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::cmp::Ordering;
15use core::hash::{Hash, Hasher};
16use core::intrinsics::abort;
17#[cfg(not(no_global_oom_handling))]
18use core::iter;
19use core::marker::{PhantomData, Unsize};
20use core::mem::{self, ManuallyDrop, align_of_val_raw};
21use core::num::NonZeroUsize;
22use core::ops::{CoerceUnsized, Deref, DerefPure, DispatchFromDyn, LegacyReceiver};
23use core::panic::{RefUnwindSafe, UnwindSafe};
24use core::pin::{Pin, PinCoerceUnsized};
25use core::ptr::{self, NonNull};
26#[cfg(not(no_global_oom_handling))]
27use core::slice::from_raw_parts_mut;
28use core::sync::atomic;
29use core::sync::atomic::Ordering::{Acquire, Relaxed, Release};
30use core::{borrow, fmt, hint};
31
32#[cfg(not(no_global_oom_handling))]
33use crate::alloc::handle_alloc_error;
34use crate::alloc::{AllocError, Allocator, Global, Layout};
35use crate::borrow::{Cow, ToOwned};
36use crate::boxed::Box;
37use crate::rc::is_dangling;
38#[cfg(not(no_global_oom_handling))]
39use crate::string::String;
40#[cfg(not(no_global_oom_handling))]
41use crate::vec::Vec;
42
43/// A soft limit on the amount of references that may be made to an `Arc`.
44///
45/// Going above this limit will abort your program (although not
46/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
47/// Trying to go above it might call a `panic` (if not actually going above it).
48///
49/// This is a global invariant, and also applies when using a compare-exchange loop.
50///
51/// See comment in `Arc::clone`.
52const MAX_REFCOUNT: usize = (isize::MAX) as usize;
53
54/// The error in case either counter reaches above `MAX_REFCOUNT`, and we can `panic` safely.
55const INTERNAL_OVERFLOW_ERROR: &str = "Arc counter overflow";
56
57#[cfg(not(sanitize = "thread"))]
58macro_rules! acquire {
59    ($x:expr) => {
60        atomic::fence(Acquire)
61    };
62}
63
64// ThreadSanitizer does not support memory fences. To avoid false positive
65// reports in Arc / Weak implementation use atomic loads for synchronization
66// instead.
67#[cfg(sanitize = "thread")]
68macro_rules! acquire {
69    ($x:expr) => {
70        $x.load(Acquire)
71    };
72}
73
74/// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
75/// Reference Counted'.
76///
77/// The type `Arc<T>` provides shared ownership of a value of type `T`,
78/// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
79/// a new `Arc` instance, which points to the same allocation on the heap as the
80/// source `Arc`, while increasing a reference count. When the last `Arc`
81/// pointer to a given allocation is destroyed, the value stored in that allocation (often
82/// referred to as "inner value") is also dropped.
83///
84/// Shared references in Rust disallow mutation by default, and `Arc` is no
85/// exception: you cannot generally obtain a mutable reference to something
86/// inside an `Arc`. If you need to mutate through an `Arc`, use
87/// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
88/// types.
89///
90/// **Note**: This type is only available on platforms that support atomic
91/// loads and stores of pointers, which includes all platforms that support
92/// the `std` crate but not all those which only support [`alloc`](crate).
93/// This may be detected at compile time using `#[cfg(target_has_atomic = "ptr")]`.
94///
95/// ## Thread Safety
96///
97/// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
98/// counting. This means that it is thread-safe. The disadvantage is that
99/// atomic operations are more expensive than ordinary memory accesses. If you
100/// are not sharing reference-counted allocations between threads, consider using
101/// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
102/// compiler will catch any attempt to send an [`Rc<T>`] between threads.
103/// However, a library might choose `Arc<T>` in order to give library consumers
104/// more flexibility.
105///
106/// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
107/// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
108/// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
109/// first: after all, isn't the point of `Arc<T>` thread safety? The key is
110/// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
111/// data, but it  doesn't add thread safety to its data. Consider
112/// <code>Arc<[RefCell\<T>]></code>. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
113/// [`Send`], <code>Arc<[RefCell\<T>]></code> would be as well. But then we'd have a problem:
114/// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
115/// non-atomic operations.
116///
117/// In the end, this means that you may need to pair `Arc<T>` with some sort of
118/// [`std::sync`] type, usually [`Mutex<T>`][mutex].
119///
120/// ## Breaking cycles with `Weak`
121///
122/// The [`downgrade`][downgrade] method can be used to create a non-owning
123/// [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
124/// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
125/// already been dropped. In other words, `Weak` pointers do not keep the value
126/// inside the allocation alive; however, they *do* keep the allocation
127/// (the backing store for the value) alive.
128///
129/// A cycle between `Arc` pointers will never be deallocated. For this reason,
130/// [`Weak`] is used to break cycles. For example, a tree could have
131/// strong `Arc` pointers from parent nodes to children, and [`Weak`]
132/// pointers from children back to their parents.
133///
134/// # Cloning references
135///
136/// Creating a new reference from an existing reference-counted pointer is done using the
137/// `Clone` trait implemented for [`Arc<T>`][Arc] and [`Weak<T>`][Weak].
138///
139/// ```
140/// use std::sync::Arc;
141/// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
142/// // The two syntaxes below are equivalent.
143/// let a = foo.clone();
144/// let b = Arc::clone(&foo);
145/// // a, b, and foo are all Arcs that point to the same memory location
146/// ```
147///
148/// ## `Deref` behavior
149///
150/// `Arc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
151/// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
152/// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
153/// functions, called using [fully qualified syntax]:
154///
155/// ```
156/// use std::sync::Arc;
157///
158/// let my_arc = Arc::new(());
159/// let my_weak = Arc::downgrade(&my_arc);
160/// ```
161///
162/// `Arc<T>`'s implementations of traits like `Clone` may also be called using
163/// fully qualified syntax. Some people prefer to use fully qualified syntax,
164/// while others prefer using method-call syntax.
165///
166/// ```
167/// use std::sync::Arc;
168///
169/// let arc = Arc::new(());
170/// // Method-call syntax
171/// let arc2 = arc.clone();
172/// // Fully qualified syntax
173/// let arc3 = Arc::clone(&arc);
174/// ```
175///
176/// [`Weak<T>`][Weak] does not auto-dereference to `T`, because the inner value may have
177/// already been dropped.
178///
179/// [`Rc<T>`]: crate::rc::Rc
180/// [clone]: Clone::clone
181/// [mutex]: ../../std/sync/struct.Mutex.html
182/// [rwlock]: ../../std/sync/struct.RwLock.html
183/// [atomic]: core::sync::atomic
184/// [downgrade]: Arc::downgrade
185/// [upgrade]: Weak::upgrade
186/// [RefCell\<T>]: core::cell::RefCell
187/// [`RefCell<T>`]: core::cell::RefCell
188/// [`std::sync`]: ../../std/sync/index.html
189/// [`Arc::clone(&from)`]: Arc::clone
190/// [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
191///
192/// # Examples
193///
194/// Sharing some immutable data between threads:
195///
196/// ```
197/// use std::sync::Arc;
198/// use std::thread;
199///
200/// let five = Arc::new(5);
201///
202/// for _ in 0..10 {
203///     let five = Arc::clone(&five);
204///
205///     thread::spawn(move || {
206///         println!("{five:?}");
207///     });
208/// }
209/// ```
210///
211/// Sharing a mutable [`AtomicUsize`]:
212///
213/// [`AtomicUsize`]: core::sync::atomic::AtomicUsize "sync::atomic::AtomicUsize"
214///
215/// ```
216/// use std::sync::Arc;
217/// use std::sync::atomic::{AtomicUsize, Ordering};
218/// use std::thread;
219///
220/// let val = Arc::new(AtomicUsize::new(5));
221///
222/// for _ in 0..10 {
223///     let val = Arc::clone(&val);
224///
225///     thread::spawn(move || {
226///         let v = val.fetch_add(1, Ordering::Relaxed);
227///         println!("{v:?}");
228///     });
229/// }
230/// ```
231///
232/// See the [`rc` documentation][rc_examples] for more examples of reference
233/// counting in general.
234///
235/// [rc_examples]: crate::rc#examples
236#[doc(search_unbox)]
237#[cfg_attr(not(test), rustc_diagnostic_item = "Arc")]
238#[stable(feature = "rust1", since = "1.0.0")]
239#[rustc_insignificant_dtor]
240pub struct Arc<
241    T: ?Sized,
242    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
243> {
244    ptr: NonNull<ArcInner<T>>,
245    phantom: PhantomData<ArcInner<T>>,
246    alloc: A,
247}
248
249#[stable(feature = "rust1", since = "1.0.0")]
250unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Arc<T, A> {}
251#[stable(feature = "rust1", since = "1.0.0")]
252unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Arc<T, A> {}
253
254#[stable(feature = "catch_unwind", since = "1.9.0")]
255impl<T: RefUnwindSafe + ?Sized, A: Allocator + UnwindSafe> UnwindSafe for Arc<T, A> {}
256
257#[unstable(feature = "coerce_unsized", issue = "18598")]
258impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Arc<U, A>> for Arc<T, A> {}
259
260#[unstable(feature = "dispatch_from_dyn", issue = "none")]
261impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}
262
263impl<T: ?Sized> Arc<T> {
264    unsafe fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
265        unsafe { Self::from_inner_in(ptr, Global) }
266    }
267
268    unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
269        unsafe { Self::from_ptr_in(ptr, Global) }
270    }
271}
272
273impl<T: ?Sized, A: Allocator> Arc<T, A> {
274    #[inline]
275    fn into_inner_with_allocator(this: Self) -> (NonNull<ArcInner<T>>, A) {
276        let this = mem::ManuallyDrop::new(this);
277        (this.ptr, unsafe { ptr::read(&this.alloc) })
278    }
279
280    #[inline]
281    unsafe fn from_inner_in(ptr: NonNull<ArcInner<T>>, alloc: A) -> Self {
282        Self { ptr, phantom: PhantomData, alloc }
283    }
284
285    #[inline]
286    unsafe fn from_ptr_in(ptr: *mut ArcInner<T>, alloc: A) -> Self {
287        unsafe { Self::from_inner_in(NonNull::new_unchecked(ptr), alloc) }
288    }
289}
290
291/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
292/// managed allocation.
293///
294/// The allocation is accessed by calling [`upgrade`] on the `Weak`
295/// pointer, which returns an <code>[Option]<[Arc]\<T>></code>.
296///
297/// Since a `Weak` reference does not count towards ownership, it will not
298/// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
299/// guarantees about the value still being present. Thus it may return [`None`]
300/// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
301/// itself (the backing store) from being deallocated.
302///
303/// A `Weak` pointer is useful for keeping a temporary reference to the allocation
304/// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
305/// prevent circular references between [`Arc`] pointers, since mutual owning references
306/// would never allow either [`Arc`] to be dropped. For example, a tree could
307/// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
308/// pointers from children back to their parents.
309///
310/// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
311///
312/// [`upgrade`]: Weak::upgrade
313#[stable(feature = "arc_weak", since = "1.4.0")]
314#[cfg_attr(not(test), rustc_diagnostic_item = "ArcWeak")]
315pub struct Weak<
316    T: ?Sized,
317    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
318> {
319    // This is a `NonNull` to allow optimizing the size of this type in enums,
320    // but it is not necessarily a valid pointer.
321    // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
322    // to allocate space on the heap. That's not a value a real pointer
323    // will ever have because RcInner has alignment at least 2.
324    // This is only possible when `T: Sized`; unsized `T` never dangle.
325    ptr: NonNull<ArcInner<T>>,
326    alloc: A,
327}
328
329#[stable(feature = "arc_weak", since = "1.4.0")]
330unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Send> Send for Weak<T, A> {}
331#[stable(feature = "arc_weak", since = "1.4.0")]
332unsafe impl<T: ?Sized + Sync + Send, A: Allocator + Sync> Sync for Weak<T, A> {}
333
334#[unstable(feature = "coerce_unsized", issue = "18598")]
335impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Weak<U, A>> for Weak<T, A> {}
336#[unstable(feature = "dispatch_from_dyn", issue = "none")]
337impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}
338
339#[stable(feature = "arc_weak", since = "1.4.0")]
340impl<T: ?Sized, A: Allocator> fmt::Debug for Weak<T, A> {
341    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
342        write!(f, "(Weak)")
343    }
344}
345
346// This is repr(C) to future-proof against possible field-reordering, which
347// would interfere with otherwise safe [into|from]_raw() of transmutable
348// inner types.
349#[repr(C)]
350struct ArcInner<T: ?Sized> {
351    strong: atomic::AtomicUsize,
352
353    // the value usize::MAX acts as a sentinel for temporarily "locking" the
354    // ability to upgrade weak pointers or downgrade strong ones; this is used
355    // to avoid races in `make_mut` and `get_mut`.
356    weak: atomic::AtomicUsize,
357
358    data: T,
359}
360
361/// Calculate layout for `ArcInner<T>` using the inner value's layout
362fn arcinner_layout_for_value_layout(layout: Layout) -> Layout {
363    // Calculate layout using the given value layout.
364    // Previously, layout was calculated on the expression
365    // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
366    // reference (see #54908).
367    Layout::new::<ArcInner<()>>().extend(layout).unwrap().0.pad_to_align()
368}
369
370unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
371unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}
372
373impl<T> Arc<T> {
374    /// Constructs a new `Arc<T>`.
375    ///
376    /// # Examples
377    ///
378    /// ```
379    /// use std::sync::Arc;
380    ///
381    /// let five = Arc::new(5);
382    /// ```
383    #[cfg(not(no_global_oom_handling))]
384    #[inline]
385    #[stable(feature = "rust1", since = "1.0.0")]
386    pub fn new(data: T) -> Arc<T> {
387        // Start the weak pointer count as 1 which is the weak pointer that's
388        // held by all the strong pointers (kinda), see std/rc.rs for more info
389        let x: Box<_> = Box::new(ArcInner {
390            strong: atomic::AtomicUsize::new(1),
391            weak: atomic::AtomicUsize::new(1),
392            data,
393        });
394        unsafe { Self::from_inner(Box::leak(x).into()) }
395    }
396
397    /// Constructs a new `Arc<T>` while giving you a `Weak<T>` to the allocation,
398    /// to allow you to construct a `T` which holds a weak pointer to itself.
399    ///
400    /// Generally, a structure circularly referencing itself, either directly or
401    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
402    /// Using this function, you get access to the weak pointer during the
403    /// initialization of `T`, before the `Arc<T>` is created, such that you can
404    /// clone and store it inside the `T`.
405    ///
406    /// `new_cyclic` first allocates the managed allocation for the `Arc<T>`,
407    /// then calls your closure, giving it a `Weak<T>` to this allocation,
408    /// and only afterwards completes the construction of the `Arc<T>` by placing
409    /// the `T` returned from your closure into the allocation.
410    ///
411    /// Since the new `Arc<T>` is not fully-constructed until `Arc<T>::new_cyclic`
412    /// returns, calling [`upgrade`] on the weak reference inside your closure will
413    /// fail and result in a `None` value.
414    ///
415    /// # Panics
416    ///
417    /// If `data_fn` panics, the panic is propagated to the caller, and the
418    /// temporary [`Weak<T>`] is dropped normally.
419    ///
420    /// # Example
421    ///
422    /// ```
423    /// # #![allow(dead_code)]
424    /// use std::sync::{Arc, Weak};
425    ///
426    /// struct Gadget {
427    ///     me: Weak<Gadget>,
428    /// }
429    ///
430    /// impl Gadget {
431    ///     /// Constructs a reference counted Gadget.
432    ///     fn new() -> Arc<Self> {
433    ///         // `me` is a `Weak<Gadget>` pointing at the new allocation of the
434    ///         // `Arc` we're constructing.
435    ///         Arc::new_cyclic(|me| {
436    ///             // Create the actual struct here.
437    ///             Gadget { me: me.clone() }
438    ///         })
439    ///     }
440    ///
441    ///     /// Returns a reference counted pointer to Self.
442    ///     fn me(&self) -> Arc<Self> {
443    ///         self.me.upgrade().unwrap()
444    ///     }
445    /// }
446    /// ```
447    /// [`upgrade`]: Weak::upgrade
448    #[cfg(not(no_global_oom_handling))]
449    #[inline]
450    #[stable(feature = "arc_new_cyclic", since = "1.60.0")]
451    pub fn new_cyclic<F>(data_fn: F) -> Arc<T>
452    where
453        F: FnOnce(&Weak<T>) -> T,
454    {
455        Self::new_cyclic_in(data_fn, Global)
456    }
457
458    /// Constructs a new `Arc` with uninitialized contents.
459    ///
460    /// # Examples
461    ///
462    /// ```
463    /// #![feature(get_mut_unchecked)]
464    ///
465    /// use std::sync::Arc;
466    ///
467    /// let mut five = Arc::<u32>::new_uninit();
468    ///
469    /// // Deferred initialization:
470    /// Arc::get_mut(&mut five).unwrap().write(5);
471    ///
472    /// let five = unsafe { five.assume_init() };
473    ///
474    /// assert_eq!(*five, 5)
475    /// ```
476    #[cfg(not(no_global_oom_handling))]
477    #[inline]
478    #[stable(feature = "new_uninit", since = "1.82.0")]
479    #[must_use]
480    pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
481        unsafe {
482            Arc::from_ptr(Arc::allocate_for_layout(
483                Layout::new::<T>(),
484                |layout| Global.allocate(layout),
485                <*mut u8>::cast,
486            ))
487        }
488    }
489
490    /// Constructs a new `Arc` with uninitialized contents, with the memory
491    /// being filled with `0` bytes.
492    ///
493    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
494    /// of this method.
495    ///
496    /// # Examples
497    ///
498    /// ```
499    /// #![feature(new_zeroed_alloc)]
500    ///
501    /// use std::sync::Arc;
502    ///
503    /// let zero = Arc::<u32>::new_zeroed();
504    /// let zero = unsafe { zero.assume_init() };
505    ///
506    /// assert_eq!(*zero, 0)
507    /// ```
508    ///
509    /// [zeroed]: mem::MaybeUninit::zeroed
510    #[cfg(not(no_global_oom_handling))]
511    #[inline]
512    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
513    #[must_use]
514    pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
515        unsafe {
516            Arc::from_ptr(Arc::allocate_for_layout(
517                Layout::new::<T>(),
518                |layout| Global.allocate_zeroed(layout),
519                <*mut u8>::cast,
520            ))
521        }
522    }
523
524    /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
525    /// `data` will be pinned in memory and unable to be moved.
526    #[cfg(not(no_global_oom_handling))]
527    #[stable(feature = "pin", since = "1.33.0")]
528    #[must_use]
529    pub fn pin(data: T) -> Pin<Arc<T>> {
530        unsafe { Pin::new_unchecked(Arc::new(data)) }
531    }
532
533    /// Constructs a new `Pin<Arc<T>>`, return an error if allocation fails.
534    #[unstable(feature = "allocator_api", issue = "32838")]
535    #[inline]
536    pub fn try_pin(data: T) -> Result<Pin<Arc<T>>, AllocError> {
537        unsafe { Ok(Pin::new_unchecked(Arc::try_new(data)?)) }
538    }
539
540    /// Constructs a new `Arc<T>`, returning an error if allocation fails.
