{"ast_node_type":"variable","tokens":[{"end":46,"kind":"keyword","start":0,"text":"let"}]}
        let ocx = ObligationCtxt::new(&infcx);
            mbcx.infcx.tcx.infer_ctxt().build_with_canonical(cause.span, &self.canonical_query);
        type_op_ascribe_user_type_with_span(&ocx, key, cause.span).ok()?;