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alloc/
sync.rs

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;
12#[cfg(not(no_global_oom_handling))]
13use core::clone::CloneToUninit;
14use core::clone::UseCloned;
15use core::cmp::Ordering;
16use core::hash::{Hash, Hasher};
17use core::intrinsics::abort;
18#[cfg(not(no_global_oom_handling))]
19use core::iter;
20use core::marker::{PhantomData, Unsize};
21use core::mem::{self, ManuallyDrop, align_of_val_raw};
22use core::num::NonZeroUsize;
23use core::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn, LegacyReceiver};
24use core::panic::{RefUnwindSafe, UnwindSafe};
25use core::pin::{Pin, PinCoerceUnsized};
26use core::ptr::{self, NonNull};
27#[cfg(not(no_global_oom_handling))]
28use core::slice::from_raw_parts_mut;
29use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
30use core::sync::atomic::{self, Atomic};
31use core::{borrow, fmt, hint};
32
33#[cfg(not(no_global_oom_handling))]
34use crate::alloc::handle_alloc_error;
35use crate::alloc::{AllocError, Allocator, Global, Layout};
36use crate::borrow::{Cow, ToOwned};
37use crate::boxed::Box;
38use crate::rc::is_dangling;
39#[cfg(not(no_global_oom_handling))]
40use crate::string::String;
41#[cfg(not(no_global_oom_handling))]
42use crate::vec::Vec;
43
44/// A soft limit on the amount of references that may be made to an `Arc`.
45///
46/// Going above this limit will abort your program (although not
47/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
48/// Trying to go above it might call a `panic` (if not actually going above it).
49///
50/// This is a global invariant, and also applies when using a compare-exchange loop.
51///
52/// See comment in `Arc::clone`.
53const MAX_REFCOUNT: usize = (isize::MAX) as usize;
54
55/// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
56const INTERNAL_OVERFLOW_ERROR: &str = "Arc counter overflow";
57
58#[cfg(not(sanitize = "thread"))]
59macro_rules! acquire {
60    ($x:expr) => {
61        atomic::fence(Acquire)
62    };
63}
64
65// ThreadSanitizer does not support memory fences. To avoid false positive
66// reports in Arc / Weak implementation use atomic loads for synchronization
67// instead.
68#[cfg(sanitize = "thread")]
69macro_rules! acquire {
70    ($x:expr) => {
71        $x.load(Acquire)
72    };
73}
74
75/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
76/// Reference Counted'.
77///
78/// The type `Arc<T>` provides shared ownership of a value of type `T`,
79/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
80/// a new `Arc` instance, which points to the same allocation on the heap as the
81/// source `Arc`, while increasing a reference count. When the last `Arc`
82/// pointer to a given allocation is destroyed, the value stored in that allocation (often
83/// referred to as "inner value") is also dropped.
84///
85/// Shared references in Rust disallow mutation by default, and `Arc` is no
86/// exception: you cannot generally obtain a mutable reference to something
87/// inside an `Arc`. If you do need to mutate through an `Arc`, you have several options:
88///
89/// 1. Use interior mutability with synchronization primitives like [`Mutex`][mutex],
90///    [`RwLock`][rwlock], or one of the [`Atomic`][atomic] types.
91///
92/// 2. Use clone-on-write semantics with [`Arc::make_mut`] which provides efficient mutation
93///    without requiring interior mutability. This approach clones the data only when
94///    needed (when there are multiple references) and can be more efficient when mutations
95///    are infrequent.
96///
97/// 3. Use [`Arc::get_mut`] when you know your `Arc` is not shared (has a reference count of 1),
98///    which provides direct mutable access to the inner value without any cloning.
99///
100/// ```
101/// use std::sync::Arc;
102///
103/// let mut data = Arc::new(vec![1, 2, 3]);
104///
105/// // This will clone the vector only if there are other references to it
106/// Arc::make_mut(&mut data).push(4);
107///
108/// assert_eq!(*data, vec![1, 2, 3, 4]);
109/// ```
110///
111/// **Note**: This type is only available on platforms that support atomic
112/// loads and stores of pointers, which includes all platforms that support
113/// the `std` crate but not all those which only support [`alloc`](crate).
114/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
115///
116/// ## Thread Safety
117///
118/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
119/// counting. This means that it is thread-safe. The disadvantage is that
120/// atomic operations are more expensive than ordinary memory accesses. If you
121/// are not sharing reference-counted allocations between threads, consider using
122/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
123/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
124/// However, a library might choose `Arc<T>` in order to give library consumers
125/// more flexibility.
126///
127/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
128/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
129/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
130/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
131/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
132/// data, but it  doesn't add thread safety to its data. Consider
133/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
134/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
135/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
136/// non-atomic operations.
137///
138/// In the end, this means that you may need to pair `Arc<T>` with some sort of
139/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
140///
141/// ## Breaking cycles with `Weak`
142///
143/// The [`downgrade`][downgrade] method can be used to create a non-owning
144/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
145/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
146/// already been dropped. In other words, `Weak` pointers do not keep the value
147/// inside the allocation alive; however, they *do* keep the allocation
148/// (the backing store for the value) alive.
149///
150/// A cycle between `Arc` pointers will never be deallocated. For this reason,
151/// [`Weak`] is used to break cycles. For example, a tree could have
152/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
153/// pointers from children back to their parents.
154///
155/// # Cloning references
156///
157/// Creating a new reference from an existing reference-counted pointer is done using the
158/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
159///
160/// ```
161/// use std::sync::Arc;
162/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
163/// // The two syntaxes below are equivalent.
164/// let a = foo.clone();
165/// let b = Arc::clone(&foo);
166/// // a, b, and foo are all Arcs that point to the same memory location
167/// ```
168///
169/// ## `Deref` behavior
170///
171/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
172/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
173/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
174/// functions, called using [fully qualified syntax]:
175///
176/// ```
177/// use std::sync::Arc;
178///
179/// let my_arc = Arc::new(());
180/// let my_weak = Arc::downgrade(&my_arc);
181/// ```
182///
183/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
184/// fully qualified syntax. Some people prefer to use fully qualified syntax,
185/// while others prefer using method-call syntax.
186///
187/// ```
188/// use std::sync::Arc;
189///
190/// let arc = Arc::new(());
191/// // Method-call syntax
192/// let arc2 = arc.clone();
193/// // Fully qualified syntax
194/// let arc3 = Arc::clone(&arc);
195/// ```
196///
197/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
198/// already been dropped.
199///
200/// [`Rc<T>`]: crate::rc::Rc
201/// [clone]: Clone::clone
202/// [mutex]: ../../std/sync/struct.Mutex.html
203/// [rwlock]: ../../std/sync/struct.RwLock.html
204/// [atomic]: core::sync::atomic
205/// [downgrade]: Arc::downgrade
206/// [upgrade]: Weak::upgrade
207/// [RefCell\<T>]: core::cell::RefCell
208/// [`RefCell<T>`]: core::cell::RefCell
209/// [`std::sync`]: ../../std/sync/index.html
210/// [`Arc::clone(&from)`]: Arc::clone
211/// [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
212///
213/// # Examples
214///
215/// Sharing some immutable data between threads:
216///
217/// ```
218/// use std::sync::Arc;
219/// use std::thread;
220///
221/// let five = Arc::new(5);
222///
223/// for _ in 0..10 {
224///     let five = Arc::clone(&five);
225///
226///     thread::spawn(move || {
227///         println!("{five:?}");
228///     });
229/// }
230/// ```
231///
232/// Sharing a mutable [`AtomicUsize`]:
233///
234/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
235///
236/// ```
237/// use std::sync::Arc;
238/// use std::sync::atomic::{AtomicUsize, Ordering};
239/// use std::thread;
240///
241/// let val = Arc::new(AtomicUsize::new(5));
242///
243/// for _ in 0..10 {
244///     let val = Arc::clone(&val);
245///
246///     thread::spawn(move || {
247///         let v = val.fetch_add(1, Ordering::Relaxed);
248///         println!("{v:?}");
249///     });
250/// }
251/// ```
252///
253/// See the [`rc` documentation][rc_examples] for more examples of reference
254/// counting in general.
255///
256/// [rc_examples]: crate::rc#examples
257#[doc(search_unbox)]
258#[rustc_diagnostic_item = "Arc"]
259#[stable(feature = "rust1", since = "1.0.0")]
260#[rustc_insignificant_dtor]
261pub struct Arc<
262    T: ?Sized,
263    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
264> {
265    ptr: NonNull<ArcInner<T>>,
266    phantom: PhantomData<ArcInner<T>>,
267    alloc: A,
268}
269
270#[stable(feature = "rust1", since = "1.0.0")]
271unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Arc<T, A> {}
272#[stable(feature = "rust1", since = "1.0.0")]
273unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Arc<T, A> {}
274
275#[stable(feature = "catch_unwind", since = "1.9.0")]
276impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A> {}
277
278#[unstable(feature = "coerce_unsized", issue = "18598")]
279impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> for Arc<T, A> {}
280
281#[unstable(feature = "dispatch_from_dyn", issue = "none")]
282impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
283
284impl<T: ?Sized> Arc<T> {
285    unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
286        unsafe { Self::from_inner_in(ptr, Global) }
287    }
288
289    unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
290        unsafe { Self::from_ptr_in(ptr, Global) }
291    }
292}
293
294impl<T: ?Sized, A: Allocator> Arc<T, A> {
295    #[inline]
296    fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
297        let this = mem::ManuallyDrop::new(this);
298        (this.ptr, unsafe { ptr::read(&this.alloc) })
299    }
300
301    #[inline]
302    unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
303        Self { ptr, phantom: PhantomData, alloc }
304    }
305
306    #[inline]
307    unsafe fn from_ptr_in(ptr: *mut ArcInner<T>, alloc: A) -> Self {
308        unsafe { Self::from_inner_in(NonNull::new_unchecked(ptr), alloc) }
309    }
310}
311
312/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
313/// managed allocation.
314///
315/// The allocation is accessed by calling [`upgrade`] on the `Weak`
316/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
317///
318/// Since a `Weak` reference does not count towards ownership, it will not
319/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
320/// guarantees about the value still being present. Thus it may return [`None`]
321/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
322/// itself (the backing store) from being deallocated.
323///
324/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
325/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
326/// prevent circular references between [`Arc`] pointers, since mutual owning references
327/// would never allow either [`Arc`] to be dropped. For example, a tree could
328/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
329/// pointers from children back to their parents.
330///
331/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
332///
333/// [`upgrade`]: Weak::upgrade
334#[stable(feature = "arc_weak", since = "1.4.0")]
335#[rustc_diagnostic_item = "ArcWeak"]
336pub struct Weak<
337    T: ?Sized,
338    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
339> {
340    // This is a `NonNull` to allow optimizing the size of this type in enums,
341    // but it is not necessarily a valid pointer.
342    // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
343    // to allocate space on the heap. That's not a value a real pointer
344    // will ever have because RcInner has alignment at least 2.
345    ptr: NonNull<ArcInner<T>>,
346    alloc: A,
347}
348
349#[stable(feature = "arc_weak", since = "1.4.0")]
350unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A> {}
351#[stable(feature = "arc_weak", since = "1.4.0")]
352unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A> {}
353
354#[unstable(feature = "coerce_unsized", issue = "18598")]
355impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {}
356#[unstable(feature = "dispatch_from_dyn", issue = "none")]
357impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
358
359#[stable(feature = "arc_weak", since = "1.4.0")]
360impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
361    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
362        write!(f, "(Weak)")
363    }
364}
365
366// This is repr(C) to future-proof against possible field-reordering, which
367// would interfere with otherwise safe [into|from]_raw() of transmutable
368// inner types.
369#[repr(C)]
370struct ArcInner<T: ?Sized> {
371    strong: Atomic<usize>,
372
373    // the value usize::MAX acts as a sentinel for temporarily "locking" the
374    // ability to upgrade weak pointers or downgrade strong ones; this is used
375    // to avoid races in `make_mut` and `get_mut`.
376    weak: Atomic<usize>,
377
378    data: T,
379}
380
381/// Calculate layout for `ArcInner<T>` using the inner value's layout
382fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
383    // Calculate layout using the given value layout.
384    // Previously, layout was calculated on the expression
385    // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
386    // reference (see #54908).
387    Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
388}
389
390unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
391unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
392
393impl<T> Arc<T> {
394    /// Constructs a new `Arc<T>`.
395    ///
396    /// # Examples
397    ///
398    /// ```
399    /// use std::sync::Arc;
400    ///
401    /// let five = Arc::new(5);
402    /// ```
403    #[cfg(not(no_global_oom_handling))]
404    #[inline]
405    #[stable(feature = "rust1", since = "1.0.0")]
406    pub fn new(data: T) -> Arc<T> {
407        // Start the weak pointer count as 1 which is the weak pointer that's
408        // held by all the strong pointers (kinda), see std/rc.rs for more info
409        let x: Box<_> = Box::new(ArcInner {
410            strong: atomic::AtomicUsize::new(1),
411            weak: atomic::AtomicUsize::new(1),
412            data,
413        });
414        unsafe { Self::from_inner(Box::leak(x).into()) }
415    }
416
417    /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
418    /// to allow you to construct a `T` which holds a weak pointer to itself.
419    ///
420    /// Generally, a structure circularly referencing itself, either directly or
421    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
422    /// Using this function, you get access to the weak pointer during the
423    /// initialization of `T`, before the `Arc<T>` is created, such that you can
424    /// clone and store it inside the `T`.
425    ///
426    /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
427    /// then calls your closure, giving it a `Weak<T>` to this allocation,
428    /// and only afterwards completes the construction of the `Arc<T>` by placing
429    /// the `T` returned from your closure into the allocation.
430    ///
431    /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
432    /// returns, calling [`upgrade`] on the weak reference inside your closure will
433    /// fail and result in a `None` value.
434    ///
435    /// # Panics
436    ///
437    /// If `data_fn` panics, the panic is propagated to the caller, and the
438    /// temporary [`Weak<T>`] is dropped normally.
439    ///
440    /// # Example
441    ///
442    /// ```
443    /// # #![allow(dead_code)]
444    /// use std::sync::{Arc, Weak};
445    ///
446    /// struct Gadget {
447    ///     me: Weak<Gadget>,
448    /// }
449    ///
450    /// impl Gadget {
451    ///     /// Constructs a reference counted Gadget.
452    ///     fn new() -> Arc<Self> {
453    ///         // `me` is a `Weak<Gadget>` pointing at the new allocation of the
454    ///         // `Arc` we're constructing.
455    ///         Arc::new_cyclic(|me| {
456    ///             // Create the actual struct here.
457    ///             Gadget { me: me.clone() }
458    ///         })
459    ///     }
460    ///
461    ///     /// Returns a reference counted pointer to Self.
462    ///     fn me(&self) -> Arc<Self> {
463    ///         self.me.upgrade().unwrap()
464    ///     }
465    /// }
466    /// ```
467    /// [`upgrade`]: Weak::upgrade
468    #[cfg(not(no_global_oom_handling))]
469    #[inline]
470    #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
471    pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
472    where
473        F: FnOnce(&Weak<T>) -> T,
474    {
475        Self::new_cyclic_in(data_fn, Global)
476    }
477
478    /// Constructs a new `Arc` with uninitialized contents.
479    ///
480    /// # Examples
481    ///
482    /// ```
483    /// use std::sync::Arc;
484    ///
485    /// let mut five = Arc::<u32>::new_uninit();
486    ///
487    /// // Deferred initialization:
488    /// Arc::get_mut(&mut five).unwrap().write(5);
489    ///
490    /// let five = unsafe { five.assume_init() };
491    ///
492    /// assert_eq!(*five, 5)
493    /// ```
494    #[cfg(not(no_global_oom_handling))]
495    #[inline]
496    #[stable(feature = "new_uninit", since = "1.82.0")]
497    #[must_use]
498    pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
499        unsafe {
500            Arc::from_ptr(Arc::allocate_for_layout(
501                Layout::new::<T>(),
502                |layout| Global.allocate(layout),
503                <*mut u8>::cast,
504            ))
505        }
506    }
507
508    /// Constructs a new `Arc` with uninitialized contents, with the memory
509    /// being filled with `0` bytes.
510    ///
511    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
512    /// of this method.
513    ///
514    /// # Examples
515    ///
516    /// ```
517    /// use std::sync::Arc;
518    ///
519    /// let zero = Arc::<u32>::new_zeroed();
520    /// let zero = unsafe { zero.assume_init() };
521    ///
522    /// assert_eq!(*zero, 0)
523    /// ```
524    ///
525    /// [zeroed]: mem::MaybeUninit::zeroed
526    #[cfg(not(no_global_oom_handling))]
527    #[inline]
528    #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
529    #[must_use]
530    pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
531        unsafe {
532            Arc::from_ptr(Arc::allocate_for_layout(
533                Layout::new::<T>(),
534                |layout| Global.allocate_zeroed(layout),
535                <*mut u8>::cast,
536            ))
537        }
538    }
539
540    /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
541    /// `data` will be pinned in memory and unable to be moved.
542    #[cfg(not(no_global_oom_handling))]
543    #[stable(feature = "pin", since = "1.33.0")]
544    #[must_use]
545    pub fn pin(data: T) -> Pin<Arc<T>> {
546        unsafe { Pin::new_unchecked(Arc::new(data)) }
547    }
548
549    /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
550    #[unstable(feature = "allocator_api", issue = "32838")]
551    #[inline]
552    pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
553        unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
554    }
555
556    /// Constructs a new `Arc<T>`, returning an error if allocation fails.
557    ///
558    /// # Examples
559    ///
560    /// ```
561    /// #![feature(allocator_api)]
562    /// use std::sync::Arc;
563    ///
564    /// let five = Arc::try_new(5)?;
565    /// # Ok::<(), std::alloc::AllocError>(())
566    /// ```
567    #[unstable(feature = "allocator_api", issue = "32838")]
568    #[inline]
569    pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
570        // Start the weak pointer count as 1 which is the weak pointer that's
571        // held by all the strong pointers (kinda), see std/rc.rs for more info
572        let x: Box<_> = Box::try_new(ArcInner {
573            strong: atomic::AtomicUsize::new(1),
574            weak: atomic::AtomicUsize::new(1),
575            data,
576        })?;
577        unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
578    }
579
580    /// Constructs a new `Arc` with uninitialized contents, returning an error
581    /// if allocation fails.
582    ///
583    /// # Examples
584    ///
585    /// ```
586    /// #![feature(allocator_api)]
587    ///
588    /// use std::sync::Arc;
589    ///
590    /// let mut five = Arc::<u32>::try_new_uninit()?;
591    ///
592    /// // Deferred initialization:
593    /// Arc::get_mut(&mut five).unwrap().write(5);
594    ///
595    /// let five = unsafe { five.assume_init() };
596    ///
597    /// assert_eq!(*five, 5);
598    /// # Ok::<(), std::alloc::AllocError>(())
599    /// ```
600    #[unstable(feature = "allocator_api", issue = "32838")]
601    pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
602        unsafe {
603            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
604                Layout::new::<T>(),
605                |layout| Global.allocate(layout),
606                <*mut u8>::cast,
607            )?))
608        }
609    }
610
611    /// Constructs a new `Arc` with uninitialized contents, with the memory
612    /// being filled with `0` bytes, returning an error if allocation fails.
613    ///
614    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
615    /// of this method.
