1#![stable(feature = "rust1", since = "1.0.0")]
2
3//! Thread-safe reference-counting pointers.
4//!
5//! See the [`Arc<T>`][Arc] documentation for more details.
6//!
7//! **Note**: This module is only available on platforms that support atomic
8//! loads and stores of pointers. This may be detected at compile time using
9//! `#[cfg(target_has_atomic = "ptr")]`.
10
11use core::any::Any;
12use core::cell::CloneFromCell;
13#[cfg(not(no_global_oom_handling))]
14use core::clone::TrivialClone;
15use core::clone::{CloneToUninit, UseCloned};
16use core::cmp::Ordering;
17use core::hash::{Hash, Hasher};
18use core::intrinsics::abort;
19#[cfg(not(no_global_oom_handling))]
20use core::iter;
21use core::marker::{PhantomData, Unsize};
22use core::mem::{self, ManuallyDrop};
23use core::num::NonZeroUsize;
24use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
25#[cfg(not(no_global_oom_handling))]
26use core::ops::{Residual, Try};
27use core::panic::{RefUnwindSafe, UnwindSafe};
28use core::pin::{Pin, PinCoerceUnsized};
29use core::ptr::{self, Alignment, NonNull};
30#[cfg(not(no_global_oom_handling))]
31use core::slice::from_raw_parts_mut;
32use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
33use core::sync::atomic::{self, Atomic};
34use core::{borrow, fmt, hint};
35
36#[cfg(not(no_global_oom_handling))]
37use crate::alloc::handle_alloc_error;
38use crate::alloc::{AllocError, Allocator, Global, Layout};
39use crate::borrow::{Cow, ToOwned};
40use crate::boxed::Box;
41use crate::rc::is_dangling;
42#[cfg(not(no_global_oom_handling))]
43use crate::string::String;
44#[cfg(not(no_global_oom_handling))]
45use crate::vec::Vec;
46
47/// A soft limit on the amount of references that may be made to an `Arc`.
48///
49/// Going above this limit will abort your program (although not
50/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
51/// Trying to go above it might call a `panic` (if not actually going above it).
52///
53/// This is a global invariant, and also applies when using a compare-exchange loop.
54///
55/// See comment in `Arc::clone`.
56const MAX_REFCOUNT: usize = (isize::MAX) as usize;
57
58/// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
59const INTERNAL_OVERFLOW_ERROR: &str = "Arc counter overflow";
60
61#[cfg(not(sanitize = "thread"))]
62macro_rules! acquire {
63 ($x:expr) => {
64 atomic::fence(Acquire)
65 };
66}
67
68// ThreadSanitizer does not support memory fences. To avoid false positive
69// reports in Arc / Weak implementation use atomic loads for synchronization
70// instead.
71#[cfg(sanitize = "thread")]
72macro_rules! acquire {
73 ($x:expr) => {
74 $x.load(Acquire)
75 };
76}
77
78/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
79/// Reference Counted'.
80///
81/// The type `Arc<T>` provides shared ownership of a value of type `T`,
82/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
83/// a new `Arc` instance, which points to the same allocation on the heap as the
84/// source `Arc`, while increasing a reference count. When the last `Arc`
85/// pointer to a given allocation is destroyed, the value stored in that allocation (often
86/// referred to as "inner value") is also dropped.
87///
88/// Shared references in Rust disallow mutation by default, and `Arc` is no
89/// exception: you cannot generally obtain a mutable reference to something
90/// inside an `Arc`. If you do need to mutate through an `Arc`, you have several options:
91///
92/// 1. Use interior mutability with synchronization primitives like [`Mutex`][mutex],
93/// [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
94///
95/// 2. Use clone-on-write semantics with [`Arc::make_mut`] which provides efficient mutation
96/// without requiring interior mutability. This approach clones the data only when
97/// needed (when there are multiple references) and can be more efficient when mutations
98/// are infrequent.
99///
100/// 3. Use [`Arc::get_mut`] when you know your `Arc` is not shared (has a reference count of 1),
101/// which provides direct mutable access to the inner value without any cloning.
102///
103/// ```
104/// use std::sync::Arc;
105///
106/// let mut data = Arc::new(vec![1, 2, 3]);
107///
108/// // This will clone the vector only if there are other references to it
109/// Arc::make_mut(&mut data).push(4);
110///
111/// assert_eq!(*data, vec![1, 2, 3, 4]);
112/// ```
113///
114/// **Note**: This type is only available on platforms that support atomic
115/// loads and stores of pointers, which includes all platforms that support
116/// the `std` crate but not all those which only support [`alloc`](crate).
117/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
118///
119/// ## Thread Safety
120///
121/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
122/// counting. This means that it is thread-safe. The disadvantage is that
123/// atomic operations are more expensive than ordinary memory accesses. If you
124/// are not sharing reference-counted allocations between threads, consider using
125/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
126/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
127/// However, a library might choose `Arc<T>` in order to give library consumers
128/// more flexibility.
129///
130/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
131/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
132/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
133/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
134/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
135/// data, but it doesn't add thread safety to its data. Consider
136/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
137/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
138/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
139/// non-atomic operations.
140///
141/// In the end, this means that you may need to pair `Arc<T>` with some sort of
142/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
143///
144/// ## Breaking cycles with `Weak`
145///
146/// The [`downgrade`][downgrade] method can be used to create a non-owning
147/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
148/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
149/// already been dropped. In other words, `Weak` pointers do not keep the value
150/// inside the allocation alive; however, they *do* keep the allocation
151/// (the backing store for the value) alive.
152///
153/// A cycle between `Arc` pointers will never be deallocated. For this reason,
154/// [`Weak`] is used to break cycles. For example, a tree could have
155/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
156/// pointers from children back to their parents.
157///
158/// # Cloning references
159///
160/// Creating a new reference from an existing reference-counted pointer is done using the
161/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
162///
163/// ```
164/// use std::sync::Arc;
165/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
166/// // The two syntaxes below are equivalent.
167/// let a = foo.clone();
168/// let b = Arc::clone(&foo);
169/// // a, b, and foo are all Arcs that point to the same memory location
170/// ```
171///
172/// ## `Deref` behavior
173///
174/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
175/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
176/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
177/// functions, called using [fully qualified syntax]:
178///
179/// ```
180/// use std::sync::Arc;
181///
182/// let my_arc = Arc::new(());
183/// let my_weak = Arc::downgrade(&my_arc);
184/// ```
185///
186/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
187/// fully qualified syntax. Some people prefer to use fully qualified syntax,
188/// while others prefer using method-call syntax.
189///
190/// ```
191/// use std::sync::Arc;
192///
193/// let arc = Arc::new(());
194/// // Method-call syntax
195/// let arc2 = arc.clone();
196/// // Fully qualified syntax
197/// let arc3 = Arc::clone(&arc);
198/// ```
199///
200/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
201/// already been dropped.
202///
203/// [`Rc<T>`]: crate::rc::Rc
204/// [clone]: Clone::clone
205/// [mutex]: ../../std/sync/struct.Mutex.html
206/// [rwlock]: ../../std/sync/struct.RwLock.html
207/// [atomic]: core::sync::atomic
208/// [downgrade]: Arc::downgrade
209/// [upgrade]: Weak::upgrade
210/// [RefCell\<T>]: core::cell::RefCell
211/// [`RefCell<T>`]: core::cell::RefCell
212/// [`std::sync`]: ../../std/sync/index.html
213/// [`Arc::clone(&from)`]: Arc::clone
214/// [fully qualified syntax]: https://doc.rust-lang.org/book/ch19-03-advanced-traits.html#fully-qualified-syntax-for-disambiguation-calling-methods-with-the-same-name
215///
216/// # Examples
217///
218/// Sharing some immutable data between threads:
219///
220/// ```
221/// use std::sync::Arc;
222/// use std::thread;
223///
224/// let five = Arc::new(5);
225///
226/// for _ in 0..10 {
227/// let five = Arc::clone(&five);
228///
229/// thread::spawn(move || {
230/// println!("{five:?}");
231/// });
232/// }
233/// ```
234///
235/// Sharing a mutable [`AtomicUsize`]:
236///
237/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
238///
239/// ```
240/// use std::sync::Arc;
241/// use std::sync::atomic::{AtomicUsize, Ordering};
242/// use std::thread;
243///
244/// let val = Arc::new(AtomicUsize::new(5));
245///
246/// for _ in 0..10 {
247/// let val = Arc::clone(&val);
248///
249/// thread::spawn(move || {
250/// let v = val.fetch_add(1, Ordering::Relaxed);
251/// println!("{v:?}");
252/// });
253/// }
254/// ```
255///
256/// See the [`rc` documentation][rc_examples] for more examples of reference
257/// counting in general.
258///
259/// [rc_examples]: crate::rc#examples
260#[doc(search_unbox)]
261#[rustc_diagnostic_item = "Arc"]
262#[stable(feature = "rust1", since = "1.0.0")]
263#[rustc_insignificant_dtor]
264pub struct Arc<
265 T: ?Sized,
266 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
267> {
268 ptr: NonNull<ArcInner<T>>,
269 phantom: PhantomData<ArcInner<T>>,
270 alloc: A,
271}
272
273#[stable(feature = "rust1", since = "1.0.0")]
274unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Arc<T, A> {}
275#[stable(feature = "rust1", since = "1.0.0")]
276unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Arc<T, A> {}
277
278#[stable(feature = "catch_unwind", since = "1.9.0")]
279impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A> {}
280
281#[unstable(feature = "coerce_unsized", issue = "18598")]
282impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> for Arc<T, A> {}
283
284#[unstable(feature = "dispatch_from_dyn", issue = "none")]
285impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
286
287// SAFETY: `Arc::clone` doesn't access any `Cell`s which could contain the `Arc` being cloned.
288#[unstable(feature = "cell_get_cloned", issue = "145329")]
289unsafe impl<T: ?Sized> CloneFromCell for Arc<T> {}
290
291impl<T: ?Sized> Arc<T> {
292 unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
293 unsafe { Self::from_inner_in(ptr, alloc:Global) }
294 }
295
296 unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
297 unsafe { Self::from_ptr_in(ptr, alloc:Global) }
298 }
299}
300
301impl<T: ?Sized, A: Allocator> Arc<T, A> {
302 #[inline]
303 fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
304 let this = mem::ManuallyDrop::new(this);
305 (this.ptr, unsafe { ptr::read(&this.alloc) })
306 }
307
308 #[inline]
309 unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
310 Self { ptr, phantom: PhantomData, alloc }
311 }
312
313 #[inline]
314 unsafe fn from_ptr_in(ptr: *mut ArcInner<T>, alloc: A) -> Self {
315 unsafe { Self::from_inner_in(ptr:NonNull::new_unchecked(ptr), alloc) }
316 }
317}
318
319/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
320/// managed allocation.
321///
322/// The allocation is accessed by calling [`upgrade`] on the `Weak`
323/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
324///
325/// Since a `Weak` reference does not count towards ownership, it will not
326/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
327/// guarantees about the value still being present. Thus it may return [`None`]
328/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
329/// itself (the backing store) from being deallocated.
330///
331/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
332/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
333/// prevent circular references between [`Arc`] pointers, since mutual owning references
334/// would never allow either [`Arc`] to be dropped. For example, a tree could
335/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
336/// pointers from children back to their parents.
337///
338/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
339///
340/// [`upgrade`]: Weak::upgrade
341#[stable(feature = "arc_weak", since = "1.4.0")]
342#[rustc_diagnostic_item = "ArcWeak"]
343pub struct Weak<
344 T: ?Sized,
345 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
346> {
347 // This is a `NonNull` to allow optimizing the size of this type in enums,
348 // but it is not necessarily a valid pointer.
349 // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
350 // to allocate space on the heap. That's not a value a real pointer
351 // will ever have because ArcInner has alignment at least 2.
352 ptr: NonNull<ArcInner<T>>,
353 alloc: A,
354}
355
356#[stable(feature = "arc_weak", since = "1.4.0")]
357unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A> {}
358#[stable(feature = "arc_weak", since = "1.4.0")]
359unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A> {}
360
361#[unstable(feature = "coerce_unsized", issue = "18598")]
362impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {}
363#[unstable(feature = "dispatch_from_dyn", issue = "none")]
364impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
365
366// SAFETY: `Weak::clone` doesn't access any `Cell`s which could contain the `Weak` being cloned.
367#[unstable(feature = "cell_get_cloned", issue = "145329")]
368unsafe impl<T: ?Sized> CloneFromCell for Weak<T> {}
369
370#[stable(feature = "arc_weak", since = "1.4.0")]
371impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
372 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
373 write!(f, "(Weak)")
374 }
375}
376
377// This is repr(C) to future-proof against possible field-reordering, which
378// would interfere with otherwise safe [into|from]_raw() of transmutable
379// inner types.
380// Unlike RcInner, repr(align(2)) is not strictly required because atomic types
381// have the alignment same as its size, but we use it for consistency and clarity.
382#[repr(C, align(2))]
383struct ArcInner<T: ?Sized> {
384 strong: Atomic<usize>,
385
386 // the value usize::MAX acts as a sentinel for temporarily "locking" the
387 // ability to upgrade weak pointers or downgrade strong ones; this is used
388 // to avoid races in `make_mut` and `get_mut`.
389 weak: Atomic<usize>,
390
391 data: T,
392}
393
394/// Calculate layout for `ArcInner<T>` using the inner value's layout
395fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
396 // Calculate layout using the given value layout.
397 // Previously, layout was calculated on the expression
398 // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
399 // reference (see #54908).
400 Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
401}
402
403unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
404unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
405
406impl<T> Arc<T> {
407 /// Constructs a new `Arc<T>`.
408 ///
409 /// # Examples
410 ///
411 /// ```
412 /// use std::sync::Arc;
413 ///
414 /// let five = Arc::new(5);
415 /// ```
416 #[cfg(not(no_global_oom_handling))]
417 #[inline]
418 #[stable(feature = "rust1", since = "1.0.0")]
419 pub fn new(data: T) -> Arc<T> {
420 // Start the weak pointer count as 1 which is the weak pointer that's
421 // held by all the strong pointers (kinda), see std/rc.rs for more info
422 let x: Box<_> = Box::new(ArcInner {
423 strong: atomic::AtomicUsize::new(1),
424 weak: atomic::AtomicUsize::new(1),
425 data,
426 });
427 unsafe { Self::from_inner(Box::leak(x).into()) }
428 }
429
430 /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
431 /// to allow you to construct a `T` which holds a weak pointer to itself.
432 ///
433 /// Generally, a structure circularly referencing itself, either directly or
434 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
435 /// Using this function, you get access to the weak pointer during the
436 /// initialization of `T`, before the `Arc<T>` is created, such that you can
437 /// clone and store it inside the `T`.
438 ///
439 /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
440 /// then calls your closure, giving it a `Weak<T>` to this allocation,
441 /// and only afterwards completes the construction of the `Arc<T>` by placing
442 /// the `T` returned from your closure into the allocation.
443 ///
444 /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
445 /// returns, calling [`upgrade`] on the weak reference inside your closure will
446 /// fail and result in a `None` value.
447 ///
448 /// # Panics
449 ///
450 /// If `data_fn` panics, the panic is propagated to the caller, and the
451 /// temporary [`Weak<T>`] is dropped normally.
452 ///
453 /// # Example
454 ///
455 /// ```
456 /// # #![allow(dead_code)]
457 /// use std::sync::{Arc, Weak};
458 ///
459 /// struct Gadget {
460 /// me: Weak<Gadget>,
461 /// }
462 ///
463 /// impl Gadget {
464 /// /// Constructs a reference counted Gadget.
465 /// fn new() -> Arc<Self> {
466 /// // `me` is a `Weak<Gadget>` pointing at the new allocation of the
467 /// // `Arc` we're constructing.
468 /// Arc::new_cyclic(|me| {
469 /// // Create the actual struct here.
470 /// Gadget { me: me.clone() }
471 /// })
472 /// }
473 ///
474 /// /// Returns a reference counted pointer to Self.
475 /// fn me(&self) -> Arc<Self> {
476 /// self.me.upgrade().unwrap()
477 /// }
478 /// }
479 /// ```
480 /// [`upgrade`]: Weak::upgrade
481 #[cfg(not(no_global_oom_handling))]
482 #[inline]
483 #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
484 pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
485 where
486 F: FnOnce(&Weak<T>) -> T,
487 {
488 Self::new_cyclic_in(data_fn, Global)
489 }
490
491 /// Constructs a new `Arc` with uninitialized contents.
492 ///
493 /// # Examples
494 ///
495 /// ```
496 /// use std::sync::Arc;
497 ///
498 /// let mut five = Arc::<u32>::new_uninit();
499 ///
500 /// // Deferred initialization:
501 /// Arc::get_mut(&mut five).unwrap().write(5);
502 ///
503 /// let five = unsafe { five.assume_init() };
504 ///
505 /// assert_eq!(*five, 5)
506 /// ```
507 #[cfg(not(no_global_oom_handling))]
508 #[inline]
509 #[stable(feature = "new_uninit", since = "1.82.0")]
510 #[must_use]
511 pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
512 unsafe {
513 Arc::from_ptr(Arc::allocate_for_layout(
514 Layout::new::<T>(),
515 |layout| Global.allocate(layout),
516 <*mut u8>::cast,
517 ))
518 }
519 }
520
521 /// Constructs a new `Arc` with uninitialized contents, with the memory
522 /// being filled with `0` bytes.
523 ///
524 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
525 /// of this method.
526 ///
527 /// # Examples
528 ///
529 /// ```
530 /// use std::sync::Arc;
531 ///
532 /// let zero = Arc::<u32>::new_zeroed();
533 /// let zero = unsafe { zero.assume_init() };
534 ///
535 /// assert_eq!(*zero, 0)
536 /// ```
537 ///
538 /// [zeroed]: mem::MaybeUninit::zeroed
539 #[cfg(not(no_global_oom_handling))]
540 #[inline]
541 #[stable(feature = "new_zeroed_alloc", since = "1.92.0")]
542 #[must_use]
543 pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
544 unsafe {
545 Arc::from_ptr(Arc::allocate_for_layout(
546 Layout::new::<T>(),
547 |layout| Global.allocate_zeroed(layout),
548 <*mut u8>::cast,
549 ))
550 }
551 }
552
553 /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
554 /// `data` will be pinned in memory and unable to be moved.
555 #[cfg(not(no_global_oom_handling))]
556 #[stable(feature = "pin", since = "1.33.0")]
557 #[must_use]
558 pub fn pin(data: T) -> Pin<Arc<T>> {
559 unsafe { Pin::new_unchecked(Arc::new(data)) }
560 }
561
562 /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
563 #[unstable(feature = "allocator_api", issue = "32838")]
564 #[inline]
565 pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
566 unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
567 }
568
569 /// Constructs a new `Arc<T>`, returning an error if allocation fails.
570 ///
571 /// # Examples
572 ///
573 /// ```
574 /// #![feature(allocator_api)]
575 /// use std::sync::Arc;
576 ///
577 /// let five = Arc::try_new(5)?;
578 /// # Ok::<(), std::alloc::AllocError>(())
579 /// ```
580 #[unstable(feature = "allocator_api", issue = "32838")]
581 #[inline]
582 pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
583 // Start the weak pointer count as 1 which is the weak pointer that's
584 // held by all the strong pointers (kinda), see std/rc.rs for more info
585 let x: Box<_> = Box::try_new(ArcInner {
586 strong: atomic::AtomicUsize::new(1),
587 weak: atomic::AtomicUsize::new(1),
588 data,
589 })?;
590 unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
591 }
592
593 /// Constructs a new `Arc` with uninitialized contents, returning an error
594 /// if allocation fails.
595 ///
596 /// # Examples
597 ///
598 /// ```
599 /// #![feature(allocator_api)]
600 ///
601 /// use std::sync::Arc;
602 ///
603 /// let mut five = Arc::<u32>::try_new_uninit()?;
604 ///
605 /// // Deferred initialization:
606 /// Arc::get_mut(&mut five).unwrap().write(5);
607 ///
608 /// let five = unsafe { five.assume_init() };
609 ///
610 /// assert_eq!(*five, 5);
611 /// # Ok::<(), std::alloc::AllocError>(())
612 /// ```
613 #[unstable(feature = "allocator_api", issue = "32838")]
614 pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
615 unsafe {
616 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
617 Layout::new::<T>(),
618 |layout| Global.allocate(layout),
619 <*mut u8>::cast,
620 )?))
621 }
622 }
623
624 /// Constructs a new `Arc` with uninitialized contents, with the memory
625 /// being filled with `0` bytes, returning an error if allocation fails.
626 ///
627 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
628 /// of this method.
629 ///
630 /// # Examples
631 ///
632 /// ```
633 /// #![feature( allocator_api)]
634 ///
635 /// use std::sync::Arc;
636 ///
637 /// let zero = Arc::<u32>::try_new_zeroed()?;
638 /// let zero = unsafe { zero.assume_init() };
639 ///
640 /// assert_eq!(*zero, 0);
641 /// # Ok::<(), std::alloc::AllocError>(())
642 /// ```
643 ///
644 /// [zeroed]: mem::MaybeUninit::zeroed
645 #[unstable(feature = "allocator_api", issue = "32838")]
646 pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
647 unsafe {
648 Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
649 Layout::new::<T>(),
650 |layout| Global.allocate_zeroed(layout),
651 <*mut u8>::cast,
652 )?))
653 }
654 }
655
656 /// Maps the value in an `Arc`, reusing the allocation if possible.
657 ///
658 /// `f` is called on a reference to the value in the `Arc`, and the result is returned, also in
659 /// an `Arc`.
660 ///
661 /// Note: this is an associated function, which means that you have
662 /// to call it as `Arc::map(a, f)` instead of `r.map(a)`. This
663 /// is so that there is no conflict with a method on the inner type.
