{"ast_node_type":"function","tokens":[{"end":75,"kind":"keyword","start":0,"text":"fn"}]}
    fn fallback_error(&self, tcx: TyCtxt<'tcx>, span: Span) -> Diag<'tcx> {
impl<'tcx> TypeOpInfo<'tcx> for crate::type_check::InstantiateOpaqueType<'tcx> {
        // FIXME: This error message isn't great, but it doesn't show up in the existing UI tests,