Add HRTB-related regression test

[JohnTitor]

Closes#59311 and cc #71546
This closes the former but the test is taken from #71546 (comment) since it seems they have the same cause and it's simplified.

[lcnr]

uh, so it makes some sense why this is happening 🤔

Thanks for pinging me here. I think what's going on here is that previously for<'a> Ty<'a>: Trait always resulted in CodeSelectionError(Unimplemented) but we now first replace 'a with a placeholder and and then successfully evaluate Ty<'_0>: Trait during borrowck.

So I think that change is fine, but @nikomatsakis should also take a quick look at this.

r? @nikomatsakis

[nikomatsakis]

I'm a bit unsure why this changed behavior, to be honest, and the error seems incorrect to me (that is, I think ideally the code would compile; obviously an error is an improvement over an ICE). It is certainly linked to the for<'a> &'a T: 'static requirement. It's true that this requirement is unsatisfiable, but I don't think it ought to be creating an error within the function body, probably more at the call-site somewhere (which would be the one obligated to prove this).

This probably has something to do with the way we sometimes try to prove that the where-clauses hold even within the callee, which is a bit silly.

[JohnTitor]

but I don't think it ought to be creating an error within the function body, probably more at the call-site somewhere (which would be the one obligated to prove this).

@nikomatsakis Makes sense. So, I think it's a good way here to add this test with a FIXME comment and follow-up the above point later so that we can prevent a regression at least.

[nikomatsakis]

@JohnTitor that sounds reasonable to me, yes

[nikomatsakis]

@JohnTitor can you add a comment to the test that says something like:

// FIXME: It's not clear that this code ought to report an error, but the regression test is here to ensure that it does not ICE. See discussion on #74889 for details.

[JohnTitor]

@nikomatsakis Done :)

[nikomatsakis]

@bors r+ rollup

Thanks!

[bors]

📌 Commit cc36c3d has been approved by nikomatsakis

[bors]

⌛ Testing commit cc36c3d with merge 4b0882c...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: nikomatsakis
Pushing 4b0882c to master...