Equate fn outputs when inferring RPITIT hidden types

[compiler-errors]

When we are trying to infer the hidden types for RPITITs, we need to equate the output tys instead of just subtyping them. For example:

trait Foo { fn bar() -> impl Sized {} }

impl Foo for () { fn bar() -> &'static str { "" } }

If we just subtype the signatures fn() -> &'static str <: fn() -> _#1t (where _#1t is the variable we've used to infer impl Sized), we'll end up &'static str <: _#1t, which causes us to infer _#1t = #'_#2r str, where '_#2r is unconstrained, which gets fixed up to ReEmpty, and which is certainly not what we want.

I can't actually think of a way to make this fail to compile, because during borrowck we've already done the method probe, and so we just look at the impl method signature and see the &'static str any time we call <() as Foo>::bar(). But this does cause the ICE here in @jackh726's "Remove ReEmpty" PR (#98559) to stop ICEing, because after that PR we were leaking unconstrained region variables into the typeck results.

r? types

[jackh726]

It's interesting. This is a place where a higher-ranked for<'a> &'a str probably makes sense. I'm trying to imagine what else we could expect this to be. 'static str obviously "works", but could be limiting if there are multiple hidden types. But maybe it's right because it's in the function signature, i.e. we're saying exactly what this signature is.

Anyways, this hack is fine for now. r=me with this issue linked in the comment

[compiler-errors]

@bors r=jackh726

[bors]

📌 Commit 022e3fe has been approved by jackh726

It is now in the queue for this repository.