{"ast_node_type":"variable","tokens":[{"end":77,"kind":"keyword","start":0,"text":"let"}]}
    let mut match_: Option<(matcher::Match<'_>, &crate::Rule, usize)> = None;
) -> ExpandResult<(tt::TopSubtree<Span>, MatchedArmIndex)> {
    for (idx, rule) in rules.iter().enumerate() {