Impl Sync for std::cell::RefMut

Is there any reason that std::cell::RefMut needs to be !Sync? I can see that std::cell::Ref needs to be !Sync in order for std::cell::Ref::clone() to be sound, but I don't think this is also the case for RefMut.

I think Iā€™d agree with your assessment.

I also found a relevant discussion on github: