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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 a7c07cf731
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27 changed files with 168 additions and 10 deletions
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6
compiler/rustc_mir_build/src/errors.rs
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@ -663,6 +663,8 @@ pub struct UnionPattern {
#[derive(Diagnostic)]
#[diag(mir_build_type_not_structural)]
#[note(mir_build_type_not_structural_tip)]
#[note(mir_build_type_not_structural_more_info)]
pub struct TypeNotStructural<'tcx> {
#[primary_span]
pub span: Span,
@ -695,12 +697,16 @@ pub struct PointerPattern;
#[derive(LintDiagnostic)]
#[diag(mir_build_indirect_structural_match)]
#[note(mir_build_type_not_structural_tip)]
#[note(mir_build_type_not_structural_more_info)]
pub struct IndirectStructuralMatch<'tcx> {
pub non_sm_ty: Ty<'tcx>,
}
#[derive(LintDiagnostic)]
#[diag(mir_build_nontrivial_structural_match)]
#[note(mir_build_type_not_structural_tip)]
#[note(mir_build_type_not_structural_more_info)]
pub struct NontrivialStructuralMatch<'tcx> {
pub non_sm_ty: Ty<'tcx>,
}