664 ///
665 /// # Examples
666 ///
667 /// ```
668 /// #![feature(smart_pointer_try_map)]
669 ///
670 /// use std::sync::Arc;
671 ///
672 /// let r = Arc::new(7);
673 /// let new = Arc::map(r, |i| i + 7);
674 /// assert_eq!(*new, 14);
675 /// ```
676 #[cfg(not(no_global_oom_handling))]
677 #[unstable(feature = "smart_pointer_try_map", issue = "144419")]
678 pub fn map<U>(this: Self, f: impl FnOnce(&T) -> U) -> Arc<U> {
679 if size_of::<T>() == size_of::<U>()
680 && align_of::<T>() == align_of::<U>()
681 && Arc::is_unique(&this)
682 {
683 unsafe {
684 let ptr = Arc::into_raw(this);
685 let value = ptr.read();
686 let mut allocation = Arc::from_raw(ptr.cast::<mem::MaybeUninit<U>>());
687
688 Arc::get_mut_unchecked(&mut allocation).write(f(&value));
689 allocation.assume_init()
690 }
691 } else {
692 Arc::new(f(&*this))
693 }
694 }
695
696 /// Attempts to map the value in an `Arc`, reusing the allocation if possible.
697 ///
698 /// `f` is called on a reference to the value in the `Arc`, and if the operation succeeds, the
699 /// result is returned, also in an `Arc`.
700 ///
701 /// Note: this is an associated function, which means that you have
702 /// to call it as `Arc::try_map(a, f)` instead of `a.try_map(f)`. This
703 /// is so that there is no conflict with a method on the inner type.
704 ///
705 /// # Examples
706 ///
707 /// ```
708 /// #![feature(smart_pointer_try_map)]
709 ///
710 /// use std::sync::Arc;
711 ///
712 /// let b = Arc::new(7);
713 /// let new = Arc::try_map(b, |&i| u32::try_from(i)).unwrap();
714 /// assert_eq!(*new, 7);
715 /// ```
716 #[cfg(not(no_global_oom_handling))]
717 #[unstable(feature = "smart_pointer_try_map", issue = "144419")]
718 pub fn try_map<R>(
719 this: Self,
720 f: impl FnOnce(&T) -> R,
721 ) -> <R::Residual as Residual<Arc<R::Output>>>::TryType
722 where
723 R: Try,
724 R::Residual: Residual<Arc<R::Output>>,
725 {
726 if size_of::<T>() == size_of::<R::Output>()
727 && align_of::<T>() == align_of::<R::Output>()
728 && Arc::is_unique(&this)
729 {
730 unsafe {
731 let ptr = Arc::into_raw(this);
732 let value = ptr.read();
733 let mut allocation = Arc::from_raw(ptr.cast::<mem::MaybeUninit<R::Output>>());
734
735 Arc::get_mut_unchecked(&mut allocation).write(f(&value)?);
736 try { allocation.assume_init() }
737 }
738 } else {
739 try { Arc::new(f(&*this)?) }
740 }
741 }
742}
743
744impl<T, A: Allocator> Arc<T, A> {
745 /// Constructs a new `Arc<T>` in the provided allocator.
746 ///
747 /// # Examples
748 ///
749 /// ```
750 /// #![feature(allocator_api)]
751 ///
752 /// use std::sync::Arc;
753 /// use std::alloc::System;
754 ///
755 /// let five = Arc::new_in(5, System);
756 /// ```
757 #[inline]
758 #[cfg(not(no_global_oom_handling))]
759 #[unstable(feature = "allocator_api", issue = "32838")]
760 pub fn new_in(data: T, alloc: A) -> Arc<T, A> {
761 // Start the weak pointer count as 1 which is the weak pointer that's
762 // held by all the strong pointers (kinda), see std/rc.rs for more info
763 let x = Box::new_in(
764 ArcInner {
765 strong: atomic::AtomicUsize::new(1),
766 weak: atomic::AtomicUsize::new(1),
767 data,
768 },
769 alloc,
770 );
771 let (ptr, alloc) = Box::into_unique(x);
772 unsafe { Self::from_inner_in(ptr.into(), alloc) }
773 }
774
775 /// Constructs a new `Arc` with uninitialized contents in the provided allocator.
776 ///
777 /// # Examples
778 ///
779 /// ```
780 /// #![feature(get_mut_unchecked)]
781 /// #![feature(allocator_api)]
782 ///
783 /// use std::sync::Arc;
784 /// use std::alloc::System;
785 ///
786 /// let mut five = Arc::<u32, _>::new_uninit_in(System);
787 ///
788 /// let five = unsafe {
789 /// // Deferred initialization:
790 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
791 ///
792 /// five.assume_init()
793 /// };
794 ///
795 /// assert_eq!(*five, 5)
796 /// ```
797 #[cfg(not(no_global_oom_handling))]
798 #[unstable(feature = "allocator_api", issue = "32838")]
799 #[inline]
800 pub fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
801 unsafe {
802 Arc::from_ptr_in(
803 Arc::allocate_for_layout(
804 Layout::new::<T>(),
805 |layout| alloc.allocate(layout),
806 <*mut u8>::cast,
807 ),
808 alloc,
809 )
810 }
811 }
812
813 /// Constructs a new `Arc` with uninitialized contents, with the memory
814 /// being filled with `0` bytes, in the provided allocator.
815 ///
816 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
817 /// of this method.
818 ///
819 /// # Examples
820 ///
821 /// ```
822 /// #![feature(allocator_api)]
823 ///
824 /// use std::sync::Arc;
825 /// use std::alloc::System;
826 ///
827 /// let zero = Arc::<u32, _>::new_zeroed_in(System);
828 /// let zero = unsafe { zero.assume_init() };
829 ///
830 /// assert_eq!(*zero, 0)
831 /// ```
832 ///
833 /// [zeroed]: mem::MaybeUninit::zeroed
834 #[cfg(not(no_global_oom_handling))]
835 #[unstable(feature = "allocator_api", issue = "32838")]
836 #[inline]
837 pub fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
838 unsafe {
839 Arc::from_ptr_in(
840 Arc::allocate_for_layout(
841 Layout::new::<T>(),
842 |layout| alloc.allocate_zeroed(layout),
843 <*mut u8>::cast,
844 ),
845 alloc,
846 )
847 }
848 }
849
850 /// Constructs a new `Arc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
851 /// to allow you to construct a `T` which holds a weak pointer to itself.
852 ///
853 /// Generally, a structure circularly referencing itself, either directly or
854 /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
855 /// Using this function, you get access to the weak pointer during the
856 /// initialization of `T`, before the `Arc<T, A>` is created, such that you can
857 /// clone and store it inside the `T`.
858 ///
859 /// `new_cyclic_in` first allocates the managed allocation for the `Arc<T, A>`,
860 /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
861 /// and only afterwards completes the construction of the `Arc<T, A>` by placing
862 /// the `T` returned from your closure into the allocation.
863 ///
864 /// Since the new `Arc<T, A>` is not fully-constructed until `Arc<T, A>::new_cyclic_in`
865 /// returns, calling [`upgrade`] on the weak reference inside your closure will
866 /// fail and result in a `None` value.
867 ///
868 /// # Panics
869 ///
870 /// If `data_fn` panics, the panic is propagated to the caller, and the
871 /// temporary [`Weak<T>`] is dropped normally.
872 ///
873 /// # Example
874 ///
875 /// See [`new_cyclic`]
876 ///
877 /// [`new_cyclic`]: Arc::new_cyclic
878 /// [`upgrade`]: Weak::upgrade
879 #[cfg(not(no_global_oom_handling))]
880 #[inline]
881 #[unstable(feature = "allocator_api", issue = "32838")]
882 pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
883 where
884 F: FnOnce(&Weak<T, A>) -> T,
885 {
886 // Construct the inner in the "uninitialized" state with a single
887 // weak reference.
888 let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in(
889 ArcInner {
890 strong: atomic::AtomicUsize::new(0),
891 weak: atomic::AtomicUsize::new(1),
892 data: mem::MaybeUninit::<T>::uninit(),
893 },
894 alloc,
895 ));
896 let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into();
897 let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
898
899 let weak = Weak { ptr: init_ptr, alloc };
900
901 // It's important we don't give up ownership of the weak pointer, or
902 // else the memory might be freed by the time `data_fn` returns. If
903 // we really wanted to pass ownership, we could create an additional
904 // weak pointer for ourselves, but this would result in additional
905 // updates to the weak reference count which might not be necessary
906 // otherwise.
907 let data = data_fn(&weak);
908
909 // Now we can properly initialize the inner value and turn our weak
910 // reference into a strong reference.
911 let strong = unsafe {
912 let inner = init_ptr.as_ptr();
913 ptr::write(&raw mut (*inner).data, data);
914
915 // The above write to the data field must be visible to any threads which
916 // observe a non-zero strong count. Therefore we need at least "Release" ordering
917 // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
918 //
919 // "Acquire" ordering is not required. When considering the possible behaviors
920 // of `data_fn` we only need to look at what it could do with a reference to a
921 // non-upgradeable `Weak`:
922 // - It can *clone* the `Weak`, increasing the weak reference count.
923 // - It can drop those clones, decreasing the weak reference count (but never to zero).
924 //
925 // These side effects do not impact us in any way, and no other side effects are
926 // possible with safe code alone.
927 let prev_value = (*inner).strong.fetch_add(1, Release);
928 debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
929
930 // Strong references should collectively own a shared weak reference,
931 // so don't run the destructor for our old weak reference.
932 // Calling into_raw_with_allocator has the double effect of giving us back the allocator,
933 // and forgetting the weak reference.
934 let alloc = weak.into_raw_with_allocator().1;
935
936 Arc::from_inner_in(init_ptr, alloc)
937 };
938
939 strong
940 }
941
942 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator. If `T` does not implement `Unpin`,
943 /// then `data` will be pinned in memory and unable to be moved.
944 #[cfg(not(no_global_oom_handling))]
945 #[unstable(feature = "allocator_api", issue = "32838")]
946 #[inline]
947 pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
948 where
949 A: 'static,
950 {
951 unsafe { Pin::new_unchecked(Arc::new_in(data, alloc)) }
952 }
953
954 /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator, return an error if allocation
955 /// fails.
956 #[inline]
957 #[unstable(feature = "allocator_api", issue = "32838")]
958 pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
959 where
960 A: 'static,
961 {
962 unsafe { Ok(Pin::new_unchecked(Arc::try_new_in(data, alloc)?)) }
963 }
964
965 /// Constructs a new `Arc<T, A>` in the provided allocator, returning an error if allocation fails.
966 ///
967 /// # Examples
968 ///
969 /// ```
970 /// #![feature(allocator_api)]
971 ///
972 /// use std::sync::Arc;
973 /// use std::alloc::System;
974 ///
975 /// let five = Arc::try_new_in(5, System)?;
976 /// # Ok::<(), std::alloc::AllocError>(())
977 /// ```
978 #[unstable(feature = "allocator_api", issue = "32838")]
979 #[inline]
980 pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError> {
981 // Start the weak pointer count as 1 which is the weak pointer that's
982 // held by all the strong pointers (kinda), see std/rc.rs for more info
983 let x = Box::try_new_in(
984 ArcInner {
985 strong: atomic::AtomicUsize::new(1),
986 weak: atomic::AtomicUsize::new(1),
987 data,
988 },
989 alloc,
990 )?;
991 let (ptr, alloc) = Box::into_unique(x);
992 Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
993 }
994
995 /// Constructs a new `Arc` with uninitialized contents, in the provided allocator, returning an
996 /// error if allocation fails.
997 ///
998 /// # Examples
999 ///
1000 /// ```
1001 /// #![feature(allocator_api)]
1002 /// #![feature(get_mut_unchecked)]
1003 ///
1004 /// use std::sync::Arc;
1005 /// use std::alloc::System;
1006 ///
1007 /// let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
1008 ///
1009 /// let five = unsafe {
1010 /// // Deferred initialization:
1011 /// Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
1012 ///
1013 /// five.assume_init()
1014 /// };
1015 ///
1016 /// assert_eq!(*five, 5);
1017 /// # Ok::<(), std::alloc::AllocError>(())
1018 /// ```
1019 #[unstable(feature = "allocator_api", issue = "32838")]
1020 #[inline]
1021 pub fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
1022 unsafe {
1023 Ok(Arc::from_ptr_in(
1024 Arc::try_allocate_for_layout(
1025 Layout::new::<T>(),
1026 |layout| alloc.allocate(layout),
1027 <*mut u8>::cast,
1028 )?,
1029 alloc,
1030 ))
1031 }
1032 }
1033
1034 /// Constructs a new `Arc` with uninitialized contents, with the memory
1035 /// being filled with `0` bytes, in the provided allocator, returning an error if allocation
1036 /// fails.
1037 ///
1038 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
1039 /// of this method.
1040 ///
1041 /// # Examples
1042 ///
1043 /// ```
1044 /// #![feature(allocator_api)]
1045 ///
1046 /// use std::sync::Arc;
1047 /// use std::alloc::System;
1048 ///
1049 /// let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
1050 /// let zero = unsafe { zero.assume_init() };
1051 ///
1052 /// assert_eq!(*zero, 0);
1053 /// # Ok::<(), std::alloc::AllocError>(())
1054 /// ```
1055 ///
1056 /// [zeroed]: mem::MaybeUninit::zeroed
1057 #[unstable(feature = "allocator_api", issue = "32838")]
1058 #[inline]
1059 pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
1060 unsafe {
1061 Ok(Arc::from_ptr_in(
1062 Arc::try_allocate_for_layout(
1063 Layout::new::<T>(),
1064 |layout| alloc.allocate_zeroed(layout),
1065 <*mut u8>::cast,
1066 )?,
1067 alloc,
1068 ))
1069 }
1070 }
1071 /// Returns the inner value, if the `Arc` has exactly one strong reference.
1072 ///
1073 /// Otherwise, an [`Err`] is returned with the same `Arc` that was
1074 /// passed in.
1075 ///
1076 /// This will succeed even if there are outstanding weak references.
1077 ///
1078 /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
1079 /// keep the `Arc` in the [`Err`] case.
1080 /// Immediately dropping the [`Err`]-value, as the expression
1081 /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
1082 /// drop to zero and the inner value of the `Arc` to be dropped.
1083 /// For instance, if two threads execute such an expression in parallel,
1084 /// there is a race condition without the possibility of unsafety:
1085 /// The threads could first both check whether they own the last instance
1086 /// in `Arc::try_unwrap`, determine that they both do not, and then both
1087 /// discard and drop their instance in the call to [`ok`][`Result::ok`].
1088 /// In this scenario, the value inside the `Arc` is safely destroyed
1089 /// by exactly one of the threads, but neither thread will ever be able
1090 /// to use the value.
1091 ///
1092 /// # Examples
1093 ///
1094 /// ```
1095 /// use std::sync::Arc;
1096 ///
1097 /// let x = Arc::new(3);
1098 /// assert_eq!(Arc::try_unwrap(x), Ok(3));
1099 ///
1100 /// let x = Arc::new(4);
1101 /// let _y = Arc::clone(&x);
1102 /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
1103 /// ```
1104 #[inline]
1105 #[stable(feature = "arc_unique", since = "1.4.0")]
1106 pub fn try_unwrap(this: Self) -> Result<T, Self> {
1107 if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
1108 return Err(this);
1109 }
1110
1111 acquire!(this.inner().strong);
1112
1113 let this = ManuallyDrop::new(this);
1114 let elem: T = unsafe { ptr::read(&this.ptr.as_ref().data) };
1115 let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator
1116
1117 // Make a weak pointer to clean up the implicit strong-weak reference
1118 let _weak = Weak { ptr: this.ptr, alloc };
1119
1120 Ok(elem)
1121 }
1122
1123 /// Returns the inner value, if the `Arc` has exactly one strong reference.
1124 ///
1125 /// Otherwise, [`None`] is returned and the `Arc` is dropped.
1126 ///
1127 /// This will succeed even if there are outstanding weak references.
1128 ///
1129 /// If `Arc::into_inner` is called on every clone of this `Arc`,
1130 /// it is guaranteed that exactly one of the calls returns the inner value.
1131 /// This means in particular that the inner value is not dropped.
1132 ///
1133 /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
1134 /// is meant for different use-cases. If used as a direct replacement
1135 /// for `Arc::into_inner` anyway, such as with the expression
1136 /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
1137 /// **not** give the same guarantee as described in the previous paragraph.
1138 /// For more information, see the examples below and read the documentation
1139 /// of [`Arc::try_unwrap`].
1140 ///
1141 /// # Examples
1142 ///
1143 /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
1144 /// ```
1145 /// use std::sync::Arc;
1146 ///
1147 /// let x = Arc::new(3);
1148 /// let y = Arc::clone(&x);
1149 ///
1150 /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
1151 /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
1152 /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
1153 ///
1154 /// let x_inner_value = x_thread.join().unwrap();
1155 /// let y_inner_value = y_thread.join().unwrap();
1156 ///
1157 /// // One of the threads is guaranteed to receive the inner value:
1158 /// assert!(matches!(
1159 /// (x_inner_value, y_inner_value),
1160 /// (None, Some(3)) | (Some(3), None)
1161 /// ));
1162 /// // The result could also be `(None, None)` if the threads called
1163 /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
1164 /// ```
1165 ///
1166 /// A more practical example demonstrating the need for `Arc::into_inner`:
1167 /// ```
1168 /// use std::sync::Arc;
1169 ///
1170 /// // Definition of a simple singly linked list using `Arc`:
1171 /// #[derive(Clone)]
1172 /// struct LinkedList<T>(Option<Arc<Node<T>>>);
1173 /// struct Node<T>(T, Option<Arc<Node<T>>>);
1174 ///
1175 /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
1176 /// // can cause a stack overflow. To prevent this, we can provide a
1177 /// // manual `Drop` implementation that does the destruction in a loop:
1178 /// impl<T> Drop for LinkedList<T> {
1179 /// fn drop(&mut self) {
1180 /// let mut link = self.0.take();
1181 /// while let Some(arc_node) = link.take() {
1182 /// if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
1183 /// link = next;
1184 /// }
1185 /// }
1186 /// }
1187 /// }
1188 ///
1189 /// // Implementation of `new` and `push` omitted
1190 /// impl<T> LinkedList<T> {
1191 /// /* ... */
1192 /// # fn new() -> Self {
1193 /// # LinkedList(None)
1194 /// # }
1195 /// # fn push(&mut self, x: T) {
1196 /// # self.0 = Some(Arc::new(Node(x, self.0.take())));
1197 /// # }
1198 /// }
1199 ///
1200 /// // The following code could have still caused a stack overflow
1201 /// // despite the manual `Drop` impl if that `Drop` impl had used
1202 /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
1203 ///
1204 /// // Create a long list and clone it
1205 /// let mut x = LinkedList::new();
1206 /// let size = 100000;
1207 /// # let size = if cfg!(miri) { 100 } else { size };
1208 /// for i in 0..size {
1209 /// x.push(i); // Adds i to the front of x
1210 /// }
1211 /// let y = x.clone();
1212 ///
1213 /// // Drop the clones in parallel
1214 /// let x_thread = std::thread::spawn(|| drop(x));
1215 /// let y_thread = std::thread::spawn(|| drop(y));
1216 /// x_thread.join().unwrap();
1217 /// y_thread.join().unwrap();
1218 /// ```
1219 #[inline]
1220 #[stable(feature = "arc_into_inner", since = "1.70.0")]
1221 pub fn into_inner(this: Self) -> Option<T> {
1222 // Make sure that the ordinary `Drop` implementation isn’t called as well
1223 let mut this = mem::ManuallyDrop::new(this);
1224
1225 // Following the implementation of `drop` and `drop_slow`
1226 if this.inner().strong.fetch_sub(1, Release) != 1 {
1227 return None;
1228 }
1229
1230 acquire!(this.inner().strong);
1231
1232 // SAFETY: This mirrors the line
1233 //
1234 // unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1235 //
1236 // in `drop_slow`. Instead of dropping the value behind the pointer,
1237 // it is read and eventually returned; `ptr::read` has the same
1238 // safety conditions as `ptr::drop_in_place`.
1239
1240 let inner = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
1241 let alloc = unsafe { ptr::read(&this.alloc) };
1242
1243 drop(Weak { ptr: this.ptr, alloc });
1244
1245 Some(inner)
1246 }
1247}
1248
1249impl<T> Arc<[T]> {
1250 /// Constructs a new atomically reference-counted slice with uninitialized contents.
1251 ///
1252 /// # Examples
1253 ///
1254 /// ```
1255 /// use std::sync::Arc;
1256 ///
1257 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1258 ///
1259 /// // Deferred initialization:
1260 /// let data = Arc::get_mut(&mut values).unwrap();
1261 /// data[0].write(1);
1262 /// data[1].write(2);
1263 /// data[2].write(3);
1264 ///
1265 /// let values = unsafe { values.assume_init() };
1266 ///
1267 /// assert_eq!(*values, [1, 2, 3])
1268 /// ```
1269 #[cfg(not(no_global_oom_handling))]
1270 #[inline]
1271 #[stable(feature = "new_uninit", since = "1.82.0")]
1272 #[must_use]
1273 pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1274 unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
1275 }
1276
1277 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1278 /// filled with `0` bytes.
1279 ///
1280 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1281 /// incorrect usage of this method.
