Check xform_ret_ty for WF in the new solver to improve method winnowing

[compiler-errors]

This is a bit interesting. Method probing in the old solver is stronger than the new solver because eagerly normalizing types causes us to check their corresponding trait goals. This is important because we don't end up checking all of the where clauses of a method when method probing; just the where clauses of the impl. i.e., for:

impl Foo
where
   WC1,
{
    fn method()
    where
        WC2,
    {}
}

We only check WC1 and not WC2. This is because at this point in probing the method is instantiated w/ infer vars, and checking the where clauses in WC2 will lead to cycles if we were to check them (at least that's my understanding; I could investigate changing that in general, incl. in the old solver, but I don't have much confidence that it won't lead to really bad overflows.)

This PR chooses to emulate the old solver by just checking that the return type is WF. This is theoretically stronger, but I'm not too worried about it. I think we alternatively have several approaches we can take here, though this one seems the simplest. Thoughts?

r? lcnr

[lcnr]

please add a non-rustc test for this 🤔 or remove the extern type stuff from this one

[lcnr]

we may want to chat about this in sync, I agree that there are a few ways to handle this. Relying on WF feels fairly straightforward and should closely match the old solver behavior 🤔 Can you add a test where we've got some struct W<T: Bound>(T) and an fn returning W<T>?

I feel like the 'correct' solution is to also check WC2 eagerly (given that we should always do so when existentially instantiating a binder). This is significantly? stronger and may cause issues with incomplete guidance and perf 🤔

I am fine with the current approach, so we can merge this as is, would like a fixme linking to a trait-system-refactor-initiative repo issue about this behavior

[compiler-errors]

@bors r=lcnr rollup

[bors]

📌 Commit 4120fdb has been approved by lcnr

It is now in the queue for this repository.