{"ast_node_type":"function","tokens":[{"end":64,"kind":"keyword","start":0,"text":"fn"}]}
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
impl<T: fmt::Debug> fmt::Debug for Arena<T> {
        fmt.debug_struct("Arena").field("len", &self.len()).field("data", &self.data).finish()