Tracking issue for RFC 2132: copy_closures/clone_closures

[aturon]

This is a tracking issue for the RFC "Copy/Clone Closures" (rust-lang/rfcs#2132).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

[nikomatsakis]

I believe that @scalexm has implemented this RFC already. =)

[arielb1]

@nmatsakis

That was for tuples, not for closures.

[nikomatsakis]

@arielb1 I was referring to #44551

[scalexm]

(I think we can tick the first box)

[nikomatsakis]

What do we want to do around stabilization here? @cramertj was floating the idea of stabilizing only the clone impl for closures, not the copy one, at least until such time as we have a workable "unexpected copy" lint that would push you to invoke clone when copying closures (but that lint probably wants a bit of design).

Personally, I think I'd be ok stabilizing copy/clone closures in their entirety, but we probably want to see a bit more usage in the wild.

Can anyone report on using this feature?

[cramertj]

I've been messing with it in some personal stuff, and I've really enjoyed having the feature. I'd be in favor of stabilizing both Copy and Clone.