message: some failures are impossible
diff: @@ -473,7 +473,7 @@ impl<'tcx> Stack {
  "no item to reborrow as {} from tag {} found in borrow stack", new_kind, derived_from,
  )))?;
  // With this we can compute the permission for the new pointer.
  - let new_perm = new_kind.new_perm(derived_from_perm)?;
  + let new_perm = new_kind.new_perm(derived_from_perm).expect("this should never fail");
  // We behave very differently for the "unsafe" case of a shared-read-write pointer
  // ("unsafe" because this also applies to shared references with interior mutability).
  