541    ///
542    /// # Examples
543    ///
544    /// ```
545    /// #![feature(allocator_api)]
546    /// use std::sync::Arc;
547    ///
548    /// let five = Arc::try_new(5)?;
549    /// # Ok::<(), std::alloc::AllocError>(())
550    /// ```
551    #[unstable(feature = "allocator_api", issue = "32838")]
552    #[inline]
553    pub fn try_new(data: T) -> Result<Arc<T>, AllocError> {
554        // Start the weak pointer count as 1 which is the weak pointer that's
555        // held by all the strong pointers (kinda), see std/rc.rs for more info
556        let x: Box<_> = Box::try_new(ArcInner {
557            strong: atomic::AtomicUsize::new(1),
558            weak: atomic::AtomicUsize::new(1),
559            data,
560        })?;
561        unsafe { Ok(Self::from_inner(Box::leak(x).into())) }
562    }
563
564    /// Constructs a new `Arc` with uninitialized contents, returning an error
565    /// if allocation fails.
566    ///
567    /// # Examples
568    ///
569    /// ```
570    /// #![feature(allocator_api)]
571    /// #![feature(get_mut_unchecked)]
572    ///
573    /// use std::sync::Arc;
574    ///
575    /// let mut five = Arc::<u32>::try_new_uninit()?;
576    ///
577    /// // Deferred initialization:
578    /// Arc::get_mut(&mut five).unwrap().write(5);
579    ///
580    /// let five = unsafe { five.assume_init() };
581    ///
582    /// assert_eq!(*five, 5);
583    /// # Ok::<(), std::alloc::AllocError>(())
584    /// ```
585    #[unstable(feature = "allocator_api", issue = "32838")]
586    // #[unstable(feature = "new_uninit", issue = "63291")]
587    pub fn try_new_uninit() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
588        unsafe {
589            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
590                Layout::new::<T>(),
591                |layout| Global.allocate(layout),
592                <*mut u8>::cast,
593            )?))
594        }
595    }
596
597    /// Constructs a new `Arc` with uninitialized contents, with the memory
598    /// being filled with `0` bytes, returning an error if allocation fails.
599    ///
600    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
601    /// of this method.
602    ///
603    /// # Examples
604    ///
605    /// ```
606    /// #![feature( allocator_api)]
607    ///
608    /// use std::sync::Arc;
609    ///
610    /// let zero = Arc::<u32>::try_new_zeroed()?;
611    /// let zero = unsafe { zero.assume_init() };
612    ///
613    /// assert_eq!(*zero, 0);
614    /// # Ok::<(), std::alloc::AllocError>(())
615    /// ```
616    ///
617    /// [zeroed]: mem::MaybeUninit::zeroed
618    #[unstable(feature = "allocator_api", issue = "32838")]
619    // #[unstable(feature = "new_uninit", issue = "63291")]
620    pub fn try_new_zeroed() -> Result<Arc<mem::MaybeUninit<T>>, AllocError> {
621        unsafe {
622            Ok(Arc::from_ptr(Arc::try_allocate_for_layout(
623                Layout::new::<T>(),
624                |layout| Global.allocate_zeroed(layout),
625                <*mut u8>::cast,
626            )?))
627        }
628    }
629}
630
631impl<T, A: Allocator> Arc<T, A> {
632    /// Constructs a new `Arc<T>` in the provided allocator.
633    ///
634    /// # Examples
635    ///
636    /// ```
637    /// #![feature(allocator_api)]
638    ///
639    /// use std::sync::Arc;
640    /// use std::alloc::System;
641    ///
642    /// let five = Arc::new_in(5, System);
643    /// ```
644    #[inline]
645    #[cfg(not(no_global_oom_handling))]
646    #[unstable(feature = "allocator_api", issue = "32838")]
647    pub fn new_in(data: T, alloc: A) -> Arc<T, A> {
648        // Start the weak pointer count as 1 which is the weak pointer that's
649        // held by all the strong pointers (kinda), see std/rc.rs for more info
650        let x = Box::new_in(
651            ArcInner {
652                strong: atomic::AtomicUsize::new(1),
653                weak: atomic::AtomicUsize::new(1),
654                data,
655            },
656            alloc,
657        );
658        let (ptr, alloc) = Box::into_unique(x);
659        unsafe { Self::from_inner_in(ptr.into(), alloc) }
660    }
661
662    /// Constructs a new `Arc` with uninitialized contents in the provided allocator.
663    ///
664    /// # Examples
665    ///
666    /// ```
667    /// #![feature(get_mut_unchecked)]
668    /// #![feature(allocator_api)]
669    ///
670    /// use std::sync::Arc;
671    /// use std::alloc::System;
672    ///
673    /// let mut five = Arc::<u32, _>::new_uninit_in(System);
674    ///
675    /// let five = unsafe {
676    ///     // Deferred initialization:
677    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
678    ///
679    ///     five.assume_init()
680    /// };
681    ///
682    /// assert_eq!(*five, 5)
683    /// ```
684    #[cfg(not(no_global_oom_handling))]
685    #[unstable(feature = "allocator_api", issue = "32838")]
686    // #[unstable(feature = "new_uninit", issue = "63291")]
687    #[inline]
688    pub fn new_uninit_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
689        unsafe {
690            Arc::from_ptr_in(
691                Arc::allocate_for_layout(
692                    Layout::new::<T>(),
693                    |layout| alloc.allocate(layout),
694                    <*mut u8>::cast,
695                ),
696                alloc,
697            )
698        }
699    }
700
701    /// Constructs a new `Arc` with uninitialized contents, with the memory
702    /// being filled with `0` bytes, in the provided allocator.
703    ///
704    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
705    /// of this method.
706    ///
707    /// # Examples
708    ///
709    /// ```
710    /// #![feature(allocator_api)]
711    ///
712    /// use std::sync::Arc;
713    /// use std::alloc::System;
714    ///
715    /// let zero = Arc::<u32, _>::new_zeroed_in(System);
716    /// let zero = unsafe { zero.assume_init() };
717    ///
718    /// assert_eq!(*zero, 0)
719    /// ```
720    ///
721    /// [zeroed]: mem::MaybeUninit::zeroed
722    #[cfg(not(no_global_oom_handling))]
723    #[unstable(feature = "allocator_api", issue = "32838")]
724    // #[unstable(feature = "new_uninit", issue = "63291")]
725    #[inline]
726    pub fn new_zeroed_in(alloc: A) -> Arc<mem::MaybeUninit<T>, A> {
727        unsafe {
728            Arc::from_ptr_in(
729                Arc::allocate_for_layout(
730                    Layout::new::<T>(),
731                    |layout| alloc.allocate_zeroed(layout),
732                    <*mut u8>::cast,
733                ),
734                alloc,
735            )
736        }
737    }
738
739    /// Constructs a new `Arc<T, A>` in the given allocator while giving you a `Weak<T, A>` to the allocation,
740    /// to allow you to construct a `T` which holds a weak pointer to itself.
741    ///
742    /// Generally, a structure circularly referencing itself, either directly or
743    /// indirectly, should not hold a strong reference to itself to prevent a memory leak.
744    /// Using this function, you get access to the weak pointer during the
745    /// initialization of `T`, before the `Arc<T, A>` is created, such that you can
746    /// clone and store it inside the `T`.
747    ///
748    /// `new_cyclic_in` first allocates the managed allocation for the `Arc<T, A>`,
749    /// then calls your closure, giving it a `Weak<T, A>` to this allocation,
750    /// and only afterwards completes the construction of the `Arc<T, A>` by placing
751    /// the `T` returned from your closure into the allocation.
752    ///
753    /// Since the new `Arc<T, A>` is not fully-constructed until `Arc<T, A>::new_cyclic_in`
754    /// returns, calling [`upgrade`] on the weak reference inside your closure will
755    /// fail and result in a `None` value.
756    ///
757    /// # Panics
758    ///
759    /// If `data_fn` panics, the panic is propagated to the caller, and the
760    /// temporary [`Weak<T>`] is dropped normally.
761    ///
762    /// # Example
763    ///
764    /// See [`new_cyclic`]
765    ///
766    /// [`new_cyclic`]: Arc::new_cyclic
767    /// [`upgrade`]: Weak::upgrade
768    #[cfg(not(no_global_oom_handling))]
769    #[inline]
770    #[unstable(feature = "allocator_api", issue = "32838")]
771    pub fn new_cyclic_in<F>(data_fn: F, alloc: A) -> Arc<T, A>
772    where
773        F: FnOnce(&Weak<T, A>) -> T,
774    {
775        // Construct the inner in the "uninitialized" state with a single
776        // weak reference.
777        let (uninit_raw_ptr, alloc) = Box::into_raw_with_allocator(Box::new_in(
778            ArcInner {
779                strong: atomic::AtomicUsize::new(0),
780                weak: atomic::AtomicUsize::new(1),
781                data: mem::MaybeUninit::<T>::uninit(),
782            },
783            alloc,
784        ));
785        let uninit_ptr: NonNull<_> = (unsafe { &mut *uninit_raw_ptr }).into();
786        let init_ptr: NonNull<ArcInner<T>> = uninit_ptr.cast();
787
788        let weak = Weak { ptr: init_ptr, alloc };
789
790        // It's important we don't give up ownership of the weak pointer, or
791        // else the memory might be freed by the time `data_fn` returns. If
792        // we really wanted to pass ownership, we could create an additional
793        // weak pointer for ourselves, but this would result in additional
794        // updates to the weak reference count which might not be necessary
795        // otherwise.
796        let data = data_fn(&weak);
797
798        // Now we can properly initialize the inner value and turn our weak
799        // reference into a strong reference.
800        let strong = unsafe {
801            let inner = init_ptr.as_ptr();
802            ptr::write(&raw mut (*inner).data, data);
803
804            // The above write to the data field must be visible to any threads which
805            // observe a non-zero strong count. Therefore we need at least "Release" ordering
806            // in order to synchronize with the `compare_exchange_weak` in `Weak::upgrade`.
807            //
808            // "Acquire" ordering is not required. When considering the possible behaviors
809            // of `data_fn` we only need to look at what it could do with a reference to a
810            // non-upgradeable `Weak`:
811            // - It can *clone* the `Weak`, increasing the weak reference count.
812            // - It can drop those clones, decreasing the weak reference count (but never to zero).
813            //
814            // These side effects do not impact us in any way, and no other side effects are
815            // possible with safe code alone.
816            let prev_value = (*inner).strong.fetch_add(1, Release);
817            debug_assert_eq!(prev_value, 0, "No prior strong references should exist");
818
819            // Strong references should collectively own a shared weak reference,
820            // so don't run the destructor for our old weak reference.
821            // Calling into_raw_with_allocator has the double effect of giving us back the allocator,
822            // and forgetting the weak reference.
823            let alloc = weak.into_raw_with_allocator().1;
824
825            Arc::from_inner_in(init_ptr, alloc)
826        };
827
828        strong
829    }
830
831    /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator. If `T` does not implement `Unpin`,
832    /// then `data` will be pinned in memory and unable to be moved.
833    #[cfg(not(no_global_oom_handling))]
834    #[unstable(feature = "allocator_api", issue = "32838")]
835    #[inline]
836    pub fn pin_in(data: T, alloc: A) -> Pin<Arc<T, A>>
837    where
838        A: 'static,
839    {
840        unsafe { Pin::new_unchecked(Arc::new_in(data, alloc)) }
841    }
842
843    /// Constructs a new `Pin<Arc<T, A>>` in the provided allocator, return an error if allocation
844    /// fails.
845    #[inline]
846    #[unstable(feature = "allocator_api", issue = "32838")]
847    pub fn try_pin_in(data: T, alloc: A) -> Result<Pin<Arc<T, A>>, AllocError>
848    where
849        A: 'static,
850    {
851        unsafe { Ok(Pin::new_unchecked(Arc::try_new_in(data, alloc)?)) }
852    }
853
854    /// Constructs a new `Arc<T, A>` in the provided allocator, returning an error if allocation fails.
855    ///
856    /// # Examples
857    ///
858    /// ```
859    /// #![feature(allocator_api)]
860    ///
861    /// use std::sync::Arc;
862    /// use std::alloc::System;
863    ///
864    /// let five = Arc::try_new_in(5, System)?;
865    /// # Ok::<(), std::alloc::AllocError>(())
866    /// ```
867    #[inline]
868    #[unstable(feature = "allocator_api", issue = "32838")]
869    #[inline]
870    pub fn try_new_in(data: T, alloc: A) -> Result<Arc<T, A>, AllocError> {
871        // Start the weak pointer count as 1 which is the weak pointer that's
872        // held by all the strong pointers (kinda), see std/rc.rs for more info
873        let x = Box::try_new_in(
874            ArcInner {
875                strong: atomic::AtomicUsize::new(1),
876                weak: atomic::AtomicUsize::new(1),
877                data,
878            },
879            alloc,
880        )?;
881        let (ptr, alloc) = Box::into_unique(x);
882        Ok(unsafe { Self::from_inner_in(ptr.into(), alloc) })
883    }
884
885    /// Constructs a new `Arc` with uninitialized contents, in the provided allocator, returning an
886    /// error if allocation fails.
887    ///
888    /// # Examples
889    ///
890    /// ```
891    /// #![feature(allocator_api)]
892    /// #![feature(get_mut_unchecked)]
893    ///
894    /// use std::sync::Arc;
895    /// use std::alloc::System;
896    ///
897    /// let mut five = Arc::<u32, _>::try_new_uninit_in(System)?;
898    ///
899    /// let five = unsafe {
900    ///     // Deferred initialization:
901    ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
902    ///
903    ///     five.assume_init()
904    /// };
905    ///
906    /// assert_eq!(*five, 5);
907    /// # Ok::<(), std::alloc::AllocError>(())
908    /// ```
909    #[unstable(feature = "allocator_api", issue = "32838")]
910    // #[unstable(feature = "new_uninit", issue = "63291")]
911    #[inline]
912    pub fn try_new_uninit_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
913        unsafe {
914            Ok(Arc::from_ptr_in(
915                Arc::try_allocate_for_layout(
916                    Layout::new::<T>(),
917                    |layout| alloc.allocate(layout),
918                    <*mut u8>::cast,
919                )?,
920                alloc,
921            ))
922        }
923    }
924
925    /// Constructs a new `Arc` with uninitialized contents, with the memory
926    /// being filled with `0` bytes, in the provided allocator, returning an error if allocation
927    /// fails.
928    ///
929    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
930    /// of this method.
931    ///
932    /// # Examples
933    ///
934    /// ```
935    /// #![feature(allocator_api)]
936    ///
937    /// use std::sync::Arc;
938    /// use std::alloc::System;
939    ///
940    /// let zero = Arc::<u32, _>::try_new_zeroed_in(System)?;
941    /// let zero = unsafe { zero.assume_init() };
942    ///
943    /// assert_eq!(*zero, 0);
944    /// # Ok::<(), std::alloc::AllocError>(())
945    /// ```
946    ///
947    /// [zeroed]: mem::MaybeUninit::zeroed
948    #[unstable(feature = "allocator_api", issue = "32838")]
949    // #[unstable(feature = "new_uninit", issue = "63291")]
950    #[inline]
951    pub fn try_new_zeroed_in(alloc: A) -> Result<Arc<mem::MaybeUninit<T>, A>, AllocError> {
952        unsafe {
953            Ok(Arc::from_ptr_in(
954                Arc::try_allocate_for_layout(
955                    Layout::new::<T>(),
956                    |layout| alloc.allocate_zeroed(layout),
957                    <*mut u8>::cast,
958                )?,
959                alloc,
960            ))
961        }
962    }
963    /// Returns the inner value, if the `Arc` has exactly one strong reference.
964    ///
965    /// Otherwise, an [`Err`] is returned with the same `Arc` that was
966    /// passed in.
967    ///
968    /// This will succeed even if there are outstanding weak references.
969    ///
970    /// It is strongly recommended to use [`Arc::into_inner`] instead if you don't
971    /// keep the `Arc` in the [`Err`] case.
972    /// Immediately dropping the [`Err`]-value, as the expression
973    /// `Arc::try_unwrap(this).ok()` does, can cause the strong count to
974    /// drop to zero and the inner value of the `Arc` to be dropped.
975    /// For instance, if two threads execute such an expression in parallel,
976    /// there is a race condition without the possibility of unsafety:
977    /// The threads could first both check whether they own the last instance
978    /// in `Arc::try_unwrap`, determine that they both do not, and then both
979    /// discard and drop their instance in the call to [`ok`][`Result::ok`].
980    /// In this scenario, the value inside the `Arc` is safely destroyed
981    /// by exactly one of the threads, but neither thread will ever be able
982    /// to use the value.
983    ///
984    /// # Examples
985    ///
986    /// ```
987    /// use std::sync::Arc;
988    ///
989    /// let x = Arc::new(3);
990    /// assert_eq!(Arc::try_unwrap(x), Ok(3));
991    ///
992    /// let x = Arc::new(4);
993    /// let _y = Arc::clone(&x);
994    /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
995    /// ```
996    #[inline]
997    #[stable(feature = "arc_unique", since = "1.4.0")]
998    pub fn try_unwrap(this: Self) -> Result<T, Self> {
999        if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
1000            return Err(this);
1001        }
1002
1003        acquire!(this.inner().strong);
1004
1005        let this = ManuallyDrop::new(this);
1006        let elem: T = unsafe { ptr::read(&this.ptr.as_ref().data) };
1007        let alloc: A = unsafe { ptr::read(&this.alloc) }; // copy the allocator
1008
1009        // Make a weak pointer to clean up the implicit strong-weak reference
1010        let _weak = Weak { ptr: this.ptr, alloc };
1011
1012        Ok(elem)
1013    }
1014
1015    /// Returns the inner value, if the `Arc` has exactly one strong reference.
1016    ///
1017    /// Otherwise, [`None`] is returned and the `Arc` is dropped.
1018    ///
1019    /// This will succeed even if there are outstanding weak references.
1020    ///
1021    /// If `Arc::into_inner` is called on every clone of this `Arc`,
1022    /// it is guaranteed that exactly one of the calls returns the inner value.
1023    /// This means in particular that the inner value is not dropped.
1024    ///
1025    /// [`Arc::try_unwrap`] is conceptually similar to `Arc::into_inner`, but it
1026    /// is meant for different use-cases. If used as a direct replacement
1027    /// for `Arc::into_inner` anyway, such as with the expression
1028    /// <code>[Arc::try_unwrap]\(this).[ok][Result::ok]()</code>, then it does
1029    /// **not** give the same guarantee as described in the previous paragraph.
1030    /// For more information, see the examples below and read the documentation
1031    /// of [`Arc::try_unwrap`].
1032    ///
1033    /// # Examples
1034    ///
1035    /// Minimal example demonstrating the guarantee that `Arc::into_inner` gives.
1036    /// ```
1037    /// use std::sync::Arc;
1038    ///
1039    /// let x = Arc::new(3);
1040    /// let y = Arc::clone(&x);
1041    ///
1042    /// // Two threads calling `Arc::into_inner` on both clones of an `Arc`:
1043    /// let x_thread = std::thread::spawn(|| Arc::into_inner(x));
1044    /// let y_thread = std::thread::spawn(|| Arc::into_inner(y));
1045    ///
1046    /// let x_inner_value = x_thread.join().unwrap();
1047    /// let y_inner_value = y_thread.join().unwrap();
1048    ///
1049    /// // One of the threads is guaranteed to receive the inner value:
1050    /// assert!(matches!(
1051    ///     (x_inner_value, y_inner_value),
1052    ///     (None, Some(3)) | (Some(3), None)
1053    /// ));
1054    /// // The result could also be `(None, None)` if the threads called
1055    /// // `Arc::try_unwrap(x).ok()` and `Arc::try_unwrap(y).ok()` instead.