1282 ///
1283 /// # Examples
1284 ///
1285 /// ```
1286 /// use std::sync::Arc;
1287 ///
1288 /// let values = Arc::<[u32]>::new_zeroed_slice(3);
1289 /// let values = unsafe { values.assume_init() };
1290 ///
1291 /// assert_eq!(*values, [0, 0, 0])
1292 /// ```
1293 ///
1294 /// [zeroed]: mem::MaybeUninit::zeroed
1295 #[cfg(not(no_global_oom_handling))]
1296 #[inline]
1297 #[stable(feature = "new_zeroed_alloc", since = "1.92.0")]
1298 #[must_use]
1299 pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1300 unsafe {
1301 Arc::from_ptr(Arc::allocate_for_layout(
1302 Layout::array::<T>(len).unwrap(),
1303 |layout| Global.allocate_zeroed(layout),
1304 |mem| {
1305 ptr::slice_from_raw_parts_mut(mem as *mut T, len)
1306 as *mut ArcInner<[mem::MaybeUninit<T>]>
1307 },
1308 ))
1309 }
1310 }
1311
1312 /// Converts the reference-counted slice into a reference-counted array.
1313 ///
1314 /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
1315 ///
1316 /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
1317 #[unstable(feature = "alloc_slice_into_array", issue = "148082")]
1318 #[inline]
1319 #[must_use]
1320 pub fn into_array<const N: usize>(self) -> Option<Arc<[T; N]>> {
1321 if self.len() == N {
1322 let ptr = Self::into_raw(self) as *const [T; N];
1323
1324 // SAFETY: The underlying array of a slice has the exact same layout as an actual array `[T; N]` if `N` is equal to the slice's length.
1325 let me = unsafe { Arc::from_raw(ptr) };
1326 Some(me)
1327 } else {
1328 None
1329 }
1330 }
1331}
1332
1333impl<T, A: Allocator> Arc<[T], A> {
1334 /// Constructs a new atomically reference-counted slice with uninitialized contents in the
1335 /// provided allocator.
1336 ///
1337 /// # Examples
1338 ///
1339 /// ```
1340 /// #![feature(get_mut_unchecked)]
1341 /// #![feature(allocator_api)]
1342 ///
1343 /// use std::sync::Arc;
1344 /// use std::alloc::System;
1345 ///
1346 /// let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
1347 ///
1348 /// let values = unsafe {
1349 /// // Deferred initialization:
1350 /// Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
1351 /// Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
1352 /// Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
1353 ///
1354 /// values.assume_init()
1355 /// };
1356 ///
1357 /// assert_eq!(*values, [1, 2, 3])
1358 /// ```
1359 #[cfg(not(no_global_oom_handling))]
1360 #[unstable(feature = "allocator_api", issue = "32838")]
1361 #[inline]
1362 pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1363 unsafe { Arc::from_ptr_in(Arc::allocate_for_slice_in(len, &alloc), alloc) }
1364 }
1365
1366 /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1367 /// filled with `0` bytes, in the provided allocator.
1368 ///
1369 /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1370 /// incorrect usage of this method.
1371 ///
1372 /// # Examples
1373 ///
1374 /// ```
1375 /// #![feature(allocator_api)]
1376 ///
1377 /// use std::sync::Arc;
1378 /// use std::alloc::System;
1379 ///
1380 /// let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
1381 /// let values = unsafe { values.assume_init() };
1382 ///
1383 /// assert_eq!(*values, [0, 0, 0])
1384 /// ```
1385 ///
1386 /// [zeroed]: mem::MaybeUninit::zeroed
1387 #[cfg(not(no_global_oom_handling))]
1388 #[unstable(feature = "allocator_api", issue = "32838")]
1389 #[inline]
1390 pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1391 unsafe {
1392 Arc::from_ptr_in(
1393 Arc::allocate_for_layout(
1394 Layout::array::<T>(len).unwrap(),
1395 |layout| alloc.allocate_zeroed(layout),
1396 |mem| {
1397 ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len)
1398 as *mut ArcInner<[mem::MaybeUninit<T>]>
1399 },
1400 ),
1401 alloc,
1402 )
1403 }
1404 }
1405}
1406
1407impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
1408 /// Converts to `Arc<T>`.
1409 ///
1410 /// # Safety
1411 ///
1412 /// As with [`MaybeUninit::assume_init`],
1413 /// it is up to the caller to guarantee that the inner value
1414 /// really is in an initialized state.
1415 /// Calling this when the content is not yet fully initialized
1416 /// causes immediate undefined behavior.
1417 ///
1418 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1419 ///
1420 /// # Examples
1421 ///
1422 /// ```
1423 /// use std::sync::Arc;
1424 ///
1425 /// let mut five = Arc::<u32>::new_uninit();
1426 ///
1427 /// // Deferred initialization:
1428 /// Arc::get_mut(&mut five).unwrap().write(5);
1429 ///
1430 /// let five = unsafe { five.assume_init() };
1431 ///
1432 /// assert_eq!(*five, 5)
1433 /// ```
1434 #[stable(feature = "new_uninit", since = "1.82.0")]
1435 #[must_use = "`self` will be dropped if the result is not used"]
1436 #[inline]
1437 pub unsafe fn assume_init(self) -> Arc<T, A> {
1438 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1439 unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
1440 }
1441}
1442
1443impl<T: ?Sized + CloneToUninit> Arc<T> {
1444 /// Constructs a new `Arc<T>` with a clone of `value`.
1445 ///
1446 /// # Examples
1447 ///
1448 /// ```
1449 /// #![feature(clone_from_ref)]
1450 /// use std::sync::Arc;
1451 ///
1452 /// let hello: Arc<str> = Arc::clone_from_ref("hello");
1453 /// ```
1454 #[cfg(not(no_global_oom_handling))]
1455 #[unstable(feature = "clone_from_ref", issue = "149075")]
1456 pub fn clone_from_ref(value: &T) -> Arc<T> {
1457 Arc::clone_from_ref_in(value, Global)
1458 }
1459
1460 /// Constructs a new `Arc<T>` with a clone of `value`, returning an error if allocation fails
1461 ///
1462 /// # Examples
1463 ///
1464 /// ```
1465 /// #![feature(clone_from_ref)]
1466 /// #![feature(allocator_api)]
1467 /// use std::sync::Arc;
1468 ///
1469 /// let hello: Arc<str> = Arc::try_clone_from_ref("hello")?;
1470 /// # Ok::<(), std::alloc::AllocError>(())
1471 /// ```
1472 #[unstable(feature = "clone_from_ref", issue = "149075")]
1473 //#[unstable(feature = "allocator_api", issue = "32838")]
1474 pub fn try_clone_from_ref(value: &T) -> Result<Arc<T>, AllocError> {
1475 Arc::try_clone_from_ref_in(value, Global)
1476 }
1477}
1478
1479impl<T: ?Sized + CloneToUninit, A: Allocator> Arc<T, A> {
1480 /// Constructs a new `Arc<T>` with a clone of `value` in the provided allocator.
1481 ///
1482 /// # Examples
1483 ///
1484 /// ```
1485 /// #![feature(clone_from_ref)]
1486 /// #![feature(allocator_api)]
1487 /// use std::sync::Arc;
1488 /// use std::alloc::System;
1489 ///
1490 /// let hello: Arc<str, System> = Arc::clone_from_ref_in("hello", System);
1491 /// ```
1492 #[cfg(not(no_global_oom_handling))]
1493 #[unstable(feature = "clone_from_ref", issue = "149075")]
1494 //#[unstable(feature = "allocator_api", issue = "32838")]
1495 pub fn clone_from_ref_in(value: &T, alloc: A) -> Arc<T, A> {
1496 // `in_progress` drops the allocation if we panic before finishing initializing it.
1497 let mut in_progress: UniqueArcUninit<T, A> = UniqueArcUninit::new(value, alloc);
1498
1499 // Initialize with clone of value.
1500 let initialized_clone = unsafe {
1501 // Clone. If the clone panics, `in_progress` will be dropped and clean up.
1502 value.clone_to_uninit(in_progress.data_ptr().cast());
1503 // Cast type of pointer, now that it is initialized.
1504 in_progress.into_arc()
1505 };
1506
1507 initialized_clone
1508 }
1509
1510 /// Constructs a new `Arc<T>` with a clone of `value` in the provided allocator, returning an error if allocation fails
1511 ///
1512 /// # Examples
1513 ///
1514 /// ```
1515 /// #![feature(clone_from_ref)]
1516 /// #![feature(allocator_api)]
1517 /// use std::sync::Arc;
1518 /// use std::alloc::System;
1519 ///
1520 /// let hello: Arc<str, System> = Arc::try_clone_from_ref_in("hello", System)?;
1521 /// # Ok::<(), std::alloc::AllocError>(())
1522 /// ```
1523 #[unstable(feature = "clone_from_ref", issue = "149075")]
1524 //#[unstable(feature = "allocator_api", issue = "32838")]
1525 pub fn try_clone_from_ref_in(value: &T, alloc: A) -> Result<Arc<T, A>, AllocError> {
1526 // `in_progress` drops the allocation if we panic before finishing initializing it.
1527 let mut in_progress: UniqueArcUninit<T, A> = UniqueArcUninit::try_new(value, alloc)?;
1528
1529 // Initialize with clone of value.
1530 let initialized_clone = unsafe {
1531 // Clone. If the clone panics, `in_progress` will be dropped and clean up.
1532 value.clone_to_uninit(in_progress.data_ptr().cast());
1533 // Cast type of pointer, now that it is initialized.
1534 in_progress.into_arc()
1535 };
1536
1537 Ok(initialized_clone)
1538 }
1539}
1540
1541impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
1542 /// Converts to `Arc<[T]>`.
1543 ///
1544 /// # Safety
1545 ///
1546 /// As with [`MaybeUninit::assume_init`],
1547 /// it is up to the caller to guarantee that the inner value
1548 /// really is in an initialized state.
1549 /// Calling this when the content is not yet fully initialized
1550 /// causes immediate undefined behavior.
1551 ///
1552 /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1553 ///
1554 /// # Examples
1555 ///
1556 /// ```
1557 /// use std::sync::Arc;
1558 ///
1559 /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1560 ///
1561 /// // Deferred initialization:
1562 /// let data = Arc::get_mut(&mut values).unwrap();
1563 /// data[0].write(1);
1564 /// data[1].write(2);
1565 /// data[2].write(3);
1566 ///
1567 /// let values = unsafe { values.assume_init() };
1568 ///
1569 /// assert_eq!(*values, [1, 2, 3])
1570 /// ```
1571 #[stable(feature = "new_uninit", since = "1.82.0")]
1572 #[must_use = "`self` will be dropped if the result is not used"]
1573 #[inline]
1574 pub unsafe fn assume_init(self) -> Arc<[T], A> {
1575 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1576 unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
1577 }
1578}
1579
1580impl<T: ?Sized> Arc<T> {
1581 /// Constructs an `Arc<T>` from a raw pointer.
1582 ///
1583 /// The raw pointer must have been previously returned by a call to
1584 /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
1585 ///
1586 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1587 /// is trivially true if `U` is `T`.
1588 /// * If `U` is unsized, its data pointer must have the same size and
1589 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1590 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1591 /// coercion].
1592 ///
1593 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1594 /// and alignment, this is basically like transmuting references of
1595 /// different types. See [`mem::transmute`][transmute] for more information
1596 /// on what restrictions apply in this case.
1597 ///
1598 /// The raw pointer must point to a block of memory allocated by the global allocator.
1599 ///
1600 /// The user of `from_raw` has to make sure a specific value of `T` is only
1601 /// dropped once.
1602 ///
1603 /// This function is unsafe because improper use may lead to memory unsafety,
1604 /// even if the returned `Arc<T>` is never accessed.
1605 ///
1606 /// [into_raw]: Arc::into_raw
1607 /// [transmute]: core::mem::transmute
1608 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1609 ///
1610 /// # Examples
1611 ///
1612 /// ```
1613 /// use std::sync::Arc;
1614 ///
1615 /// let x = Arc::new("hello".to_owned());
1616 /// let x_ptr = Arc::into_raw(x);
1617 ///
1618 /// unsafe {
1619 /// // Convert back to an `Arc` to prevent leak.
1620 /// let x = Arc::from_raw(x_ptr);
1621 /// assert_eq!(&*x, "hello");
1622 ///
1623 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1624 /// }
1625 ///
1626 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1627 /// ```
1628 ///
1629 /// Convert a slice back into its original array:
1630 ///
1631 /// ```
1632 /// use std::sync::Arc;
1633 ///
1634 /// let x: Arc<[u32]> = Arc::new([1, 2, 3]);
1635 /// let x_ptr: *const [u32] = Arc::into_raw(x);
1636 ///
1637 /// unsafe {
1638 /// let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
1639 /// assert_eq!(&*x, &[1, 2, 3]);
1640 /// }
1641 /// ```
1642 #[inline]
1643 #[stable(feature = "rc_raw", since = "1.17.0")]
1644 pub unsafe fn from_raw(ptr: *const T) -> Self {
1645 unsafe { Arc::from_raw_in(ptr, Global) }
1646 }
1647
1648 /// Consumes the `Arc`, returning the wrapped pointer.
1649 ///
1650 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1651 /// [`Arc::from_raw`].
1652 ///
1653 /// # Examples
1654 ///
1655 /// ```
1656 /// use std::sync::Arc;
1657 ///
1658 /// let x = Arc::new("hello".to_owned());
1659 /// let x_ptr = Arc::into_raw(x);
1660 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1661 /// # // Prevent leaks for Miri.
1662 /// # drop(unsafe { Arc::from_raw(x_ptr) });
1663 /// ```
1664 #[must_use = "losing the pointer will leak memory"]
1665 #[stable(feature = "rc_raw", since = "1.17.0")]
1666 #[rustc_never_returns_null_ptr]
1667 pub fn into_raw(this: Self) -> *const T {
1668 let this = ManuallyDrop::new(this);
1669 Self::as_ptr(&*this)
1670 }
1671
1672 /// Increments the strong reference count on the `Arc<T>` associated with the
1673 /// provided pointer by one.
1674 ///
1675 /// # Safety
1676 ///
1677 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1678 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1679 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1680 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1681 /// allocated by the global allocator.
1682 ///
1683 /// [from_raw_in]: Arc::from_raw_in
1684 ///
1685 /// # Examples
1686 ///
1687 /// ```
1688 /// use std::sync::Arc;
1689 ///
1690 /// let five = Arc::new(5);
1691 ///
1692 /// unsafe {
1693 /// let ptr = Arc::into_raw(five);
1694 /// Arc::increment_strong_count(ptr);
1695 ///
1696 /// // This assertion is deterministic because we haven't shared
1697 /// // the `Arc` between threads.
1698 /// let five = Arc::from_raw(ptr);
1699 /// assert_eq!(2, Arc::strong_count(&five));
1700 /// # // Prevent leaks for Miri.
1701 /// # Arc::decrement_strong_count(ptr);
1702 /// }
1703 /// ```
1704 #[inline]
1705 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1706 pub unsafe fn increment_strong_count(ptr: *const T) {
1707 unsafe { Arc::increment_strong_count_in(ptr, Global) }
1708 }
1709
1710 /// Decrements the strong reference count on the `Arc<T>` associated with the
1711 /// provided pointer by one.
1712 ///
1713 /// # Safety
1714 ///
1715 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1716 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1717 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1718 /// least 1) when invoking this method, and `ptr` must point to a block of memory
1719 /// allocated by the global allocator. This method can be used to release the final
1720 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1721 /// released.
1722 ///
1723 /// [from_raw_in]: Arc::from_raw_in
1724 ///
1725 /// # Examples
1726 ///
1727 /// ```
1728 /// use std::sync::Arc;
1729 ///
1730 /// let five = Arc::new(5);
1731 ///
1732 /// unsafe {
1733 /// let ptr = Arc::into_raw(five);
1734 /// Arc::increment_strong_count(ptr);
1735 ///
1736 /// // Those assertions are deterministic because we haven't shared
1737 /// // the `Arc` between threads.
1738 /// let five = Arc::from_raw(ptr);
1739 /// assert_eq!(2, Arc::strong_count(&five));
1740 /// Arc::decrement_strong_count(ptr);
1741 /// assert_eq!(1, Arc::strong_count(&five));
1742 /// }
1743 /// ```
1744 #[inline]
1745 #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1746 pub unsafe fn decrement_strong_count(ptr: *const T) {
1747 unsafe { Arc::decrement_strong_count_in(ptr, Global) }
1748 }
1749}
1750
1751impl<T: ?Sized, A: Allocator> Arc<T, A> {
1752 /// Returns a reference to the underlying allocator.
1753 ///
1754 /// Note: this is an associated function, which means that you have
1755 /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
1756 /// is so that there is no conflict with a method on the inner type.
1757 #[inline]
1758 #[unstable(feature = "allocator_api", issue = "32838")]
1759 pub fn allocator(this: &Self) -> &A {
1760 &this.alloc
1761 }
1762
1763 /// Consumes the `Arc`, returning the wrapped pointer and allocator.
1764 ///
1765 /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1766 /// [`Arc::from_raw_in`].
1767 ///
1768 /// # Examples
1769 ///
1770 /// ```
1771 /// #![feature(allocator_api)]
1772 /// use std::sync::Arc;
1773 /// use std::alloc::System;
1774 ///
1775 /// let x = Arc::new_in("hello".to_owned(), System);
1776 /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
1777 /// assert_eq!(unsafe { &*ptr }, "hello");
1778 /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
1779 /// assert_eq!(&*x, "hello");
1780 /// ```
1781 #[must_use = "losing the pointer will leak memory"]
1782 #[unstable(feature = "allocator_api", issue = "32838")]
1783 pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
1784 let this = mem::ManuallyDrop::new(this);
1785 let ptr = Self::as_ptr(&this);
1786 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
1787 let alloc = unsafe { ptr::read(&this.alloc) };
1788 (ptr, alloc)
1789 }
1790
1791 /// Provides a raw pointer to the data.
1792 ///
1793 /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
1794 /// as long as there are strong counts in the `Arc`.
1795 ///
1796 /// # Examples
1797 ///
1798 /// ```
1799 /// use std::sync::Arc;
1800 ///
1801 /// let x = Arc::new("hello".to_owned());
1802 /// let y = Arc::clone(&x);
1803 /// let x_ptr = Arc::as_ptr(&x);
1804 /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1805 /// assert_eq!(unsafe { &*x_ptr }, "hello");
1806 /// ```
1807 #[must_use]
1808 #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1809 #[rustc_never_returns_null_ptr]
1810 pub fn as_ptr(this: &Self) -> *const T {
1811 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
1812
1813 // SAFETY: This cannot go through Deref::deref or ArcInnerPtr::inner because
1814 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1815 // write through the pointer after the Arc is recovered through `from_raw`.
1816 unsafe { &raw mut (*ptr).data }
1817 }
1818
1819 /// Constructs an `Arc<T, A>` from a raw pointer.
1820 ///
1821 /// The raw pointer must have been previously returned by a call to [`Arc<U,
1822 /// A>::into_raw`][into_raw] with the following requirements:
1823 ///
1824 /// * If `U` is sized, it must have the same size and alignment as `T`. This
1825 /// is trivially true if `U` is `T`.
1826 /// * If `U` is unsized, its data pointer must have the same size and
1827 /// alignment as `T`. This is trivially true if `Arc<U>` was constructed
1828 /// through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1829 /// coercion].
1830 ///
1831 /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1832 /// and alignment, this is basically like transmuting references of
1833 /// different types. See [`mem::transmute`][transmute] for more information
1834 /// on what restrictions apply in this case.
1835 ///
1836 /// The raw pointer must point to a block of memory allocated by `alloc`
1837 ///
1838 /// The user of `from_raw` has to make sure a specific value of `T` is only
1839 /// dropped once.
1840 ///
1841 /// This function is unsafe because improper use may lead to memory unsafety,
1842 /// even if the returned `Arc<T>` is never accessed.
1843 ///
1844 /// [into_raw]: Arc::into_raw
1845 /// [transmute]: core::mem::transmute
1846 /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1847 ///
1848 /// # Examples
1849 ///
1850 /// ```
1851 /// #![feature(allocator_api)]
1852 ///
1853 /// use std::sync::Arc;
1854 /// use std::alloc::System;
1855 ///
1856 /// let x = Arc::new_in("hello".to_owned(), System);
1857 /// let (x_ptr, alloc) = Arc::into_raw_with_allocator(x);
1858 ///
1859 /// unsafe {
1860 /// // Convert back to an `Arc` to prevent leak.
1861 /// let x = Arc::from_raw_in(x_ptr, System);
1862 /// assert_eq!(&*x, "hello");
1863 ///
1864 /// // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1865 /// }
1866 ///
1867 /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1868 /// ```
1869 ///
1870 /// Convert a slice back into its original array:
1871 ///
1872 /// ```
1873 /// #![feature(allocator_api)]
1874 ///
1875 /// use std::sync::Arc;
1876 /// use std::alloc::System;
1877 ///
1878 /// let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
1879 /// let x_ptr: *const [u32] = Arc::into_raw_with_allocator(x).0;
1880 ///
1881 /// unsafe {
1882 /// let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
1883 /// assert_eq!(&*x, &[1, 2, 3]);
1884 /// }
1885 /// ```
1886 #[inline]
1887 #[unstable(feature = "allocator_api", issue = "32838")]
1888 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
1889 unsafe {
1890 let offset = data_offset(ptr);
1891
1892 // Reverse the offset to find the original ArcInner.
1893 let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
1894
1895 Self::from_ptr_in(arc_ptr, alloc)
1896 }
1897 }
1898
1899 /// Creates a new [`Weak`] pointer to this allocation.
1900 ///
1901 /// # Examples
1902 ///
1903 /// ```
1904 /// use std::sync::Arc;
1905 ///
1906 /// let five = Arc::new(5);
1907 ///
1908 /// let weak_five = Arc::downgrade(&five);
1909 /// ```
1910 #[must_use = "this returns a new `Weak` pointer, \
1911 without modifying the original `Arc`"]
1912 #[stable(feature = "arc_weak", since = "1.4.0")]
1913 pub fn downgrade(this: &Self) -> Weak<T, A>
1914 where
1915 A: Clone,
1916 {
1917 // This Relaxed is OK because we're checking the value in the CAS
1918 // below.