616    ///
617    /// # Examples
618    ///
619    /// ```
620    /// #![feature( allocator_api)]
621    ///
622    /// use std::sync::Arc;
623    ///
624    /// let zero = Arc::<u32>::try_new_zeroed()?;
625    /// let zero = unsafe { zero.assume_init() };
626    ///
627    /// assert_eq!(*zero, 0);
628    /// # Ok::<(), std::alloc::AllocError>(())
629    /// ```
630    ///
631    /// [zeroed]: mem::MaybeUninit::zeroed
632    #[unstable(feature = "allocator_api", issue = "32838")]
633    pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
634        unsafe {
635            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
636                Layout::new::<T>(),
637                |layout| Global.allocate_zeroed(layout),
638                <*mut u8>::cast,
639            )?))
640        }
641    }
642}
643
644impl<T, A: Allocator> Arc<T, A> {
645    /// Constructs a new `Arc<T>` in the provided allocator.
646    ///
647    /// # Examples
648    ///
649    /// ```
650    /// #![feature(allocator_api)]
651    ///
652    /// use std::sync::Arc;
653    /// use std::alloc::System;
654    ///
655    /// let five = Arc::new_in(5, System);
656    /// ```
657    #[inline]
658    #[cfg(not(no_global_oom_handling))]
659    #[unstable(feature = "allocator_api", issue = "32838")]
660    pub fn new_in(data: T, alloc: A) -> Arc<T, A> {
661        // Start the weak pointer count as 1 which is the weak pointer that's
662        // held by all the strong pointers (kinda), see std/rc.rs for more info
663        let x = Box::new_in(
664            ArcInner {
665                strong: atomic::AtomicUsize::new(1),
666                weak: atomic::AtomicUsize::new(1),
667                data,
668            },
669            alloc,
670        );
671        let (ptr, alloc) = Box::into_unique(x);
672        unsafe { Self::from_inner_in(ptr.into(), alloc) }
673    }
674
675    /// Constructs a new `Arc` with uninitialized contents in the provided allocator.
676    ///
677    /// # Examples
678    ///
679    /// ```
680    /// #![feature(get_mut_unchecked)]
681    /// #![feature(allocator_api)]
682    ///
683    /// use std::sync::Arc;
684    /// use std::alloc::System;
685    ///
686    /// let mut five = Arc::<u32, _>::new_uninit_in(System);
687    ///
688    /// let five = unsafe {
689    ///     // Deferred initialization:
690    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
691    ///
692    ///     five.assume_init()
693    /// };
694    ///
695    /// assert_eq!(*five, 5)
696    /// ```
697    #[cfg(not(no_global_oom_handling))]
698    #[unstable(feature = "allocator_api", issue = "32838")]
699    #[inline]
700    pub fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
701        unsafe {
702            Arc::from_ptr_in(
703                Arc::allocate_for_layout(
704                    Layout::new::<T>(),
705                    |layout| alloc.allocate(layout),
706                    <*mut u8>::cast,
707                ),
708                alloc,
709            )
710        }
711    }
712
713    /// Constructs a new `Arc` with uninitialized contents, with the memory
714    /// being filled with `0` bytes, in the provided allocator.
715    ///
716    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
717    /// of this method.
718    ///
719    /// # Examples
720    ///
721    /// ```
722    /// #![feature(allocator_api)]
723    ///
724    /// use std::sync::Arc;
725    /// use std::alloc::System;
726    ///
727    /// let zero = Arc::<u32, _>::new_zeroed_in(System);
728    /// let zero = unsafe { zero.assume_init() };
729    ///
730    /// assert_eq!(*zero, 0)
731    /// ```
732    ///
733    /// [zeroed]: mem::MaybeUninit::zeroed
734    #[cfg(not(no_global_oom_handling))]
735    #[unstable(feature = "allocator_api", issue = "32838")]
736    #[inline]
737    pub fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
738        unsafe {
739            Arc::from_ptr_in(
740                Arc::allocate_for_layout(
741                    Layout::new::<T>(),
742                    |layout| alloc.allocate_zeroed(layout),
743                    <*mut u8>::cast,
744                ),
745                alloc,
746            )
747        }
748    }
749
750    /// Constructs a new `Arc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
751    /// to allow you to construct a `T` which holds a weak pointer to itself.
752    ///
753    /// Generally, a structure circularly referencing itself, either directly or
754    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
755    /// Using this function, you get access to the weak pointer during the
756    /// initialization of `T`, before the `Arc<T, A>` is created, such that you can
757    /// clone and store it inside the `T`.
758    ///
759    /// `new_cyclic_in` first allocates the managed allocation for the `Arc<T, A>`,
760    /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
761    /// and only afterwards completes the construction of the `Arc<T, A>` by placing
762    /// the `T` returned from your closure into the allocation.
763    ///
764    /// Since the new `Arc<T, A>` is not fully-constructed until `Arc<T, A>::new_cyclic_in`
765    /// returns, calling [`upgrade`] on the weak reference inside your closure will
766    /// fail and result in a `None` value.
767    ///
768    /// # Panics
769    ///
770    /// If `data_fn` panics, the panic is propagated to the caller, and the
771    /// temporary [`Weak<T>`] is dropped normally.
772    ///
773    /// # Example
774    ///
775    /// See [`new_cyclic`]
776    ///
777    /// [`new_cyclic`]: Arc::new_cyclic
778    /// [`upgrade`]: Weak::upgrade
779    #[cfg(not(no_global_oom_handling))]
780    #[inline]
781    #[unstable(feature = "allocator_api", issue = "32838")]
782    pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
783    where
784        F: FnOnce(&Weak<T, A>) -> T,
785    {
786        // Construct the inner in the "uninitialized" state with a single
787        // weak reference.
788        let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in(
789            ArcInner {
790                strong: atomic::AtomicUsize::new(0),
791                weak: atomic::AtomicUsize::new(1),
792                data: mem::MaybeUninit::<T>::uninit(),
793            },
794            alloc,
795        ));
796        let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into();
797        let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
798
799        let weak = Weak { ptr: init_ptr, alloc };
800
801        // It's important we don't give up ownership of the weak pointer, or
802        // else the memory might be freed by the time `data_fn` returns. If
803        // we really wanted to pass ownership, we could create an additional
804        // weak pointer for ourselves, but this would result in additional
805        // updates to the weak reference count which might not be necessary
806        // otherwise.
807        let data = data_fn(&weak);
808
809        // Now we can properly initialize the inner value and turn our weak
810        // reference into a strong reference.
811        let strong = unsafe {
812            let inner = init_ptr.as_ptr();
813            ptr::write(&raw mut (*inner).data, data);
814
815            // The above write to the data field must be visible to any threads which
816            // observe a non-zero strong count. Therefore we need at least "Release" ordering
817            // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
818            //
819            // "Acquire" ordering is not required. When considering the possible behaviors
820            // of `data_fn` we only need to look at what it could do with a reference to a
821            // non-upgradeable `Weak`:
822            // - It can *clone* the `Weak`, increasing the weak reference count.
823            // - It can drop those clones, decreasing the weak reference count (but never to zero).
824            //
825            // These side effects do not impact us in any way, and no other side effects are
826            // possible with safe code alone.
827            let prev_value = (*inner).strong.fetch_add(1, Release);
828            debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
829
830            // Strong references should collectively own a shared weak reference,
831            // so don't run the destructor for our old weak reference.
832            // Calling into_raw_with_allocator has the double effect of giving us back the allocator,
833            // and forgetting the weak reference.
834            let alloc = weak.into_raw_with_allocator().1;
835
836            Arc::from_inner_in(init_ptr, alloc)
837        };
838
839        strong
840    }
841
842    /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator. If `T` does not implement `Unpin`,
843    /// then `data` will be pinned in memory and unable to be moved.
844    #[cfg(not(no_global_oom_handling))]
845    #[unstable(feature = "allocator_api", issue = "32838")]
846    #[inline]
847    pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
848    where
849        A: 'static,
850    {
851        unsafe { Pin::new_unchecked(Arc::new_in(data, alloc)) }
852    }
853
854    /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator, return an error if allocation
855    /// fails.
856    #[inline]
857    #[unstable(feature = "allocator_api", issue = "32838")]
858    pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
859    where
860        A: 'static,
861    {
862        unsafe { Ok(Pin::new_unchecked(Arc::try_new_in(data, alloc)?)) }
863    }
864
865    /// Constructs a new `Arc<T, A>` in the provided allocator, returning an error if allocation fails.
866    ///
867    /// # Examples
868    ///
869    /// ```
870    /// #![feature(allocator_api)]
871    ///
872    /// use std::sync::Arc;
873    /// use std::alloc::System;
874    ///
875    /// let five = Arc::try_new_in(5, System)?;
876    /// # Ok::<(), std::alloc::AllocError>(())
877    /// ```
878    #[inline]
879    #[unstable(feature = "allocator_api", issue = "32838")]
880    #[inline]
881    pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError> {
882        // Start the weak pointer count as 1 which is the weak pointer that's
883        // held by all the strong pointers (kinda), see std/rc.rs for more info
884        let x = Box::try_new_in(
885            ArcInner {
886                strong: atomic::AtomicUsize::new(1),
887                weak: atomic::AtomicUsize::new(1),
888                data,
889            },
890            alloc,
891        )?;
892        let (ptr, alloc) = Box::into_unique(x);
893        Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
894    }
895
896    /// Constructs a new `Arc` with uninitialized contents, in the provided allocator, returning an
897    /// error if allocation fails.
898    ///
899    /// # Examples
900    ///
901    /// ```
902    /// #![feature(allocator_api)]
903    /// #![feature(get_mut_unchecked)]
904    ///
905    /// use std::sync::Arc;
906    /// use std::alloc::System;
907    ///
908    /// let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
909    ///
910    /// let five = unsafe {
911    ///     // Deferred initialization:
912    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
913    ///
914    ///     five.assume_init()
915    /// };
916    ///
917    /// assert_eq!(*five, 5);
918    /// # Ok::<(), std::alloc::AllocError>(())
919    /// ```
920    #[unstable(feature = "allocator_api", issue = "32838")]
921    #[inline]
922    pub fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
923        unsafe {
924            Ok(Arc::from_ptr_in(
925                Arc::try_allocate_for_layout(
926                    Layout::new::<T>(),
927                    |layout| alloc.allocate(layout),
928                    <*mut u8>::cast,
929                )?,
930                alloc,
931            ))
932        }
933    }
934
935    /// Constructs a new `Arc` with uninitialized contents, with the memory
936    /// being filled with `0` bytes, in the provided allocator, returning an error if allocation
937    /// fails.
938    ///
939    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
940    /// of this method.
941    ///
942    /// # Examples
943    ///
944    /// ```
945    /// #![feature(allocator_api)]
946    ///
947    /// use std::sync::Arc;
948    /// use std::alloc::System;
949    ///
950    /// let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
951    /// let zero = unsafe { zero.assume_init() };
952    ///
953    /// assert_eq!(*zero, 0);
954    /// # Ok::<(), std::alloc::AllocError>(())
955    /// ```
956    ///
957    /// [zeroed]: mem::MaybeUninit::zeroed
958    #[unstable(feature = "allocator_api", issue = "32838")]
959    #[inline]
960    pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
961        unsafe {
962            Ok(Arc::from_ptr_in(
963                Arc::try_allocate_for_layout(
964                    Layout::new::<T>(),
965                    |layout| alloc.allocate_zeroed(layout),
966                    <*mut u8>::cast,
967                )?,
968                alloc,
969            ))
970        }
971    }
972    /// Returns the inner value, if the `Arc` has exactly one strong reference.
973    ///
974    /// Otherwise, an [`Err`] is returned with the same `Arc` that was
975    /// passed in.
976    ///
977    /// This will succeed even if there are outstanding weak references.
978    ///
979    /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
980    /// keep the `Arc` in the [`Err`] case.
981    /// Immediately dropping the [`Err`]-value, as the expression
982    /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
983    /// drop to zero and the inner value of the `Arc` to be dropped.
984    /// For instance, if two threads execute such an expression in parallel,
985    /// there is a race condition without the possibility of unsafety:
986    /// The threads could first both check whether they own the last instance
987    /// in `Arc::try_unwrap`, determine that they both do not, and then both
988    /// discard and drop their instance in the call to [`ok`][`Result::ok`].
989    /// In this scenario, the value inside the `Arc` is safely destroyed
990    /// by exactly one of the threads, but neither thread will ever be able
991    /// to use the value.
992    ///
993    /// # Examples
994    ///
995    /// ```
996    /// use std::sync::Arc;
997    ///
998    /// let x = Arc::new(3);
999    /// assert_eq!(Arc::try_unwrap(x), Ok(3));
1000    ///
1001    /// let x = Arc::new(4);
1002    /// let _y = Arc::clone(&x);
1003    /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
1004    /// ```
1005    #[inline]
1006    #[stable(feature = "arc_unique", since = "1.4.0")]
1007    pub fn try_unwrap(this: Self) -> Result<T, Self> {
1008        if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
1009            return Err(this);
1010        }
1011
1012        acquire!(this.inner().strong);
1013
1014        let this = ManuallyDrop::new(this);
1015        let elem: T = unsafe { ptr::read(&this.ptr.as_ref().data) };
1016        let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator
1017
1018        // Make a weak pointer to clean up the implicit strong-weak reference
1019        let _weak = Weak { ptr: this.ptr, alloc };
1020
1021        Ok(elem)
1022    }
1023
1024    /// Returns the inner value, if the `Arc` has exactly one strong reference.
1025    ///
1026    /// Otherwise, [`None`] is returned and the `Arc` is dropped.
1027    ///
1028    /// This will succeed even if there are outstanding weak references.
1029    ///
1030    /// If `Arc::into_inner` is called on every clone of this `Arc`,
1031    /// it is guaranteed that exactly one of the calls returns the inner value.
1032    /// This means in particular that the inner value is not dropped.
1033    ///
1034    /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
1035    /// is meant for different use-cases. If used as a direct replacement
1036    /// for `Arc::into_inner` anyway, such as with the expression
1037    /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
1038    /// **not** give the same guarantee as described in the previous paragraph.
1039    /// For more information, see the examples below and read the documentation
1040    /// of [`Arc::try_unwrap`].
1041    ///
1042    /// # Examples
1043    ///
1044    /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
1045    /// ```
1046    /// use std::sync::Arc;
1047    ///
1048    /// let x = Arc::new(3);
1049    /// let y = Arc::clone(&x);
1050    ///
1051    /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
1052    /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
1053    /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
1054    ///
1055    /// let x_inner_value = x_thread.join().unwrap();
1056    /// let y_inner_value = y_thread.join().unwrap();
1057    ///
1058    /// // One of the threads is guaranteed to receive the inner value:
1059    /// assert!(matches!(
1060    ///     (x_inner_value, y_inner_value),
1061    ///     (None, Some(3)) | (Some(3), None)
1062    /// ));
1063    /// // The result could also be `(None, None)` if the threads called
1064    /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
1065    /// ```
1066    ///
1067    /// A more practical example demonstrating the need for `Arc::into_inner`:
1068    /// ```
1069    /// use std::sync::Arc;
1070    ///
1071    /// // Definition of a simple singly linked list using `Arc`:
1072    /// #[derive(Clone)]
1073    /// struct LinkedList<T>(Option<Arc<Node<T>>>);
1074    /// struct Node<T>(T, Option<Arc<Node<T>>>);
1075    ///
1076    /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
1077    /// // can cause a stack overflow. To prevent this, we can provide a
1078    /// // manual `Drop` implementation that does the destruction in a loop:
1079    /// impl<T> Drop for LinkedList<T> {
1080    ///     fn drop(&mut self) {
1081    ///         let mut link = self.0.take();
1082    ///         while let Some(arc_node) = link.take() {
1083    ///             if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
1084    ///                 link = next;
1085    ///             }
1086    ///         }
1087    ///     }
1088    /// }
1089    ///
1090    /// // Implementation of `new` and `push` omitted
1091    /// impl<T> LinkedList<T> {
1092    ///     /* ... */
1093    /// #   fn new() -> Self {
1094    /// #       LinkedList(None)
1095    /// #   }
1096    /// #   fn push(&mut self, x: T) {
1097    /// #       self.0 = Some(Arc::new(Node(x, self.0.take())));
1098    /// #   }
1099    /// }
1100    ///
1101    /// // The following code could have still caused a stack overflow
1102    /// // despite the manual `Drop` impl if that `Drop` impl had used
1103    /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
1104    ///
1105    /// // Create a long list and clone it
1106    /// let mut x = LinkedList::new();
1107    /// let size = 100000;
1108    /// # let size = if cfg!(miri) { 100 } else { size };
1109    /// for i in 0..size {
1110    ///     x.push(i); // Adds i to the front of x
1111    /// }
1112    /// let y = x.clone();
1113    ///
1114    /// // Drop the clones in parallel
1115    /// let x_thread = std::thread::spawn(|| drop(x));
1116    /// let y_thread = std::thread::spawn(|| drop(y));
1117    /// x_thread.join().unwrap();
1118    /// y_thread.join().unwrap();
1119    /// ```
1120    #[inline]
1121    #[stable(feature = "arc_into_inner", since = "1.70.0")]
1122    pub fn into_inner(this: Self) -> Option<T> {
1123        // Make sure that the ordinary `Drop` implementation isn’t called as well
1124        let mut this = mem::ManuallyDrop::new(this);
1125
1126        // Following the implementation of `drop` and `drop_slow`
1127        if this.inner().strong.fetch_sub(1, Release) != 1 {
1128            return None;
1129        }
1130
1131        acquire!(this.inner().strong);
1132
1133        // SAFETY: This mirrors the line
1134        //
1135        //     unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1136        //
1137        // in `drop_slow`. Instead of dropping the value behind the pointer,
1138        // it is read and eventually returned; `ptr::read` has the same
1139        // safety conditions as `ptr::drop_in_place`.
1140
1141        let inner = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
1142        let alloc = unsafe { ptr::read(&this.alloc) };
1143
1144        drop(Weak { ptr: this.ptr, alloc });
1145
1146        Some(inner)
1147    }
1148}
1149
1150impl<T> Arc<[T]> {
1151    /// Constructs a new atomically reference-counted slice with uninitialized contents.
1152    ///
1153    /// # Examples
1154    ///
1155    /// ```
1156    /// use std::sync::Arc;
1157    ///
1158    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1159    ///
1160    /// // Deferred initialization:
1161    /// let data = Arc::get_mut(&mut values).unwrap();
1162    /// data[0].write(1);
1163    /// data[1].write(2);
1164    /// data[2].write(3);
1165    ///
1166    /// let values = unsafe { values.assume_init() };
1167    ///
1168    /// assert_eq!(*values, [1, 2, 3])
1169    /// ```
1170    #[cfg(not(no_global_oom_handling))]
1171    #[inline]
1172    #[stable(feature = "new_uninit", since = "1.82.0")]
1173    #[must_use]
1174    pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1175        unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
1176    }
1177
1178    /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1179    /// filled with `0` bytes.
1180    ///
1181    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1182    /// incorrect usage of this method.
1183    ///
1184    /// # Examples
1185    ///
1186    /// ```
1187    /// use std::sync::Arc;
1188    ///
1189    /// let values = Arc::<[u32]>::new_zeroed_slice(3);
1190    /// let values = unsafe { values.assume_init() };
1191    ///
1192    /// assert_eq!(*values, [0, 0, 0])
1193    /// ```
1194    ///
1195    /// [zeroed]: mem::MaybeUninit::zeroed
1196    #[cfg(not(no_global_oom_handling))]
1197    #[inline]
1198    #[stable(feature = "new_zeroed_alloc", since = "CURRENT_RUSTC_VERSION")]
1199    #[must_use]
1200    pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1201        unsafe {
1202            Arc::from_ptr(Arc::allocate_for_layout(
1203                Layout::array::<T>(len).unwrap(),
1204                |layout| Global.allocate_zeroed(layout),
1205                |mem| {
1206                    ptr::slice_from_raw_parts_mut(mem as *mut T, len)
1207                        as *mut ArcInner<[mem::MaybeUninit<T>]>
1208                },
1209            ))
1210        }
1211    }
1212
1213    /// Converts the reference-counted slice into a reference-counted array.
