Implement Copy/Clone for closures

[scalexm]

Implement RFC #2132 (tracking issue: #44490).

NB: I'm not totally sure about the whole feature gates thing, that's my first PR of this kind...

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[aidanhs]

Thanks for the PR @scalexm, we'll check in now and again to make sure @pnkfelix or another reviewer gets to this soon!

[arielb1]

Don't you need to check the feature-gate status of the crate that defined the closure, to avoid ICEs?

Crate A:

#![feature(clone_closures)]pubtraitGeneric{fnfoo<T:Clone>(self,t:T);}pubfnexample<G:Generic>(g:G){
g.foo(|| {});}

Crate B:

// no feature flags!externcrate a;structS;implGenericforS{fnfoo<T:Clone>(self,t:T){let _ = t.clone();}}fnmain(){// trans will monomorphize this, and will try to evaluate `[closure]: Clone`, which// will fail and cause an ICE.
foo::example(S);}

Another option would be to make all closures Copy & Clone in trans, but that would be visible through specialization etc. so I'm not sure it's the better idea.

[scalexm]

@arielb1 Yes you're right. I'm working on it.

[arielb1]

Is there a reason for this assertion? This method works just as well for structs and univariant enums.

[scalexm]

Indeed, but it won't work for arrays. I guess the match can be extended in the future if needed.

[bors]

☔ The latest upstream changes (presumably #44634) made this pull request unmergeable. Please resolve the merge conflicts.

[arielb1]

@bors r+

[bors]

📌 Commit 3fa3fe0 has been approved by arielb1

[bors]

⌛ Testing commit 3fa3fe0 with merge e2504cf...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: arielb1
Pushing e2504cf to master...