1056    /// ```
1057    ///
1058    /// A more practical example demonstrating the need for `Arc::into_inner`:
1059    /// ```
1060    /// use std::sync::Arc;
1061    ///
1062    /// // Definition of a simple singly linked list using `Arc`:
1063    /// #[derive(Clone)]
1064    /// struct LinkedList<T>(Option<Arc<Node<T>>>);
1065    /// struct Node<T>(T, Option<Arc<Node<T>>>);
1066    ///
1067    /// // Dropping a long `LinkedList<T>` relying on the destructor of `Arc`
1068    /// // can cause a stack overflow. To prevent this, we can provide a
1069    /// // manual `Drop` implementation that does the destruction in a loop:
1070    /// impl<T> Drop for LinkedList<T> {
1071    ///     fn drop(&mut self) {
1072    ///         let mut link = self.0.take();
1073    ///         while let Some(arc_node) = link.take() {
1074    ///             if let Some(Node(_value, next)) = Arc::into_inner(arc_node) {
1075    ///                 link = next;
1076    ///             }
1077    ///         }
1078    ///     }
1079    /// }
1080    ///
1081    /// // Implementation of `new` and `push` omitted
1082    /// impl<T> LinkedList<T> {
1083    ///     /* ... */
1084    /// #   fn new() -> Self {
1085    /// #       LinkedList(None)
1086    /// #   }
1087    /// #   fn push(&mut self, x: T) {
1088    /// #       self.0 = Some(Arc::new(Node(x, self.0.take())));
1089    /// #   }
1090    /// }
1091    ///
1092    /// // The following code could have still caused a stack overflow
1093    /// // despite the manual `Drop` impl if that `Drop` impl had used
1094    /// // `Arc::try_unwrap(arc).ok()` instead of `Arc::into_inner(arc)`.
1095    ///
1096    /// // Create a long list and clone it
1097    /// let mut x = LinkedList::new();
1098    /// let size = 100000;
1099    /// # let size = if cfg!(miri) { 100 } else { size };
1100    /// for i in 0..size {
1101    ///     x.push(i); // Adds i to the front of x
1102    /// }
1103    /// let y = x.clone();
1104    ///
1105    /// // Drop the clones in parallel
1106    /// let x_thread = std::thread::spawn(|| drop(x));
1107    /// let y_thread = std::thread::spawn(|| drop(y));
1108    /// x_thread.join().unwrap();
1109    /// y_thread.join().unwrap();
1110    /// ```
1111    #[inline]
1112    #[stable(feature = "arc_into_inner", since = "1.70.0")]
1113    pub fn into_inner(this: Self) -> Option<T> {
1114        // Make sure that the ordinary `Drop` implementation isn’t called as well
1115        let mut this = mem::ManuallyDrop::new(this);
1116
1117        // Following the implementation of `drop` and `drop_slow`
1118        if this.inner().strong.fetch_sub(1, Release) != 1 {
1119            return None;
1120        }
1121
1122        acquire!(this.inner().strong);
1123
1124        // SAFETY: This mirrors the line
1125        //
1126        //     unsafe { ptr::drop_in_place(Self::get_mut_unchecked(self)) };
1127        //
1128        // in `drop_slow`. Instead of dropping the value behind the pointer,
1129        // it is read and eventually returned; `ptr::read` has the same
1130        // safety conditions as `ptr::drop_in_place`.
1131
1132        let inner = unsafe { ptr::read(Self::get_mut_unchecked(&mut this)) };
1133        let alloc = unsafe { ptr::read(&this.alloc) };
1134
1135        drop(Weak { ptr: this.ptr, alloc });
1136
1137        Some(inner)
1138    }
1139}
1140
1141impl<T> Arc<[T]> {
1142    /// Constructs a new atomically reference-counted slice with uninitialized contents.
1143    ///
1144    /// # Examples
1145    ///
1146    /// ```
1147    /// #![feature(get_mut_unchecked)]
1148    ///
1149    /// use std::sync::Arc;
1150    ///
1151    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1152    ///
1153    /// // Deferred initialization:
1154    /// let data = Arc::get_mut(&mut values).unwrap();
1155    /// data[0].write(1);
1156    /// data[1].write(2);
1157    /// data[2].write(3);
1158    ///
1159    /// let values = unsafe { values.assume_init() };
1160    ///
1161    /// assert_eq!(*values, [1, 2, 3])
1162    /// ```
1163    #[cfg(not(no_global_oom_handling))]
1164    #[inline]
1165    #[stable(feature = "new_uninit", since = "1.82.0")]
1166    #[must_use]
1167    pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1168        unsafe { Arc::from_ptr(Arc::allocate_for_slice(len)) }
1169    }
1170
1171    /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1172    /// filled with `0` bytes.
1173    ///
1174    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1175    /// incorrect usage of this method.
1176    ///
1177    /// # Examples
1178    ///
1179    /// ```
1180    /// #![feature(new_zeroed_alloc)]
1181    ///
1182    /// use std::sync::Arc;
1183    ///
1184    /// let values = Arc::<[u32]>::new_zeroed_slice(3);
1185    /// let values = unsafe { values.assume_init() };
1186    ///
1187    /// assert_eq!(*values, [0, 0, 0])
1188    /// ```
1189    ///
1190    /// [zeroed]: mem::MaybeUninit::zeroed
1191    #[cfg(not(no_global_oom_handling))]
1192    #[inline]
1193    #[unstable(feature = "new_zeroed_alloc", issue = "129396")]
1194    #[must_use]
1195    pub fn new_zeroed_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
1196        unsafe {
1197            Arc::from_ptr(Arc::allocate_for_layout(
1198                Layout::array::<T>(len).unwrap(),
1199                |layout| Global.allocate_zeroed(layout),
1200                |mem| {
1201                    ptr::slice_from_raw_parts_mut(mem as *mut T, len)
1202                        as *mut ArcInner<[mem::MaybeUninit<T>]>
1203                },
1204            ))
1205        }
1206    }
1207
1208    /// Converts the reference-counted slice into a reference-counted array.
1209    ///
1210    /// This operation does not reallocate; the underlying array of the slice is simply reinterpreted as an array type.
1211    ///
1212    /// If `N` is not exactly equal to the length of `self`, then this method returns `None`.
1213    #[unstable(feature = "slice_as_array", issue = "133508")]
1214    #[inline]
1215    #[must_use]
1216    pub fn into_array<const N: usize>(self) -> Option<Arc<[T; N]>> {
1217        if self.len() == N {
1218            let ptr = Self::into_raw(self) as *const [T; N];
1219
1220            // 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.
1221            let me = unsafe { Arc::from_raw(ptr) };
1222            Some(me)
1223        } else {
1224            None
1225        }
1226    }
1227}
1228
1229impl<T, A: Allocator> Arc<[T], A> {
1230    /// Constructs a new atomically reference-counted slice with uninitialized contents in the
1231    /// provided allocator.
1232    ///
1233    /// # Examples
1234    ///
1235    /// ```
1236    /// #![feature(get_mut_unchecked)]
1237    /// #![feature(allocator_api)]
1238    ///
1239    /// use std::sync::Arc;
1240    /// use std::alloc::System;
1241    ///
1242    /// let mut values = Arc::<[u32], _>::new_uninit_slice_in(3, System);
1243    ///
1244    /// let values = unsafe {
1245    ///     // Deferred initialization:
1246    ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
1247    ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
1248    ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
1249    ///
1250    ///     values.assume_init()
1251    /// };
1252    ///
1253    /// assert_eq!(*values, [1, 2, 3])
1254    /// ```
1255    #[cfg(not(no_global_oom_handling))]
1256    #[unstable(feature = "allocator_api", issue = "32838")]
1257    #[inline]
1258    pub fn new_uninit_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1259        unsafe { Arc::from_ptr_in(Arc::allocate_for_slice_in(len, &alloc), alloc) }
1260    }
1261
1262    /// Constructs a new atomically reference-counted slice with uninitialized contents, with the memory being
1263    /// filled with `0` bytes, in the provided allocator.
1264    ///
1265    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
1266    /// incorrect usage of this method.
1267    ///
1268    /// # Examples
1269    ///
1270    /// ```
1271    /// #![feature(allocator_api)]
1272    ///
1273    /// use std::sync::Arc;
1274    /// use std::alloc::System;
1275    ///
1276    /// let values = Arc::<[u32], _>::new_zeroed_slice_in(3, System);
1277    /// let values = unsafe { values.assume_init() };
1278    ///
1279    /// assert_eq!(*values, [0, 0, 0])
1280    /// ```
1281    ///
1282    /// [zeroed]: mem::MaybeUninit::zeroed
1283    #[cfg(not(no_global_oom_handling))]
1284    #[unstable(feature = "allocator_api", issue = "32838")]
1285    #[inline]
1286    pub fn new_zeroed_slice_in(len: usize, alloc: A) -> Arc<[mem::MaybeUninit<T>], A> {
1287        unsafe {
1288            Arc::from_ptr_in(
1289                Arc::allocate_for_layout(
1290                    Layout::array::<T>(len).unwrap(),
1291                    |layout| alloc.allocate_zeroed(layout),
1292                    |mem| {
1293                        ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len)
1294                            as *mut ArcInner<[mem::MaybeUninit<T>]>
1295                    },
1296                ),
1297                alloc,
1298            )
1299        }
1300    }
1301}
1302
1303impl<T, A: Allocator> Arc<mem::MaybeUninit<T>, A> {
1304    /// Converts to `Arc<T>`.
1305    ///
1306    /// # Safety
1307    ///
1308    /// As with [`MaybeUninit::assume_init`],
1309    /// it is up to the caller to guarantee that the inner value
1310    /// really is in an initialized state.
1311    /// Calling this when the content is not yet fully initialized
1312    /// causes immediate undefined behavior.
1313    ///
1314    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1315    ///
1316    /// # Examples
1317    ///
1318    /// ```
1319    /// #![feature(get_mut_unchecked)]
1320    ///
1321    /// use std::sync::Arc;
1322    ///
1323    /// let mut five = Arc::<u32>::new_uninit();
1324    ///
1325    /// // Deferred initialization:
1326    /// Arc::get_mut(&mut five).unwrap().write(5);
1327    ///
1328    /// let five = unsafe { five.assume_init() };
1329    ///
1330    /// assert_eq!(*five, 5)
1331    /// ```
1332    #[stable(feature = "new_uninit", since = "1.82.0")]
1333    #[must_use = "`self` will be dropped if the result is not used"]
1334    #[inline]
1335    pub unsafe fn assume_init(self) -> Arc<T, A> {
1336        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1337        unsafe { Arc::from_inner_in(ptr.cast(), alloc) }
1338    }
1339}
1340
1341impl<T, A: Allocator> Arc<[mem::MaybeUninit<T>], A> {
1342    /// Converts to `Arc<[T]>`.
1343    ///
1344    /// # Safety
1345    ///
1346    /// As with [`MaybeUninit::assume_init`],
1347    /// it is up to the caller to guarantee that the inner value
1348    /// really is in an initialized state.
1349    /// Calling this when the content is not yet fully initialized
1350    /// causes immediate undefined behavior.
1351    ///
1352    /// [`MaybeUninit::assume_init`]: mem::MaybeUninit::assume_init
1353    ///
1354    /// # Examples
1355    ///
1356    /// ```
1357    /// #![feature(get_mut_unchecked)]
1358    ///
1359    /// use std::sync::Arc;
1360    ///
1361    /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
1362    ///
1363    /// // Deferred initialization:
1364    /// let data = Arc::get_mut(&mut values).unwrap();
1365    /// data[0].write(1);
1366    /// data[1].write(2);
1367    /// data[2].write(3);
1368    ///
1369    /// let values = unsafe { values.assume_init() };
1370    ///
1371    /// assert_eq!(*values, [1, 2, 3])
1372    /// ```
1373    #[stable(feature = "new_uninit", since = "1.82.0")]
1374    #[must_use = "`self` will be dropped if the result is not used"]
1375    #[inline]
1376    pub unsafe fn assume_init(self) -> Arc<[T], A> {
1377        let (ptr, alloc) = Arc::into_inner_with_allocator(self);
1378        unsafe { Arc::from_ptr_in(ptr.as_ptr() as _, alloc) }
1379    }
1380}
1381
1382impl<T: ?Sized> Arc<T> {
1383    /// Constructs an `Arc<T>` from a raw pointer.
1384    ///
1385    /// The raw pointer must have been previously returned by a call to
1386    /// [`Arc<U>::into_raw`][into_raw] with the following requirements:
1387    ///
1388    /// * If `U` is sized, it must have the same size and alignment as `T`. This
1389    ///   is trivially true if `U` is `T`.
1390    /// * If `U` is unsized, its data pointer must have the same size and
1391    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
1392    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1393    ///   coercion].
1394    ///
1395    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1396    /// and alignment, this is basically like transmuting references of
1397    /// different types. See [`mem::transmute`][transmute] for more information
1398    /// on what restrictions apply in this case.
1399    ///
1400    /// The raw pointer must point to a block of memory allocated by the global allocator.
1401    ///
1402    /// The user of `from_raw` has to make sure a specific value of `T` is only
1403    /// dropped once.
1404    ///
1405    /// This function is unsafe because improper use may lead to memory unsafety,
1406    /// even if the returned `Arc<T>` is never accessed.
1407    ///
1408    /// [into_raw]: Arc::into_raw
1409    /// [transmute]: core::mem::transmute
1410    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1411    ///
1412    /// # Examples
1413    ///
1414    /// ```
1415    /// use std::sync::Arc;
1416    ///
1417    /// let x = Arc::new("hello".to_owned());
1418    /// let x_ptr = Arc::into_raw(x);
1419    ///
1420    /// unsafe {
1421    ///     // Convert back to an `Arc` to prevent leak.
1422    ///     let x = Arc::from_raw(x_ptr);
1423    ///     assert_eq!(&*x, "hello");
1424    ///
1425    ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1426    /// }
1427    ///
1428    /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1429    /// ```
1430    ///
1431    /// Convert a slice back into its original array:
1432    ///
1433    /// ```
1434    /// use std::sync::Arc;
1435    ///
1436    /// let x: Arc<[u32]> = Arc::new([1, 2, 3]);
1437    /// let x_ptr: *const [u32] = Arc::into_raw(x);
1438    ///
1439    /// unsafe {
1440    ///     let x: Arc<[u32; 3]> = Arc::from_raw(x_ptr.cast::<[u32; 3]>());
1441    ///     assert_eq!(&*x, &[1, 2, 3]);
1442    /// }
1443    /// ```
1444    #[inline]
1445    #[stable(feature = "rc_raw", since = "1.17.0")]
1446    pub unsafe fn from_raw(ptr: *const T) -> Self {
1447        unsafe { Arc::from_raw_in(ptr, Global) }
1448    }
1449
1450    /// Increments the strong reference count on the `Arc<T>` associated with the
1451    /// provided pointer by one.
1452    ///
1453    /// # Safety
1454    ///
1455    /// The pointer must have been obtained through `Arc::into_raw`, and the
1456    /// associated `Arc` instance must be valid (i.e. the strong count must be at
1457    /// least 1) for the duration of this method, and `ptr` must point to a block of memory
1458    /// allocated by the global allocator.
1459    ///
1460    /// # Examples
1461    ///
1462    /// ```
1463    /// use std::sync::Arc;
1464    ///
1465    /// let five = Arc::new(5);
1466    ///
1467    /// unsafe {
1468    ///     let ptr = Arc::into_raw(five);
1469    ///     Arc::increment_strong_count(ptr);
1470    ///
1471    ///     // This assertion is deterministic because we haven't shared
1472    ///     // the `Arc` between threads.
1473    ///     let five = Arc::from_raw(ptr);
1474    ///     assert_eq!(2, Arc::strong_count(&five));
1475    /// #   // Prevent leaks for Miri.
1476    /// #   Arc::decrement_strong_count(ptr);
1477    /// }
1478    /// ```
1479    #[inline]
1480    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1481    pub unsafe fn increment_strong_count(ptr: *const T) {
1482        unsafe { Arc::increment_strong_count_in(ptr, Global) }
1483    }
1484
1485    /// Decrements the strong reference count on the `Arc<T>` associated with the
1486    /// provided pointer by one.
1487    ///
1488    /// # Safety
1489    ///
1490    /// The pointer must have been obtained through `Arc::into_raw`, and the
1491    /// associated `Arc` instance must be valid (i.e. the strong count must be at
1492    /// least 1) when invoking this method, and `ptr` must point to a block of memory
1493    /// allocated by the global allocator. This method can be used to release the final
1494    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1495    /// released.
1496    ///
1497    /// # Examples
1498    ///
1499    /// ```
1500    /// use std::sync::Arc;
1501    ///
1502    /// let five = Arc::new(5);
1503    ///
1504    /// unsafe {
1505    ///     let ptr = Arc::into_raw(five);
1506    ///     Arc::increment_strong_count(ptr);
1507    ///
1508    ///     // Those assertions are deterministic because we haven't shared
1509    ///     // the `Arc` between threads.
1510    ///     let five = Arc::from_raw(ptr);
1511    ///     assert_eq!(2, Arc::strong_count(&five));
1512    ///     Arc::decrement_strong_count(ptr);
1513    ///     assert_eq!(1, Arc::strong_count(&five));
1514    /// }
1515    /// ```
1516    #[inline]
1517    #[stable(feature = "arc_mutate_strong_count", since = "1.51.0")]
1518    pub unsafe fn decrement_strong_count(ptr: *const T) {
1519        unsafe { Arc::decrement_strong_count_in(ptr, Global) }
1520    }
1521}
1522
1523impl<T: ?Sized, A: Allocator> Arc<T, A> {
1524    /// Returns a reference to the underlying allocator.
1525    ///
1526    /// Note: this is an associated function, which means that you have
1527    /// to call it as `Arc::allocator(&a)` instead of `a.allocator()`. This
1528    /// is so that there is no conflict with a method on the inner type.
1529    #[inline]
1530    #[unstable(feature = "allocator_api", issue = "32838")]
1531    pub fn allocator(this: &Self) -> &A {
1532        &this.alloc
1533    }
1534
1535    /// Consumes the `Arc`, returning the wrapped pointer.
1536    ///
1537    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1538    /// [`Arc::from_raw`].
1539    ///
1540    /// # Examples
1541    ///
1542    /// ```
1543    /// use std::sync::Arc;
1544    ///
1545    /// let x = Arc::new("hello".to_owned());
1546    /// let x_ptr = Arc::into_raw(x);
1547    /// assert_eq!(unsafe { &*x_ptr }, "hello");
1548    /// # // Prevent leaks for Miri.
1549    /// # drop(unsafe { Arc::from_raw(x_ptr) });
1550    /// ```
1551    #[must_use = "losing the pointer will leak memory"]
1552    #[stable(feature = "rc_raw", since = "1.17.0")]
1553    #[rustc_never_returns_null_ptr]
1554    pub fn into_raw(this: Self) -> *const T {
1555        let this = ManuallyDrop::new(this);
1556        Self::as_ptr(&*this)
1557    }
1558
1559    /// Consumes the `Arc`, returning the wrapped pointer and allocator.
1560    ///
1561    /// To avoid a memory leak the pointer must be converted back to an `Arc` using
1562    /// [`Arc::from_raw_in`].
