Advertise matches! as an alternative to PartialEq on E0369

[lukaslueg]

Code

enum Foobar {
    Foo,
    Bar
}


fn main() {
    // This does not compile due to E0369
    // This only advertised remedy is to impl PartialEq
    assert!(Foobar::Foo != Foobar::Bar);
    
    // ... but this is also possible, and does not require PartialEq
    assert!(!matches!(Foobar::Foo, Foobar::Bar));
}

Current output

Compiling playground v0.0.1 (/playground)
error[E0369]: binary operation `!=` cannot be applied to type `Foobar`
  --> src/main.rs:10:25
   |
10 |     assert!(Foobar::Foo != Foobar::Bar);
   |             ----------- ^^ ----------- Foobar
   |             |
   |             Foobar
   |
note: an implementation of `PartialEq` might be missing for `Foobar`
  --> src/main.rs:1:1
   |
1  | enum Foobar {
   | ^^^^^^^^^^^ must implement `PartialEq`
help: consider annotating `Foobar` with `#[derive(PartialEq)]`
   |
1  + #[derive(PartialEq)]
2  | enum Foobar {
   |

For more information about this error, try `rustc --explain E0369`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Desired output

The binary operation can be avoided altogether by using structural equality (e.g. `matches!(Foobar::Foo, Foobar::Bar)`). It'd be nice if E0369 could detect that there are two remedies to the problem:

* Implement `PartialEq` as currently advertised, and keep the binary operation
* Use `matches!` instead of `PartialEq`.

Rationale and extra context

People coming from other programming languages might not be aware that by-value comparison (requiring PartialEq) and pattern-matching aren't the same thing. Clippy warns-by-default specifically on Option<T> == None (looking at you, Python!).

IMHO the help on E0369 should advertise both solutions here; it's an anti-pattern to teach people that by-value comparison is required when they are actually doing structural comparison.

Other cases

Rust Version

Rust 1.86 stable

Anything else?

playground