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.