1 #![stable(feature = "rust1", since = "1.0.0")]
3 //! Thread-safe reference-counting pointers.
5 //! See the [`Arc<T>`][arc] documentation for more details.
7 //! [arc]: struct.Arc.html
10 use core
::array
::LengthAtMost32
;
12 use core
::cmp
::Ordering
;
13 use core
::convert
::{From, TryFrom}
;
15 use core
::hash
::{Hash, Hasher}
;
16 use core
::intrinsics
::abort
;
18 use core
::marker
::{PhantomData, Unpin, Unsize}
;
19 use core
::mem
::{self, align_of, align_of_val, size_of_val}
;
20 use core
::ops
::{CoerceUnsized, Deref, DispatchFromDyn, Receiver}
;
22 use core
::ptr
::{self, NonNull}
;
23 use core
::slice
::{self, from_raw_parts_mut}
;
24 use core
::sync
::atomic
;
25 use core
::sync
::atomic
::Ordering
::{Acquire, Relaxed, Release, SeqCst}
;
26 use core
::{isize, usize}
;
28 use crate::alloc
::{box_free, handle_alloc_error, AllocRef, Global, Layout}
;
29 use crate::boxed
::Box
;
30 use crate::rc
::is_dangling
;
31 use crate::string
::String
;
37 /// A soft limit on the amount of references that may be made to an `Arc`.
39 /// Going above this limit will abort your program (although not
40 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
41 const MAX_REFCOUNT
: usize = (isize::MAX
) as usize;
43 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
44 /// Reference Counted'.
46 /// The type `Arc<T>` provides shared ownership of a value of type `T`,
47 /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
48 /// a new `Arc` instance, which points to the same allocation on the heap as the
49 /// source `Arc`, while increasing a reference count. When the last `Arc`
50 /// pointer to a given allocation is destroyed, the value stored in that allocation (often
51 /// referred to as "inner value") is also dropped.
53 /// Shared references in Rust disallow mutation by default, and `Arc` is no
54 /// exception: you cannot generally obtain a mutable reference to something
55 /// inside an `Arc`. If you need to mutate through an `Arc`, use
56 /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
61 /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
62 /// counting. This means that it is thread-safe. The disadvantage is that
63 /// atomic operations are more expensive than ordinary memory accesses. If you
64 /// are not sharing reference-counted allocations between threads, consider using
65 /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
66 /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
67 /// However, a library might choose `Arc<T>` in order to give library consumers
70 /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
71 /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
72 /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
73 /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
74 /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
75 /// data, but it doesn't add thread safety to its data. Consider
76 /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
77 /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
78 /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
79 /// non-atomic operations.
81 /// In the end, this means that you may need to pair `Arc<T>` with some sort of
82 /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
84 /// ## Breaking cycles with `Weak`
86 /// The [`downgrade`][downgrade] method can be used to create a non-owning
87 /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
88 /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
89 /// already been dropped. In other words, `Weak` pointers do not keep the value
90 /// inside the allocation alive; however, they *do* keep the allocation
91 /// (the backing store for the value) alive.
93 /// A cycle between `Arc` pointers will never be deallocated. For this reason,
94 /// [`Weak`][weak] is used to break cycles. For example, a tree could have
95 /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
96 /// pointers from children back to their parents.
98 /// # Cloning references
100 /// Creating a new reference from an existing reference counted pointer is done using the
101 /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
104 /// use std::sync::Arc;
105 /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
106 /// // The two syntaxes below are equivalent.
107 /// let a = foo.clone();
108 /// let b = Arc::clone(&foo);
109 /// // a, b, and foo are all Arcs that point to the same memory location
112 /// ## `Deref` behavior
114 /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
115 /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
116 /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
117 /// functions, called using function-like syntax:
120 /// use std::sync::Arc;
121 /// let my_arc = Arc::new(());
123 /// Arc::downgrade(&my_arc);
126 /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the inner value may have
127 /// already been dropped.
129 /// [arc]: struct.Arc.html
130 /// [weak]: struct.Weak.html
131 /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
132 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
133 /// [mutex]: ../../std/sync/struct.Mutex.html
134 /// [rwlock]: ../../std/sync/struct.RwLock.html
135 /// [atomic]: ../../std/sync/atomic/index.html
136 /// [`Send`]: ../../std/marker/trait.Send.html
137 /// [`Sync`]: ../../std/marker/trait.Sync.html
138 /// [deref]: ../../std/ops/trait.Deref.html
139 /// [downgrade]: struct.Arc.html#method.downgrade
140 /// [upgrade]: struct.Weak.html#method.upgrade
141 /// [`None`]: ../../std/option/enum.Option.html#variant.None
142 /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
143 /// [`std::sync`]: ../../std/sync/index.html
144 /// [`Arc::clone(&from)`]: #method.clone
148 /// Sharing some immutable data between threads:
150 // Note that we **do not** run these tests here. The windows builders get super
151 // unhappy if a thread outlives the main thread and then exits at the same time
152 // (something deadlocks) so we just avoid this entirely by not running these
155 /// use std::sync::Arc;
158 /// let five = Arc::new(5);
161 /// let five = Arc::clone(&five);
163 /// thread::spawn(move || {
164 /// println!("{:?}", five);
169 /// Sharing a mutable [`AtomicUsize`]:
171 /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
174 /// use std::sync::Arc;
175 /// use std::sync::atomic::{AtomicUsize, Ordering};
178 /// let val = Arc::new(AtomicUsize::new(5));
181 /// let val = Arc::clone(&val);
183 /// thread::spawn(move || {
184 /// let v = val.fetch_add(1, Ordering::SeqCst);
185 /// println!("{:?}", v);
190 /// See the [`rc` documentation][rc_examples] for more examples of reference
191 /// counting in general.
193 /// [rc_examples]: ../../std/rc/index.html#examples
194 #[cfg_attr(not(test), lang = "arc")]
195 #[stable(feature = "rust1", since = "1.0.0")]
196 pub struct Arc
<T
: ?Sized
> {
197 ptr
: NonNull
<ArcInner
<T
>>,
198 phantom
: PhantomData
<ArcInner
<T
>>,
201 #[stable(feature = "rust1", since = "1.0.0")]
202 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Arc
<T
> {}
203 #[stable(feature = "rust1", since = "1.0.0")]
204 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Arc
<T
> {}
206 #[unstable(feature = "coerce_unsized", issue = "27732")]
207 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Arc
<U
>> for Arc
<T
> {}
209 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
210 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Arc
<U
>> for Arc
<T
> {}
212 impl<T
: ?Sized
> Arc
<T
> {
213 fn from_inner(ptr
: NonNull
<ArcInner
<T
>>) -> Self {
214 Self { ptr, phantom: PhantomData }
217 unsafe fn from_ptr(ptr
: *mut ArcInner
<T
>) -> Self {
218 Self::from_inner(NonNull
::new_unchecked(ptr
))
222 /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
223 /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
224 /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
226 /// Since a `Weak` reference does not count towards ownership, it will not
227 /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
228 /// guarantees about the value still being present. Thus it may return [`None`]
229 /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
230 /// itself (the backing store) from being deallocated.
232 /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
233 /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
234 /// prevent circular references between [`Arc`] pointers, since mutual owning references
235 /// would never allow either [`Arc`] to be dropped. For example, a tree could
236 /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
237 /// pointers from children back to their parents.
239 /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
241 /// [`Arc`]: struct.Arc.html
242 /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
243 /// [`upgrade`]: struct.Weak.html#method.upgrade
244 /// [`Option`]: ../../std/option/enum.Option.html
245 /// [`None`]: ../../std/option/enum.Option.html#variant.None
246 #[stable(feature = "arc_weak", since = "1.4.0")]
247 pub struct Weak
<T
: ?Sized
> {
248 // This is a `NonNull` to allow optimizing the size of this type in enums,
249 // but it is not necessarily a valid pointer.
