Rollup merge of #58196 - varkor:const-fn-feature-gate-error, r=oli-obk

Add specific feature gate error for const-unstable features

Before:
```
error: `impl Trait` in const fn is unstable
 --> src/lib.rs:7:19
  |
7 | const fn foo() -> impl T {
  |                   ^^^^^^

error: aborting due to previous error
```

After:
```
error[E0723]: `impl Trait` in const fn is unstable (see issue #57563)
 --> src/lib.rs:7:19
  |
7 | const fn foo() -> impl T {
  |                   ^^^^^^
  = help: add #![feature(const_fn)] to the crate attributes to enable

error: aborting due to previous error
```

This improves the situation with https://github.com/rust-lang/rust/issues/57563. Fixes https://github.com/rust-lang/rust/issues/57544. Fixes https://github.com/rust-lang/rust/issues/54469.

r? @oli-obk

0 files changed, 0 insertions, 0 deletions