Add implementations of AsFd/AsHandle/AsSocket

[sfackler]

The new release of socket2 works with these traits rather than the old AsRaw* ones.

[Thomasdezeeuw]

The implementation LGTM, I'll leave the feature detection/enabling to @Darksonn

[Darksonn]

I made this review back when you posted it, but somehow I failed to click the button to submit it.

[Darksonn]

This would only be correct if AsFd was introduced in the exact same nightly as const mutex new. Can you have it use a separate probe here?

Generally, the purpose behind these checks is that some nightlies claim to be 1.63 without supporting the feature, and we need to make sure we actually compile on those nightlies too.

[sfackler]

It seems like it'd be safer to just constrain the impls to 1.64 or greater to avoid having to deal with the nightly probe. Does that seem workable to you? Since there are different traits on different targets, the probe block is a bit more awkward here than for something like const Mutex::new.

[Darksonn]

You can just do this:

const UNIX_AS_FD_PROBE: &str = r#"
{
    #[allow(unused_imports)]
    use std::os::unix::io::AsFd as _;
}
"#;

const WINDOWS_AS_HANDLE_PROBE: &str = r#"
{
    #[allow(unused_imports)]
    use std::os::windows::io::AsHandle as _;
}
"#;

if ac.probe_expression(UNIX_AS_FD_PROBE) || ac.probe_expression(WINDOWS_AS_HANDLE_PROBE) {
    enable_as_fd = true;
}

The first nightly where the traits are considered stable is 2022-06-16 on both platforms, so it doesn't matter if the build script mixes up the platform during cross compilation.

[sfackler]

Updated

[Darksonn]

Thanks.