1563    ///
1564    /// # Examples
1565    ///
1566    /// ```
1567    /// #![feature(allocator_api)]
1568    /// use std::sync::Arc;
1569    /// use std::alloc::System;
1570    ///
1571    /// let x = Arc::new_in("hello".to_owned(), System);
1572    /// let (ptr, alloc) = Arc::into_raw_with_allocator(x);
1573    /// assert_eq!(unsafe { &*ptr }, "hello");
1574    /// let x = unsafe { Arc::from_raw_in(ptr, alloc) };
1575    /// assert_eq!(&*x, "hello");
1576    /// ```
1577    #[must_use = "losing the pointer will leak memory"]
1578    #[unstable(feature = "allocator_api", issue = "32838")]
1579    pub fn into_raw_with_allocator(this: Self) -> (*const T, A) {
1580        let this = mem::ManuallyDrop::new(this);
1581        let ptr = Self::as_ptr(&this);
1582        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
1583        let alloc = unsafe { ptr::read(&this.alloc) };
1584        (ptr, alloc)
1585    }
1586
1587    /// Provides a raw pointer to the data.
1588    ///
1589    /// The counts are not affected in any way and the `Arc` is not consumed. The pointer is valid for
1590    /// as long as there are strong counts in the `Arc`.
1591    ///
1592    /// # Examples
1593    ///
1594    /// ```
1595    /// use std::sync::Arc;
1596    ///
1597    /// let x = Arc::new("hello".to_owned());
1598    /// let y = Arc::clone(&x);
1599    /// let x_ptr = Arc::as_ptr(&x);
1600    /// assert_eq!(x_ptr, Arc::as_ptr(&y));
1601    /// assert_eq!(unsafe { &*x_ptr }, "hello");
1602    /// ```
1603    #[must_use]
1604    #[stable(feature = "rc_as_ptr", since = "1.45.0")]
1605    #[rustc_never_returns_null_ptr]
1606    pub fn as_ptr(this: &Self) -> *const T {
1607        let ptr: *mut ArcInner<T> = NonNull::as_ptr(this.ptr);
1608
1609        // SAFETY: This cannot go through Deref::deref or RcInnerPtr::inner because
1610        // this is required to retain raw/mut provenance such that e.g. `get_mut` can
1611        // write through the pointer after the Rc is recovered through `from_raw`.
1612        unsafe { &raw mut (*ptr).data }
1613    }
1614
1615    /// Constructs an `Arc<T, A>` from a raw pointer.
1616    ///
1617    /// The raw pointer must have been previously returned by a call to [`Arc<U,
1618    /// A>::into_raw`][into_raw] with the following requirements:
1619    ///
1620    /// * If `U` is sized, it must have the same size and alignment as `T`. This
1621    ///   is trivially true if `U` is `T`.
1622    /// * If `U` is unsized, its data pointer must have the same size and
1623    ///   alignment as `T`. This is trivially true if `Arc<U>` was constructed
1624    ///   through `Arc<T>` and then converted to `Arc<U>` through an [unsized
1625    ///   coercion].
1626    ///
1627    /// Note that if `U` or `U`'s data pointer is not `T` but has the same size
1628    /// and alignment, this is basically like transmuting references of
1629    /// different types. See [`mem::transmute`][transmute] for more information
1630    /// on what restrictions apply in this case.
1631    ///
1632    /// The raw pointer must point to a block of memory allocated by `alloc`
1633    ///
1634    /// The user of `from_raw` has to make sure a specific value of `T` is only
1635    /// dropped once.
1636    ///
1637    /// This function is unsafe because improper use may lead to memory unsafety,
1638    /// even if the returned `Arc<T>` is never accessed.
1639    ///
1640    /// [into_raw]: Arc::into_raw
1641    /// [transmute]: core::mem::transmute
1642    /// [unsized coercion]: https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions
1643    ///
1644    /// # Examples
1645    ///
1646    /// ```
1647    /// #![feature(allocator_api)]
1648    ///
1649    /// use std::sync::Arc;
1650    /// use std::alloc::System;
1651    ///
1652    /// let x = Arc::new_in("hello".to_owned(), System);
1653    /// let x_ptr = Arc::into_raw(x);
1654    ///
1655    /// unsafe {
1656    ///     // Convert back to an `Arc` to prevent leak.
1657    ///     let x = Arc::from_raw_in(x_ptr, System);
1658    ///     assert_eq!(&*x, "hello");
1659    ///
1660    ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
1661    /// }
1662    ///
1663    /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
1664    /// ```
1665    ///
1666    /// Convert a slice back into its original array:
1667    ///
1668    /// ```
1669    /// #![feature(allocator_api)]
1670    ///
1671    /// use std::sync::Arc;
1672    /// use std::alloc::System;
1673    ///
1674    /// let x: Arc<[u32], _> = Arc::new_in([1, 2, 3], System);
1675    /// let x_ptr: *const [u32] = Arc::into_raw(x);
1676    ///
1677    /// unsafe {
1678    ///     let x: Arc<[u32; 3], _> = Arc::from_raw_in(x_ptr.cast::<[u32; 3]>(), System);
1679    ///     assert_eq!(&*x, &[1, 2, 3]);
1680    /// }
1681    /// ```
1682    #[inline]
1683    #[unstable(feature = "allocator_api", issue = "32838")]
1684    pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
1685        unsafe {
1686            let offset = data_offset(ptr);
1687
1688            // Reverse the offset to find the original ArcInner.
1689            let arc_ptr = ptr.byte_sub(offset) as *mut ArcInner<T>;
1690
1691            Self::from_ptr_in(arc_ptr, alloc)
1692        }
1693    }
1694
1695    /// Creates a new [`Weak`] pointer to this allocation.
1696    ///
1697    /// # Examples
1698    ///
1699    /// ```
1700    /// use std::sync::Arc;
1701    ///
1702    /// let five = Arc::new(5);
1703    ///
1704    /// let weak_five = Arc::downgrade(&five);
1705    /// ```
1706    #[must_use = "this returns a new `Weak` pointer, \
1707                  without modifying the original `Arc`"]
1708    #[stable(feature = "arc_weak", since = "1.4.0")]
1709    pub fn downgrade(this: &Self) -> Weak<T, A>
1710    where
1711        A: Clone,
1712    {
1713        // This Relaxed is OK because we're checking the value in the CAS
1714        // below.
1715        let mut cur = this.inner().weak.load(Relaxed);
1716
1717        loop {
1718            // check if the weak counter is currently "locked"; if so, spin.
1719            if cur == usize::MAX {
1720                hint::spin_loop();
1721                cur = this.inner().weak.load(Relaxed);
1722                continue;
1723            }
1724
1725            // We can't allow the refcount to increase much past `MAX_REFCOUNT`.
1726            assert!(cur <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
1727
1728            // NOTE: this code currently ignores the possibility of overflow
1729            // into usize::MAX; in general both Rc and Arc need to be adjusted
1730            // to deal with overflow.
1731
1732            // Unlike with Clone(), we need this to be an Acquire read to
1733            // synchronize with the write coming from `is_unique`, so that the
1734            // events prior to that write happen before this read.
1735            match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
1736                Ok(_) => {
1737                    // Make sure we do not create a dangling Weak
1738                    debug_assert!(!is_dangling(this.ptr.as_ptr()));
1739                    return Weak { ptr: this.ptr, alloc: this.alloc.clone() };
1740                }
1741                Err(old) => cur = old,
1742            }
1743        }
1744    }
1745
1746    /// Gets the number of [`Weak`] pointers to this allocation.
1747    ///
1748    /// # Safety
1749    ///
1750    /// This method by itself is safe, but using it correctly requires extra care.
1751    /// Another thread can change the weak count at any time,
1752    /// including potentially between calling this method and acting on the result.
1753    ///
1754    /// # Examples
1755    ///
1756    /// ```
1757    /// use std::sync::Arc;
1758    ///
1759    /// let five = Arc::new(5);
1760    /// let _weak_five = Arc::downgrade(&five);
1761    ///
1762    /// // This assertion is deterministic because we haven't shared
1763    /// // the `Arc` or `Weak` between threads.
1764    /// assert_eq!(1, Arc::weak_count(&five));
1765    /// ```
1766    #[inline]
1767    #[must_use]
1768    #[stable(feature = "arc_counts", since = "1.15.0")]
1769    pub fn weak_count(this: &Self) -> usize {
1770        let cnt = this.inner().weak.load(Relaxed);
1771        // If the weak count is currently locked, the value of the
1772        // count was 0 just before taking the lock.
1773        if cnt == usize::MAX { 0 } else { cnt - 1 }
1774    }
1775
1776    /// Gets the number of strong (`Arc`) pointers to this allocation.
1777    ///
1778    /// # Safety
1779    ///
1780    /// This method by itself is safe, but using it correctly requires extra care.
1781    /// Another thread can change the strong count at any time,
1782    /// including potentially between calling this method and acting on the result.
1783    ///
1784    /// # Examples
1785    ///
1786    /// ```
1787    /// use std::sync::Arc;
1788    ///
1789    /// let five = Arc::new(5);
1790    /// let _also_five = Arc::clone(&five);
1791    ///
1792    /// // This assertion is deterministic because we haven't shared
1793    /// // the `Arc` between threads.
1794    /// assert_eq!(2, Arc::strong_count(&five));
1795    /// ```
1796    #[inline]
1797    #[must_use]
1798    #[stable(feature = "arc_counts", since = "1.15.0")]
1799    pub fn strong_count(this: &Self) -> usize {
1800        this.inner().strong.load(Relaxed)
1801    }
1802
1803    /// Increments the strong reference count on the `Arc<T>` associated with the
1804    /// provided pointer by one.
1805    ///
1806    /// # Safety
1807    ///
1808    /// The pointer must have been obtained through `Arc::into_raw`, and the
1809    /// associated `Arc` instance must be valid (i.e. the strong count must be at
1810    /// least 1) for the duration of this method,, and `ptr` must point to a block of memory
1811    /// allocated by `alloc`.
1812    ///
1813    /// # Examples
1814    ///
1815    /// ```
1816    /// #![feature(allocator_api)]
1817    ///
1818    /// use std::sync::Arc;
1819    /// use std::alloc::System;
1820    ///
1821    /// let five = Arc::new_in(5, System);
1822    ///
1823    /// unsafe {
1824    ///     let ptr = Arc::into_raw(five);
1825    ///     Arc::increment_strong_count_in(ptr, System);
1826    ///
1827    ///     // This assertion is deterministic because we haven't shared
1828    ///     // the `Arc` between threads.
1829    ///     let five = Arc::from_raw_in(ptr, System);
1830    ///     assert_eq!(2, Arc::strong_count(&five));
1831    /// #   // Prevent leaks for Miri.
1832    /// #   Arc::decrement_strong_count_in(ptr, System);
1833    /// }
1834    /// ```
1835    #[inline]
1836    #[unstable(feature = "allocator_api", issue = "32838")]
1837    pub unsafe fn increment_strong_count_in(ptr: *const T, alloc: A)
1838    where
1839        A: Clone,
1840    {
1841        // Retain Arc, but don't touch refcount by wrapping in ManuallyDrop
1842        let arc = unsafe { mem::ManuallyDrop::new(Arc::from_raw_in(ptr, alloc)) };
1843        // Now increase refcount, but don't drop new refcount either
1844        let _arc_clone: mem::ManuallyDrop<_> = arc.clone();
1845    }
1846
1847    /// Decrements the strong reference count on the `Arc<T>` associated with the
1848    /// provided pointer by one.
1849    ///
1850    /// # Safety
1851    ///
1852    /// The pointer must have been obtained through `Arc::into_raw`,  the
1853    /// associated `Arc` instance must be valid (i.e. the strong count must be at
1854    /// least 1) when invoking this method, and `ptr` must point to a block of memory
1855    /// allocated by `alloc`. This method can be used to release the final
1856    /// `Arc` and backing storage, but **should not** be called after the final `Arc` has been
1857    /// released.
1858    ///
1859    /// # Examples
1860    ///
1861    /// ```
1862    /// #![feature(allocator_api)]
1863    ///
1864    /// use std::sync::Arc;
1865    /// use std::alloc::System;
1866    ///
1867    /// let five = Arc::new_in(5, System);
1868    ///
1869    /// unsafe {
1870    ///     let ptr = Arc::into_raw(five);
1871    ///     Arc::increment_strong_count_in(ptr, System);
1872    ///
1873    ///     // Those assertions are deterministic because we haven't shared
1874    ///     // the `Arc` between threads.
1875    ///     let five = Arc::from_raw_in(ptr, System);
1876    ///     assert_eq!(2, Arc::strong_count(&five));
1877    ///     Arc::decrement_strong_count_in(ptr, System);
1878    ///     assert_eq!(1, Arc::strong_count(&five));
1879    /// }
1880    /// ```
1881    #[inline]
1882    #[unstable(feature = "allocator_api", issue = "32838")]
1883    pub unsafe fn decrement_strong_count_in(ptr: *const T, alloc: A) {
1884        unsafe { drop(Arc::from_raw_in(ptr, alloc)) };
1885    }
1886
1887    #[inline]
1888    fn inner(&self) -> &ArcInner<T> {
1889        // This unsafety is ok because while this arc is alive we're guaranteed
1890        // that the inner pointer is valid. Furthermore, we know that the
1891        // `ArcInner` structure itself is `Sync` because the inner data is
1892        // `Sync` as well, so we're ok loaning out an immutable pointer to these
1893        // contents.
1894        unsafe { self.ptr.as_ref() }
1895    }
1896
1897    // Non-inlined part of `drop`.
1898    #[inline(never)]
1899    unsafe fn drop_slow(&mut self) {
1900        // Drop the weak ref collectively held by all strong references when this
1901        // variable goes out of scope. This ensures that the memory is deallocated
1902        // even if the destructor of `T` panics.
1903        // Take a reference to `self.alloc` instead of cloning because 1. it'll last long
1904        // enough, and 2. you should be able to drop `Arc`s with unclonable allocators
1905        let _weak = Weak { ptr: self.ptr, alloc: &self.alloc };
1906
1907        // Destroy the data at this time, even though we must not free the box
1908        // allocation itself (there might still be weak pointers lying around).
1909        // We cannot use `get_mut_unchecked` here, because `self.alloc` is borrowed.
1910        unsafe { ptr::drop_in_place(&mut (*self.ptr.as_ptr()).data) };
1911    }
1912
1913    /// Returns `true` if the two `Arc`s point to the same allocation in a vein similar to
1914    /// [`ptr::eq`]. This function ignores the metadata of  `dyn Trait` pointers.
1915    ///
1916    /// # Examples
1917    ///
1918    /// ```
1919    /// use std::sync::Arc;
1920    ///
1921    /// let five = Arc::new(5);
1922    /// let same_five = Arc::clone(&five);
1923    /// let other_five = Arc::new(5);
1924    ///
1925    /// assert!(Arc::ptr_eq(&five, &same_five));
1926    /// assert!(!Arc::ptr_eq(&five, &other_five));
1927    /// ```
1928    ///
1929    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
1930    #[inline]
1931    #[must_use]
1932    #[stable(feature = "ptr_eq", since = "1.17.0")]
1933    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
1934        ptr::addr_eq(this.ptr.as_ptr(), other.ptr.as_ptr())
1935    }
1936}
1937
1938impl<T: ?Sized> Arc<T> {
1939    /// Allocates an `ArcInner<T>` with sufficient space for
1940    /// a possibly-unsized inner value where the value has the layout provided.
1941    ///
1942    /// The function `mem_to_arcinner` is called with the data pointer
1943    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1944    #[cfg(not(no_global_oom_handling))]
1945    unsafe fn allocate_for_layout(
1946        value_layout: Layout,
1947        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1948        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1949    ) -> *mut ArcInner<T> {
1950        let layout = arcinner_layout_for_value_layout(value_layout);
1951
1952        let ptr = allocate(layout).unwrap_or_else(|_| handle_alloc_error(layout));
1953
1954        unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) }
1955    }
1956
1957    /// Allocates an `ArcInner<T>` with sufficient space for
1958    /// a possibly-unsized inner value where the value has the layout provided,
1959    /// returning an error if allocation fails.
1960    ///
1961    /// The function `mem_to_arcinner` is called with the data pointer
1962    /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
1963    unsafe fn try_allocate_for_layout(
1964        value_layout: Layout,
1965        allocate: impl FnOnce(Layout) -> Result<NonNull<[u8]>, AllocError>,
1966        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1967    ) -> Result<*mut ArcInner<T>, AllocError> {
1968        let layout = arcinner_layout_for_value_layout(value_layout);
1969
1970        let ptr = allocate(layout)?;
1971
1972        let inner = unsafe { Self::initialize_arcinner(ptr, layout, mem_to_arcinner) };
1973
1974        Ok(inner)
1975    }
1976
1977    unsafe fn initialize_arcinner(
1978        ptr: NonNull<[u8]>,
1979        layout: Layout,
1980        mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>,
1981    ) -> *mut ArcInner<T> {
1982        let inner = mem_to_arcinner(ptr.as_non_null_ptr().as_ptr());
1983        debug_assert_eq!(unsafe { Layout::for_value_raw(inner) }, layout);
1984
1985        unsafe {
1986            (&raw mut (*inner).strong).write(atomic::AtomicUsize::new(1));
1987            (&raw mut (*inner).weak).write(atomic::AtomicUsize::new(1));
1988        }
1989
1990        inner
1991    }
1992}
1993
1994impl<T: ?Sized, A: Allocator> Arc<T, A> {
1995    /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
1996    #[inline]
1997    #[cfg(not(no_global_oom_handling))]
1998    unsafe fn allocate_for_ptr_in(ptr: *const T, alloc: &A) -> *mut ArcInner<T> {
1999        // Allocate for the `ArcInner<T>` using the given value.
2000        unsafe {
2001            Arc::allocate_for_layout(
2002                Layout::for_value_raw(ptr),
2003                |layout| alloc.allocate(layout),
2004                |mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
2005            )
2006        }
2007    }
2008
2009    #[cfg(not(no_global_oom_handling))]
2010    fn from_box_in(src: Box<T, A>) -> Arc<T, A> {
2011        unsafe {
2012            let value_size = size_of_val(&*src);
2013            let ptr = Self::allocate_for_ptr_in(&*src, Box::allocator(&src));
2014
2015            // Copy value as bytes
2016            ptr::copy_nonoverlapping(
2017                (&raw const *src) as *const u8,
2018                (&raw mut (*ptr).data) as *mut u8,
2019                value_size,
2020            );
2021
2022            // Free the allocation without dropping its contents
2023            let (bptr, alloc) = Box::into_raw_with_allocator(src);
2024            let src = Box::from_raw_in(bptr as *mut mem::ManuallyDrop<T>, alloc.by_ref());
2025            drop(src);
2026
2027            Self::from_ptr_in(ptr, alloc)
2028        }
2029    }
2030}
2031
2032impl<T> Arc<[T]> {
2033    /// Allocates an `ArcInner<[T]>` with the given length.
2034    #[cfg(not(no_global_oom_handling))]
2035    unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
2036        unsafe {
2037            Self::allocate_for_layout(
2038                Layout::array::<T>(len).unwrap(),
2039                |layout| Global.allocate(layout),
2040                |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2041            )
2042        }
2043    }
2044
2045    /// Copy elements from slice into newly allocated `Arc<[T]>`
2046    ///
2047    /// Unsafe because the caller must either take ownership or bind `T: Copy`.
