pub unsafe auto trait Sync { }Expand description
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:
&TisSendif and only ifTisSync&mut TisSendif and only ifTisSend&Tand&mut TareSyncif and only ifTisSync
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 Args
impl !Sync for ArgsOs
impl Sync for alloc::string::Drain<'_>
impl Sync for core::ffi::c_str::Bytes<'_>
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 Sync for bytes::bytes::Bytes
impl Sync for BytesMut
impl<'a> Sync for IoSlice<'a>
impl<'a> Sync for IoSliceMut<'a>
impl<'a, T, const CAP: usize> Sync for arrayvec::arrayvec::Drain<'a, T, CAP>where
T: Sync,
impl<Dyn> Sync for DynMetadata<Dyn>where
Dyn: ?Sized,
impl<T> !Sync for *const Twhere
T: ?Sized,
impl<T> !Sync for *mut Twhere
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 std::sync::mpsc::Receiver<T>
impl<T> Sync for ThinBox<T>
ThinBox<T> is Sync if T is Sync because the data is owned.
impl<T> Sync for alloc::collections::linked_list::Iter<'_, T>where
T: Sync,
impl<T> Sync for alloc::collections::linked_list::IterMut<'_, T>where
T: Sync,
impl<T> Sync for SyncUnsafeCell<T>
impl<T> Sync for NonZero<T>where
T: ZeroablePrimitive + Sync,
impl<T> Sync for ChunksExactMut<'_, T>where
T: Sync,
impl<T> Sync for ChunksMut<'_, T>where
T: Sync,
impl<T> Sync for core::slice::iter::Iter<'_, T>where
T: Sync,
impl<T> Sync for core::slice::iter::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,
impl<T> Sync for std::sync::mpmc::Receiver<T>where
T: Send,
impl<T> Sync for std::sync::mpmc::Sender<T>where
T: Send,
impl<T> Sync for std::sync::mpsc::Sender<T>where
T: Send,
impl<T> Sync for OnceLock<T>
impl<T> Sync for MappedMutexGuard<'_, T>
impl<T> Sync for Mutex<T>
T must be Send for Mutex to be Sync.
This ensures that the protected data can be accessed safely from multiple threads
without causing data races or other unsafe behavior.
Mutex<T> provides mutable access to T to one thread at a time. However, it’s essential
for T to be Send because it’s not safe for non-Send structures to be accessed in
this manner. For instance, consider Rc, a non-atomic reference counted smart pointer,
which is not Send. With Rc, we can have multiple copies pointing to the same heap
allocation with a non-atomic reference count. If we were to use Mutex<Rc<_>>, it would
only protect one instance of Rc from shared access, leaving other copies vulnerable
to potential data races.
Also note that it is not necessary for T to be Sync as &T is only made available
to one thread at a time if T is not Sync.
impl<T> Sync for MutexGuard<'_, T>
T must be Sync for a MutexGuard<T> to be Sync
because it is possible to get a &T from &MutexGuard (via Deref).