250 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
251 // to allocate space on the heap. That's not a value a real pointer
252 // will ever have because RcBox has alignment at least 2.
253 ptr
: NonNull
<ArcInner
<T
>>,
256 #[stable(feature = "arc_weak", since = "1.4.0")]
257 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for Weak
<T
> {}
258 #[stable(feature = "arc_weak", since = "1.4.0")]
259 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for Weak
<T
> {}
261 #[unstable(feature = "coerce_unsized", issue = "27732")]
262 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Weak
<U
>> for Weak
<T
> {}
263 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
264 impl<T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> DispatchFromDyn
<Weak
<U
>> for Weak
<T
> {}
266 #[stable(feature = "arc_weak", since = "1.4.0")]
267 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Weak
<T
> {
268 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
273 struct ArcInner
<T
: ?Sized
> {
274 strong
: atomic
::AtomicUsize
,
276 // the value usize::MAX acts as a sentinel for temporarily "locking" the
277 // ability to upgrade weak pointers or downgrade strong ones; this is used
278 // to avoid races in `make_mut` and `get_mut`.
279 weak
: atomic
::AtomicUsize
,
284 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Send
for ArcInner
<T
> {}
285 unsafe impl<T
: ?Sized
+ Sync
+ Send
> Sync
for ArcInner
<T
> {}
288 /// Constructs a new `Arc<T>`.
293 /// use std::sync::Arc;
295 /// let five = Arc::new(5);
298 #[stable(feature = "rust1", since = "1.0.0")]
299 pub fn new(data
: T
) -> Arc
<T
> {
300 // Start the weak pointer count as 1 which is the weak pointer that's
301 // held by all the strong pointers (kinda), see std/rc.rs for more info
302 let x
: Box
<_
> = box ArcInner
{
303 strong
: atomic
::AtomicUsize
::new(1),
304 weak
: atomic
::AtomicUsize
::new(1),
307 Self::from_inner(Box
::into_raw_non_null(x
))
310 /// Constructs a new `Arc` with uninitialized contents.
315 /// #![feature(new_uninit)]
316 /// #![feature(get_mut_unchecked)]
318 /// use std::sync::Arc;
320 /// let mut five = Arc::<u32>::new_uninit();
322 /// let five = unsafe {
323 /// // Deferred initialization:
324 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
326 /// five.assume_init()
329 /// assert_eq!(*five, 5)
331 #[unstable(feature = "new_uninit", issue = "63291")]
332 pub fn new_uninit() -> Arc
<mem
::MaybeUninit
<T
>> {
334 Arc
::from_ptr(Arc
::allocate_for_layout(Layout
::new
::<T
>(), |mem
| {
335 mem
as *mut ArcInner
<mem
::MaybeUninit
<T
>>
340 /// Constructs a new `Arc` with uninitialized contents, with the memory
341 /// being filled with `0` bytes.
343 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
349 /// #![feature(new_uninit)]
351 /// use std::sync::Arc;
353 /// let zero = Arc::<u32>::new_zeroed();
354 /// let zero = unsafe { zero.assume_init() };
356 /// assert_eq!(*zero, 0)
359 /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
360 #[unstable(feature = "new_uninit", issue = "63291")]
361 pub fn new_zeroed() -> Arc
<mem
::MaybeUninit
<T
>> {
363 let mut uninit
= Self::new_uninit();
364 ptr
::write_bytes
::<T
>(Arc
::get_mut_unchecked(&mut uninit
).as_mut_ptr(), 0, 1);
369 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
370 /// `data` will be pinned in memory and unable to be moved.
371 #[stable(feature = "pin", since = "1.33.0")]
372 pub fn pin(data
: T
) -> Pin
<Arc
<T
>> {
373 unsafe { Pin::new_unchecked(Arc::new(data)) }
376 /// Returns the inner value, if the `Arc` has exactly one strong reference.
378 /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
381 /// This will succeed even if there are outstanding weak references.
383 /// [result]: ../../std/result/enum.Result.html
388 /// use std::sync::Arc;
390 /// let x = Arc::new(3);
391 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
393 /// let x = Arc::new(4);
394 /// let _y = Arc::clone(&x);
395 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
398 #[stable(feature = "arc_unique", since = "1.4.0")]
399 pub fn try_unwrap(this
: Self) -> Result
<T
, Self> {
400 // See `drop` for why all these atomics are like this
401 if this
.inner().strong
.compare_exchange(1, 0, Release
, Relaxed
).is_err() {
405 atomic
::fence(Acquire
);
408 let elem
= ptr
::read(&this
.ptr
.as_ref().data
);
410 // Make a weak pointer to clean up the implicit strong-weak reference
411 let _weak
= Weak { ptr: this.ptr }
;
420 /// Constructs a new reference-counted slice with uninitialized contents.
425 /// #![feature(new_uninit)]
426 /// #![feature(get_mut_unchecked)]
428 /// use std::sync::Arc;
430 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
432 /// let values = unsafe {
433 /// // Deferred initialization:
434 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
435 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
436 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
438 /// values.assume_init()
441 /// assert_eq!(*values, [1, 2, 3])
443 #[unstable(feature = "new_uninit", issue = "63291")]
444 pub fn new_uninit_slice(len
: usize) -> Arc
<[mem
::MaybeUninit
<T
>]> {
445 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
449 impl<T
> Arc
<mem
::MaybeUninit
<T
>> {
450 /// Converts to `Arc<T>`.
454 /// As with [`MaybeUninit::assume_init`],
455 /// it is up to the caller to guarantee that the inner value
456 /// really is in an initialized state.
457 /// Calling this when the content is not yet fully initialized
458 /// causes immediate undefined behavior.
460 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
465 /// #![feature(new_uninit)]
466 /// #![feature(get_mut_unchecked)]
468 /// use std::sync::Arc;
470 /// let mut five = Arc::<u32>::new_uninit();
472 /// let five = unsafe {
473 /// // Deferred initialization:
474 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
476 /// five.assume_init()
479 /// assert_eq!(*five, 5)
481 #[unstable(feature = "new_uninit", issue = "63291")]
483 pub unsafe fn assume_init(self) -> Arc
<T
> {
484 Arc
::from_inner(mem
::ManuallyDrop
::new(self).ptr
.cast())
488 impl<T
> Arc
<[mem
::MaybeUninit
<T
>]> {
489 /// Converts to `Arc<[T]>`.
493 /// As with [`MaybeUninit::assume_init`],
494 /// it is up to the caller to guarantee that the inner value
495 /// really is in an initialized state.
496 /// Calling this when the content is not yet fully initialized
497 /// causes immediate undefined behavior.
499 /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
504 /// #![feature(new_uninit)]
505 /// #![feature(get_mut_unchecked)]
507 /// use std::sync::Arc;
509 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
511 /// let values = unsafe {
512 /// // Deferred initialization:
513 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
514 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
515 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
517 /// values.assume_init()
520 /// assert_eq!(*values, [1, 2, 3])
522 #[unstable(feature = "new_uninit", issue = "63291")]
524 pub unsafe fn assume_init(self) -> Arc
<[T
]> {
525 Arc
::from_ptr(mem
::ManuallyDrop
::new(self).ptr
.as_ptr() as _
)
529 impl<T
: ?Sized
> Arc
<T
> {
530 /// Consumes the `Arc`, returning the wrapped pointer.
532 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
533 /// [`Arc::from_raw`][from_raw].
535 /// [from_raw]: struct.Arc.html#method.from_raw
540 /// use std::sync::Arc;
542 /// let x = Arc::new("hello".to_owned());
543 /// let x_ptr = Arc::into_raw(x);
544 /// assert_eq!(unsafe { &*x_ptr }, "hello");
546 #[stable(feature = "rc_raw", since = "1.17.0")]
547 pub fn into_raw(this
: Self) -> *const T
{
548 let ptr
: *mut ArcInner
<T
> = NonNull
::as_ptr(this
.ptr
);
549 let fake_ptr
= ptr
as *mut T
;
552 // SAFETY: This cannot go through Deref::deref.
