Add cygwin support

[Berrysoft]

Description

rust-lang/rust#134999

Most changes are ported from https://github.com/ookiineko-cygport/libc

Sources

https://github.com/cygwin/cygwin/tree/main/winsup/cygwin/include

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

I'm not sure how to test a no_std target. Actually I need these changes to implement std for cygwin target.

[rustbot]

r? @tgross35

rustbot has assigned @tgross35.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Berrysoft]

Stable compilers failed because cygwin target is only supported by nightly ones nowadays. Are there some ways to solve this problem?

[tgross35]

Stable compilers failed because cygwin target is only supported by nightly ones nowadays. Are there some ways to solve this problem?

If it's just check-cfg failing, then it needs an update in

libc/build.rs

Line 25 in 3695157

const CHECK_CFG_EXTRA: &'static [(&'static str, &'static [&'static str])] = &[

[tgross35]

Most of this looks pretty good but there are a handful of changes to get this more in line with the other OSs.

Does cygwin only ever use newlib, or are there other libc implementations?

[mati865]

AFAIK Nnwlib is the only one ported.

[Berrysoft]

Newlib is the only libc on cygwin, but some definitions are replaced, e.g., signal-related structures.

[tgross35]

I did a more thorough read through of everything here, I did not check the definitions for correctness but everything looks pretty good for implementation. I have a handful of small requests then this should be good to merge.

Please squash after these changes are done.

[Berrysoft]

The windows CI failed because the self-update command of rustup failed. I have also met this problem in my other repos, and it's not caused by this PR.

[tgross35]

Thanks for the updates, everything looks good. Looks like CI is going again.

[Berrysoft]

Thank you! I don't understand why the arm64 android CI failed. Maybe a rerun would be fine?

[tgross35]

Android is just flaky #4297, restarted

[Berrysoft]

I see. It reached the timeout (25 minutes) and was cancelled again.

[tgross35]

The windows CI failed because the self-update command of rustup failed. I have also met this problem in my other repos, and it's not caused by this PR.

Fwiw things still seem broken, passing --no-self-update corrected the issue.

[tgross35]

This is in the latest release fyi https://github.com/rust-lang/libc/releases/tag/0.2.171