Change derive macro for Eq to not impl Eq trait fn

[KiChjang]

Fixes #152504.

r? @cyrgani

[rustbot]

Changes to the code generated for builtin derived traits.

cc @nnethercote

[theemathas]

A test case that seems like might break with this:

trait Trait<T> {}

#[derive(PartialEq, Eq)]
struct Thing<T: Trait<Self>>(T);

[theemathas]

A less exotic test case:

#[derive(PartialEq, Eq)]
struct Thing(Option<Box<Self>>);

[Kivooeo]

Nick, I would reassign to you (feel free to reroll) since @cyrgani is a triage member and can't approve compiler changes

r? nnethercote

[KiChjang]

A less exotic test case:

#[derive(PartialEq, Eq)]
struct Thing(Option<Box<Self>>);

Gah, because we're now generating a const item instead of a trait impl block, we'd need to rewrite the Self reference to its concrete type, but this is a bit tough as it would require us to supply the correct generic params to the Self concrete type if it has any.

A better solution now would be to emit the following:

impl<...> Type<...> {
    const fn assert_fields_are_eq() {
        let _: ::core::cmp::AssertParamIsEq<Option<Box<Self>>>;
    }
}

This would side-step the issue of rewriting Self into its concrete type.

[KiChjang]

This is expected since the Eq derive macro now respans generics in the where clause, which causes the same E0107 error to be reported in 2 different contexts instead of simply the original one (i.e. in the derived trait impl for Eq)

[KiChjang]

Looks like CI is passing now, r? @nnethercote

[rustbot]

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[cyrgani]

You should be able to delete the fn assert_fields_are_eq(&self) {} method from the Eq trait again now.

[cyrgani]

Could you also add a test that something like

fn main() {
    X::assert_fields_are_eq();
}
#[derive(PartialEq, Eq)]
struct X(u8);

will not compile?

[cyrgani]

I guess this will also need #[inline] like #153157?