message: Correctly handle integer overflow in alloca
  Summary:
  Here `wrapping_add` may overflow, and this branch won't be taken.
  ```
  let mut stop = start.wrapping_add(size_words);
  if unlikely(stop > self.end.get()) {
  ```
  I don't think it is practically possible to trigger this bug, but better fix it.
diff: @@ -100,13 +100,15 @@ impl Alloca {
  let size_words = layout.size() / mem::size_of::<usize>();
  let mut start = self.alloc.get();
  - let mut stop = start.wrapping_add(size_words);
  - if unlikely(stop > self.end.get()) {
  +
  + let rem_words = unsafe { self.end.get().offset_from(start) as usize };
  + if unlikely(size_words > rem_words) {
  self.allocate_more(layout);
  start = self.alloc.get();
  - stop = start.wrapping_add(size_words);
  }
  - let old = self.alloc.get();
  +
  + let stop = start.wrapping_add(size_words);
  + let old = start;
  self.alloc.set(stop);
  let data = start as *mut MaybeUninit<T>;
  let slice = unsafe { slice::from_raw_parts_mut(data, len) };
  