Automated rollback of commit 116cf4a41d.

PiperOrigin-RevId: 514495606

src/google/protobuf/descriptor.cc

@@ -8417,7 +8417,18 @@ bool HasPreservingUnknownEnumSemantics(const FieldDescriptor* field) {
 }
 
 bool HasHasbit(const FieldDescriptor* field) {
-  return field->has_presence() && !field->real_containing_oneof() &&
+  // This predicate includes proto3 message fields only if they have "optional".
+  //   Foo submsg1 = 1;           // HasHasbit() == false
+  //   optional Foo submsg2 = 2;  // HasHasbit() == true
+  // This is slightly odd, as adding "optional" to a singular proto3 field does
+  // not change the semantics or API. However whenever any field in a message
+  // has a hasbit, it forces reflection to include hasbit offsets for *all*
+  // fields, even if almost all of them are set to -1 (no hasbit). So to avoid
+  // causing a sudden size regression for ~all proto3 messages, we give proto3
+  // message fields a hasbit only if "optional" is present. If the user is
+  // explicitly writing "optional", it is likely they are writing it on
+  // primitive fields also.
+  return (field->has_optional_keyword() || field->is_required()) &&
          !field->options().weak();
 }