add PhantomConst

crates/fayalite/src/expr.rs

@@ -16,6 +16,7 @@ use crate::{
         transform::visit::{Fold, Folder, Visit, Visitor},
         Instance, ModuleIO,
     },
+    phantom_const::PhantomConst,
     reg::Reg,
     reset::{AsyncReset, Reset, ResetType, ResetTypeDispatch, SyncReset},
     ty::{CanonicalType, StaticType, Type, TypeWithDeref},
@@ -109,6 +110,7 @@ expr_enum! {
         UIntLiteral(Interned<UIntValue>),
         SIntLiteral(Interned<SIntValue>),
         BoolLiteral(bool),
+        PhantomConst(PhantomConst),
         BundleLiteral(ops::BundleLiteral),
         ArrayLiteral(ops::ArrayLiteral<CanonicalType, DynSize>),
         EnumLiteral(ops::EnumLiteral),
@@ -755,3 +757,27 @@ pub fn repeat<T: Type, L: SizeType>(
     )
     .to_expr()
 }
+
+impl<T: ?Sized + crate::phantom_const::PhantomConstValue> ToExpr for PhantomConst<T> {
+    type Type = Self;
+
+    fn to_expr(&self) -> Expr<Self::Type> {
+        Expr {
+            __enum: ExprEnum::PhantomConst(self.canonical_phantom_const()).intern_sized(),
+            __ty: *self,
+            __flow: Flow::Source,
+        }
+    }
+}
+
+impl<T: ?Sized + crate::phantom_const::PhantomConstValue> GetTarget for PhantomConst<T> {
+    fn target(&self) -> Option<Interned<Target>> {
+        None
+    }
+}
+
+impl<T: ?Sized + crate::phantom_const::PhantomConstValue> ToLiteralBits for PhantomConst<T> {
+    fn to_literal_bits(&self) -> Result<Interned<BitSlice>, NotALiteralExpr> {
+        Ok(Interned::default())
+    }
+}

crates/fayalite/src/expr/ops.rs

@@ -11,14 +11,15 @@ use crate::{
             GetTarget, Target, TargetPathArrayElement, TargetPathBundleField,
             TargetPathDynArrayElement, TargetPathElement,
         },
-        CastTo, Expr, ExprEnum, Flow, HdlPartialEq, HdlPartialOrd, NotALiteralExpr, ReduceBits,
-        ToExpr, ToLiteralBits,
+        CastBitsTo as _, CastTo, CastToBits as _, Expr, ExprEnum, Flow, HdlPartialEq,
+        HdlPartialOrd, NotALiteralExpr, ReduceBits, ToExpr, ToLiteralBits,
     },
     int::{
         Bool, BoolOrIntType, DynSize, IntType, KnownSize, SInt, SIntType, SIntValue, Size, UInt,
         UIntType, UIntValue,
     },
     intern::{Intern, Interned},
+    phantom_const::{PhantomConst, PhantomConstValue},
     reset::{
         AsyncReset, Reset, ResetType, ResetTypeDispatch, SyncReset, ToAsyncReset, ToReset,
         ToSyncReset,
@@ -1892,6 +1893,26 @@ impl ExprCastTo<Clock> for Clock {
     }
 }
 
+impl<T: ?Sized + PhantomConstValue> ExprCastTo<()> for PhantomConst<T> {
+    fn cast_to(src: Expr<Self>, to_type: ()) -> Expr<()> {
+        src.cast_to_bits().cast_bits_to(to_type)
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> ExprCastTo<PhantomConst<T>> for () {
+    fn cast_to(src: Expr<Self>, to_type: PhantomConst<T>) -> Expr<PhantomConst<T>> {
+        src.cast_to_bits().cast_bits_to(to_type)
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue, U: ?Sized + PhantomConstValue> ExprCastTo<PhantomConst<T>>
+    for PhantomConst<U>
+{
+    fn cast_to(src: Expr<Self>, to_type: PhantomConst<T>) -> Expr<PhantomConst<T>> {
+        src.cast_to_bits().cast_bits_to(to_type)
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
 pub struct FieldAccess<FieldType: Type = CanonicalType> {
     base: Expr<Bundle>,

