6 +997,9 @@ 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
     }
@@ -1143,3 +1301,413 @@ 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/sim.rs

@@ -1,12 +1,19 @@
 // SPDX-License-Identifier: LGPL-3.0-or-later
 // See Notices.txt for copyright information
 
+use bitvec::{order::Lsb0, view::BitView};
 use fayalite::{
-    memory::{ReadStruct, ReadWriteStruct, WriteStruct},
-    module::{instance_with_loc, memory_with_init_and_loc, reg_builder_with_loc},
+    assert_export_firrtl,
+    firrtl::ExportOptions,
+    memory::{ReadStruct, ReadWriteStruct, WriteStruct, splat_mask},
+    module::{
+        instance_with_loc, memory_with_init_and_loc, reg_builder_with_loc,
+        transform::simplify_enums::SimplifyEnumsKind,
+    },
     prelude::*,
     reset::ResetType,
     sim::vcd::VcdWriterDecls,
+    ty::{SimValueDebug, StaticType, TraceAsString, TraceAsStringSimValue},
     util::{RcWriter, ready_valid::queue},
 };
 use std::{collections::BTreeMap, num::NonZeroUsize, rc::Rc};
@@ -2985,3 +2992,621 @@ fn test_queue_4_true_true() {
         include_str!("sim/expected/queue_4_true_true.txt"),
     );
 }
+
+#[hdl(outline_generated, custom_debug(sim), cmp_eq)]
+pub enum HasCustomDebug {
+    Text(UInt<512>),
+    FmtError,
+}
+
+impl HasCustomDebug {
+    #[hdl]
+    pub fn new_sim(text: Result<&str, std::fmt::Error>) -> SimValue<Self> {
+        match text {
+            Ok(text) => {
+                let mut retval = HasCustomDebug.Text.zero();
+                let src = text.as_bytes().view_bits::<Lsb0>();
+                let dest = retval.bits_mut();
+                let len = src.len().min(dest.len());
+                dest[..len].clone_from_bitslice(&src[..len]);
+                #[hdl(sim)]
+                HasCustomDebug.Text(retval)
+            }
+            Err(std::fmt::Error) =>
+            {
+                #[hdl(sim)]
+                HasCustomDebug.FmtError()
+            }
+        }
+    }
+}
+
+impl SimValueDebug for HasCustomDebug {
+    #[hdl]
+    fn sim_value_debug(
+        value: &<Self as Type>::SimValue,
+        f: &mut std::fmt::Formatter<'_>,
+    ) -> std::fmt::Result {
+        if f.alternate() {
+            return #[hdl(sim)]
+            match value {
+                Self::FmtError => f.write_str("FmtError"),
+                Self::Text(text) => f.debug_tuple("Text").field(text).finish(),
+            };
+        }
+        #[hdl(sim)]
+        match value {
+            Self::FmtError => Err(std::fmt::Error),
+            Self::Text(text) => {
+                assert_eq!(text.ty().width() % u8::BITS as usize, 0);
+                let mut bytes = vec![0u8; text.ty().width() / u8::BITS as usize];
+                bytes
+                    .view_bits_mut::<Lsb0>()
+                    .clone_from_bitslice(text.bits());
+                if let Some(len) = bytes.iter().position(|b| *b == 0) {
+                    bytes.truncate(len);
+                }
+                f.write_str(&String::from_utf8_lossy(&bytes))
+            }
+        }
+    }
+}
+
+#[hdl_module(outline_generated)]
+pub fn sim_trace_as_string() {
+    #[hdl]
+    let clk: Clock = m.input();
+    #[hdl]
+    let read: ReadStruct<TraceAsString<Array<HasCustomDebug, 2>>, ConstUsize<8>> = m.input();
+    #[hdl]
+    let write: WriteStruct<Array<TraceAsString<HasCustomDebug>, 2>, ConstUsize<8>> = m.input();
+    #[hdl]
+    let mut mem = memory_with_init(
+        [[Expr::as_trace_as_string(HasCustomDebug::new_sim(Ok("")).to_expr(), StaticType::TYPE); 2];
+            4],
+    );
+    let read_port = mem.new_read_port();
+    connect(read_port.clk, clk);
+    connect_any(read_port.addr, read.addr);
+    connect(read_port.en, read.en);
+    for (l, r) in read.data.iter().zip(read_port.data.iter()) {
+        connect(l, &**r);
+    }
+    let write_port = mem.new_write_port();
+    connect(write_port.clk, clk);
+    connect_any(write_port.addr, write.addr);
+    connect(write_port.data, write.data);
+    connect(write_port.en, write.en);
+    connect(write_port.mask, write.mask);
+}
+
+#[hdl]
+#[test]
+fn test_sim_trace_as_string() {
+    let _n = SourceLocation::normalize_files_for_tests();
+    let m = sim_trace_as_string();
