Make RefMut Sync

[ChayimFriedman2]

This is safe because a &RefMut cannot change the borrow flags.

We cannot impl Send for RefMut because of map_split(); we cannot impl Sync for Ref because of clone().

This is insta-stable and needs a FCP.

This precludes the addition of an API that transforms &RefMut to Ref, so we need to choose what we want.

[rustbot]

r? @m-ou-se

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[m-ou-se]

Do you have a use case for this? Wouldn't it be better to just share the &T or &mut T rather than the RefMut<T>?

[ChayimFriedman2]

@m-ou-se I don't, but I think that we tend to impl traits as we can, no?

[dtolnay]

This precludes the addition of an API that transforms &RefMut to Ref, so we need to choose what we want.

I'd be interested in seeing a prototype of what a sound API for this might look like, to better understand what options we are closing off.

Is it fn(&'short RefMut<'long, T>) -> Ref<'short, T>? I think I can see why that would be sound. The original RefMut remains borrowed for 'short, ensuring it cannot be used to mutate the T, consequently ensuring that no mutation of T is possible during 'short so having other Ref is okay.

And indeed, that would make it problematic to have RefMut<T>: Sync because one could send &RefMut<T> to 2 different threads, each of which performs the conversion to Ref, racing on the reference count.

[ChayimFriedman2]

@dtolnay Yes, that is what I had in mind.

[dtolnay]

@rust-lang/libs-api:
@rfcbot fcp close

Given that there is no known use case benefiting from RefMut<T>: Sync, keeping options open for the API described in #115157 (review) seems more valuable.

Workaround: send the &T instead of &RefMut<T>.

[rfcbot]

Team member @dtolnay has proposed to close this. The next step is review by the rest of the tagged team members:

• @Amanieu
• @BurntSushi
• @dtolnay
• @joshtriplett
• @m-ou-se

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rfcbot]

The final comment period, with a disposition to close, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[dtolnay]

Thanks anyway for the PR!