syntax_data: {"ast_node_type":"variable","tokens":[{"end":55,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:             let expn_data = self.expn_data(outer_expn);
context_before:             debug!("walk_chain_collapsed({:?}): outer_expn={:?}", span, outer_expn);
context_after:             debug!("walk_chain_collapsed({:?}): expn_data={:?}", span, expn_data);