{"ast_node_type":"other","tokens":[{"end":25,"kind":"keyword","start":0,"text":"idx_ty:"}]}
        idx_ty: Ty<'tcx>,
        index_expr: &'tcx hir::Expr<'tcx>,
    ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {