Auto merge of #77464 - ecstatic-morse:const-fn-impl-trait, r=oli-obk

Give `impl Trait` in a `const fn` its own feature gate

...previously it was gated under `#![feature(const_fn)]`.

I think we actually want to do this in all const-contexts? If so, this should be `#![feature(const_impl_trait)]` instead. I don't think there's any way to make use of `impl Trait` within a `const` initializer.

cc #77463

r? `@oli-obk`

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -558,12 +558,17 @@ pub mod ty {
     #[derive(Debug)]
     pub struct ImplTrait;
     impl NonConstOp for ImplTrait {
-        fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-            mcf_status_in_item(ccx)
+        fn status_in_item(&self, _: &ConstCx<'_, '_>) -> Status {
+            Status::Unstable(sym::const_impl_trait)
         }
 
         fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-            mcf_build_error(ccx, span, "`impl Trait` in const fn is unstable")
+            feature_err(
+                &ccx.tcx.sess.parse_sess,
+                sym::const_impl_trait,
+                span,
+                &format!("`impl Trait` is not allowed in {}s", ccx.const_kind()),
+            )
         }
     }