1214    ///
1215    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
1216    ///
1217    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
1218    #[unstable(feature = "slice_as_array", issue = "133508")]
1219    #[inline]
1220    #[must_use]
1221    pub fn into_array<const N: usize>(self) -> Option<Arc<[T; N]>> {
1222        if self.len() == N {
1223            let ptr = Self::into_raw(self) as *const [T; N];
1224
1225            // 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.
1226            let me = unsafe { Arc::from_raw(ptr) };
1227            Some(me)
1228        } else {
1229            None
1230        }
1231    }
1232}
1233
1234impl<T, A: Allocator> Arc<[T], A> {
1235    /// Constructs a new atomically reference-counted slice with uninitialized contents in the
1236    /// provided allocator.
1237    ///
1238    /// # Examples
1239    ///
1240    /// ```
1241    /// #![feature(get_mut_unchecked)]
1242    /// #![feature(allocator_api)]
1243    ///
1244    /// use std::sync::Arc;
1245    /// use std::alloc::System;
1246    ///
1247    /// let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
1248    ///
1249    /// let values = unsafe {
1250    ///     // Deferred initialization:
1251    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
1252    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
1253    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
1254    ///
1255    ///     values.assume_init()
1256    /// };
1257    ///
1258    /// assert_eq!(*values, [1, 2, 3])
1259    /// ```
1260    #[cfg(not(no_global_oom_handling))]
1261    #[unstable(feature = "allocator_api", issue = "32838")]
1262    #[inline]
1263    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1264        unsafe { Arc::from_ptr_in(Arc::allocate_for_slice_in(len, &alloc), alloc) }
1265    }
1266
1267    /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1268    /// filled with `0` bytes, in the provided allocator.
1269    ///
1270    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1271    /// incorrect usage of this method.
1272    ///
1273    /// # Examples
1274    ///
1275    /// ```
1276    /// #![feature(allocator_api)]
1277    ///
1278    /// use std::sync::Arc;
1279    /// use std::alloc::System;
1280    ///
1281    /// let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
1282    /// let values = unsafe { values.assume_init() };
1283    ///
1284    /// assert_eq!(*values, [0, 0, 0])
1285    /// ```
1286    ///
1287    /// [zeroed]: mem::MaybeUninit::zeroed
1288    #[cfg(not(no_global_oom_handling))]
1289    #[unstable(feature = "allocator_api", issue = "32838")]
1290    #[inline]
1291    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1292        unsafe {
1293            Arc::from_ptr_in(
1294                Arc::allocate_for_layout(
1295                    Layout::array::<T>(len).unwrap(),
1296                    |layout| alloc.allocate_zeroed(layout),
1297                    |mem| {
1298                        ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len)
1299                            as *mut ArcInner<[mem::MaybeUninit<T>]>
1300                    },
1301                ),
1302                alloc,
1303            )
1304        }
1305    }
1306}
1307
1308impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
1309    /// Converts to `Arc<T>`.
1310    ///
1311    /// # Safety
1312    ///
1313    /// As with [`MaybeUninit::assume_init`],
1314    /// it is up to the caller to guarantee that the inner value
1315    /// really is in an initialized state.
1316    /// Calling this when the content is not yet fully initialized
1317    /// causes immediate undefined behavior.
1318    ///
1319    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1320    ///
1321    /// # Examples
1322    ///
1323    /// ```
1324    /// use std::sync::Arc;
1325    ///
1326    /// let mut five = Arc::<u32>::new_uninit();
1327    ///
1328    /// // Deferred initialization:
1329    /// Arc::get_mut(&mut five).unwrap().write(5);
1330    ///
1331    /// let five = unsafe { five.assume_init() };
1332    ///
1333    /// assert_eq!(*five, 5)
1334    /// ```
1335    #[stable(feature = "new_uninit", since = "1.82.0")]
1336    #[must_use = "`self` will be dropped if the result is not used"]
1337    #[inline]
1338    pub unsafe fn assume_init(self) -> Arc<T, A> {
1339        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1340        unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
1341    }
1342}
1343
1344impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
1345    /// Converts to `Arc<[T]>`.
1346    ///
1347    /// # Safety
1348    ///
1349    /// As with [`MaybeUninit::assume_init`],
1350    /// it is up to the caller to guarantee that the inner value
1351    /// really is in an initialized state.
1352    /// Calling this when the content is not yet fully initialized
1353    /// causes immediate undefined behavior.
1354    ///
1355    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1356    ///
1357    /// # Examples
1358    ///
1359    /// ```
1360    /// use std::sync::Arc;
1361    ///
1362    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1363    ///
1364    /// // Deferred initialization:
1365    /// let data = Arc::get_mut(&mut values).unwrap();
1366    /// data[0].write(1);
1367    /// data[1].write(2);
1368    /// data[2].write(3);
1369    ///
1370    /// let values = unsafe { values.assume_init() };
1371    ///
1372    /// assert_eq!(*values, [1, 2, 3])
1373    /// ```
1374    #[stable(feature = "new_uninit", since = "1.82.0")]
1375    #[must_use = "`self` will be dropped if the result is not used"]
1376    #[inline]
1377    pub unsafe fn assume_init(self) -> Arc<[T], A> {
1378        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1379        unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
1380    }
1381}
1382
1383impl<T: ?Sized> Arc<T> {
1384    /// Constructs an `Arc<T>` from a raw pointer.
1385    ///
1386    /// The raw pointer must have been previously returned by a call to
1387    /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
1388    ///
1389    /// * If `U` is sized, it must have the same size and alignment as `T`. This
1390    ///   is trivially true if `U` is `T`.
1391    /// * If `U` is unsized, its data pointer must have the same size and
1392    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
1393    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1394    ///   coercion].
1395    ///
1396    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1397    /// and alignment, this is basically like transmuting references of
1398    /// different types. See [`mem::transmute`][transmute] for more information
1399    /// on what restrictions apply in this case.
1400    ///
1401    /// The raw pointer must point to a block of memory allocated by the global allocator.
1402    ///
1403    /// The user of `from_raw` has to make sure a specific value of `T` is only
1404    /// dropped once.
1405    ///
1406    /// This function is unsafe because improper use may lead to memory unsafety,
1407    /// even if the returned `Arc<T>` is never accessed.
1408    ///
1409    /// [into_raw]: Arc::into_raw
1410    /// [transmute]: core::mem::transmute
1411    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1412    ///
1413    /// # Examples
1414    ///
1415    /// ```
1416    /// use std::sync::Arc;
1417    ///
1418    /// let x = Arc::new("hello".to_owned());
1419    /// let x_ptr = Arc::into_raw(x);
1420    ///
1421    /// unsafe {
1422    ///     // Convert back to an `Arc` to prevent leak.
1423    ///     let x = Arc::from_raw(x_ptr);
1424    ///     assert_eq!(&*x, "hello");
1425    ///
1426    ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1427    /// }
1428    ///
1429    /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1430    /// ```
1431    ///
1432    /// Convert a slice back into its original array:
1433    ///
1434    /// ```
1435    /// use std::sync::Arc;
1436    ///
1437    /// let x: Arc<[u32]> = Arc::new([1, 2, 3]);
1438    /// let x_ptr: *const [u32] = Arc::into_raw(x);
1439    ///
1440    /// unsafe {
1441    ///     let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
1442    ///     assert_eq!(&*x, &[1, 2, 3]);
1443    /// }
1444    /// ```
1445    #[inline]
1446    #[stable(feature = "rc_raw", since = "1.17.0")]
1447    pub unsafe fn from_raw(ptr: *const T) -> Self {
1448        unsafe { Arc::from_raw_in(ptr, Global) }
1449    }
1450
1451    /// Consumes the `Arc`, returning the wrapped pointer.
1452    ///
1453    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1454    /// [`Arc::from_raw`].
1455    ///
1456    /// # Examples
1457    ///
1458    /// ```
1459    /// use std::sync::Arc;
1460    ///
1461    /// let x = Arc::new("hello".to_owned());
1462    /// let x_ptr = Arc::into_raw(x);
1463    /// assert_eq!(unsafe { &*x_ptr }, "hello");
1464    /// # // Prevent leaks for Miri.
1465    /// # drop(unsafe { Arc::from_raw(x_ptr) });
1466    /// ```
1467    #[must_use = "losing the pointer will leak memory"]
1468    #[stable(feature = "rc_raw", since = "1.17.0")]
1469    #[rustc_never_returns_null_ptr]
1470    pub fn into_raw(this: Self) -> *const T {
1471        let this = ManuallyDrop::new(this);
1472        Self::as_ptr(&*this)
1473    }
1474
1475    /// Increments the strong reference count on the `Arc<T>` associated with the
1476    /// provided pointer by one.
1477    ///
1478    /// # Safety
1479    ///
1480    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1481    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1482    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1483    /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1484    /// allocated by the global allocator.
1485    ///
1486    /// [from_raw_in]: Arc::from_raw_in
1487    ///
1488    /// # Examples
1489    ///
1490    /// ```
1491    /// use std::sync::Arc;
1492    ///
1493    /// let five = Arc::new(5);
1494    ///
1495    /// unsafe {
1496    ///     let ptr = Arc::into_raw(five);
1497    ///     Arc::increment_strong_count(ptr);
1498    ///
1499    ///     // This assertion is deterministic because we haven't shared
1500    ///     // the `Arc` between threads.
1501    ///     let five = Arc::from_raw(ptr);
1502    ///     assert_eq!(2, Arc::strong_count(&five));
1503    /// #   // Prevent leaks for Miri.
1504    /// #   Arc::decrement_strong_count(ptr);
1505    /// }
1506    /// ```
1507    #[inline]
1508    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1509    pub unsafe fn increment_strong_count(ptr: *const T) {
1510        unsafe { Arc::increment_strong_count_in(ptr, Global) }
1511    }
1512
1513    /// Decrements the strong reference count on the `Arc<T>` associated with the
1514    /// provided pointer by one.
1515    ///
1516    /// # Safety
1517    ///
1518    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1519    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1520    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1521    /// least 1) when invoking this method, and `ptr` must point to a block of memory
1522    /// allocated by the global allocator. This method can be used to release the final
1523    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1524    /// released.
1525    ///
1526    /// [from_raw_in]: Arc::from_raw_in
1527    ///
1528    /// # Examples
1529    ///
1530    /// ```
1531    /// use std::sync::Arc;
1532    ///
1533    /// let five = Arc::new(5);
1534    ///
1535    /// unsafe {
1536    ///     let ptr = Arc::into_raw(five);
1537    ///     Arc::increment_strong_count(ptr);
1538    ///
1539    ///     // Those assertions are deterministic because we haven't shared
1540    ///     // the `Arc` between threads.
1541    ///     let five = Arc::from_raw(ptr);
1542    ///     assert_eq!(2, Arc::strong_count(&five));
1543    ///     Arc::decrement_strong_count(ptr);
1544    ///     assert_eq!(1, Arc::strong_count(&five));
1545    /// }
1546    /// ```
1547    #[inline]
1548    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1549    pub unsafe fn decrement_strong_count(ptr: *const T) {
1550        unsafe { Arc::decrement_strong_count_in(ptr, Global) }
1551    }
1552}
1553
1554impl<T: ?Sized, A: Allocator> Arc<T, A> {
1555    /// Returns a reference to the underlying allocator.
1556    ///
1557    /// Note: this is an associated function, which means that you have
1558    /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
1559    /// is so that there is no conflict with a method on the inner type.
1560    #[inline]
1561    #[unstable(feature = "allocator_api", issue = "32838")]
1562    pub fn allocator(this: &Self) -> &A {
1563        &this.alloc
1564    }
1565
1566    /// Consumes the `Arc`, returning the wrapped pointer and allocator.
1567    ///
1568    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1569    /// [`Arc::from_raw_in`].
1570    ///
1571    /// # Examples
1572    ///
1573    /// ```
1574    /// #![feature(allocator_api)]
1575    /// use std::sync::Arc;
1576    /// use std::alloc::System;
1577    ///
1578    /// let x = Arc::new_in("hello".to_owned(), System);
1579    /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
1580    /// assert_eq!(unsafe { &*ptr }, "hello");
1581    /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
1582    /// assert_eq!(&*x, "hello");
1583    /// ```
1584    #[must_use = "losing the pointer will leak memory"]
1585    #[unstable(feature = "allocator_api", issue = "32838")]
1586    pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
1587        let this = mem::ManuallyDrop::new(this);
1588        let ptr = Self::as_ptr(&this);
1589        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
1590        let alloc = unsafe { ptr::read(&this.alloc) };
1591        (ptr, alloc)
1592    }
1593
1594    /// Provides a raw pointer to the data.
1595    ///
1596    /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
1597    /// as long as there are strong counts in the `Arc`.
1598    ///
1599    /// # Examples
1600    ///
1601    /// ```
1602    /// use std::sync::Arc;
1603    ///
1604    /// let x = Arc::new("hello".to_owned());
1605    /// let y = Arc::clone(&x);
1606    /// let x_ptr = Arc::as_ptr(&x);
1607    /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1608    /// assert_eq!(unsafe { &*x_ptr }, "hello");
1609    /// ```
1610    #[must_use]
1611    #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1612    #[rustc_never_returns_null_ptr]
1613    pub fn as_ptr(this: &Self) -> *const T {
1614        let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
1615
1616        // SAFETY: This cannot go through Deref::deref or RcInnerPtr::inner because
1617        // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1618        // write through the pointer after the Rc is recovered through `from_raw`.
1619        unsafe { &raw mut (*ptr).data }
1620    }
1621
1622    /// Constructs an `Arc<T, A>` from a raw pointer.
1623    ///
1624    /// The raw pointer must have been previously returned by a call to [`Arc<U,
1625    /// A>::into_raw`][into_raw] with the following requirements:
1626    ///
1627    /// * If `U` is sized, it must have the same size and alignment as `T`. This
1628    ///   is trivially true if `U` is `T`.
1629    /// * If `U` is unsized, its data pointer must have the same size and
1630    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
1631    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1632    ///   coercion].
1633    ///
1634    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1635    /// and alignment, this is basically like transmuting references of
1636    /// different types. See [`mem::transmute`][transmute] for more information
1637    /// on what restrictions apply in this case.
1638    ///
1639    /// The raw pointer must point to a block of memory allocated by `alloc`
1640    ///
1641    /// The user of `from_raw` has to make sure a specific value of `T` is only
1642    /// dropped once.
1643    ///
1644    /// This function is unsafe because improper use may lead to memory unsafety,
1645    /// even if the returned `Arc<T>` is never accessed.
1646    ///
1647    /// [into_raw]: Arc::into_raw
1648    /// [transmute]: core::mem::transmute
1649    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1650    ///
1651    /// # Examples
1652    ///
1653    /// ```
1654    /// #![feature(allocator_api)]
1655    ///
1656    /// use std::sync::Arc;
1657    /// use std::alloc::System;
1658    ///
1659    /// let x = Arc::new_in("hello".to_owned(), System);
1660    /// let (x_ptr, alloc) = Arc::into_raw_with_allocator(x);
1661    ///
1662    /// unsafe {
1663    ///     // Convert back to an `Arc` to prevent leak.
1664    ///     let x = Arc::from_raw_in(x_ptr, System);
1665    ///     assert_eq!(&*x, "hello");
1666    ///
1667    ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1668    /// }
1669    ///
1670    /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1671    /// ```
1672    ///
1673    /// Convert a slice back into its original array:
1674    ///
1675    /// ```
1676    /// #![feature(allocator_api)]
1677    ///
1678    /// use std::sync::Arc;
1679    /// use std::alloc::System;
1680    ///
1681    /// let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
1682    /// let x_ptr: *const [u32] = Arc::into_raw_with_allocator(x).0;
1683    ///
1684    /// unsafe {
1685    ///     let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
1686    ///     assert_eq!(&*x, &[1, 2, 3]);
1687    /// }
1688    /// ```
1689    #[inline]
1690    #[unstable(feature = "allocator_api", issue = "32838")]
1691    pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
1692        unsafe {
1693            let offset = data_offset(ptr);
1694
1695            // Reverse the offset to find the original ArcInner.
1696            let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
1697
1698            Self::from_ptr_in(arc_ptr, alloc)
1699        }
1700    }
1701
1702    /// Creates a new [`Weak`] pointer to this allocation.
1703    ///
1704    /// # Examples
1705    ///
1706    /// ```
1707    /// use std::sync::Arc;
1708    ///
1709    /// let five = Arc::new(5);
1710    ///
1711    /// let weak_five = Arc::downgrade(&five);
1712    /// ```
1713    #[must_use = "this returns a new `Weak` pointer, \
1714                  without modifying the original `Arc`"]
1715    #[stable(feature = "arc_weak", since = "1.4.0")]
1716    pub fn downgrade(this: &Self) -> Weak<T, A>
1717    where
1718        A: Clone,
1719    {
1720        // This Relaxed is OK because we're checking the value in the CAS
1721        // below.
1722        let mut cur = this.inner().weak.load(Relaxed);
1723
1724        loop {
1725            // check if the weak counter is currently "locked"; if so, spin.
1726            if cur == usize::MAX {
1727                hint::spin_loop();
1728                cur = this.inner().weak.load(Relaxed);
1729                continue;
1730            }
1731
1732            // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1733            assert!(cur <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
1734
1735            // NOTE: this code currently ignores the possibility of overflow
1736            // into usize::MAX; in general both Rc and Arc need to be adjusted
1737            // to deal with overflow.
1738
1739            // Unlike with Clone(), we need this to be an Acquire read to
1740            // synchronize with the write coming from `is_unique`, so that the
1741            // events prior to that write happen before this read.
1742            match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
1743                Ok(_) => {
1744                    // Make sure we do not create a dangling Weak
1745                    debug_assert!(!is_dangling(this.ptr.as_ptr()));
1746                    return Weak { ptr: this.ptr, alloc: this.alloc.clone() };
1747                }
1748                Err(old) => cur = old,
1749            }
1750        }
1751    }
1752
1753    /// Gets the number of [`Weak`] pointers to this allocation.
1754    ///
1755    /// # Safety
1756    ///
1757    /// This method by itself is safe, but using it correctly requires extra care.
1758    /// Another thread can change the weak count at any time,
1759    /// including potentially between calling this method and acting on the result.
1760    ///
1761    /// # Examples
1762    ///
1763    /// ```
1764    /// use std::sync::Arc;
1765    ///
1766    /// let five = Arc::new(5);
1767    /// let _weak_five = Arc::downgrade(&five);
1768    ///
1769    /// // This assertion is deterministic because we haven't shared
1770    /// // the `Arc` or `Weak` between threads.
1771    /// assert_eq!(1, Arc::weak_count(&five));
1772    /// ```
1773    #[inline]
1774    #[must_use]
1775    #[stable(feature = "arc_counts", since = "1.15.0")]
1776    pub fn weak_count(this: &Self) -> usize {
1777        let cnt = this.inner().weak.load(Relaxed);
1778        // If the weak count is currently locked, the value of the
1779        // count was 0 just before taking the lock.
1780        if cnt == usize::MAX { 0 } else { cnt - 1 }
1781    }
1782
1783    /// Gets the number of strong (`Arc`) pointers to this allocation.
1784    ///
1785    /// # Safety
1786    ///
1787    /// This method by itself is safe, but using it correctly requires extra care.
1788    /// Another thread can change the strong count at any time,
1789    /// including potentially between calling this method and acting on the result.
1790    ///
1791    /// # Examples
1792    ///
1793    /// ```
1794    /// use std::sync::Arc;
1795    ///
1796    /// let five = Arc::new(5);
1797    /// let _also_five = Arc::clone(&five);
1798    ///
1799    /// // This assertion is deterministic because we haven't shared
1800    /// // the `Arc` between threads.
1801    /// assert_eq!(2, Arc::strong_count(&five));
1802    /// ```
1803    #[inline]
1804    #[must_use]
1805    #[stable(feature = "arc_counts", since = "1.15.0")]
1806    pub fn strong_count(this: &Self) -> usize {
1807        this.inner().strong.load(Relaxed)
1808    }
1809
1810    /// Increments the strong reference count on the `Arc<T>` associated with the
1811    /// provided pointer by one.
1812    ///
1813    /// # Safety
1814    ///
1815    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1816    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1817    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1818    /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1819    /// allocated by `alloc`.
1820    ///
1821    /// [from_raw_in]: Arc::from_raw_in
1822    ///
1823    /// # Examples
1824    ///
1825    /// ```
1826    /// #![feature(allocator_api)]
1827    ///
1828    /// use std::sync::Arc;
1829    /// use std::alloc::System;
1830    ///
1831    /// let five = Arc::new_in(5, System);
1832    ///
1833    /// unsafe {
1834    ///     let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
1835    ///     Arc::increment_strong_count_in(ptr, System);
1836    ///
1837    ///     // This assertion is deterministic because we haven't shared
1838    ///     // the `Arc` between threads.
1839    ///     let five = Arc::from_raw_in(ptr, System);
1840    ///     assert_eq!(2, Arc::strong_count(&five));
1841    /// #   // Prevent leaks for Miri.
1842    /// #   Arc::decrement_strong_count_in(ptr, System);
1843    /// }
1844    /// ```
1845    #[inline]
1846    #[unstable(feature = "allocator_api", issue = "32838")]
1847    pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
1848    where
1849        A: Clone,
1850    {
1851        // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1852        let arc = unsafe { mem::ManuallyDrop::new(Arc::from_raw_in(ptr, alloc)) };
1853        // Now increase refcount, but don't drop new refcount either
1854        let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1855    }
1856
1857    /// Decrements the strong reference count on the `Arc<T>` associated with the
1858    /// provided pointer by one.
1859    ///
1860    /// # Safety
1861    ///
1862    /// The pointer must have been obtained through `Arc::into_raw` and must satisfy the
1863    /// same layout requirements specified in [`Arc::from_raw_in`][from_raw_in].
1864    /// The associated `Arc` instance must be valid (i.e. the strong count must be at
1865    /// least 1) when invoking this method, and `ptr` must point to a block of memory
1866    /// allocated by `alloc`. This method can be used to release the final
1867    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1868    /// released.
1869    ///
1870    /// [from_raw_in]: Arc::from_raw_in
1871    ///
1872    /// # Examples
1873    ///
1874    /// ```
1875    /// #![feature(allocator_api)]
1876    ///
1877    /// use std::sync::Arc;
1878    /// use std::alloc::System;
1879    ///
1880    /// let five = Arc::new_in(5, System);
1881    ///
1882    /// unsafe {
1883    ///     let (ptr, _alloc) = Arc::into_raw_with_allocator(five);
1884    ///     Arc::increment_strong_count_in(ptr, System);
1885    ///
1886    ///     // Those assertions are deterministic because we haven't shared
1887    ///     // the `Arc` between threads.
1888    ///     let five = Arc::from_raw_in(ptr, System);
1889    ///     assert_eq!(2, Arc::strong_count(&five));
1890    ///     Arc::decrement_strong_count_in(ptr, System);
1891    ///     assert_eq!(1, Arc::strong_count(&five));
1892    /// }
1893    /// ```
1894    #[inline]
1895    #[unstable(feature = "allocator_api", issue = "32838")]
1896    pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) {
1897        unsafe { drop(Arc::from_raw_in(ptr, alloc)) };
1898    }
1899
1900    #[inline]
1901    fn inner(&self) -> &ArcInner<T> {
1902        // This unsafety is ok because while this arc is alive we're guaranteed
1903        // that the inner pointer is valid. Furthermore, we know that the
1904        // `ArcInner` structure itself is `Sync` because the inner data is
1905        // `Sync` as well, so we're ok loaning out an immutable pointer to these
1906        // contents.
1907        unsafe { self.ptr.as_ref() }
1908    }
1909
1910    // Non-inlined part of `drop`.
1911    #[inline(never)]
1912    unsafe fn drop_slow(&mut self) {
1913        // Drop the weak ref collectively held by all strong references when this
1914        // variable goes out of scope. This ensures that the memory is deallocated
1915        // even if the destructor of `T` panics.
1916        // Take a reference to `self.alloc` instead of cloning because 1. it'll last long
1917        // enough, and 2. you should be able to drop `Arc`s with unclonable allocators
1918        let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
1919
1920        // Destroy the data at this time, even though we must not free the box
1921        // allocation itself (there might still be weak pointers lying around).
1922        // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed.
1923        unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
1924    }
1925
1926    /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1927    /// [`ptr::eq`]. This function ignores the metadata of  `dyn Trait` pointers.
1928    ///
1929    /// # Examples
1930    ///
1931    /// ```
1932    /// use std::sync::Arc;
1933    ///
1934    /// let five = Arc::new(5);
1935    /// let same_five = Arc::clone(&five);
1936    /// let other_five = Arc::new(5);
1937    ///
1938    /// assert!(Arc::ptr_eq(&five, &same_five));
1939    /// assert!(!Arc::ptr_eq(&five, &other_five));
1940    /// ```
1941    ///
1942    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1943    #[inline]
1944    #[must_use]
1945    #[stable(feature = "ptr_eq", since = "1.17.0")]
1946    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1947        ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr())
1948    }
1949}
1950
1951impl<T: ?Sized> Arc<T> {
1952    /// Allocates an `ArcInner<T>` with sufficient space for
1953    /// a possibly-unsized inner value where the value has the layout provided.
1954    ///
1955    /// The function `mem_to_arcinner` is called with the data pointer
1956    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1957    #[cfg(not(no_global_oom_handling))]
1958    unsafe fn allocate_for_layout(
1959        value_layout: Layout,
1960        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1961        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1962    ) -> *mut ArcInner<T> {
1963        let layout = arcinner_layout_for_value_layout(value_layout);
1964
1965        let ptr = allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
1966
1967        unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) }
1968    }
1969
1970    /// Allocates an `ArcInner<T>` with sufficient space for
1971    /// a possibly-unsized inner value where the value has the layout provided,
1972    /// returning an error if allocation fails.
1973    ///
1974    /// The function `mem_to_arcinner` is called with the data pointer
1975    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1976    unsafe fn try_allocate_for_layout(
1977        value_layout: Layout,
1978        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1979        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1980    ) -> Result<*mut ArcInner<T>, AllocError> {
1981        let layout = arcinner_layout_for_value_layout(value_layout);
1982
1983        let ptr = allocate(layout)?;
1984
1985        let inner = unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) };
1986
1987        Ok(inner)
1988    }
1989
1990    unsafe fn initialize_arcinner(
1991        ptr: NonNull<[u8]>,
1992        layout: Layout,
1993        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1994    ) -> *mut ArcInner<T> {
1995        let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1996        debug_assert_eq!(unsafe { Layout::for_value_raw(inner) }, layout);
1997
1998        unsafe {
1999            (&raw mut (*inner).strong).write(atomic::AtomicUsize::new(1));
2000            (&raw mut (*inner).weak).write(atomic::AtomicUsize::new(1));
2001        }
2002
2003        inner
2004    }
2005}
2006
2007impl<T: ?Sized, A: Allocator> Arc<T, A> {
2008    /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
2009    #[inline]
2010    #[cfg(not(no_global_oom_handling))]
2011    unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut ArcInner<T> {
2012        // Allocate for the `ArcInner<T>` using the given value.
2013        unsafe {
2014            Arc::allocate_for_layout(
2015                Layout::for_value_raw(ptr),
2016                |layout| alloc.allocate(layout),
2017                |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
2018            )
2019        }
2020    }
2021
2022    #[cfg(not(no_global_oom_handling))]
2023    fn from_box_in(src: Box<T, A>) -> Arc<T, A> {
2024        unsafe {
2025            let value_size = size_of_val(&*src);
2026            let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src));
2027
2028            // Copy value as bytes
2029            ptr::copy_nonoverlapping(
2030                (&raw const *src) as *const u8,
2031                (&raw mut (*ptr).data) as *mut u8,
2032                value_size,
2033            );
2034
2035            // Free the allocation without dropping its contents
2036            let (bptr, alloc) = Box::into_raw_with_allocator(src);
2037            let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
2038            drop(src);
2039
2040            Self::from_ptr_in(ptr, alloc)
2041        }
2042    }
2043}
2044
2045impl<T> Arc<[T]> {
2046    /// Allocates an `ArcInner<[T]>` with the given length.
2047    #[cfg(not(no_global_oom_handling))]
2048    unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
2049        unsafe {
2050            Self::allocate_for_layout(
2051                Layout::array::<T>(len).unwrap(),
2052                |layout| Global.allocate(layout),
2053                |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2054            )
2055        }
2056    }
2057
2058    /// Copy elements from slice into newly allocated `Arc<[T]>`
2059    ///
2060    /// Unsafe because the caller must either take ownership or bind `T: Copy`.
2061    #[cfg(not(no_global_oom_handling))]
2062    unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
2063        unsafe {
2064            let ptr = Self::allocate_for_slice(v.len());
2065
2066            ptr::copy_nonoverlapping(v.as_ptr(), (&raw mut (*ptr).data) as *mut T, v.len());
2067
2068            Self::from_ptr(ptr)
2069        }
2070    }
2071
2072    /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
2073    ///
2074    /// Behavior is undefined should the size be wrong.
2075    #[cfg(not(no_global_oom_handling))]
2076    unsafe fn from_iter_exact(iter: impl Iterator<Item = T>, len: usize) -> Arc<[T]> {
2077        // Panic guard while cloning T elements.
2078        // In the event of a panic, elements that have been written
2079        // into the new ArcInner will be dropped, then the memory freed.
2080        struct Guard<T> {
2081            mem: NonNull<u8>,
2082            elems: *mut T,
2083            layout: Layout,
2084            n_elems: usize,
2085        }
2086
2087        impl<T> Drop for Guard<T> {
2088            fn drop(&mut self) {
2089                unsafe {
2090                    let slice = from_raw_parts_mut(self.elems, self.n_elems);
2091                    ptr::drop_in_place(slice);
2092
2093                    Global.deallocate(self.mem, self.layout);
2094                }
2095            }
2096        }
2097
2098        unsafe {
2099            let ptr = Self::allocate_for_slice(len);
2100
2101            let mem = ptr as *mut _ as *mut u8;
2102            let layout = Layout::for_value_raw(ptr);
2103
2104            // Pointer to first element
2105            let elems = (&raw mut (*ptr).data) as *mut T;
2106
2107            let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
2108
2109            for (i, item) in iter.enumerate() {
2110                ptr::write(elems.add(i), item);
2111                guard.n_elems += 1;
2112            }
2113
2114            // All clear. Forget the guard so it doesn't free the new ArcInner.
2115            mem::forget(guard);
2116
2117            Self::from_ptr(ptr)
2118        }
2119    }
2120}
2121
2122impl<T, A: Allocator> Arc<[T], A> {
2123    /// Allocates an `ArcInner<[T]>` with the given length.
2124    #[inline]
2125    #[cfg(not(no_global_oom_handling))]
2126    unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut ArcInner<[T]> {
2127        unsafe {
2128            Arc::allocate_for_layout(
2129                Layout::array::<T>(len).unwrap(),
2130                |layout| alloc.allocate(layout),
2131                |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2132            )
2133        }
2134    }
2135}
2136
2137/// Specialization trait used for `From<&[T]>`.
2138#[cfg(not(no_global_oom_handling))]
2139trait ArcFromSlice<T> {
2140    fn from_slice(slice: &[T]) -> Self;
2141}
2142
2143#[cfg(not(no_global_oom_handling))]
2144impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
2145    #[inline]
2146    default fn from_slice(v: &[T]) -> Self {
2147        unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
2148    }
2149}
2150
2151#[cfg(not(no_global_oom_handling))]
2152impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
2153    #[inline]
2154    fn from_slice(v: &[T]) -> Self {
2155        unsafe { Arc::copy_from_slice(v) }
2156    }
2157}
2158
2159#[stable(feature = "rust1", since = "1.0.0")]
2160impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
2161    /// Makes a clone of the `Arc` pointer.
2162    ///
2163    /// This creates another pointer to the same allocation, increasing the
2164    /// strong reference count.
2165    ///
2166    /// # Examples
2167    ///
2168    /// ```
2169    /// use std::sync::Arc;
2170    ///
2171    /// let five = Arc::new(5);
2172    ///
2173    /// let _ = Arc::clone(&five);
2174    /// ```
2175    #[inline]
2176    fn clone(&self) -> Arc<T, A> {
2177        // Using a relaxed ordering is alright here, as knowledge of the
2178        // original reference prevents other threads from erroneously deleting
2179        // the object.
2180        //
2181        // As explained in the [Boost documentation][1], Increasing the
2182        // reference counter can always be done with memory_order_relaxed: New
2183        // references to an object can only be formed from an existing
2184        // reference, and passing an existing reference from one thread to
2185        // another must already provide any required synchronization.
2186        //
2187        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2188        let old_size = self.inner().strong.fetch_add(1, Relaxed);
2189
2190        // However we need to guard against massive refcounts in case someone is `mem::forget`ing
2191        // Arcs. If we don't do this the count can overflow and users will use-after free. This
2192        // branch will never be taken in any realistic program. We abort because such a program is
2193        // incredibly degenerate, and we don't care to support it.
2194        //
2195        // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
2196        // But we do that check *after* having done the increment, so there is a chance here that
2197        // the worst already happened and we actually do overflow the `usize` counter. However, that
2198        // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
2199        // above and the `abort` below, which seems exceedingly unlikely.
2200        //
2201        // This is a global invariant, and also applies when using a compare-exchange loop to increment
2202        // counters in other methods.
2203        // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
2204        // and then overflow using a few `fetch_add`s.
2205        if old_size > MAX_REFCOUNT {
2206            abort();
2207        }
2208
2209        unsafe { Self::from_inner_in(self.ptr, self.alloc.clone()) }
2210    }
2211}
2212
2213#[unstable(feature = "ergonomic_clones", issue = "132290")]
2214impl<T: ?Sized, A: Allocator + Clone> UseCloned for Arc<T, A> {}
2215
2216#[stable(feature = "rust1", since = "1.0.0")]
2217impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
2218    type Target = T;
2219
2220    #[inline]
2221    fn deref(&self) -> &T {
2222        &self.inner().data
2223    }
2224}
2225
2226#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2227unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
2228
2229#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2230unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {}
2231
2232#[unstable(feature = "deref_pure_trait", issue = "87121")]
2233unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
2234
2235#[unstable(feature = "legacy_receiver_trait", issue = "none")]
2236impl<T: ?Sized> LegacyReceiver for Arc<T> {}
2237
2238#[cfg(not(no_global_oom_handling))]
2239impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
2240    /// Makes a mutable reference into the given `Arc`.
2241    ///
2242    /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
2243    /// [`clone`] the inner value to a new allocation to ensure unique ownership.  This is also
2244    /// referred to as clone-on-write.
2245    ///
2246    /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
2247    /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
2248    /// be cloned.
2249    ///
2250    /// See also [`get_mut`], which will fail rather than cloning the inner value
2251    /// or dissociating [`Weak`] pointers.
2252    ///
2253    /// [`clone`]: Clone::clone
2254    /// [`get_mut`]: Arc::get_mut
2255    ///
2256    /// # Examples
2257    ///
2258    /// ```
2259    /// use std::sync::Arc;
2260    ///
2261    /// let mut data = Arc::new(5);
2262    ///
2263    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
2264    /// let mut other_data = Arc::clone(&data); // Won't clone inner data
2265    /// *Arc::make_mut(&mut data) += 1;         // Clones inner data
2266    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
2267    /// *Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything
2268    ///
2269    /// // Now `data` and `other_data` point to different allocations.
2270    /// assert_eq!(*data, 8);
2271    /// assert_eq!(*other_data, 12);
2272    /// ```
2273    ///
2274    /// [`Weak`] pointers will be dissociated:
2275    ///
2276    /// ```
2277    /// use std::sync::Arc;
2278    ///
2279    /// let mut data = Arc::new(75);
2280    /// let weak = Arc::downgrade(&data);
2281    ///
2282    /// assert!(75 == *data);
2283    /// assert!(75 == *weak.upgrade().unwrap());
2284    ///
2285    /// *Arc::make_mut(&mut data) += 1;
2286    ///
2287    /// assert!(76 == *data);
2288    /// assert!(weak.upgrade().is_none());
2289    /// ```
2290    #[inline]
2291    #[stable(feature = "arc_unique", since = "1.4.0")]
2292    pub fn make_mut(this: &mut Self) -> &mut T {
2293        let size_of_val = size_of_val::<T>(&**this);
2294
2295        // Note that we hold both a strong reference and a weak reference.
2296        // Thus, releasing our strong reference only will not, by itself, cause
2297        // the memory to be deallocated.
2298        //
2299        // Use Acquire to ensure that we see any writes to `weak` that happen
2300        // before release writes (i.e., decrements) to `strong`. Since we hold a
2301        // weak count, there's no chance the ArcInner itself could be
2302        // deallocated.
2303        if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
2304            // Another strong pointer exists, so we must clone.
2305
2306            let this_data_ref: &T = &**this;
2307            // `in_progress` drops the allocation if we panic before finishing initializing it.
2308            let mut in_progress: UniqueArcUninit<T, A> =
2309                UniqueArcUninit::new(this_data_ref, this.alloc.clone());
2310
2311            let initialized_clone = unsafe {
2312                // Clone. If the clone panics, `in_progress` will be dropped and clean up.
2313                this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
2314                // Cast type of pointer, now that it is initialized.
2315                in_progress.into_arc()
2316            };
2317            *this = initialized_clone;
2318        } else if this.inner().weak.load(Relaxed) != 1 {
2319            // Relaxed suffices in the above because this is fundamentally an
2320            // optimization: we are always racing with weak pointers being
2321            // dropped. Worst case, we end up allocated a new Arc unnecessarily.
2322
2323            // We removed the last strong ref, but there are additional weak
2324            // refs remaining. We'll move the contents to a new Arc, and
2325            // invalidate the other weak refs.
2326
2327            // Note that it is not possible for the read of `weak` to yield
2328            // usize::MAX (i.e., locked), since the weak count can only be
2329            // locked by a thread with a strong reference.
2330
2331            // Materialize our own implicit weak pointer, so that it can clean
2332            // up the ArcInner as needed.
2333            let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
2334
2335            // Can just steal the data, all that's left is Weaks
2336            //
2337            // We don't need panic-protection like the above branch does, but we might as well
2338            // use the same mechanism.
2339            let mut in_progress: UniqueArcUninit<T, A> =
2340                UniqueArcUninit::new(&**this, this.alloc.clone());
2341            unsafe {
2342                // Initialize `in_progress` with move of **this.
