message: Make unused stack slots zero sized in stack2reg
diff: @@ -237,7 +237,7 @@ pub(super) fn optimize_function<T: std::fmt::Debug>(
  }
  if users.stack_store.is_empty() && users.stack_load.is_empty() {
  - // FIXME make stack_slot zero sized.
  + opt_ctx.ctx.func.stack_slots[stack_slot.0].size = 0;
  }
  }
  