syntax_data: {"ast_node_type":"other","tokens":[{"end":73,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:                 // the ID we generate for the SourceFile we just created.
context_before:                 // Let's make sure the file_id we generated above actually matches
context_after:                 debug_assert_eq!(source_file.stable_id, stable_id);