1919 let mut cur = this.inner().weak.load(Relaxed);
1920
1921 loop {
1922 // check if the weak counter is currently "locked"; if so, spin.
1923 if cur == usize::MAX {
1924 hint::spin_loop();
1925 cur = this.inner().weak.load(Relaxed);
1926 continue;
1927 }
1928
1929 // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1930 assert!(cur <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
1931
1932 // NOTE: this code currently ignores the possibility of overflow
1933 // into usize::MAX; in general both Rc and Arc need to be adjusted
1934 // to deal with overflow.
1935
1936 // Unlike with Clone(), we need this to be an Acquire read to
1937 // synchronize with the write coming from `is_unique`, so that the
1938 // events prior to that write happen before this read.
1939 match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
1940 Ok(_) => {
1941 // Make sure we do not create a dangling Weak
1942 debug_assert!(!is_dangling(this.ptr.as_ptr()));
1943 return Weak { ptr: this.ptr, alloc: this.alloc.clone() };
1944 }
1945 Err(old) => cur = old,
1946 }
1947 }
1948 }
1949
1950 /// Gets the number of [`Weak`] pointers to this allocation.
1951 ///
1952 /// # Safety
1953 ///
1954 /// This method by itself is safe, but using it correctly requires extra care.
1955 /// Another thread can change the weak count at any time,
1956 /// including potentially between calling this method and acting on the result.
1957 ///
1958 /// # Examples
1959 ///
1960 /// ```
1961 /// use std::sync::Arc;
1962 ///
1963 /// let five = Arc::new(5);
1964 /// let _weak_five = Arc::downgrade(&five);
1965 ///
1966 /// // This assertion is deterministic because we haven't shared
1967 /// // the `Arc` or `Weak` between threads.
1968 /// assert_eq!(1, Arc::weak_count(&five));
1969 /// ```
1970 #[inline]
1971 #[must_use]
1972 #[stable(feature = "arc_counts", since = "1.15.0")]
1973 pub fn weak_count(this: &Self) -> usize {
1974 let cnt = this.inner().weak.load(Relaxed);
1975 // If the weak count is currently locked, the value of the
1976 // count was 0 just before taking the lock.
1977 if cnt == usize::MAX { 0 } else { cnt - 1 }
1978 }
1979
1980 /// Gets the number of strong (`Arc`) pointers to this allocation.
1981 ///
1982 /// # Safety
1983 ///
1984 /// This method by itself is safe, but using it correctly requires extra care.
1985 /// Another thread can change the strong count at any time,
1986 /// including potentially between calling this method and acting on the result.
1987 ///
1988 /// # Examples
1989 ///
1990 /// ```
1991 /// use std::sync::Arc;
1992 ///
1993 /// let five = Arc::new(5);
1994 /// let _also_five = Arc::clone(&five);
1995 ///
1996 /// // This assertion is deterministic because we haven't shared
1997 /// // the `Arc` between threads.
1998 /// assert_eq!(2, Arc::strong_count(&five));
1999 /// ```
2000 #[inline]
2001 #[must_use]
2002 #[stable(feature = "arc_counts", since = "1.15.0")]
2003 pub fn strong_count(this: &Self) -> usize {
2004 this.inner().strong.load(Relaxed)
2005 }
2006
2007 /// Increments the strong reference count on the `Arc<T>` associated with the
2008 /// provided pointer by one.
2009 ///
2010 /// # Safety
2011 ///
2012 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
2013 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
2014 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
2015 /// least 1) for the duration of this method, and `ptr` must point to a block of memory
2016 /// allocated by `alloc`.
2017 ///
2018 /// [from_raw_in]: Arc::from_raw_in
2019 ///
2020 /// # Examples
2021 ///
2022 /// ```
2023 /// #![feature(allocator_api)]
2024 ///
2025 /// use std::sync::Arc;
2026 /// use std::alloc::System;
2027 ///
2028 /// let five = Arc::new_in(5, System);
2029 ///
2030 /// unsafe {
2031 /// let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
2032 /// Arc::increment_strong_count_in(ptr, System);
2033 ///
2034 /// // This assertion is deterministic because we haven't shared
2035 /// // the `Arc` between threads.
2036 /// let five = Arc::from_raw_in(ptr, System);
2037 /// assert_eq!(2, Arc::strong_count(&five));
2038 /// # // Prevent leaks for Miri.
2039 /// # Arc::decrement_strong_count_in(ptr, System);
2040 /// }
2041 /// ```
2042 #[inline]
2043 #[unstable(feature = "allocator_api", issue = "32838")]
2044 pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
2045 where
2046 A: Clone,
2047 {
2048 // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
2049 let arc = unsafe { mem::ManuallyDrop::new(Arc::from_raw_in(ptr, alloc)) };
2050 // Now increase refcount, but don't drop new refcount either
2051 let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
2052 }
2053
2054 /// Decrements the strong reference count on the `Arc<T>` associated with the
2055 /// provided pointer by one.
2056 ///
2057 /// # Safety
2058 ///
2059 /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
2060 /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
2061 /// The associated `Arc` instance must be valid (i.e. the strong count must be at
2062 /// least 1) when invoking this method, and `ptr` must point to a block of memory
2063 /// allocated by `alloc`. This method can be used to release the final
2064 /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
2065 /// released.
2066 ///
2067 /// [from_raw_in]: Arc::from_raw_in
2068 ///
2069 /// # Examples
2070 ///
2071 /// ```
2072 /// #![feature(allocator_api)]
2073 ///
2074 /// use std::sync::Arc;
2075 /// use std::alloc::System;
2076 ///
2077 /// let five = Arc::new_in(5, System);
2078 ///
2079 /// unsafe {
2080 /// let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
2081 /// Arc::increment_strong_count_in(ptr, System);
2082 ///
2083 /// // Those assertions are deterministic because we haven't shared
2084 /// // the `Arc` between threads.
2085 /// let five = Arc::from_raw_in(ptr, System);
2086 /// assert_eq!(2, Arc::strong_count(&five));
2087 /// Arc::decrement_strong_count_in(ptr, System);
2088 /// assert_eq!(1, Arc::strong_count(&five));
2089 /// }
2090 /// ```
2091 #[inline]
2092 #[unstable(feature = "allocator_api", issue = "32838")]
2093 pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) {
2094 unsafe { drop(Arc::from_raw_in(ptr, alloc)) };
2095 }
2096
2097 #[inline]
2098 fn inner(&self) -> &ArcInner<T> {
2099 // This unsafety is ok because while this arc is alive we're guaranteed
2100 // that the inner pointer is valid. Furthermore, we know that the
2101 // `ArcInner` structure itself is `Sync` because the inner data is
2102 // `Sync` as well, so we're ok loaning out an immutable pointer to these
2103 // contents.
2104 unsafe { self.ptr.as_ref() }
2105 }
2106
2107 // Non-inlined part of `drop`.
2108 #[inline(never)]
2109 unsafe fn drop_slow(&mut self) {
2110 // Drop the weak ref collectively held by all strong references when this
2111 // variable goes out of scope. This ensures that the memory is deallocated
2112 // even if the destructor of `T` panics.
2113 // Take a reference to `self.alloc` instead of cloning because 1. it'll last long
2114 // enough, and 2. you should be able to drop `Arc`s with unclonable allocators
2115 let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
2116
2117 // Destroy the data at this time, even though we must not free the box
2118 // allocation itself (there might still be weak pointers lying around).
2119 // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed.
2120 unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
2121 }
2122
2123 /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
2124 /// [`ptr::eq`]. This function ignores the metadata of `dyn Trait` pointers.
2125 ///
2126 /// # Examples
2127 ///
2128 /// ```
2129 /// use std::sync::Arc;
2130 ///
2131 /// let five = Arc::new(5);
2132 /// let same_five = Arc::clone(&five);
2133 /// let other_five = Arc::new(5);
2134 ///
2135 /// assert!(Arc::ptr_eq(&five, &same_five));
2136 /// assert!(!Arc::ptr_eq(&five, &other_five));
2137 /// ```
2138 ///
2139 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
2140 #[inline]
2141 #[must_use]
2142 #[stable(feature = "ptr_eq", since = "1.17.0")]
2143 pub fn ptr_eq(this: &Self, other: &Self) -> bool {
2144 ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr())
2145 }
2146}
2147
2148impl<T: ?Sized> Arc<T> {
2149 /// Allocates an `ArcInner<T>` with sufficient space for
2150 /// a possibly-unsized inner value where the value has the layout provided.
2151 ///
2152 /// The function `mem_to_arcinner` is called with the data pointer
2153 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
2154 #[cfg(not(no_global_oom_handling))]
2155 unsafe fn allocate_for_layout(
2156 value_layout: Layout,
2157 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
2158 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2159 ) -> *mut ArcInner<T> {
2160 let layout = arcinner_layout_for_value_layout(value_layout);
2161
2162 let ptr = allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
2163
2164 unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) }
2165 }
2166
2167 /// Allocates an `ArcInner<T>` with sufficient space for
2168 /// a possibly-unsized inner value where the value has the layout provided,
2169 /// returning an error if allocation fails.
2170 ///
2171 /// The function `mem_to_arcinner` is called with the data pointer
2172 /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
2173 unsafe fn try_allocate_for_layout(
2174 value_layout: Layout,
2175 allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
2176 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2177 ) -> Result<*mut ArcInner<T>, AllocError> {
2178 let layout = arcinner_layout_for_value_layout(value_layout);
2179
2180 let ptr = allocate(layout)?;
2181
2182 let inner = unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) };
2183
2184 Ok(inner)
2185 }
2186
2187 unsafe fn initialize_arcinner(
2188 ptr: NonNull<[u8]>,
2189 layout: Layout,
2190 mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
2191 ) -> *mut ArcInner<T> {
2192 let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
2193 debug_assert_eq!(unsafe { Layout::for_value_raw(inner) }, layout);
2194
2195 unsafe {
2196 (&raw mut (*inner).strong).write(atomic::AtomicUsize::new(1));
2197 (&raw mut (*inner).weak).write(atomic::AtomicUsize::new(1));
2198 }
2199
2200 inner
2201 }
2202}
2203
2204impl<T: ?Sized, A: Allocator> Arc<T, A> {
2205 /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
2206 #[inline]
2207 #[cfg(not(no_global_oom_handling))]
2208 unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut ArcInner<T> {
2209 // Allocate for the `ArcInner<T>` using the given value.
2210 unsafe {
2211 Arc::allocate_for_layout(
2212 Layout::for_value_raw(ptr),
2213 |layout| alloc.allocate(layout),
2214 |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
2215 )
2216 }
2217 }
2218
2219 #[cfg(not(no_global_oom_handling))]
2220 fn from_box_in(src: Box<T, A>) -> Arc<T, A> {
2221 unsafe {
2222 let value_size = size_of_val(&*src);
2223 let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src));
2224
2225 // Copy value as bytes
2226 ptr::copy_nonoverlapping(
2227 (&raw const *src) as *const u8,
2228 (&raw mut (*ptr).data) as *mut u8,
2229 value_size,
2230 );
2231
2232 // Free the allocation without dropping its contents
2233 let (bptr, alloc) = Box::into_raw_with_allocator(src);
2234 let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
2235 drop(src);
2236
2237 Self::from_ptr_in(ptr, alloc)
2238 }
2239 }
2240}
2241
2242impl<T> Arc<[T]> {
2243 /// Allocates an `ArcInner<[T]>` with the given length.
2244 #[cfg(not(no_global_oom_handling))]
2245 unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
2246 unsafe {
2247 Self::allocate_for_layout(
2248 Layout::array::<T>(len).unwrap(),
2249 |layout| Global.allocate(layout),
2250 |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2251 )
2252 }
2253 }
2254
2255 /// Copy elements from slice into newly allocated `Arc<[T]>`
2256 ///
2257 /// Unsafe because the caller must either take ownership, bind `T: Copy` or
2258 /// bind `T: TrivialClone`.
2259 #[cfg(not(no_global_oom_handling))]
2260 unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
2261 unsafe {
2262 let ptr = Self::allocate_for_slice(v.len());
2263
2264 ptr::copy_nonoverlapping(v.as_ptr(), (&raw mut (*ptr).data) as *mut T, v.len());
2265
2266 Self::from_ptr(ptr)
2267 }
2268 }
2269
2270 /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
2271 ///
2272 /// Behavior is undefined should the size be wrong.
2273 #[cfg(not(no_global_oom_handling))]
2274 unsafe fn from_iter_exact(iter: impl Iterator<Item = T>, len: usize) -> Arc<[T]> {
2275 // Panic guard while cloning T elements.
2276 // In the event of a panic, elements that have been written
2277 // into the new ArcInner will be dropped, then the memory freed.
2278 struct Guard<T> {
2279 mem: NonNull<u8>,
2280 elems: *mut T,
2281 layout: Layout,
2282 n_elems: usize,
2283 }
2284
2285 impl<T> Drop for Guard<T> {
2286 fn drop(&mut self) {
2287 unsafe {
2288 let slice = from_raw_parts_mut(self.elems, self.n_elems);
2289 ptr::drop_in_place(slice);
2290
2291 Global.deallocate(self.mem, self.layout);
2292 }
2293 }
2294 }
2295
2296 unsafe {
2297 let ptr = Self::allocate_for_slice(len);
2298
2299 let mem = ptr as *mut _ as *mut u8;
2300 let layout = Layout::for_value_raw(ptr);
2301
2302 // Pointer to first element
2303 let elems = (&raw mut (*ptr).data) as *mut T;
2304
2305 let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
2306
2307 for (i, item) in iter.enumerate() {
2308 ptr::write(elems.add(i), item);
2309 guard.n_elems += 1;
2310 }
2311
2312 // All clear. Forget the guard so it doesn't free the new ArcInner.
2313 mem::forget(guard);
2314
2315 Self::from_ptr(ptr)
2316 }
2317 }
2318}
2319
2320impl<T, A: Allocator> Arc<[T], A> {
2321 /// Allocates an `ArcInner<[T]>` with the given length.
2322 #[inline]
2323 #[cfg(not(no_global_oom_handling))]
2324 unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut ArcInner<[T]> {
2325 unsafe {
2326 Arc::allocate_for_layout(
2327 value_layout:Layout::array::<T>(len).unwrap(),
2328 |layout| alloc.allocate(layout),
2329 |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2330 )
2331 }
2332 }
2333}
2334
2335/// Specialization trait used for `From<&[T]>`.
2336#[cfg(not(no_global_oom_handling))]
2337trait ArcFromSlice<T> {
2338 fn from_slice(slice: &[T]) -> Self;
2339}
2340
2341#[cfg(not(no_global_oom_handling))]
2342impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
2343 #[inline]
2344 default fn from_slice(v: &[T]) -> Self {
2345 unsafe { Self::from_iter_exact(iter:v.iter().cloned(), v.len()) }
2346 }
2347}
2348
2349#[cfg(not(no_global_oom_handling))]
2350impl<T: TrivialClone> ArcFromSlice<T> for Arc<[T]> {
2351 #[inline]
2352 fn from_slice(v: &[T]) -> Self {
2353 // SAFETY: `T` implements `TrivialClone`, so this is sound and equivalent
2354 // to the above.
2355 unsafe { Arc::copy_from_slice(v) }
2356 }
2357}
2358
2359#[stable(feature = "rust1", since = "1.0.0")]
2360impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
2361 /// Makes a clone of the `Arc` pointer.
2362 ///
2363 /// This creates another pointer to the same allocation, increasing the
2364 /// strong reference count.
2365 ///
2366 /// # Examples
2367 ///
2368 /// ```
2369 /// use std::sync::Arc;
2370 ///
2371 /// let five = Arc::new(5);
2372 ///
2373 /// let _ = Arc::clone(&five);
2374 /// ```
2375 #[inline]
2376 fn clone(&self) -> Arc<T, A> {
2377 // Using a relaxed ordering is alright here, as knowledge of the
2378 // original reference prevents other threads from erroneously deleting
2379 // the object.
2380 //
2381 // As explained in the [Boost documentation][1], Increasing the
2382 // reference counter can always be done with memory_order_relaxed: New
2383 // references to an object can only be formed from an existing
2384 // reference, and passing an existing reference from one thread to
2385 // another must already provide any required synchronization.
2386 //
2387 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2388 let old_size = self.inner().strong.fetch_add(1, Relaxed);
2389
2390 // However we need to guard against massive refcounts in case someone is `mem::forget`ing
2391 // Arcs. If we don't do this the count can overflow and users will use-after free. This
2392 // branch will never be taken in any realistic program. We abort because such a program is
2393 // incredibly degenerate, and we don't care to support it.
2394 //
2395 // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
2396 // But we do that check *after* having done the increment, so there is a chance here that
2397 // the worst already happened and we actually do overflow the `usize` counter. However, that
2398 // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
2399 // above and the `abort` below, which seems exceedingly unlikely.
2400 //
2401 // This is a global invariant, and also applies when using a compare-exchange loop to increment
2402 // counters in other methods.
2403 // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
2404 // and then overflow using a few `fetch_add`s.
2405 if old_size > MAX_REFCOUNT {
2406 abort();
2407 }
2408
2409 unsafe { Self::from_inner_in(self.ptr, self.alloc.clone()) }
2410 }
2411}
2412
2413#[unstable(feature = "ergonomic_clones", issue = "132290")]
2414impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A> {}
2415
2416#[stable(feature = "rust1", since = "1.0.0")]
2417impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
2418 type Target = T;
2419
2420 #[inline]
2421 fn deref(&self) -> &T {
2422 &self.inner().data
2423 }
2424}
2425
2426#[unstable(feature = "pin_coerce_unsized_trait", issue = "150112")]
2427unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
2428
2429#[unstable(feature = "pin_coerce_unsized_trait", issue = "150112")]
2430unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {}
2431
2432#[unstable(feature = "deref_pure_trait", issue = "87121")]
2433unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
2434
2435#[unstable(feature = "legacy_receiver_trait", issue = "none")]
2436impl<T: ?Sized> LegacyReceiver for Arc<T> {}
2437
2438#[cfg(not(no_global_oom_handling))]
2439impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
2440 /// Makes a mutable reference into the given `Arc`.
2441 ///
2442 /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
2443 /// [`clone`] the inner value to a new allocation to ensure unique ownership. This is also
2444 /// referred to as clone-on-write.
2445 ///
2446 /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
2447 /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
2448 /// be cloned.
2449 ///
2450 /// See also [`get_mut`], which will fail rather than cloning the inner value
2451 /// or dissociating [`Weak`] pointers.
2452 ///
2453 /// [`clone`]: Clone::clone
2454 /// [`get_mut`]: Arc::get_mut
2455 ///
2456 /// # Examples
2457 ///
2458 /// ```
2459 /// use std::sync::Arc;
2460 ///
2461 /// let mut data = Arc::new(5);
2462 ///
2463 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2464 /// let mut other_data = Arc::clone(&data); // Won't clone inner data
2465 /// *Arc::make_mut(&mut data) += 1; // Clones inner data
2466 /// *Arc::make_mut(&mut data) += 1; // Won't clone anything
2467 /// *Arc::make_mut(&mut other_data) *= 2; // Won't clone anything
2468 ///
2469 /// // Now `data` and `other_data` point to different allocations.
2470 /// assert_eq!(*data, 8);
2471 /// assert_eq!(*other_data, 12);
2472 /// ```
2473 ///
2474 /// [`Weak`] pointers will be dissociated:
2475 ///
2476 /// ```
2477 /// use std::sync::Arc;
2478 ///
2479 /// let mut data = Arc::new(75);
2480 /// let weak = Arc::downgrade(&data);
2481 ///
2482 /// assert!(75 == *data);
2483 /// assert!(75 == *weak.upgrade().unwrap());
2484 ///
2485 /// *Arc::make_mut(&mut data) += 1;
2486 ///
2487 /// assert!(76 == *data);
2488 /// assert!(weak.upgrade().is_none());
2489 /// ```
2490 #[inline]
2491 #[stable(feature = "arc_unique", since = "1.4.0")]
2492 pub fn make_mut(this: &mut Self) -> &mut T {
2493 let size_of_val = size_of_val::<T>(&**this);
2494
2495 // Note that we hold both a strong reference and a weak reference.
2496 // Thus, releasing our strong reference only will not, by itself, cause
2497 // the memory to be deallocated.
2498 //
2499 // Use Acquire to ensure that we see any writes to `weak` that happen
2500 // before release writes (i.e., decrements) to `strong`. Since we hold a
2501 // weak count, there's no chance the ArcInner itself could be
2502 // deallocated.
2503 if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
2504 // Another strong pointer exists, so we must clone.
2505 *this = Arc::clone_from_ref_in(&**this, this.alloc.clone());
2506 } else if this.inner().weak.load(Relaxed) != 1 {
2507 // Relaxed suffices in the above because this is fundamentally an
2508 // optimization: we are always racing with weak pointers being
2509 // dropped. Worst case, we end up allocated a new Arc unnecessarily.
2510
2511 // We removed the last strong ref, but there are additional weak
2512 // refs remaining. We'll move the contents to a new Arc, and
2513 // invalidate the other weak refs.
2514
2515 // Note that it is not possible for the read of `weak` to yield
2516 // usize::MAX (i.e., locked), since the weak count can only be
2517 // locked by a thread with a strong reference.
2518
2519 // Materialize our own implicit weak pointer, so that it can clean
2520 // up the ArcInner as needed.
2521 let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
2522
2523 // Can just steal the data, all that's left is Weaks
2524 //
2525 // We don't need panic-protection like the above branch does, but we might as well
2526 // use the same mechanism.