crates/fayalite/src/firrtl.rs

@@ -15,7 +15,7 @@ use crate::{
         target::{
             Target, TargetBase, TargetPathArrayElement, TargetPathBundleField, TargetPathElement,
         },
-        Expr, ExprEnum,
+        CastBitsTo, Expr, ExprEnum,
     },
     formal::FormalKind,
     int::{Bool, DynSize, IntType, SIntValue, UInt, UIntValue},
@@ -447,6 +447,7 @@ impl TypeState {
             CanonicalType::AsyncReset(AsyncReset {}) => "AsyncReset".into(),
             CanonicalType::SyncReset(SyncReset {}) => "UInt<1>".into(),
             CanonicalType::Reset(Reset {}) => "Reset".into(),
+            CanonicalType::PhantomConst(_) => "{}".into(),
         }
     }
 }
@@ -1152,6 +1153,7 @@ impl<'a> Exporter<'a> {
             | CanonicalType::Clock(_)
             | CanonicalType::AsyncReset(_)
             | CanonicalType::Reset(_) => format!("asUInt({value_str})"),
+            CanonicalType::PhantomConst(_) => "UInt<0>(0)".into(),
         }
     }
     fn expr_cast_bits_to_bundle(
@@ -1357,6 +1359,12 @@ impl<'a> Exporter<'a> {
             CanonicalType::AsyncReset(_) => format!("asAsyncReset({value_str})"),
             CanonicalType::SyncReset(_) => value_str,
             CanonicalType::Reset(_) => unreachable!("Reset is not bit castable to"),
+            CanonicalType::PhantomConst(_) => {
+                let retval = self.module.ns.make_new("_cast_bits_to_phantom_const_expr");
+                definitions.add_definition_line(format_args!("{extra_indent}wire {retval}: {{}}"));
+                definitions.add_definition_line(format_args!("{extra_indent}invalidate {retval}"));
+                return retval.to_string();
+            }
         }
     }
     fn expr_unary<T: Type>(
@@ -1395,6 +1403,11 @@ impl<'a> Exporter<'a> {
             ExprEnum::UIntLiteral(literal) => self.uint_literal(&literal),
             ExprEnum::SIntLiteral(literal) => self.sint_literal(&literal),
             ExprEnum::BoolLiteral(literal) => self.bool_literal(literal),
+            ExprEnum::PhantomConst(ty) => self.expr(
+                UInt[0].zero().cast_bits_to(ty.canonical()),
+                definitions,
+                const_ty,
+            ),
             ExprEnum::ArrayLiteral(array_literal) => {
                 self.array_literal_expr(array_literal, definitions, const_ty)
             }

crates/fayalite/src/lib.rs

@@ -96,6 +96,7 @@ pub mod int;
 pub mod intern;
 pub mod memory;
 pub mod module;
+pub mod phantom_const;
 pub mod prelude;
 pub mod reg;
 pub mod reset;

crates/fayalite/src/memory.rs

@@ -1082,6 +1082,7 @@ 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())),
     }
 }
 

crates/fayalite/src/module.rs

@@ -1490,6 +1490,9 @@ impl TargetState {
                         })
                         .collect(),
                 },
+                CanonicalType::PhantomConst(_) => TargetStateInner::Decomposed {
+                    subtargets: HashMap::new(),
+                },
                 CanonicalType::Array(ty) => TargetStateInner::Decomposed {
                     subtargets: (0..ty.len())
                         .map(|index| {

