message: remove a dead optimization
diff: @@ -559,16 +559,9 @@ impl<'tcx> Stack {
  self.len()
  };
  - // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
  - // `new_idx` might be 0 if we just cleared the entire stack.
  - if self.get(new_idx) == Some(new) || (new_idx > 0 && self.get(new_idx - 1).unwrap() == new)
  - {
  - // Optimization applies, done.
  - trace!("reborrow: avoiding adding redundant item {:?}", new);
  - } else {
  + // Put the new item there.
  trace!("reborrow: adding item {:?}", new);
  self.insert(new_idx, new);
  - }
  Ok(())
  }
  }
  