handle_opaque_type likely mis-uses `eq` relation

[oli-obk]

I was unable to reproduce the issue in the defining scope (that doesn't mean it's not possible, just that I gave up for now ^^). I found this absolutely useless diagnostic though: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b80473c86172959763c139535154c3f9

Did some more thinking and I'm wondering now if the did.is_local() check should actually be infcx.opaque_type_origin(did).is_some() so we only generate those generalized inference vars if we're actually able to handle the opaque type. I was kinda hoping to keep that isolated in handle_opaque_type, but that appears to have been a misplaced hope.

One more thing is that we're using eq inside handle_opaque_types when it should probably be using the relation that is calling handle_opaque_types. So maybe we could scrap the entire generalization logic and just always invoke handle_opaque_types by passing the relation as an argument and forwarding to that instead of using eq.

so many possibilities to try out. Let's merge this PR for now and I'm moving this comment to an issue so we can continue there.

Originally posted by @oli-obk in #99928 (comment)

[compiler-errors]

Did some more thinking and I'm wondering now if the did.is_local() check should actually be infcx.opaque_type_origin(did).is_some() so we only generate those generalized inference vars if we're actually able to handle the opaque type.

Yeah, I've considered this too.

One more thing is that we're using eq inside handle_opaque_types when it should probably be using the relation that is calling handle_opaque_types.

Do we even need to pass down the relation? We just re-invoke eq just to turn it into an error -- could we just return Err(Sorts(..)) right there?

[oli-obk]

while we could fix that site, the reason we are generalizing at all is the other eq in that file that merges the hidden types. If we avoided the generalization and instead replaced that eq with invoking the relation, we may just get generalization "for free" and thus avoid most of the issues here.

[compiler-errors]

Ah yeah, totally overlooked the success case and was focusing too much on the error case 😅

[oli-obk]

I believe we've cleaned up all we can here