fix: add fallback case in generated PartialEq impl

[rami3l]

Partially fixes #13727.

When generating PartialEq implementations for enums, the original code can already generate the following fallback case:

_ => std::mem::discriminant(self) == std::mem::discriminant(other),

However, it has been suppressed in the following example for no good reason:

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.

[lowr]

LGTM :) Maintainers that have write permissions should review in a few days.

[jonas-schievink]

@bors r+

[bors]

📌 Commit 57fb18e has been approved by jonas-schievink

It is now in the queue for this repository.

[bors]

⌛ Testing commit 57fb18e with merge 3a7215b...

[bors]

☀️ Test successful - checks-actions
Approved by: jonas-schievink
Pushing 3a7215b to master...

[lnicola]

changelog fix (first contribution) add fallback case to generated PartialEq impl