syntax_data: {"ast_node_type":"variable","tokens":[{"end":58,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:             let outer_expn = self.outer_expn(span.ctxt());
context_before:         while span.ctxt() != to && span.from_expansion() {
context_after:             debug!("walk_chain({:?}): outer_expn={:?}", span, outer_expn);