553 // Instead, we manually offset the pointer rather than manifesting a reference.
554 // This is so that the returned pointer retains the same provenance as our pointer.
555 // This is required so that e.g. `get_mut` can write through the pointer
556 // after the Arc is recovered through `from_raw`.
558 let offset
= data_offset(&(*ptr
).data
);
559 set_data_ptr(fake_ptr
, (ptr
as *mut u8).offset(offset
))
563 /// Constructs an `Arc` from a raw pointer.
565 /// The raw pointer must have been previously returned by a call to a
566 /// [`Arc::into_raw`][into_raw].
568 /// This function is unsafe because improper use may lead to memory problems. For example, a
569 /// double-free may occur if the function is called twice on the same raw pointer.
571 /// [into_raw]: struct.Arc.html#method.into_raw
576 /// use std::sync::Arc;
578 /// let x = Arc::new("hello".to_owned());
579 /// let x_ptr = Arc::into_raw(x);
582 /// // Convert back to an `Arc` to prevent leak.
583 /// let x = Arc::from_raw(x_ptr);
584 /// assert_eq!(&*x, "hello");
586 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
589 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
591 #[stable(feature = "rc_raw", since = "1.17.0")]
592 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
593 let offset
= data_offset(ptr
);
595 // Reverse the offset to find the original ArcInner.
596 let fake_ptr
= ptr
as *mut ArcInner
<T
>;
597 let arc_ptr
= set_data_ptr(fake_ptr
, (ptr
as *mut u8).offset(-offset
));
599 Self::from_ptr(arc_ptr
)
602 /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
607 /// #![feature(rc_into_raw_non_null)]
609 /// use std::sync::Arc;
611 /// let x = Arc::new("hello".to_owned());
612 /// let ptr = Arc::into_raw_non_null(x);
613 /// let deref = unsafe { ptr.as_ref() };
614 /// assert_eq!(deref, "hello");
616 #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
618 pub fn into_raw_non_null(this
: Self) -> NonNull
<T
> {
619 // safe because Arc guarantees its pointer is non-null
620 unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
623 /// Creates a new [`Weak`][weak] pointer to this allocation.
625 /// [weak]: struct.Weak.html
630 /// use std::sync::Arc;
632 /// let five = Arc::new(5);
634 /// let weak_five = Arc::downgrade(&five);
636 #[stable(feature = "arc_weak", since = "1.4.0")]
637 pub fn downgrade(this
: &Self) -> Weak
<T
> {
638 // This Relaxed is OK because we're checking the value in the CAS
640 let mut cur
= this
.inner().weak
.load(Relaxed
);
643 // check if the weak counter is currently "locked"; if so, spin.
644 if cur
== usize::MAX
{
645 cur
= this
.inner().weak
.load(Relaxed
);
649 // NOTE: this code currently ignores the possibility of overflow
650 // into usize::MAX; in general both Rc and Arc need to be adjusted
651 // to deal with overflow.
653 // Unlike with Clone(), we need this to be an Acquire read to
654 // synchronize with the write coming from `is_unique`, so that the
655 // events prior to that write happen before this read.
656 match this
.inner().weak
.compare_exchange_weak(cur
, cur
+ 1, Acquire
, Relaxed
) {
658 // Make sure we do not create a dangling Weak
659 debug_assert
!(!is_dangling(this
.ptr
));
660 return Weak { ptr: this.ptr }
;
662 Err(old
) => cur
= old
,
667 /// Gets the number of [`Weak`][weak] pointers to this allocation.
669 /// [weak]: struct.Weak.html
673 /// This method by itself is safe, but using it correctly requires extra care.
674 /// Another thread can change the weak count at any time,
675 /// including potentially between calling this method and acting on the result.
680 /// use std::sync::Arc;
682 /// let five = Arc::new(5);
683 /// let _weak_five = Arc::downgrade(&five);
685 /// // This assertion is deterministic because we haven't shared
686 /// // the `Arc` or `Weak` between threads.
687 /// assert_eq!(1, Arc::weak_count(&five));
690 #[stable(feature = "arc_counts", since = "1.15.0")]
691 pub fn weak_count(this
: &Self) -> usize {
692 let cnt
= this
.inner().weak
.load(SeqCst
);
693 // If the weak count is currently locked, the value of the
694 // count was 0 just before taking the lock.
695 if cnt
== usize::MAX { 0 }
else { cnt - 1 }
698 /// Gets the number of strong (`Arc`) pointers to this allocation.
702 /// This method by itself is safe, but using it correctly requires extra care.
703 /// Another thread can change the strong count at any time,
704 /// including potentially between calling this method and acting on the result.
709 /// use std::sync::Arc;
711 /// let five = Arc::new(5);
712 /// let _also_five = Arc::clone(&five);
714 /// // This assertion is deterministic because we haven't shared
715 /// // the `Arc` between threads.
716 /// assert_eq!(2, Arc::strong_count(&five));
719 #[stable(feature = "arc_counts", since = "1.15.0")]
720 pub fn strong_count(this
: &Self) -> usize {
721 this
.inner().strong
.load(SeqCst
)
725 fn inner(&self) -> &ArcInner
<T
> {
726 // This unsafety is ok because while this arc is alive we're guaranteed
727 // that the inner pointer is valid. Furthermore, we know that the
728 // `ArcInner` structure itself is `Sync` because the inner data is
729 // `Sync` as well, so we're ok loaning out an immutable pointer to these
731 unsafe { self.ptr.as_ref() }
734 // Non-inlined part of `drop`.
736 unsafe fn drop_slow(&mut self) {
737 // Destroy the data at this time, even though we may not free the box
738 // allocation itself (there may still be weak pointers lying around).
739 ptr
::drop_in_place(&mut self.ptr
.as_mut().data
);
741 if self.inner().weak
.fetch_sub(1, Release
) == 1 {
742 atomic
::fence(Acquire
);
743 Global
.dealloc(self.ptr
.cast(), Layout
::for_value(self.ptr
.as_ref()))
748 #[stable(feature = "ptr_eq", since = "1.17.0")]
749 /// Returns `true` if the two `Arc`s point to the same allocation
750 /// (in a vein similar to [`ptr::eq`]).
755 /// use std::sync::Arc;
757 /// let five = Arc::new(5);
758 /// let same_five = Arc::clone(&five);
759 /// let other_five = Arc::new(5);
761 /// assert!(Arc::ptr_eq(&five, &same_five));
762 /// assert!(!Arc::ptr_eq(&five, &other_five));
765 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
766 pub fn ptr_eq(this
: &Self, other
: &Self) -> bool
{
767 this
.ptr
.as_ptr() == other
.ptr
.as_ptr()
771 impl<T
: ?Sized
> Arc
<T
> {
772 /// Allocates an `ArcInner<T>` with sufficient space for
773 /// a possibly-unsized inner value where the value has the layout provided.
775 /// The function `mem_to_arcinner` is called with the data pointer
776 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
777 unsafe fn allocate_for_layout(
778 value_layout
: Layout
,
779 mem_to_arcinner
: impl FnOnce(*mut u8) -> *mut ArcInner
<T
>,
780 ) -> *mut ArcInner
<T
> {
781 // Calculate layout using the given value layout.
782 // Previously, layout was calculated on the expression
783 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
784 // reference (see #54908).
785 let layout
= Layout
::new
::<ArcInner
<()>>().extend(value_layout
).unwrap().0.pad_to_align();
787 let (mem
, _
) = Global
.alloc(layout
).unwrap_or_else(|_
| handle_alloc_error(layout
));
789 // Initialize the ArcInner
790 let inner
= mem_to_arcinner(mem
.as_ptr());
791 debug_assert_eq
!(Layout
::for_value(&*inner
), layout
);
793 ptr
::write(&mut (*inner
).strong
, atomic
::AtomicUsize
::new(1));
794 ptr
::write(&mut (*inner
).weak
, atomic
::AtomicUsize
::new(1));
799 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
800 unsafe fn allocate_for_ptr(ptr
: *const T
) -> *mut ArcInner
<T
> {
801 // Allocate for the `ArcInner<T>` using the given value.