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

@@ -155,6 +155,7 @@ impl ResetsLayout {
                     CanonicalType::SyncReset(_) => ResetsLayout::SyncReset,
                     CanonicalType::Reset(_) => ResetsLayout::Reset,
                     CanonicalType::Clock(_) => ResetsLayout::NoResets,
+                    CanonicalType::PhantomConst(_) => ResetsLayout::NoResets,
                 }
             }
         }
@@ -407,7 +408,8 @@ impl Resets {
             | CanonicalType::Bool(_)
             | CanonicalType::AsyncReset(_)
             | CanonicalType::SyncReset(_)
-            | CanonicalType::Clock(_) => Ok(self.ty),
+            | CanonicalType::Clock(_)
+            | CanonicalType::PhantomConst(_) => Ok(self.ty),
             CanonicalType::Array(ty) => Ok(CanonicalType::Array(Array::new_dyn(
                 self.array_elements().substituted_type(
                     reset_graph,
@@ -998,7 +1000,8 @@ fn cast_bit_op<P: Pass, T: Type, A: Type>(
                             CanonicalType::Array(_)
                             | CanonicalType::Enum(_)
                             | CanonicalType::Bundle(_)
-                            | CanonicalType::Reset(_) => unreachable!(),
+                            | CanonicalType::Reset(_)
+                            | CanonicalType::PhantomConst(_) => unreachable!(),
                             $(CanonicalType::$Variant(ty) => Expr::expr_enum($arg.cast_to(ty)),)*
                         }
                     };
@@ -1010,6 +1013,7 @@ fn cast_bit_op<P: Pass, T: Type, A: Type>(
                             | CanonicalType::Enum(_)
                             | CanonicalType::Bundle(_)
                             | CanonicalType::Reset(_) => unreachable!(),
+                            CanonicalType::PhantomConst(_) => Expr::expr_enum(arg),
                             $(CanonicalType::$Variant(_) => {
                                 let arg = Expr::<$Variant>::from_canonical(arg);
                                 match_expr_ty!(arg, UInt, SInt, Bool, AsyncReset, SyncReset, Clock)
@@ -1040,6 +1044,7 @@ impl<P: Pass> RunPass<P> for ExprEnum {
             ExprEnum::UIntLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::SIntLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::BoolLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
+            ExprEnum::PhantomConst(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::BundleLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::ArrayLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
             ExprEnum::EnumLiteral(expr) => Ok(expr.run_pass(pass_args)?.map(ExprEnum::from)),
@@ -1670,7 +1675,8 @@ impl RunPassDispatch for AnyReg {
                 | CanonicalType::Enum(_)
                 | CanonicalType::Bundle(_)
                 | CanonicalType::Reset(_)
-                | CanonicalType::Clock(_) => unreachable!(),
+                | CanonicalType::Clock(_)
+                | CanonicalType::PhantomConst(_) => unreachable!(),
             }
         })
     }
@@ -1769,6 +1775,7 @@ impl_run_pass_copy!([] SVAttributeAnnotation);
 impl_run_pass_copy!([] UInt);
 impl_run_pass_copy!([] usize);
 impl_run_pass_copy!([] FormalKind);
+impl_run_pass_copy!([] PhantomConst);
 
 macro_rules! impl_run_pass_for_struct {
     (

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

@@ -69,7 +69,8 @@ fn contains_any_enum_types(ty: CanonicalType) -> bool {
                 | CanonicalType::AsyncReset(_)
                 | CanonicalType::SyncReset(_)
                 | CanonicalType::Reset(_)
-                | CanonicalType::Clock(_) => false,
+                | CanonicalType::Clock(_)
+                | CanonicalType::PhantomConst(_) => false,
             }
         }
     }
@@ -512,7 +513,8 @@ impl State {
             | CanonicalType::AsyncReset(_)
             | CanonicalType::SyncReset(_)
             | CanonicalType::Reset(_)
-            | CanonicalType::Clock(_) => unreachable!(),
+            | CanonicalType::Clock(_)
+            | CanonicalType::PhantomConst(_) => unreachable!(),
         }
     }
 }
@@ -577,7 +579,8 @@ fn connect_port(
         | (CanonicalType::Clock(_), _)
         | (CanonicalType::AsyncReset(_), _)
         | (CanonicalType::SyncReset(_), _)
-        | (CanonicalType::Reset(_), _) => unreachable!(
+        | (CanonicalType::Reset(_), _)
+        | (CanonicalType::PhantomConst(_), _) => unreachable!(
             "trying to connect memory ports:\n{:?}\n{:?}",
             Expr::ty(lhs),
             Expr::ty(rhs),
@@ -665,6 +668,7 @@ impl Folder for State {
             ExprEnum::UIntLiteral(_)
             | ExprEnum::SIntLiteral(_)
             | ExprEnum::BoolLiteral(_)
+            | ExprEnum::PhantomConst(_)
             | ExprEnum::BundleLiteral(_)
             | ExprEnum::ArrayLiteral(_)
             | ExprEnum::Uninit(_)
@@ -923,7 +927,8 @@ impl Folder for State {
             | CanonicalType::Clock(_)
             | CanonicalType::AsyncReset(_)
             | CanonicalType::SyncReset(_)
-            | CanonicalType::Reset(_) => canonical_type.default_fold(self),
+            | CanonicalType::Reset(_)
+            | CanonicalType::PhantomConst(_) => canonical_type.default_fold(self),
         }
     }
 

