syntax_data: {"ast_node_type":"variable","tokens":[{"end":50,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let span = tracing::debug_span!("arg_of");
context_before:     let arg_of = |ty: Ty<'tcx>, arg_idx: Option<usize>| -> Result<_, &'tcx FnAbiError<'tcx>> {
context_after:         let _entered = span.enter();