message: remove unnecessary enumerate
diff: @@ -24,7 +24,7 @@ impl GrowingBufferArena {
  let size = align(self.alignment, size);
  assert!(size <= self.desc.size);
  - for (_, (buffer, cursor)) in self.buffers.iter_mut().enumerate() {
  + for (buffer, cursor) in &mut self.buffers {
  if size <= self.desc.size - *cursor {
  let offset = *cursor;
  *cursor += size;
  