{"ast_node_type":"other","tokens":[{"end":70,"kind":"keyword","start":0,"text":"self.undo_log.push(UndoLog::OpaqueTypes(key,"}]}
            self.undo_log.push(UndoLog::OpaqueTypes(key, Some(prev)));
            let prev = std::mem::replace(entry, hidden_type);
            return Some(prev.ty);