Return err instead of ICE

[ouz-a]

Having escaping_bound_vars results in ICE when trying to create ty::Binder::dummy, to avoid it we return err like the line above. I think this requires a more sophisticated fix, I would love to investigate if mentorship is available 🤓

Fixes #95023 and #85350

[rust-highfive]

r? @davidtwco

(rust-highfive has picked a reviewer for you, use r? to override)

[jackh726]

This isn't the right fix for this. Roughly, when we see an impl like impl Fn(&isize) for Error {}, that should be impl<'a> Fn<(&'a isze,)> for Error { type Output = (); }. What's actually happening is we are that elided lifetime in the function arg as late-bound, becuse we enter "fn-like elision" here.

So, our final impl_trait_ref really should be Error: Fn<(&'_ isize, )>. Alternatively, We could in theory make impl_trait_ref return a PolyTraitRef and have it return for<'a> Error: Fn(&'a isize,)>, but that's more invasive and not really correct.

See #85477 for a previous (abandoned) attempt at this. And my comment for solutions.

I think probably the simplest option is either to 1) Check if the self ty is an error as in #85477 2) Treat this lifetime as 'static:

Here is resolve lifetime in scope for an impl. Interestingly, we are storing late-bound lifetimes here but probably shouldn't be. (You can try to remove that line and see if anything fails; would be good to know.) But, we can probably add a allow_late_bound field to Scope::Binder, then here only break when that is true (and delay_span_bug otherwise)

[jackh726]

r? @jackh726

[ouz-a]

Thanks for the great review I was not happy with the fix itself 😅 , I will try to implement new solution as you suggested.

[jackh726]

Let me know if you have questions. I'm around on Zulip most of the time.

[jackh726]

This can just be in the same match arm as Scope::ObjectLifetimeDefault etc.

[ouz-a]

How ? Scope::ObjectLifetimeDefault does a completely different thing, that would cause an infinite loop I think.

[jackh726]

Oops, I mean Root/Body

[jackh726]

Sorry, just a couple more things! Also, can you squash the commits?

[jackh726]

@bors r+

[bors]

📌 Commit be566f1 has been approved by jackh726