802 Self::allocate_for_layout(Layout
::for_value(&*ptr
), |mem
| {
803 set_data_ptr(ptr
as *mut T
, mem
) as *mut ArcInner
<T
>
807 fn from_box(v
: Box
<T
>) -> Arc
<T
> {
809 let box_unique
= Box
::into_unique(v
);
810 let bptr
= box_unique
.as_ptr();
812 let value_size
= size_of_val(&*bptr
);
813 let ptr
= Self::allocate_for_ptr(bptr
);
815 // Copy value as bytes
816 ptr
::copy_nonoverlapping(
817 bptr
as *const T
as *const u8,
818 &mut (*ptr
).data
as *mut _
as *mut u8,
822 // Free the allocation without dropping its contents
823 box_free(box_unique
);
831 /// Allocates an `ArcInner<[T]>` with the given length.
832 unsafe fn allocate_for_slice(len
: usize) -> *mut ArcInner
<[T
]> {
833 Self::allocate_for_layout(Layout
::array
::<T
>(len
).unwrap(), |mem
| {
834 ptr
::slice_from_raw_parts_mut(mem
as *mut T
, len
) as *mut ArcInner
<[T
]>
839 /// Sets the data pointer of a `?Sized` raw pointer.
841 /// For a slice/trait object, this sets the `data` field and leaves the rest
842 /// unchanged. For a sized raw pointer, this simply sets the pointer.
843 unsafe fn set_data_ptr
<T
: ?Sized
, U
>(mut ptr
: *mut T
, data
: *mut U
) -> *mut T
{
844 ptr
::write(&mut ptr
as *mut _
as *mut *mut u8, data
as *mut u8);
849 /// Copy elements from slice into newly allocated Arc<[T]>
851 /// Unsafe because the caller must either take ownership or bind `T: Copy`.
852 unsafe fn copy_from_slice(v
: &[T
]) -> Arc
<[T
]> {
853 let ptr
= Self::allocate_for_slice(v
.len());
855 ptr
::copy_nonoverlapping(v
.as_ptr(), &mut (*ptr
).data
as *mut [T
] as *mut T
, v
.len());
860 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
862 /// Behavior is undefined should the size be wrong.
863 unsafe fn from_iter_exact(iter
: impl iter
::Iterator
<Item
= T
>, len
: usize) -> Arc
<[T
]> {
864 // Panic guard while cloning T elements.
865 // In the event of a panic, elements that have been written
866 // into the new ArcInner will be dropped, then the memory freed.
874 impl<T
> Drop
for Guard
<T
> {
877 let slice
= from_raw_parts_mut(self.elems
, self.n_elems
);
878 ptr
::drop_in_place(slice
);
880 Global
.dealloc(self.mem
.cast(), self.layout
);
885 let ptr
= Self::allocate_for_slice(len
);
887 let mem
= ptr
as *mut _
as *mut u8;
888 let layout
= Layout
::for_value(&*ptr
);
890 // Pointer to first element
891 let elems
= &mut (*ptr
).data
as *mut [T
] as *mut T
;
893 let mut guard
= Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 }
;
895 for (i
, item
) in iter
.enumerate() {
896 ptr
::write(elems
.add(i
), item
);
900 // All clear. Forget the guard so it doesn't free the new ArcInner.
907 /// Specialization trait used for `From<&[T]>`.
908 trait ArcFromSlice
<T
> {
909 fn from_slice(slice
: &[T
]) -> Self;
912 impl<T
: Clone
> ArcFromSlice
<T
> for Arc
<[T
]> {
914 default fn from_slice(v
: &[T
]) -> Self {
915 unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
919 impl<T
: Copy
> ArcFromSlice
<T
> for Arc
<[T
]> {
921 fn from_slice(v
: &[T
]) -> Self {
922 unsafe { Arc::copy_from_slice(v) }
926 #[stable(feature = "rust1", since = "1.0.0")]
927 impl<T
: ?Sized
> Clone
for Arc
<T
> {
928 /// Makes a clone of the `Arc` pointer.
930 /// This creates another pointer to the same allocation, increasing the
931 /// strong reference count.
936 /// use std::sync::Arc;
938 /// let five = Arc::new(5);
940 /// let _ = Arc::clone(&five);
943 fn clone(&self) -> Arc
<T
> {
944 // Using a relaxed ordering is alright here, as knowledge of the
945 // original reference prevents other threads from erroneously deleting
948 // As explained in the [Boost documentation][1], Increasing the
949 // reference counter can always be done with memory_order_relaxed: New
950 // references to an object can only be formed from an existing
951 // reference, and passing an existing reference from one thread to
952 // another must already provide any required synchronization.
954 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
955 let old_size
= self.inner().strong
.fetch_add(1, Relaxed
);
957 // However we need to guard against massive refcounts in case someone
958 // is `mem::forget`ing Arcs. If we don't do this the count can overflow
959 // and users will use-after free. We racily saturate to `isize::MAX` on
960 // the assumption that there aren't ~2 billion threads incrementing
961 // the reference count at once. This branch will never be taken in
962 // any realistic program.
964 // We abort because such a program is incredibly degenerate, and we
965 // don't care to support it.
966 if old_size
> MAX_REFCOUNT
{
972 Self::from_inner(self.ptr
)
976 #[stable(feature = "rust1", since = "1.0.0")]
977 impl<T
: ?Sized
> Deref
for Arc
<T
> {
981 fn deref(&self) -> &T
{
986 #[unstable(feature = "receiver_trait", issue = "none")]
987 impl<T
: ?Sized
> Receiver
for Arc
<T
> {}
989 impl<T
: Clone
> Arc
<T
> {
990 /// Makes a mutable reference into the given `Arc`.
992 /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
993 /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
994 /// to ensure unique ownership. This is also referred to as clone-on-write.
996 /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
997 /// any remaining `Weak` pointers.
999 /// See also [`get_mut`][get_mut], which will fail rather than cloning.
1001 /// [weak]: struct.Weak.html
1002 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1003 /// [get_mut]: struct.Arc.html#method.get_mut
1004 /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
1009 /// use std::sync::Arc;
1011 /// let mut data = Arc::new(5);
1013 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1014 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
1015 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
1016 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
1017 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
1019 /// // Now `data` and `other_data` point to different allocations.
1020 /// assert_eq!(*data, 8);
1021 /// assert_eq!(*other_data, 12);
1024 #[stable(feature = "arc_unique", since = "1.4.0")]
1025 pub fn make_mut(this
: &mut Self) -> &mut T
{
1026 // Note that we hold both a strong reference and a weak reference.
1027 // Thus, releasing our strong reference only will not, by itself, cause
1028 // the memory to be deallocated.
1030 // Use Acquire to ensure that we see any writes to `weak` that happen
1031 // before release writes (i.e., decrements) to `strong`. Since we hold a
1032 // weak count, there's no chance the ArcInner itself could be
1034 if this
.inner().strong
.compare_exchange(1, 0, Acquire
, Relaxed
).is_err() {
1035 // Another strong pointer exists; clone
1036 *this
= Arc
::new((**this
).clone());
1037 } else if this
.inner().weak
.load(Relaxed
) != 1 {
1038 // Relaxed suffices in the above because this is fundamentally an
1039 // optimization: we are always racing with weak pointers being
1040 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
1042 // We removed the last strong ref, but there are additional weak
1043 // refs remaining. We'll move the contents to a new Arc, and
1044 // invalidate the other weak refs.
1046 // Note that it is not possible for the read of `weak` to yield
1047 // usize::MAX (i.e., locked), since the weak count can only be
1048 // locked by a thread with a strong reference.
1050 // Materialize our own implicit weak pointer, so that it can clean
1051 // up the ArcInner as needed.
