message: Use Vec::resize in LocalSlots::reserve
  Summary: [Rust compiler cannot optimize pushes in loop to memset](https://rust.godbolt.org/z/6z8ds4fWT)
diff: @@ -66,10 +66,7 @@ impl<'v> LocalSlots<'v> {
  pub fn reserve(&mut self, len: usize) -> LocalSlotBase {
  let res = LocalSlotBase(self.slots.len());
  - self.slots.reserve(len);
  - for _ in 0..len {
  - self.slots.push(Cell::new(None));
  - }
  + self.slots.resize(self.slots.len() + len, Cell::new(None));
  res
  }
  