{"ast_node_type":"variable","tokens":[{"end":73,"kind":"keyword","start":0,"text":"let"}]}
    let mut ctx = ExpandCtx { bindings, nesting: Vec::new(), call_site };
) -> ExpandResult<tt::TopSubtree<Span>> {
    let mut builder = tt::TopSubtreeBuilder::new(tt::Delimiter::invisible_spanned(ctx.call_site));