2343                // We have to express this in terms of bytes because `T: ?Sized`; there is no
2344                // operation that just copies a value based on its `size_of_val()`.
2345                ptr::copy_nonoverlapping(
2346                    ptr::from_ref(&**this).cast::<u8>(),
2347                    in_progress.data_ptr().cast::<u8>(),
2348                    size_of_val,
2349                );
2350
2351                ptr::write(this, in_progress.into_arc());
2352            }
2353        } else {
2354            // We were the sole reference of either kind; bump back up the
2355            // strong ref count.
2356            this.inner().strong.store(1, Release);
2357        }
2358
2359        // As with `get_mut()`, the unsafety is ok because our reference was
2360        // either unique to begin with, or became one upon cloning the contents.
2361        unsafe { Self::get_mut_unchecked(this) }
2362    }
2363}
2364
2365impl<T: Clone, A: Allocator> Arc<T, A> {
2366    /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
2367    /// clone.
2368    ///
2369    /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
2370    /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
2371    ///
2372    /// # Examples
2373    ///
2374    /// ```
2375    /// # use std::{ptr, sync::Arc};
2376    /// let inner = String::from("test");
2377    /// let ptr = inner.as_ptr();
2378    ///
2379    /// let arc = Arc::new(inner);
2380    /// let inner = Arc::unwrap_or_clone(arc);
2381    /// // The inner value was not cloned
2382    /// assert!(ptr::eq(ptr, inner.as_ptr()));
2383    ///
2384    /// let arc = Arc::new(inner);
2385    /// let arc2 = arc.clone();
2386    /// let inner = Arc::unwrap_or_clone(arc);
2387    /// // Because there were 2 references, we had to clone the inner value.
2388    /// assert!(!ptr::eq(ptr, inner.as_ptr()));
2389    /// // `arc2` is the last reference, so when we unwrap it we get back
2390    /// // the original `String`.
2391    /// let inner = Arc::unwrap_or_clone(arc2);
2392    /// assert!(ptr::eq(ptr, inner.as_ptr()));
2393    /// ```
2394    #[inline]
2395    #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")]
2396    pub fn unwrap_or_clone(this: Self) -> T {
2397        Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
2398    }
2399}
2400
2401impl<T: ?Sized, A: Allocator> Arc<T, A> {
2402    /// Returns a mutable reference into the given `Arc`, if there are
2403    /// no other `Arc` or [`Weak`] pointers to the same allocation.
2404    ///
2405    /// Returns [`None`] otherwise, because it is not safe to
2406    /// mutate a shared value.
2407    ///
2408    /// See also [`make_mut`][make_mut], which will [`clone`][clone]
2409    /// the inner value when there are other `Arc` pointers.
2410    ///
2411    /// [make_mut]: Arc::make_mut
2412    /// [clone]: Clone::clone
2413    ///
2414    /// # Examples
2415    ///
2416    /// ```
2417    /// use std::sync::Arc;
2418    ///
2419    /// let mut x = Arc::new(3);
2420    /// *Arc::get_mut(&mut x).unwrap() = 4;
2421    /// assert_eq!(*x, 4);
2422    ///
2423    /// let _y = Arc::clone(&x);
2424    /// assert!(Arc::get_mut(&mut x).is_none());
2425    /// ```
2426    #[inline]
2427    #[stable(feature = "arc_unique", since = "1.4.0")]
2428    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
2429        if Self::is_unique(this) {
2430            // This unsafety is ok because we're guaranteed that the pointer
2431            // returned is the *only* pointer that will ever be returned to T. Our
2432            // reference count is guaranteed to be 1 at this point, and we required
2433            // the Arc itself to be `mut`, so we're returning the only possible
2434            // reference to the inner data.
2435            unsafe { Some(Arc::get_mut_unchecked(this)) }
2436        } else {
2437            None
2438        }
2439    }
2440
2441    /// Returns a mutable reference into the given `Arc`,
2442    /// without any check.
2443    ///
2444    /// See also [`get_mut`], which is safe and does appropriate checks.
2445    ///
2446    /// [`get_mut`]: Arc::get_mut
2447    ///
2448    /// # Safety
2449    ///
2450    /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
2451    /// they must not be dereferenced or have active borrows for the duration
2452    /// of the returned borrow, and their inner type must be exactly the same as the
2453    /// inner type of this Rc (including lifetimes). This is trivially the case if no
2454    /// such pointers exist, for example immediately after `Arc::new`.
2455    ///
2456    /// # Examples
2457    ///
2458    /// ```
2459    /// #![feature(get_mut_unchecked)]
2460    ///
2461    /// use std::sync::Arc;
2462    ///
2463    /// let mut x = Arc::new(String::new());
2464    /// unsafe {
2465    ///     Arc::get_mut_unchecked(&mut x).push_str("foo")
2466    /// }
2467    /// assert_eq!(*x, "foo");
2468    /// ```
2469    /// Other `Arc` pointers to the same allocation must be to the same type.
2470    /// ```no_run
2471    /// #![feature(get_mut_unchecked)]
2472    ///
2473    /// use std::sync::Arc;
2474    ///
2475    /// let x: Arc<str> = Arc::from("Hello, world!");
2476    /// let mut y: Arc<[u8]> = x.clone().into();
2477    /// unsafe {
2478    ///     // this is Undefined Behavior, because x's inner type is str, not [u8]
2479    ///     Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
2480    /// }
2481    /// println!("{}", &*x); // Invalid UTF-8 in a str
2482    /// ```
2483    /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
2484    /// ```no_run
2485    /// #![feature(get_mut_unchecked)]
2486    ///
2487    /// use std::sync::Arc;
2488    ///
2489    /// let x: Arc<&str> = Arc::new("Hello, world!");
2490    /// {
2491    ///     let s = String::from("Oh, no!");
2492    ///     let mut y: Arc<&str> = x.clone();
2493    ///     unsafe {
2494    ///         // this is Undefined Behavior, because x's inner type
2495    ///         // is &'long str, not &'short str
2496    ///         *Arc::get_mut_unchecked(&mut y) = &s;
2497    ///     }
2498    /// }
2499    /// println!("{}", &*x); // Use-after-free
2500    /// ```
2501    #[inline]
2502    #[unstable(feature = "get_mut_unchecked", issue = "63292")]
2503    pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
2504        // We are careful to *not* create a reference covering the "count" fields, as
2505        // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
2506        unsafe { &mut (*this.ptr.as_ptr()).data }
2507    }
2508
2509    /// Determine whether this is the unique reference to the underlying data.
2510    ///
2511    /// Returns `true` if there are no other `Arc` or [`Weak`] pointers to the same allocation;
2512    /// returns `false` otherwise.
2513    ///
2514    /// If this function returns `true`, then is guaranteed to be safe to call [`get_mut_unchecked`]
2515    /// on this `Arc`, so long as no clones occur in between.
2516    ///
2517    /// # Examples
2518    ///
2519    /// ```
2520    /// #![feature(arc_is_unique)]
2521    ///
2522    /// use std::sync::Arc;
2523    ///
2524    /// let x = Arc::new(3);
2525    /// assert!(Arc::is_unique(&x));
2526    ///
2527    /// let y = Arc::clone(&x);
2528    /// assert!(!Arc::is_unique(&x));
2529    /// drop(y);
2530    ///
2531    /// // Weak references also count, because they could be upgraded at any time.
2532    /// let z = Arc::downgrade(&x);
2533    /// assert!(!Arc::is_unique(&x));
2534    /// ```
2535    ///
2536    /// # Pointer invalidation
2537    ///
2538    /// This function will always return the same value as `Arc::get_mut(arc).is_some()`. However,
2539    /// unlike that operation it does not produce any mutable references to the underlying data,
2540    /// meaning no pointers to the data inside the `Arc` are invalidated by the call. Thus, the
2541    /// following code is valid, even though it would be UB if it used `Arc::get_mut`:
2542    ///
2543    /// ```
2544    /// #![feature(arc_is_unique)]
2545    ///
2546    /// use std::sync::Arc;
2547    ///
2548    /// let arc = Arc::new(5);
2549    /// let pointer: *const i32 = &*arc;
2550    /// assert!(Arc::is_unique(&arc));
2551    /// assert_eq!(unsafe { *pointer }, 5);
2552    /// ```
2553    ///
2554    /// # Atomic orderings
2555    ///
2556    /// Concurrent drops to other `Arc` pointers to the same allocation will synchronize with this
2557    /// call - that is, this call performs an `Acquire` operation on the underlying strong and weak
2558    /// ref counts. This ensures that calling `get_mut_unchecked` is safe.
2559    ///
2560    /// Note that this operation requires locking the weak ref count, so concurrent calls to
2561    /// `downgrade` may spin-loop for a short period of time.
2562    ///
2563    /// [`get_mut_unchecked`]: Self::get_mut_unchecked
2564    #[inline]
2565    #[unstable(feature = "arc_is_unique", issue = "138938")]
2566    pub fn is_unique(this: &Self) -> bool {
2567        // lock the weak pointer count if we appear to be the sole weak pointer
2568        // holder.
2569        //
2570        // The acquire label here ensures a happens-before relationship with any
2571        // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
2572        // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
2573        // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
2574        if this.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
2575            // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
2576            // counter in `drop` -- the only access that happens when any but the last reference
2577            // is being dropped.
2578            let unique = this.inner().strong.load(Acquire) == 1;
2579
2580            // The release write here synchronizes with a read in `downgrade`,
2581            // effectively preventing the above read of `strong` from happening
2582            // after the write.
2583            this.inner().weak.store(1, Release); // release the lock
2584            unique
2585        } else {
2586            false
2587        }
2588    }
2589}
2590
2591#[stable(feature = "rust1", since = "1.0.0")]
2592unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
2593    /// Drops the `Arc`.
2594    ///
2595    /// This will decrement the strong reference count. If the strong reference
2596    /// count reaches zero then the only other references (if any) are
2597    /// [`Weak`], so we `drop` the inner value.
2598    ///
2599    /// # Examples
2600    ///
2601    /// ```
2602    /// use std::sync::Arc;
2603    ///
2604    /// struct Foo;
2605    ///
2606    /// impl Drop for Foo {
2607    ///     fn drop(&mut self) {
2608    ///         println!("dropped!");
2609    ///     }
2610    /// }
2611    ///
2612    /// let foo  = Arc::new(Foo);
2613    /// let foo2 = Arc::clone(&foo);
2614    ///
2615    /// drop(foo);    // Doesn't print anything
2616    /// drop(foo2);   // Prints "dropped!"
2617    /// ```
2618    #[inline]
2619    fn drop(&mut self) {
2620        // Because `fetch_sub` is already atomic, we do not need to synchronize
2621        // with other threads unless we are going to delete the object. This
2622        // same logic applies to the below `fetch_sub` to the `weak` count.
2623        if self.inner().strong.fetch_sub(1, Release) != 1 {
2624            return;
2625        }
2626
2627        // This fence is needed to prevent reordering of use of the data and
2628        // deletion of the data. Because it is marked `Release`, the decreasing
2629        // of the reference count synchronizes with this `Acquire` fence. This
2630        // means that use of the data happens before decreasing the reference
2631        // count, which happens before this fence, which happens before the
2632        // deletion of the data.
2633        //
2634        // As explained in the [Boost documentation][1],
2635        //
2636        // > It is important to enforce any possible access to the object in one
2637        // > thread (through an existing reference) to *happen before* deleting
2638        // > the object in a different thread. This is achieved by a "release"
2639        // > operation after dropping a reference (any access to the object
2640        // > through this reference must obviously happened before), and an
2641        // > "acquire" operation before deleting the object.
2642        //
2643        // In particular, while the contents of an Arc are usually immutable, it's
2644        // possible to have interior writes to something like a Mutex<T>. Since a
2645        // Mutex is not acquired when it is deleted, we can't rely on its
2646        // synchronization logic to make writes in thread A visible to a destructor
2647        // running in thread B.
2648        //
2649        // Also note that the Acquire fence here could probably be replaced with an
2650        // Acquire load, which could improve performance in highly-contended
2651        // situations. See [2].
2652        //
2653        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2654        // [2]: (https://github.com/rust-lang/rust/pull/41714)
2655        acquire!(self.inner().strong);
2656
2657        // Make sure we aren't trying to "drop" the shared static for empty slices
2658        // used by Default::default.
2659        debug_assert!(
2660            !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
2661            "Arcs backed by a static should never reach a strong count of 0. \
2662            Likely decrement_strong_count or from_raw were called too many times.",
2663        );
2664
2665        unsafe {
2666            self.drop_slow();
2667        }
2668    }
2669}
2670
2671impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
2672    /// Attempts to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
2673    ///
2674    /// # Examples
2675    ///
2676    /// ```
2677    /// use std::any::Any;
2678    /// use std::sync::Arc;
2679    ///
2680    /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
2681    ///     if let Ok(string) = value.downcast::<String>() {
2682    ///         println!("String ({}): {}", string.len(), string);
2683    ///     }
2684    /// }
2685    ///
2686    /// let my_string = "Hello World".to_string();
2687    /// print_if_string(Arc::new(my_string));
2688    /// print_if_string(Arc::new(0i8));
2689    /// ```
2690    #[inline]
2691    #[stable(feature = "rc_downcast", since = "1.29.0")]
2692    pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
2693    where
2694        T: Any + Send + Sync,
2695    {
2696        if (*self).is::<T>() {
2697            unsafe {
2698                let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2699                Ok(Arc::from_inner_in(ptr.cast(), alloc))
2700            }
2701        } else {
2702            Err(self)
2703        }
2704    }
2705
2706    /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
2707    ///
2708    /// For a safe alternative see [`downcast`].
2709    ///
2710    /// # Examples
2711    ///
2712    /// ```
2713    /// #![feature(downcast_unchecked)]
2714    ///
2715    /// use std::any::Any;
2716    /// use std::sync::Arc;
2717    ///
2718    /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
2719    ///
2720    /// unsafe {
2721    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
2722    /// }
2723    /// ```
2724    ///
2725    /// # Safety
2726    ///
2727    /// The contained value must be of type `T`. Calling this method
2728    /// with the incorrect type is *undefined behavior*.
2729    ///
2730    ///
2731    /// [`downcast`]: Self::downcast
2732    #[inline]
2733    #[unstable(feature = "downcast_unchecked", issue = "90850")]
2734    pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
2735    where
2736        T: Any + Send + Sync,
2737    {
2738        unsafe {
2739            let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2740            Arc::from_inner_in(ptr.cast(), alloc)
2741        }
2742    }
2743}
2744
2745impl<T> Weak<T> {
2746    /// Constructs a new `Weak<T>`, without allocating any memory.
2747    /// Calling [`upgrade`] on the return value always gives [`None`].
2748    ///
2749    /// [`upgrade`]: Weak::upgrade
2750    ///
2751    /// # Examples
2752    ///
2753    /// ```
2754    /// use std::sync::Weak;
2755    ///
2756    /// let empty: Weak<i64> = Weak::new();
2757    /// assert!(empty.upgrade().is_none());
2758    /// ```
2759    #[inline]
2760    #[stable(feature = "downgraded_weak", since = "1.10.0")]
2761    #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
2762    #[must_use]
2763    pub const fn new() -> Weak<T> {
2764        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
2765    }
2766}
2767
2768impl<T, A: Allocator> Weak<T, A> {
2769    /// Constructs a new `Weak<T, A>`, without allocating any memory, technically in the provided
2770    /// allocator.
2771    /// Calling [`upgrade`] on the return value always gives [`None`].
2772    ///
2773    /// [`upgrade`]: Weak::upgrade
2774    ///
2775    /// # Examples
2776    ///
2777    /// ```
2778    /// #![feature(allocator_api)]
2779    ///
2780    /// use std::sync::Weak;
2781    /// use std::alloc::System;
2782    ///
2783    /// let empty: Weak<i64, _> = Weak::new_in(System);
2784    /// assert!(empty.upgrade().is_none());
2785    /// ```
2786    #[inline]
2787    #[unstable(feature = "allocator_api", issue = "32838")]
2788    pub fn new_in(alloc: A) -> Weak<T, A> {
2789        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
2790    }
2791}
2792
2793/// Helper type to allow accessing the reference counts without
2794/// making any assertions about the data field.
2795struct WeakInner<'a> {
2796    weak: &'a Atomic<usize>,
2797    strong: &'a Atomic<usize>,
2798}
2799
2800impl<T: ?Sized> Weak<T> {
2801    /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2802    ///
2803    /// This can be used to safely get a strong reference (by calling [`upgrade`]
2804    /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2805    ///
2806    /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2807    /// as these don't own anything; the method still works on them).
2808    ///
2809    /// # Safety
2810    ///
2811    /// The pointer must have originated from the [`into_raw`] and must still own its potential
2812    /// weak reference, and must point to a block of memory allocated by global allocator.
2813    ///
2814    /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2815    /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2816    /// count is not modified by this operation) and therefore it must be paired with a previous
2817    /// call to [`into_raw`].
2818    /// # Examples
2819    ///
2820    /// ```
2821    /// use std::sync::{Arc, Weak};
2822    ///
2823    /// let strong = Arc::new("hello".to_owned());
2824    ///
2825    /// let raw_1 = Arc::downgrade(&strong).into_raw();
2826    /// let raw_2 = Arc::downgrade(&strong).into_raw();
2827    ///
2828    /// assert_eq!(2, Arc::weak_count(&strong));
2829    ///
2830    /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2831    /// assert_eq!(1, Arc::weak_count(&strong));
2832    ///
2833    /// drop(strong);
2834    ///
2835    /// // Decrement the last weak count.
2836    /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2837    /// ```
2838    ///
2839    /// [`new`]: Weak::new
2840    /// [`into_raw`]: Weak::into_raw
2841    /// [`upgrade`]: Weak::upgrade
2842    #[inline]
2843    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2844    pub unsafe fn from_raw(ptr: *const T) -> Self {
2845        unsafe { Weak::from_raw_in(ptr, Global) }
2846    }
2847
2848    /// Consumes the `Weak<T>` and turns it into a raw pointer.
2849    ///
2850    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2851    /// one weak reference (the weak count is not modified by this operation). It can be turned
2852    /// back into the `Weak<T>` with [`from_raw`].
2853    ///
2854    /// The same restrictions of accessing the target of the pointer as with
2855    /// [`as_ptr`] apply.
2856    ///
2857    /// # Examples
2858    ///
2859    /// ```
2860    /// use std::sync::{Arc, Weak};
2861    ///
2862    /// let strong = Arc::new("hello".to_owned());
2863    /// let weak = Arc::downgrade(&strong);
2864    /// let raw = weak.into_raw();
2865    ///
2866    /// assert_eq!(1, Arc::weak_count(&strong));
2867    /// assert_eq!("hello", unsafe { &*raw });
2868    ///
2869    /// drop(unsafe { Weak::from_raw(raw) });
2870    /// assert_eq!(0, Arc::weak_count(&strong));
2871    /// ```
2872    ///
2873    /// [`from_raw`]: Weak::from_raw
2874    /// [`as_ptr`]: Weak::as_ptr
2875    #[must_use = "losing the pointer will leak memory"]
2876    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2877    pub fn into_raw(self) -> *const T {
2878        ManuallyDrop::new(self).as_ptr()
2879    }
2880}
2881
2882impl<T: ?Sized, A: Allocator> Weak<T, A> {
2883    /// Returns a reference to the underlying allocator.
2884    #[inline]
2885    #[unstable(feature = "allocator_api", issue = "32838")]
2886    pub fn allocator(&self) -> &A {
2887        &self.alloc
2888    }
2889
2890    /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2891    ///
2892    /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2893    /// unaligned or even [`null`] otherwise.