2527 let mut in_progress: UniqueArcUninit<T, A> =
2528 UniqueArcUninit::new(&**this, this.alloc.clone());
2529 unsafe {
2530 // Initialize `in_progress` with move of **this.
2531 // We have to express this in terms of bytes because `T: ?Sized`; there is no
2532 // operation that just copies a value based on its `size_of_val()`.
2533 ptr::copy_nonoverlapping(
2534 ptr::from_ref(&**this).cast::<u8>(),
2535 in_progress.data_ptr().cast::<u8>(),
2536 size_of_val,
2537 );
2538
2539 ptr::write(this, in_progress.into_arc());
2540 }
2541 } else {
2542 // We were the sole reference of either kind; bump back up the
2543 // strong ref count.
2544 this.inner().strong.store(1, Release);
2545 }
2546
2547 // As with `get_mut()`, the unsafety is ok because our reference was
2548 // either unique to begin with, or became one upon cloning the contents.
2549 unsafe { Self::get_mut_unchecked(this) }
2550 }
2551}
2552
2553impl<T: Clone, A: Allocator> Arc<T, A> {
2554 /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
2555 /// clone.
2556 ///
2557 /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
2558 /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
2559 ///
2560 /// # Examples
2561 ///
2562 /// ```
2563 /// # use std::{ptr, sync::Arc};
2564 /// let inner = String::from("test");
2565 /// let ptr = inner.as_ptr();
2566 ///
2567 /// let arc = Arc::new(inner);
2568 /// let inner = Arc::unwrap_or_clone(arc);
2569 /// // The inner value was not cloned
2570 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2571 ///
2572 /// let arc = Arc::new(inner);
2573 /// let arc2 = arc.clone();
2574 /// let inner = Arc::unwrap_or_clone(arc);
2575 /// // Because there were 2 references, we had to clone the inner value.
2576 /// assert!(!ptr::eq(ptr, inner.as_ptr()));
2577 /// // `arc2` is the last reference, so when we unwrap it we get back
2578 /// // the original `String`.
2579 /// let inner = Arc::unwrap_or_clone(arc2);
2580 /// assert!(ptr::eq(ptr, inner.as_ptr()));
2581 /// ```
2582 #[inline]
2583 #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")]
2584 pub fn unwrap_or_clone(this: Self) -> T {
2585 Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
2586 }
2587}
2588
2589impl<T: ?Sized, A: Allocator> Arc<T, A> {
2590 /// Returns a mutable reference into the given `Arc`, if there are
2591 /// no other `Arc` or [`Weak`] pointers to the same allocation.
2592 ///
2593 /// Returns [`None`] otherwise, because it is not safe to
2594 /// mutate a shared value.
2595 ///
2596 /// See also [`make_mut`][make_mut], which will [`clone`][clone]
2597 /// the inner value when there are other `Arc` pointers.
2598 ///
2599 /// [make_mut]: Arc::make_mut
2600 /// [clone]: Clone::clone
2601 ///
2602 /// # Examples
2603 ///
2604 /// ```
2605 /// use std::sync::Arc;
2606 ///
2607 /// let mut x = Arc::new(3);
2608 /// *Arc::get_mut(&mut x).unwrap() = 4;
2609 /// assert_eq!(*x, 4);
2610 ///
2611 /// let _y = Arc::clone(&x);
2612 /// assert!(Arc::get_mut(&mut x).is_none());
2613 /// ```
2614 #[inline]
2615 #[stable(feature = "arc_unique", since = "1.4.0")]
2616 pub fn get_mut(this: &mut Self) -> Option<&mut T> {
2617 if Self::is_unique(this) {
2618 // This unsafety is ok because we're guaranteed that the pointer
2619 // returned is the *only* pointer that will ever be returned to T. Our
2620 // reference count is guaranteed to be 1 at this point, and we required
2621 // the Arc itself to be `mut`, so we're returning the only possible
2622 // reference to the inner data.
2623 unsafe { Some(Arc::get_mut_unchecked(this)) }
2624 } else {
2625 None
2626 }
2627 }
2628
2629 /// Returns a mutable reference into the given `Arc`,
2630 /// without any check.
2631 ///
2632 /// See also [`get_mut`], which is safe and does appropriate checks.
2633 ///
2634 /// [`get_mut`]: Arc::get_mut
2635 ///
2636 /// # Safety
2637 ///
2638 /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
2639 /// they must not be dereferenced or have active borrows for the duration
2640 /// of the returned borrow, and their inner type must be exactly the same as the
2641 /// inner type of this Arc (including lifetimes). This is trivially the case if no
2642 /// such pointers exist, for example immediately after `Arc::new`.
2643 ///
2644 /// # Examples
2645 ///
2646 /// ```
2647 /// #![feature(get_mut_unchecked)]
2648 ///
2649 /// use std::sync::Arc;
2650 ///
2651 /// let mut x = Arc::new(String::new());
2652 /// unsafe {
2653 /// Arc::get_mut_unchecked(&mut x).push_str("foo")
2654 /// }
2655 /// assert_eq!(*x, "foo");
2656 /// ```
2657 /// Other `Arc` pointers to the same allocation must be to the same type.
2658 /// ```no_run
2659 /// #![feature(get_mut_unchecked)]
2660 ///
2661 /// use std::sync::Arc;
2662 ///
2663 /// let x: Arc<str> = Arc::from("Hello, world!");
2664 /// let mut y: Arc<[u8]> = x.clone().into();
2665 /// unsafe {
2666 /// // this is Undefined Behavior, because x's inner type is str, not [u8]
2667 /// Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
2668 /// }
2669 /// println!("{}", &*x); // Invalid UTF-8 in a str
2670 /// ```
2671 /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
2672 /// ```no_run
2673 /// #![feature(get_mut_unchecked)]
2674 ///
2675 /// use std::sync::Arc;
2676 ///
2677 /// let x: Arc<&str> = Arc::new("Hello, world!");
2678 /// {
2679 /// let s = String::from("Oh, no!");
2680 /// let mut y: Arc<&str> = x.clone();
2681 /// unsafe {
2682 /// // this is Undefined Behavior, because x's inner type
2683 /// // is &'long str, not &'short str
2684 /// *Arc::get_mut_unchecked(&mut y) = &s;
2685 /// }
2686 /// }
2687 /// println!("{}", &*x); // Use-after-free
2688 /// ```
2689 #[inline]
2690 #[unstable(feature = "get_mut_unchecked", issue = "63292")]
2691 pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
2692 // We are careful to *not* create a reference covering the "count" fields, as
2693 // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
2694 unsafe { &mut (*this.ptr.as_ptr()).data }
2695 }
2696
2697 /// Determine whether this is the unique reference to the underlying data.
2698 ///
2699 /// Returns `true` if there are no other `Arc` or [`Weak`] pointers to the same allocation;
2700 /// returns `false` otherwise.
2701 ///
2702 /// If this function returns `true`, then is guaranteed to be safe to call [`get_mut_unchecked`]
2703 /// on this `Arc`, so long as no clones occur in between.
2704 ///
2705 /// # Examples
2706 ///
2707 /// ```
2708 /// #![feature(arc_is_unique)]
2709 ///
2710 /// use std::sync::Arc;
2711 ///
2712 /// let x = Arc::new(3);
2713 /// assert!(Arc::is_unique(&x));
2714 ///
2715 /// let y = Arc::clone(&x);
2716 /// assert!(!Arc::is_unique(&x));
2717 /// drop(y);
2718 ///
2719 /// // Weak references also count, because they could be upgraded at any time.
2720 /// let z = Arc::downgrade(&x);
2721 /// assert!(!Arc::is_unique(&x));
2722 /// ```
2723 ///
2724 /// # Pointer invalidation
2725 ///
2726 /// This function will always return the same value as `Arc::get_mut(arc).is_some()`. However,
2727 /// unlike that operation it does not produce any mutable references to the underlying data,
2728 /// meaning no pointers to the data inside the `Arc` are invalidated by the call. Thus, the
2729 /// following code is valid, even though it would be UB if it used `Arc::get_mut`:
2730 ///
2731 /// ```
2732 /// #![feature(arc_is_unique)]
2733 ///
2734 /// use std::sync::Arc;
2735 ///
2736 /// let arc = Arc::new(5);
2737 /// let pointer: *const i32 = &*arc;
2738 /// assert!(Arc::is_unique(&arc));
2739 /// assert_eq!(unsafe { *pointer }, 5);
2740 /// ```
2741 ///
2742 /// # Atomic orderings
2743 ///
2744 /// Concurrent drops to other `Arc` pointers to the same allocation will synchronize with this
2745 /// call - that is, this call performs an `Acquire` operation on the underlying strong and weak
2746 /// ref counts. This ensures that calling `get_mut_unchecked` is safe.
2747 ///
2748 /// Note that this operation requires locking the weak ref count, so concurrent calls to
2749 /// `downgrade` may spin-loop for a short period of time.
2750 ///
2751 /// [`get_mut_unchecked`]: Self::get_mut_unchecked
2752 #[inline]
2753 #[unstable(feature = "arc_is_unique", issue = "138938")]
2754 pub fn is_unique(this: &Self) -> bool {
2755 // lock the weak pointer count if we appear to be the sole weak pointer
2756 // holder.
2757 //
2758 // The acquire label here ensures a happens-before relationship with any
2759 // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
2760 // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
2761 // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
2762 if this.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
2763 // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
2764 // counter in `drop` -- the only access that happens when any but the last reference
2765 // is being dropped.
2766 let unique = this.inner().strong.load(Acquire) == 1;
2767
2768 // The release write here synchronizes with a read in `downgrade`,
2769 // effectively preventing the above read of `strong` from happening
2770 // after the write.
2771 this.inner().weak.store(1, Release); // release the lock
2772 unique
2773 } else {
2774 false
2775 }
2776 }
2777}
2778
2779#[stable(feature = "rust1", since = "1.0.0")]
2780unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
2781 /// Drops the `Arc`.
2782 ///
2783 /// This will decrement the strong reference count. If the strong reference
2784 /// count reaches zero then the only other references (if any) are
2785 /// [`Weak`], so we `drop` the inner value.
2786 ///
2787 /// # Examples
2788 ///
2789 /// ```
2790 /// use std::sync::Arc;
2791 ///
2792 /// struct Foo;
2793 ///
2794 /// impl Drop for Foo {
2795 /// fn drop(&mut self) {
2796 /// println!("dropped!");
2797 /// }
2798 /// }
2799 ///
2800 /// let foo = Arc::new(Foo);
2801 /// let foo2 = Arc::clone(&foo);
2802 ///
2803 /// drop(foo); // Doesn't print anything
2804 /// drop(foo2); // Prints "dropped!"
2805 /// ```
2806 #[inline]
2807 fn drop(&mut self) {
2808 // Because `fetch_sub` is already atomic, we do not need to synchronize
2809 // with other threads unless we are going to delete the object. This
2810 // same logic applies to the below `fetch_sub` to the `weak` count.
2811 if self.inner().strong.fetch_sub(1, Release) != 1 {
2812 return;
2813 }
2814
2815 // This fence is needed to prevent reordering of use of the data and
2816 // deletion of the data. Because it is marked `Release`, the decreasing
2817 // of the reference count synchronizes with this `Acquire` fence. This
2818 // means that use of the data happens before decreasing the reference
2819 // count, which happens before this fence, which happens before the
2820 // deletion of the data.
2821 //
2822 // As explained in the [Boost documentation][1],
2823 //
2824 // > It is important to enforce any possible access to the object in one
2825 // > thread (through an existing reference) to *happen before* deleting
2826 // > the object in a different thread. This is achieved by a "release"
2827 // > operation after dropping a reference (any access to the object
2828 // > through this reference must obviously happened before), and an
2829 // > "acquire" operation before deleting the object.
2830 //
2831 // In particular, while the contents of an Arc are usually immutable, it's
2832 // possible to have interior writes to something like a Mutex<T>. Since a
2833 // Mutex is not acquired when it is deleted, we can't rely on its
2834 // synchronization logic to make writes in thread A visible to a destructor
2835 // running in thread B.
2836 //
2837 // Also note that the Acquire fence here could probably be replaced with an
2838 // Acquire load, which could improve performance in highly-contended
2839 // situations. See [2].
2840 //
2841 // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2842 // [2]: (https://github.com/rust-lang/rust/pull/41714)
2843 acquire!(self.inner().strong);
2844
2845 // Make sure we aren't trying to "drop" the shared static for empty slices
2846 // used by Default::default.
2847 debug_assert!(
2848 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
2849 "Arcs backed by a static should never reach a strong count of 0. \
2850 Likely decrement_strong_count or from_raw were called too many times.",
2851 );
2852
2853 unsafe {
2854 self.drop_slow();
2855 }
2856 }
2857}
2858
2859impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
2860 /// Attempts to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
2861 ///
2862 /// # Examples
2863 ///
2864 /// ```
2865 /// use std::any::Any;
2866 /// use std::sync::Arc;
2867 ///
2868 /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
2869 /// if let Ok(string) = value.downcast::<String>() {
2870 /// println!("String ({}): {}", string.len(), string);
2871 /// }
2872 /// }
2873 ///
2874 /// let my_string = "Hello World".to_string();
2875 /// print_if_string(Arc::new(my_string));
2876 /// print_if_string(Arc::new(0i8));
2877 /// ```
2878 #[inline]
2879 #[stable(feature = "rc_downcast", since = "1.29.0")]
2880 pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
2881 where
2882 T: Any + Send + Sync,
2883 {
2884 if (*self).is::<T>() {
2885 unsafe {
2886 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2887 Ok(Arc::from_inner_in(ptr.cast(), alloc))
2888 }
2889 } else {
2890 Err(self)
2891 }
2892 }
2893
2894 /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
2895 ///
2896 /// For a safe alternative see [`downcast`].
2897 ///
2898 /// # Examples
2899 ///
2900 /// ```
2901 /// #![feature(downcast_unchecked)]
2902 ///
2903 /// use std::any::Any;
2904 /// use std::sync::Arc;
2905 ///
2906 /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
2907 ///
2908 /// unsafe {
2909 /// assert_eq!(*x.downcast_unchecked::<usize>(), 1);
2910 /// }
2911 /// ```
2912 ///
2913 /// # Safety
2914 ///
2915 /// The contained value must be of type `T`. Calling this method
2916 /// with the incorrect type is *undefined behavior*.
2917 ///
2918 ///
2919 /// [`downcast`]: Self::downcast
2920 #[inline]
2921 #[unstable(feature = "downcast_unchecked", issue = "90850")]
2922 pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
2923 where
2924 T: Any + Send + Sync,
2925 {
2926 unsafe {
2927 let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2928 Arc::from_inner_in(ptr.cast(), alloc)
2929 }
2930 }
2931}
2932
2933impl<T> Weak<T> {
2934 /// Constructs a new `Weak<T>`, without allocating any memory.
2935 /// Calling [`upgrade`] on the return value always gives [`None`].
2936 ///
2937 /// [`upgrade`]: Weak::upgrade
2938 ///
2939 /// # Examples
2940 ///
2941 /// ```
2942 /// use std::sync::Weak;
2943 ///
2944 /// let empty: Weak<i64> = Weak::new();
2945 /// assert!(empty.upgrade().is_none());
2946 /// ```
2947 #[inline]
2948 #[stable(feature = "downgraded_weak", since = "1.10.0")]
2949 #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
2950 #[must_use]
2951 pub const fn new() -> Weak<T> {
2952 Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
2953 }
2954}
2955
2956impl<T, A: Allocator> Weak<T, A> {
2957 /// Constructs a new `Weak<T, A>`, without allocating any memory, technically in the provided
2958 /// allocator.
2959 /// Calling [`upgrade`] on the return value always gives [`None`].
2960 ///
2961 /// [`upgrade`]: Weak::upgrade
2962 ///
2963 /// # Examples
2964 ///
2965 /// ```
2966 /// #![feature(allocator_api)]
2967 ///
2968 /// use std::sync::Weak;
2969 /// use std::alloc::System;
2970 ///
2971 /// let empty: Weak<i64, _> = Weak::new_in(System);
2972 /// assert!(empty.upgrade().is_none());
2973 /// ```
2974 #[inline]
2975 #[unstable(feature = "allocator_api", issue = "32838")]
2976 pub fn new_in(alloc: A) -> Weak<T, A> {
2977 Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
2978 }
2979}
2980
2981/// Helper type to allow accessing the reference counts without
2982/// making any assertions about the data field.
2983struct WeakInner<'a> {
2984 weak: &'a Atomic<usize>,
2985 strong: &'a Atomic<usize>,
2986}
2987
2988impl<T: ?Sized> Weak<T> {
2989 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2990 ///
2991 /// This can be used to safely get a strong reference (by calling [`upgrade`]
2992 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2993 ///
2994 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2995 /// as these don't own anything; the method still works on them).
2996 ///
2997 /// # Safety
2998 ///
2999 /// The pointer must have originated from the [`into_raw`] and must still own its potential
3000 /// weak reference, and must point to a block of memory allocated by global allocator.
3001 ///
3002 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
3003 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
3004 /// count is not modified by this operation) and therefore it must be paired with a previous
3005 /// call to [`into_raw`].
3006 /// # Examples
3007 ///
3008 /// ```
3009 /// use std::sync::{Arc, Weak};
3010 ///
3011 /// let strong = Arc::new("hello".to_owned());
3012 ///
3013 /// let raw_1 = Arc::downgrade(&strong).into_raw();
3014 /// let raw_2 = Arc::downgrade(&strong).into_raw();
3015 ///
3016 /// assert_eq!(2, Arc::weak_count(&strong));
3017 ///
3018 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
3019 /// assert_eq!(1, Arc::weak_count(&strong));
3020 ///
3021 /// drop(strong);
3022 ///
3023 /// // Decrement the last weak count.
3024 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
3025 /// ```
3026 ///
3027 /// [`new`]: Weak::new
3028 /// [`into_raw`]: Weak::into_raw
3029 /// [`upgrade`]: Weak::upgrade
3030 #[inline]
3031 #[stable(feature = "weak_into_raw", since = "1.45.0")]
3032 pub unsafe fn from_raw(ptr: *const T) -> Self {
3033 unsafe { Weak::from_raw_in(ptr, Global) }
3034 }
3035
3036 /// Consumes the `Weak<T>` and turns it into a raw pointer.
3037 ///
3038 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
3039 /// one weak reference (the weak count is not modified by this operation). It can be turned
3040 /// back into the `Weak<T>` with [`from_raw`].
3041 ///
3042 /// The same restrictions of accessing the target of the pointer as with
3043 /// [`as_ptr`] apply.
3044 ///
3045 /// # Examples
3046 ///
3047 /// ```
3048 /// use std::sync::{Arc, Weak};
3049 ///
3050 /// let strong = Arc::new("hello".to_owned());
3051 /// let weak = Arc::downgrade(&strong);
3052 /// let raw = weak.into_raw();
3053 ///
3054 /// assert_eq!(1, Arc::weak_count(&strong));
3055 /// assert_eq!("hello", unsafe { &*raw });
3056 ///
3057 /// drop(unsafe { Weak::from_raw(raw) });
3058 /// assert_eq!(0, Arc::weak_count(&strong));
3059 /// ```
3060 ///
3061 /// [`from_raw`]: Weak::from_raw
3062 /// [`as_ptr`]: Weak::as_ptr
3063 #[must_use = "losing the pointer will leak memory"]
3064 #[stable(feature = "weak_into_raw", since = "1.45.0")]
3065 pub fn into_raw(self) -> *const T {
3066 ManuallyDrop::new(self).as_ptr()
3067 }
3068}
3069
3070impl<T: ?Sized, A: Allocator> Weak<T, A> {
3071 /// Returns a reference to the underlying allocator.
3072 #[inline]
3073 #[unstable(feature = "allocator_api", issue = "32838")]
3074 pub fn allocator(&self) -> &A {
3075 &self.alloc
3076 }
3077
3078 /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
3079 ///
3080 /// The pointer is valid only if there are some strong references. The pointer may be dangling,
3081 /// unaligned or even [`null`] otherwise.
3082 ///
3083 /// # Examples
3084 ///
3085 /// ```
3086 /// use std::sync::Arc;
3087 /// use std::ptr;
3088 ///
3089 /// let strong = Arc::new("hello".to_owned());
3090 /// let weak = Arc::downgrade(&strong);
3091 /// // Both point to the same object
3092 /// assert!(ptr::eq(&*strong, weak.as_ptr()));
3093 /// // The strong here keeps it alive, so we can still access the object.
3094 /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
3095 ///
3096 /// drop(strong);
3097 /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
3098 /// // undefined behavior.
3099 /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
3100 /// ```
3101 ///
3102 /// [`null`]: core::ptr::null "ptr::null"
3103 #[must_use]
3104 #[stable(feature = "weak_into_raw", since = "1.45.0")]
3105 pub fn as_ptr(&self) -> *const T {
3106 let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
3107
3108 if is_dangling(ptr) {
3109 // If the pointer is dangling, we return the sentinel directly. This cannot be
3110 // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
3111 ptr as *const T
3112 } else {
3113 // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
3114 // The payload may be dropped at this point, and we have to maintain provenance,
3115 // so use raw pointer manipulation.
3116 unsafe { &raw mut (*ptr).data }
3117 }
3118 }
3119
3120 /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
3121 ///
3122 /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
3123 /// one weak reference (the weak count is not modified by this operation). It can be turned
3124 /// back into the `Weak<T>` with [`from_raw_in`].
3125 ///
3126 /// The same restrictions of accessing the target of the pointer as with
3127 /// [`as_ptr`] apply.
