Wasmtime(gc): Add support for supertypes and finality (#8595)

This is the final type system change for Wasm GC: the ability to explicitly
declare supertypes and finality. A final type may not be a supertype of another
type. A concrete heap type matches another concrete heap type if its concrete
type is a subtype (potentially transitively) of the other heap type's concrete
type.

Next, I'll begin support for allocating GC structs and arrays at runtime.

I've also implemented `O(1)` subtype checking in the types registry:

In a type system with single inheritance, the subtyping relationships between
all types form a set of trees. The root of each tree is a type that has no
supertype; each node's immediate children are the types that directly subtype
that node.

For example, consider these types:

    class Base {}
    class A subtypes Base {}
    class B subtypes Base {}
    class C subtypes A {}
    class D subtypes A {}
    class E subtypes C {}

These types produce the following tree:

               Base
              /    \
             A      B
            / \
           C   D
          /
         E

Note the following properties:

1. If `sub` is a subtype of `sup` (either directly or transitively) then
`sup` *must* be on the path from `sub` up to the root of `sub`'s tree.

2. Additionally, `sup` *must* be the `i`th node down from the root in that path,
where `i` is the length of the path from `sup` to its tree's root.

Therefore, if we maintain a vector containing the path to the root for each
type, then we can simply check if `sup` is at index `supertypes(sup).len()`
within `supertypes(sub)`.

crates/cranelift/src/func_environ.rs

@@ -1642,6 +1642,11 @@ impl TypeConvert for FuncEnvironment<'_> {
     fn lookup_heap_type(&self, ty: wasmparser::UnpackedIndex) -> WasmHeapType {
         wasmtime_environ::WasmparserTypeConverter::new(self.types, self.module).lookup_heap_type(ty)
     }
+
+    fn lookup_type_index(&self, index: wasmparser::UnpackedIndex) -> EngineOrModuleTypeIndex {
+        wasmtime_environ::WasmparserTypeConverter::new(self.types, self.module)
+            .lookup_type_index(index)
+    }
 }
 
 impl<'module_environment> TargetEnvironment for FuncEnvironment<'module_environment> {

crates/environ/src/compile/module_environ.rs

@@ -965,6 +965,13 @@ impl TypeConvert for ModuleEnvironment<'_, '_> {
     fn lookup_heap_type(&self, index: wasmparser::UnpackedIndex) -> WasmHeapType {
         WasmparserTypeConverter::new(&self.types, &self.result.module).lookup_heap_type(index)
     }
+
+    fn lookup_type_index(
+        &self,
+        index: wasmparser::UnpackedIndex,
+    ) -> wasmtime_types::EngineOrModuleTypeIndex {
+        WasmparserTypeConverter::new(&self.types, &self.result.module).lookup_type_index(index)
+    }
 }
 
 impl ModuleTranslation<'_> {

crates/environ/src/compile/module_types.rs

@@ -113,7 +113,7 @@ impl ModuleTypesBuilder {
             let ty = &validator_types[id];
             let wasm_ty = WasmparserTypeConverter::new(self, module)
                 .with_rec_group(validator_types, rec_group_id)
-                .convert_sub_type(ty)?;
+                .convert_sub_type(ty);
             self.wasm_sub_type_in_rec_group(id, wasm_ty);
         }
 
@@ -161,6 +161,8 @@ impl ModuleTypesBuilder {
                 // `trampoline_types` so we can reuse it in the future.
                 Cow::Owned(f) => {
                     let idx = self.types.push(WasmSubType {
+                        is_final: true,
+                        supertype: None,
                         composite_type: WasmCompositeType::Func(f.clone()),
                     });
 
