Mark OpaqueWrappable specializations as final

For better perf optimization of dynamic_cast<T*>

Author: jasnell

src/workerd/jsg/promise.h

@@ -34,8 +34,13 @@ struct OpaqueWrappableBase: public Wrappable {
   }
 };
 
+// We declare OpaqueWrappable<T> specializations here as final to allow the compiler
+// to optimize dynamic_cast<T*> to simply check if the vtable is equal to T's vtable.
+// This provides a minor performance improvement when interacting with opaque-wrapped
+// values.
+
 template <typename T>
-struct OpaqueWrappable<T, false>: public OpaqueWrappableBase {
+struct OpaqueWrappable<T, false> final: public OpaqueWrappableBase {
   // Used to implement wrapOpaque().
 
   OpaqueWrappable(T&& value): value(kj::mv(value)) {}
@@ -49,10 +54,17 @@ struct OpaqueWrappable<T, false>: public OpaqueWrappableBase {
 };
 
 template <typename T>
-struct OpaqueWrappable<T, true>: public OpaqueWrappable<T, false> {
+struct OpaqueWrappable<T, true> final: public OpaqueWrappableBase {
   // When T is GC-visitable, make sure to implement visitation.
 
-  using OpaqueWrappable<T, false>::OpaqueWrappable;
+  OpaqueWrappable(T&& value): value(kj::mv(value)) {}
+
+  T value;
+  bool movedAway = false;
+
+  size_t jsgGetMemorySelfSize() const override final {
+    return sizeof(OpaqueWrappable);
+  }
 
   void jsgVisitForGc(GcVisitor& visitor) override {
     if (!this->movedAway) {