pthread_exit return type fix

[newpavlov]

Closes: #1520

[rust-highfive]

r? @gnzlbg

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[gnzlbg]

Thank you. I think we should do this, but I want to be careful that we can revert this easily if this breaks too much code.

I hope that nobody is using the return value of pthread_exit in any way, but who knows, so I'm going to land this in the following way:

• I'm going to do a libc release (0.2.64):
  • this is currently blocked on deciding what to do with Add FreeBSD10 support #1518
• I'm going to then update libc on rust-lang rust to 0.2.64
  • and wait a couple of nightlies to see if that goes smoothly
• At this point we have a 0.2.64 version that's known to work
• Merge this change, and release 0.2.65 only with this change
  • if people complain too much, we can yank 0.2.65
• Update rust-lang/rust to 0.2.65.
  • if something breaks, we can yank, and then revert the PR
• Done, this change is good.

This is sadly a lot of steps, and will take ~1 week for the 0.2.64 release, and ~1 week for the 0.2.65 release, but such is life.

If after doing the 0.2.64 thing something high-priority must land upstream (new target, critical target upgrade), I'll prioritize that, and do this in the next release.

[newpavlov]

I don't think it should break any code. This snippet works without any issues:

fn foo() -> ! { panic!(); }
fn main() { let a: u64 = foo(); }

And during type inference right now Rust defaults to (), e.g. this code:

fn foo() -> ! { panic!(); }
fn main() { let a = foo(); a + 1; }

Returns "cannot add i32 to ()" compilation error.

[gnzlbg]

Ah awesome, I thought that let x = ! would fail to compile.

@bors: r+ then :)

[bors]

📌 Commit 7d1907e has been approved by gnzlbg

[bors]

⌛ Testing commit 7d1907e with merge 8dfdfec...

[bors]

💔 Test failed - status-azure

[gnzlbg]

@bors: retry

[bors]

⌛ Testing commit 7d1907e with merge 2f5ed5d...

[bors]

☀️ Test successful - checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, status-azure
Approved by: gnzlbg
Pushing 2f5ed5d to master...