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

@@ -62,6 +62,7 @@ enum MemSplit {
     Bundle {
         fields: Rc<[MemSplit]>,
     },
+    PhantomConst,
     Single {
         output_mem: Option<Mem>,
         element_type: SingleType,
@@ -76,6 +77,7 @@ impl MemSplit {
     fn mark_changed_element_type(self) -> Self {
         match self {
             MemSplit::Bundle { fields: _ } => self,
+            MemSplit::PhantomConst => self,
             MemSplit::Single {
                 output_mem,
                 element_type,
@@ -97,6 +99,7 @@ impl MemSplit {
                     .map(|field| Self::new(field.ty).mark_changed_element_type())
                     .collect(),
             },
+            CanonicalType::PhantomConst(_) => MemSplit::PhantomConst,
             CanonicalType::Array(ty) => {
                 let element = MemSplit::new(ty.element());
                 if let Self::Single {
@@ -339,6 +342,7 @@ impl SplitMemState<'_, '_> {
                     self.split_state_stack.pop();
                 }
             }
+            MemSplit::PhantomConst => {}
             MemSplit::Single {
                 output_mem,
                 element_type: single_type,
@@ -538,7 +542,12 @@ impl ModuleState {
             };
         loop {
             match input_element_type {
-                CanonicalType::Bundle(_) => unreachable!("bundle types are always split"),
+                CanonicalType::Bundle(_) => {
+                    unreachable!("bundle types are always split")
+                }
+                CanonicalType::PhantomConst(_) => {
+                    unreachable!("PhantomConst are always removed")
+                }
                 CanonicalType::Enum(_)
                     if input_array_types
                         .first()
@@ -743,7 +752,8 @@ impl ModuleState {
                 ..
             }
             | MemSplit::Bundle { .. }
-            | MemSplit::Array { .. } => {
+            | MemSplit::Array { .. }
+            | MemSplit::PhantomConst => {
                 let mut replacement_ports = Vec::with_capacity(input_mem.ports().len());
                 let mut wire_port_rdata = Vec::with_capacity(input_mem.ports().len());
                 let mut wire_port_wdata = Vec::with_capacity(input_mem.ports().len());

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

@@ -28,6 +28,7 @@ use crate::{
         NormalModuleBody, ScopedNameId, Stmt, StmtConnect, StmtDeclaration, StmtFormal, StmtIf,
         StmtInstance, StmtMatch, StmtReg, StmtWire,
     },
+    phantom_const::PhantomConst,
     reg::Reg,
     reset::{AsyncReset, Reset, ResetType, SyncReset},
     source_location::SourceLocation,

