{"ast_node_type":"variable","tokens":[{"end":75,"kind":"keyword","start":0,"text":"let"}]}
        let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) else {
    ) -> (Vec<NeededMigration>, MigrationWarningReason) {
            return (Vec::new(), MigrationWarningReason::default());