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Rollup merge of #108548 - jamen:master, r=compiler-errors

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Clarify the 'use a constant in a pattern' error message

```rs
use std::borrow::Cow;

const ERROR_CODE: Cow<'_, str> = Cow::Borrowed("23505");

fn main() {
    let x = Cow::from("23505");

    match x {
        ERROR_CODE => {}
    }
}
```
```
error: to use a constant of type `Cow` in a pattern, `Cow` must be annotated with `#[derive(PartialEq, Eq)]`
 --> src/main.rs:9:9
  |
9 |         ERROR_CODE => {}
  |         ^^^^^^^^^^

error: could not compile `playground` due to previous error
```

It seems helpful to link to StructuralEq in this message. I was a little confused, because `Cow<'_, str>` implements PartialEq and Eq, but they're not derived, which I learned is necessary for structural equality and using constants in patterns (thanks to the Rust community Discord server)

For tests, should I update every occurrence of this message? I see tests where this is still a warning and I'm not sure if I should update those.
This commit is contained in:
Matthias Krüger 2023-03-28 07:01:08 +02:00 committed by GitHub
commit d57abf382b
1 changed files with 2 additions and 0 deletions
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tests/ui/crashes/ice-6254.stderr
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@ -6,6 +6,8 @@ LL | FOO_REF_REF => {},
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #62411 <https://github.com/rust-lang/rust/issues/62411>
= note: the traits must be derived, manual `impl`s are not sufficient
= note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
= note: `-D indirect-structural-match` implied by `-D warnings`
error: aborting due to previous error