A handful of little tweaks for GC types (#8925)

* A handful of little tweaks for GC types

Splitting this out from a larger PR so that it is easier to review and
everything can land quicker.

* Don't use `Cow`, just use references

crates/wasmtime/src/runtime/types.rs

@@ -127,6 +127,9 @@ impl ValType {
     /// The `externref` type, aka `(ref null extern)`.
     pub const EXTERNREF: Self = ValType::Ref(RefType::EXTERNREF);
 
+    /// The `nullexternref` type, aka `(ref null noextern)`.
+    pub const NULLEXTERNREF: Self = ValType::Ref(RefType::NULLEXTERNREF);
+
     /// The `funcref` type, aka `(ref null func)`.
     pub const FUNCREF: Self = ValType::Ref(RefType::FUNCREF);
 
@@ -139,6 +142,9 @@ impl ValType {
     /// The `i31ref` type, aka `(ref null i31)`.
     pub const I31REF: Self = ValType::Ref(RefType::I31REF);
 
+    /// The `structref` type, aka `(ref null struct)`.
+    pub const STRUCTREF: Self = ValType::Ref(RefType::STRUCTREF);
+
     /// The `nullref` type, aka `(ref null none)`.
     pub const NULLREF: Self = ValType::Ref(RefType::NULLREF);
 
@@ -371,6 +377,12 @@ impl RefType {
         heap_type: HeapType::Extern,
     };
 
+    /// The `nullexternref` type, aka `(ref null noextern)`.
+    pub const NULLEXTERNREF: Self = RefType {
+        is_nullable: true,
+        heap_type: HeapType::NoExtern,
+    };
+
     /// The `funcref` type, aka `(ref null func)`.
     pub const FUNCREF: Self = RefType {
         is_nullable: true,
@@ -395,6 +407,12 @@ impl RefType {
         heap_type: HeapType::I31,
     };
 
+    /// The `structref` type, aka `(ref null struct)`.
+    pub const STRUCTREF: Self = RefType {
+        is_nullable: true,
+        heap_type: HeapType::Struct,
+    };
+
     /// The `nullref` type, aka `(ref null none)`.
     pub const NULLREF: Self = RefType {
         is_nullable: true,
@@ -752,7 +770,10 @@ impl HeapType {
     /// Types that are not concrete, user-defined types are abstract types.
     #[inline]
     pub fn is_concrete(&self) -> bool {
-        matches!(self, HeapType::ConcreteFunc(_) | HeapType::ConcreteArray(_))
+        matches!(
+            self,
+            HeapType::ConcreteFunc(_) | HeapType::ConcreteArray(_) | HeapType::ConcreteStruct(_)
+        )
     }
 
     /// Is this a concrete, user-defined function type?
@@ -797,6 +818,27 @@ impl HeapType {
         self.as_concrete_array().unwrap()
     }
 
+    /// Is this a concrete, user-defined struct type?
+    pub fn is_concrete_struct(&self) -> bool {
+        matches!(self, HeapType::ConcreteStruct(_))
+    }
+
+    /// Get the underlying concrete, user-defined struct type, if any.
+    ///
+    /// Returns `None` for if this is not a concrete struct type.
+    pub fn as_concrete_struct(&self) -> Option<&StructType> {
+        match self {
+            HeapType::ConcreteStruct(f) => Some(f),
+            _ => None,
+        }
+    }
+
+    /// Get the underlying concrete, user-defined type, panicking if this is not
+    /// a concrete struct type.
+    pub fn unwrap_concrete_struct(&self) -> &StructType {
+        self.as_concrete_struct().unwrap()
+    }
+
     /// Get the top type of this heap type's type hierarchy.
     ///
     /// The returned heap type is a supertype of all types in this heap type's
@@ -1198,6 +1240,16 @@ pub enum StorageType {
     ValType(ValType),
 }
 
