syntax_data: {"ast_node_type":"other","tokens":[{"end":20,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:     // other ExpnIds
context_before:     // Don't decode the data inside `HygieneData::with`, since we need to recursively decode
context_after:     let (expn_data, hash) = decode_data(expn_id);