A trait method was declared const.

Erroneous code example:

```compile_fail,E0379
trait Foo {
    const fn bar() -> u32; // error!
}

impl Foo for () {
    const fn bar() -> u32 { 0 } // error!
}
```

Trait methods cannot be declared `const` by design. For more information, see
[RFC 911].

[RFC 911]: https://github.com/rust-lang/rfcs/pull/911
