pub unsafe auto trait Send { }
Expand description
Types that can be transferred across thread boundaries.
This trait is automatically implemented when the compiler determines it’s appropriate.
An example of a non-Send
type is the reference-counting pointer
rc::Rc
. If two threads attempt to clone Rc
s that point to the same
reference-counted value, they might try to update the reference count at the
same time, which is undefined behavior because Rc
doesn’t use atomic
operations. Its cousin sync::Arc
does use atomic operations (incurring
some overhead) and thus is Send
.
See the Nomicon for more details.
Implementations on Foreign Types
impl Send for Argument
impl Send for FormatSpec
impl Send for Alignment
impl Send for Count
impl Send for OwnedHandle
impl<'_, T> !Send for RwLockReadGuard<'_, T> where
T: ?Sized,
impl<'a> Send for IoSliceMut<'a>
impl<'a> Send for IoSlice<'a>
impl<T> Send for Sender<T> where
T: Send,
impl<T> Send for Mutex<T> where
T: Send + ?Sized,
impl !Send for ArgsOs
impl<'_, T> !Send for MutexGuard<'_, T> where
T: ?Sized,
impl<T> Send for SyncSender<T> where
T: Send,
impl Send for HandleOrInvalid
impl<T> Send for SyncOnceCell<T> where
T: Send,
impl Send for HandleOrNull
impl<T> Send for RwLock<T> where
T: Send + ?Sized,
impl<'_, T> !Send for RwLockWriteGuard<'_, T> where
T: ?Sized,
impl Send for Once
impl<T> Send for JoinHandle<T>
impl !Send for Args
impl<T> Send for Receiver<T> where
T: Send,
impl<'_> Send for BorrowedHandle<'_>
impl<T> !Send for NonNull<T> where
T: ?Sized,
NonNull
pointers are not Send
because the data they reference may be aliased.
impl<Dyn> Send for DynMetadata<Dyn> where
Dyn: ?Sized,
impl Send for Waker
impl<T> Send for RefCell<T> where
T: Send + ?Sized,
impl<'_, T> Send for Iter<'_, T> where
T: Sync,
impl<'_, T> Send for &'_ T where
T: Sync + ?Sized,
impl<T> Send for AtomicPtr<T>
impl<'_, T> Send for IterMut<'_, T> where
T: Send,
impl<'_, T> Send for &'_ mut T where
T: Send + ?Sized,
impl<T> !Send for *mut T where
T: ?Sized,
impl<T> !Send for *const T where
T: ?Sized,
impl<T> Send for Cell<T> where
T: Send + ?Sized,
impl<T> Send for Arc<T> where
T: Sync + Send + ?Sized,
impl<T> !Send for Rc<T> where
T: ?Sized,
impl<T> Send for Weak<T> where
T: Sync + Send + ?Sized,
impl<T> !Send for Weak<T> where
T: ?Sized,
impl<'a, T, const CAP: usize> Send for Drain<'a, T, CAP> where
T: Send,
impl<'a, T, O> Send for Iter<'a, T, O> where
T: BitStore,
O: BitOrder,
&'a mut BitSlice<T, O>: Send,
impl<'_, M, T, O> Send for BitRef<'_, M, T, O> where
M: Mutability,
T: BitStore + Sync,
O: BitOrder,
impl<'_, T, O> Send for Drain<'_, T, O> where
T: BitStore,
O: BitOrder,
&'a mut BitSlice<T, O>: for<'a> Send,
impl<T> Send for BitSpanError<T> where
T: BitStore,
impl<T, O> Send for BitSlice<T, O> where
T: BitStore + Sync,
O: BitOrder,
Bit-Slice Thread Safety
This allows bit-slice references to be moved across thread boundaries only when
the underlying T
element can tolerate concurrency.
All BitSlice
references, shared or exclusive, are only threadsafe if the T
element type is Send
, because any given bit-slice reference may only have
partial control of a memory element that is also being shared by a bit-slice
reference on another thread. As such, this is never implemented for Cell<U>
,
but always implemented for AtomicU
and U
for a given unsigned integer type
U
.
Atomic integers safely handle concurrent writes, cells do not allow concurrency
at all, so the only missing piece is &mut BitSlice<_, U: Unsigned>
. This is
handled by the aliasing system that the mutable splitters employ: a mutable
reference to an unsynchronized bit-slice can only cross threads when no other
handle is able to exist to the elements it governs. Splitting a mutable
bit-slice causes the split halves to change over to either atomics or cells, so
concurrency is either safe or impossible.