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

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.

Author: matthiaskrgr

compiler/rustc_mir_build/messages.ftl

@@ -331,6 +331,10 @@ mir_build_indirect_structural_match =
 mir_build_nontrivial_structural_match =
     to use a constant of type `{$non_sm_ty}` in a pattern, the constant's initializer must be trivial or `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`
 
+mir_build_type_not_structural_tip = the traits must be derived, manual `impl`s are not sufficient
+
+mir_build_type_not_structural_more_info = see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
+
 mir_build_overlapping_range_endpoints = multiple patterns overlap on their endpoints
     .range = ... with this range
     .note = you likely meant to write mutually exclusive ranges

compiler/rustc_mir_build/src/errors.rs

@@ -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>,
 }

src/tools/clippy/tests/ui/crashes/ice-6254.stderr

@@ -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

tests/ui/consts/const_in_pattern/cross-crate-fail.stderr

@@ -3,12 +3,18 @@ error: to use a constant of type `CustomEq` in a pattern, `CustomEq` must be ann
    |
 LL |         consts::SOME => panic!(),
    |         ^^^^^^^^^^^^
+   |
+   = 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
 
 error: to use a constant of type `CustomEq` in a pattern, `CustomEq` must be annotated with `#[derive(PartialEq, Eq)]`
   --> $DIR/cross-crate-fail.rs:20:9
    |
 LL |         <Defaulted as consts::AssocConst>::SOME  => panic!(),
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = 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
 
 error: aborting due to 2 previous errors
 

tests/ui/consts/const_in_pattern/custom-eq-branch-warn.rs

@@ -28,6 +28,8 @@ fn main() {
     match Foo::Qux(CustomEq) {
         BAR_BAZ => panic!(),
         //~^ WARN must be annotated with `#[derive(PartialEq, Eq)]`
+        //~| NOTE the traits must be derived
+        //~| NOTE StructuralEq.html for details
         //~| WARN this was previously accepted
         //~| NOTE see issue #73448
         //~| NOTE `#[warn(nontrivial_structural_match)]` on by default

tests/ui/consts/const_in_pattern/custom-eq-branch-warn.stderr

@@ -6,6 +6,8 @@ LL |         BAR_BAZ => panic!(),
    |
    = 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 #73448 <https://github.com/rust-lang/rust/issues/73448>
+   = 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: `#[warn(nontrivial_structural_match)]` on by default
 
 warning: 1 warning emitted

tests/ui/consts/const_in_pattern/incomplete-slice.stderr

@@ -6,6 +6,8 @@ LL |         E_SL => {}
    |
    = 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: `#[warn(indirect_structural_match)]` on by default
 
 error[E0004]: non-exhaustive patterns: `&_` not covered

tests/ui/consts/const_in_pattern/issue-78057.stderr

@@ -3,6 +3,9 @@ error: to use a constant of type `Opaque` in a pattern, `Opaque` must be annotat
    |
 LL |         FOO => {},
    |         ^^^
+   |
+   = 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
 
 error: unreachable pattern
   --> $DIR/issue-78057.rs:14:9

tests/ui/consts/const_in_pattern/no-eq-branch-fail.stderr

@@ -3,6 +3,9 @@ error: to use a constant of type `Foo` in a pattern, `Foo` must be annotated wit
    |
 LL |         BAR_BAZ => panic!(),
    |         ^^^^^^^
+   |
+   = 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
 
 error: aborting due to previous error
 

tests/ui/consts/const_in_pattern/reject_non_partial_eq.stderr

@@ -3,6 +3,9 @@ error: to use a constant of type `NoPartialEq` in a pattern, `NoPartialEq` must
    |
 LL |         NO_PARTIAL_EQ_NONE => println!("NO_PARTIAL_EQ_NONE"),
    |         ^^^^^^^^^^^^^^^^^^
+   |
+   = 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
 
 error: aborting due to previous error