1052 let weak
= Weak { ptr: this.ptr }
;
1054 // mark the data itself as already deallocated
1056 // there is no data race in the implicit write caused by `read`
1057 // here (due to zeroing) because data is no longer accessed by
1058 // other threads (due to there being no more strong refs at this
1060 let mut swap
= Arc
::new(ptr
::read(&weak
.ptr
.as_ref().data
));
1061 mem
::swap(this
, &mut swap
);
1065 // We were the sole reference of either kind; bump back up the
1066 // strong ref count.
1067 this
.inner().strong
.store(1, Release
);
1070 // As with `get_mut()`, the unsafety is ok because our reference was
1071 // either unique to begin with, or became one upon cloning the contents.
1072 unsafe { &mut this.ptr.as_mut().data }
1076 impl<T
: ?Sized
> Arc
<T
> {
1077 /// Returns a mutable reference into the given `Arc`, if there are
1078 /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
1080 /// Returns [`None`][option] otherwise, because it is not safe to
1081 /// mutate a shared value.
1083 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
1084 /// the inner value when there are other pointers.
1086 /// [weak]: struct.Weak.html
1087 /// [option]: ../../std/option/enum.Option.html
1088 /// [make_mut]: struct.Arc.html#method.make_mut
1089 /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
1094 /// use std::sync::Arc;
1096 /// let mut x = Arc::new(3);
1097 /// *Arc::get_mut(&mut x).unwrap() = 4;
1098 /// assert_eq!(*x, 4);
1100 /// let _y = Arc::clone(&x);
1101 /// assert!(Arc::get_mut(&mut x).is_none());
1104 #[stable(feature = "arc_unique", since = "1.4.0")]
1105 pub fn get_mut(this
: &mut Self) -> Option
<&mut T
> {
1106 if this
.is_unique() {
1107 // This unsafety is ok because we're guaranteed that the pointer
1108 // returned is the *only* pointer that will ever be returned to T. Our
1109 // reference count is guaranteed to be 1 at this point, and we required
1110 // the Arc itself to be `mut`, so we're returning the only possible
1111 // reference to the inner data.
1112 unsafe { Some(Arc::get_mut_unchecked(this)) }
1118 /// Returns a mutable reference into the given `Arc`,
1119 /// without any check.
1121 /// See also [`get_mut`], which is safe and does appropriate checks.
1123 /// [`get_mut`]: struct.Arc.html#method.get_mut
1127 /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
1128 /// for the duration of the returned borrow.
1129 /// This is trivially the case if no such pointers exist,
1130 /// for example immediately after `Arc::new`.
1135 /// #![feature(get_mut_unchecked)]
1137 /// use std::sync::Arc;
1139 /// let mut x = Arc::new(String::new());
1141 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
1143 /// assert_eq!(*x, "foo");
1146 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
1147 pub unsafe fn get_mut_unchecked(this
: &mut Self) -> &mut T
{
1148 &mut this
.ptr
.as_mut().data
1151 /// Determine whether this is the unique reference (including weak refs) to
1152 /// the underlying data.
1154 /// Note that this requires locking the weak ref count.
1155 fn is_unique(&mut self) -> bool
{
1156 // lock the weak pointer count if we appear to be the sole weak pointer
1159 // The acquire label here ensures a happens-before relationship with any
1160 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
1161 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
1162 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
1163 if self.inner().weak
.compare_exchange(1, usize::MAX
, Acquire
, Relaxed
).is_ok() {
1164 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
1165 // counter in `drop` -- the only access that happens when any but the last reference
1166 // is being dropped.
1167 let unique
= self.inner().strong
.load(Acquire
) == 1;
1169 // The release write here synchronizes with a read in `downgrade`,
1170 // effectively preventing the above read of `strong` from happening
1172 self.inner().weak
.store(1, Release
); // release the lock
1180 #[stable(feature = "rust1", since = "1.0.0")]
1181 unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
1182 /// Drops the `Arc`.
1184 /// This will decrement the strong reference count. If the strong reference
1185 /// count reaches zero then the only other references (if any) are
1186 /// [`Weak`], so we `drop` the inner value.
1191 /// use std::sync::Arc;
1195 /// impl Drop for Foo {
1196 /// fn drop(&mut self) {
1197 /// println!("dropped!");
1201 /// let foo = Arc::new(Foo);
1202 /// let foo2 = Arc::clone(&foo);
1204 /// drop(foo); // Doesn't print anything
1205 /// drop(foo2); // Prints "dropped!"
1208 /// [`Weak`]: ../../std/sync/struct.Weak.html
1210 fn drop(&mut self) {
1211 // Because `fetch_sub` is already atomic, we do not need to synchronize
1212 // with other threads unless we are going to delete the object. This
1213 // same logic applies to the below `fetch_sub` to the `weak` count.
1214 if self.inner().strong
.fetch_sub(1, Release
) != 1 {
1218 // This fence is needed to prevent reordering of use of the data and
1219 // deletion of the data. Because it is marked `Release`, the decreasing
1220 // of the reference count synchronizes with this `Acquire` fence. This
1221 // means that use of the data happens before decreasing the reference
1222 // count, which happens before this fence, which happens before the
1223 // deletion of the data.
1225 // As explained in the [Boost documentation][1],
1227 // > It is important to enforce any possible access to the object in one
1228 // > thread (through an existing reference) to *happen before* deleting
1229 // > the object in a different thread. This is achieved by a "release"
1230 // > operation after dropping a reference (any access to the object
1231 // > through this reference must obviously happened before), and an
1232 // > "acquire" operation before deleting the object.
1234 // In particular, while the contents of an Arc are usually immutable, it's
1235 // possible to have interior writes to something like a Mutex<T>. Since a
1236 // Mutex is not acquired when it is deleted, we can't rely on its
1237 // synchronization logic to make writes in thread A visible to a destructor
1238 // running in thread B.
1240 // Also note that the Acquire fence here could probably be replaced with an
1241 // Acquire load, which could improve performance in highly-contended
1242 // situations. See [2].
1244 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
1245 // [2]: (https://github.com/rust-lang/rust/pull/41714)
1246 atomic
::fence(Acquire
);
1254 impl Arc
<dyn Any
+ Send
+ Sync
> {
1256 #[stable(feature = "rc_downcast", since = "1.29.0")]
1257 /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
1262 /// use std::any::Any;
1263 /// use std::sync::Arc;
1265 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
1266 /// if let Ok(string) = value.downcast::<String>() {
1267 /// println!("String ({}): {}", string.len(), string);
1271 /// let my_string = "Hello World".to_string();
1272 /// print_if_string(Arc::new(my_string));
1273 /// print_if_string(Arc::new(0i8));
1275 pub fn downcast
<T
>(self) -> Result
<Arc
<T
>, Self>
1277 T
: Any
+ Send
+ Sync
+ '
static,
1279 if (*self).is
::<T
>() {
1280 let ptr
= self.ptr
.cast
::<ArcInner
<T
>>();
1282 Ok(Arc
::from_inner(ptr
))
1290 /// Constructs a new `Weak<T>`, without allocating any memory.
1291 /// Calling [`upgrade`] on the return value always gives [`None`].
1293 /// [`upgrade`]: struct.Weak.html#method.upgrade
1294 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1299 /// use std::sync::Weak;
1301 /// let empty: Weak<i64> = Weak::new();
1302 /// assert!(empty.upgrade().is_none());
1304 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1305 pub fn new() -> Weak
<T
> {
1306 Weak { ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0") }
1309 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
1311 /// The pointer is valid only if there are some strong references. The pointer may be dangling
1312 /// or even [`null`] otherwise.
1317 /// #![feature(weak_into_raw)]
1319 /// use std::sync::Arc;
1322 /// let strong = Arc::new("hello".to_owned());
1323 /// let weak = Arc::downgrade(&strong);
1324 /// // Both point to the same object
1325 /// assert!(ptr::eq(&*strong, weak.as_raw()));
1326 /// // The strong here keeps it alive, so we can still access the object.
