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.
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.
Implementations on Foreign Types
impl Sync for Argument
impl Sync for FormatSpec
impl Sync for Alignment
impl Sync for Count
impl<T> Sync for Key<T>
impl<T> Sync for SyncOnceCell<T> where
T: Sync + Send,
impl<T> Sync for JoinHandle<T>
impl !Sync for Args
impl<'a> Sync for IoSliceMut<'a>
impl<'a> Sync for IoSlice<'a>
impl !Sync for ArgsOs
impl<T, F> Sync for SyncLazy<T, F> where
F: Send,
SyncOnceCell<T>: Sync,
impl Sync for Waker
impl<T> !Sync for *const T where
T: ?Sized,
impl<T> !Sync for *mut T where
T: ?Sized,
impl<'_> Sync for Drain<'_>
impl<'_, T> Sync for Cursor<'_, T> where
T: Sync,
impl<T> Sync for LinkedList<T> where
T: Sync,
impl<'_, T> Sync for Iter<'_, T> where
T: Sync,
impl<'_, T> Sync for CursorMut<'_, T> where
T: Sync,
impl<'_, T> Sync for IterMut<'_, T> where
T: Sync,
impl Sync for isize
impl<T> Sync for [T] where
T: Sync,
impl Sync for [u8]
impl Sync for char
impl Sync for u128
impl Sync for u16
impl Sync for i128
impl Sync for i16
impl Sync for str
impl Sync for f64
impl Sync for u64
impl Sync for u8
impl Sync for i64
impl Sync for i8
impl<T, const N: usize> Sync for [T; N] where
T: Sync,
impl Sync for bool
impl Sync for f32
impl Sync for u32
impl Sync for usize
impl Sync for i32
Implementors
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 sp_std::sync::Once
impl<'_, T> Sync for sp_std::collections::vec_deque::IterMut<'_, T> where
T: Sync,
impl<'_, T> Sync for sp_std::slice::Iter<'_, T> where
T: Sync,
impl<'_, T> Sync for sp_std::slice::IterMut<'_, T> where
T: Sync,
impl<'_, T> Sync for MutexGuard<'_, T> where
T: Sync + ?Sized,
impl<'_, T> Sync for RwLockReadGuard<'_, T> where
T: Sync + ?Sized,
impl<'_, T> Sync for RwLockWriteGuard<'_, T> where
T: Sync + ?Sized,
impl<'_, T, A> Sync for sp_std::collections::vec_deque::Drain<'_, T, A> where
T: Sync,
A: Allocator + Sync,
impl<'_, T, A> Sync for sp_std::vec::Drain<'_, T, A> where
T: Sync,
A: Sync + Allocator,
impl<Dyn> Sync for DynMetadata<Dyn> where
Dyn: ?Sized,
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.