give a hint which feature is missing

compiler/rustc_session/src/parse.rs

@@ -155,6 +155,7 @@ pub fn feature_warn_issue(
 }
 
 /// Adds the diagnostics for a feature to an existing error.
+/// Must be a language feature!
 pub fn add_feature_diagnostics<G: EmissionGuarantee>(
     err: &mut Diag<'_, G>,
     sess: &Session,

compiler/rustc_session/src/session.rs

@@ -300,6 +300,7 @@ impl Session {
         self.opts.test
     }
 
+    /// `feature` must be a language feature.
     #[track_caller]
     pub fn create_feature_err<'a>(&'a self, err: impl Diagnostic<'a>, feature: Symbol) -> Diag<'a> {
         let mut err = self.dcx().create_err(err);