Remove extra traits

[rj00a]

This PR removes the extra_traits feature and all implementations of Eq, Hash, and Debug as mentioned in #3248.

The decision to remove Debug impls was left as an open question, but I've gone ahead and removed them here in the interest of maintainability and consistency.

Closes #2700, #2064, #2816

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[rustbot]

Some changes occurred in OpenBSD module

cc @semarie

Some changes occurred in solarish module

cc @jclulow, @pfmooney

Some changes occurred in OpenBSD module

cc @semarie

[rj00a]

I had to allow multiple s! macros in one module because the s_no_extra_traits! macro no longer exists.

[bors]

☔ The latest upstream changes (presumably #3617) made this pull request unmergeable. Please resolve the merge conflicts.

[myl7]

Hi, I am the one who is working on the project to modernize the libc crate (Related: OSPP Idea OSPP Zulip).
I would like to ask that if I could rebase your commits and develop upon them, to complete the "trait implementations" change for libc v1?

[SteveLauC]

I would like to ask that if I could rebase your commits and develop upon them, to complete the "trait implementations" change for libc v1?

Hi, would you like to show me what these trait impl would look like in libc 1.0, this is not covered in the tracking issue and that OSPP page. Nix, as a big consumer of the libc crate, uses them a lot: nix-rust/nix#2468

[erickt]

As an alternative to getting rid of extra_traits, another direction could be to use or build a library like zerocopy to assert at compile time that all the fields of a union can be referenced without UB? That could be done through a test-only cfg to reduce compile times, or if the MSRV didn't support this or something.

[myl7]

what these trait impl would look like in libc 1.0

According to the "Trait implementations" section, I think Eq and Hash impl would be completely removed.
Debug impl may be kept, but at least future code would not impl it.