+    let mut sim = Simulation::new(m);
+    // sim.set_breakpoints_unstable(Default::default(), true);
+    let mut writer = RcWriter::default();
+    sim.add_trace_writer(VcdWriterDecls::new(writer.clone()));
+    sim.write(sim.io().clk, false);
+    sim.write(sim.io().read.clk, false);
+    sim.write(sim.io().write.clk, false);
+    #[derive(Debug)]
+    struct TestCase {
+        read: Option<(u8, [Result<&'static str, std::fmt::Error>; 2])>,
+        write_addr: Option<u8>,
+        write_data: [Result<&'static str, std::fmt::Error>; 2],
+        write_mask: [bool; 2],
+    }
+    const TEST_CASES: &[TestCase] = &[
+        TestCase {
+            read: None,
+            write_addr: None,
+            write_data: [Ok(""); 2],
+            write_mask: [false; 2],
+        },
+        TestCase {
+            read: None,
+            write_addr: Some(0),
+            write_data: [Ok("mem[0][0]"), Ok("mem[0][1]")],
+            write_mask: [true; 2],
+        },
+        TestCase {
+            read: None,
+            write_addr: Some(1),
+            write_data: [Ok("mem[1][0]"), Ok("mem[1][1]")],
+            write_mask: [true; 2],
+        },
+        TestCase {
+            read: None,
+            write_addr: Some(2),
+            write_data: [Ok("mem[2][0]"), Ok("mem[2][1]")],
+            write_mask: [true; 2],
+        },
+        TestCase {
+            read: None,
+            write_addr: Some(3),
+            write_data: [Ok("mem[3][0]"), Ok("mem[3][1]")],
+            write_mask: [true; 2],
+        },
+        TestCase {
+            read: Some((1, [Ok("mem[1][0]"), Ok("mem[1][1]")])),
+            write_addr: None,
+            write_data: [Err(std::fmt::Error), Err(std::fmt::Error)],
+            write_mask: [true; 2],
+        },
+        TestCase {
+            read: Some((1, [Err(std::fmt::Error), Err(std::fmt::Error)])),
+            write_addr: Some(1),
+            write_data: [Err(std::fmt::Error), Err(std::fmt::Error)],
+            write_mask: [true; 2],
+        },
+    ];
+    for test_case in TEST_CASES {
+        let TestCase {
+            read,
+            write_addr,
+            write_data,
+            write_mask,
+        } = test_case;
+        sim.write(sim.io().read.addr, read.map(|v| v.0).unwrap_or(0));
+        sim.write(sim.io().read.en, read.is_some());
+        sim.write(sim.io().write.addr, write_addr.unwrap_or(0));
+        sim.write(sim.io().write.en, write_addr.is_some());
+        sim.write(
+            sim.io().write.data,
+            write_data.map(|v| TraceAsStringSimValue::new(HasCustomDebug::new_sim(v))),
+        );
+        sim.write(
+            sim.io().write.mask,
+            write_mask.map(|v| splat_mask(TraceAsString[HasCustomDebug], v.to_expr())),
+        );
+        sim.write(sim.io().clk, false);
+        sim.advance_time(SimDuration::from_nanos(500));
+        sim.write(sim.io().clk, true);
+        sim.advance_time(SimDuration::from_nanos(500));
+        if let Some((_, expected_read_data)) = read {
+            let read_data = sim.read(sim.io().read.data);
+            let expected_read_data = expected_read_data
+                .map(HasCustomDebug::new_sim)
+                .into_sim_value();
+            assert!(
+                **read_data == *expected_read_data,
+                "{read_data:#?}\n!= {expected_read_data:#?}",
+            );
+        }
+    }
+    sim.flush_traces().unwrap();
+    let vcd = String::from_utf8(writer.take()).unwrap();
+    println!("####### VCD:\n{vcd}\n#######");
+    if vcd != include_str!("sim/expected/sim_trace_as_string.vcd") {
+        panic!();
+    }
+    let sim_debug = format!("{sim:#?}");
+    println!("#######\n{sim_debug}\n#######");
+    if sim_debug != include_str!("sim/expected/sim_trace_as_string.txt") {
+        panic!();
+    }
+}
+
+#[test]
+fn test_firrtl_trace_as_string() {
+    let _n = SourceLocation::normalize_files_for_tests();
+    let m = sim_trace_as_string();
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_memories: false,
+            simplify_enums: None,
+            ..ExportOptions::default()
+        },
+        "/test/sim_trace_as_string.fir": r#"FIRRTL version 3.2.0
