UnsafeCell::into_inner is safe regressions

[Mark-Simulacrum]

This is likely going to be closed as expected breakage. This means those cc-ed below should probably aim to fix their crate and release a new point release.

Caused by #47204.

• oncemutex 0.1.1 cc @reem
• shared-mutex 0.3.1 cc @reem

Estimated breakage: 43 versions (0.05% versions); 11 crates (0.08% crates).

[leoyvens]

I'm pretty sure @reem doesn't maintain things anymore.

[sfackler]

Those are just lint failures so it won't prevent anyone from depending on those crates, fortunately.

[alexcrichton]

I'm going to close this as we don't typically back things out due to lint changes and otherwise the impact here looks relatively small