syntax_data: {"ast_node_type":"other","tokens":[{"end":51,"kind":"keyword","start":0,"text":"debug!(\"walk_chain({:?},"}]}
source_snippet:         debug!("walk_chain({:?}, {:?})", span, to);
context_before:         let orig_span = span;
context_after:         debug!("walk_chain: span ctxt = {:?}", span.ctxt());