3128 ///
3129 /// # Examples
3130 ///
3131 /// ```
3132 /// #![feature(allocator_api)]
3133 /// use std::sync::{Arc, Weak};
3134 /// use std::alloc::System;
3135 ///
3136 /// let strong = Arc::new_in("hello".to_owned(), System);
3137 /// let weak = Arc::downgrade(&strong);
3138 /// let (raw, alloc) = weak.into_raw_with_allocator();
3139 ///
3140 /// assert_eq!(1, Arc::weak_count(&strong));
3141 /// assert_eq!("hello", unsafe { &*raw });
3142 ///
3143 /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
3144 /// assert_eq!(0, Arc::weak_count(&strong));
3145 /// ```
3146 ///
3147 /// [`from_raw_in`]: Weak::from_raw_in
3148 /// [`as_ptr`]: Weak::as_ptr
3149 #[must_use = "losing the pointer will leak memory"]
3150 #[unstable(feature = "allocator_api", issue = "32838")]
3151 pub fn into_raw_with_allocator(self) -> (*const T, A) {
3152 let this = mem::ManuallyDrop::new(self);
3153 let result = this.as_ptr();
3154 // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
3155 let alloc = unsafe { ptr::read(&this.alloc) };
3156 (result, alloc)
3157 }
3158
3159 /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
3160 /// allocator.
3161 ///
3162 /// This can be used to safely get a strong reference (by calling [`upgrade`]
3163 /// later) or to deallocate the weak count by dropping the `Weak<T>`.
3164 ///
3165 /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
3166 /// as these don't own anything; the method still works on them).
3167 ///
3168 /// # Safety
3169 ///
3170 /// The pointer must have originated from the [`into_raw`] and must still own its potential
3171 /// weak reference, and must point to a block of memory allocated by `alloc`.
3172 ///
3173 /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
3174 /// takes ownership of one weak reference currently represented as a raw pointer (the weak
3175 /// count is not modified by this operation) and therefore it must be paired with a previous
3176 /// call to [`into_raw`].
3177 /// # Examples
3178 ///
3179 /// ```
3180 /// use std::sync::{Arc, Weak};
3181 ///
3182 /// let strong = Arc::new("hello".to_owned());
3183 ///
3184 /// let raw_1 = Arc::downgrade(&strong).into_raw();
3185 /// let raw_2 = Arc::downgrade(&strong).into_raw();
3186 ///
3187 /// assert_eq!(2, Arc::weak_count(&strong));
3188 ///
3189 /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
3190 /// assert_eq!(1, Arc::weak_count(&strong));
3191 ///
3192 /// drop(strong);
3193 ///
3194 /// // Decrement the last weak count.
3195 /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
3196 /// ```
3197 ///
3198 /// [`new`]: Weak::new
3199 /// [`into_raw`]: Weak::into_raw
3200 /// [`upgrade`]: Weak::upgrade
3201 #[inline]
3202 #[unstable(feature = "allocator_api", issue = "32838")]
3203 pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
3204 // See Weak::as_ptr for context on how the input pointer is derived.
3205
3206 let ptr = if is_dangling(ptr) {
3207 // This is a dangling Weak.
3208 ptr as *mut ArcInner<T>
3209 } else {
3210 // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
3211 // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
3212 let offset = unsafe { data_offset(ptr) };
3213 // Thus, we reverse the offset to get the whole ArcInner.
3214 // SAFETY: the pointer originated from a Weak, so this offset is safe.
3215 unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
3216 };
3217
3218 // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
3219 Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc }
3220 }
3221}
3222
3223impl<T: ?Sized, A: Allocator> Weak<T, A> {
3224 /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
3225 /// dropping of the inner value if successful.
3226 ///
3227 /// Returns [`None`] if the inner value has since been dropped.
3228 ///
3229 /// # Examples
3230 ///
3231 /// ```
3232 /// use std::sync::Arc;
3233 ///
3234 /// let five = Arc::new(5);
3235 ///
3236 /// let weak_five = Arc::downgrade(&five);
3237 ///
3238 /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
3239 /// assert!(strong_five.is_some());
3240 ///
3241 /// // Destroy all strong pointers.
3242 /// drop(strong_five);
3243 /// drop(five);
3244 ///
3245 /// assert!(weak_five.upgrade().is_none());
3246 /// ```
3247 #[must_use = "this returns a new `Arc`, \
3248 without modifying the original weak pointer"]
3249 #[stable(feature = "arc_weak", since = "1.4.0")]
3250 pub fn upgrade(&self) -> Option<Arc<T, A>>
3251 where
3252 A: Clone,
3253 {
3254 #[inline]
3255 fn checked_increment(n: usize) -> Option<usize> {
3256 // Any write of 0 we can observe leaves the field in permanently zero state.
3257 if n == 0 {
3258 return None;
3259 }
3260 // See comments in `Arc::clone` for why we do this (for `mem::forget`).
3261 assert!(n <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
3262 Some(n + 1)
3263 }
3264
3265 // We use a CAS loop to increment the strong count instead of a
3266 // fetch_add as this function should never take the reference count
3267 // from zero to one.
3268 //
3269 // Relaxed is fine for the failure case because we don't have any expectations about the new state.
3270 // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
3271 // value can be initialized after `Weak` references have already been created. In that case, we
3272 // expect to observe the fully initialized value.
3273 if self.inner()?.strong.try_update(Acquire, Relaxed, checked_increment).is_ok() {
3274 // SAFETY: pointer is not null, verified in checked_increment
3275 unsafe { Some(Arc::from_inner_in(self.ptr, self.alloc.clone())) }
3276 } else {
3277 None
3278 }
3279 }
3280
3281 /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
3282 ///
3283 /// If `self` was created using [`Weak::new`], this will return 0.
3284 #[must_use]
3285 #[stable(feature = "weak_counts", since = "1.41.0")]
3286 pub fn strong_count(&self) -> usize {
3287 if let Some(inner) = self.inner() { inner.strong.load(Relaxed) } else { 0 }
3288 }
3289
3290 /// Gets an approximation of the number of `Weak` pointers pointing to this
3291 /// allocation.
3292 ///
3293 /// If `self` was created using [`Weak::new`], or if there are no remaining
3294 /// strong pointers, this will return 0.
3295 ///
3296 /// # Accuracy
3297 ///
3298 /// Due to implementation details, the returned value can be off by 1 in
3299 /// either direction when other threads are manipulating any `Arc`s or
3300 /// `Weak`s pointing to the same allocation.
3301 #[must_use]
3302 #[stable(feature = "weak_counts", since = "1.41.0")]
3303 pub fn weak_count(&self) -> usize {
3304 if let Some(inner) = self.inner() {
3305 let weak = inner.weak.load(Acquire);
3306 let strong = inner.strong.load(Relaxed);
3307 if strong == 0 {
3308 0
3309 } else {
3310 // Since we observed that there was at least one strong pointer
3311 // after reading the weak count, we know that the implicit weak
3312 // reference (present whenever any strong references are alive)
3313 // was still around when we observed the weak count, and can
3314 // therefore safely subtract it.
3315 weak - 1
3316 }
3317 } else {
3318 0
3319 }
3320 }
3321
3322 /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
3323 /// (i.e., when this `Weak` was created by `Weak::new`).
3324 #[inline]
3325 fn inner(&self) -> Option<WeakInner<'_>> {
3326 let ptr = self.ptr.as_ptr();
3327 if is_dangling(ptr) {
3328 None
3329 } else {
3330 // We are careful to *not* create a reference covering the "data" field, as
3331 // the field may be mutated concurrently (for example, if the last `Arc`
3332 // is dropped, the data field will be dropped in-place).
3333 Some(unsafe { WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak } })
3334 }
3335 }
3336
3337 /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
3338 /// both don't point to any allocation (because they were created with `Weak::new()`). However,
3339 /// this function ignores the metadata of `dyn Trait` pointers.
3340 ///
3341 /// # Notes
3342 ///
3343 /// Since this compares pointers it means that `Weak::new()` will equal each
3344 /// other, even though they don't point to any allocation.
3345 ///
3346 /// # Examples
3347 ///
3348 /// ```
3349 /// use std::sync::Arc;
3350 ///
3351 /// let first_rc = Arc::new(5);
3352 /// let first = Arc::downgrade(&first_rc);
3353 /// let second = Arc::downgrade(&first_rc);
3354 ///
3355 /// assert!(first.ptr_eq(&second));
3356 ///
3357 /// let third_rc = Arc::new(5);
3358 /// let third = Arc::downgrade(&third_rc);
3359 ///
3360 /// assert!(!first.ptr_eq(&third));
3361 /// ```
3362 ///
3363 /// Comparing `Weak::new`.
3364 ///
3365 /// ```
3366 /// use std::sync::{Arc, Weak};
3367 ///
3368 /// let first = Weak::new();
3369 /// let second = Weak::new();
3370 /// assert!(first.ptr_eq(&second));
3371 ///
3372 /// let third_rc = Arc::new(());
3373 /// let third = Arc::downgrade(&third_rc);
3374 /// assert!(!first.ptr_eq(&third));
3375 /// ```
3376 ///
3377 /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
3378 #[inline]
3379 #[must_use]
3380 #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
3381 pub fn ptr_eq(&self, other: &Self) -> bool {
3382 ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr())
3383 }
3384}
3385
3386#[stable(feature = "arc_weak", since = "1.4.0")]
3387impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
3388 /// Makes a clone of the `Weak` pointer that points to the same allocation.
3389 ///
3390 /// # Examples
3391 ///
3392 /// ```
3393 /// use std::sync::{Arc, Weak};
3394 ///
3395 /// let weak_five = Arc::downgrade(&Arc::new(5));
3396 ///
3397 /// let _ = Weak::clone(&weak_five);
3398 /// ```
3399 #[inline]
3400 fn clone(&self) -> Weak<T, A> {
3401 if let Some(inner) = self.inner() {
3402 // See comments in Arc::clone() for why this is relaxed. This can use a
3403 // fetch_add (ignoring the lock) because the weak count is only locked
3404 // where are *no other* weak pointers in existence. (So we can't be
3405 // running this code in that case).
3406 let old_size = inner.weak.fetch_add(1, Relaxed);
3407
3408 // See comments in Arc::clone() for why we do this (for mem::forget).
3409 if old_size > MAX_REFCOUNT {
3410 abort();
3411 }
3412 }
3413
3414 Weak { ptr: self.ptr, alloc: self.alloc.clone() }
3415 }
3416}
3417
3418#[unstable(feature = "ergonomic_clones", issue = "132290")]
3419impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
3420
3421#[stable(feature = "downgraded_weak", since = "1.10.0")]
3422impl<T> Default for Weak<T> {
3423 /// Constructs a new `Weak<T>`, without allocating memory.
3424 /// Calling [`upgrade`] on the return value always
3425 /// gives [`None`].
3426 ///
3427 /// [`upgrade`]: Weak::upgrade
3428 ///
3429 /// # Examples
3430 ///
3431 /// ```
3432 /// use std::sync::Weak;
3433 ///
3434 /// let empty: Weak<i64> = Default::default();
3435 /// assert!(empty.upgrade().is_none());
3436 /// ```
3437 fn default() -> Weak<T> {
3438 Weak::new()
3439 }
3440}
3441
3442#[stable(feature = "arc_weak", since = "1.4.0")]
3443unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> {
3444 /// Drops the `Weak` pointer.
3445 ///
3446 /// # Examples
3447 ///
3448 /// ```
3449 /// use std::sync::{Arc, Weak};
3450 ///
3451 /// struct Foo;
3452 ///
3453 /// impl Drop for Foo {
3454 /// fn drop(&mut self) {
3455 /// println!("dropped!");
3456 /// }
3457 /// }
3458 ///
3459 /// let foo = Arc::new(Foo);
3460 /// let weak_foo = Arc::downgrade(&foo);
3461 /// let other_weak_foo = Weak::clone(&weak_foo);
3462 ///
3463 /// drop(weak_foo); // Doesn't print anything
3464 /// drop(foo); // Prints "dropped!"
3465 ///
3466 /// assert!(other_weak_foo.upgrade().is_none());
3467 /// ```
3468 fn drop(&mut self) {
3469 // If we find out that we were the last weak pointer, then its time to
3470 // deallocate the data entirely. See the discussion in Arc::drop() about
3471 // the memory orderings
3472 //
3473 // It's not necessary to check for the locked state here, because the
3474 // weak count can only be locked if there was precisely one weak ref,
3475 // meaning that drop could only subsequently run ON that remaining weak
3476 // ref, which can only happen after the lock is released.
3477 let inner = if let Some(inner) = self.inner() { inner } else { return };
3478
3479 if inner.weak.fetch_sub(1, Release) == 1 {
3480 acquire!(inner.weak);
3481
3482 // Make sure we aren't trying to "deallocate" the shared static for empty slices
3483 // used by Default::default.
3484 debug_assert!(
3485 !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
3486 "Arc/Weaks backed by a static should never be deallocated. \
3487 Likely decrement_strong_count or from_raw were called too many times.",
3488 );
3489
3490 unsafe {
3491 self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()))
3492 }
3493 }
3494 }
3495}
3496
3497#[stable(feature = "rust1", since = "1.0.0")]
3498trait ArcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
3499 fn eq(&self, other: &Arc<T, A>) -> bool;
3500 fn ne(&self, other: &Arc<T, A>) -> bool;
3501}
3502
3503#[stable(feature = "rust1", since = "1.0.0")]
3504impl<T: ?Sized + PartialEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3505 #[inline]
3506 default fn eq(&self, other: &Arc<T, A>) -> bool {
3507 **self == **other
3508 }
3509 #[inline]
3510 default fn ne(&self, other: &Arc<T, A>) -> bool {
3511 **self != **other
3512 }
3513}
3514
3515/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
3516/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
3517/// store large values, that are slow to clone, but also heavy to check for equality, causing this
3518/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
3519/// the same value, than two `&T`s.
3520///
3521/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
3522#[stable(feature = "rust1", since = "1.0.0")]
3523impl<T: ?Sized + crate::rc::MarkerEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3524 #[inline]
3525 fn eq(&self, other: &Arc<T, A>) -> bool {
3526 Arc::ptr_eq(self, other) || **self == **other
3527 }
3528
3529 #[inline]
3530 fn ne(&self, other: &Arc<T, A>) -> bool {
3531 !Arc::ptr_eq(self, other) && **self != **other
3532 }
3533}
3534
3535#[stable(feature = "rust1", since = "1.0.0")]
3536impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A> {
3537 /// Equality for two `Arc`s.
3538 ///
3539 /// Two `Arc`s are equal if their inner values are equal, even if they are
3540 /// stored in different allocation.
3541 ///
3542 /// If `T` also implements `Eq` (implying reflexivity of equality),
3543 /// two `Arc`s that point to the same allocation are always equal.
3544 ///
3545 /// # Examples
3546 ///
3547 /// ```
3548 /// use std::sync::Arc;
3549 ///
3550 /// let five = Arc::new(5);
3551 ///
3552 /// assert!(five == Arc::new(5));
3553 /// ```
3554 #[inline]
3555 fn eq(&self, other: &Arc<T, A>) -> bool {
3556 ArcEqIdent::eq(self, other)
3557 }
3558
3559 /// Inequality for two `Arc`s.
3560 ///
3561 /// Two `Arc`s are not equal if their inner values are not equal.
3562 ///
3563 /// If `T` also implements `Eq` (implying reflexivity of equality),
3564 /// two `Arc`s that point to the same value are always equal.
3565 ///
3566 /// # Examples
3567 ///
3568 /// ```
3569 /// use std::sync::Arc;
3570 ///
3571 /// let five = Arc::new(5);
3572 ///
3573 /// assert!(five != Arc::new(6));
3574 /// ```
3575 #[inline]
3576 fn ne(&self, other: &Arc<T, A>) -> bool {
3577 ArcEqIdent::ne(self, other)
3578 }
3579}
3580
3581#[stable(feature = "rust1", since = "1.0.0")]
3582impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A> {
3583 /// Partial comparison for two `Arc`s.
3584 ///
3585 /// The two are compared by calling `partial_cmp()` on their inner values.
3586 ///
3587 /// # Examples
3588 ///
3589 /// ```
3590 /// use std::sync::Arc;
3591 /// use std::cmp::Ordering;
3592 ///
3593 /// let five = Arc::new(5);
3594 ///
3595 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
3596 /// ```
3597 fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering> {
3598 (**self).partial_cmp(&**other)
3599 }
3600
3601 /// Less-than comparison for two `Arc`s.
3602 ///
3603 /// The two are compared by calling `<` on their inner values.
3604 ///
3605 /// # Examples
3606 ///
3607 /// ```
3608 /// use std::sync::Arc;
3609 ///
3610 /// let five = Arc::new(5);
3611 ///
3612 /// assert!(five < Arc::new(6));
3613 /// ```
3614 fn lt(&self, other: &Arc<T, A>) -> bool {
3615 *(*self) < *(*other)
3616 }
3617
3618 /// 'Less than or equal to' comparison for two `Arc`s.
3619 ///
3620 /// The two are compared by calling `<=` on their inner values.
3621 ///
3622 /// # Examples
3623 ///
3624 /// ```
3625 /// use std::sync::Arc;
3626 ///
3627 /// let five = Arc::new(5);
3628 ///
3629 /// assert!(five <= Arc::new(5));
3630 /// ```
3631 fn le(&self, other: &Arc<T, A>) -> bool {
3632 *(*self) <= *(*other)
3633 }
3634
3635 /// Greater-than comparison for two `Arc`s.
3636 ///
3637 /// The two are compared by calling `>` on their inner values.
3638 ///
3639 /// # Examples
3640 ///
3641 /// ```
3642 /// use std::sync::Arc;
3643 ///
3644 /// let five = Arc::new(5);
3645 ///
3646 /// assert!(five > Arc::new(4));
3647 /// ```
3648 fn gt(&self, other: &Arc<T, A>) -> bool {
3649 *(*self) > *(*other)
3650 }
3651
3652 /// 'Greater than or equal to' comparison for two `Arc`s.
3653 ///
3654 /// The two are compared by calling `>=` on their inner values.
3655 ///
3656 /// # Examples
3657 ///
3658 /// ```
3659 /// use std::sync::Arc;
3660 ///
3661 /// let five = Arc::new(5);
3662 ///
3663 /// assert!(five >= Arc::new(5));
3664 /// ```
3665 fn ge(&self, other: &Arc<T, A>) -> bool {
3666 *(*self) >= *(*other)
3667 }
3668}
3669#[stable(feature = "rust1", since = "1.0.0")]
3670impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A> {
3671 /// Comparison for two `Arc`s.
3672 ///
3673 /// The two are compared by calling `cmp()` on their inner values.
3674 ///
3675 /// # Examples
3676 ///
3677 /// ```
3678 /// use std::sync::Arc;
3679 /// use std::cmp::Ordering;
3680 ///
3681 /// let five = Arc::new(5);
3682 ///
3683 /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
3684 /// ```
3685 fn cmp(&self, other: &Arc<T, A>) -> Ordering {
3686 (**self).cmp(&**other)
3687 }
3688}
3689#[stable(feature = "rust1", since = "1.0.0")]
3690impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A> {}
3691
3692#[stable(feature = "rust1", since = "1.0.0")]
3693impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Arc<T, A> {
3694 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3695 fmt::Display::fmt(&**self, f)
3696 }
3697}
3698
3699#[stable(feature = "rust1", since = "1.0.0")]
3700impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for Arc<T, A> {
3701 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3702 fmt::Debug::fmt(&**self, f)
3703 }
3704}
3705
3706#[stable(feature = "rust1", since = "1.0.0")]
3707impl<T: ?Sized, A: Allocator> fmt::Pointer for Arc<T, A> {
3708 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3709 fmt::Pointer::fmt(&(&raw const **self), f)
3710 }
3711}
3712
3713#[cfg(not(no_global_oom_handling))]
3714#[stable(feature = "rust1", since = "1.0.0")]
3715impl<T: Default> Default for Arc<T> {
3716 /// Creates a new `Arc<T>`, with the `Default` value for `T`.
3717 ///
3718 /// # Examples
3719 ///
3720 /// ```
3721 /// use std::sync::Arc;
3722 ///
3723 /// let x: Arc<i32> = Default::default();
3724 /// assert_eq!(*x, 0);
3725 /// ```
3726 fn default() -> Arc<T> {
3727 unsafe {
3728 Self::from_inner(
3729 Box::leak(Box::write(
3730 Box::new_uninit(),
3731 ArcInner {
3732 strong: atomic::AtomicUsize::new(1),
3733 weak: atomic::AtomicUsize::new(1),
3734 data: T::default(),
3735 },
3736 ))
3737 .into(),
3738 )
3739 }
3740 }
3741}
3742
3743/// Struct to hold the static `ArcInner` used for empty `Arc<str/CStr/[T]>` as
3744/// returned by `Default::default`.
3745///
3746/// Layout notes:
3747/// * `repr(align(16))` so we can use it for `[T]` with `align_of::<T>() <= 16`.
3748/// * `repr(C)` so `inner` is at offset 0 (and thus guaranteed to actually be aligned to 16).
3749/// * `[u8; 1]` (to be initialized with 0) so it can be used for `Arc<CStr>`.
3750#[repr(C, align(16))]
3751struct SliceArcInnerForStatic {
3752 inner: ArcInner<[u8; 1]>,
3753}
3754#[cfg(not(no_global_oom_handling))]
3755const MAX_STATIC_INNER_SLICE_ALIGNMENT: usize = 16;
3756
3757static STATIC_INNER_SLICE: SliceArcInnerForStatic = SliceArcInnerForStatic {
3758 inner: ArcInner {
3759 strong: atomic::AtomicUsize::new(1),
3760 weak: atomic::AtomicUsize::new(1),
3761 data: [0],
3762 },
3763};
3764
3765#[cfg(not(no_global_oom_handling))]
3766#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3767impl Default for Arc<str> {
3768 /// Creates an empty str inside an Arc
3769 ///
3770 /// This may or may not share an allocation with other Arcs.