crates/fayalite/src/phantom_const.rs

@@ -0,0 +1,273 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use serde::{de::DeserializeOwned, Deserialize, Serialize};
+
+use crate::{
+    intern::{Intern, Interned, InternedCompare, LazyInterned, Memoize},
+    source_location::SourceLocation,
+    ty::{impl_match_variant_as_self, CanonicalType, StaticType, Type, TypeProperties},
+};
+use std::{
+    any::Any,
+    fmt,
+    hash::{Hash, Hasher},
+    marker::PhantomData,
+};
+
+#[derive(Clone)]
+pub struct PhantomConstCanonicalValue {
+    parsed: serde_json::Value,
+    serialized: Interned<str>,
+}
+
+impl PhantomConstCanonicalValue {
+    pub fn from_json_value(parsed: serde_json::Value) -> Self {
+        let serialized = Intern::intern_owned(
+            serde_json::to_string(&parsed)
+                .expect("conversion from json value to text shouldn't fail"),
+        );
+        Self { parsed, serialized }
+    }
+    pub fn as_json_value(&self) -> &serde_json::Value {
+        &self.parsed
+    }
+    pub fn as_str(&self) -> Interned<str> {
+        self.serialized
+    }
+}
+
+impl fmt::Debug for PhantomConstCanonicalValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.serialized)
+    }
+}
+
+impl fmt::Display for PhantomConstCanonicalValue {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.serialized)
+    }
+}
+
+impl PartialEq for PhantomConstCanonicalValue {
+    fn eq(&self, other: &Self) -> bool {
+        self.serialized == other.serialized
+    }
+}
+
+impl Eq for PhantomConstCanonicalValue {}
+
+impl Hash for PhantomConstCanonicalValue {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.serialized.hash(state);
+    }
+}
+
+impl Serialize for PhantomConstCanonicalValue {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        self.parsed.serialize(serializer)
+    }
+}
+
+impl<'de> Deserialize<'de> for PhantomConstCanonicalValue {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        Ok(Self::from_json_value(serde_json::Value::deserialize(
+            deserializer,
+        )?))
+    }
+}
+
+pub trait PhantomConstValue: Intern + InternedCompare + Serialize + fmt::Debug {
+    fn deserialize<'de, D>(deserializer: D) -> Result<Interned<Self>, D::Error>
+    where
+        D: serde::Deserializer<'de>;
+}
+
+impl<T> PhantomConstValue for T
+where
+    T: ?Sized + Intern + InternedCompare + Serialize + fmt::Debug,
+    Interned<T>: DeserializeOwned,
+{
+    fn deserialize<'de, D>(deserializer: D) -> Result<Interned<Self>, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        <Interned<T> as Deserialize<'de>>::deserialize(deserializer)
+    }
+}
+
+/// Wrapper type that allows any Rust value to be smuggled as a HDL [`Type`].
+/// This only works for values that can be [serialized][Serialize] to and [deserialized][Deserialize] from [JSON][serde_json].
+pub struct PhantomConst<T: ?Sized + PhantomConstValue = PhantomConstCanonicalValue> {
+    value: LazyInterned<T>,
+}
+
+impl<T: ?Sized + PhantomConstValue> fmt::Debug for PhantomConst<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("PhantomConst").field(&self.get()).finish()
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> Clone for PhantomConst<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> Copy for PhantomConst<T> {}
+
+impl<T: ?Sized + PhantomConstValue> PartialEq for PhantomConst<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.get() == other.get()
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> Eq for PhantomConst<T> {}
+
+impl<T: ?Sized + PhantomConstValue> Hash for PhantomConst<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.get().hash(state);
+    }
+}
+
+struct PhantomConstCanonicalMemoize<T: ?Sized, const IS_FROM_CANONICAL: bool>(PhantomData<T>);
+
+impl<T: ?Sized, const IS_FROM_CANONICAL: bool> Copy
+    for PhantomConstCanonicalMemoize<T, IS_FROM_CANONICAL>
+{
+}
+
+impl<T: ?Sized, const IS_FROM_CANONICAL: bool> Clone