2894    ///
2895    /// # Examples
2896    ///
2897    /// ```
2898    /// use std::sync::Arc;
2899    /// use std::ptr;
2900    ///
2901    /// let strong = Arc::new("hello".to_owned());
2902    /// let weak = Arc::downgrade(&strong);
2903    /// // Both point to the same object
2904    /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2905    /// // The strong here keeps it alive, so we can still access the object.
2906    /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2907    ///
2908    /// drop(strong);
2909    /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2910    /// // undefined behavior.
2911    /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2912    /// ```
2913    ///
2914    /// [`null`]: core::ptr::null "ptr::null"
2915    #[must_use]
2916    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2917    pub fn as_ptr(&self) -> *const T {
2918        let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
2919
2920        if is_dangling(ptr) {
2921            // If the pointer is dangling, we return the sentinel directly. This cannot be
2922            // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
2923            ptr as *const T
2924        } else {
2925            // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2926            // The payload may be dropped at this point, and we have to maintain provenance,
2927            // so use raw pointer manipulation.
2928            unsafe { &raw mut (*ptr).data }
2929        }
2930    }
2931
2932    /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
2933    ///
2934    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2935    /// one weak reference (the weak count is not modified by this operation). It can be turned
2936    /// back into the `Weak<T>` with [`from_raw_in`].
2937    ///
2938    /// The same restrictions of accessing the target of the pointer as with
2939    /// [`as_ptr`] apply.
2940    ///
2941    /// # Examples
2942    ///
2943    /// ```
2944    /// #![feature(allocator_api)]
2945    /// use std::sync::{Arc, Weak};
2946    /// use std::alloc::System;
2947    ///
2948    /// let strong = Arc::new_in("hello".to_owned(), System);
2949    /// let weak = Arc::downgrade(&strong);
2950    /// let (raw, alloc) = weak.into_raw_with_allocator();
2951    ///
2952    /// assert_eq!(1, Arc::weak_count(&strong));
2953    /// assert_eq!("hello", unsafe { &*raw });
2954    ///
2955    /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
2956    /// assert_eq!(0, Arc::weak_count(&strong));
2957    /// ```
2958    ///
2959    /// [`from_raw_in`]: Weak::from_raw_in
2960    /// [`as_ptr`]: Weak::as_ptr
2961    #[must_use = "losing the pointer will leak memory"]
2962    #[unstable(feature = "allocator_api", issue = "32838")]
2963    pub fn into_raw_with_allocator(self) -> (*const T, A) {
2964        let this = mem::ManuallyDrop::new(self);
2965        let result = this.as_ptr();
2966        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
2967        let alloc = unsafe { ptr::read(&this.alloc) };
2968        (result, alloc)
2969    }
2970
2971    /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
2972    /// allocator.
2973    ///
2974    /// This can be used to safely get a strong reference (by calling [`upgrade`]
2975    /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2976    ///
2977    /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2978    /// as these don't own anything; the method still works on them).
2979    ///
2980    /// # Safety
2981    ///
2982    /// The pointer must have originated from the [`into_raw`] and must still own its potential
2983    /// weak reference, and must point to a block of memory allocated by `alloc`.
2984    ///
2985    /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2986    /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2987    /// count is not modified by this operation) and therefore it must be paired with a previous
2988    /// call to [`into_raw`].
2989    /// # Examples
2990    ///
2991    /// ```
2992    /// use std::sync::{Arc, Weak};
2993    ///
2994    /// let strong = Arc::new("hello".to_owned());
2995    ///
2996    /// let raw_1 = Arc::downgrade(&strong).into_raw();
2997    /// let raw_2 = Arc::downgrade(&strong).into_raw();
2998    ///
2999    /// assert_eq!(2, Arc::weak_count(&strong));
3000    ///
3001    /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
3002    /// assert_eq!(1, Arc::weak_count(&strong));
3003    ///
3004    /// drop(strong);
3005    ///
3006    /// // Decrement the last weak count.
3007    /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
3008    /// ```
3009    ///
3010    /// [`new`]: Weak::new
3011    /// [`into_raw`]: Weak::into_raw
3012    /// [`upgrade`]: Weak::upgrade
3013    #[inline]
3014    #[unstable(feature = "allocator_api", issue = "32838")]
3015    pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
3016        // See Weak::as_ptr for context on how the input pointer is derived.
3017
3018        let ptr = if is_dangling(ptr) {
3019            // This is a dangling Weak.
3020            ptr as *mut ArcInner<T>
3021        } else {
3022            // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
3023            // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
3024            let offset = unsafe { data_offset(ptr) };
3025            // Thus, we reverse the offset to get the whole RcInner.
3026            // SAFETY: the pointer originated from a Weak, so this offset is safe.
3027            unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
3028        };
3029
3030        // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
3031        Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc }
3032    }
3033}
3034
3035impl<T: ?Sized, A: Allocator> Weak<T, A> {
3036    /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
3037    /// dropping of the inner value if successful.
3038    ///
3039    /// Returns [`None`] if the inner value has since been dropped.
3040    ///
3041    /// # Examples
3042    ///
3043    /// ```
3044    /// use std::sync::Arc;
3045    ///
3046    /// let five = Arc::new(5);
3047    ///
3048    /// let weak_five = Arc::downgrade(&five);
3049    ///
3050    /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
3051    /// assert!(strong_five.is_some());
3052    ///
3053    /// // Destroy all strong pointers.
3054    /// drop(strong_five);
3055    /// drop(five);
3056    ///
3057    /// assert!(weak_five.upgrade().is_none());
3058    /// ```
3059    #[must_use = "this returns a new `Arc`, \
3060                  without modifying the original weak pointer"]
3061    #[stable(feature = "arc_weak", since = "1.4.0")]
3062    pub fn upgrade(&self) -> Option<Arc<T, A>>
3063    where
3064        A: Clone,
3065    {
3066        #[inline]
3067        fn checked_increment(n: usize) -> Option<usize> {
3068            // Any write of 0 we can observe leaves the field in permanently zero state.
3069            if n == 0 {
3070                return None;
3071            }
3072            // See comments in `Arc::clone` for why we do this (for `mem::forget`).
3073            assert!(n <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
3074            Some(n + 1)
3075        }
3076
3077        // We use a CAS loop to increment the strong count instead of a
3078        // fetch_add as this function should never take the reference count
3079        // from zero to one.
3080        //
3081        // Relaxed is fine for the failure case because we don't have any expectations about the new state.
3082        // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
3083        // value can be initialized after `Weak` references have already been created. In that case, we
3084        // expect to observe the fully initialized value.
3085        if self.inner()?.strong.fetch_update(Acquire, Relaxed, checked_increment).is_ok() {
3086            // SAFETY: pointer is not null, verified in checked_increment
3087            unsafe { Some(Arc::from_inner_in(self.ptr, self.alloc.clone())) }
3088        } else {
3089            None
3090        }
3091    }
3092
3093    /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
3094    ///
3095    /// If `self` was created using [`Weak::new`], this will return 0.
3096    #[must_use]
3097    #[stable(feature = "weak_counts", since = "1.41.0")]
3098    pub fn strong_count(&self) -> usize {
3099        if let Some(inner) = self.inner() { inner.strong.load(Relaxed) } else { 0 }
3100    }
3101
3102    /// Gets an approximation of the number of `Weak` pointers pointing to this
3103    /// allocation.
3104    ///
3105    /// If `self` was created using [`Weak::new`], or if there are no remaining
3106    /// strong pointers, this will return 0.
3107    ///
3108    /// # Accuracy
3109    ///
3110    /// Due to implementation details, the returned value can be off by 1 in
3111    /// either direction when other threads are manipulating any `Arc`s or
3112    /// `Weak`s pointing to the same allocation.
3113    #[must_use]
3114    #[stable(feature = "weak_counts", since = "1.41.0")]
3115    pub fn weak_count(&self) -> usize {
3116        if let Some(inner) = self.inner() {
3117            let weak = inner.weak.load(Acquire);
3118            let strong = inner.strong.load(Relaxed);
3119            if strong == 0 {
3120                0
3121            } else {
3122                // Since we observed that there was at least one strong pointer
3123                // after reading the weak count, we know that the implicit weak
3124                // reference (present whenever any strong references are alive)
3125                // was still around when we observed the weak count, and can
3126                // therefore safely subtract it.
3127                weak - 1
3128            }
3129        } else {
3130            0
3131        }
3132    }
3133
3134    /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
3135    /// (i.e., when this `Weak` was created by `Weak::new`).
3136    #[inline]
3137    fn inner(&self) -> Option<WeakInner<'_>> {
3138        let ptr = self.ptr.as_ptr();
3139        if is_dangling(ptr) {
3140            None
3141        } else {
3142            // We are careful to *not* create a reference covering the "data" field, as
3143            // the field may be mutated concurrently (for example, if the last `Arc`
3144            // is dropped, the data field will be dropped in-place).
3145            Some(unsafe { WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak } })
3146        }
3147    }
3148
3149    /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
3150    /// both don't point to any allocation (because they were created with `Weak::new()`). However,
3151    /// this function ignores the metadata of  `dyn Trait` pointers.
3152    ///
3153    /// # Notes
3154    ///
3155    /// Since this compares pointers it means that `Weak::new()` will equal each
3156    /// other, even though they don't point to any allocation.
3157    ///
3158    /// # Examples
3159    ///
3160    /// ```
3161    /// use std::sync::Arc;
3162    ///
3163    /// let first_rc = Arc::new(5);
3164    /// let first = Arc::downgrade(&first_rc);
3165    /// let second = Arc::downgrade(&first_rc);
3166    ///
3167    /// assert!(first.ptr_eq(&second));
3168    ///
3169    /// let third_rc = Arc::new(5);
3170    /// let third = Arc::downgrade(&third_rc);
3171    ///
3172    /// assert!(!first.ptr_eq(&third));
3173    /// ```
3174    ///
3175    /// Comparing `Weak::new`.
3176    ///
3177    /// ```
3178    /// use std::sync::{Arc, Weak};
3179    ///
3180    /// let first = Weak::new();
3181    /// let second = Weak::new();
3182    /// assert!(first.ptr_eq(&second));
3183    ///
3184    /// let third_rc = Arc::new(());
3185    /// let third = Arc::downgrade(&third_rc);
3186    /// assert!(!first.ptr_eq(&third));
3187    /// ```
3188    ///
3189    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
3190    #[inline]
3191    #[must_use]
3192    #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
3193    pub fn ptr_eq(&self, other: &Self) -> bool {
3194        ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr())
3195    }
3196}
3197
3198#[stable(feature = "arc_weak", since = "1.4.0")]
3199impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
3200    /// Makes a clone of the `Weak` pointer that points to the same allocation.
3201    ///
3202    /// # Examples
3203    ///
3204    /// ```
3205    /// use std::sync::{Arc, Weak};
3206    ///
3207    /// let weak_five = Arc::downgrade(&Arc::new(5));
3208    ///
3209    /// let _ = Weak::clone(&weak_five);
3210    /// ```
3211    #[inline]
3212    fn clone(&self) -> Weak<T, A> {
3213        if let Some(inner) = self.inner() {
3214            // See comments in Arc::clone() for why this is relaxed. This can use a
3215            // fetch_add (ignoring the lock) because the weak count is only locked
3216            // where are *no other* weak pointers in existence. (So we can't be
3217            // running this code in that case).
3218            let old_size = inner.weak.fetch_add(1, Relaxed);
3219
3220            // See comments in Arc::clone() for why we do this (for mem::forget).
3221            if old_size > MAX_REFCOUNT {
3222                abort();
3223            }
3224        }
3225
3226        Weak { ptr: self.ptr, alloc: self.alloc.clone() }
3227    }
3228}
3229
3230#[unstable(feature = "ergonomic_clones", issue = "132290")]
3231impl<T: ?Sized, A: Allocator + Clone> UseCloned for Weak<T, A> {}
3232
3233#[stable(feature = "downgraded_weak", since = "1.10.0")]
3234impl<T> Default for Weak<T> {
3235    /// Constructs a new `Weak<T>`, without allocating memory.
3236    /// Calling [`upgrade`] on the return value always
3237    /// gives [`None`].
3238    ///
3239    /// [`upgrade`]: Weak::upgrade
3240    ///
3241    /// # Examples
3242    ///
3243    /// ```
3244    /// use std::sync::Weak;
3245    ///
3246    /// let empty: Weak<i64> = Default::default();
3247    /// assert!(empty.upgrade().is_none());
3248    /// ```
3249    fn default() -> Weak<T> {
3250        Weak::new()
3251    }
3252}
3253
3254#[stable(feature = "arc_weak", since = "1.4.0")]
3255unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> {
3256    /// Drops the `Weak` pointer.
3257    ///
3258    /// # Examples
3259    ///
3260    /// ```
3261    /// use std::sync::{Arc, Weak};
3262    ///
3263    /// struct Foo;
3264    ///
3265    /// impl Drop for Foo {
3266    ///     fn drop(&mut self) {
3267    ///         println!("dropped!");
3268    ///     }
3269    /// }
3270    ///
3271    /// let foo = Arc::new(Foo);
3272    /// let weak_foo = Arc::downgrade(&foo);
3273    /// let other_weak_foo = Weak::clone(&weak_foo);
3274    ///
3275    /// drop(weak_foo);   // Doesn't print anything
3276    /// drop(foo);        // Prints "dropped!"
3277    ///
3278    /// assert!(other_weak_foo.upgrade().is_none());
3279    /// ```
3280    fn drop(&mut self) {
3281        // If we find out that we were the last weak pointer, then its time to
3282        // deallocate the data entirely. See the discussion in Arc::drop() about
3283        // the memory orderings
3284        //
3285        // It's not necessary to check for the locked state here, because the
3286        // weak count can only be locked if there was precisely one weak ref,
3287        // meaning that drop could only subsequently run ON that remaining weak
3288        // ref, which can only happen after the lock is released.
3289        let inner = if let Some(inner) = self.inner() { inner } else { return };
3290
3291        if inner.weak.fetch_sub(1, Release) == 1 {
3292            acquire!(inner.weak);
3293
3294            // Make sure we aren't trying to "deallocate" the shared static for empty slices
3295            // used by Default::default.
3296            debug_assert!(
3297                !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
3298                "Arc/Weaks backed by a static should never be deallocated. \
3299                Likely decrement_strong_count or from_raw were called too many times.",
3300            );
3301
3302            unsafe {
3303                self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()))
3304            }
3305        }
3306    }
3307}
3308
3309#[stable(feature = "rust1", since = "1.0.0")]
3310trait ArcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
3311    fn eq(&self, other: &Arc<T, A>) -> bool;
3312    fn ne(&self, other: &Arc<T, A>) -> bool;
3313}
3314
3315#[stable(feature = "rust1", since = "1.0.0")]
3316impl<T: ?Sized + PartialEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3317    #[inline]
3318    default fn eq(&self, other: &Arc<T, A>) -> bool {
3319        **self == **other
3320    }
3321    #[inline]
3322    default fn ne(&self, other: &Arc<T, A>) -> bool {
3323        **self != **other
3324    }
3325}
3326
3327/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
3328/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
3329/// store large values, that are slow to clone, but also heavy to check for equality, causing this
3330/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
3331/// the same value, than two `&T`s.
3332///
3333/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
3334#[stable(feature = "rust1", since = "1.0.0")]
3335impl<T: ?Sized + crate::rc::MarkerEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3336    #[inline]
3337    fn eq(&self, other: &Arc<T, A>) -> bool {
3338        Arc::ptr_eq(self, other) || **self == **other
3339    }
3340
3341    #[inline]
3342    fn ne(&self, other: &Arc<T, A>) -> bool {
3343        !Arc::ptr_eq(self, other) && **self != **other
3344    }
3345}
3346
3347#[stable(feature = "rust1", since = "1.0.0")]
3348impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A> {
3349    /// Equality for two `Arc`s.
3350    ///
3351    /// Two `Arc`s are equal if their inner values are equal, even if they are
3352    /// stored in different allocation.
3353    ///
3354    /// If `T` also implements `Eq` (implying reflexivity of equality),
3355    /// two `Arc`s that point to the same allocation are always equal.
3356    ///
3357    /// # Examples
3358    ///
3359    /// ```
3360    /// use std::sync::Arc;
3361    ///
3362    /// let five = Arc::new(5);
3363    ///
3364    /// assert!(five == Arc::new(5));
3365    /// ```
3366    #[inline]
3367    fn eq(&self, other: &Arc<T, A>) -> bool {
3368        ArcEqIdent::eq(self, other)
3369    }
3370
3371    /// Inequality for two `Arc`s.
3372    ///
3373    /// Two `Arc`s are not equal if their inner values are not equal.
3374    ///
3375    /// If `T` also implements `Eq` (implying reflexivity of equality),
3376    /// two `Arc`s that point to the same value are always equal.
3377    ///
3378    /// # Examples
3379    ///
3380    /// ```
3381    /// use std::sync::Arc;
3382    ///
3383    /// let five = Arc::new(5);
3384    ///
3385    /// assert!(five != Arc::new(6));
3386    /// ```
3387    #[inline]
3388    fn ne(&self, other: &Arc<T, A>) -> bool {
3389        ArcEqIdent::ne(self, other)
3390    }
3391}
3392
3393#[stable(feature = "rust1", since = "1.0.0")]
3394impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A> {
3395    /// Partial comparison for two `Arc`s.
3396    ///
3397    /// The two are compared by calling `partial_cmp()` on their inner values.
3398    ///
3399    /// # Examples
3400    ///
3401    /// ```
3402    /// use std::sync::Arc;
3403    /// use std::cmp::Ordering;
3404    ///
3405    /// let five = Arc::new(5);
3406    ///
3407    /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
3408    /// ```
3409    fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering> {
3410        (**self).partial_cmp(&**other)
3411    }
3412
3413    /// Less-than comparison for two `Arc`s.
3414    ///
3415    /// The two are compared by calling `<` on their inner values.
3416    ///
3417    /// # Examples
3418    ///
3419    /// ```
3420    /// use std::sync::Arc;
3421    ///
3422    /// let five = Arc::new(5);
3423    ///
3424    /// assert!(five < Arc::new(6));
3425    /// ```
3426    fn lt(&self, other: &Arc<T, A>) -> bool {
3427        *(*self) < *(*other)
3428    }
3429
3430    /// 'Less than or equal to' comparison for two `Arc`s.
3431    ///
3432    /// The two are compared by calling `<=` on their inner values.
3433    ///
3434    /// # Examples
3435    ///
3436    /// ```
3437    /// use std::sync::Arc;
3438    ///
3439    /// let five = Arc::new(5);
3440    ///
3441    /// assert!(five <= Arc::new(5));
3442    /// ```
3443    fn le(&self, other: &Arc<T, A>) -> bool {
3444        *(*self) <= *(*other)
3445    }
3446
3447    /// Greater-than comparison for two `Arc`s.
3448    ///
3449    /// The two are compared by calling `>` on their inner values.
3450    ///
3451    /// # Examples
3452    ///
3453    /// ```
3454    /// use std::sync::Arc;
3455    ///
3456    /// let five = Arc::new(5);
3457    ///
3458    /// assert!(five > Arc::new(4));
3459    /// ```
3460    fn gt(&self, other: &Arc<T, A>) -> bool {
3461        *(*self) > *(*other)
3462    }
3463
3464    /// 'Greater than or equal to' comparison for two `Arc`s.
3465    ///
3466    /// The two are compared by calling `>=` on their inner values.
