Suggest using equality comparison instead of pattern matching on non-structural constant in pattern

[estebank]

error: constant of non-structural type `partial_eq::S` in a pattern
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:16:18
   |
LL |     struct S;
   |     -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL |     const C: S = S;
   |     ---------- constant defined here
...
LL |             Some(C) => {}
   |                  ^ constant of non-structural type
   |
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
   |
LL |     impl PartialEq<S> for S {
   |     ^^^^^^^^^^^^^^^^^^^^^^^
help: add a condition to the match arm checking for equality
   |
LL -             Some(C) => {}
LL +             Some(binding) if binding == C => {}
   |

error: constant of non-structural type `partial_eq::S` in a pattern
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:22:18
   |
LL |     struct S;
   |     -------- `partial_eq::S` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
LL |     const C: S = S;
   |     ---------- constant defined here
...
LL |         let Some(C) = Some(S) else { return; };
   |                  ^ constant of non-structural type
   |
note: the `PartialEq` trait must be derived, manual `impl`s are not sufficient; see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
  --> $DIR/suggest_equality_comparison_instead_of_pattern_matching.rs:5:5
   |
LL |     impl PartialEq<S> for S {
   |     ^^^^^^^^^^^^^^^^^^^^^^^
help: check for equality instead of pattern matching
   |
LL -         let Some(C) = Some(S) else { return; };
LL +         if Some(C) == Some(S) { return; };
   |

Fix #42753.

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

[rustbot]

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 69 candidates
• Random selection from 14 candidates