I am going through Rust atomics and locks book where it builds spin locks.

Here is the structure:

struct SpinLock<T> {
    locked: AtomicBool,
    value: UnsafeCell<T>,
}

unsafe impl<T> Sync for SpinLock<T> where T: Send {}

Now I see the documentation says T is Send if it can be safely moved across thread and T is Sync if &T can be shared with threads concurrently.

It requires T be Send only and not Sync. Why?

but below requires it to be Send + Sync -

struct RwLock<T> { 
    state: AtomicU32, 
    value: UnsafeCell<T>, 
} 

unsafe impl<T> Sync for RwLock<T> where T: Send + Sync {}

Could anyone please explain with an example how these two traits change the behavior of implementers?

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