{"ast_node_type":"impl","tokens":[{"end":83,"kind":"keyword","start":0,"text":"//"}]}
        //   In this case, the builtin impl will have no nested subgoals. This is a
        //   if the ADTs definition implies that it is {meta,}sized by for all possible args.
        //   "best effort" optimization and `{meta,pointee,}sized_constraint` may return `Some`,