message: make NOP dyn casts not require anything about the vtable
diff: @@ -29,6 +29,7 @@ pub(crate) fn unsized_info<'tcx>(
  let old_info =
  old_info.expect("unsized_info: missing old info for trait upcasting coercion");
  if data_a.principal_def_id() == data_b.principal_def_id() {
  + // A NOP cast that doesn't actually change anything, should be allowed even with invalid vtables.
  return old_info;
  }
  