Send + Sync

Most types you come across are Send + Sync:

  • i8, f32, bool, char, &str, …
  • (T1, T2), [T; N], &[T], struct { x: T }, …
  • String, Option<T>, Vec<T>, Box<T>, …
  • Arc<T>: Explicitly thread-safe via atomic reference count.
  • Mutex<T>: Explicitly thread-safe via internal locking.
  • mpsc::Sender<T>: As of 1.72.0.
  • AtomicBool, AtomicU8, …: Uses special atomic instructions.

The generic types are typically Send + Sync when the type parameters are Send + Sync.

Send + !Sync

These types can be moved to other threads, but they’re not thread-safe. Typically because of interior mutability:

  • mpsc::Receiver<T>
  • Cell<T>
  • RefCell<T>

!Send + Sync

These types are thread-safe, but they cannot be moved to another thread:

  • MutexGuard<T: Sync>: Uses OS level primitives which must be deallocated on the thread which created them.

!Send + !Sync

These types are not thread-safe and cannot be moved to other threads:

  • Rc<T>: each Rc<T> has a reference to an RcBox<T>, which contains a non-atomic reference count.
  • *const T, *mut T: Rust assumes raw pointers may have special concurrency considerations.