2048    #[cfg(not(no_global_oom_handling))]
2049    unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
2050        unsafe {
2051            let ptr = Self::allocate_for_slice(v.len());
2052
2053            ptr::copy_nonoverlapping(v.as_ptr(), (&raw mut (*ptr).data) as *mut T, v.len());
2054
2055            Self::from_ptr(ptr)
2056        }
2057    }
2058
2059    /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
2060    ///
2061    /// Behavior is undefined should the size be wrong.
2062    #[cfg(not(no_global_oom_handling))]
2063    unsafe fn from_iter_exact(iter: impl Iterator<Item = T>, len: usize) -> Arc<[T]> {
2064        // Panic guard while cloning T elements.
2065        // In the event of a panic, elements that have been written
2066        // into the new ArcInner will be dropped, then the memory freed.
2067        struct Guard<T> {
2068            mem: NonNull<u8>,
2069            elems: *mut T,
2070            layout: Layout,
2071            n_elems: usize,
2072        }
2073
2074        impl<T> Drop for Guard<T> {
2075            fn drop(&mut self) {
2076                unsafe {
2077                    let slice = from_raw_parts_mut(self.elems, self.n_elems);
2078                    ptr::drop_in_place(slice);
2079
2080                    Global.deallocate(self.mem, self.layout);
2081                }
2082            }
2083        }
2084
2085        unsafe {
2086            let ptr = Self::allocate_for_slice(len);
2087
2088            let mem = ptr as *mut _ as *mut u8;
2089            let layout = Layout::for_value_raw(ptr);
2090
2091            // Pointer to first element
2092            let elems = (&raw mut (*ptr).data) as *mut T;
2093
2094            let mut guard = Guard { mem: NonNull::new_unchecked(mem), elems, layout, n_elems: 0 };
2095
2096            for (i, item) in iter.enumerate() {
2097                ptr::write(elems.add(i), item);
2098                guard.n_elems += 1;
2099            }
2100
2101            // All clear. Forget the guard so it doesn't free the new ArcInner.
2102            mem::forget(guard);
2103
2104            Self::from_ptr(ptr)
2105        }
2106    }
2107}
2108
2109impl<T, A: Allocator> Arc<[T], A> {
2110    /// Allocates an `ArcInner<[T]>` with the given length.
2111    #[inline]
2112    #[cfg(not(no_global_oom_handling))]
2113    unsafe fn allocate_for_slice_in(len: usize, alloc: &A) -> *mut ArcInner<[T]> {
2114        unsafe {
2115            Arc::allocate_for_layout(
2116                Layout::array::<T>(len).unwrap(),
2117                |layout| alloc.allocate(layout),
2118                |mem| ptr::slice_from_raw_parts_mut(mem.cast::<T>(), len) as *mut ArcInner<[T]>,
2119            )
2120        }
2121    }
2122}
2123
2124/// Specialization trait used for `From<&[T]>`.
2125#[cfg(not(no_global_oom_handling))]
2126trait ArcFromSlice<T> {
2127    fn from_slice(slice: &[T]) -> Self;
2128}
2129
2130#[cfg(not(no_global_oom_handling))]
2131impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
2132    #[inline]
2133    default fn from_slice(v: &[T]) -> Self {
2134        unsafe { Self::from_iter_exact(v.iter().cloned(), v.len()) }
2135    }
2136}
2137
2138#[cfg(not(no_global_oom_handling))]
2139impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
2140    #[inline]
2141    fn from_slice(v: &[T]) -> Self {
2142        unsafe { Arc::copy_from_slice(v) }
2143    }
2144}
2145
2146#[stable(feature = "rust1", since = "1.0.0")]
2147impl<T: ?Sized, A: Allocator + Clone> Clone for Arc<T, A> {
2148    /// Makes a clone of the `Arc` pointer.
2149    ///
2150    /// This creates another pointer to the same allocation, increasing the
2151    /// strong reference count.
2152    ///
2153    /// # Examples
2154    ///
2155    /// ```
2156    /// use std::sync::Arc;
2157    ///
2158    /// let five = Arc::new(5);
2159    ///
2160    /// let _ = Arc::clone(&five);
2161    /// ```
2162    #[inline]
2163    fn clone(&self) -> Arc<T, A> {
2164        // Using a relaxed ordering is alright here, as knowledge of the
2165        // original reference prevents other threads from erroneously deleting
2166        // the object.
2167        //
2168        // As explained in the [Boost documentation][1], Increasing the
2169        // reference counter can always be done with memory_order_relaxed: New
2170        // references to an object can only be formed from an existing
2171        // reference, and passing an existing reference from one thread to
2172        // another must already provide any required synchronization.
2173        //
2174        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2175        let old_size = self.inner().strong.fetch_add(1, Relaxed);
2176
2177        // However we need to guard against massive refcounts in case someone is `mem::forget`ing
2178        // Arcs. If we don't do this the count can overflow and users will use-after free. This
2179        // branch will never be taken in any realistic program. We abort because such a program is
2180        // incredibly degenerate, and we don't care to support it.
2181        //
2182        // This check is not 100% water-proof: we error when the refcount grows beyond `isize::MAX`.
2183        // But we do that check *after* having done the increment, so there is a chance here that
2184        // the worst already happened and we actually do overflow the `usize` counter. However, that
2185        // requires the counter to grow from `isize::MAX` to `usize::MAX` between the increment
2186        // above and the `abort` below, which seems exceedingly unlikely.
2187        //
2188        // This is a global invariant, and also applies when using a compare-exchange loop to increment
2189        // counters in other methods.
2190        // Otherwise, the counter could be brought to an almost-overflow using a compare-exchange loop,
2191        // and then overflow using a few `fetch_add`s.
2192        if old_size > MAX_REFCOUNT {
2193            abort();
2194        }
2195
2196        unsafe { Self::from_inner_in(self.ptr, self.alloc.clone()) }
2197    }
2198}
2199
2200#[stable(feature = "rust1", since = "1.0.0")]
2201impl<T: ?Sized, A: Allocator> Deref for Arc<T, A> {
2202    type Target = T;
2203
2204    #[inline]
2205    fn deref(&self) -> &T {
2206        &self.inner().data
2207    }
2208}
2209
2210#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2211unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Arc<T, A> {}
2212
2213#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2214unsafe impl<T: ?Sized, A: Allocator> PinCoerceUnsized for Weak<T, A> {}
2215
2216#[unstable(feature = "deref_pure_trait", issue = "87121")]
2217unsafe impl<T: ?Sized, A: Allocator> DerefPure for Arc<T, A> {}
2218
2219#[unstable(feature = "legacy_receiver_trait", issue = "none")]
2220impl<T: ?Sized> LegacyReceiver for Arc<T> {}
2221
2222#[cfg(not(no_global_oom_handling))]
2223impl<T: ?Sized + CloneToUninit, A: Allocator + Clone> Arc<T, A> {
2224    /// Makes a mutable reference into the given `Arc`.
2225    ///
2226    /// If there are other `Arc` pointers to the same allocation, then `make_mut` will
2227    /// [`clone`] the inner value to a new allocation to ensure unique ownership.  This is also
2228    /// referred to as clone-on-write.
2229    ///
2230    /// However, if there are no other `Arc` pointers to this allocation, but some [`Weak`]
2231    /// pointers, then the [`Weak`] pointers will be dissociated and the inner value will not
2232    /// be cloned.
2233    ///
2234    /// See also [`get_mut`], which will fail rather than cloning the inner value
2235    /// or dissociating [`Weak`] pointers.
2236    ///
2237    /// [`clone`]: Clone::clone
2238    /// [`get_mut`]: Arc::get_mut
2239    ///
2240    /// # Examples
2241    ///
2242    /// ```
2243    /// use std::sync::Arc;
2244    ///
2245    /// let mut data = Arc::new(5);
2246    ///
2247    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
2248    /// let mut other_data = Arc::clone(&data); // Won't clone inner data
2249    /// *Arc::make_mut(&mut data) += 1;         // Clones inner data
2250    /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
2251    /// *Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything
2252    ///
2253    /// // Now `data` and `other_data` point to different allocations.
2254    /// assert_eq!(*data, 8);
2255    /// assert_eq!(*other_data, 12);
2256    /// ```
2257    ///
2258    /// [`Weak`] pointers will be dissociated:
2259    ///
2260    /// ```
2261    /// use std::sync::Arc;
2262    ///
2263    /// let mut data = Arc::new(75);
2264    /// let weak = Arc::downgrade(&data);
2265    ///
2266    /// assert!(75 == *data);
2267    /// assert!(75 == *weak.upgrade().unwrap());
2268    ///
2269    /// *Arc::make_mut(&mut data) += 1;
2270    ///
2271    /// assert!(76 == *data);
2272    /// assert!(weak.upgrade().is_none());
2273    /// ```
2274    #[inline]
2275    #[stable(feature = "arc_unique", since = "1.4.0")]
2276    pub fn make_mut(this: &mut Self) -> &mut T {
2277        let size_of_val = mem::size_of_val::<T>(&**this);
2278
2279        // Note that we hold both a strong reference and a weak reference.
2280        // Thus, releasing our strong reference only will not, by itself, cause
2281        // the memory to be deallocated.
2282        //
2283        // Use Acquire to ensure that we see any writes to `weak` that happen
2284        // before release writes (i.e., decrements) to `strong`. Since we hold a
2285        // weak count, there's no chance the ArcInner itself could be
2286        // deallocated.
2287        if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
2288            // Another strong pointer exists, so we must clone.
2289
2290            let this_data_ref: &T = &**this;
2291            // `in_progress` drops the allocation if we panic before finishing initializing it.
2292            let mut in_progress: UniqueArcUninit<T, A> =
2293                UniqueArcUninit::new(this_data_ref, this.alloc.clone());
2294
2295            let initialized_clone = unsafe {
2296                // Clone. If the clone panics, `in_progress` will be dropped and clean up.
2297                this_data_ref.clone_to_uninit(in_progress.data_ptr().cast());
2298                // Cast type of pointer, now that it is initialized.
2299                in_progress.into_arc()
2300            };
2301            *this = initialized_clone;
2302        } else if this.inner().weak.load(Relaxed) != 1 {
2303            // Relaxed suffices in the above because this is fundamentally an
2304            // optimization: we are always racing with weak pointers being
2305            // dropped. Worst case, we end up allocated a new Arc unnecessarily.
2306
2307            // We removed the last strong ref, but there are additional weak
2308            // refs remaining. We'll move the contents to a new Arc, and
2309            // invalidate the other weak refs.
2310
2311            // Note that it is not possible for the read of `weak` to yield
2312            // usize::MAX (i.e., locked), since the weak count can only be
2313            // locked by a thread with a strong reference.
2314
2315            // Materialize our own implicit weak pointer, so that it can clean
2316            // up the ArcInner as needed.
2317            let _weak = Weak { ptr: this.ptr, alloc: this.alloc.clone() };
2318
2319            // Can just steal the data, all that's left is Weaks
2320            //
2321            // We don't need panic-protection like the above branch does, but we might as well
2322            // use the same mechanism.
2323            let mut in_progress: UniqueArcUninit<T, A> =
2324                UniqueArcUninit::new(&**this, this.alloc.clone());
2325            unsafe {
2326                // Initialize `in_progress` with move of **this.
2327                // We have to express this in terms of bytes because `T: ?Sized`; there is no
2328                // operation that just copies a value based on its `size_of_val()`.
2329                ptr::copy_nonoverlapping(
2330                    ptr::from_ref(&**this).cast::<u8>(),
2331                    in_progress.data_ptr().cast::<u8>(),
2332                    size_of_val,
2333                );
2334
2335                ptr::write(this, in_progress.into_arc());
2336            }
2337        } else {
2338            // We were the sole reference of either kind; bump back up the
2339            // strong ref count.
2340            this.inner().strong.store(1, Release);
2341        }
2342
2343        // As with `get_mut()`, the unsafety is ok because our reference was
2344        // either unique to begin with, or became one upon cloning the contents.
2345        unsafe { Self::get_mut_unchecked(this) }
2346    }
2347}
2348
2349impl<T: Clone, A: Allocator> Arc<T, A> {
2350    /// If we have the only reference to `T` then unwrap it. Otherwise, clone `T` and return the
2351    /// clone.
2352    ///
2353    /// Assuming `arc_t` is of type `Arc<T>`, this function is functionally equivalent to
2354    /// `(*arc_t).clone()`, but will avoid cloning the inner value where possible.
2355    ///
2356    /// # Examples
2357    ///
2358    /// ```
2359    /// # use std::{ptr, sync::Arc};
2360    /// let inner = String::from("test");
2361    /// let ptr = inner.as_ptr();
2362    ///
2363    /// let arc = Arc::new(inner);
2364    /// let inner = Arc::unwrap_or_clone(arc);
2365    /// // The inner value was not cloned
2366    /// assert!(ptr::eq(ptr, inner.as_ptr()));
2367    ///
2368    /// let arc = Arc::new(inner);
2369    /// let arc2 = arc.clone();
2370    /// let inner = Arc::unwrap_or_clone(arc);
2371    /// // Because there were 2 references, we had to clone the inner value.
2372    /// assert!(!ptr::eq(ptr, inner.as_ptr()));
2373    /// // `arc2` is the last reference, so when we unwrap it we get back
2374    /// // the original `String`.
2375    /// let inner = Arc::unwrap_or_clone(arc2);
2376    /// assert!(ptr::eq(ptr, inner.as_ptr()));
2377    /// ```
2378    #[inline]
2379    #[stable(feature = "arc_unwrap_or_clone", since = "1.76.0")]
2380    pub fn unwrap_or_clone(this: Self) -> T {
2381        Arc::try_unwrap(this).unwrap_or_else(|arc| (*arc).clone())
2382    }
2383}
2384
2385impl<T: ?Sized, A: Allocator> Arc<T, A> {
2386    /// Returns a mutable reference into the given `Arc`, if there are
2387    /// no other `Arc` or [`Weak`] pointers to the same allocation.
2388    ///
2389    /// Returns [`None`] otherwise, because it is not safe to
2390    /// mutate a shared value.
2391    ///
2392    /// See also [`make_mut`][make_mut], which will [`clone`][clone]
2393    /// the inner value when there are other `Arc` pointers.
2394    ///
2395    /// [make_mut]: Arc::make_mut
2396    /// [clone]: Clone::clone
2397    ///
2398    /// # Examples
2399    ///
2400    /// ```
2401    /// use std::sync::Arc;
2402    ///
2403    /// let mut x = Arc::new(3);
2404    /// *Arc::get_mut(&mut x).unwrap() = 4;
2405    /// assert_eq!(*x, 4);
2406    ///
2407    /// let _y = Arc::clone(&x);
2408    /// assert!(Arc::get_mut(&mut x).is_none());
2409    /// ```
2410    #[inline]
2411    #[stable(feature = "arc_unique", since = "1.4.0")]
2412    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
2413        if this.is_unique() {
2414            // This unsafety is ok because we're guaranteed that the pointer
2415            // returned is the *only* pointer that will ever be returned to T. Our
2416            // reference count is guaranteed to be 1 at this point, and we required
2417            // the Arc itself to be `mut`, so we're returning the only possible
2418            // reference to the inner data.
2419            unsafe { Some(Arc::get_mut_unchecked(this)) }
2420        } else {
2421            None
2422        }
2423    }
2424
2425    /// Returns a mutable reference into the given `Arc`,
2426    /// without any check.
2427    ///
2428    /// See also [`get_mut`], which is safe and does appropriate checks.
2429    ///
2430    /// [`get_mut`]: Arc::get_mut
2431    ///
2432    /// # Safety
2433    ///
2434    /// If any other `Arc` or [`Weak`] pointers to the same allocation exist, then
2435    /// they must not be dereferenced or have active borrows for the duration
2436    /// of the returned borrow, and their inner type must be exactly the same as the
2437    /// inner type of this Rc (including lifetimes). This is trivially the case if no
2438    /// such pointers exist, for example immediately after `Arc::new`.
2439    ///
2440    /// # Examples
2441    ///
2442    /// ```
2443    /// #![feature(get_mut_unchecked)]
2444    ///
2445    /// use std::sync::Arc;
2446    ///
2447    /// let mut x = Arc::new(String::new());
2448    /// unsafe {
2449    ///     Arc::get_mut_unchecked(&mut x).push_str("foo")
2450    /// }
2451    /// assert_eq!(*x, "foo");
2452    /// ```
2453    /// Other `Arc` pointers to the same allocation must be to the same type.
2454    /// ```no_run
2455    /// #![feature(get_mut_unchecked)]
2456    ///
2457    /// use std::sync::Arc;
2458    ///
2459    /// let x: Arc<str> = Arc::from("Hello, world!");
2460    /// let mut y: Arc<[u8]> = x.clone().into();
2461    /// unsafe {
2462    ///     // this is Undefined Behavior, because x's inner type is str, not [u8]
2463    ///     Arc::get_mut_unchecked(&mut y).fill(0xff); // 0xff is invalid in UTF-8
2464    /// }
2465    /// println!("{}", &*x); // Invalid UTF-8 in a str
2466    /// ```
2467    /// Other `Arc` pointers to the same allocation must be to the exact same type, including lifetimes.
2468    /// ```no_run
2469    /// #![feature(get_mut_unchecked)]
2470    ///
2471    /// use std::sync::Arc;
2472    ///
2473    /// let x: Arc<&str> = Arc::new("Hello, world!");
2474    /// {
2475    ///     let s = String::from("Oh, no!");
2476    ///     let mut y: Arc<&str> = x.clone();
2477    ///     unsafe {
2478    ///         // this is Undefined Behavior, because x's inner type
2479    ///         // is &'long str, not &'short str
2480    ///         *Arc::get_mut_unchecked(&mut y) = &s;
2481    ///     }
2482    /// }
2483    /// println!("{}", &*x); // Use-after-free
2484    /// ```
2485    #[inline]
2486    #[unstable(feature = "get_mut_unchecked", issue = "63292")]
2487    pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
2488        // We are careful to *not* create a reference covering the "count" fields, as
2489        // this would alias with concurrent access to the reference counts (e.g. by `Weak`).
2490        unsafe { &mut (*this.ptr.as_ptr()).data }
2491    }
2492
2493    /// Determine whether this is the unique reference (including weak refs) to
2494    /// the underlying data.
2495    ///
2496    /// Note that this requires locking the weak ref count.
2497    fn is_unique(&mut self) -> bool {
2498        // lock the weak pointer count if we appear to be the sole weak pointer
2499        // holder.
2500        //
2501        // The acquire label here ensures a happens-before relationship with any
2502        // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
2503        // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded
2504        // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
2505        if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
2506            // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
2507            // counter in `drop` -- the only access that happens when any but the last reference
2508            // is being dropped.
2509            let unique = self.inner().strong.load(Acquire) == 1;
2510
2511            // The release write here synchronizes with a read in `downgrade`,
2512            // effectively preventing the above read of `strong` from happening
2513            // after the write.
2514            self.inner().weak.store(1, Release); // release the lock
2515            unique
2516        } else {
2517            false
2518        }
2519    }
2520}
2521
2522#[stable(feature = "rust1", since = "1.0.0")]
2523unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Arc<T, A> {
2524    /// Drops the `Arc`.
2525    ///
2526    /// This will decrement the strong reference count. If the strong reference
2527    /// count reaches zero then the only other references (if any) are
2528    /// [`Weak`], so we `drop` the inner value.
