rustc-ap-rustc_error_codes 638.0.0

Automatically published version of the package `rustc_error_codes` in the rust-lang/rust repository from commit 30ca215b4e38b32aa7abdd635c5e2d56f5724494 The publishing script for this crate lives at: https://github.com/alexcrichton/rustc-auto-publish
Documentation 
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An feature unstable 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
    ()
}
```

To enable this feature on a nightly version of rustc, add the `const_fn`
feature flag:

```
#![feature(const_fn)]

trait T {}

impl T for () {}

const fn foo() -> impl T {
    ()
}
```