+circuit sim_trace_as_string: %[[
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem.mem",
+    "hexOrBinary": "b",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem"
+  }
+]]
+    type Ty0 = {|Text: UInt<512>, FmtError|}
+    type Ty1 = {addr: UInt<8>, en: UInt<1>, clk: Clock, flip data: Ty0[2]}
+    type Ty2 = {addr: UInt<8>, en: UInt<1>, clk: Clock, data: Ty0[2], mask: UInt<1>[2]}
+    type Ty3 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: Ty0[2]}
+    type Ty4 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty0[2], mask: UInt<1>[2]}
+    module sim_trace_as_string: @[module-XXXXXXXXXX.rs 1:1]
+        input clk: Clock @[module-XXXXXXXXXX.rs 2:1]
+        input `read`: Ty1 @[module-XXXXXXXXXX.rs 3:1]
+        input `write`: Ty2 @[module-XXXXXXXXXX.rs 4:1]
+        mem `mem`: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => Ty0[2]
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        connect `mem`.r0.clk, clk @[module-XXXXXXXXXX.rs 7:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect `mem`.r0.addr, `read`.addr @[module-XXXXXXXXXX.rs 8:1]
+        connect `mem`.r0.en, `read`.en @[module-XXXXXXXXXX.rs 9:1]
+        connect `read`.data[0], `mem`.r0.data[0] @[module-XXXXXXXXXX.rs 10:1]
+        connect `read`.data[1], `mem`.r0.data[1] @[module-XXXXXXXXXX.rs 10:1]
+        connect `mem`.w1.clk, clk @[module-XXXXXXXXXX.rs 12:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect `mem`.w1.addr, `write`.addr @[module-XXXXXXXXXX.rs 13:1]
+        connect `mem`.w1.data, `write`.data @[module-XXXXXXXXXX.rs 14:1]
+        connect `mem`.w1.en, `write`.en @[module-XXXXXXXXXX.rs 15:1]
+        connect `mem`.w1.mask, `write`.mask @[module-XXXXXXXXXX.rs 16:1]
+"#,
+        "/test/sim_trace_as_string/mem.mem": r"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_memories: true,
+            simplify_enums: None,
+            ..ExportOptions::default()
+        },
+        "/test/sim_trace_as_string.fir": r#"FIRRTL version 3.2.0
+circuit sim_trace_as_string: %[[
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem.mem",
+    "hexOrBinary": "b",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem"
+  }
+]]
+    type Ty0 = {|Text: UInt<512>, FmtError|}
+    type Ty1 = {addr: UInt<8>, en: UInt<1>, clk: Clock, flip data: Ty0[2]}
+    type Ty2 = {addr: UInt<8>, en: UInt<1>, clk: Clock, data: Ty0[2], mask: UInt<1>[2]}
+    type Ty3 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: Ty0[2]}
+    type Ty4 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty0[2], mask: UInt<1>[2]}
+    type Ty5 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: UInt<513>[2]}
+    type Ty6 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: UInt<513>[2], mask: UInt<1>[2]}
+    module sim_trace_as_string: @[module-XXXXXXXXXX.rs 1:1]
+        input clk: Clock @[module-XXXXXXXXXX.rs 2:1]
+        input `read`: Ty1 @[module-XXXXXXXXXX.rs 3:1]
+        input `write`: Ty2 @[module-XXXXXXXXXX.rs 4:1]
+        mem `mem`: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => UInt<513>[2]
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        wire mem_r0: Ty3 @[module-XXXXXXXXXX.rs 6:1]
+        wire mem_w1: Ty4 @[module-XXXXXXXXXX.rs 11:1]
+        wire _cast_bits_to_enum_expr: Ty0
+        wire _cast_bits_to_enum_expr_body: UInt<512>
+        connect _cast_bits_to_enum_expr_body, head(`mem`.r0.data[0], 512)
+        when eq(UInt<1>(0), tail(`mem`.r0.data[0], 512)):
+            connect _cast_bits_to_enum_expr, {|Text: UInt<512>, FmtError|}(Text, _cast_bits_to_enum_expr_body)
+        else:
+            connect _cast_bits_to_enum_expr, {|Text: UInt<512>, FmtError|}(FmtError)
+        connect mem_r0.data[0], _cast_bits_to_enum_expr @[module-XXXXXXXXXX.rs 6:1]
+        wire _cast_bits_to_enum_expr_1: Ty0
+        wire _cast_bits_to_enum_expr_body_1: UInt<512>
+        connect _cast_bits_to_enum_expr_body_1, head(`mem`.r0.data[1], 512)
+        when eq(UInt<1>(0), tail(`mem`.r0.data[1], 512)):
+            connect _cast_bits_to_enum_expr_1, {|Text: UInt<512>, FmtError|}(Text, _cast_bits_to_enum_expr_body_1)
+        else:
+            connect _cast_bits_to_enum_expr_1, {|Text: UInt<512>, FmtError|}(FmtError)