3467    ///
3468    /// # Examples
3469    ///
3470    /// ```
3471    /// use std::sync::Arc;
3472    ///
3473    /// let five = Arc::new(5);
3474    ///
3475    /// assert!(five >= Arc::new(5));
3476    /// ```
3477    fn ge(&self, other: &Arc<T, A>) -> bool {
3478        *(*self) >= *(*other)
3479    }
3480}
3481#[stable(feature = "rust1", since = "1.0.0")]
3482impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A> {
3483    /// Comparison for two `Arc`s.
3484    ///
3485    /// The two are compared by calling `cmp()` on their inner values.
3486    ///
3487    /// # Examples
3488    ///
3489    /// ```
3490    /// use std::sync::Arc;
3491    /// use std::cmp::Ordering;
3492    ///
3493    /// let five = Arc::new(5);
3494    ///
3495    /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
3496    /// ```
3497    fn cmp(&self, other: &Arc<T, A>) -> Ordering {
3498        (**self).cmp(&**other)
3499    }
3500}
3501#[stable(feature = "rust1", since = "1.0.0")]
3502impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A> {}
3503
3504#[stable(feature = "rust1", since = "1.0.0")]
3505impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Arc<T, A> {
3506    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3507        fmt::Display::fmt(&**self, f)
3508    }
3509}
3510
3511#[stable(feature = "rust1", since = "1.0.0")]
3512impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for Arc<T, A> {
3513    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3514        fmt::Debug::fmt(&**self, f)
3515    }
3516}
3517
3518#[stable(feature = "rust1", since = "1.0.0")]
3519impl<T: ?Sized, A: Allocator> fmt::Pointer for Arc<T, A> {
3520    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3521        fmt::Pointer::fmt(&(&raw const **self), f)
3522    }
3523}
3524
3525#[cfg(not(no_global_oom_handling))]
3526#[stable(feature = "rust1", since = "1.0.0")]
3527impl<T: Default> Default for Arc<T> {
3528    /// Creates a new `Arc<T>`, with the `Default` value for `T`.
3529    ///
3530    /// # Examples
3531    ///
3532    /// ```
3533    /// use std::sync::Arc;
3534    ///
3535    /// let x: Arc<i32> = Default::default();
3536    /// assert_eq!(*x, 0);
3537    /// ```
3538    fn default() -> Arc<T> {
3539        unsafe {
3540            Self::from_inner(
3541                Box::leak(Box::write(
3542                    Box::new_uninit(),
3543                    ArcInner {
3544                        strong: atomic::AtomicUsize::new(1),
3545                        weak: atomic::AtomicUsize::new(1),
3546                        data: T::default(),
3547                    },
3548                ))
3549                .into(),
3550            )
3551        }
3552    }
3553}
3554
3555/// Struct to hold the static `ArcInner` used for empty `Arc<str/CStr/[T]>` as
3556/// returned by `Default::default`.
3557///
3558/// Layout notes:
3559/// * `repr(align(16))` so we can use it for `[T]` with `align_of::<T>() <= 16`.
3560/// * `repr(C)` so `inner` is at offset 0 (and thus guaranteed to actually be aligned to 16).
3561/// * `[u8; 1]` (to be initialized with 0) so it can be used for `Arc<CStr>`.
3562#[repr(C, align(16))]
3563struct SliceArcInnerForStatic {
3564    inner: ArcInner<[u8; 1]>,
3565}
3566#[cfg(not(no_global_oom_handling))]
3567const MAX_STATIC_INNER_SLICE_ALIGNMENT: usize = 16;
3568
3569static STATIC_INNER_SLICE: SliceArcInnerForStatic = SliceArcInnerForStatic {
3570    inner: ArcInner {
3571        strong: atomic::AtomicUsize::new(1),
3572        weak: atomic::AtomicUsize::new(1),
3573        data: [0],
3574    },
3575};
3576
3577#[cfg(not(no_global_oom_handling))]
3578#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3579impl Default for Arc<str> {
3580    /// Creates an empty str inside an Arc
3581    ///
3582    /// This may or may not share an allocation with other Arcs.
3583    #[inline]
3584    fn default() -> Self {
3585        let arc: Arc<[u8]> = Default::default();
3586        debug_assert!(core::str::from_utf8(&*arc).is_ok());
3587        let (ptr, alloc) = Arc::into_inner_with_allocator(arc);
3588        unsafe { Arc::from_ptr_in(ptr.as_ptr() as *mut ArcInner<str>, alloc) }
3589    }
3590}
3591
3592#[cfg(not(no_global_oom_handling))]
3593#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3594impl Default for Arc<core::ffi::CStr> {
3595    /// Creates an empty CStr inside an Arc
3596    ///
3597    /// This may or may not share an allocation with other Arcs.
3598    #[inline]
3599    fn default() -> Self {
3600        use core::ffi::CStr;
3601        let inner: NonNull<ArcInner<[u8]>> = NonNull::from(&STATIC_INNER_SLICE.inner);
3602        let inner: NonNull<ArcInner<CStr>> =
3603            NonNull::new(inner.as_ptr() as *mut ArcInner<CStr>).unwrap();
3604        // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3605        let this: mem::ManuallyDrop<Arc<CStr>> =
3606            unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3607        (*this).clone()
3608    }
3609}
3610
3611#[cfg(not(no_global_oom_handling))]
3612#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3613impl<T> Default for Arc<[T]> {
3614    /// Creates an empty `[T]` inside an Arc
3615    ///
3616    /// This may or may not share an allocation with other Arcs.
3617    #[inline]
3618    fn default() -> Self {
3619        if align_of::<T>() <= MAX_STATIC_INNER_SLICE_ALIGNMENT {
3620            // We take a reference to the whole struct instead of the ArcInner<[u8; 1]> inside it so
3621            // we don't shrink the range of bytes the ptr is allowed to access under Stacked Borrows.
3622            // (Miri complains on 32-bit targets with Arc<[Align16]> otherwise.)
3623            // (Note that NonNull::from(&STATIC_INNER_SLICE.inner) is fine under Tree Borrows.)
3624            let inner: NonNull<SliceArcInnerForStatic> = NonNull::from(&STATIC_INNER_SLICE);
3625            let inner: NonNull<ArcInner<[T; 0]>> = inner.cast();
3626            // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3627            let this: mem::ManuallyDrop<Arc<[T; 0]>> =
3628                unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3629            return (*this).clone();
3630        }
3631
3632        // If T's alignment is too large for the static, make a new unique allocation.
3633        let arr: [T; 0] = [];
3634        Arc::from(arr)
3635    }
3636}
3637
3638#[cfg(not(no_global_oom_handling))]
3639#[stable(feature = "pin_default_impls", since = "CURRENT_RUSTC_VERSION")]
3640impl<T> Default for Pin<Arc<T>>
3641where
3642    T: ?Sized,
3643    Arc<T>: Default,
3644{
3645    #[inline]
3646    fn default() -> Self {
3647        unsafe { Pin::new_unchecked(Arc::<T>::default()) }
3648    }
3649}
3650
3651#[stable(feature = "rust1", since = "1.0.0")]
3652impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
3653    fn hash<H: Hasher>(&self, state: &mut H) {
3654        (**self).hash(state)
3655    }
3656}
3657
3658#[cfg(not(no_global_oom_handling))]
3659#[stable(feature = "from_for_ptrs", since = "1.6.0")]
3660impl<T> From<T> for Arc<T> {
3661    /// Converts a `T` into an `Arc<T>`
3662    ///
3663    /// The conversion moves the value into a
3664    /// newly allocated `Arc`. It is equivalent to
3665    /// calling `Arc::new(t)`.
3666    ///
3667    /// # Example
3668    /// ```rust
3669    /// # use std::sync::Arc;
3670    /// let x = 5;
3671    /// let arc = Arc::new(5);
3672    ///
3673    /// assert_eq!(Arc::from(x), arc);
3674    /// ```
3675    fn from(t: T) -> Self {
3676        Arc::new(t)
3677    }
3678}
3679
3680#[cfg(not(no_global_oom_handling))]
3681#[stable(feature = "shared_from_array", since = "1.74.0")]
3682impl<T, const N: usize> From<[T; N]> for Arc<[T]> {
3683    /// Converts a [`[T; N]`](prim@array) into an `Arc<[T]>`.
3684    ///
3685    /// The conversion moves the array into a newly allocated `Arc`.
3686    ///
3687    /// # Example
3688    ///
3689    /// ```
3690    /// # use std::sync::Arc;
3691    /// let original: [i32; 3] = [1, 2, 3];
3692    /// let shared: Arc<[i32]> = Arc::from(original);
3693    /// assert_eq!(&[1, 2, 3], &shared[..]);
3694    /// ```
3695    #[inline]
3696    fn from(v: [T; N]) -> Arc<[T]> {
3697        Arc::<[T; N]>::from(v)
3698    }
3699}
3700
3701#[cfg(not(no_global_oom_handling))]
3702#[stable(feature = "shared_from_slice", since = "1.21.0")]
3703impl<T: Clone> From<&[T]> for Arc<[T]> {
3704    /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3705    ///
3706    /// # Example
3707    ///
3708    /// ```
3709    /// # use std::sync::Arc;
3710    /// let original: &[i32] = &[1, 2, 3];
3711    /// let shared: Arc<[i32]> = Arc::from(original);
3712    /// assert_eq!(&[1, 2, 3], &shared[..]);
3713    /// ```
3714    #[inline]
3715    fn from(v: &[T]) -> Arc<[T]> {
3716        <Self as ArcFromSlice<T>>::from_slice(v)
3717    }
3718}
3719
3720#[cfg(not(no_global_oom_handling))]
3721#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3722impl<T: Clone> From<&mut [T]> for Arc<[T]> {
3723    /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3724    ///
3725    /// # Example
3726    ///
3727    /// ```
3728    /// # use std::sync::Arc;
3729    /// let mut original = [1, 2, 3];
3730    /// let original: &mut [i32] = &mut original;
3731    /// let shared: Arc<[i32]> = Arc::from(original);
3732    /// assert_eq!(&[1, 2, 3], &shared[..]);
3733    /// ```
3734    #[inline]
3735    fn from(v: &mut [T]) -> Arc<[T]> {
3736        Arc::from(&*v)
3737    }
3738}
3739
3740#[cfg(not(no_global_oom_handling))]
3741#[stable(feature = "shared_from_slice", since = "1.21.0")]
3742impl From<&str> for Arc<str> {
3743    /// Allocates a reference-counted `str` and copies `v` into it.
3744    ///
3745    /// # Example
3746    ///
3747    /// ```
3748    /// # use std::sync::Arc;
3749    /// let shared: Arc<str> = Arc::from("eggplant");
3750    /// assert_eq!("eggplant", &shared[..]);
3751    /// ```
3752    #[inline]
3753    fn from(v: &str) -> Arc<str> {
3754        let arc = Arc::<[u8]>::from(v.as_bytes());
3755        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
3756    }
3757}
3758
3759#[cfg(not(no_global_oom_handling))]
3760#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3761impl From<&mut str> for Arc<str> {
3762    /// Allocates a reference-counted `str` and copies `v` into it.
3763    ///
3764    /// # Example
3765    ///
3766    /// ```
3767    /// # use std::sync::Arc;
3768    /// let mut original = String::from("eggplant");
3769    /// let original: &mut str = &mut original;
3770    /// let shared: Arc<str> = Arc::from(original);
3771    /// assert_eq!("eggplant", &shared[..]);
3772    /// ```
3773    #[inline]
3774    fn from(v: &mut str) -> Arc<str> {
3775        Arc::from(&*v)
3776    }
3777}
3778
3779#[cfg(not(no_global_oom_handling))]
3780#[stable(feature = "shared_from_slice", since = "1.21.0")]
3781impl From<String> for Arc<str> {
3782    /// Allocates a reference-counted `str` and copies `v` into it.
3783    ///
3784    /// # Example
3785    ///
3786    /// ```
3787    /// # use std::sync::Arc;
3788    /// let unique: String = "eggplant".to_owned();
3789    /// let shared: Arc<str> = Arc::from(unique);
3790    /// assert_eq!("eggplant", &shared[..]);
3791    /// ```
3792    #[inline]
3793    fn from(v: String) -> Arc<str> {
3794        Arc::from(&v[..])
3795    }
3796}
3797
3798#[cfg(not(no_global_oom_handling))]
3799#[stable(feature = "shared_from_slice", since = "1.21.0")]
3800impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
3801    /// Move a boxed object to a new, reference-counted allocation.
3802    ///
3803    /// # Example
3804    ///
3805    /// ```
3806    /// # use std::sync::Arc;
3807    /// let unique: Box<str> = Box::from("eggplant");
3808    /// let shared: Arc<str> = Arc::from(unique);
3809    /// assert_eq!("eggplant", &shared[..]);
3810    /// ```
3811    #[inline]
3812    fn from(v: Box<T, A>) -> Arc<T, A> {
3813        Arc::from_box_in(v)
3814    }
3815}
3816
3817#[cfg(not(no_global_oom_handling))]
3818#[stable(feature = "shared_from_slice", since = "1.21.0")]
3819impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
3820    /// Allocates a reference-counted slice and moves `v`'s items into it.
3821    ///
3822    /// # Example
3823    ///
3824    /// ```
3825    /// # use std::sync::Arc;
3826    /// let unique: Vec<i32> = vec![1, 2, 3];
3827    /// let shared: Arc<[i32]> = Arc::from(unique);
3828    /// assert_eq!(&[1, 2, 3], &shared[..]);
3829    /// ```
3830    #[inline]
3831    fn from(v: Vec<T, A>) -> Arc<[T], A> {
3832        unsafe {
3833            let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
3834
3835            let rc_ptr = Self::allocate_for_slice_in(len, &alloc);
3836            ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).data) as *mut T, len);
3837
3838            // Create a `Vec<T, &A>` with length 0, to deallocate the buffer
3839            // without dropping its contents or the allocator
3840            let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc);
3841
3842            Self::from_ptr_in(rc_ptr, alloc)
3843        }
3844    }
3845}
3846
3847#[stable(feature = "shared_from_cow", since = "1.45.0")]
3848impl<'a, B> From<Cow<'a, B>> for Arc<B>
3849where
3850    B: ToOwned + ?Sized,
3851    Arc<B>: From<&'a B> + From<B::Owned>,
3852{
3853    /// Creates an atomically reference-counted pointer from a clone-on-write
3854    /// pointer by copying its content.
3855    ///
3856    /// # Example
3857    ///
3858    /// ```rust
3859    /// # use std::sync::Arc;
3860    /// # use std::borrow::Cow;
3861    /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
3862    /// let shared: Arc<str> = Arc::from(cow);
3863    /// assert_eq!("eggplant", &shared[..]);
3864    /// ```
3865    #[inline]
3866    fn from(cow: Cow<'a, B>) -> Arc<B> {
3867        match cow {
3868            Cow::Borrowed(s) => Arc::from(s),
3869            Cow::Owned(s) => Arc::from(s),
3870        }
3871    }
3872}
3873
3874#[stable(feature = "shared_from_str", since = "1.62.0")]
3875impl From<Arc<str>> for Arc<[u8]> {
3876    /// Converts an atomically reference-counted string slice into a byte slice.
3877    ///
3878    /// # Example
3879    ///
3880    /// ```
3881    /// # use std::sync::Arc;
3882    /// let string: Arc<str> = Arc::from("eggplant");
3883    /// let bytes: Arc<[u8]> = Arc::from(string);
3884    /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
3885    /// ```
3886    #[inline]
3887    fn from(rc: Arc<str>) -> Self {
3888        // SAFETY: `str` has the same layout as `[u8]`.
3889        unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
3890    }
3891}
3892
3893#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
3894impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
3895    type Error = Arc<[T], A>;
3896
3897    fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
3898        if boxed_slice.len() == N {
3899            let (ptr, alloc) = Arc::into_inner_with_allocator(boxed_slice);
3900            Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
3901        } else {
3902            Err(boxed_slice)
3903        }
3904    }
3905}
3906
3907#[cfg(not(no_global_oom_handling))]
3908#[stable(feature = "shared_from_iter", since = "1.37.0")]
3909impl<T> FromIterator<T> for Arc<[T]> {
3910    /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
3911    ///
3912    /// # Performance characteristics
3913    ///
3914    /// ## The general case
3915    ///
3916    /// In the general case, collecting into `Arc<[T]>` is done by first
3917    /// collecting into a `Vec<T>`. That is, when writing the following:
3918    ///
3919    /// ```rust
3920    /// # use std::sync::Arc;
3921    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
3922    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3923    /// ```
3924    ///
3925    /// this behaves as if we wrote:
3926    ///
3927    /// ```rust
3928    /// # use std::sync::Arc;
3929    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
3930    ///     .collect::<Vec<_>>() // The first set of allocations happens here.
3931    ///     .into(); // A second allocation for `Arc<[T]>` happens here.
3932    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3933    /// ```
3934    ///
3935    /// This will allocate as many times as needed for constructing the `Vec<T>`
3936    /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
3937    ///
3938    /// ## Iterators of known length
3939    ///
3940    /// When your `Iterator` implements `TrustedLen` and is of an exact size,
3941    /// a single allocation will be made for the `Arc<[T]>`. For example:
3942    ///
3943    /// ```rust
3944    /// # use std::sync::Arc;
3945    /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
3946    /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
3947    /// ```
3948    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
3949        ToArcSlice::to_arc_slice(iter.into_iter())
3950    }
3951}
3952
3953#[cfg(not(no_global_oom_handling))]
3954/// Specialization trait used for collecting into `Arc<[T]>`.
3955trait ToArcSlice<T>: Iterator<Item = T> + Sized {
3956    fn to_arc_slice(self) -> Arc<[T]>;
3957}
3958
3959#[cfg(not(no_global_oom_handling))]
3960impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
3961    default fn to_arc_slice(self) -> Arc<[T]> {
3962        self.collect::<Vec<T>>().into()
3963    }
3964}
3965
3966#[cfg(not(no_global_oom_handling))]
3967impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
3968    fn to_arc_slice(self) -> Arc<[T]> {
3969        // This is the case for a `TrustedLen` iterator.
3970        let (low, high) = self.size_hint();
3971        if let Some(high) = high {
3972            debug_assert_eq!(
3973                low,
3974                high,
3975                "TrustedLen iterator's size hint is not exact: {:?}",
3976                (low, high)
3977            );
3978
3979            unsafe {
3980                // SAFETY: We need to ensure that the iterator has an exact length and we have.
3981                Arc::from_iter_exact(self, low)
3982            }
3983        } else {
3984            // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
3985            // length exceeding `usize::MAX`.
3986            // The default implementation would collect into a vec which would panic.
3987            // Thus we panic here immediately without invoking `Vec` code.
3988            panic!("capacity overflow");
3989        }
3990    }
3991}
3992
3993#[stable(feature = "rust1", since = "1.0.0")]
3994impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Arc<T, A> {
3995    fn borrow(&self) -> &T {
3996        &**self
3997    }
3998}
3999
4000#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
4001impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> {
4002    fn as_ref(&self) -> &T {
4003        &**self
4004    }
4005}
4006
4007#[stable(feature = "pin", since = "1.33.0")]
4008impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
4009
4010/// Gets the offset within an `ArcInner` for the payload behind a pointer.
4011///
4012/// # Safety
4013///
4014/// The pointer must point to (and have valid metadata for) a previously
4015/// valid instance of T, but the T is allowed to be dropped.
4016unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
4017    // Align the unsized value to the end of the ArcInner.
4018    // Because RcInner is repr(C), it will always be the last field in memory.
