message: slightly more verbose error msg
diff: @@ -153,7 +153,7 @@ impl<'tcx> Stack {
  fn deref(&self, bor: Borrow, kind: RefKind) -> Result<Option<usize>, String> {
  // Exclude unique ref with frozen tag.
  if let (RefKind::Unique, Borrow::Shr(Some(_))) = (kind, bor) {
  - return Err(format!("Encountered mutable reference with frozen tag"));
  + return Err(format!("Encountered mutable reference with frozen tag ({:?})", bor));
  }
  // Checks related to freezing
  match bor {
  