Make UnsafeCell::into_inner safe

[varkor]

This fixes#35067. It will require a Crater run as discussed in that
issue.

[rust-highfive]

r? @Mark-Simulacrum

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

[Mark-Simulacrum]

r? @BurntSushi

@bors try

cc @aidanhs for crater

[bors]

⌛ Trying commit 4829d50 with merge 68950fc...

[cramertj]

This shouldn't need a crater anymore since this works now:

let x:unsafefn(_) -> _ = UnsafeCell::<u32>::into_inner;

I added that conversion way back in #37389.

[bors]

💔 Test failed - status-travis

[nikomatsakis]

It still technically needs a crater run. There are other ways to expose the type. e.g. you might have a trait implemented only for unsafe fn() and so forth.

[cramertj]

@nikomatsakis Even then you'd still have to trigger the coercion to unsafe fn() first, wouldn't you? For example:

traitFoo{fnbar(&self){}}implFooforunsafefn(){}unsafefnbaz(){}fnmain(){// These do not work:
baz.bar();Foo::bar(&baz);// These do:let x:unsafefn() = baz;
x.bar();Foo::bar(&x);}

[nikomatsakis]

@cramertj ah, hmm, good point. You may be right. =)

[carols10cents]

errrrr try failed because of... cargo? I'm going to see if retrying works...

@bors try

[kennytm]

@bors clean retry try

[bors]

⌛ Trying commit 4829d50 with merge 15e0ec4...

[bors]

💔 Test failed - status-travis

[kennytm]

Nope doesn't work. Cargo needs to add #[allow(unused_unsafe)] for their LazyCell.

[Mark-Simulacrum]

This is a breaking change because of the deny(warnings) resulting the compilation failure, though it is minor. We have done this in the past, though (most recently with the ASCII extension traits). @rust-lang/libs: Do we want to do this considering the breaking change aspect?

[alexcrichton]

@rfcbot fcp merge

Curious to see what the libs team thinks!

[rfcbot]

Team member @alexcrichton has proposed to merge this. The next step is review by the rest of the tagged teams:

• @BurntSushi
• @Kimundi
• @alexcrichton
• @aturon
• @dtolnay
• @sfackler

No concerns currently listed.

Once these reviewers reach consensus, this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[alexcrichton]

@bors: r+

[bors]

📌 Commit 4829d50 has been approved by alexcrichton

[bors]

⌛ Testing commit 4829d50 with merge ea2b106d7305b530c34266858ecbd55bfcae03b3...

[bors]

💔 Test failed - status-travis

[kennytm]

Same error as #47204 (comment), please fix cargo first.

(Triage: Blocked by rust-lang/cargo#4972)

[kennytm]

@bors retry

The cargo on master contained rust-lang/cargo#4972 already.

[bors]

⌛ Testing commit 4829d50 with merge 21882aa...

[bors]

💔 Test failed - status-appveyor

[alexcrichton]

Er this passed, just a 3 hour timeout, merged manually