2529    ///
2530    /// # Examples
2531    ///
2532    /// ```
2533    /// use std::sync::Arc;
2534    ///
2535    /// struct Foo;
2536    ///
2537    /// impl Drop for Foo {
2538    ///     fn drop(&mut self) {
2539    ///         println!("dropped!");
2540    ///     }
2541    /// }
2542    ///
2543    /// let foo  = Arc::new(Foo);
2544    /// let foo2 = Arc::clone(&foo);
2545    ///
2546    /// drop(foo);    // Doesn't print anything
2547    /// drop(foo2);   // Prints "dropped!"
2548    /// ```
2549    #[inline]
2550    fn drop(&mut self) {
2551        // Because `fetch_sub` is already atomic, we do not need to synchronize
2552        // with other threads unless we are going to delete the object. This
2553        // same logic applies to the below `fetch_sub` to the `weak` count.
2554        if self.inner().strong.fetch_sub(1, Release) != 1 {
2555            return;
2556        }
2557
2558        // This fence is needed to prevent reordering of use of the data and
2559        // deletion of the data. Because it is marked `Release`, the decreasing
2560        // of the reference count synchronizes with this `Acquire` fence. This
2561        // means that use of the data happens before decreasing the reference
2562        // count, which happens before this fence, which happens before the
2563        // deletion of the data.
2564        //
2565        // As explained in the [Boost documentation][1],
2566        //
2567        // > It is important to enforce any possible access to the object in one
2568        // > thread (through an existing reference) to *happen before* deleting
2569        // > the object in a different thread. This is achieved by a "release"
2570        // > operation after dropping a reference (any access to the object
2571        // > through this reference must obviously happened before), and an
2572        // > "acquire" operation before deleting the object.
2573        //
2574        // In particular, while the contents of an Arc are usually immutable, it's
2575        // possible to have interior writes to something like a Mutex<T>. Since a
2576        // Mutex is not acquired when it is deleted, we can't rely on its
2577        // synchronization logic to make writes in thread A visible to a destructor
2578        // running in thread B.
2579        //
2580        // Also note that the Acquire fence here could probably be replaced with an
2581        // Acquire load, which could improve performance in highly-contended
2582        // situations. See [2].
2583        //
2584        // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
2585        // [2]: (https://github.com/rust-lang/rust/pull/41714)
2586        acquire!(self.inner().strong);
2587
2588        // Make sure we aren't trying to "drop" the shared static for empty slices
2589        // used by Default::default.
2590        debug_assert!(
2591            !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
2592            "Arcs backed by a static should never reach a strong count of 0. \
2593            Likely decrement_strong_count or from_raw were called too many times.",
2594        );
2595
2596        unsafe {
2597            self.drop_slow();
2598        }
2599    }
2600}
2601
2602impl<A: Allocator> Arc<dyn Any + Send + Sync, A> {
2603    /// Attempts to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
2604    ///
2605    /// # Examples
2606    ///
2607    /// ```
2608    /// use std::any::Any;
2609    /// use std::sync::Arc;
2610    ///
2611    /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
2612    ///     if let Ok(string) = value.downcast::<String>() {
2613    ///         println!("String ({}): {}", string.len(), string);
2614    ///     }
2615    /// }
2616    ///
2617    /// let my_string = "Hello World".to_string();
2618    /// print_if_string(Arc::new(my_string));
2619    /// print_if_string(Arc::new(0i8));
2620    /// ```
2621    #[inline]
2622    #[stable(feature = "rc_downcast", since = "1.29.0")]
2623    pub fn downcast<T>(self) -> Result<Arc<T, A>, Self>
2624    where
2625        T: Any + Send + Sync,
2626    {
2627        if (*self).is::<T>() {
2628            unsafe {
2629                let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2630                Ok(Arc::from_inner_in(ptr.cast(), alloc))
2631            }
2632        } else {
2633            Err(self)
2634        }
2635    }
2636
2637    /// Downcasts the `Arc<dyn Any + Send + Sync>` to a concrete type.
2638    ///
2639    /// For a safe alternative see [`downcast`].
2640    ///
2641    /// # Examples
2642    ///
2643    /// ```
2644    /// #![feature(downcast_unchecked)]
2645    ///
2646    /// use std::any::Any;
2647    /// use std::sync::Arc;
2648    ///
2649    /// let x: Arc<dyn Any + Send + Sync> = Arc::new(1_usize);
2650    ///
2651    /// unsafe {
2652    ///     assert_eq!(*x.downcast_unchecked::<usize>(), 1);
2653    /// }
2654    /// ```
2655    ///
2656    /// # Safety
2657    ///
2658    /// The contained value must be of type `T`. Calling this method
2659    /// with the incorrect type is *undefined behavior*.
2660    ///
2661    ///
2662    /// [`downcast`]: Self::downcast
2663    #[inline]
2664    #[unstable(feature = "downcast_unchecked", issue = "90850")]
2665    pub unsafe fn downcast_unchecked<T>(self) -> Arc<T, A>
2666    where
2667        T: Any + Send + Sync,
2668    {
2669        unsafe {
2670            let (ptr, alloc) = Arc::into_inner_with_allocator(self);
2671            Arc::from_inner_in(ptr.cast(), alloc)
2672        }
2673    }
2674}
2675
2676impl<T> Weak<T> {
2677    /// Constructs a new `Weak<T>`, without allocating any memory.
2678    /// Calling [`upgrade`] on the return value always gives [`None`].
2679    ///
2680    /// [`upgrade`]: Weak::upgrade
2681    ///
2682    /// # Examples
2683    ///
2684    /// ```
2685    /// use std::sync::Weak;
2686    ///
2687    /// let empty: Weak<i64> = Weak::new();
2688    /// assert!(empty.upgrade().is_none());
2689    /// ```
2690    #[inline]
2691    #[stable(feature = "downgraded_weak", since = "1.10.0")]
2692    #[rustc_const_stable(feature = "const_weak_new", since = "1.73.0")]
2693    #[must_use]
2694    pub const fn new() -> Weak<T> {
2695        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc: Global }
2696    }
2697}
2698
2699impl<T, A: Allocator> Weak<T, A> {
2700    /// Constructs a new `Weak<T, A>`, without allocating any memory, technically in the provided
2701    /// allocator.
2702    /// Calling [`upgrade`] on the return value always gives [`None`].
2703    ///
2704    /// [`upgrade`]: Weak::upgrade
2705    ///
2706    /// # Examples
2707    ///
2708    /// ```
2709    /// #![feature(allocator_api)]
2710    ///
2711    /// use std::sync::Weak;
2712    /// use std::alloc::System;
2713    ///
2714    /// let empty: Weak<i64, _> = Weak::new_in(System);
2715    /// assert!(empty.upgrade().is_none());
2716    /// ```
2717    #[inline]
2718    #[unstable(feature = "allocator_api", issue = "32838")]
2719    pub fn new_in(alloc: A) -> Weak<T, A> {
2720        Weak { ptr: NonNull::without_provenance(NonZeroUsize::MAX), alloc }
2721    }
2722}
2723
2724/// Helper type to allow accessing the reference counts without
2725/// making any assertions about the data field.
2726struct WeakInner<'a> {
2727    weak: &'a atomic::AtomicUsize,
2728    strong: &'a atomic::AtomicUsize,
2729}
2730
2731impl<T: ?Sized> Weak<T> {
2732    /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>`.
2733    ///
2734    /// This can be used to safely get a strong reference (by calling [`upgrade`]
2735    /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2736    ///
2737    /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2738    /// as these don't own anything; the method still works on them).
2739    ///
2740    /// # Safety
2741    ///
2742    /// The pointer must have originated from the [`into_raw`] and must still own its potential
2743    /// weak reference, and must point to a block of memory allocated by global allocator.
2744    ///
2745    /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2746    /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2747    /// count is not modified by this operation) and therefore it must be paired with a previous
2748    /// call to [`into_raw`].
2749    /// # Examples
2750    ///
2751    /// ```
2752    /// use std::sync::{Arc, Weak};
2753    ///
2754    /// let strong = Arc::new("hello".to_owned());
2755    ///
2756    /// let raw_1 = Arc::downgrade(&strong).into_raw();
2757    /// let raw_2 = Arc::downgrade(&strong).into_raw();
2758    ///
2759    /// assert_eq!(2, Arc::weak_count(&strong));
2760    ///
2761    /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2762    /// assert_eq!(1, Arc::weak_count(&strong));
2763    ///
2764    /// drop(strong);
2765    ///
2766    /// // Decrement the last weak count.
2767    /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2768    /// ```
2769    ///
2770    /// [`new`]: Weak::new
2771    /// [`into_raw`]: Weak::into_raw
2772    /// [`upgrade`]: Weak::upgrade
2773    #[inline]
2774    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2775    pub unsafe fn from_raw(ptr: *const T) -> Self {
2776        unsafe { Weak::from_raw_in(ptr, Global) }
2777    }
2778}
2779
2780impl<T: ?Sized, A: Allocator> Weak<T, A> {
2781    /// Returns a reference to the underlying allocator.
2782    #[inline]
2783    #[unstable(feature = "allocator_api", issue = "32838")]
2784    pub fn allocator(&self) -> &A {
2785        &self.alloc
2786    }
2787
2788    /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
2789    ///
2790    /// The pointer is valid only if there are some strong references. The pointer may be dangling,
2791    /// unaligned or even [`null`] otherwise.
2792    ///
2793    /// # Examples
2794    ///
2795    /// ```
2796    /// use std::sync::Arc;
2797    /// use std::ptr;
2798    ///
2799    /// let strong = Arc::new("hello".to_owned());
2800    /// let weak = Arc::downgrade(&strong);
2801    /// // Both point to the same object
2802    /// assert!(ptr::eq(&*strong, weak.as_ptr()));
2803    /// // The strong here keeps it alive, so we can still access the object.
2804    /// assert_eq!("hello", unsafe { &*weak.as_ptr() });
2805    ///
2806    /// drop(strong);
2807    /// // But not any more. We can do weak.as_ptr(), but accessing the pointer would lead to
2808    /// // undefined behavior.
2809    /// // assert_eq!("hello", unsafe { &*weak.as_ptr() });
2810    /// ```
2811    ///
2812    /// [`null`]: core::ptr::null "ptr::null"
2813    #[must_use]
2814    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2815    pub fn as_ptr(&self) -> *const T {
2816        let ptr: *mut ArcInner<T> = NonNull::as_ptr(self.ptr);
2817
2818        if is_dangling(ptr) {
2819            // If the pointer is dangling, we return the sentinel directly. This cannot be
2820            // a valid payload address, as the payload is at least as aligned as ArcInner (usize).
2821            ptr as *const T
2822        } else {
2823            // SAFETY: if is_dangling returns false, then the pointer is dereferenceable.
2824            // The payload may be dropped at this point, and we have to maintain provenance,
2825            // so use raw pointer manipulation.
2826            unsafe { &raw mut (*ptr).data }
2827        }
2828    }
2829
2830    /// Consumes the `Weak<T>` and turns it into a raw pointer.
2831    ///
2832    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2833    /// one weak reference (the weak count is not modified by this operation). It can be turned
2834    /// back into the `Weak<T>` with [`from_raw`].
2835    ///
2836    /// The same restrictions of accessing the target of the pointer as with
2837    /// [`as_ptr`] apply.
2838    ///
2839    /// # Examples
2840    ///
2841    /// ```
2842    /// use std::sync::{Arc, Weak};
2843    ///
2844    /// let strong = Arc::new("hello".to_owned());
2845    /// let weak = Arc::downgrade(&strong);
2846    /// let raw = weak.into_raw();
2847    ///
2848    /// assert_eq!(1, Arc::weak_count(&strong));
2849    /// assert_eq!("hello", unsafe { &*raw });
2850    ///
2851    /// drop(unsafe { Weak::from_raw(raw) });
2852    /// assert_eq!(0, Arc::weak_count(&strong));
2853    /// ```
2854    ///
2855    /// [`from_raw`]: Weak::from_raw
2856    /// [`as_ptr`]: Weak::as_ptr
2857    #[must_use = "losing the pointer will leak memory"]
2858    #[stable(feature = "weak_into_raw", since = "1.45.0")]
2859    pub fn into_raw(self) -> *const T {
2860        ManuallyDrop::new(self).as_ptr()
2861    }
2862
2863    /// Consumes the `Weak<T>`, returning the wrapped pointer and allocator.
2864    ///
2865    /// This converts the weak pointer into a raw pointer, while still preserving the ownership of
2866    /// one weak reference (the weak count is not modified by this operation). It can be turned
2867    /// back into the `Weak<T>` with [`from_raw_in`].
2868    ///
2869    /// The same restrictions of accessing the target of the pointer as with
2870    /// [`as_ptr`] apply.
2871    ///
2872    /// # Examples
2873    ///
2874    /// ```
2875    /// #![feature(allocator_api)]
2876    /// use std::sync::{Arc, Weak};
2877    /// use std::alloc::System;
2878    ///
2879    /// let strong = Arc::new_in("hello".to_owned(), System);
2880    /// let weak = Arc::downgrade(&strong);
2881    /// let (raw, alloc) = weak.into_raw_with_allocator();
2882    ///
2883    /// assert_eq!(1, Arc::weak_count(&strong));
2884    /// assert_eq!("hello", unsafe { &*raw });
2885    ///
2886    /// drop(unsafe { Weak::from_raw_in(raw, alloc) });
2887    /// assert_eq!(0, Arc::weak_count(&strong));
2888    /// ```
2889    ///
2890    /// [`from_raw_in`]: Weak::from_raw_in
2891    /// [`as_ptr`]: Weak::as_ptr
2892    #[must_use = "losing the pointer will leak memory"]
2893    #[unstable(feature = "allocator_api", issue = "32838")]
2894    pub fn into_raw_with_allocator(self) -> (*const T, A) {
2895        let this = mem::ManuallyDrop::new(self);
2896        let result = this.as_ptr();
2897        // Safety: `this` is ManuallyDrop so the allocator will not be double-dropped
2898        let alloc = unsafe { ptr::read(&this.alloc) };
2899        (result, alloc)
2900    }
2901
2902    /// Converts a raw pointer previously created by [`into_raw`] back into `Weak<T>` in the provided
2903    /// allocator.
2904    ///
2905    /// This can be used to safely get a strong reference (by calling [`upgrade`]
2906    /// later) or to deallocate the weak count by dropping the `Weak<T>`.
2907    ///
2908    /// It takes ownership of one weak reference (with the exception of pointers created by [`new`],
2909    /// as these don't own anything; the method still works on them).
2910    ///
2911    /// # Safety
2912    ///
2913    /// The pointer must have originated from the [`into_raw`] and must still own its potential
2914    /// weak reference, and must point to a block of memory allocated by `alloc`.
2915    ///
2916    /// It is allowed for the strong count to be 0 at the time of calling this. Nevertheless, this
2917    /// takes ownership of one weak reference currently represented as a raw pointer (the weak
2918    /// count is not modified by this operation) and therefore it must be paired with a previous
2919    /// call to [`into_raw`].
2920    /// # Examples
2921    ///
2922    /// ```
2923    /// use std::sync::{Arc, Weak};
2924    ///
2925    /// let strong = Arc::new("hello".to_owned());
2926    ///
2927    /// let raw_1 = Arc::downgrade(&strong).into_raw();
2928    /// let raw_2 = Arc::downgrade(&strong).into_raw();
2929    ///
2930    /// assert_eq!(2, Arc::weak_count(&strong));
2931    ///
2932    /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
2933    /// assert_eq!(1, Arc::weak_count(&strong));
2934    ///
2935    /// drop(strong);
2936    ///
2937    /// // Decrement the last weak count.
2938    /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
2939    /// ```
2940    ///
2941    /// [`new`]: Weak::new
2942    /// [`into_raw`]: Weak::into_raw
2943    /// [`upgrade`]: Weak::upgrade
2944    #[inline]
2945    #[unstable(feature = "allocator_api", issue = "32838")]
2946    pub unsafe fn from_raw_in(ptr: *const T, alloc: A) -> Self {
2947        // See Weak::as_ptr for context on how the input pointer is derived.
2948
2949        let ptr = if is_dangling(ptr) {
2950            // This is a dangling Weak.
2951            ptr as *mut ArcInner<T>
2952        } else {
2953            // Otherwise, we're guaranteed the pointer came from a nondangling Weak.
2954            // SAFETY: data_offset is safe to call, as ptr references a real (potentially dropped) T.
2955            let offset = unsafe { data_offset(ptr) };
2956            // Thus, we reverse the offset to get the whole RcInner.
2957            // SAFETY: the pointer originated from a Weak, so this offset is safe.
2958            unsafe { ptr.byte_sub(offset) as *mut ArcInner<T> }
2959        };
2960
2961        // SAFETY: we now have recovered the original Weak pointer, so can create the Weak.
2962        Weak { ptr: unsafe { NonNull::new_unchecked(ptr) }, alloc }
2963    }
2964}
2965
2966impl<T: ?Sized, A: Allocator> Weak<T, A> {
2967    /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
2968    /// dropping of the inner value if successful.
2969    ///
2970    /// Returns [`None`] if the inner value has since been dropped.
2971    ///
2972    /// # Examples
2973    ///
2974    /// ```
2975    /// use std::sync::Arc;
2976    ///
2977    /// let five = Arc::new(5);
2978    ///
2979    /// let weak_five = Arc::downgrade(&five);
2980    ///
2981    /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
2982    /// assert!(strong_five.is_some());
2983    ///
2984    /// // Destroy all strong pointers.
2985    /// drop(strong_five);
2986    /// drop(five);
2987    ///
2988    /// assert!(weak_five.upgrade().is_none());
2989    /// ```
2990    #[must_use = "this returns a new `Arc`, \
2991                  without modifying the original weak pointer"]
2992    #[stable(feature = "arc_weak", since = "1.4.0")]
2993    pub fn upgrade(&self) -> Option<Arc<T, A>>
2994    where
2995        A: Clone,
2996    {
2997        #[inline]
2998        fn checked_increment(n: usize) -> Option<usize> {
2999            // Any write of 0 we can observe leaves the field in permanently zero state.
3000            if n == 0 {
3001                return None;
3002            }
3003            // See comments in `Arc::clone` for why we do this (for `mem::forget`).
3004            assert!(n <= MAX_REFCOUNT, "{}", INTERNAL_OVERFLOW_ERROR);
3005            Some(n + 1)
3006        }
3007
3008        // We use a CAS loop to increment the strong count instead of a
3009        // fetch_add as this function should never take the reference count
3010        // from zero to one.
3011        //
3012        // Relaxed is fine for the failure case because we don't have any expectations about the new state.
3013        // Acquire is necessary for the success case to synchronise with `Arc::new_cyclic`, when the inner
3014        // value can be initialized after `Weak` references have already been created. In that case, we
3015        // expect to observe the fully initialized value.
3016        if self.inner()?.strong.fetch_update(Acquire, Relaxed, checked_increment).is_ok() {
3017            // SAFETY: pointer is not null, verified in checked_increment
3018            unsafe { Some(Arc::from_inner_in(self.ptr, self.alloc.clone())) }
3019        } else {
3020            None
3021        }
3022    }
3023
3024    /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
3025    ///
3026    /// If `self` was created using [`Weak::new`], this will return 0.
