Use Pin to pin RWLock.

[m-ou-se]

Use Pin to guarantee the no-moving requirement of sys_common::RWLock.

This makes some things safe that were unsafe before.

sys_common::RWLock::destroy is now changed to a regular (safe) Drop implementation, because it no longer has unsafe requirements.

This change leaves the unsafe sys::RwLock untouched. Those implementations should probably also use a Pin to make them a bit safer, and move their destroy() to Drop. But that can be a later change. (I want to wait for #77666 and #77657 before touching sys.)

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r? @withoutboats

Following our conversation on Zulip, this is a better example where Pin<&Self> is useful.

[bors]

☔ The latest upstream changes (presumably #82982) made this pull request unmergeable. Please resolve the merge conflicts.

[jyn514]

I want to wait for #77666 and #77657 before touching sys.

Both those PRs have been merged.

[RalfJung]

Isn't this changing the safety preconditions? Previously there was something about "active users", which is no longer mentioned.

This seems related to #85434. The "no active users" condition is not actually upheld by the users of this code. So it makes sense to remove this comment here, but IMO then there should be a comment one layer down -- because there pthread_mutex_destroy is being called and the safety precondition does not ensure all that needs to be ensured. (Basically, this moves the "active users" comment instead of removing it entirely, and acknowledges that we have a gap in our safety reasoning here.)

[Dylan-DPC]

@m-ou-se any updates on this?

[Dylan-DPC]

Closing this due to inactivity.