message: add a sanity assertion
diff: @@ -318,6 +318,7 @@ impl<'tcx> Stacks {
  new_bor: Borrow,
  new_kind: RefKind,
  ) -> EvalResult<'tcx> {
  + assert_eq!(new_bor.is_unique(), new_kind == RefKind::Unique);
  trace!("reborrow for tag {:?} to {:?} as {:?}: {:?}, size {}",
  ptr.tag, new_bor, new_kind, ptr, size.bytes());
  let mut stacks = self.stacks.borrow_mut();
  