syntax_data: {"ast_node_type":"other","tokens":[{"end":61,"kind":"keyword","start":0,"text":"err.subdiagnostic(FeatureDiagnosticHelp"}]}
source_snippet:         err.subdiagnostic(FeatureDiagnosticHelp { feature });
context_before:     if sess.psess.unstable_features.is_nightly_build() {