syntax_data: {"ast_node_type":"other","tokens":[{"end":56,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:         // a `SyntaxContext` that we haven't seen before
context_before:         // When we serialize a `SyntaxContextData`, we may end up serializing
context_after:         while !self.latest_ctxts.lock().is_empty() || !self.latest_expns.lock().is_empty() {