Unsafe impls

[Ericson2314]

While working on Once, I noticed that our unsafe Send and Sync impls are different from std---I am not sure why or whether its sound. [I ended up going with the RwLock ones for Once, since both give access to a &T to potentially multiple threads.]

• https://github.com/mvdnes/spinlock-rs/blob/master/src/mutex.rs#L95
• https://github.com/mvdnes/spinlock-rs/blob/master/src/rw_lock.rs#L72-L73
• https://doc.rust-lang.org/nightly/src/std/up/src/libstd/sync/mutex.rs.html#129-131
• https://doc.rust-lang.org/nightly/src/std/up/src/libstd/sync/rwlock.rs.html#75-77

[mvdnes]

At one point, I thought I understood how Send and Sync work, but the more I read the less sure I am.
I do not really know why you would need the Send bound for Mutex to be Sync for example.

Also, I think Mutex is already Send when T: Send, but that needs to be tested.

Any thoughts on this matter would be welcome.

[Ericson2314]

I'm a bit rusty too. Last I heard T: Sync means its safe to share &T across threads, and Sync => Send.

I'd just make them match their std equivalents as I can't think of any reason why differences in implementation would matter here.

[mvdnes]

Seems like a reasonable thing to do, yes