Use Rust union types

[kellda]

For #1020

Replace some types with unions now that rustc supports them.

I also found sigevent.sigev_notify_thread_id, siginfo_t.__data and sigaction.sa_sigaction that I think should be unions, but I wasn't sure.

[rust-highfive]

r? @Amanieu

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[kellda]

Also I had to ignore iffaddrs.ifa_ifu on Linux to pass CI

[Amanieu]

Unfortunately I believe this is a breaking change, which means we can't actually do this until we bump the crate version to 1.0.

cc @JohnTitor

[kellda]

I'm aware that it is a breaking change and that's not a problem for me to wait until a new major version for this to be merged.

[bors]

☔ The latest upstream changes (presumably #1975) made this pull request unmergeable. Please resolve the merge conflicts.

[tgross35]

@kellda main is now intended for libc 1.0. Are you able to rebase this?

@rustbot author

[rustbot]

Some changes occurred in solarish module

cc @jclulow, @pfmooney

[kellda]

I'm unsure how to find out what's wrong with sigval on emscripten...

[kellda]

Btw, I noted (when I originally opened this PR) that the following fields may also be wroth considering turning into unions:

• sigevent.sigev_notify_thread_id
• siginfo_t.{_pad/__data}
• sigaction.sa_sigaction

[tgross35]

I'm unsure how to find out what's wrong with sigval on emscripten...

I also don't know. I'll mark this as review as a reminder that I need to take a look at this.

@rustbot review

Btw, I noted (when I originally opened this PR) that the following fields may also be wroth considering turning into unions:

* sigevent.sigev_notify_thread_id

* siginfo_t.{_pad/__data}

* sigaction.sa_sigaction

Feel free to submit another PR!

[tgross35]

This needs another rebase, feel free to just leave emscripten disabled in build.rs as a FIXME if needed.

Regarding traits: don't add Hash/PartialEq/Eq for unions, or leave them but make the bodies unimplemented!("traits") if some other structs depend on them. We are likely going to drop these in 1.0 so we aren't doing any unsafe union field access.

For Debug, make the unions fmt.debug_struct("type_name").finish_non_exhaustive().

@rustbot author