syntax_data: {"ast_node_type":"variable","tokens":[{"end":92,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:     let mut err = sess.dcx().create_err(FeatureGateError { span, explain: explain.into() });
context_after:     add_feature_diagnostics_for_issue(&mut err, sess, feature, issue, false, None);