Make makedev, major, minor const

[igankevich]

Description

This PR marks all makedev, major, minor re-implementations (except Solarish) as const fn and tests major and minor against their C counterparts (macros). This PR closely follows the work that was done in this commit.

The motivation behind this PR is to make Rust code that uses these three functions consistent across different platforms. To give a concrete example: currently one has to wrap major and minor in unsafe blocks on MacOS, whereas on Linux these blocks are not needed. Unnecessary unsafe blocks produce a warning that one silences with #[allow(unused_unsafe)]. All of that confuses code reviewers and creates unnecessary noise. makedev, major and minor are macros on most Unixes, there is no need for them to be unsafe.

As far as I understand these changes do not break existing code: changing unsafe functions to const only produce warnings but not compilation errors.

@rustbot label stable-nominated

Sources

None.

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

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @tgross35 (or someone else) some time within the next two weeks.

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[rustbot]

Some changes occurred in OpenBSD module

cc @semarie

[igankevich]

After testing with the CI I found and fixed a bug in emscripten's major and minor: bit shifts and masking were done in the reverse order. The original macros are here, the same link is in the crate's source code.

[tgross35]

This also changes from unsafe to safe, which should be fine and fixes #3759.

@hoodmane could you double check the emscripten changes?

[tgross35]

Could you name the types here rather than using _?

[igankevich]

I wish I could, but the return type can be c_int, c_uint, major_t, minor_t depending on the platform.

[hoodmane]

Maybe it would be nice to keep the bitwise or terms of the rust code in the same order as they appear in musl for easier validation?

[hoodmane]

Emscripten has actual functions for these in addition to macros so it would be possible to call out to them. But it would be a lot slower than this.

[hoodmane]

Otherwise it looks good to me.

[igankevich]

Maybe it would be nice to keep the bitwise or terms of the rust code in the same order as they appear in musl for easier validation?

Thanks for pointing this out! I've changed the order. Kindly re-requesting the review 🙏