Trait MaybeSync 

pub unsafe auto trait MaybeSync { }

Types for which it is safe to share references between threads.

This trait is automatically implemented when the compiler determines it’s appropriate.

The precise definition is: a type T is Sync if and only if &T is Send. In other words, if there is no possibility of undefined behavior (including data races) when passing &T references between threads.

As one would expect, primitive types like u8 and f64 are all Sync, and so are simple aggregate types containing them, like tuples, structs and enums. More examples of basic Sync types include “immutable” types like &T, and those with simple inherited mutability, such as Box<T>, Vec<T> and most other collection types. (Generic parameters need to be Sync for their container to be Sync.)

A somewhat surprising consequence of the definition is that &mut T is Sync (if T is Sync) even though it seems like that might provide unsynchronized mutation. The trick is that a mutable reference behind a shared reference (that is, & &mut T) becomes read-only, as if it were a & &T. Hence there is no risk of a data race.

A shorter overview of how Sync and Send relate to referencing:

• &T is Send if and only if T is Sync
• &mut T is Send if and only if T is Send
• &T and &mut T are Sync if and only if T is Sync

Types that are not Sync are those that have “interior mutability” in a non-thread-safe form, such as Cell and RefCell. These types allow for mutation of their contents even through an immutable, shared reference. For example the set method on Cell<T> takes &self, so it requires only a shared reference &Cell<T>. The method performs no synchronization, thus Cell cannot be Sync.

Another example of a non-Sync type is the reference-counting pointer Rc. Given any reference &Rc<T>, you can clone a new Rc<T>, modifying the reference counts in a non-atomic way.

For cases when one does need thread-safe interior mutability, Rust provides atomic data types, as well as explicit locking via sync::Mutex and sync::RwLock. These types ensure that any mutation cannot cause data races, hence the types are Sync. Likewise, sync::Arc provides a thread-safe analogue of Rc.

Any types with interior mutability must also use the cell::UnsafeCell wrapper around the value(s) which can be mutated through a shared reference. Failing to doing this is undefined behavior. For example, transmute-ing from &T to &mut T is invalid.

See the Nomicon for more details about Sync.

Implementors

impl !Sync for Arguments<'_>

impl !Sync for LocalWaker

impl Sync for TypeId

impl Sync for Bytes<'_>

impl Sync for Location<'_>

impl Sync for AtomicBool

impl Sync for AtomicI8

impl Sync for AtomicI16

impl Sync for AtomicI32

impl Sync for AtomicI64

impl Sync for AtomicIsize

impl Sync for AtomicU8

impl Sync for AtomicU16

impl Sync for AtomicU32

impl Sync for AtomicU64

impl Sync for AtomicUsize

impl Sync for Waker

impl<Dyn> Sync for DynMetadata<Dyn>

where Dyn: ?Sized,

impl<T> !Sync for *const T

where T: ?Sized,

impl<T> !Sync for *mut T

where T: ?Sized,

impl<T> !Sync for OnceCell<T>

impl<T> !Sync for Cell<T>

where T: ?Sized,

impl<T> !Sync for RefCell<T>

where T: ?Sized,

impl<T> !Sync for UnsafeCell<T>

where T: ?Sized,

impl<T> !Sync for NonNull<T>

where T: ?Sized,

NonNull pointers are not Sync because the data they reference may be aliased.

impl<T> Sync for SyncUnsafeCell<T>

where T: Sync + ?Sized,

impl<T> Sync for NonZero<T>

where T: ZeroablePrimitive + Sync,

impl<T> Sync for UnsafePinned<T>

where T: Sync + ?Sized,

impl<T> Sync for ChunksExactMut<'_, T>

where T: Sync,

impl<T> Sync for ChunksMut<'_, T>

where T: Sync,

impl<T> Sync for Iter<'_, T>

where T: Sync,

impl<T> Sync for IterMut<'_, T>

where T: Sync,

impl<T> Sync for RChunksExactMut<'_, T>

where T: Sync,

impl<T> Sync for RChunksMut<'_, T>

where T: Sync,

impl<T> Sync for AtomicPtr<T>

impl<T> Sync for Exclusive<T>

where T: ?Sized,

Auto implementors