filter lifetimes in impl Trait

[nikomatsakis]

currently, impl Trait usages capture all lifetimes that are in scope (and this kind of fails around late-bound lifetimes). In the newer RFC, the proposal was that they should only capture those lifetimes that are used in the impl trait itself.

Here are some mentoring instructions I wrote-up from before on how one might do this. But another way to think about this might be to think about implementing abstract type (perhaps in a limited form) and desugaring to that.

However, work on this issue is blocked on #44727.

[nikomatsakis]

See also: #44727

[ErichDonGubler]

This issue is now ready to be worked on, right? :)

[Yoric]

Since #44727 is closed, I'm interested in working on this. @nikomatsakis a few pointers on which files I'm supposed to dig into would be helpful, though.

[nikomatsakis]

Er, sorry, this is done actually. =)