+        connect mem_r0.data[1], _cast_bits_to_enum_expr_1 @[module-XXXXXXXXXX.rs 6:1]
+        wire _cast_enum_to_bits_expr: UInt<513>
+        match mem_w1.data[0]:
+            Text(_cast_enum_to_bits_expr_Text):
+                connect _cast_enum_to_bits_expr, pad(cat(_cast_enum_to_bits_expr_Text, UInt<1>(0)), 513)
+            FmtError:
+                connect _cast_enum_to_bits_expr, UInt<513>(1)
+        connect `mem`.w1.data[0], _cast_enum_to_bits_expr @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[0], mem_w1.mask[0] @[module-XXXXXXXXXX.rs 11:1]
+        wire _cast_enum_to_bits_expr_1: UInt<513>
+        match mem_w1.data[1]:
+            Text(_cast_enum_to_bits_expr_Text_1):
+                connect _cast_enum_to_bits_expr_1, pad(cat(_cast_enum_to_bits_expr_Text_1, UInt<1>(0)), 513)
+            FmtError:
+                connect _cast_enum_to_bits_expr_1, UInt<513>(1)
+        connect `mem`.w1.data[1], _cast_enum_to_bits_expr_1 @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[1], mem_w1.mask[1] @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.r0.addr, mem_r0.addr @[module-XXXXXXXXXX.rs 6:1]
+        connect `mem`.r0.clk, mem_r0.clk @[module-XXXXXXXXXX.rs 6:1]
+        connect `mem`.r0.en, mem_r0.en @[module-XXXXXXXXXX.rs 6:1]
+        connect `mem`.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_r0.clk, clk @[module-XXXXXXXXXX.rs 7:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect mem_r0.addr, `read`.addr @[module-XXXXXXXXXX.rs 8:1]
+        connect mem_r0.en, `read`.en @[module-XXXXXXXXXX.rs 9:1]
+        connect `read`.data[0], mem_r0.data[0] @[module-XXXXXXXXXX.rs 10:1]
+        connect `read`.data[1], mem_r0.data[1] @[module-XXXXXXXXXX.rs 10:1]
+        connect mem_w1.clk, clk @[module-XXXXXXXXXX.rs 12:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect mem_w1.addr, `write`.addr @[module-XXXXXXXXXX.rs 13:1]
+        connect mem_w1.data, `write`.data @[module-XXXXXXXXXX.rs 14:1]
+        connect mem_w1.en, `write`.en @[module-XXXXXXXXXX.rs 15:1]
+        connect mem_w1.mask, `write`.mask @[module-XXXXXXXXXX.rs 16:1]
+"#,
+        "/test/sim_trace_as_string/mem.mem": r"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_memories: false,
+            simplify_enums: Some(SimplifyEnumsKind::SimplifyToEnumsWithNoBody),
+            ..ExportOptions::default()
+        },
+        "/test/sim_trace_as_string.fir": r#"FIRRTL version 3.2.0
+circuit sim_trace_as_string: %[[
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem.mem",
+    "hexOrBinary": "b",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem"
+  }
+]]
+    type Ty0 = {|Text, FmtError|}
+    type Ty1 = {tag: Ty0, body: UInt<512>}
+    type Ty2 = {addr: UInt<8>, en: UInt<1>, clk: Clock, flip data: Ty1[2]}
+    type Ty3 = {addr: UInt<8>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: UInt<1>[2]}
+    type Ty4 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: UInt<1>[2]}
+    type Ty5 = {tag: UInt<1>, body: UInt<1>}
+    type Ty6 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: Ty5[2]}
+    type Ty7 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: Ty1[2]}
+    module sim_trace_as_string: @[module-XXXXXXXXXX.rs 1:1]
+        input clk: Clock @[module-XXXXXXXXXX.rs 2:1]
+        input `read`: Ty2 @[module-XXXXXXXXXX.rs 3:1]
+        input `write`: Ty3 @[module-XXXXXXXXXX.rs 4:1]
+        mem `mem`: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => Ty1[2]
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        wire mem_w1: Ty4 @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.data, mem_w1.data @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[0].tag, mem_w1.mask[0] @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[0].body, mem_w1.mask[0] @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[1].tag, mem_w1.mask[1] @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.w1.mask[1].body, mem_w1.mask[1] @[module-XXXXXXXXXX.rs 11:1]
+        connect `mem`.r0.clk, clk @[module-XXXXXXXXXX.rs 7:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect `mem`.r0.addr, `read`.addr @[module-XXXXXXXXXX.rs 8:1]
+        connect `mem`.r0.en, `read`.en @[module-XXXXXXXXXX.rs 9:1]