+    for PhantomConstCanonicalMemoize<T, IS_FROM_CANONICAL>
+{
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+impl<T: ?Sized, const IS_FROM_CANONICAL: bool> Eq
+    for PhantomConstCanonicalMemoize<T, IS_FROM_CANONICAL>
+{
+}
+
+impl<T: ?Sized, const IS_FROM_CANONICAL: bool> PartialEq
+    for PhantomConstCanonicalMemoize<T, IS_FROM_CANONICAL>
+{
+    fn eq(&self, _other: &Self) -> bool {
+        true
+    }
+}
+
+impl<T: ?Sized, const IS_FROM_CANONICAL: bool> Hash
+    for PhantomConstCanonicalMemoize<T, IS_FROM_CANONICAL>
+{
+    fn hash<H: Hasher>(&self, _state: &mut H) {}
+}
+
+impl<T: ?Sized + PhantomConstValue> Memoize for PhantomConstCanonicalMemoize<T, false> {
+    type Input = Interned<T>;
+    type InputOwned = Interned<T>;
+    type Output = Interned<PhantomConstCanonicalValue>;
+
+    fn inner(self, input: &Self::Input) -> Self::Output {
+        Intern::intern_sized(PhantomConstCanonicalValue::from_json_value(
+            serde_json::to_value(input)
+                .expect("serialization failed when constructing a canonical PhantomConst"),
+        ))
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> Memoize for PhantomConstCanonicalMemoize<T, true> {
+    type Input = Interned<PhantomConstCanonicalValue>;
+    type InputOwned = Interned<PhantomConstCanonicalValue>;
+    type Output = Interned<T>;
+
+    fn inner(self, input: &Self::Input) -> Self::Output {
+        PhantomConstValue::deserialize(input.as_json_value()).expect("deserialization failed ")
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> PhantomConst<T>
+where
+    Interned<T>: Default,
+{
+    pub const fn default() -> Self {
+        PhantomConst {
+            value: LazyInterned::new_lazy(&Interned::<T>::default),
+        }
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> PhantomConst<T> {
+    pub fn new(value: Interned<T>) -> Self {
+        Self {
+            value: LazyInterned::Interned(value),
+        }
+    }
+    pub fn get(self) -> Interned<T> {
+        self.value.interned()
+    }
+    pub fn type_properties(self) -> TypeProperties {
+        <()>::TYPE_PROPERTIES
+    }
+    pub fn can_connect(self, other: Self) -> bool {
+        self == other
+    }
+    pub fn canonical_phantom_const(self) -> PhantomConst {
+        if let Some(&retval) = <dyn Any>::downcast_ref::<PhantomConst>(&self) {
+            return retval;
+        }
+        <PhantomConst>::new(
+            PhantomConstCanonicalMemoize::<T, false>(PhantomData).get_owned(self.get()),
+        )
+    }
+    pub fn from_canonical_phantom_const(canonical_type: PhantomConst) -> Self {
+        if let Some(&retval) = <dyn Any>::downcast_ref::<Self>(&canonical_type) {
+            return retval;
+        }
+        Self::new(
+            PhantomConstCanonicalMemoize::<T, true>(PhantomData).get_owned(canonical_type.get()),
+        )
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> Type for PhantomConst<T> {
+    type BaseType = PhantomConst;
+    type MaskType = ();
+    impl_match_variant_as_self!();
+
+    fn mask_type(&self) -> Self::MaskType {
+        ()
+    }
+
+    fn canonical(&self) -> CanonicalType {
+        CanonicalType::PhantomConst(self.canonical_phantom_const())
+    }
+
+    fn from_canonical(canonical_type: CanonicalType) -> Self {
+        let CanonicalType::PhantomConst(phantom_const) = canonical_type else {
+            panic!("expected PhantomConst");
+        };
+        Self::from_canonical_phantom_const(phantom_const)
+    }
+
+    fn source_location() -> SourceLocation {
+        SourceLocation::builtin()
+    }
+}
+
+impl<T: ?Sized + PhantomConstValue> StaticType for PhantomConst<T>
+where
+    Interned<T>: Default,
+{
+    const TYPE: Self = Self::default();
+    const MASK_TYPE: Self::MaskType = ();
+    const TYPE_PROPERTIES: TypeProperties = <()>::TYPE_PROPERTIES;
+    const MASK_TYPE_PROPERTIES: TypeProperties = <()>::TYPE_PROPERTIES;
+}