1327 /// assert_eq!("hello", unsafe { &*weak.as_raw() });
1330 /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
1331 /// // undefined behaviour.
1332 /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
1335 /// [`null`]: ../../std/ptr/fn.null.html
1336 #[unstable(feature = "weak_into_raw", issue = "60728")]
1337 pub fn as_raw(&self) -> *const T
{
1338 match self.inner() {
1339 None
=> ptr
::null(),
1341 let offset
= data_offset_sized
::<T
>();
1342 let ptr
= inner
as *const ArcInner
<T
>;
1343 // Note: while the pointer we create may already point to dropped value, the
1344 // allocation still lives (it must hold the weak point as long as we are alive).
1345 // Therefore, the offset is OK to do, it won't get out of the allocation.
1346 let ptr
= unsafe { (ptr as *const u8).offset(offset) }
;
1352 /// Consumes the `Weak<T>` and turns it into a raw pointer.
1354 /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
1355 /// can be turned back into the `Weak<T>` with [`from_raw`].
1357 /// The same restrictions of accessing the target of the pointer as with
1358 /// [`as_raw`] apply.
1363 /// #![feature(weak_into_raw)]
1365 /// use std::sync::{Arc, Weak};
1367 /// let strong = Arc::new("hello".to_owned());
1368 /// let weak = Arc::downgrade(&strong);
1369 /// let raw = weak.into_raw();
1371 /// assert_eq!(1, Arc::weak_count(&strong));
1372 /// assert_eq!("hello", unsafe { &*raw });
1374 /// drop(unsafe { Weak::from_raw(raw) });
1375 /// assert_eq!(0, Arc::weak_count(&strong));
1378 /// [`from_raw`]: struct.Weak.html#method.from_raw
1379 /// [`as_raw`]: struct.Weak.html#method.as_raw
1380 #[unstable(feature = "weak_into_raw", issue = "60728")]
1381 pub fn into_raw(self) -> *const T
{
1382 let result
= self.as_raw();
1387 /// Converts a raw pointer previously created by [`into_raw`] back into
1390 /// This can be used to safely get a strong reference (by calling [`upgrade`]
1391 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
1393 /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
1394 /// as these don't have any corresponding weak count).
1398 /// The pointer must have originated from the [`into_raw`] (or [`as_raw'], provided there was
1399 /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
1402 /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
1403 /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
1409 /// #![feature(weak_into_raw)]
1411 /// use std::sync::{Arc, Weak};
1413 /// let strong = Arc::new("hello".to_owned());
1415 /// let raw_1 = Arc::downgrade(&strong).into_raw();
1416 /// let raw_2 = Arc::downgrade(&strong).into_raw();
1418 /// assert_eq!(2, Arc::weak_count(&strong));
1420 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
1421 /// assert_eq!(1, Arc::weak_count(&strong));
1425 /// // Decrement the last weak count.
1426 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
1429 /// [`as_raw`]: struct.Weak.html#method.as_raw
1430 /// [`new`]: struct.Weak.html#method.new
1431 /// [`into_raw`]: struct.Weak.html#method.into_raw
1432 /// [`upgrade`]: struct.Weak.html#method.upgrade
1433 /// [`Weak`]: struct.Weak.html
1434 /// [`Arc`]: struct.Arc.html
1435 /// [`forget`]: ../../std/mem/fn.forget.html
1436 #[unstable(feature = "weak_into_raw", issue = "60728")]
1437 pub unsafe fn from_raw(ptr
: *const T
) -> Self {
1441 // See Arc::from_raw for details
1442 let offset
= data_offset(ptr
);
1443 let fake_ptr
= ptr
as *mut ArcInner
<T
>;
1444 let ptr
= set_data_ptr(fake_ptr
, (ptr
as *mut u8).offset(-offset
));
1445 Weak { ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw") }
1450 impl<T
: ?Sized
> Weak
<T
> {
1451 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
1452 /// dropping of the inner value if successful.
1454 /// Returns [`None`] if the inner value has since been dropped.
1456 /// [`Arc`]: struct.Arc.html
1457 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1462 /// use std::sync::Arc;
1464 /// let five = Arc::new(5);
1466 /// let weak_five = Arc::downgrade(&five);
1468 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
1469 /// assert!(strong_five.is_some());
1471 /// // Destroy all strong pointers.
1472 /// drop(strong_five);
1475 /// assert!(weak_five.upgrade().is_none());
1477 #[stable(feature = "arc_weak", since = "1.4.0")]
1478 pub fn upgrade(&self) -> Option
<Arc
<T
>> {
1479 // We use a CAS loop to increment the strong count instead of a
1480 // fetch_add because once the count hits 0 it must never be above 0.
1481 let inner
= self.inner()?
;
1483 // Relaxed load because any write of 0 that we can observe
1484 // leaves the field in a permanently zero state (so a
1485 // "stale" read of 0 is fine), and any other value is
1486 // confirmed via the CAS below.
1487 let mut n
= inner
.strong
.load(Relaxed
);
1494 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
1495 if n
> MAX_REFCOUNT
{
1501 // Relaxed is valid for the same reason it is on Arc's Clone impl
1502 match inner
.strong
.compare_exchange_weak(n
, n
+ 1, Relaxed
, Relaxed
) {
1503 Ok(_
) => return Some(Arc
::from_inner(self.ptr
)), // null checked above
1504 Err(old
) => n
= old
,
1509 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
1511 /// If `self` was created using [`Weak::new`], this will return 0.
1513 /// [`Weak::new`]: #method.new
1514 #[stable(feature = "weak_counts", since = "1.41.0")]
1515 pub fn strong_count(&self) -> usize {
1516 if let Some(inner
) = self.inner() { inner.strong.load(SeqCst) }
else { 0 }
1519 /// Gets an approximation of the number of `Weak` pointers pointing to this
1522 /// If `self` was created using [`Weak::new`], or if there are no remaining
1523 /// strong pointers, this will return 0.
1527 /// Due to implementation details, the returned value can be off by 1 in
1528 /// either direction when other threads are manipulating any `Arc`s or
1529 /// `Weak`s pointing to the same allocation.
1531 /// [`Weak::new`]: #method.new
1532 #[stable(feature = "weak_counts", since = "1.41.0")]
1533 pub fn weak_count(&self) -> usize {
1536 let weak
= inner
.weak
.load(SeqCst
);
1537 let strong
= inner
.strong
.load(SeqCst
);
1541 // Since we observed that there was at least one strong pointer
1542 // after reading the weak count, we know that the implicit weak
1543 // reference (present whenever any strong references are alive)
1544 // was still around when we observed the weak count, and can
1545 // therefore safely subtract it.
1552 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
1553 /// (i.e., when this `Weak` was created by `Weak::new`).
1555 fn inner(&self) -> Option
<&ArcInner
<T
>> {
1556 if is_dangling(self.ptr
) { None }
else { Some(unsafe { self.ptr.as_ref() }
) }
1559 /// Returns `true` if the two `Weak`s point to the same allocation (similar to
1560 /// [`ptr::eq`]), or if both don't point to any allocation
1561 /// (because they were created with `Weak::new()`).
1565 /// Since this compares pointers it means that `Weak::new()` will equal each
1566 /// other, even though they don't point to any allocation.
1571 /// use std::sync::Arc;
1573 /// let first_rc = Arc::new(5);
1574 /// let first = Arc::downgrade(&first_rc);
1575 /// let second = Arc::downgrade(&first_rc);
1577 /// assert!(first.ptr_eq(&second));
1579 /// let third_rc = Arc::new(5);
1580 /// let third = Arc::downgrade(&third_rc);
1582 /// assert!(!first.ptr_eq(&third));
1585 /// Comparing `Weak::new`.