3771 #[inline]
3772 fn default() -> Self {
3773 let arc: Arc<[u8]> = Default::default();
3774 debug_assert!(core::str::from_utf8(&*arc).is_ok());
3775 let (ptr, alloc: Global) = Arc::into_inner_with_allocator(this:arc);
3776 unsafe { Arc::from_ptr_in(ptr.as_ptr() as *mut ArcInner<str>, alloc) }
3777 }
3778}
3779
3780#[cfg(not(no_global_oom_handling))]
3781#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3782impl Default for Arc<core::ffi::CStr> {
3783 /// Creates an empty CStr inside an Arc
3784 ///
3785 /// This may or may not share an allocation with other Arcs.
3786 #[inline]
3787 fn default() -> Self {
3788 use core::ffi::CStr;
3789 let inner: NonNull<ArcInner<[u8]>> = NonNull::from(&STATIC_INNER_SLICE.inner);
3790 let inner: NonNull<ArcInner<CStr>> =
3791 NonNull::new(inner.as_ptr() as *mut ArcInner<CStr>).unwrap();
3792 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3793 let this: mem::ManuallyDrop<Arc<CStr>> =
3794 unsafe { mem::ManuallyDrop::new(Arc::from_inner(ptr:inner)) };
3795 (*this).clone()
3796 }
3797}
3798
3799#[cfg(not(no_global_oom_handling))]
3800#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3801impl<T> Default for Arc<[T]> {
3802 /// Creates an empty `[T]` inside an Arc
3803 ///
3804 /// This may or may not share an allocation with other Arcs.
3805 #[inline]
3806 fn default() -> Self {
3807 if align_of::<T>() <= MAX_STATIC_INNER_SLICE_ALIGNMENT {
3808 // We take a reference to the whole struct instead of the ArcInner<[u8; 1]> inside it so
3809 // we don't shrink the range of bytes the ptr is allowed to access under Stacked Borrows.
3810 // (Miri complains on 32-bit targets with Arc<[Align16]> otherwise.)
3811 // (Note that NonNull::from(&STATIC_INNER_SLICE.inner) is fine under Tree Borrows.)
3812 let inner: NonNull<SliceArcInnerForStatic> = NonNull::from(&STATIC_INNER_SLICE);
3813 let inner: NonNull<ArcInner<[T; 0]>> = inner.cast();
3814 // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3815 let this: mem::ManuallyDrop<Arc<[T; 0]>> =
3816 unsafe { mem::ManuallyDrop::new(Arc::from_inner(ptr:inner)) };
3817 return (*this).clone();
3818 }
3819
3820 // If T's alignment is too large for the static, make a new unique allocation.
3821 let arr: [T; 0] = [];
3822 Arc::from(arr)
3823 }
3824}
3825
3826#[cfg(not(no_global_oom_handling))]
3827#[stable(feature = "pin_default_impls", since = "1.91.0")]
3828impl<T> Default for Pin<Arc<T>>
3829where
3830 T: ?Sized,
3831 Arc<T>: Default,
3832{
3833 #[inline]
3834 fn default() -> Self {
3835 unsafe { Pin::new_unchecked(Arc::<T>::default()) }
3836 }
3837}
3838
3839#[stable(feature = "rust1", since = "1.0.0")]
3840impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
3841 fn hash<H: Hasher>(&self, state: &mut H) {
3842 (**self).hash(state)
3843 }
3844}
3845
3846#[cfg(not(no_global_oom_handling))]
3847#[stable(feature = "from_for_ptrs", since = "1.6.0")]
3848impl<T> From<T> for Arc<T> {
3849 /// Converts a `T` into an `Arc<T>`
3850 ///
3851 /// The conversion moves the value into a
3852 /// newly allocated `Arc`. It is equivalent to
3853 /// calling `Arc::new(t)`.
3854 ///
3855 /// # Example
3856 /// ```rust
3857 /// # use std::sync::Arc;
3858 /// let x = 5;
3859 /// let arc = Arc::new(5);
3860 ///
3861 /// assert_eq!(Arc::from(x), arc);
3862 /// ```
3863 fn from(t: T) -> Self {
3864 Arc::new(data:t)
3865 }
3866}
3867
3868#[cfg(not(no_global_oom_handling))]
3869#[stable(feature = "shared_from_array", since = "1.74.0")]
3870impl<T, const N: usize> From<[T; N]> for Arc<[T]> {
3871 /// Converts a [`[T; N]`](prim@array) into an `Arc<[T]>`.
3872 ///
3873 /// The conversion moves the array into a newly allocated `Arc`.
3874 ///
3875 /// # Example
3876 ///
3877 /// ```
3878 /// # use std::sync::Arc;
3879 /// let original: [i32; 3] = [1, 2, 3];
3880 /// let shared: Arc<[i32]> = Arc::from(original);
3881 /// assert_eq!(&[1, 2, 3], &shared[..]);
3882 /// ```
3883 #[inline]
3884 fn from(v: [T; N]) -> Arc<[T]> {
3885 Arc::<[T; N]>::from(v)
3886 }
3887}
3888
3889#[cfg(not(no_global_oom_handling))]
3890#[stable(feature = "shared_from_slice", since = "1.21.0")]
3891impl<T: Clone> From<&[T]> for Arc<[T]> {
3892 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3893 ///
3894 /// # Example
3895 ///
3896 /// ```
3897 /// # use std::sync::Arc;
3898 /// let original: &[i32] = &[1, 2, 3];
3899 /// let shared: Arc<[i32]> = Arc::from(original);
3900 /// assert_eq!(&[1, 2, 3], &shared[..]);
3901 /// ```
3902 #[inline]
3903 fn from(v: &[T]) -> Arc<[T]> {
3904 <Self as ArcFromSlice<T>>::from_slice(v)
3905 }
3906}
3907
3908#[cfg(not(no_global_oom_handling))]
3909#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3910impl<T: Clone> From<&mut [T]> for Arc<[T]> {
3911 /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3912 ///
3913 /// # Example
3914 ///
3915 /// ```
3916 /// # use std::sync::Arc;
3917 /// let mut original = [1, 2, 3];
3918 /// let original: &mut [i32] = &mut original;
3919 /// let shared: Arc<[i32]> = Arc::from(original);
3920 /// assert_eq!(&[1, 2, 3], &shared[..]);
3921 /// ```
3922 #[inline]
3923 fn from(v: &mut [T]) -> Arc<[T]> {
3924 Arc::from(&*v)
3925 }
3926}
3927
3928#[cfg(not(no_global_oom_handling))]
3929#[stable(feature = "shared_from_slice", since = "1.21.0")]
3930impl From<&str> for Arc<str> {
3931 /// Allocates a reference-counted `str` and copies `v` into it.
3932 ///
3933 /// # Example
3934 ///
3935 /// ```
3936 /// # use std::sync::Arc;
3937 /// let shared: Arc<str> = Arc::from("eggplant");
3938 /// assert_eq!("eggplant", &shared[..]);
3939 /// ```
3940 #[inline]
3941 fn from(v: &str) -> Arc<str> {
3942 let arc: Arc = Arc::<[u8]>::from(v.as_bytes());
3943 unsafe { Arc::from_raw(ptr:Arc::into_raw(this:arc) as *const str) }
3944 }
3945}
3946
3947#[cfg(not(no_global_oom_handling))]
3948#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3949impl From<&mut str> for Arc<str> {
3950 /// Allocates a reference-counted `str` and copies `v` into it.
3951 ///
3952 /// # Example
3953 ///
3954 /// ```
3955 /// # use std::sync::Arc;
3956 /// let mut original = String::from("eggplant");
3957 /// let original: &mut str = &mut original;
3958 /// let shared: Arc<str> = Arc::from(original);
3959 /// assert_eq!("eggplant", &shared[..]);
3960 /// ```
3961 #[inline]
3962 fn from(v: &mut str) -> Arc<str> {
3963 Arc::from(&*v)
3964 }
3965}
3966
3967#[cfg(not(no_global_oom_handling))]
3968#[stable(feature = "shared_from_slice", since = "1.21.0")]
3969impl From<String> for Arc<str> {
3970 /// Allocates a reference-counted `str` and copies `v` into it.
3971 ///
3972 /// # Example
3973 ///
3974 /// ```
3975 /// # use std::sync::Arc;
3976 /// let unique: String = "eggplant".to_owned();
3977 /// let shared: Arc<str> = Arc::from(unique);
3978 /// assert_eq!("eggplant", &shared[..]);
3979 /// ```
3980 #[inline]
3981 fn from(v: String) -> Arc<str> {
3982 Arc::from(&v[..])
3983 }
3984}
3985
3986#[cfg(not(no_global_oom_handling))]
3987#[stable(feature = "shared_from_slice", since = "1.21.0")]
3988impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
3989 /// Move a boxed object to a new, reference-counted allocation.
3990 ///
3991 /// # Example
3992 ///
3993 /// ```
3994 /// # use std::sync::Arc;
3995 /// let unique: Box<str> = Box::from("eggplant");
3996 /// let shared: Arc<str> = Arc::from(unique);
3997 /// assert_eq!("eggplant", &shared[..]);
3998 /// ```
3999 #[inline]
4000 fn from(v: Box<T, A>) -> Arc<T, A> {
4001 Arc::from_box_in(src:v)
4002 }
4003}
4004
4005#[cfg(not(no_global_oom_handling))]
4006#[stable(feature = "shared_from_slice", since = "1.21.0")]
4007impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
4008 /// Allocates a reference-counted slice and moves `v`'s items into it.
4009 ///
4010 /// # Example
4011 ///
4012 /// ```
4013 /// # use std::sync::Arc;
4014 /// let unique: Vec<i32> = vec![1, 2, 3];
4015 /// let shared: Arc<[i32]> = Arc::from(unique);
4016 /// assert_eq!(&[1, 2, 3], &shared[..]);
4017 /// ```
4018 #[inline]
4019 fn from(v: Vec<T, A>) -> Arc<[T], A> {
4020 unsafe {
4021 let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
4022
4023 let rc_ptr = Self::allocate_for_slice_in(len, &alloc);
4024 ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).data) as *mut T, len);
4025
4026 // Create a `Vec<T, &A>` with length 0, to deallocate the buffer
4027 // without dropping its contents or the allocator
4028 let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc);
4029
4030 Self::from_ptr_in(rc_ptr, alloc)
4031 }
4032 }
4033}
4034
4035#[stable(feature = "shared_from_cow", since = "1.45.0")]
4036impl<'a, B> From<Cow<'a, B>> for Arc<B>
4037where
4038 B: ToOwned + ?Sized,
4039 Arc<B>: From<&'a B> + From<B::Owned>,
4040{
4041 /// Creates an atomically reference-counted pointer from a clone-on-write
4042 /// pointer by copying its content.
4043 ///
4044 /// # Example
4045 ///
4046 /// ```rust
4047 /// # use std::sync::Arc;
4048 /// # use std::borrow::Cow;
4049 /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
4050 /// let shared: Arc<str> = Arc::from(cow);
4051 /// assert_eq!("eggplant", &shared[..]);
4052 /// ```
4053 #[inline]
4054 fn from(cow: Cow<'a, B>) -> Arc<B> {
4055 match cow {
4056 Cow::Borrowed(s: &B) => Arc::from(s),
4057 Cow::Owned(s: B) => Arc::from(s),
4058 }
4059 }
4060}
4061
4062#[stable(feature = "shared_from_str", since = "1.62.0")]
4063impl From<Arc<str>> for Arc<[u8]> {
4064 /// Converts an atomically reference-counted string slice into a byte slice.
4065 ///
4066 /// # Example
4067 ///
4068 /// ```
4069 /// # use std::sync::Arc;
4070 /// let string: Arc<str> = Arc::from("eggplant");
4071 /// let bytes: Arc<[u8]> = Arc::from(string);
4072 /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
4073 /// ```
4074 #[inline]
4075 fn from(rc: Arc<str>) -> Self {
4076 // SAFETY: `str` has the same layout as `[u8]`.
4077 unsafe { Arc::from_raw(ptr:Arc::into_raw(this:rc) as *const [u8]) }
4078 }
4079}
4080
4081#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
4082impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
4083 type Error = Arc<[T], A>;
4084
4085 fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
4086 if boxed_slice.len() == N {
4087 let (ptr, alloc: A) = Arc::into_inner_with_allocator(this:boxed_slice);
4088 Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
4089 } else {
4090 Err(boxed_slice)
4091 }
4092 }
4093}
4094
4095#[cfg(not(no_global_oom_handling))]
4096#[stable(feature = "shared_from_iter", since = "1.37.0")]
4097impl<T> FromIterator<T> for Arc<[T]> {
4098 /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
4099 ///
4100 /// # Performance characteristics
4101 ///
4102 /// ## The general case
4103 ///
4104 /// In the general case, collecting into `Arc<[T]>` is done by first
4105 /// collecting into a `Vec<T>`. That is, when writing the following:
4106 ///
4107 /// ```rust
4108 /// # use std::sync::Arc;
4109 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
4110 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
4111 /// ```
4112 ///
4113 /// this behaves as if we wrote:
4114 ///
4115 /// ```rust
4116 /// # use std::sync::Arc;
4117 /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
4118 /// .collect::<Vec<_>>() // The first set of allocations happens here.
4119 /// .into(); // A second allocation for `Arc<[T]>` happens here.
4120 /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
4121 /// ```
4122 ///
4123 /// This will allocate as many times as needed for constructing the `Vec<T>`
4124 /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
4125 ///
4126 /// ## Iterators of known length
4127 ///
4128 /// When your `Iterator` implements `TrustedLen` and is of an exact size,
4129 /// a single allocation will be made for the `Arc<[T]>`. For example:
4130 ///
4131 /// ```rust
4132 /// # use std::sync::Arc;
4133 /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
4134 /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
4135 /// ```
4136 fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
4137 ToArcSlice::to_arc_slice(iter.into_iter())
4138 }
4139}
4140
4141#[cfg(not(no_global_oom_handling))]
4142/// Specialization trait used for collecting into `Arc<[T]>`.
4143trait ToArcSlice<T>: Iterator<Item = T> + Sized {
4144 fn to_arc_slice(self) -> Arc<[T]>;
4145}
4146
4147#[cfg(not(no_global_oom_handling))]
4148impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
4149 default fn to_arc_slice(self) -> Arc<[T]> {
4150 self.collect::<Vec<T>>().into()
4151 }
4152}
4153
4154#[cfg(not(no_global_oom_handling))]
4155impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
4156 fn to_arc_slice(self) -> Arc<[T]> {
4157 // This is the case for a `TrustedLen` iterator.
4158 let (low, high) = self.size_hint();
4159 if let Some(high) = high {
4160 debug_assert_eq!(
4161 low,
4162 high,
4163 "TrustedLen iterator's size hint is not exact: {:?}",
4164 (low, high)
4165 );
4166
4167 unsafe {
4168 // SAFETY: We need to ensure that the iterator has an exact length and we have.
4169 Arc::from_iter_exact(self, low)
4170 }
4171 } else {
4172 // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
4173 // length exceeding `usize::MAX`.
4174 // The default implementation would collect into a vec which would panic.
4175 // Thus we panic here immediately without invoking `Vec` code.
4176 panic!("capacity overflow");
4177 }
4178 }
4179}
4180
4181#[stable(feature = "rust1", since = "1.0.0")]
4182impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Arc<T, A> {
4183 fn borrow(&self) -> &T {
4184 &**self
4185 }
4186}
4187
4188#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
4189impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> {
4190 fn as_ref(&self) -> &T {
4191 &**self
4192 }
4193}
4194
4195#[stable(feature = "pin", since = "1.33.0")]
4196impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
4197
4198/// Gets the offset within an `ArcInner` for the payload behind a pointer.
4199///
4200/// # Safety
4201///
4202/// The pointer must point to (and have valid metadata for) a previously
4203/// valid instance of T, but the T is allowed to be dropped.
4204unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
4205 // Align the unsized value to the end of the ArcInner.
4206 // Because ArcInner is repr(C), it will always be the last field in memory.
4207 // SAFETY: since the only unsized types possible are slices, trait objects,
4208 // and extern types, the input safety requirement is currently enough to
4209 // satisfy the requirements of Alignment::of_val_raw; this is an implementation
4210 // detail of the language that must not be relied upon outside of std.
4211 unsafe { data_offset_alignment(Alignment::of_val_raw(ptr)) }
4212}
4213
4214#[inline]
4215fn data_offset_alignment(alignment: Alignment) -> usize {
4216 let layout = Layout::new::<ArcInner<()>>();
4217 layout.size() + layout.padding_needed_for(alignment)
4218}
4219
4220/// A unique owning pointer to an [`ArcInner`] **that does not imply the contents are initialized,**
4221/// but will deallocate it (without dropping the value) when dropped.
4222///
4223/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
4224struct UniqueArcUninit<T: ?Sized, A: Allocator> {
4225 ptr: NonNull<ArcInner<T>>,
4226 layout_for_value: Layout,
4227 alloc: Option<A>,
4228}
4229
4230impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
4231 /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it.
4232 #[cfg(not(no_global_oom_handling))]
4233 fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
4234 let layout = Layout::for_value(for_value);
4235 let ptr = unsafe {
4236 Arc::allocate_for_layout(
4237 layout,
4238 |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
4239 |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
4240 )
4241 };
4242 Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
4243 }
4244
4245 /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it,
4246 /// returning an error if allocation fails.
4247 fn try_new(for_value: &T, alloc: A) -> Result<UniqueArcUninit<T, A>, AllocError> {
4248 let layout = Layout::for_value(for_value);
4249 let ptr = unsafe {
4250 Arc::try_allocate_for_layout(
4251 layout,
4252 |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
4253 |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
4254 )?
4255 };
4256 Ok(Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) })
4257 }
4258
4259 /// Returns the pointer to be written into to initialize the [`Arc`].
4260 fn data_ptr(&mut self) -> *mut T {
4261 let offset = data_offset_alignment(self.layout_for_value.alignment());
4262 unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
4263 }
4264
4265 /// Upgrade this into a normal [`Arc`].
4266 ///
4267 /// # Safety
4268 ///
4269 /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
4270 unsafe fn into_arc(self) -> Arc<T, A> {
4271 let mut this = ManuallyDrop::new(self);
4272 let ptr = this.ptr.as_ptr();
4273 let alloc = this.alloc.take().unwrap();
4274
4275 // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
4276 // for having initialized the data.
4277 unsafe { Arc::from_ptr_in(ptr, alloc) }
4278 }
4279}
4280
4281#[cfg(not(no_global_oom_handling))]
4282impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
4283 fn drop(&mut self) {
4284 // SAFETY:
4285 // * new() produced a pointer safe to deallocate.
4286 // * We own the pointer unless into_arc() was called, which forgets us.
4287 unsafe {
4288 self.alloc.take().unwrap().deallocate(
4289 self.ptr.cast(),
4290 arcinner_layout_for_value_layout(self.layout_for_value),
4291 );
4292 }
4293 }
4294}
4295
4296#[stable(feature = "arc_error", since = "1.52.0")]
4297impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
4298 #[allow(deprecated)]
4299 fn cause(&self) -> Option<&dyn core::error::Error> {
4300 core::error::Error::cause(&**self)
4301 }
4302
4303 fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
4304 core::error::Error::source(&**self)
4305 }
4306
4307 fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
4308 core::error::Error::provide(&**self, req);
4309 }
4310}
4311
4312/// A uniquely owned [`Arc`].
4313///
4314/// This represents an `Arc` that is known to be uniquely owned -- that is, have exactly one strong
4315/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong
4316/// references will fail unless the `UniqueArc` they point to has been converted into a regular `Arc`.
4317///
4318/// Because it is uniquely owned, the contents of a `UniqueArc` can be freely mutated. A common
4319/// use case is to have an object be mutable during its initialization phase but then have it become
4320/// immutable and converted to a normal `Arc`.
4321///
4322/// This can be used as a flexible way to create cyclic data structures, as in the example below.
4323///
4324/// ```
4325/// #![feature(unique_rc_arc)]
4326/// use std::sync::{Arc, Weak, UniqueArc};
4327///
4328/// struct Gadget {
4329/// me: Weak<Gadget>,
4330/// }
4331///
4332/// fn create_gadget() -> Option<Arc<Gadget>> {
4333/// let mut rc = UniqueArc::new(Gadget {
4334/// me: Weak::new(),
4335/// });
4336/// rc.me = UniqueArc::downgrade(&rc);
4337/// Some(UniqueArc::into_arc(rc))
4338/// }
4339///
4340/// create_gadget().unwrap();
4341/// ```
4342///
4343/// An advantage of using `UniqueArc` over [`Arc::new_cyclic`] to build cyclic data structures is that
4344/// [`Arc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the
4345/// previous example, `UniqueArc` allows for more flexibility in the construction of cyclic data,
4346/// including fallible or async constructors.
4347#[unstable(feature = "unique_rc_arc", issue = "112566")]
4348pub struct UniqueArc<
4349 T: ?Sized,
4350 #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
4351> {
4352 ptr: NonNull<ArcInner<T>>,
4353 // Define the ownership of `ArcInner<T>` for drop-check
4354 _marker: PhantomData<ArcInner<T>>,
4355 // Invariance is necessary for soundness: once other `Weak`
4356 // references exist, we already have a form of shared mutability!
