{"ast_node_type":"function","tokens":[{"end":81,"kind":"keyword","start":0,"text":"fn"}]}
    fn parse_expr_box(&mut self, box_kw: Span) -> PResult<'a, (Span, ExprKind)> {
    /// for now to provide a more useful error
        let (span, expr) = self.parse_expr_prefix_common(box_kw)?;