4019    // SAFETY: since the only unsized types possible are slices, trait objects,
4020    // and extern types, the input safety requirement is currently enough to
4021    // satisfy the requirements of align_of_val_raw; this is an implementation
4022    // detail of the language that must not be relied upon outside of std.
4023    unsafe { data_offset_align(align_of_val_raw(ptr)) }
4024}
4025
4026#[inline]
4027fn data_offset_align(align: usize) -> usize {
4028    let layout = Layout::new::<ArcInner<()>>();
4029    layout.size() + layout.padding_needed_for(align)
4030}
4031
4032/// A unique owning pointer to an [`ArcInner`] **that does not imply the contents are initialized,**
4033/// but will deallocate it (without dropping the value) when dropped.
4034///
4035/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
4036#[cfg(not(no_global_oom_handling))]
4037struct UniqueArcUninit<T: ?Sized, A: Allocator> {
4038    ptr: NonNull<ArcInner<T>>,
4039    layout_for_value: Layout,
4040    alloc: Option<A>,
4041}
4042
4043#[cfg(not(no_global_oom_handling))]
4044impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
4045    /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it.
4046    fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
4047        let layout = Layout::for_value(for_value);
4048        let ptr = unsafe {
4049            Arc::allocate_for_layout(
4050                layout,
4051                |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
4052                |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
4053            )
4054        };
4055        Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
4056    }
4057
4058    /// Returns the pointer to be written into to initialize the [`Arc`].
4059    fn data_ptr(&mut self) -> *mut T {
4060        let offset = data_offset_align(self.layout_for_value.align());
4061        unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
4062    }
4063
4064    /// Upgrade this into a normal [`Arc`].
4065    ///
4066    /// # Safety
4067    ///
4068    /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
4069    unsafe fn into_arc(self) -> Arc<T, A> {
4070        let mut this = ManuallyDrop::new(self);
4071        let ptr = this.ptr.as_ptr();
4072        let alloc = this.alloc.take().unwrap();
4073
4074        // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
4075        // for having initialized the data.
4076        unsafe { Arc::from_ptr_in(ptr, alloc) }
4077    }
4078}
4079
4080#[cfg(not(no_global_oom_handling))]
4081impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
4082    fn drop(&mut self) {
4083        // SAFETY:
4084        // * new() produced a pointer safe to deallocate.
4085        // * We own the pointer unless into_arc() was called, which forgets us.
4086        unsafe {
4087            self.alloc.take().unwrap().deallocate(
4088                self.ptr.cast(),
4089                arcinner_layout_for_value_layout(self.layout_for_value),
4090            );
4091        }
4092    }
4093}
4094
4095#[stable(feature = "arc_error", since = "1.52.0")]
4096impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
4097    #[allow(deprecated)]
4098    fn cause(&self) -> Option<&dyn core::error::Error> {
4099        core::error::Error::cause(&**self)
4100    }
4101
4102    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
4103        core::error::Error::source(&**self)
4104    }
4105
4106    fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
4107        core::error::Error::provide(&**self, req);
4108    }
4109}
4110
4111/// A uniquely owned [`Arc`].
4112///
4113/// This represents an `Arc` that is known to be uniquely owned -- that is, have exactly one strong
4114/// reference. Multiple weak pointers can be created, but attempts to upgrade those to strong
4115/// references will fail unless the `UniqueArc` they point to has been converted into a regular `Arc`.
4116///
4117/// Because it is uniquely owned, the contents of a `UniqueArc` can be freely mutated. A common
4118/// use case is to have an object be mutable during its initialization phase but then have it become
4119/// immutable and converted to a normal `Arc`.
4120///
4121/// This can be used as a flexible way to create cyclic data structures, as in the example below.
4122///
4123/// ```
4124/// #![feature(unique_rc_arc)]
4125/// use std::sync::{Arc, Weak, UniqueArc};
4126///
4127/// struct Gadget {
4128///     me: Weak<Gadget>,
4129/// }
4130///
4131/// fn create_gadget() -> Option<Arc<Gadget>> {
4132///     let mut rc = UniqueArc::new(Gadget {
4133///         me: Weak::new(),
4134///     });
4135///     rc.me = UniqueArc::downgrade(&rc);
4136///     Some(UniqueArc::into_arc(rc))
4137/// }
4138///
4139/// create_gadget().unwrap();
4140/// ```
4141///
4142/// An advantage of using `UniqueArc` over [`Arc::new_cyclic`] to build cyclic data structures is that
4143/// [`Arc::new_cyclic`]'s `data_fn` parameter cannot be async or return a [`Result`]. As shown in the
4144/// previous example, `UniqueArc` allows for more flexibility in the construction of cyclic data,
4145/// including fallible or async constructors.
4146#[unstable(feature = "unique_rc_arc", issue = "112566")]
4147pub struct UniqueArc<
4148    T: ?Sized,
4149    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
4150> {
4151    ptr: NonNull<ArcInner<T>>,
4152    // Define the ownership of `ArcInner<T>` for drop-check
4153    _marker: PhantomData<ArcInner<T>>,
4154    // Invariance is necessary for soundness: once other `Weak`
4155    // references exist, we already have a form of shared mutability!
4156    _marker2: PhantomData<*mut T>,
4157    alloc: A,
4158}
4159
4160#[unstable(feature = "unique_rc_arc", issue = "112566")]
4161unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for UniqueArc<T, A> {}
4162
4163#[unstable(feature = "unique_rc_arc", issue = "112566")]
4164unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for UniqueArc<T, A> {}
4165
4166#[unstable(feature = "unique_rc_arc", issue = "112566")]
4167// #[unstable(feature = "coerce_unsized", issue = "18598")]
4168impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<UniqueArc<U, A>>
4169    for UniqueArc<T, A>
4170{
4171}
4172
4173//#[unstable(feature = "unique_rc_arc", issue = "112566")]
4174#[unstable(feature = "dispatch_from_dyn", issue = "none")]
4175impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<UniqueArc<U>> for UniqueArc<T> {}
4176
4177#[unstable(feature = "unique_rc_arc", issue = "112566")]
4178impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for UniqueArc<T, A> {
4179    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4180        fmt::Display::fmt(&**self, f)
4181    }
4182}
4183
4184#[unstable(feature = "unique_rc_arc", issue = "112566")]
4185impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for UniqueArc<T, A> {
4186    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4187        fmt::Debug::fmt(&**self, f)
4188    }
4189}
4190
4191#[unstable(feature = "unique_rc_arc", issue = "112566")]
4192impl<T: ?Sized, A: Allocator> fmt::Pointer for UniqueArc<T, A> {
4193    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
4194        fmt::Pointer::fmt(&(&raw const **self), f)
4195    }
4196}
4197
4198#[unstable(feature = "unique_rc_arc", issue = "112566")]
4199impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for UniqueArc<T, A> {
4200    fn borrow(&self) -> &T {
4201        &**self
4202    }
4203}
4204
4205#[unstable(feature = "unique_rc_arc", issue = "112566")]
4206impl<T: ?Sized, A: Allocator> borrow::BorrowMut<T> for UniqueArc<T, A> {
4207    fn borrow_mut(&mut self) -> &mut T {
4208        &mut **self
4209    }
4210}
4211
4212#[unstable(feature = "unique_rc_arc", issue = "112566")]
4213impl<T: ?Sized, A: Allocator> AsRef<T> for UniqueArc<T, A> {
4214    fn as_ref(&self) -> &T {
4215        &**self
4216    }
4217}
4218
4219#[unstable(feature = "unique_rc_arc", issue = "112566")]
4220impl<T: ?Sized, A: Allocator> AsMut<T> for UniqueArc<T, A> {
4221    fn as_mut(&mut self) -> &mut T {
4222        &mut **self
4223    }
4224}
4225
4226#[unstable(feature = "unique_rc_arc", issue = "112566")]
4227impl<T: ?Sized, A: Allocator> Unpin for UniqueArc<T, A> {}
4228
4229#[unstable(feature = "unique_rc_arc", issue = "112566")]
4230impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for UniqueArc<T, A> {
4231    /// Equality for two `UniqueArc`s.
4232    ///
4233    /// Two `UniqueArc`s are equal if their inner values are equal.
4234    ///
4235    /// # Examples
4236    ///
4237    /// ```
4238    /// #![feature(unique_rc_arc)]
4239    /// use std::sync::UniqueArc;
4240    ///
4241    /// let five = UniqueArc::new(5);
4242    ///
4243    /// assert!(five == UniqueArc::new(5));
4244    /// ```
4245    #[inline]
4246    fn eq(&self, other: &Self) -> bool {
4247        PartialEq::eq(&**self, &**other)
4248    }
4249}
4250
4251#[unstable(feature = "unique_rc_arc", issue = "112566")]
4252impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for UniqueArc<T, A> {
4253    /// Partial comparison for two `UniqueArc`s.
4254    ///
4255    /// The two are compared by calling `partial_cmp()` on their inner values.
4256    ///
4257    /// # Examples
4258    ///
4259    /// ```
4260    /// #![feature(unique_rc_arc)]
4261    /// use std::sync::UniqueArc;
4262    /// use std::cmp::Ordering;
4263    ///
4264    /// let five = UniqueArc::new(5);
4265    ///
4266    /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&UniqueArc::new(6)));
4267    /// ```
4268    #[inline(always)]
4269    fn partial_cmp(&self, other: &UniqueArc<T, A>) -> Option<Ordering> {
4270        (**self).partial_cmp(&**other)
4271    }
4272
4273    /// Less-than comparison for two `UniqueArc`s.
4274    ///
4275    /// The two are compared by calling `<` on their inner values.
4276    ///
4277    /// # Examples
4278    ///
4279    /// ```
4280    /// #![feature(unique_rc_arc)]
4281    /// use std::sync::UniqueArc;
4282    ///
4283    /// let five = UniqueArc::new(5);
4284    ///
4285    /// assert!(five < UniqueArc::new(6));
4286    /// ```
4287    #[inline(always)]
4288    fn lt(&self, other: &UniqueArc<T, A>) -> bool {
4289        **self < **other
4290    }
4291
4292    /// 'Less than or equal to' comparison for two `UniqueArc`s.
4293    ///
4294    /// The two are compared by calling `<=` on their inner values.
4295    ///
4296    /// # Examples
4297    ///
4298    /// ```
4299    /// #![feature(unique_rc_arc)]
4300    /// use std::sync::UniqueArc;
4301    ///
4302    /// let five = UniqueArc::new(5);
4303    ///
4304    /// assert!(five <= UniqueArc::new(5));
4305    /// ```
4306    #[inline(always)]
4307    fn le(&self, other: &UniqueArc<T, A>) -> bool {
4308        **self <= **other
4309    }
4310
4311    /// Greater-than comparison for two `UniqueArc`s.
4312    ///
4313    /// The two are compared by calling `>` on their inner values.
4314    ///
4315    /// # Examples
4316    ///
4317    /// ```
4318    /// #![feature(unique_rc_arc)]
4319    /// use std::sync::UniqueArc;
4320    ///
4321    /// let five = UniqueArc::new(5);
4322    ///
4323    /// assert!(five > UniqueArc::new(4));
4324    /// ```
4325    #[inline(always)]
4326    fn gt(&self, other: &UniqueArc<T, A>) -> bool {
4327        **self > **other
4328    }
4329
4330    /// 'Greater than or equal to' comparison for two `UniqueArc`s.
4331    ///
4332    /// The two are compared by calling `>=` on their inner values.
4333    ///
4334    /// # Examples
4335    ///
4336    /// ```
4337    /// #![feature(unique_rc_arc)]
4338    /// use std::sync::UniqueArc;
4339    ///
4340    /// let five = UniqueArc::new(5);
4341    ///
4342    /// assert!(five >= UniqueArc::new(5));
4343    /// ```
4344    #[inline(always)]
4345    fn ge(&self, other: &UniqueArc<T, A>) -> bool {
4346        **self >= **other
4347    }
4348}
4349
4350#[unstable(feature = "unique_rc_arc", issue = "112566")]
4351impl<T: ?Sized + Ord, A: Allocator> Ord for UniqueArc<T, A> {
4352    /// Comparison for two `UniqueArc`s.
4353    ///
4354    /// The two are compared by calling `cmp()` on their inner values.
4355    ///
4356    /// # Examples
4357    ///
4358    /// ```
4359    /// #![feature(unique_rc_arc)]
4360    /// use std::sync::UniqueArc;
4361    /// use std::cmp::Ordering;
4362    ///
4363    /// let five = UniqueArc::new(5);
4364    ///
4365    /// assert_eq!(Ordering::Less, five.cmp(&UniqueArc::new(6)));
4366    /// ```
4367    #[inline]
4368    fn cmp(&self, other: &UniqueArc<T, A>) -> Ordering {
4369        (**self).cmp(&**other)
4370    }
4371}
4372
4373#[unstable(feature = "unique_rc_arc", issue = "112566")]
4374impl<T: ?Sized + Eq, A: Allocator> Eq for UniqueArc<T, A> {}
4375
4376#[unstable(feature = "unique_rc_arc", issue = "112566")]
4377impl<T: ?Sized + Hash, A: Allocator> Hash for UniqueArc<T, A> {
4378    fn hash<H: Hasher>(&self, state: &mut H) {
4379        (**self).hash(state);
4380    }
4381}
4382
4383impl<T> UniqueArc<T, Global> {
4384    /// Creates a new `UniqueArc`.
4385    ///
4386    /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4387    /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4388    /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4389    /// point to the new [`Arc`].
4390    #[cfg(not(no_global_oom_handling))]
4391    #[unstable(feature = "unique_rc_arc", issue = "112566")]
4392    #[must_use]
4393    pub fn new(value: T) -> Self {
4394        Self::new_in(value, Global)
4395    }
4396}
4397
4398impl<T, A: Allocator> UniqueArc<T, A> {
4399    /// Creates a new `UniqueArc` in the provided allocator.
4400    ///
4401    /// Weak references to this `UniqueArc` can be created with [`UniqueArc::downgrade`]. Upgrading
4402    /// these weak references will fail before the `UniqueArc` has been converted into an [`Arc`].
4403    /// After converting the `UniqueArc` into an [`Arc`], any weak references created beforehand will
4404    /// point to the new [`Arc`].
4405    #[cfg(not(no_global_oom_handling))]
4406    #[unstable(feature = "unique_rc_arc", issue = "112566")]
4407    #[must_use]
4408    // #[unstable(feature = "allocator_api", issue = "32838")]
4409    pub fn new_in(data: T, alloc: A) -> Self {
4410        let (ptr, alloc) = Box::into_unique(Box::new_in(
4411            ArcInner {
4412                strong: atomic::AtomicUsize::new(0),
4413                // keep one weak reference so if all the weak pointers that are created are dropped
4414                // the UniqueArc still stays valid.
4415                weak: atomic::AtomicUsize::new(1),
4416                data,
4417            },
4418            alloc,
4419        ));
4420        Self { ptr: ptr.into(), _marker: PhantomData, _marker2: PhantomData, alloc }
4421    }
4422}
4423
4424impl<T: ?Sized, A: Allocator> UniqueArc<T, A> {
4425    /// Converts the `UniqueArc` into a regular [`Arc`].
4426    ///
4427    /// This consumes the `UniqueArc` and returns a regular [`Arc`] that contains the `value` that
4428    /// is passed to `into_arc`.
4429    ///
4430    /// Any weak references created before this method is called can now be upgraded to strong
4431    /// references.
4432    #[unstable(feature = "unique_rc_arc", issue = "112566")]
4433    #[must_use]
4434    pub fn into_arc(this: Self) -> Arc<T, A> {
4435        let this = ManuallyDrop::new(this);
4436
4437        // Move the allocator out.
4438        // SAFETY: `this.alloc` will not be accessed again, nor dropped because it is in
4439        // a `ManuallyDrop`.
4440        let alloc: A = unsafe { ptr::read(&this.alloc) };
4441
4442        // SAFETY: This pointer was allocated at creation time so we know it is valid.
4443        unsafe {
4444            // Convert our weak reference into a strong reference
4445            (*this.ptr.as_ptr()).strong.store(1, Release);
4446            Arc::from_inner_in(this.ptr, alloc)
4447        }
4448    }
4449}
4450
4451impl<T: ?Sized, A: Allocator + Clone> UniqueArc<T, A> {
4452    /// Creates a new weak reference to the `UniqueArc`.
4453    ///
4454    /// Attempting to upgrade this weak reference will fail before the `UniqueArc` has been converted
4455    /// to a [`Arc`] using [`UniqueArc::into_arc`].
4456    #[unstable(feature = "unique_rc_arc", issue = "112566")]
4457    #[must_use]
4458    pub fn downgrade(this: &Self) -> Weak<T, A> {
4459        // Using a relaxed ordering is alright here, as knowledge of the
4460        // original reference prevents other threads from erroneously deleting
4461        // the object or converting the object to a normal `Arc<T, A>`.
4462        //
4463        // Note that we don't need to test if the weak counter is locked because there
4464        // are no such operations like `Arc::get_mut` or `Arc::make_mut` that will lock
4465        // the weak counter.
4466        //
4467        // SAFETY: This pointer was allocated at creation time so we know it is valid.
4468        let old_size = unsafe { (*this.ptr.as_ptr()).weak.fetch_add(1, Relaxed) };
4469
4470        // See comments in Arc::clone() for why we do this (for mem::forget).
4471        if old_size > MAX_REFCOUNT {
4472            abort();
4473        }
4474
4475        Weak { ptr: this.ptr, alloc: this.alloc.clone() }
4476    }
4477}
4478
4479#[unstable(feature = "unique_rc_arc", issue = "112566")]
4480impl<T: ?Sized, A: Allocator> Deref for UniqueArc<T, A> {
4481    type Target = T;
4482
4483    fn deref(&self) -> &T {
4484        // SAFETY: This pointer was allocated at creation time so we know it is valid.
4485        unsafe { &self.ptr.as_ref().data }
4486    }
4487}
4488
4489// #[unstable(feature = "unique_rc_arc", issue = "112566")]
4490#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
4491unsafe impl<T: ?Sized> PinCoerceUnsized for UniqueArc<T> {}
4492
4493#[unstable(feature = "unique_rc_arc", issue = "112566")]
4494impl<T: ?Sized, A: Allocator> DerefMut for UniqueArc<T, A> {
4495    fn deref_mut(&mut self) -> &mut T {
4496        // SAFETY: This pointer was allocated at creation time so we know it is valid. We know we
4497        // have unique ownership and therefore it's safe to make a mutable reference because
4498        // `UniqueArc` owns the only strong reference to itself.
4499        // We also need to be careful to only create a mutable reference to the `data` field,
4500        // as a mutable reference to the entire `ArcInner` would assert uniqueness over the
4501        // ref count fields too, invalidating any attempt by `Weak`s to access the ref count.
4502        unsafe { &mut (*self.ptr.as_ptr()).data }
4503    }
4504}
4505
4506#[unstable(feature = "unique_rc_arc", issue = "112566")]
4507// #[unstable(feature = "deref_pure_trait", issue = "87121")]
4508unsafe impl<T: ?Sized, A: Allocator> DerefPure for UniqueArc<T, A> {}
4509
4510#[unstable(feature = "unique_rc_arc", issue = "112566")]
4511unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for UniqueArc<T, A> {
4512    fn drop(&mut self) {
4513        // See `Arc::drop_slow` which drops an `Arc` with a strong count of 0.
4514        // SAFETY: This pointer was allocated at creation time so we know it is valid.
4515        let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
4516
4517        unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
4518    }
4519}