+impl fmt::Display for StorageType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            StorageType::I8 => write!(f, "i8"),
+            StorageType::I16 => write!(f, "i16"),
+            StorageType::ValType(ty) => fmt::Display::fmt(ty, f),
+        }
+    }
+}
+
 impl From<ValType> for StorageType {
     #[inline]
     fn from(v: ValType) -> Self {
@@ -1241,6 +1293,20 @@ impl StorageType {
         self.as_val_type().unwrap()
     }
 
+    /// Unpack this (possibly packed) storage type into a full `ValType`.
+    ///
+    /// If this is a `StorageType::ValType`, then the inner `ValType` is
+    /// returned as-is.
+    ///
+    /// If this is a packed `StorageType::I8` or `StorageType::I16, then a
+    /// `ValType::I32` is returned.
+    pub fn unpack(&self) -> &ValType {
+        match self {
+            StorageType::I8 | StorageType::I16 => &ValType::I32,
+            StorageType::ValType(ty) => ty,
+        }
+    }
+
     /// Does this field type match the other field type?
     ///
     /// That is, is this field type a subtype of the other field type?
@@ -1307,6 +1373,16 @@ pub struct FieldType {
     element_type: StorageType,
 }
 
+impl fmt::Display for FieldType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.mutability.is_var() {
+            write!(f, "(mut {})", self.element_type)
+        } else {
+            fmt::Display::fmt(&self.element_type, f)
+        }
+    }
+}
+
 impl FieldType {
     /// Construct a new field type from the given parts.
     #[inline]
@@ -1401,6 +1477,17 @@ pub struct StructType {
     registered_type: RegisteredType,
 }
 
+impl fmt::Display for StructType {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "(struct")?;
+        for field in self.fields() {
+            write!(f, " (field {field})")?;
+        }
+        write!(f, ")")?;
+        Ok(())
+    }
+}
+
 impl StructType {
     /// Construct a new `StructType` with the given field types.
     ///
@@ -1576,15 +1663,19 @@ impl StructType {
     }
 
     pub(crate) fn comes_from_same_engine(&self, engine: &Engine) -> bool {
-        Engine::same(self.registered_type.engine(), engine)
+        Engine::same(self.registered_type().engine(), engine)
     }
 
     pub(crate) fn type_index(&self) -> VMSharedTypeIndex {
-        self.registered_type.index()
+        self.registered_type().index()
     }
 
     pub(crate) fn as_wasm_struct_type(&self) -> &WasmStructType {
-        self.registered_type.unwrap_struct()
+        self.registered_type().unwrap_struct()
+    }
+
+    pub(crate) fn registered_type(&self) -> &RegisteredType {
+        &self.registered_type
     }
 
     /// Construct a `StructType` from a `WasmStructType`.
@@ -1630,10 +1721,12 @@ impl StructType {
             "VMSharedTypeIndex is not registered in the Engine! Wrong \
              engine? Didn't root the index somewhere?",
         );
-        assert!(ty.is_struct());
-        Self {
-            registered_type: ty,
+        Self::from_registered_type(ty)
     }
+
+    pub(crate) fn from_registered_type(registered_type: RegisteredType) -> Self {
+        debug_assert!(registered_type.is_struct());
+        Self { registered_type }
     }
 }
 
@@ -1860,10 +1953,12 @@ impl ArrayType {
             "VMSharedTypeIndex is not registered in the Engine! Wrong \
              engine? Didn't root the index somewhere?",
         );
-        assert!(ty.is_array());
-        Self {
-            registered_type: ty,
+        Self::from_registered_type(ty)
     }
+
+    pub(crate) fn from_registered_type(registered_type: RegisteredType) -> Self {
+        debug_assert!(registered_type.is_array());
+        Self { registered_type }
     }
 }
 
@@ -2214,10 +2309,12 @@ impl FuncType {
             "VMSharedTypeIndex is not registered in the Engine! Wrong \
              engine? Didn't root the index somewhere?",
         );
-        assert!(ty.is_func());
-        Self {
-            registered_type: ty,
+        Self::from_registered_type(ty)
     }
+
+    pub(crate) fn from_registered_type(registered_type: RegisteredType) -> Self {
+        debug_assert!(registered_type.is_func());
+        Self { registered_type }
     }
 }