syntax_data: {"ast_node_type":"other","tokens":[{"end":84,"kind":"keyword","start":0,"text":"debug!(\"walk_chain_collapsed({:?}):"}]}
source_snippet:             debug!("walk_chain_collapsed({:?}): outer_expn={:?}", span, outer_expn);
context_before:             let outer_expn = self.outer_expn(ctxt);
context_after:             let expn_data = self.expn_data(outer_expn);