3027    #[must_use]
3028    #[stable(feature = "weak_counts", since = "1.41.0")]
3029    pub fn strong_count(&self) -> usize {
3030        if let Some(inner) = self.inner() { inner.strong.load(Relaxed) } else { 0 }
3031    }
3032
3033    /// Gets an approximation of the number of `Weak` pointers pointing to this
3034    /// allocation.
3035    ///
3036    /// If `self` was created using [`Weak::new`], or if there are no remaining
3037    /// strong pointers, this will return 0.
3038    ///
3039    /// # Accuracy
3040    ///
3041    /// Due to implementation details, the returned value can be off by 1 in
3042    /// either direction when other threads are manipulating any `Arc`s or
3043    /// `Weak`s pointing to the same allocation.
3044    #[must_use]
3045    #[stable(feature = "weak_counts", since = "1.41.0")]
3046    pub fn weak_count(&self) -> usize {
3047        if let Some(inner) = self.inner() {
3048            let weak = inner.weak.load(Acquire);
3049            let strong = inner.strong.load(Relaxed);
3050            if strong == 0 {
3051                0
3052            } else {
3053                // Since we observed that there was at least one strong pointer
3054                // after reading the weak count, we know that the implicit weak
3055                // reference (present whenever any strong references are alive)
3056                // was still around when we observed the weak count, and can
3057                // therefore safely subtract it.
3058                weak - 1
3059            }
3060        } else {
3061            0
3062        }
3063    }
3064
3065    /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
3066    /// (i.e., when this `Weak` was created by `Weak::new`).
3067    #[inline]
3068    fn inner(&self) -> Option<WeakInner<'_>> {
3069        let ptr = self.ptr.as_ptr();
3070        if is_dangling(ptr) {
3071            None
3072        } else {
3073            // We are careful to *not* create a reference covering the "data" field, as
3074            // the field may be mutated concurrently (for example, if the last `Arc`
3075            // is dropped, the data field will be dropped in-place).
3076            Some(unsafe { WeakInner { strong: &(*ptr).strong, weak: &(*ptr).weak } })
3077        }
3078    }
3079
3080    /// Returns `true` if the two `Weak`s point to the same allocation similar to [`ptr::eq`], or if
3081    /// both don't point to any allocation (because they were created with `Weak::new()`). However,
3082    /// this function ignores the metadata of  `dyn Trait` pointers.
3083    ///
3084    /// # Notes
3085    ///
3086    /// Since this compares pointers it means that `Weak::new()` will equal each
3087    /// other, even though they don't point to any allocation.
3088    ///
3089    /// # Examples
3090    ///
3091    /// ```
3092    /// use std::sync::Arc;
3093    ///
3094    /// let first_rc = Arc::new(5);
3095    /// let first = Arc::downgrade(&first_rc);
3096    /// let second = Arc::downgrade(&first_rc);
3097    ///
3098    /// assert!(first.ptr_eq(&second));
3099    ///
3100    /// let third_rc = Arc::new(5);
3101    /// let third = Arc::downgrade(&third_rc);
3102    ///
3103    /// assert!(!first.ptr_eq(&third));
3104    /// ```
3105    ///
3106    /// Comparing `Weak::new`.
3107    ///
3108    /// ```
3109    /// use std::sync::{Arc, Weak};
3110    ///
3111    /// let first = Weak::new();
3112    /// let second = Weak::new();
3113    /// assert!(first.ptr_eq(&second));
3114    ///
3115    /// let third_rc = Arc::new(());
3116    /// let third = Arc::downgrade(&third_rc);
3117    /// assert!(!first.ptr_eq(&third));
3118    /// ```
3119    ///
3120    /// [`ptr::eq`]: core::ptr::eq "ptr::eq"
3121    #[inline]
3122    #[must_use]
3123    #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
3124    pub fn ptr_eq(&self, other: &Self) -> bool {
3125        ptr::addr_eq(self.ptr.as_ptr(), other.ptr.as_ptr())
3126    }
3127}
3128
3129#[stable(feature = "arc_weak", since = "1.4.0")]
3130impl<T: ?Sized, A: Allocator + Clone> Clone for Weak<T, A> {
3131    /// Makes a clone of the `Weak` pointer that points to the same allocation.
3132    ///
3133    /// # Examples
3134    ///
3135    /// ```
3136    /// use std::sync::{Arc, Weak};
3137    ///
3138    /// let weak_five = Arc::downgrade(&Arc::new(5));
3139    ///
3140    /// let _ = Weak::clone(&weak_five);
3141    /// ```
3142    #[inline]
3143    fn clone(&self) -> Weak<T, A> {
3144        if let Some(inner) = self.inner() {
3145            // See comments in Arc::clone() for why this is relaxed. This can use a
3146            // fetch_add (ignoring the lock) because the weak count is only locked
3147            // where are *no other* weak pointers in existence. (So we can't be
3148            // running this code in that case).
3149            let old_size = inner.weak.fetch_add(1, Relaxed);
3150
3151            // See comments in Arc::clone() for why we do this (for mem::forget).
3152            if old_size > MAX_REFCOUNT {
3153                abort();
3154            }
3155        }
3156
3157        Weak { ptr: self.ptr, alloc: self.alloc.clone() }
3158    }
3159}
3160
3161#[stable(feature = "downgraded_weak", since = "1.10.0")]
3162impl<T> Default for Weak<T> {
3163    /// Constructs a new `Weak<T>`, without allocating memory.
3164    /// Calling [`upgrade`] on the return value always
3165    /// gives [`None`].
3166    ///
3167    /// [`upgrade`]: Weak::upgrade
3168    ///
3169    /// # Examples
3170    ///
3171    /// ```
3172    /// use std::sync::Weak;
3173    ///
3174    /// let empty: Weak<i64> = Default::default();
3175    /// assert!(empty.upgrade().is_none());
3176    /// ```
3177    fn default() -> Weak<T> {
3178        Weak::new()
3179    }
3180}
3181
3182#[stable(feature = "arc_weak", since = "1.4.0")]
3183unsafe impl<#[may_dangle] T: ?Sized, A: Allocator> Drop for Weak<T, A> {
3184    /// Drops the `Weak` pointer.
3185    ///
3186    /// # Examples
3187    ///
3188    /// ```
3189    /// use std::sync::{Arc, Weak};
3190    ///
3191    /// struct Foo;
3192    ///
3193    /// impl Drop for Foo {
3194    ///     fn drop(&mut self) {
3195    ///         println!("dropped!");
3196    ///     }
3197    /// }
3198    ///
3199    /// let foo = Arc::new(Foo);
3200    /// let weak_foo = Arc::downgrade(&foo);
3201    /// let other_weak_foo = Weak::clone(&weak_foo);
3202    ///
3203    /// drop(weak_foo);   // Doesn't print anything
3204    /// drop(foo);        // Prints "dropped!"
3205    ///
3206    /// assert!(other_weak_foo.upgrade().is_none());
3207    /// ```
3208    fn drop(&mut self) {
3209        // If we find out that we were the last weak pointer, then its time to
3210        // deallocate the data entirely. See the discussion in Arc::drop() about
3211        // the memory orderings
3212        //
3213        // It's not necessary to check for the locked state here, because the
3214        // weak count can only be locked if there was precisely one weak ref,
3215        // meaning that drop could only subsequently run ON that remaining weak
3216        // ref, which can only happen after the lock is released.
3217        let inner = if let Some(inner) = self.inner() { inner } else { return };
3218
3219        if inner.weak.fetch_sub(1, Release) == 1 {
3220            acquire!(inner.weak);
3221
3222            // Make sure we aren't trying to "deallocate" the shared static for empty slices
3223            // used by Default::default.
3224            debug_assert!(
3225                !ptr::addr_eq(self.ptr.as_ptr(), &STATIC_INNER_SLICE.inner),
3226                "Arc/Weaks backed by a static should never be deallocated. \
3227                Likely decrement_strong_count or from_raw were called too many times.",
3228            );
3229
3230            unsafe {
3231                self.alloc.deallocate(self.ptr.cast(), Layout::for_value_raw(self.ptr.as_ptr()))
3232            }
3233        }
3234    }
3235}
3236
3237#[stable(feature = "rust1", since = "1.0.0")]
3238trait ArcEqIdent<T: ?Sized + PartialEq, A: Allocator> {
3239    fn eq(&self, other: &Arc<T, A>) -> bool;
3240    fn ne(&self, other: &Arc<T, A>) -> bool;
3241}
3242
3243#[stable(feature = "rust1", since = "1.0.0")]
3244impl<T: ?Sized + PartialEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3245    #[inline]
3246    default fn eq(&self, other: &Arc<T, A>) -> bool {
3247        **self == **other
3248    }
3249    #[inline]
3250    default fn ne(&self, other: &Arc<T, A>) -> bool {
3251        **self != **other
3252    }
3253}
3254
3255/// We're doing this specialization here, and not as a more general optimization on `&T`, because it
3256/// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
3257/// store large values, that are slow to clone, but also heavy to check for equality, causing this
3258/// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
3259/// the same value, than two `&T`s.
3260///
3261/// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
3262#[stable(feature = "rust1", since = "1.0.0")]
3263impl<T: ?Sized + crate::rc::MarkerEq, A: Allocator> ArcEqIdent<T, A> for Arc<T, A> {
3264    #[inline]
3265    fn eq(&self, other: &Arc<T, A>) -> bool {
3266        Arc::ptr_eq(self, other) || **self == **other
3267    }
3268
3269    #[inline]
3270    fn ne(&self, other: &Arc<T, A>) -> bool {
3271        !Arc::ptr_eq(self, other) && **self != **other
3272    }
3273}
3274
3275#[stable(feature = "rust1", since = "1.0.0")]
3276impl<T: ?Sized + PartialEq, A: Allocator> PartialEq for Arc<T, A> {
3277    /// Equality for two `Arc`s.
3278    ///
3279    /// Two `Arc`s are equal if their inner values are equal, even if they are
3280    /// stored in different allocation.
3281    ///
3282    /// If `T` also implements `Eq` (implying reflexivity of equality),
3283    /// two `Arc`s that point to the same allocation are always equal.
3284    ///
3285    /// # Examples
3286    ///
3287    /// ```
3288    /// use std::sync::Arc;
3289    ///
3290    /// let five = Arc::new(5);
3291    ///
3292    /// assert!(five == Arc::new(5));
3293    /// ```
3294    #[inline]
3295    fn eq(&self, other: &Arc<T, A>) -> bool {
3296        ArcEqIdent::eq(self, other)
3297    }
3298
3299    /// Inequality for two `Arc`s.
3300    ///
3301    /// Two `Arc`s are not equal if their inner values are not equal.
3302    ///
3303    /// If `T` also implements `Eq` (implying reflexivity of equality),
3304    /// two `Arc`s that point to the same value are always equal.
3305    ///
3306    /// # Examples
3307    ///
3308    /// ```
3309    /// use std::sync::Arc;
3310    ///
3311    /// let five = Arc::new(5);
3312    ///
3313    /// assert!(five != Arc::new(6));
3314    /// ```
3315    #[inline]
3316    fn ne(&self, other: &Arc<T, A>) -> bool {
3317        ArcEqIdent::ne(self, other)
3318    }
3319}
3320
3321#[stable(feature = "rust1", since = "1.0.0")]
3322impl<T: ?Sized + PartialOrd, A: Allocator> PartialOrd for Arc<T, A> {
3323    /// Partial comparison for two `Arc`s.
3324    ///
3325    /// The two are compared by calling `partial_cmp()` on their inner values.
3326    ///
3327    /// # Examples
3328    ///
3329    /// ```
3330    /// use std::sync::Arc;
3331    /// use std::cmp::Ordering;
3332    ///
3333    /// let five = Arc::new(5);
3334    ///
3335    /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
3336    /// ```
3337    fn partial_cmp(&self, other: &Arc<T, A>) -> Option<Ordering> {
3338        (**self).partial_cmp(&**other)
3339    }
3340
3341    /// Less-than comparison for two `Arc`s.
3342    ///
3343    /// The two are compared by calling `<` on their inner values.
3344    ///
3345    /// # Examples
3346    ///
3347    /// ```
3348    /// use std::sync::Arc;
3349    ///
3350    /// let five = Arc::new(5);
3351    ///
3352    /// assert!(five < Arc::new(6));
3353    /// ```
3354    fn lt(&self, other: &Arc<T, A>) -> bool {
3355        *(*self) < *(*other)
3356    }
3357
3358    /// 'Less than or equal to' comparison for two `Arc`s.
3359    ///
3360    /// The two are compared by calling `<=` on their inner values.
3361    ///
3362    /// # Examples
3363    ///
3364    /// ```
3365    /// use std::sync::Arc;
3366    ///
3367    /// let five = Arc::new(5);
3368    ///
3369    /// assert!(five <= Arc::new(5));
3370    /// ```
3371    fn le(&self, other: &Arc<T, A>) -> bool {
3372        *(*self) <= *(*other)
3373    }
3374
3375    /// Greater-than comparison for two `Arc`s.
3376    ///
3377    /// The two are compared by calling `>` on their inner values.
3378    ///
3379    /// # Examples
3380    ///
3381    /// ```
3382    /// use std::sync::Arc;
3383    ///
3384    /// let five = Arc::new(5);
3385    ///
3386    /// assert!(five > Arc::new(4));
3387    /// ```
3388    fn gt(&self, other: &Arc<T, A>) -> bool {
3389        *(*self) > *(*other)
3390    }
3391
3392    /// 'Greater than or equal to' comparison for two `Arc`s.
3393    ///
3394    /// The two are compared by calling `>=` on their inner values.
3395    ///
3396    /// # Examples
3397    ///
3398    /// ```
3399    /// use std::sync::Arc;
3400    ///
3401    /// let five = Arc::new(5);
3402    ///
3403    /// assert!(five >= Arc::new(5));
3404    /// ```
3405    fn ge(&self, other: &Arc<T, A>) -> bool {
3406        *(*self) >= *(*other)
3407    }
3408}
3409#[stable(feature = "rust1", since = "1.0.0")]
3410impl<T: ?Sized + Ord, A: Allocator> Ord for Arc<T, A> {
3411    /// Comparison for two `Arc`s.
3412    ///
3413    /// The two are compared by calling `cmp()` on their inner values.
3414    ///
3415    /// # Examples
3416    ///
3417    /// ```
3418    /// use std::sync::Arc;
3419    /// use std::cmp::Ordering;
3420    ///
3421    /// let five = Arc::new(5);
3422    ///
3423    /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
3424    /// ```
3425    fn cmp(&self, other: &Arc<T, A>) -> Ordering {
3426        (**self).cmp(&**other)
3427    }
3428}
3429#[stable(feature = "rust1", since = "1.0.0")]
3430impl<T: ?Sized + Eq, A: Allocator> Eq for Arc<T, A> {}
3431
3432#[stable(feature = "rust1", since = "1.0.0")]
3433impl<T: ?Sized + fmt::Display, A: Allocator> fmt::Display for Arc<T, A> {
3434    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3435        fmt::Display::fmt(&**self, f)
3436    }
3437}
3438
3439#[stable(feature = "rust1", since = "1.0.0")]
3440impl<T: ?Sized + fmt::Debug, A: Allocator> fmt::Debug for Arc<T, A> {
3441    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3442        fmt::Debug::fmt(&**self, f)
3443    }
3444}
3445
3446#[stable(feature = "rust1", since = "1.0.0")]
3447impl<T: ?Sized, A: Allocator> fmt::Pointer for Arc<T, A> {
3448    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
3449        fmt::Pointer::fmt(&(&raw const **self), f)
3450    }
3451}
3452
3453#[cfg(not(no_global_oom_handling))]
3454#[stable(feature = "rust1", since = "1.0.0")]
3455impl<T: Default> Default for Arc<T> {
3456    /// Creates a new `Arc<T>`, with the `Default` value for `T`.
3457    ///
3458    /// # Examples
3459    ///
3460    /// ```
3461    /// use std::sync::Arc;
3462    ///
3463    /// let x: Arc<i32> = Default::default();
3464    /// assert_eq!(*x, 0);
3465    /// ```
3466    fn default() -> Arc<T> {
3467        unsafe {
3468            Self::from_inner(
3469                Box::leak(Box::write(
3470                    Box::new_uninit(),
3471                    ArcInner {
3472                        strong: atomic::AtomicUsize::new(1),
3473                        weak: atomic::AtomicUsize::new(1),
3474                        data: T::default(),
3475                    },
3476                ))
3477                .into(),
3478            )
3479        }
3480    }
3481}
3482
3483/// Struct to hold the static `ArcInner` used for empty `Arc<str/CStr/[T]>` as
3484/// returned by `Default::default`.
3485///
3486/// Layout notes:
3487/// * `repr(align(16))` so we can use it for `[T]` with `align_of::<T>() <= 16`.
3488/// * `repr(C)` so `inner` is at offset 0 (and thus guaranteed to actually be aligned to 16).
3489/// * `[u8; 1]` (to be initialized with 0) so it can be used for `Arc<CStr>`.
3490#[repr(C, align(16))]
3491struct SliceArcInnerForStatic {
3492    inner: ArcInner<[u8; 1]>,
3493}
3494#[cfg(not(no_global_oom_handling))]
3495const MAX_STATIC_INNER_SLICE_ALIGNMENT: usize = 16;
3496
3497static STATIC_INNER_SLICE: SliceArcInnerForStatic = SliceArcInnerForStatic {
3498    inner: ArcInner {
3499        strong: atomic::AtomicUsize::new(1),
3500        weak: atomic::AtomicUsize::new(1),
3501        data: [0],
3502    },
3503};
3504
3505#[cfg(not(no_global_oom_handling))]
3506#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3507impl Default for Arc<str> {
3508    /// Creates an empty str inside an Arc
3509    ///
3510    /// This may or may not share an allocation with other Arcs.
3511    #[inline]
3512    fn default() -> Self {
3513        let arc: Arc<[u8]> = Default::default();
3514        debug_assert!(core::str::from_utf8(&*arc).is_ok());
3515        let (ptr, alloc) = Arc::into_inner_with_allocator(arc);
3516        unsafe { Arc::from_ptr_in(ptr.as_ptr() as *mut ArcInner<str>, alloc) }
3517    }
3518}
3519
3520#[cfg(not(no_global_oom_handling))]
3521#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3522impl Default for Arc<core::ffi::CStr> {
3523    /// Creates an empty CStr inside an Arc
3524    ///
3525    /// This may or may not share an allocation with other Arcs.
3526    #[inline]
3527    fn default() -> Self {
3528        use core::ffi::CStr;
3529        let inner: NonNull<ArcInner<[u8]>> = NonNull::from(&STATIC_INNER_SLICE.inner);
3530        let inner: NonNull<ArcInner<CStr>> =
3531            NonNull::new(inner.as_ptr() as *mut ArcInner<CStr>).unwrap();
3532        // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3533        let this: mem::ManuallyDrop<Arc<CStr>> =
3534            unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3535        (*this).clone()
3536    }
3537}
3538
3539#[cfg(not(no_global_oom_handling))]
3540#[stable(feature = "more_rc_default_impls", since = "1.80.0")]
3541impl<T> Default for Arc<[T]> {
3542    /// Creates an empty `[T]` inside an Arc
3543    ///
3544    /// This may or may not share an allocation with other Arcs.
3545    #[inline]
3546    fn default() -> Self {
3547        if mem::align_of::<T>() <= MAX_STATIC_INNER_SLICE_ALIGNMENT {
3548            // We take a reference to the whole struct instead of the ArcInner<[u8; 1]> inside it so
3549            // we don't shrink the range of bytes the ptr is allowed to access under Stacked Borrows.
3550            // (Miri complains on 32-bit targets with Arc<[Align16]> otherwise.)
3551            // (Note that NonNull::from(&STATIC_INNER_SLICE.inner) is fine under Tree Borrows.)
3552            let inner: NonNull<SliceArcInnerForStatic> = NonNull::from(&STATIC_INNER_SLICE);
3553            let inner: NonNull<ArcInner<[T; 0]>> = inner.cast();
3554            // `this` semantically is the Arc "owned" by the static, so make sure not to drop it.
3555            let this: mem::ManuallyDrop<Arc<[T; 0]>> =
3556                unsafe { mem::ManuallyDrop::new(Arc::from_inner(inner)) };
3557            return (*this).clone();
3558        }
3559
3560        // If T's alignment is too large for the static, make a new unique allocation.
3561        let arr: [T; 0] = [];
3562        Arc::from(arr)
3563    }
3564}
3565
3566#[stable(feature = "rust1", since = "1.0.0")]
3567impl<T: ?Sized + Hash, A: Allocator> Hash for Arc<T, A> {
3568    fn hash<H: Hasher>(&self, state: &mut H) {
3569        (**self).hash(state)
3570    }
3571}
3572
3573#[cfg(not(no_global_oom_handling))]
3574#[stable(feature = "from_for_ptrs", since = "1.6.0")]
3575impl<T> From<T> for Arc<T> {
3576    /// Converts a `T` into an `Arc<T>`
3577    ///
3578    /// The conversion moves the value into a
3579    /// newly allocated `Arc`. It is equivalent to
3580    /// calling `Arc::new(t)`.
3581    ///
3582    /// # Example
3583    /// ```rust
3584    /// # use std::sync::Arc;
3585    /// let x = 5;
3586    /// let arc = Arc::new(5);
3587    ///
3588    /// assert_eq!(Arc::from(x), arc);
3589    /// ```
3590    fn from(t: T) -> Self {
3591        Arc::new(t)
3592    }
3593}
3594
3595#[cfg(not(no_global_oom_handling))]
3596#[stable(feature = "shared_from_array", since = "1.74.0")]
3597impl<T, const N: usize> From<[T; N]> for Arc<[T]> {
3598    /// Converts a [`[T; N]`](prim@array) into an `Arc<[T]>`.
3599    ///
3600    /// The conversion moves the array into a newly allocated `Arc`.
3601    ///
3602    /// # Example
3603    ///
3604    /// ```
3605    /// # use std::sync::Arc;
3606    /// let original: [i32; 3] = [1, 2, 3];
3607    /// let shared: Arc<[i32]> = Arc::from(original);
3608    /// assert_eq!(&[1, 2, 3], &shared[..]);
3609    /// ```
3610    #[inline]
3611    fn from(v: [T; N]) -> Arc<[T]> {
3612        Arc::<[T; N]>::from(v)
3613    }
3614}
3615
3616#[cfg(not(no_global_oom_handling))]
3617#[stable(feature = "shared_from_slice", since = "1.21.0")]
3618impl<T: Clone> From<&[T]> for Arc<[T]> {
3619    /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3620    ///
3621    /// # Example
3622    ///
3623    /// ```
3624    /// # use std::sync::Arc;
3625    /// let original: &[i32] = &[1, 2, 3];
3626    /// let shared: Arc<[i32]> = Arc::from(original);
3627    /// assert_eq!(&[1, 2, 3], &shared[..]);
3628    /// ```
3629    #[inline]
3630    fn from(v: &[T]) -> Arc<[T]> {
3631        <Self as ArcFromSlice<T>>::from_slice(v)
3632    }
3633}
3634
3635#[cfg(not(no_global_oom_handling))]
3636#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3637impl<T: Clone> From<&mut [T]> for Arc<[T]> {
3638    /// Allocates a reference-counted slice and fills it by cloning `v`'s items.
3639    ///
3640    /// # Example
3641    ///
3642    /// ```
3643    /// # use std::sync::Arc;
3644    /// let mut original = [1, 2, 3];
3645    /// let original: &mut [i32] = &mut original;
3646    /// let shared: Arc<[i32]> = Arc::from(original);
3647    /// assert_eq!(&[1, 2, 3], &shared[..]);
3648    /// ```
3649    #[inline]
3650    fn from(v: &mut [T]) -> Arc<[T]> {
3651        Arc::from(&*v)
3652    }
3653}
3654
3655#[cfg(not(no_global_oom_handling))]
3656#[stable(feature = "shared_from_slice", since = "1.21.0")]
3657impl From<&str> for Arc<str> {
3658    /// Allocates a reference-counted `str` and copies `v` into it.
3659    ///
3660    /// # Example
3661    ///
3662    /// ```
3663    /// # use std::sync::Arc;
3664    /// let shared: Arc<str> = Arc::from("eggplant");
3665    /// assert_eq!("eggplant", &shared[..]);
3666    /// ```
3667    #[inline]
3668    fn from(v: &str) -> Arc<str> {
3669        let arc = Arc::<[u8]>::from(v.as_bytes());
3670        unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
3671    }
3672}
3673
3674#[cfg(not(no_global_oom_handling))]
3675#[stable(feature = "shared_from_mut_slice", since = "1.84.0")]
3676impl From<&mut str> for Arc<str> {
3677    /// Allocates a reference-counted `str` and copies `v` into it.
3678    ///
3679    /// # Example
3680    ///
3681    /// ```
3682    /// # use std::sync::Arc;
3683    /// let mut original = String::from("eggplant");
3684    /// let original: &mut str = &mut original;
3685    /// let shared: Arc<str> = Arc::from(original);
3686    /// assert_eq!("eggplant", &shared[..]);
3687    /// ```
3688    #[inline]
3689    fn from(v: &mut str) -> Arc<str> {
3690        Arc::from(&*v)
3691    }
3692}
3693
3694#[cfg(not(no_global_oom_handling))]
3695#[stable(feature = "shared_from_slice", since = "1.21.0")]
3696impl From<String> for Arc<str> {
3697    /// Allocates a reference-counted `str` and copies `v` into it.
3698    ///
3699    /// # Example
3700    ///
3701    /// ```
3702    /// # use std::sync::Arc;
3703    /// let unique: String = "eggplant".to_owned();
3704    /// let shared: Arc<str> = Arc::from(unique);
3705    /// assert_eq!("eggplant", &shared[..]);
3706    /// ```
3707    #[inline]
3708    fn from(v: String) -> Arc<str> {
3709        Arc::from(&v[..])
3710    }
3711}
3712
3713#[cfg(not(no_global_oom_handling))]
3714#[stable(feature = "shared_from_slice", since = "1.21.0")]
3715impl<T: ?Sized, A: Allocator> From<Box<T, A>> for Arc<T, A> {
3716    /// Move a boxed object to a new, reference-counted allocation.
3717    ///
3718    /// # Example
3719    ///
3720    /// ```
3721    /// # use std::sync::Arc;
3722    /// let unique: Box<str> = Box::from("eggplant");
3723    /// let shared: Arc<str> = Arc::from(unique);
3724    /// assert_eq!("eggplant", &shared[..]);
3725    /// ```
3726    #[inline]
3727    fn from(v: Box<T, A>) -> Arc<T, A> {
3728        Arc::from_box_in(v)
3729    }
3730}
3731
3732#[cfg(not(no_global_oom_handling))]
3733#[stable(feature = "shared_from_slice", since = "1.21.0")]
3734impl<T, A: Allocator + Clone> From<Vec<T, A>> for Arc<[T], A> {
3735    /// Allocates a reference-counted slice and moves `v`'s items into it.
3736    ///
3737    /// # Example
3738    ///
3739    /// ```
3740    /// # use std::sync::Arc;
3741    /// let unique: Vec<i32> = vec![1, 2, 3];
3742    /// let shared: Arc<[i32]> = Arc::from(unique);
3743    /// assert_eq!(&[1, 2, 3], &shared[..]);
3744    /// ```
3745    #[inline]
3746    fn from(v: Vec<T, A>) -> Arc<[T], A> {
3747        unsafe {
3748            let (vec_ptr, len, cap, alloc) = v.into_raw_parts_with_alloc();
3749
3750            let rc_ptr = Self::allocate_for_slice_in(len, &alloc);
3751            ptr::copy_nonoverlapping(vec_ptr, (&raw mut (*rc_ptr).data) as *mut T, len);
3752
3753            // Create a `Vec<T, &A>` with length 0, to deallocate the buffer
3754            // without dropping its contents or the allocator
3755            let _ = Vec::from_raw_parts_in(vec_ptr, 0, cap, &alloc);
3756
3757            Self::from_ptr_in(rc_ptr, alloc)
3758        }
3759    }
3760}
3761
3762#[stable(feature = "shared_from_cow", since = "1.45.0")]
3763impl<'a, B> From<Cow<'a, B>> for Arc<B>
3764where
3765    B: ToOwned + ?Sized,
3766    Arc<B>: From<&'a B> + From<B::Owned>,
3767{
3768    /// Creates an atomically reference-counted pointer from a clone-on-write
3769    /// pointer by copying its content.
3770    ///
3771    /// # Example
3772    ///
3773    /// ```rust
3774    /// # use std::sync::Arc;
3775    /// # use std::borrow::Cow;
3776    /// let cow: Cow<'_, str> = Cow::Borrowed("eggplant");
3777    /// let shared: Arc<str> = Arc::from(cow);
3778    /// assert_eq!("eggplant", &shared[..]);
3779    /// ```
3780    #[inline]
3781    fn from(cow: Cow<'a, B>) -> Arc<B> {
3782        match cow {
3783            Cow::Borrowed(s) => Arc::from(s),
3784            Cow::Owned(s) => Arc::from(s),
3785        }
3786    }
3787}
3788
3789#[stable(feature = "shared_from_str", since = "1.62.0")]
3790impl From<Arc<str>> for Arc<[u8]> {
3791    /// Converts an atomically reference-counted string slice into a byte slice.
3792    ///
3793    /// # Example
3794    ///
3795    /// ```
3796    /// # use std::sync::Arc;
3797    /// let string: Arc<str> = Arc::from("eggplant");
3798    /// let bytes: Arc<[u8]> = Arc::from(string);
3799    /// assert_eq!("eggplant".as_bytes(), bytes.as_ref());
3800    /// ```
3801    #[inline]
3802    fn from(rc: Arc<str>) -> Self {
3803        // SAFETY: `str` has the same layout as `[u8]`.
3804        unsafe { Arc::from_raw(Arc::into_raw(rc) as *const [u8]) }
3805    }
3806}
3807
3808#[stable(feature = "boxed_slice_try_from", since = "1.43.0")]
3809impl<T, A: Allocator, const N: usize> TryFrom<Arc<[T], A>> for Arc<[T; N], A> {
3810    type Error = Arc<[T], A>;
3811
3812    fn try_from(boxed_slice: Arc<[T], A>) -> Result<Self, Self::Error> {
3813        if boxed_slice.len() == N {
3814            let (ptr, alloc) = Arc::into_inner_with_allocator(boxed_slice);
3815            Ok(unsafe { Arc::from_inner_in(ptr.cast(), alloc) })
3816        } else {
3817            Err(boxed_slice)
3818        }
3819    }
3820}
3821
3822#[cfg(not(no_global_oom_handling))]
3823#[stable(feature = "shared_from_iter", since = "1.37.0")]
3824impl<T> FromIterator<T> for Arc<[T]> {
3825    /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
3826    ///
3827    /// # Performance characteristics
3828    ///
3829    /// ## The general case
3830    ///
3831    /// In the general case, collecting into `Arc<[T]>` is done by first
3832    /// collecting into a `Vec<T>`. That is, when writing the following:
3833    ///
3834    /// ```rust
3835    /// # use std::sync::Arc;
3836    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
3837    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3838    /// ```
3839    ///
3840    /// this behaves as if we wrote:
3841    ///
3842    /// ```rust
3843    /// # use std::sync::Arc;
3844    /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
3845    ///     .collect::<Vec<_>>() // The first set of allocations happens here.
3846    ///     .into(); // A second allocation for `Arc<[T]>` happens here.
3847    /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
3848    /// ```
3849    ///
3850    /// This will allocate as many times as needed for constructing the `Vec<T>`
3851    /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
3852    ///
3853    /// ## Iterators of known length
3854    ///
3855    /// When your `Iterator` implements `TrustedLen` and is of an exact size,
3856    /// a single allocation will be made for the `Arc<[T]>`. For example:
3857    ///
3858    /// ```rust
3859    /// # use std::sync::Arc;
3860    /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
3861    /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
3862    /// ```
3863    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
3864        ToArcSlice::to_arc_slice(iter.into_iter())
3865    }
3866}
3867
3868#[cfg(not(no_global_oom_handling))]
3869/// Specialization trait used for collecting into `Arc<[T]>`.
3870trait ToArcSlice<T>: Iterator<Item = T> + Sized {
3871    fn to_arc_slice(self) -> Arc<[T]>;
3872}
3873
3874#[cfg(not(no_global_oom_handling))]
3875impl<T, I: Iterator<Item = T>> ToArcSlice<T> for I {
3876    default fn to_arc_slice(self) -> Arc<[T]> {
3877        self.collect::<Vec<T>>().into()
3878    }
3879}
3880
3881#[cfg(not(no_global_oom_handling))]
3882impl<T, I: iter::TrustedLen<Item = T>> ToArcSlice<T> for I {
3883    fn to_arc_slice(self) -> Arc<[T]> {
3884        // This is the case for a `TrustedLen` iterator.
3885        let (low, high) = self.size_hint();
3886        if let Some(high) = high {
3887            debug_assert_eq!(
3888                low,
3889                high,
3890                "TrustedLen iterator's size hint is not exact: {:?}",
3891                (low, high)
3892            );
3893
3894            unsafe {
3895                // SAFETY: We need to ensure that the iterator has an exact length and we have.
3896                Arc::from_iter_exact(self, low)
3897            }
3898        } else {
3899            // TrustedLen contract guarantees that `upper_bound == None` implies an iterator
3900            // length exceeding `usize::MAX`.
3901            // The default implementation would collect into a vec which would panic.
3902            // Thus we panic here immediately without invoking `Vec` code.
3903            panic!("capacity overflow");
3904        }
3905    }
3906}
3907
3908#[stable(feature = "rust1", since = "1.0.0")]
3909impl<T: ?Sized, A: Allocator> borrow::Borrow<T> for Arc<T, A> {
3910    fn borrow(&self) -> &T {
3911        &**self
3912    }
3913}
3914
3915#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
3916impl<T: ?Sized, A: Allocator> AsRef<T> for Arc<T, A> {
3917    fn as_ref(&self) -> &T {
3918        &**self
3919    }
3920}
3921
3922#[stable(feature = "pin", since = "1.33.0")]
3923impl<T: ?Sized, A: Allocator> Unpin for Arc<T, A> {}
3924
3925/// Gets the offset within an `ArcInner` for the payload behind a pointer.
3926///
3927/// # Safety
3928///
3929/// The pointer must point to (and have valid metadata for) a previously
3930/// valid instance of T, but the T is allowed to be dropped.
3931unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> usize {
3932    // Align the unsized value to the end of the ArcInner.
3933    // Because RcInner is repr(C), it will always be the last field in memory.
3934    // SAFETY: since the only unsized types possible are slices, trait objects,
3935    // and extern types, the input safety requirement is currently enough to
3936    // satisfy the requirements of align_of_val_raw; this is an implementation
3937    // detail of the language that must not be relied upon outside of std.
3938    unsafe { data_offset_align(align_of_val_raw(ptr)) }
3939}
3940
3941#[inline]
3942fn data_offset_align(align: usize) -> usize {
3943    let layout = Layout::new::<ArcInner<()>>();
3944    layout.size() + layout.padding_needed_for(align)
3945}
3946
3947/// A unique owning pointer to an [`ArcInner`] **that does not imply the contents are initialized,**
3948/// but will deallocate it (without dropping the value) when dropped.
3949///
3950/// This is a helper for [`Arc::make_mut()`] to ensure correct cleanup on panic.
3951#[cfg(not(no_global_oom_handling))]
3952struct UniqueArcUninit<T: ?Sized, A: Allocator> {
3953    ptr: NonNull<ArcInner<T>>,
3954    layout_for_value: Layout,
3955    alloc: Option<A>,
3956}
3957
3958#[cfg(not(no_global_oom_handling))]
3959impl<T: ?Sized, A: Allocator> UniqueArcUninit<T, A> {
3960    /// Allocates an ArcInner with layout suitable to contain `for_value` or a clone of it.
3961    fn new(for_value: &T, alloc: A) -> UniqueArcUninit<T, A> {
3962        let layout = Layout::for_value(for_value);
3963        let ptr = unsafe {
3964            Arc::allocate_for_layout(
3965                layout,
3966                |layout_for_arcinner| alloc.allocate(layout_for_arcinner),
3967                |mem| mem.with_metadata_of(ptr::from_ref(for_value) as *const ArcInner<T>),
3968            )
3969        };
3970        Self { ptr: NonNull::new(ptr).unwrap(), layout_for_value: layout, alloc: Some(alloc) }
3971    }
3972
3973    /// Returns the pointer to be written into to initialize the [`Arc`].
3974    fn data_ptr(&mut self) -> *mut T {
3975        let offset = data_offset_align(self.layout_for_value.align());
3976        unsafe { self.ptr.as_ptr().byte_add(offset) as *mut T }
3977    }
3978
3979    /// Upgrade this into a normal [`Arc`].
3980    ///
3981    /// # Safety
3982    ///
3983    /// The data must have been initialized (by writing to [`Self::data_ptr()`]).
3984    unsafe fn into_arc(self) -> Arc<T, A> {
3985        let mut this = ManuallyDrop::new(self);
3986        let ptr = this.ptr.as_ptr();
3987        let alloc = this.alloc.take().unwrap();
3988
3989        // SAFETY: The pointer is valid as per `UniqueArcUninit::new`, and the caller is responsible
3990        // for having initialized the data.
3991        unsafe { Arc::from_ptr_in(ptr, alloc) }
3992    }
3993}
3994
3995#[cfg(not(no_global_oom_handling))]
3996impl<T: ?Sized, A: Allocator> Drop for UniqueArcUninit<T, A> {
3997    fn drop(&mut self) {
3998        // SAFETY:
3999        // * new() produced a pointer safe to deallocate.
4000        // * We own the pointer unless into_arc() was called, which forgets us.
4001        unsafe {
4002            self.alloc.take().unwrap().deallocate(
4003                self.ptr.cast(),
4004                arcinner_layout_for_value_layout(self.layout_for_value),
4005            );
4006        }
4007    }
4008}
4009
4010#[stable(feature = "arc_error", since = "1.52.0")]
4011impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
4012    #[allow(deprecated, deprecated_in_future)]
4013    fn description(&self) -> &str {
4014        core::error::Error::description(&**self)
4015    }
4016
4017    #[allow(deprecated)]
4018    fn cause(&self) -> Option<&dyn core::error::Error> {
4019        core::error::Error::cause(&**self)
4020    }
4021
4022    fn source(&self) -> Option<&(dyn core::error::Error + 'static)> {
4023        core::error::Error::source(&**self)
4024    }
4025
4026    fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
4027        core::error::Error::provide(&**self, req);
4028    }
4029}