@@ -435,4 +437,19 @@ impl TypeConvert for WasmparserTypeConverter<'_> {
             UnpackedIndex::RecGroup(_) => unreachable!(),
         }
     }
+
+    fn lookup_type_index(&self, index: wasmparser::UnpackedIndex) -> EngineOrModuleTypeIndex {
+        match index {
+            UnpackedIndex::Id(id) => {
+                let interned = self.types.wasmparser_to_wasmtime[&id];
+                EngineOrModuleTypeIndex::Module(interned)
+            }
+            UnpackedIndex::Module(module_index) => {
+                let module_index = TypeIndex::from_u32(module_index);
+                let interned = self.module.types[module_index];
+                EngineOrModuleTypeIndex::Module(interned)
+            }
+            UnpackedIndex::RecGroup(_) => unreachable!(),
+        }
+    }
 }

crates/environ/src/component/translate.rs

@@ -973,6 +973,13 @@ mod pre_inlining {
         fn lookup_heap_type(&self, index: wasmparser::UnpackedIndex) -> WasmHeapType {
             self.types.lookup_heap_type(index)
         }
+
+        fn lookup_type_index(
+            &self,
+            index: wasmparser::UnpackedIndex,
+        ) -> wasmtime_types::EngineOrModuleTypeIndex {
+            self.types.lookup_type_index(index)
+        }
     }
 }
 use pre_inlining::PreInliningComponentTypes;

crates/environ/src/component/types_builder.rs

@@ -703,6 +703,13 @@ impl TypeConvert for ComponentTypesBuilder {
     fn lookup_heap_type(&self, _index: wasmparser::UnpackedIndex) -> WasmHeapType {
         panic!("heap types are not supported yet")
     }
+
+    fn lookup_type_index(
+        &self,
+        _index: wasmparser::UnpackedIndex,
+    ) -> wasmtime_types::EngineOrModuleTypeIndex {
+        panic!("typed references are not supported yet")
+    }
 }
 
 fn intern<T, U>(map: &mut HashMap<T, U>, list: &mut PrimaryMap<U, T>, item: T) -> U

crates/types/src/lib.rs

@@ -326,11 +326,26 @@ pub enum EngineOrModuleTypeIndex {
 }
 
 impl From<ModuleInternedTypeIndex> for EngineOrModuleTypeIndex {
+    #[inline]
     fn from(i: ModuleInternedTypeIndex) -> Self {
         Self::Module(i)
     }
 }
 
+impl From<VMSharedTypeIndex> for EngineOrModuleTypeIndex {
+    #[inline]
+    fn from(i: VMSharedTypeIndex) -> Self {
+        Self::Engine(i)
+    }
+}
+
+impl From<RecGroupRelativeTypeIndex> for EngineOrModuleTypeIndex {
+    #[inline]
+    fn from(i: RecGroupRelativeTypeIndex) -> Self {
+        Self::RecGroup(i)
+    }
+}
+
 impl fmt::Display for EngineOrModuleTypeIndex {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
@@ -868,8 +883,13 @@ impl TypeTrace for WasmCompositeType {
 /// A concrete, user-defined (or host-defined) Wasm type.
 #[derive(Debug, Clone, Eq, PartialEq, Hash, Serialize, Deserialize)]
 pub struct WasmSubType {
-    // TODO: is_final, supertype
-    //
+    /// Whether this type is forbidden from being the supertype of any other
+    /// type.
+    pub is_final: bool,
+
+    /// This type's supertype, if any.
+    pub supertype: Option<EngineOrModuleTypeIndex>,
+
     /// The array, function, or struct that is defined.
     pub composite_type: WasmCompositeType,
 }
@@ -926,13 +946,19 @@ impl TypeTrace for WasmSubType {
     where
         F: FnMut(EngineOrModuleTypeIndex) -> Result<(), E>,
     {
+        if let Some(sup) = self.supertype {
+            func(sup)?;
+        }
         self.composite_type.trace(func)
     }
 
     fn trace_mut<F, E>(&mut self, func: &mut F) -> Result<(), E>
     where
         F: FnMut(&mut EngineOrModuleTypeIndex) -> Result<(), E>,
     {
+        if let Some(sup) = self.supertype.as_mut() {
+            func(sup)?;
+        }
         self.composite_type.trace_mut(func)
     }
 }
@@ -1513,27 +1539,24 @@ pub trait TypeConvert {
         })
     }
 
