Pass translation closure to add_to_diag_with() as reference

compiler/rustc_parse/src/errors.rs

@@ -1463,7 +1463,7 @@ impl Subdiagnostic for FnTraitMissingParen {
     fn add_to_diag_with<G: EmissionGuarantee, F: SubdiagMessageOp<G>>(
         self,
         diag: &mut Diag<'_, G>,
-        _: F,
+        _: &F,
     ) {
         diag.span_label(self.span, crate::fluent_generated::parse_fn_trait_missing_paren);
         let applicability = if self.machine_applicable {