crates/fayalite/src/prelude.rs

@@ -26,6 +26,7 @@ pub use crate::{
         annotate, connect, connect_any, incomplete_wire, instance, memory, memory_array,
         memory_with_init, reg_builder, wire, Instance, Module, ModuleBuilder,
     },
+    phantom_const::PhantomConst,
     reg::Reg,
     reset::{AsyncReset, Reset, SyncReset, ToAsyncReset, ToReset, ToSyncReset},
     source_location::SourceLocation,

crates/fayalite/src/sim.rs

@@ -167,6 +167,14 @@ impl<T: Type> CompiledTypeLayout<T> {
                             body: CompiledTypeLayoutBody::Array { element },
                         }
                     }
+                    CanonicalType::PhantomConst(_) => {
+                        let unit_layout = CompiledTypeLayout::get(());
+                        CompiledTypeLayout {
+                            ty: *input,
+                            layout: unit_layout.layout,
+                            body: unit_layout.body,
+                        }
+                    }
                     CanonicalType::Bundle(bundle) => {
                         let mut layout = TypeLayout::empty();
                         let fields = bundle
@@ -1792,7 +1800,7 @@ impl Compiler {
                 }
                 .into()
             }
-            CanonicalType::Bundle(_) => unreachable!(),
+            CanonicalType::Bundle(_) | CanonicalType::PhantomConst(_) => unreachable!(),
             CanonicalType::AsyncReset(_) => TraceAsyncReset {
                 location: self.make_trace_scalar_helper(
                     instantiated_module,
@@ -2009,6 +2017,13 @@ impl Compiler {
             | CanonicalType::Clock(_) => {
                 self.make_trace_scalar(instantiated_module, target, name, source_location)
             }
+            CanonicalType::PhantomConst(_) => TraceBundle {
+                name,
+                fields: Interned::default(),
+                ty: Bundle::new(Interned::default()),
+                flow: target.flow(),
+            }
+            .into(),
         }
     }
     fn make_trace_decl(
@@ -2469,6 +2484,9 @@ impl Compiler {
                     Expr::field(Expr::<Bundle>::from_canonical(expr.arg()), &field.name)
                 }),
             ),
+            CanonicalType::PhantomConst(_) => {
+                self.compile_cast_aggregate_to_bits(instantiated_module, [])
+            }
         }
     }
     fn compile_cast_bits_to(
@@ -2518,6 +2536,10 @@ impl Compiler {
             CanonicalType::SyncReset(ty) => Expr::canonical(expr.arg().cast_to(ty)),
             CanonicalType::Reset(_) => unreachable!(),
             CanonicalType::Clock(ty) => Expr::canonical(expr.arg().cast_to(ty)),
+            CanonicalType::PhantomConst(ty) => {
+                let _ = self.compile_expr(instantiated_module, Expr::canonical(expr.arg()));
+                Expr::canonical(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())
@@ -2567,6 +2589,7 @@ impl Compiler {
                 CanonicalType::SyncReset(_) => false,
                 CanonicalType::Reset(_) => false,
                 CanonicalType::Clock(_) => false,
+                CanonicalType::PhantomConst(_) => unreachable!(),
             };
             let dest_signed = match Expr::ty(expr) {
                 CanonicalType::UInt(_) => false,
@@ -2579,6 +2602,7 @@ impl Compiler {
                 CanonicalType::SyncReset(_) => false,
                 CanonicalType::Reset(_) => false,
                 CanonicalType::Clock(_) => false,
+                CanonicalType::PhantomConst(_) => unreachable!(),
             };
             self.simple_nary_big_expr(instantiated_module, Expr::ty(expr), [arg], |dest, [src]| {
                 match (src_signed, dest_signed) {
@@ -2634,6 +2658,9 @@ impl Compiler {
                     }]
                 })
                 .into(),
+            ExprEnum::PhantomConst(_) => self
+                .compile_aggregate_literal(instantiated_module, Expr::ty(expr), Interned::default())
+                .into(),
             ExprEnum::BundleLiteral(literal) => self
                 .compile_aggregate_literal(
                     instantiated_module,
@@ -3537,6 +3564,7 @@ impl Compiler {
                 CanonicalType::SyncReset(_) => unreachable!(),
                 CanonicalType::Reset(_) => unreachable!(),
                 CanonicalType::Clock(_) => unreachable!(),
+                CanonicalType::PhantomConst(_) => unreachable!("PhantomConst mismatch"),
             }
         }
         let Some(target) = lhs.target() else {
@@ -3901,6 +3929,7 @@ impl Compiler {
                     CanonicalType::SyncReset(_) => false,
                     CanonicalType::Reset(_) => false,
                     CanonicalType::Clock(_) => false,
+                    CanonicalType::PhantomConst(_) => unreachable!(),
                 };
                 let width = data_layout.ty.bit_width();
                 if let Some(MemoryPortReadInsns {
@@ -5909,6 +5938,7 @@ impl SimValue<CanonicalType> {
             CanonicalType::Clock(ty) => {
                 Expr::canonical(Bool::bits_to_expr(Cow::Borrowed(bits)).cast_to(ty))
             }
+            CanonicalType::PhantomConst(ty) => Expr::canonical(ty.to_expr()),
         }
     }
 }
@@ -6312,7 +6342,8 @@ impl ToSimValue<CanonicalType> for bool {
             | CanonicalType::SInt(_)
             | CanonicalType::Array(_)
             | CanonicalType::Enum(_)
-            | CanonicalType::Bundle(_) => {
+            | CanonicalType::Bundle(_)
+            | CanonicalType::PhantomConst(_) => {
                 panic!("can't create SimValue from bool: expected value of type: {ty:?}");
             }
             CanonicalType::Bool(_)
@@ -6977,6 +7008,7 @@ impl SimulationImpl {
                     CanonicalType::SyncReset(_) => false,
                     CanonicalType::Reset(_) => false,
                     CanonicalType::Clock(_) => false,
+                    CanonicalType::PhantomConst(_) => unreachable!(),
                 };
                 match compiled_value.range.len() {
                     TypeLen::A_SMALL_SLOT => read_write_small_scalar(