-    fn convert_sub_type(&self, ty: &wasmparser::SubType) -> WasmResult<WasmSubType> {
-        if ty.supertype_idx.is_some() {
-            return Err(wasm_unsupported!("wasm gc: explicit subtyping"));
+    fn convert_sub_type(&self, ty: &wasmparser::SubType) -> WasmSubType {
+        WasmSubType {
+            is_final: ty.is_final,
+            supertype: ty.supertype_idx.map(|i| self.lookup_type_index(i.unpack())),
+            composite_type: self.convert_composite_type(&ty.composite_type),
         }
-        let composite_type = self.convert_composite_type(&ty.composite_type)?;
-        Ok(WasmSubType { composite_type })
     }
 
-    fn convert_composite_type(
-        &self,
-        ty: &wasmparser::CompositeType,
-    ) -> WasmResult<WasmCompositeType> {
+    fn convert_composite_type(&self, ty: &wasmparser::CompositeType) -> WasmCompositeType {
         match ty {
             wasmparser::CompositeType::Func(f) => {
-                Ok(WasmCompositeType::Func(self.convert_func_type(f)))
+                WasmCompositeType::Func(self.convert_func_type(f))
             }
             wasmparser::CompositeType::Array(a) => {
-                Ok(WasmCompositeType::Array(self.convert_array_type(a)))
+                WasmCompositeType::Array(self.convert_array_type(a))
             }
             wasmparser::CompositeType::Struct(s) => {
-                Ok(WasmCompositeType::Struct(self.convert_struct_type(s)))
+                WasmCompositeType::Struct(self.convert_struct_type(s))
             }
         }
     }
@@ -1626,4 +1649,8 @@ pub trait TypeConvert {
     /// Converts the specified type index from a heap type into a canonicalized
     /// heap type.
     fn lookup_heap_type(&self, index: wasmparser::UnpackedIndex) -> WasmHeapType;
+
+    /// Converts the specified type index from a heap type into a canonicalized
+    /// heap type.
+    fn lookup_type_index(&self, index: wasmparser::UnpackedIndex) -> EngineOrModuleTypeIndex;
 }

crates/wasmtime/src/runtime/component/types.rs

@@ -871,10 +871,15 @@ impl ComponentItem {
             TypeDef::ComponentFunc(idx) => Self::ComponentFunc(ComponentFunc::from(*idx, ty)),
             TypeDef::Interface(iface_ty) => Self::Type(Type::from(iface_ty, ty)),
             TypeDef::Module(idx) => Self::Module(Module::from(*idx, ty)),
-            TypeDef::CoreFunc(idx) => Self::CoreFunc(FuncType::from_wasm_func_type(
-                engine,
-                ty.types[*idx].unwrap_func().clone(),
-            )),
+            TypeDef::CoreFunc(idx) => {
+                let subty = &ty.types[*idx];
+                Self::CoreFunc(FuncType::from_wasm_func_type(
+                    engine,
+                    subty.is_final,
+                    subty.supertype,
+                    subty.unwrap_func().clone(),
+                ))
+            }
             TypeDef::Resource(idx) => {
                 let resource_index = ty.types[*idx].ty;
                 let ty = match ty.resources.get(resource_index) {

crates/wasmtime/src/runtime/module.rs

@@ -24,7 +24,7 @@ use std::path::Path;
 use wasmparser::{Parser, ValidPayload, Validator};
 use wasmtime_environ::{
     CompiledModuleInfo, DefinedFuncIndex, DefinedMemoryIndex, EntityIndex, HostPtr, ModuleTypes,
-    ObjectKind, VMOffsets, VMSharedTypeIndex,
+    ObjectKind, TypeTrace, VMOffsets, VMSharedTypeIndex,
 };
 mod registry;
 
@@ -687,7 +687,12 @@ impl Module {
         let engine = self.engine();
         module
             .imports()
-            .map(move |(module, field, ty)| ImportType::new(module, field, ty, types, engine))
+            .map(move |(imp_mod, imp_field, mut ty)| {
+                ty.canonicalize_for_runtime_usage(&mut |i| {
+                    self.signatures().shared_type(i).unwrap()
+                });
+                ImportType::new(imp_mod, imp_field, ty, types, engine)
+            })
             .collect::<Vec<_>>()
             .into_iter()
     }