+        connect `read`.data[0], `mem`.r0.data[0] @[module-XXXXXXXXXX.rs 10:1]
+        connect `read`.data[1], `mem`.r0.data[1] @[module-XXXXXXXXXX.rs 10:1]
+        connect mem_w1.clk, clk @[module-XXXXXXXXXX.rs 12:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect mem_w1.addr, `write`.addr @[module-XXXXXXXXXX.rs 13:1]
+        connect mem_w1.data, `write`.data @[module-XXXXXXXXXX.rs 14:1]
+        connect mem_w1.en, `write`.en @[module-XXXXXXXXXX.rs 15:1]
+        connect mem_w1.mask, `write`.mask @[module-XXXXXXXXXX.rs 16:1]
+"#,
+        "/test/sim_trace_as_string/mem.mem": r"000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+",
+    };
+    #[rustfmt::skip] // work around https://github.com/rust-lang/rustfmt/issues/6161
+    assert_export_firrtl! {
+        m =>
+        options: ExportOptions {
+            simplify_memories: true,
+            simplify_enums: Some(SimplifyEnumsKind::SimplifyToEnumsWithNoBody),
+            ..ExportOptions::default()
+        },
+        "/test/sim_trace_as_string.fir": r#"FIRRTL version 3.2.0
+circuit sim_trace_as_string: %[[
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem_0_tag.mem",
+    "hexOrBinary": "b",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem_0_tag"
+  },
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem_0_body.mem",
+    "hexOrBinary": "h",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem_0_body"
+  },
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem_1_tag.mem",
+    "hexOrBinary": "b",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem_1_tag"
+  },
+  {
+    "class": "firrtl.annotations.MemoryFileInlineAnnotation",
+    "filename": "/test/sim_trace_as_string/mem_1_body.mem",
+    "hexOrBinary": "h",
+    "target": "~sim_trace_as_string|sim_trace_as_string>mem_1_body"
+  }
+]]
+    type Ty0 = {|Text, FmtError|}
+    type Ty1 = {tag: Ty0, body: UInt<512>}
+    type Ty2 = {addr: UInt<8>, en: UInt<1>, clk: Clock, flip data: Ty1[2]}
+    type Ty3 = {addr: UInt<8>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: UInt<1>[2]}
+    type Ty4 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: Ty1[2]}
+    type Ty5 = {tag: UInt<1>, body: UInt<1>}
+    type Ty6 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: Ty5[2]}
+    type Ty7 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: UInt<1>}
+    type Ty8 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: UInt<1>, mask: UInt<1>}
+    type Ty9 = {addr: UInt<2>, en: UInt<1>, clk: Clock, flip data: UInt<512>}
+    type Ty10 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: UInt<512>, mask: UInt<1>}
+    type Ty11 = {addr: UInt<2>, en: UInt<1>, clk: Clock, data: Ty1[2], mask: UInt<1>[2]}
+    module sim_trace_as_string: @[module-XXXXXXXXXX.rs 1:1]
+        input clk: Clock @[module-XXXXXXXXXX.rs 2:1]
+        input `read`: Ty2 @[module-XXXXXXXXXX.rs 3:1]
+        input `write`: Ty3 @[module-XXXXXXXXXX.rs 4:1]
+        mem mem_0_tag: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => UInt<1>
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        mem mem_0_body: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => UInt<512>
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        mem mem_1_tag: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => UInt<1>
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        mem mem_1_body: @[module-XXXXXXXXXX.rs 5:1]
+            data-type => UInt<512>
+            depth => 4
+            read-latency => 0
+            write-latency => 1
+            read-under-write => old
+            reader => r0
+            writer => w1
+        wire mem_r0: Ty4 @[module-XXXXXXXXXX.rs 6:1]
+        wire mem_w1: Ty6 @[module-XXXXXXXXXX.rs 11:1]
+        wire _cast_bits_to_enum_expr: Ty0
+        when eq(UInt<1>(0), tail(mem_0_tag.r0.data, 0)):
+            connect _cast_bits_to_enum_expr, {|Text, FmtError|}(Text)
+        else:
+            connect _cast_bits_to_enum_expr, {|Text, FmtError|}(FmtError)
+        connect mem_r0.data[0].tag, _cast_bits_to_enum_expr @[module-XXXXXXXXXX.rs 6:1]
+        wire _cast_enum_to_bits_expr: UInt<1>
+        match mem_w1.data[0].tag:
+            Text:
+                connect _cast_enum_to_bits_expr, UInt<1>(0)
