syntax_data: {"ast_node_type":"other","tokens":[{"end":74,"kind":"keyword","start":0,"text":"debug!(\"walk_chain({:?}):"}]}
source_snippet:             debug!("walk_chain({:?}): outer_expn={:?}", span, outer_expn);
context_before:             let outer_expn = self.outer_expn(span.ctxt());
context_after:             let expn_data = self.expn_data(outer_expn);