{"ast_node_type":"other","tokens":[{"end":71,"kind":"keyword","start":0,"text":")"}]}
    ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {
        idx_ty: Ty<'tcx>,
        // FIXME(#18741) -- this is almost but not quite the same as the