fix: add fallback case in generated `PartialEq` impl
Partially fixes#13727.
When generating `PartialEq` implementations for enums, the original code can already generate the following fallback case:
```rs
_ => std::mem::discriminant(self) == std::mem::discriminant(other),
```
However, it has been suppressed in the following example for no good reason:
```rs
enum Either<T, U> {
Left(T),
Right(U),
}
impl<T, U> PartialEq for Either<T, U> {
fn eq(&self, other: &Self) -> bool {
match (self, other) {
(Self::Left(l0), Self::Left(r0)) => l0 == r0,
(Self::Right(l0), Self::Right(r0)) => l0 == r0,
// _ => std::mem::discriminant(self) == std::mem::discriminant(other),
// ^ this completes the match arms!
}
}
}
```
This PR has removed that suppression logic.
~~Of course, the PR could have suppressed the fallback case generation for single-variant enums instead, but I believe that this case is quite rare and should be caught by `#[warn(unreachable_patterns)]` anyway.~~
After this fix, when the enum has >1 variants, the following fallback arm will be generated :
* `_ => false,` if we've already gone through every case where the variants of `self` and `other` match;
* The original one (as stated above) in other cases.
---
Note: The code example is still wrong after the fix due to incorrect trait bounds.
3a7215b92e