{"ast_node_type":"function","tokens":[{"end":85,"kind":"keyword","start":0,"text":"//"}]}
        // (FIXME: technically we only need to check this if the type is a fn ptr...)
        // A built-in `AsyncFn` impl only holds if the output is sized.
        let output_is_sized_pred = tupled_inputs_and_output_and_coroutine.map_bound(