1588 /// use std::sync::{Arc, Weak};
1590 /// let first = Weak::new();
1591 /// let second = Weak::new();
1592 /// assert!(first.ptr_eq(&second));
1594 /// let third_rc = Arc::new(());
1595 /// let third = Arc::downgrade(&third_rc);
1596 /// assert!(!first.ptr_eq(&third));
1599 /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
1601 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
1602 pub fn ptr_eq(&self, other
: &Self) -> bool
{
1603 self.ptr
.as_ptr() == other
.ptr
.as_ptr()
1607 #[stable(feature = "arc_weak", since = "1.4.0")]
1608 impl<T
: ?Sized
> Clone
for Weak
<T
> {
1609 /// Makes a clone of the `Weak` pointer that points to the same allocation.
1614 /// use std::sync::{Arc, Weak};
1616 /// let weak_five = Arc::downgrade(&Arc::new(5));
1618 /// let _ = Weak::clone(&weak_five);
1621 fn clone(&self) -> Weak
<T
> {
1622 let inner
= if let Some(inner
) = self.inner() {
1625 return Weak { ptr: self.ptr }
;
1627 // See comments in Arc::clone() for why this is relaxed. This can use a
1628 // fetch_add (ignoring the lock) because the weak count is only locked
1629 // where are *no other* weak pointers in existence. (So we can't be
1630 // running this code in that case).
1631 let old_size
= inner
.weak
.fetch_add(1, Relaxed
);
1633 // See comments in Arc::clone() for why we do this (for mem::forget).
1634 if old_size
> MAX_REFCOUNT
{
1640 Weak { ptr: self.ptr }
1644 #[stable(feature = "downgraded_weak", since = "1.10.0")]
1645 impl<T
> Default
for Weak
<T
> {
1646 /// Constructs a new `Weak<T>`, without allocating memory.
1647 /// Calling [`upgrade`] on the return value always
1650 /// [`None`]: ../../std/option/enum.Option.html#variant.None
1651 /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
1656 /// use std::sync::Weak;
1658 /// let empty: Weak<i64> = Default::default();
1659 /// assert!(empty.upgrade().is_none());
1661 fn default() -> Weak
<T
> {
1666 #[stable(feature = "arc_weak", since = "1.4.0")]
1667 impl<T
: ?Sized
> Drop
for Weak
<T
> {
1668 /// Drops the `Weak` pointer.
1673 /// use std::sync::{Arc, Weak};
1677 /// impl Drop for Foo {
1678 /// fn drop(&mut self) {
1679 /// println!("dropped!");
1683 /// let foo = Arc::new(Foo);
1684 /// let weak_foo = Arc::downgrade(&foo);
1685 /// let other_weak_foo = Weak::clone(&weak_foo);
1687 /// drop(weak_foo); // Doesn't print anything
1688 /// drop(foo); // Prints "dropped!"
1690 /// assert!(other_weak_foo.upgrade().is_none());
1692 fn drop(&mut self) {
1693 // If we find out that we were the last weak pointer, then its time to
1694 // deallocate the data entirely. See the discussion in Arc::drop() about
1695 // the memory orderings
1697 // It's not necessary to check for the locked state here, because the
1698 // weak count can only be locked if there was precisely one weak ref,
1699 // meaning that drop could only subsequently run ON that remaining weak
1700 // ref, which can only happen after the lock is released.
1701 let inner
= if let Some(inner
) = self.inner() { inner }
else { return }
;
1703 if inner
.weak
.fetch_sub(1, Release
) == 1 {
1704 atomic
::fence(Acquire
);
1705 unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
1710 #[stable(feature = "rust1", since = "1.0.0")]
1711 trait ArcEqIdent
<T
: ?Sized
+ PartialEq
> {
1712 fn eq(&self, other
: &Arc
<T
>) -> bool
;
1713 fn ne(&self, other
: &Arc
<T
>) -> bool
;
1716 #[stable(feature = "rust1", since = "1.0.0")]
1717 impl<T
: ?Sized
+ PartialEq
> ArcEqIdent
<T
> for Arc
<T
> {
1719 default fn eq(&self, other
: &Arc
<T
>) -> bool
{
1723 default fn ne(&self, other
: &Arc
<T
>) -> bool
{
1728 /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
1729 /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
1730 /// store large values, that are slow to clone, but also heavy to check for equality, causing this
1731 /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
1732 /// the same value, than two `&T`s.
1734 /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
1735 #[stable(feature = "rust1", since = "1.0.0")]
1736 impl<T
: ?Sized
+ Eq
> ArcEqIdent
<T
> for Arc
<T
> {
1738 fn eq(&self, other
: &Arc
<T
>) -> bool
{
1739 Arc
::ptr_eq(self, other
) || **self == **other
1743 fn ne(&self, other
: &Arc
<T
>) -> bool
{
1744 !Arc
::ptr_eq(self, other
) && **self != **other
1748 #[stable(feature = "rust1", since = "1.0.0")]
1749 impl<T
: ?Sized
+ PartialEq
> PartialEq
for Arc
<T
> {
1750 /// Equality for two `Arc`s.
1752 /// Two `Arc`s are equal if their inner values are equal, even if they are
1753 /// stored in different allocation.
1755 /// If `T` also implements `Eq` (implying reflexivity of equality),
1756 /// two `Arc`s that point to the same allocation are always equal.
1761 /// use std::sync::Arc;
1763 /// let five = Arc::new(5);
1765 /// assert!(five == Arc::new(5));
1768 fn eq(&self, other
: &Arc
<T
>) -> bool
{
1769 ArcEqIdent
::eq(self, other
)
1772 /// Inequality for two `Arc`s.
1774 /// Two `Arc`s are unequal if their inner values are unequal.
1776 /// If `T` also implements `Eq` (implying reflexivity of equality),
1777 /// two `Arc`s that point to the same value are never unequal.
1782 /// use std::sync::Arc;
1784 /// let five = Arc::new(5);
1786 /// assert!(five != Arc::new(6));
1789 fn ne(&self, other
: &Arc
<T
>) -> bool
{
1790 ArcEqIdent
::ne(self, other
)
1794 #[stable(feature = "rust1", since = "1.0.0")]
1795 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for Arc
<T
> {
1796 /// Partial comparison for two `Arc`s.
1798 /// The two are compared by calling `partial_cmp()` on their inner values.
1803 /// use std::sync::Arc;
1804 /// use std::cmp::Ordering;
1806 /// let five = Arc::new(5);
1808 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
1810 fn partial_cmp(&self, other
: &Arc
<T
>) -> Option
<Ordering
> {
1811 (**self).partial_cmp(&**other
)
1814 /// Less-than comparison for two `Arc`s.
1816 /// The two are compared by calling `<` on their inner values.
1821 /// use std::sync::Arc;
1823 /// let five = Arc::new(5);
1825 /// assert!(five < Arc::new(6));
1827 fn lt(&self, other
: &Arc
<T
>) -> bool
{
1828 *(*self) < *(*other
)
1831 /// 'Less than or equal to' comparison for two `Arc`s.
1833 /// The two are compared by calling `<=` on their inner values.
1838 /// use std::sync::Arc;
1840 /// let five = Arc::new(5);
1842 /// assert!(five <= Arc::new(5));
1844 fn le(&self, other
: &Arc
<T
>) -> bool
{
1845 *(*self) <= *(*other
)
1848 /// Greater-than comparison for two `Arc`s.
1850 /// The two are compared by calling `>` on their inner values.
1855 /// use std::sync::Arc;
1857 /// let five = Arc::new(5);
1859 /// assert!(five > Arc::new(4));
1861 fn gt(&self, other
: &Arc
<T
>) -> bool
{
1862 *(*self) > *(*other
)
1865 /// 'Greater than or equal to' comparison for two `Arc`s.
1867 /// The two are compared by calling `>=` on their inner values.
1872 /// use std::sync::Arc;
1874 /// let five = Arc::new(5);
1876 /// assert!(five >= Arc::new(5));
1878 fn ge(&self, other
: &Arc
<T
>) -> bool
{
1879 *(*self) >= *(*other
)
1882 #[stable(feature = "rust1", since = "1.0.0")]
1883 impl<T
: ?Sized
+ Ord
> Ord
for Arc
<T
> {
1884 /// Comparison for two `Arc`s.
