{"ast_node_type":"variable","tokens":[{"end":47,"kind":"keyword","start":0,"text":"let"}]}
        let span = lo.to(self.prev_token.span);
        let kind = ExprKind::Yield(YieldKind::Prefix(self.parse_expr_opt()?));
        self.psess.gated_spans.gate(sym::yield_expr, span);