Initial support for `final` types (#5803)

Implement support in the type system for final types, which are not allowed to
have any subtypes. Final types are syntactically different from similar
non-final types, so type canonicalization is made aware of finality. Similarly,
TypeMerging and TypeSSA are updated to work correctly in the presence of final
types as well.

Implement binary and text parsing and emitting of final types. Use the standard
text format to represent final types and interpret the non-standard
"struct_subtype" and friends as non-final. This allows a graceful upgrade path
for users currently using the non-standard text format, where they can update
their code to use final types correctly at the point when they update to use the
standard format. Once users have migrated to using the fully expanded standard
text format, we can update update Binaryen's parsers to interpret the MVP
shorthands as final types to match the spec without breaking those users.

To make it safe for V8 to independently start interpreting types declared
without `sub` as final, also reserve that shorthand encoding only for types that
have no strict subtypes.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/wasm-type.h b/src/wasm-type.h
index a8a331ace..323ef0207 100644
--- a/src/wasm-type.h
+++ b/src/wasm-type.h
@@ -359,6 +359,7 @@ public:
   bool isArray() const;
   bool isString() const;
   bool isBottom() const;
+  bool isFinal() const;
 
   Signature getSignature() const;
   const Struct& getStruct() const;
@@ -584,6 +585,8 @@ struct TypeBuilder {
   // not overlap or go out of bounds.
   void createRecGroup(size_t i, size_t length);
 
+  void setFinal(size_t i, bool final = true);
+
   enum class ErrorReason {
     // There is a cycle in the supertype relation.
     SelfSupertype,
@@ -645,6 +648,10 @@ struct TypeBuilder {
       builder.setSubType(index, other);
       return *this;
     }
+    Entry& setFinal(bool final = true) {
+      builder.setFinal(index, final);
+      return *this;
+    }
   };
 
   Entry operator[](size_t i) { return Entry{*this, i}; }