Miss Dada  🏳️‍⚧️

No, the compiler relies (only) on stuff being Sync to impl Sync. There is additionally an impl of &T: Send for T:Sync.

"if you follow through", this is a combination of features interacting together

MutexGuard contains an &Mutex. Mutex itself is Sync+Send when T is Send, due to an impl. So is &Mutex, then. So is MutexGuard, then.

rustc only inferred the second two steps. The rest was explicitly provided to it; we *told* the compiler "T: Send => Mutex<T>: Sync+Send"

relevant line: unsafe impl<T: ?Sized + Send> Sync for Mutex<T> { }

It really does seem like using `PhantomData` would have made more sense than opting out of the autotrait impl

PhantomData<&mut T> I suppose? Perhaps. I wonder how the variance of that works out. Actually, there's a chance variance is wrong there.