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_span/src/symbol.rs

@@ -359,6 +359,7 @@ symbols! {
         const_fn_union,
         const_generics,
         const_if_match,
+        const_impl_trait,
         const_in_array_repeat_expressions,
         const_indexing,
         const_let,