4357 _marker2: PhantomData<*mut T>,
4358 alloc: A,
4359}
4360
4361#[unstable(feature = "unique_rc_arc", issue = "112566")]
4362unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for UniqueArc<T, A> {}
4363
4364#[unstable(feature = "unique_rc_arc", issue = "112566")]
4365unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for UniqueArc<T, A> {}
4366
4367#[unstable(feature = "unique_rc_arc", issue = "112566")]
4368// #[unstable(feature = "coerce_unsized", issue = "18598")]
4369impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueArc<U, A>>
4370 for UniqueArc<T, A>
4371{
4372}
4373
4374//#[unstable(feature = "unique_rc_arc", issue = "112566")]
4375#[unstable(feature = "dispatch_from_dyn", issue = "none")]
4376impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueArc<U>> for UniqueArc<T> {}
4377
4378#[unstable(feature = "unique_rc_arc", issue = "112566")]
4379impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for UniqueArc<T, A> {
4380 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4381 fmt::Display::fmt(&**self, f)
4382 }
4383}
4384
4385#[unstable(feature = "unique_rc_arc", issue = "112566")]
4386impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for UniqueArc<T, A> {
4387 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4388 fmt::Debug::fmt(&**self, f)
4389 }
4390}
4391
4392#[unstable(feature = "unique_rc_arc", issue = "112566")]
4393impl<T: ?Sized, A: Allocator> fmt::Pointer for UniqueArc<T, A> {
4394 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4395 fmt::Pointer::fmt(&(&raw const **self), f)
4396 }
4397}
4398
4399#[unstable(feature = "unique_rc_arc", issue = "112566")]
4400impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for UniqueArc<T, A> {
4401 fn borrow(&self) -> &T {
4402 &**self
4403 }
4404}
4405
4406#[unstable(feature = "unique_rc_arc", issue = "112566")]
4407impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for UniqueArc<T, A> {
4408 fn borrow_mut(&mut self) -> &mut T {
4409 &mut **self
4410 }
4411}
4412
4413#[unstable(feature = "unique_rc_arc", issue = "112566")]
4414impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueArc<T, A> {
4415 fn as_ref(&self) -> &T {
4416 &**self
4417 }
4418}
4419
4420#[unstable(feature = "unique_rc_arc", issue = "112566")]
4421impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueArc<T, A> {
4422 fn as_mut(&mut self) -> &mut T {
4423 &mut **self
4424 }
4425}
4426
4427#[unstable(feature = "unique_rc_arc", issue = "112566")]
4428impl<T: ?Sized, A: Allocator> Unpin for UniqueArc<T, A> {}
4429
4430#[unstable(feature = "unique_rc_arc", issue = "112566")]
4431impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueArc<T, A> {
4432 /// Equality for two `UniqueArc`s.
4433 ///
4434 /// Two `UniqueArc`s are equal if their inner values are equal.
4435 ///
4436 /// # Examples
4437 ///
4438 /// ```
4439 /// #![feature(unique_rc_arc)]
4440 /// use std::sync::UniqueArc;
4441 ///
4442 /// let five = UniqueArc::new(5);
4443 ///
4444 /// assert!(five == UniqueArc::new(5));
4445 /// ```
4446 #[inline]
4447 fn eq(&self, other: &Self) -> bool {
4448 PartialEq::eq(&**self, &**other)
4449 }
4450}
4451
4452#[unstable(feature = "unique_rc_arc", issue = "112566")]
4453impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueArc<T, A> {
4454 /// Partial comparison for two `UniqueArc`s.
4455 ///
4456 /// The two are compared by calling `partial_cmp()` on their inner values.
4457 ///
4458 /// # Examples
4459 ///
4460 /// ```
4461 /// #![feature(unique_rc_arc)]
4462 /// use std::sync::UniqueArc;
4463 /// use std::cmp::Ordering;
4464 ///
4465 /// let five = UniqueArc::new(5);
4466 ///
4467 /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueArc::new(6)));
4468 /// ```
4469 #[inline(always)]
4470 fn partial_cmp(&self, other: &UniqueArc<T, A>) -> Option<Ordering> {
4471 (**self).partial_cmp(&**other)
4472 }
4473
4474 /// Less-than comparison for two `UniqueArc`s.
4475 ///
4476 /// The two are compared by calling `<` on their inner values.
4477 ///
4478 /// # Examples
4479 ///
4480 /// ```
4481 /// #![feature(unique_rc_arc)]
4482 /// use std::sync::UniqueArc;
4483 ///
4484 /// let five = UniqueArc::new(5);
4485 ///
4486 /// assert!(five < UniqueArc::new(6));
4487 /// ```
4488 #[inline(always)]
4489 fn lt(&self, other: &UniqueArc<T, A>) -> bool {
4490 **self < **other
4491 }
4492
4493 /// 'Less than or equal to' comparison for two `UniqueArc`s.
4494 ///
4495 /// The two are compared by calling `<=` on their inner values.
4496 ///
4497 /// # Examples
4498 ///
4499 /// ```
4500 /// #![feature(unique_rc_arc)]
4501 /// use std::sync::UniqueArc;
4502 ///
4503 /// let five = UniqueArc::new(5);
4504 ///
4505 /// assert!(five <= UniqueArc::new(5));
4506 /// ```
4507 #[inline(always)]
4508 fn le(&self, other: &UniqueArc<T, A>) -> bool {
4509 **self <= **other
4510 }
4511
4512 /// Greater-than comparison for two `UniqueArc`s.
4513 ///
4514 /// The two are compared by calling `>` on their inner values.
4515 ///
4516 /// # Examples
4517 ///
4518 /// ```
4519 /// #![feature(unique_rc_arc)]
4520 /// use std::sync::UniqueArc;
4521 ///
4522 /// let five = UniqueArc::new(5);
4523 ///
4524 /// assert!(five > UniqueArc::new(4));
4525 /// ```
4526 #[inline(always)]
4527 fn gt(&self, other: &UniqueArc<T, A>) -> bool {
4528 **self > **other
4529 }
4530
4531 /// 'Greater than or equal to' comparison for two `UniqueArc`s.
4532 ///
4533 /// The two are compared by calling `>=` on their inner values.
4534 ///
4535 /// # Examples
4536 ///
4537 /// ```
4538 /// #![feature(unique_rc_arc)]
4539 /// use std::sync::UniqueArc;
4540 ///
4541 /// let five = UniqueArc::new(5);
4542 ///
4543 /// assert!(five >= UniqueArc::new(5));
4544 /// ```
4545 #[inline(always)]
4546 fn ge(&self, other: &UniqueArc<T, A>) -> bool {
4547 **self >= **other
4548 }
4549}
4550
4551#[unstable(feature = "unique_rc_arc", issue = "112566")]
4552impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueArc<T, A> {
4553 /// Comparison for two `UniqueArc`s.
4554 ///
4555 /// The two are compared by calling `cmp()` on their inner values.
4556 ///
4557 /// # Examples
4558 ///
4559 /// ```
4560 /// #![feature(unique_rc_arc)]
4561 /// use std::sync::UniqueArc;
4562 /// use std::cmp::Ordering;
4563 ///
4564 /// let five = UniqueArc::new(5);
4565 ///
4566 /// assert_eq!(Ordering::Less, five.cmp(&UniqueArc::new(6)));
4567 /// ```
4568 #[inline]
4569 fn cmp(&self, other: &UniqueArc<T, A>) -> Ordering {
4570 (**self).cmp(&**other)
4571 }
4572}
4573
4574#[unstable(feature = "unique_rc_arc", issue = "112566")]
4575impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueArc<T, A> {}
4576
4577#[unstable(feature = "unique_rc_arc", issue = "112566")]
4578impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueArc<T, A> {
4579 fn hash<H: Hasher>(&self, state: &mut H) {
4580 (**self).hash(state);
4581 }
4582}
4583
4584impl<T> UniqueArc<T, Global> {
4585 /// Creates a new `UniqueArc`.
4586 ///
4587 /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4588 /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4589 /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4590 /// point to the new [`Arc`].
4591 #[cfg(not(no_global_oom_handling))]
4592 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4593 #[must_use]
4594 pub fn new(value: T) -> Self {
4595 Self::new_in(value, Global)
4596 }
4597
4598 /// Maps the value in a `UniqueArc`, reusing the allocation if possible.
4599 ///
4600 /// `f` is called on a reference to the value in the `UniqueArc`, and the result is returned,
4601 /// also in a `UniqueArc`.
4602 ///
4603 /// Note: this is an associated function, which means that you have
4604 /// to call it as `UniqueArc::map(u, f)` instead of `u.map(f)`. This
4605 /// is so that there is no conflict with a method on the inner type.
4606 ///
4607 /// # Examples
4608 ///
4609 /// ```
4610 /// #![feature(smart_pointer_try_map)]
4611 /// #![feature(unique_rc_arc)]
4612 ///
4613 /// use std::sync::UniqueArc;
4614 ///
4615 /// let r = UniqueArc::new(7);
4616 /// let new = UniqueArc::map(r, |i| i + 7);
4617 /// assert_eq!(*new, 14);
4618 /// ```
4619 #[cfg(not(no_global_oom_handling))]
4620 #[unstable(feature = "smart_pointer_try_map", issue = "144419")]
4621 pub fn map<U>(this: Self, f: impl FnOnce(T) -> U) -> UniqueArc<U> {
4622 if size_of::<T>() == size_of::<U>()
4623 && align_of::<T>() == align_of::<U>()
4624 && UniqueArc::weak_count(&this) == 0
4625 {
4626 unsafe {
4627 let ptr = UniqueArc::into_raw(this);
4628 let value = ptr.read();
4629 let mut allocation = UniqueArc::from_raw(ptr.cast::<mem::MaybeUninit<U>>());
4630
4631 allocation.write(f(value));
4632 allocation.assume_init()
4633 }
4634 } else {
4635 UniqueArc::new(f(UniqueArc::unwrap(this)))
4636 }
4637 }
4638
4639 /// Attempts to map the value in a `UniqueArc`, reusing the allocation if possible.
4640 ///
4641 /// `f` is called on a reference to the value in the `UniqueArc`, and if the operation succeeds,
4642 /// the result is returned, also in a `UniqueArc`.
4643 ///
4644 /// Note: this is an associated function, which means that you have
4645 /// to call it as `UniqueArc::try_map(u, f)` instead of `u.try_map(f)`. This
4646 /// is so that there is no conflict with a method on the inner type.
4647 ///
4648 /// # Examples
4649 ///
4650 /// ```
4651 /// #![feature(smart_pointer_try_map)]
4652 /// #![feature(unique_rc_arc)]
4653 ///
4654 /// use std::sync::UniqueArc;
4655 ///
4656 /// let b = UniqueArc::new(7);
4657 /// let new = UniqueArc::try_map(b, u32::try_from).unwrap();
4658 /// assert_eq!(*new, 7);
4659 /// ```
4660 #[cfg(not(no_global_oom_handling))]
4661 #[unstable(feature = "smart_pointer_try_map", issue = "144419")]
4662 pub fn try_map<R>(
4663 this: Self,
4664 f: impl FnOnce(T) -> R,
4665 ) -> <R::Residual as Residual<UniqueArc<R::Output>>>::TryType
4666 where
4667 R: Try,
4668 R::Residual: Residual<UniqueArc<R::Output>>,
4669 {
4670 if size_of::<T>() == size_of::<R::Output>()
4671 && align_of::<T>() == align_of::<R::Output>()
4672 && UniqueArc::weak_count(&this) == 0
4673 {
4674 unsafe {
4675 let ptr = UniqueArc::into_raw(this);
4676 let value = ptr.read();
4677 let mut allocation = UniqueArc::from_raw(ptr.cast::<mem::MaybeUninit<R::Output>>());
4678
4679 allocation.write(f(value)?);
4680 try { allocation.assume_init() }
4681 }
4682 } else {
4683 try { UniqueArc::new(f(UniqueArc::unwrap(this))?) }
4684 }
4685 }
4686
4687 #[cfg(not(no_global_oom_handling))]
4688 fn unwrap(this: Self) -> T {
4689 let this = ManuallyDrop::new(this);
4690 let val: T = unsafe { ptr::read(&**this) };
4691
4692 let _weak = Weak { ptr: this.ptr, alloc: Global };
4693
4694 val
4695 }
4696}
4697
4698impl<T: ?Sized> UniqueArc<T> {
4699 #[cfg(not(no_global_oom_handling))]
4700 unsafe fn from_raw(ptr: *const T) -> Self {
4701 let offset: usize = unsafe { data_offset(ptr) };
4702
4703 // Reverse the offset to find the original ArcInner.
4704 let rc_ptr: *mut ArcInner = unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> };
4705
4706 Self {
4707 ptr: unsafe { NonNull::new_unchecked(rc_ptr) },
4708 _marker: PhantomData,
4709 _marker2: PhantomData,
4710 alloc: Global,
4711 }
4712 }
4713
4714 #[cfg(not(no_global_oom_handling))]
4715 fn into_raw(this: Self) -> *const T {
4716 let this = ManuallyDrop::new(this);
4717 Self::as_ptr(&*this)
4718 }
4719}
4720
4721impl<T, A: Allocator> UniqueArc<T, A> {
4722 /// Creates a new `UniqueArc` in the provided allocator.
4723 ///
4724 /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4725 /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4726 /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4727 /// point to the new [`Arc`].
4728 #[cfg(not(no_global_oom_handling))]
4729 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4730 #[must_use]
4731 // #[unstable(feature = "allocator_api", issue = "32838")]
4732 pub fn new_in(data: T, alloc: A) -> Self {
4733 let (ptr, alloc) = Box::into_unique(Box::new_in(
4734 ArcInner {
4735 strong: atomic::AtomicUsize::new(0),
4736 // keep one weak reference so if all the weak pointers that are created are dropped
4737 // the UniqueArc still stays valid.
4738 weak: atomic::AtomicUsize::new(1),
4739 data,
4740 },
4741 alloc,
4742 ));
4743 Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
4744 }
4745}
4746
4747impl<T: ?Sized, A: Allocator> UniqueArc<T, A> {
4748 /// Converts the `UniqueArc` into a regular [`Arc`].
4749 ///
4750 /// This consumes the `UniqueArc` and returns a regular [`Arc`] that contains the `value` that
4751 /// is passed to `into_arc`.
4752 ///
4753 /// Any weak references created before this method is called can now be upgraded to strong
4754 /// references.
4755 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4756 #[must_use]
4757 pub fn into_arc(this: Self) -> Arc<T, A> {
4758 let this = ManuallyDrop::new(this);
4759
4760 // Move the allocator out.
4761 // SAFETY: `this.alloc` will not be accessed again, nor dropped because it is in
4762 // a `ManuallyDrop`.
4763 let alloc: A = unsafe { ptr::read(&this.alloc) };
4764
4765 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4766 unsafe {
4767 // Convert our weak reference into a strong reference
4768 (*this.ptr.as_ptr()).strong.store(1, Release);
4769 Arc::from_inner_in(this.ptr, alloc)
4770 }
4771 }
4772
4773 #[cfg(not(no_global_oom_handling))]
4774 fn weak_count(this: &Self) -> usize {
4775 this.inner().weak.load(Acquire) - 1
4776 }
4777
4778 #[cfg(not(no_global_oom_handling))]
4779 fn inner(&self) -> &ArcInner<T> {
4780 // SAFETY: while this UniqueArc is alive we're guaranteed that the inner pointer is valid.
4781 unsafe { self.ptr.as_ref() }
4782 }
4783
4784 #[cfg(not(no_global_oom_handling))]
4785 fn as_ptr(this: &Self) -> *const T {
4786 let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
4787
4788 // SAFETY: This cannot go through Deref::deref or UniqueArc::inner because
4789 // this is required to retain raw/mut provenance such that e.g. `get_mut` can
4790 // write through the pointer after the Rc is recovered through `from_raw`.
4791 unsafe { &raw mut (*ptr).data }
4792 }
4793
4794 #[inline]
4795 #[cfg(not(no_global_oom_handling))]
4796 fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
4797 let this = mem::ManuallyDrop::new(this);
4798 (this.ptr, unsafe { ptr::read(&this.alloc) })
4799 }
4800
4801 #[inline]
4802 #[cfg(not(no_global_oom_handling))]
4803 unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
4804 Self { ptr, _marker: PhantomData, _marker2: PhantomData, alloc }
4805 }
4806}
4807
4808impl<T: ?Sized, A: Allocator + Clone> UniqueArc<T, A> {
4809 /// Creates a new weak reference to the `UniqueArc`.
4810 ///
4811 /// Attempting to upgrade this weak reference will fail before the `UniqueArc` has been converted
4812 /// to a [`Arc`] using [`UniqueArc::into_arc`].
4813 #[unstable(feature = "unique_rc_arc", issue = "112566")]
4814 #[must_use]
4815 pub fn downgrade(this: &Self) -> Weak<T, A> {
4816 // Using a relaxed ordering is alright here, as knowledge of the
4817 // original reference prevents other threads from erroneously deleting
4818 // the object or converting the object to a normal `Arc<T, A>`.
4819 //
4820 // Note that we don't need to test if the weak counter is locked because there
4821 // are no such operations like `Arc::get_mut` or `Arc::make_mut` that will lock
4822 // the weak counter.
4823 //
4824 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4825 let old_size = unsafe { (*this.ptr.as_ptr()).weak.fetch_add(1, Relaxed) };
4826
4827 // See comments in Arc::clone() for why we do this (for mem::forget).
4828 if old_size > MAX_REFCOUNT {
4829 abort();
4830 }
4831
4832 Weak { ptr: this.ptr, alloc: this.alloc.clone() }
4833 }
4834}
4835
4836#[cfg(not(no_global_oom_handling))]
4837impl<T, A: Allocator> UniqueArc<mem::MaybeUninit<T>, A> {
4838 unsafe fn assume_init(self) -> UniqueArc<T, A> {
4839 let (ptr, alloc: A) = UniqueArc::into_inner_with_allocator(self);
4840 unsafe { UniqueArc::from_inner_in(ptr.cast(), alloc) }
4841 }
4842}
4843
4844#[unstable(feature = "unique_rc_arc", issue = "112566")]
4845impl<T: ?Sized, A: Allocator> Deref for UniqueArc<T, A> {
4846 type Target = T;
4847
4848 fn deref(&self) -> &T {
4849 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4850 unsafe { &self.ptr.as_ref().data }
4851 }
4852}
4853
4854// #[unstable(feature = "unique_rc_arc", issue = "112566")]
4855#[unstable(feature = "pin_coerce_unsized_trait", issue = "150112")]
4856unsafe impl<T: ?Sized> PinCoerceUnsized for UniqueArc<T> {}
4857
4858#[unstable(feature = "unique_rc_arc", issue = "112566")]
4859impl<T: ?Sized, A: Allocator> DerefMut for UniqueArc<T, A> {
4860 fn deref_mut(&mut self) -> &mut T {
4861 // SAFETY: This pointer was allocated at creation time so we know it is valid. We know we
4862 // have unique ownership and therefore it's safe to make a mutable reference because
4863 // `UniqueArc` owns the only strong reference to itself.
4864 // We also need to be careful to only create a mutable reference to the `data` field,
4865 // as a mutable reference to the entire `ArcInner` would assert uniqueness over the
4866 // ref count fields too, invalidating any attempt by `Weak`s to access the ref count.
4867 unsafe { &mut (*self.ptr.as_ptr()).data }
4868 }
4869}
4870
4871#[unstable(feature = "unique_rc_arc", issue = "112566")]
4872// #[unstable(feature = "deref_pure_trait", issue = "87121")]
4873unsafe impl<T: ?Sized, A: Allocator> DerefPure for UniqueArc<T, A> {}
4874
4875#[unstable(feature = "unique_rc_arc", issue = "112566")]
4876unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueArc<T, A> {
4877 fn drop(&mut self) {
4878 // See `Arc::drop_slow` which drops an `Arc` with a strong count of 0.
4879 // SAFETY: This pointer was allocated at creation time so we know it is valid.
4880 let _weak: Weak<{unknown}, &A> = Weak { ptr: self.ptr, alloc: &self.alloc };
4881
4882 unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
4883 }
4884}
4885
4886#[unstable(feature = "allocator_api", issue = "32838")]
4887unsafe impl<T: ?Sized + Allocator, A: Allocator> Allocator for Arc<T, A> {
4888 #[inline]
4889 fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
4890 (**self).allocate(layout)
4891 }
4892
4893 #[inline]
4894 fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
4895 (**self).allocate_zeroed(layout)
4896 }
4897
4898 #[inline]
4899 unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
4900 // SAFETY: the safety contract must be upheld by the caller
4901 unsafe { (**self).deallocate(ptr, layout) }
4902 }
4903
4904 #[inline]
4905 unsafe fn grow(
4906 &self,
4907 ptr: NonNull<u8>,
4908 old_layout: Layout,
4909 new_layout: Layout,
4910 ) -> Result<NonNull<[u8]>, AllocError> {
4911 // SAFETY: the safety contract must be upheld by the caller
4912 unsafe { (**self).grow(ptr, old_layout, new_layout) }
4913 }
4914
4915 #[inline]
4916 unsafe fn grow_zeroed(
4917 &self,
4918 ptr: NonNull<u8>,
4919 old_layout: Layout,
4920 new_layout: Layout,
4921 ) -> Result<NonNull<[u8]>, AllocError> {
4922 // SAFETY: the safety contract must be upheld by the caller
4923 unsafe { (**self).grow_zeroed(ptr, old_layout, new_layout) }
4924 }
4925
4926 #[inline]
4927 unsafe fn shrink(
4928 &self,
4929 ptr: NonNull<u8>,
4930 old_layout: Layout,
4931 new_layout: Layout,
4932 ) -> Result<NonNull<[u8]>, AllocError> {
4933 // SAFETY: the safety contract must be upheld by the caller
4934 unsafe { (**self).shrink(ptr, old_layout, new_layout) }
4935 }
4936}
4937