+            FmtError:
+                connect _cast_enum_to_bits_expr, UInt<1>(1)
+        connect mem_0_tag.w1.data, _cast_enum_to_bits_expr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_tag.w1.mask, mem_w1.mask[0].tag @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_tag.r0.addr, mem_r0.addr @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_tag.r0.clk, mem_r0.clk @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_tag.r0.en, mem_r0.en @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_tag.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_tag.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_tag.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_r0.data[0].body, mem_0_body.r0.data @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_body.w1.data, mem_w1.data[0].body @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_body.w1.mask, mem_w1.mask[0].body @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_body.r0.addr, mem_r0.addr @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_body.r0.clk, mem_r0.clk @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_body.r0.en, mem_r0.en @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_0_body.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_body.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_0_body.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        wire _cast_bits_to_enum_expr_1: Ty0
+        when eq(UInt<1>(0), tail(mem_1_tag.r0.data, 0)):
+            connect _cast_bits_to_enum_expr_1, {|Text, FmtError|}(Text)
+        else:
+            connect _cast_bits_to_enum_expr_1, {|Text, FmtError|}(FmtError)
+        connect mem_r0.data[1].tag, _cast_bits_to_enum_expr_1 @[module-XXXXXXXXXX.rs 6:1]
+        wire _cast_enum_to_bits_expr_1: UInt<1>
+        match mem_w1.data[1].tag:
+            Text:
+                connect _cast_enum_to_bits_expr_1, UInt<1>(0)
+            FmtError:
+                connect _cast_enum_to_bits_expr_1, UInt<1>(1)
+        connect mem_1_tag.w1.data, _cast_enum_to_bits_expr_1 @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_tag.w1.mask, mem_w1.mask[1].tag @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_tag.r0.addr, mem_r0.addr @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_tag.r0.clk, mem_r0.clk @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_tag.r0.en, mem_r0.en @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_tag.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_tag.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_tag.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_r0.data[1].body, mem_1_body.r0.data @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_body.w1.data, mem_w1.data[1].body @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_body.w1.mask, mem_w1.mask[1].body @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_body.r0.addr, mem_r0.addr @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_body.r0.clk, mem_r0.clk @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_body.r0.en, mem_r0.en @[module-XXXXXXXXXX.rs 6:1]
+        connect mem_1_body.w1.addr, mem_w1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_body.w1.clk, mem_w1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_1_body.w1.en, mem_w1.en @[module-XXXXXXXXXX.rs 11:1]
+        wire mem_w1_1: Ty11 @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.addr, mem_w1_1.addr @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.en, mem_w1_1.en @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.clk, mem_w1_1.clk @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.data, mem_w1_1.data @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.mask[0].tag, mem_w1_1.mask[0] @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.mask[0].body, mem_w1_1.mask[0] @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.mask[1].tag, mem_w1_1.mask[1] @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_w1.mask[1].body, mem_w1_1.mask[1] @[module-XXXXXXXXXX.rs 11:1]
+        connect mem_r0.clk, clk @[module-XXXXXXXXXX.rs 7:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect mem_r0.addr, `read`.addr @[module-XXXXXXXXXX.rs 8:1]
+        connect mem_r0.en, `read`.en @[module-XXXXXXXXXX.rs 9:1]