1886 /// The two are compared by calling `cmp()` on their inner values.
1891 /// use std::sync::Arc;
1892 /// use std::cmp::Ordering;
1894 /// let five = Arc::new(5);
1896 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
1898 fn cmp(&self, other
: &Arc
<T
>) -> Ordering
{
1899 (**self).cmp(&**other
)
1902 #[stable(feature = "rust1", since = "1.0.0")]
1903 impl<T
: ?Sized
+ Eq
> Eq
for Arc
<T
> {}
1905 #[stable(feature = "rust1", since = "1.0.0")]
1906 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Arc
<T
> {
1907 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1908 fmt
::Display
::fmt(&**self, f
)
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl<T
: ?Sized
+ fmt
::Debug
> fmt
::Debug
for Arc
<T
> {
1914 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1915 fmt
::Debug
::fmt(&**self, f
)
1919 #[stable(feature = "rust1", since = "1.0.0")]
1920 impl<T
: ?Sized
> fmt
::Pointer
for Arc
<T
> {
1921 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1922 fmt
::Pointer
::fmt(&(&**self as *const T
), f
)
1926 #[stable(feature = "rust1", since = "1.0.0")]
1927 impl<T
: Default
> Default
for Arc
<T
> {
1928 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
1933 /// use std::sync::Arc;
1935 /// let x: Arc<i32> = Default::default();
1936 /// assert_eq!(*x, 0);
1938 fn default() -> Arc
<T
> {
1939 Arc
::new(Default
::default())
1943 #[stable(feature = "rust1", since = "1.0.0")]
1944 impl<T
: ?Sized
+ Hash
> Hash
for Arc
<T
> {
1945 fn hash
<H
: Hasher
>(&self, state
: &mut H
) {
1946 (**self).hash(state
)
1950 #[stable(feature = "from_for_ptrs", since = "1.6.0")]
1951 impl<T
> From
<T
> for Arc
<T
> {
1952 fn from(t
: T
) -> Self {
1957 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1958 impl<T
: Clone
> From
<&[T
]> for Arc
<[T
]> {
1960 fn from(v
: &[T
]) -> Arc
<[T
]> {
1961 <Self as ArcFromSlice
<T
>>::from_slice(v
)
1965 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1966 impl From
<&str> for Arc
<str> {
1968 fn from(v
: &str) -> Arc
<str> {
1969 let arc
= Arc
::<[u8]>::from(v
.as_bytes());
1970 unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
1974 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1975 impl From
<String
> for Arc
<str> {
1977 fn from(v
: String
) -> Arc
<str> {
1982 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1983 impl<T
: ?Sized
> From
<Box
<T
>> for Arc
<T
> {
1985 fn from(v
: Box
<T
>) -> Arc
<T
> {
1990 #[stable(feature = "shared_from_slice", since = "1.21.0")]
1991 impl<T
> From
<Vec
<T
>> for Arc
<[T
]> {
1993 fn from(mut v
: Vec
<T
>) -> Arc
<[T
]> {
1995 let arc
= Arc
::copy_from_slice(&v
);
1997 // Allow the Vec to free its memory, but not destroy its contents
2005 #[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
2006 impl<T
, const N
: usize> TryFrom
<Arc
<[T
]>> for Arc
<[T
; N
]>
2008 [T
; N
]: LengthAtMost32
,
2010 type Error
= Arc
<[T
]>;
2012 fn try_from(boxed_slice
: Arc
<[T
]>) -> Result
<Self, Self::Error
> {
2013 if boxed_slice
.len() == N
{
2014 Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) }
)
2021 #[stable(feature = "shared_from_iter", since = "1.37.0")]
2022 impl<T
> iter
::FromIterator
<T
> for Arc
<[T
]> {
2023 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
2025 /// # Performance characteristics
2027 /// ## The general case
2029 /// In the general case, collecting into `Arc<[T]>` is done by first
2030 /// collecting into a `Vec<T>`. That is, when writing the following:
2033 /// # use std::sync::Arc;
2034 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
2035 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2038 /// this behaves as if we wrote:
2041 /// # use std::sync::Arc;
2042 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
2043 /// .collect::<Vec<_>>() // The first set of allocations happens here.
2044 /// .into(); // A second allocation for `Arc<[T]>` happens here.
2045 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
2048 /// This will allocate as many times as needed for constructing the `Vec<T>`
2049 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
2051 /// ## Iterators of known length
2053 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
2054 /// a single allocation will be made for the `Arc<[T]>`. For example:
2057 /// # use std::sync::Arc;
2058 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
2059 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
2061 fn from_iter
<I
: iter
::IntoIterator
<Item
= T
>>(iter
: I
) -> Self {
2062 ArcFromIter
::from_iter(iter
.into_iter())
2066 /// Specialization trait used for collecting into `Arc<[T]>`.
2067 trait ArcFromIter
<T
, I
> {
2068 fn from_iter(iter
: I
) -> Self;
2071 impl<T
, I
: Iterator
<Item
= T
>> ArcFromIter
<T
, I
> for Arc
<[T
]> {
2072 default fn from_iter(iter
: I
) -> Self {
2073 iter
.collect
::<Vec
<T
>>().into()
2077 impl<T
, I
: iter
::TrustedLen
<Item
= T
>> ArcFromIter
<T
, I
> for Arc
<[T
]> {
2078 default fn from_iter(iter
: I
) -> Self {
2079 // This is the case for a `TrustedLen` iterator.
2080 let (low
, high
) = iter
.size_hint();
2081 if let Some(high
) = high
{
2085 "TrustedLen iterator's size hint is not exact: {:?}",
2090 // SAFETY: We need to ensure that the iterator has an exact length and we have.
2091 Arc
::from_iter_exact(iter
, low
)
2094 // Fall back to normal implementation.
2095 iter
.collect
::<Vec
<T
>>().into()
2100 impl<'a
, T
: 'a
+ Clone
> ArcFromIter
<&'a T
, slice
::Iter
<'a
, T
>> for Arc
<[T
]> {
2101 fn from_iter(iter
: slice
::Iter
<'a
, T
>) -> Self {
2102 // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
2104 // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
2105 // which is even more performant.
2107 // In the fall-back case we have `T: Clone`. This is still better
2108 // than the `TrustedLen` implementation as slices have a known length
2109 // and so we get to avoid calling `size_hint` and avoid the branching.
2110 iter
.as_slice().into()
2114 #[stable(feature = "rust1", since = "1.0.0")]
2115 impl<T
: ?Sized
> borrow
::Borrow
<T
> for Arc
<T
> {
2116 fn borrow(&self) -> &T
{
2121 #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
2122 impl<T
: ?Sized
> AsRef
<T
> for Arc
<T
> {
2123 fn as_ref(&self) -> &T
{
2128 #[stable(feature = "pin", since = "1.33.0")]
2129 impl<T
: ?Sized
> Unpin
for Arc
<T
> {}
2131 /// Computes the offset of the data field within `ArcInner`.
2132 unsafe fn data_offset
<T
: ?Sized
>(ptr
: *const T
) -> isize {
2133 // Align the unsized value to the end of the `ArcInner`.
2134 // Because it is `?Sized`, it will always be the last field in memory.
2135 // Note: This is a detail of the current implementation of the compiler,
2136 // and is not a guaranteed language detail. Do not rely on it outside of std.
2137 data_offset_align(align_of_val(&*ptr
))
2140 /// Computes the offset of the data field within `ArcInner`.
2142 /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
2143 fn data_offset_sized
<T
>() -> isize {
2144 data_offset_align(align_of
::<T
>())
2148 fn data_offset_align(align
: usize) -> isize {
2149 let layout
= Layout
::new
::<ArcInner
<()>>();
2150 (layout
.size() + layout
.padding_needed_for(align
)) as isize