syntax_data: {"ast_node_type":"other","tokens":[{"end":43,"kind":"keyword","start":0,"text":"//"}]}
source_snippet: // manually and avoid the fluent machinery.
context_before: // natural language, and (b) it's only used in tests. So we construct it
context_after: impl<G: EmissionGuarantee> Diagnostic<'_, G> for TestOutput {