+        connect `read`.data[0], mem_r0.data[0] @[module-XXXXXXXXXX.rs 10:1]
+        connect `read`.data[1], mem_r0.data[1] @[module-XXXXXXXXXX.rs 10:1]
+        connect mem_w1_1.clk, clk @[module-XXXXXXXXXX.rs 12:1]
+        ; connect different types:
+        ; lhs: UInt<2>
+        ; rhs: UInt<8>
+        connect mem_w1_1.addr, `write`.addr @[module-XXXXXXXXXX.rs 13:1]
+        connect mem_w1_1.data, `write`.data @[module-XXXXXXXXXX.rs 14:1]
+        connect mem_w1_1.en, `write`.en @[module-XXXXXXXXXX.rs 15:1]
+        connect mem_w1_1.mask, `write`.mask @[module-XXXXXXXXXX.rs 16:1]
+"#,
+        "/test/sim_trace_as_string/mem_0_body.mem": r"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+",
+        "/test/sim_trace_as_string/mem_0_tag.mem": r"0
+0
+0
+0
+",
+        "/test/sim_trace_as_string/mem_1_body.mem": r"00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
+",
+        "/test/sim_trace_as_string/mem_1_tag.mem": r"0
+0
+0
+0
+",
+    };
+}

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

@@ -0,0 +1,221 @@
+$timescale 1 ps $end
+$scope module sim_trace_as_string $end
+$var wire 1 J(7*b clk $end
+$scope struct read $end
+$var wire 8 @t0}\ addr $end
+$var wire 1 78"T5 en $end
+$var wire 1 G7v@m clk $end
+$var string 1 F&^FN data $end
+$upscope $end
+$scope struct write $end
+$var wire 8 "fUdW addr $end
+$var wire 1 r1OK) en $end
+$var wire 1 ,ADvU clk $end
+$scope struct data $end
+$var string 1 pD.mP \[0] $end
+$var string 1 !V!em \[1] $end
+$upscope $end
+$scope struct mask $end
+$var wire 1 l8dgD \[0] $end
+$var wire 1 1/sDs \[1] $end
+$upscope $end
+$upscope $end
+$scope struct mem $end
+$scope struct contents $end
+$scope struct \[0] $end
+$scope struct mem $end
+$var string 1 sz>#| \[0] $end
+$var string 1 G._83 \[1] $end
+$upscope $end
+$upscope $end
+$scope struct \[1] $end
+$scope struct mem $end
+$var string 1 2r3#W \[0] $end
+$var string 1 AbGF% \[1] $end
+$upscope $end
+$upscope $end
+$scope struct \[2] $end
+$scope struct mem $end
+$var string 1 .^<$p \[0] $end
+$var string 1 ?s@Dc \[1] $end
+$upscope $end
+$upscope $end
+$scope struct \[3] $end
+$scope struct mem $end
+$var string 1 {*||o \[0] $end
+$var string 1 Bg,vB \[1] $end
+$upscope $end
+$upscope $end
+$upscope $end
+$scope struct r0 $end
+$var wire 2 .0()- addr $end
+$var wire 1 GEbRA en $end
+$var wire 1 ;`9BK clk $end
+$scope struct data $end
+$var string 1 _Xe"P \[0] $end
+$var string 1 jXrsx \[1] $end
+$upscope $end
+$upscope $end
+$scope struct w1 $end
+$var wire 2 '8u?z addr $end
+$var wire 1 ~o=`& en $end
+$var wire 1 *q>M1 clk $end
+$scope struct data $end
+$var string 1 N\zBe \[0] $end
+$var string 1 c3h8{ \[1] $end
+$upscope $end
+$scope struct mask $end
+$var wire 1 .SYGD \[0] $end
+$var wire 1 />wYd \[1] $end
+$upscope $end
+$upscope $end
+$upscope $end
+$upscope $end
+$enddefinitions $end
+$dumpvars
+s sz>#|
+s G._83
+s 2r3#W
+s AbGF%
+s .^<$p
+s ?s@Dc
+s {*||o
+s Bg,vB
+0J(7*b
+b0 @t0}\
+078"T5
+0G7v@m
+s[,\x20] F&^FN
+b0 "fUdW
+0r1OK)
+0,ADvU
+s pD.mP
+s !V!em
+0l8dgD
+01/sDs
+b0 .0()-
+0GEbRA
+0;`9BK
+s _Xe"P
+s jXrsx
+b0 '8u?z
+0~o=`&
+0*q>M1
+s N\zBe
+s c3h8{
+0.SYGD
+0/>wYd
+$end
+#500000
+1J(7*b
+1;`9BK
+1*q>M1
+#1000000
+0J(7*b
+1r1OK)
+smem[0][0] pD.mP
+smem[0][1] !V!em
+1l8dgD
+11/sDs
+0;`9BK
+1~o=`&
+0*q>M1
+smem[0][0] N\zBe
+smem[0][1] c3h8{
+1.SYGD
+1/>wYd
+#1500000
+smem[0][0] sz>#|
+smem[0][1] G._83
+1J(7*b
+1;`9BK
+1*q>M1
+#2000000
+0J(7*b
+b1 "fUdW
+smem[1][0] pD.mP
+smem[1][1] !V!em
+0;`9BK
+b1 '8u?z
+0*q>M1
+smem[1][0] N\zBe
+smem[1][1] c3h8{
+#2500000
+smem[1][0] 2r3#W
+smem[1][1] AbGF%
+1J(7*b
+1;`9BK
+1*q>M1
+#3000000
+0J(7*b
+b10 "fUdW
+smem[2][0] pD.mP
+smem[2][1] !V!em
+0;`9BK
+b10 '8u?z
+0*q>M1
+smem[2][0] N\zBe
+smem[2][1] c3h8{
+#3500000
+smem[2][0] .^<$p
+smem[2][1] ?s@Dc
+1J(7*b
+1;`9BK
+1*q>M1
+#4000000
+0J(7*b
+b11 "fUdW
+smem[3][0] pD.mP
+smem[3][1] !V!em
+0;`9BK
+b11 '8u?z
+0*q>M1
+smem[3][0] N\zBe
+smem[3][1] c3h8{
+#4500000
+smem[3][0] {*||o
+smem[3][1] Bg,vB
+1J(7*b
+1;`9BK
+1*q>M1
+#5000000
+0J(7*b
+b1 @t0}\
+178"T5
+s[mem[1][0],\x20mem[1][1]] F&^FN
+b0 "fUdW
+0r1OK)
+s<!!!failed\x20to\x20format!!!> pD.mP
+s<!!!failed\x20to\x20format!!!> !V!em
+b1 .0()-
+1GEbRA
