message: Assert !needs_drop in Arena:: alloc_extra_non_drop
  Summary: Safety and documentation.
diff: @@ -192,6 +192,8 @@ impl Arena {
  x: T,
  extra: usize,
  ) -> &'v AValueHeader {
  + assert!(!mem::needs_drop::<T>());
  +
  let p = Self::alloc_empty::<T>(&self.non_drop, extra);
  unsafe {
  ptr::write(
  