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, 14 insertions, 7 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 5d49ad900..67d0dbdcc 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -3017,13 +3017,20 @@ struct PrintSExpression : public UnifiedExpressionVisitor<PrintSExpression> {
     o << ')';
   }
   void handleHeapType(HeapType type) {
-    bool hasSuper = false;
-    // TODO: Consider finality once we support that.
-    if (auto super = type.getSuperType()) {
-      hasSuper = true;
+    auto super = type.getSuperType();
+    bool useSub = false;
+    // TODO: Once we parse MVP signature types as final, use the MVP shorthand
+    // for final types without supertypes.
+    if (super || type.isFinal()) {
+      useSub = true;
       o << "(sub ";
-      TypeNamePrinter(o, currModule).print(*super);
-      o << ' ';
+      if (type.isFinal()) {
+        o << "final ";
+      }
+      if (super) {
+        TypeNamePrinter(o, currModule).print(*super);
+        o << ' ';
+      }
     }
     if (type.isSignature()) {
       handleSignature(type);
@@ -3034,7 +3041,7 @@ struct PrintSExpression : public UnifiedExpressionVisitor<PrintSExpression> {
     } else {
       o << type;
     }
-    if (hasSuper) {
+    if (useSub) {
       o << ')';
     }
   }