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