fix: Correctly generate default PartialEq::ne

[SpecialMike]

Fixes #12779

For the Generate default members assist on the PartialEq trait, the assist will now give the default implementation instead of generating a function.

[Veykril]

Thanks!
@bors r+

[bors]

📌 Commit 1c32fcf has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1c32fcf with merge c001b9c...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing c001b9c to master...

[lowr]

Sorry to bother after this is merged, but I think this is inadequate. While this fixes the exact problem described in #12779, Implement default members on PartialOrd gives you wrong implementation as well.

Considering those codgen don't take traits' default members into account, I'd argue they were never intended for Implement default members but only for Implement missing members. How about not calling try_gen_trait_body() for Implement default members instead of filtering out every default members in codegen?

[lnicola]

Implement default members on PartialOrd gives you wrong implementation as well.

Wrong? Just wondering, I'm probably missing something.

[lowr]

I get the following with the latest commit:

struct S { data: i32 }

// generated by `Implement default members`
impl PartialOrd for S {
    fn lt(&self, other: &Self) -> bool {
        self.data.partial_cmp(&other.data)
    }

    fn le(&self, other: &Self) -> bool {
        // Pattern `Some(Less | Eq)` optimizes worse than negating `None | Some(Greater)`.
        // FIXME: The root cause was fixed upstream in LLVM with:
        // https://github.com/llvm/llvm-project/commit/9bad7de9a3fb844f1ca2965f35d0c2a3d1e11775
        // Revert this workaround once support for LLVM 12 gets dropped.
        !matches!(self.partial_cmp(other), None | Some(Greater))
    }

    fn gt(&self, other: &Self) -> bool {
        matches!(self.partial_cmp(other), Some(Greater))
    }

    fn ge(&self, other: &Self) -> bool {
        matches!(self.partial_cmp(other), Some(Greater | Equal))
    }
}

The body of PartialOrd::lt() is for PartialOrd::partial_cmp(). I expect that to be like this: matches!(self.partial_cmp(other), Some(Less)).

[lnicola]

Oh, right, good point.

[SpecialMike]

I had that same thought this morning about other traits, I can take on the change to not call try_gen_trait_body for default members as well. Does a new issue need to be made for this, or should the original issue be reopened?

[lowr]

@SpecialMike I should have mentioned I was already working on this, sorry about that.