message: add `salsa_struct_deleted` for input fields
diff: @@ -93,4 +93,9 @@ where
  fn mark_validated_output(&self, _db: &DB, _executor: DatabaseKeyIndex, _output_key: Id) {}
  fn remove_stale_output(&self, _db: &DB, _executor: DatabaseKeyIndex, _stale_output_key: Id) {}
  +
  + fn salsa_struct_deleted(&self, _db: &DB, _id: Id) {
  + panic!("unexpected call: input fields are never deleted");
  + }
  +
  }
  