crates/fayalite/src/ty.rs

@@ -9,6 +9,7 @@ use crate::{
     expr::Expr,
     int::{Bool, SInt, UInt},
     intern::{Intern, Interned},
+    phantom_const::PhantomConst,
     reset::{AsyncReset, Reset, SyncReset},
     source_location::SourceLocation,
     util::ConstUsize,
@@ -36,6 +37,7 @@ pub enum CanonicalType {
     SyncReset(SyncReset),
     Reset(Reset),
     Clock(Clock),
+    PhantomConst(PhantomConst),
 }
 
 impl fmt::Debug for CanonicalType {
@@ -51,6 +53,7 @@ impl fmt::Debug for CanonicalType {
             Self::SyncReset(v) => v.fmt(f),
             Self::Reset(v) => v.fmt(f),
             Self::Clock(v) => v.fmt(f),
+            Self::PhantomConst(v) => v.fmt(f),
         }
     }
 }
@@ -68,6 +71,7 @@ impl CanonicalType {
             CanonicalType::SyncReset(v) => v.type_properties(),
             CanonicalType::Reset(v) => v.type_properties(),
             CanonicalType::Clock(v) => v.type_properties(),
+            CanonicalType::PhantomConst(v) => v.type_properties(),
         }
     }
     pub fn is_passive(self) -> bool {
@@ -144,6 +148,12 @@ impl CanonicalType {
                 };
                 lhs.can_connect(rhs)
             }
+            CanonicalType::PhantomConst(lhs) => {
+                let CanonicalType::PhantomConst(rhs) = rhs else {
+                    return false;
+                };
+                lhs.can_connect(rhs)
+            }
         }
     }
 }
@@ -222,6 +232,7 @@ impl_base_type!(AsyncReset);
 impl_base_type!(SyncReset);
 impl_base_type!(Reset);
 impl_base_type!(Clock);
+impl_base_type!(PhantomConst);
 
 impl sealed::BaseTypeSealed for CanonicalType {}
 
@@ -316,6 +327,7 @@ impl Type for CanonicalType {
             CanonicalType::SyncReset(v) => v.mask_type().canonical(),
             CanonicalType::Reset(v) => v.mask_type().canonical(),
             CanonicalType::Clock(v) => v.mask_type().canonical(),
+            CanonicalType::PhantomConst(v) => v.mask_type().canonical(),
         }
     }
     fn canonical(&self) -> CanonicalType {

crates/fayalite/visit_types.json

@@ -49,7 +49,8 @@
                 "AsyncReset": "Visible",
                 "SyncReset": "Visible",
                 "Reset": "Visible",
-                "Clock": "Visible"
+                "Clock": "Visible",
+                "PhantomConst": "Visible"
             }
         },
         "Bundle": {
@@ -1262,6 +1263,12 @@
                 "ArrayElement": "Visible",
                 "DynArrayElement": "Visible"
             }
+        },
+        "PhantomConst": {
+            "data": {
+                "$kind": "Opaque"
+            },
+            "generics": "<T: ?Sized + crate::phantom_const::PhantomConstValue>"
         }
     }
 }