{"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 `Fn` impl only holds if the output is sized.
        let output_is_sized_pred = inputs_and_output.map_bound(|(_, output)| {