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`

1 files changed, 4 insertions, 12 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0723.md b/compiler/rustc_error_codes/src/error_codes/E0723.md
index 95d47ab21cb..bc224421915 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0723.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0723.md
@@ -3,12 +3,8 @@ An unstable feature in `const` contexts was used.
 Erroneous code example:
 
 ```compile_fail,E0723
-trait T {}
-
-impl T for () {}
-
-const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable
-    ()
+const fn foo<T: Copy>(_: T) { // error!
+   // ...
 }
 ```
 
@@ -18,11 +14,7 @@ feature flag:
 ```
 #![feature(const_fn)]
 
-trait T {}
-
-impl T for () {}
-
-const fn foo() -> impl T {
-    ()
+const fn foo<T: Copy>(_: T) { // ok!
+   // ...
 }
 ```