+0;`9BK
+smem[1][0] _Xe"P
+smem[1][1] jXrsx
+b0 '8u?z
+0~o=`&
+0*q>M1
+s<!!!failed\x20to\x20format!!!> N\zBe
+s<!!!failed\x20to\x20format!!!> c3h8{
+#5500000
+1J(7*b
+1;`9BK
+1*q>M1
+#6000000
+0J(7*b
+b1 "fUdW
+1r1OK)
+0;`9BK
+b1 '8u?z
+1~o=`&
+0*q>M1
+#6500000
+s<!!!failed\x20to\x20format!!!> 2r3#W
+s<!!!failed\x20to\x20format!!!> AbGF%
+1J(7*b
+s<!!!failed\x20to\x20format!!!> F&^FN
+1;`9BK
+s<!!!failed\x20to\x20format!!!> _Xe"P
+s<!!!failed\x20to\x20format!!!> jXrsx
+1*q>M1
+#7000000

crates/fayalite/tests/ui/simvalue_is_not_internable.stderr

@@ -75,7 +75,7 @@ note: required because it appears within the type `Vec<DynSimOnlyValue>`
 note: required because it appears within the type `OpaqueSimValue`
     --> src/ty.rs
      |
- 761 | pub struct OpaqueSimValue {
+ 895 | pub struct OpaqueSimValue {
      |            ^^^^^^^^^^^^^^
 note: required because it appears within the type `value::SimValueInner<()>`
     --> src/sim/value.rs
@@ -214,7 +214,7 @@ note: required because it appears within the type `Vec<DynSimOnlyValue>`
 note: required because it appears within the type `OpaqueSimValue`
     --> src/ty.rs
      |
- 761 | pub struct OpaqueSimValue {
+ 895 | pub struct OpaqueSimValue {
      |            ^^^^^^^^^^^^^^
 note: required because it appears within the type `value::SimValueInner<()>`
     --> src/sim/value.rs
@@ -326,7 +326,7 @@ note: required because it appears within the type `Vec<DynSimOnlyValue>`
 note: required because it appears within the type `OpaqueSimValue`
     --> src/ty.rs
      |
- 761 | pub struct OpaqueSimValue {
+ 895 | pub struct OpaqueSimValue {
      |            ^^^^^^^^^^^^^^
 note: required because it appears within the type `value::SimValueInner<()>`
     --> src/sim/value.rs

crates/fayalite/visit_types.json

@@ -51,7 +51,8 @@
                 "Reset": "Visible",
                 "Clock": "Visible",
                 "PhantomConst": "Visible",
-                "DynSimOnly": "Visible"
+                "DynSimOnly": "Visible",
+                "TraceAsString": "Visible"
             }
         },
         "Bundle": {
@@ -1021,6 +1022,27 @@
             "fold_where": "T: Fold<State>",
             "visit_where": "T: Visit<State>"
         },
+        "ops::AsTraceAsString": {
+            "data": {
+                "$kind": "Struct",
+                "$constructor": "ops::AsTraceAsString::new",
+                "inner()": "Visible",
+                "ty()": "Visible"
+            },
+            "generics": "<T: Type>",
+            "fold_where": "T: Fold<State>",
+            "visit_where": "T: Visit<State>"
+        },
+        "ops::TraceAsStringAsInner": {
+            "data": {
+                "$kind": "Struct",
+                "$constructor": "ops::TraceAsStringAsInner::new",
+                "arg_typed()": "Visible"
+            },
+            "generics": "<T: Type>",
+            "fold_where": "T: Fold<State>",
+            "visit_where": "T: Visit<State>"
+        },
         "BlockId": {
             "data": {
                 "$kind": "Opaque"
@@ -1283,12 +1305,25 @@
                 "$kind": "Struct"
             }
         },
+        "TargetPathTraceAsStringInner": {
+            "data": {
+                "$kind": "Struct"
+            }
+        },
+        "TargetPathAsTraceAsString": {
+            "data": {
+                "$kind": "Struct",
+                "ty": "Visible"
+            }
+        },
         "TargetPathElement": {
             "data": {
                 "$kind": "Enum",
                 "BundleField": "Visible",
                 "ArrayElement": "Visible",
-                "DynArrayElement": "Visible"
+                "DynArrayElement": "Visible",
+                "TraceAsStringInner": "Visible",
+                "AsTraceAsString": "Visible"
             }
         },
         "PhantomConst": {
@@ -1306,6 +1341,14 @@
             "data": {
                 "$kind": "ManualImpl"
             }
+        },
+        "TraceAsString": {
+            "data": {
+                "$kind": "ManualImpl"
+            },
+            "generics": "<T: Type>",
+            "fold_where": "T: Fold<State>",
+            "visit_where": "T: Visit<State>"
         }
     }
 }