Auto merge of rust-lang#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 rust-lang#77463

r? `@oli-obk`

Author: bors

core/src/lib.rs

@@ -83,6 +83,7 @@
 #![feature(const_fn)]
 #![feature(const_fn_union)]
 #![feature(const_assume)]
+#![cfg_attr(not(bootstrap), feature(const_impl_trait))]
 #![cfg_attr(not(bootstrap), feature(const_fn_floating_point_arithmetic))]
 #![cfg_attr(not(bootstrap), feature(const_fn_fn_ptr_basics))]
 #![feature(const_generics)]