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    #![stable(feature = "rust1", since = "1.0.0")]

    //! Thread-safe reference-counting pointers.
    //!
    //! See the [`Arc<T>`][arc] documentation for more details.
    //!
    //! [arc]: struct.Arc.html

    use core::any::Any;
    use core::array::LengthAtMost32;
    use core::sync::atomic;
    use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
    use core::borrow;
    use core::fmt;
    use core::cmp::{self, Ordering};
    use core::iter;
    use core::intrinsics::abort;
    use core::mem::{self, align_of, align_of_val, size_of_val};
    use core::ops::{Deref, Receiver, CoerceUnsized, DispatchFromDyn};
    use core::pin::Pin;
    use core::ptr::{self, NonNull};
    use core::marker::{Unpin, Unsize, PhantomData};
    use core::hash::{Hash, Hasher};
    use core::{isize, usize};
    use core::convert::{From, TryFrom};
    use core::slice::{self, from_raw_parts_mut};

    use crate::alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
    use crate::boxed::Box;
    use crate::rc::is_dangling;
    use crate::string::String;
    use crate::vec::Vec;

    #[cfg(test)]
    mod tests;

    /// A soft limit on the amount of references that may be made to an `Arc`.
    ///
    /// Going above this limit will abort your program (although not
    /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
    const MAX_REFCOUNT: usize = (isize::MAX) as usize;

    /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
    /// Reference Counted'.
    ///
    /// The type `Arc<T>` provides shared ownership of a value of type `T`,
    /// allocated in the heap. Invoking [`clone`][clone] on `Arc` produces
    /// a new `Arc` instance, which points to the same allocation on the heap as the
    /// source `Arc`, while increasing a reference count. When the last `Arc`
    /// pointer to a given allocation is destroyed, the value stored in that allocation (often
    /// referred to as "inner value") is also dropped.
    ///
    /// Shared references in Rust disallow mutation by default, and `Arc` is no
    /// exception: you cannot generally obtain a mutable reference to something
    /// inside an `Arc`. If you need to mutate through an `Arc`, use
    /// [`Mutex`][mutex], [`RwLock`][rwlock], or one of the [`Atomic`][atomic]
    /// types.
    ///
    /// ## Thread Safety
    ///
    /// Unlike [`Rc<T>`], `Arc<T>` uses atomic operations for its reference
    /// counting. This means that it is thread-safe. The disadvantage is that
    /// atomic operations are more expensive than ordinary memory accesses. If you
    /// are not sharing reference-counted allocations between threads, consider using
    /// [`Rc<T>`] for lower overhead. [`Rc<T>`] is a safe default, because the
    /// compiler will catch any attempt to send an [`Rc<T>`] between threads.
    /// However, a library might choose `Arc<T>` in order to give library consumers
    /// more flexibility.
    ///
    /// `Arc<T>` will implement [`Send`] and [`Sync`] as long as the `T` implements
    /// [`Send`] and [`Sync`]. Why can't you put a non-thread-safe type `T` in an
    /// `Arc<T>` to make it thread-safe? This may be a bit counter-intuitive at
    /// first: after all, isn't the point of `Arc<T>` thread safety? The key is
    /// this: `Arc<T>` makes it thread safe to have multiple ownership of the same
    /// data, but it  doesn't add thread safety to its data. Consider
    /// `Arc<`[`RefCell<T>`]`>`. [`RefCell<T>`] isn't [`Sync`], and if `Arc<T>` was always
    /// [`Send`], `Arc<`[`RefCell<T>`]`>` would be as well. But then we'd have a problem:
    /// [`RefCell<T>`] is not thread safe; it keeps track of the borrowing count using
    /// non-atomic operations.
    ///
    /// In the end, this means that you may need to pair `Arc<T>` with some sort of
    /// [`std::sync`] type, usually [`Mutex<T>`][mutex].
    ///
    /// ## Breaking cycles with `Weak`
    ///
    /// The [`downgrade`][downgrade] method can be used to create a non-owning
    /// [`Weak`][weak] pointer. A [`Weak`][weak] pointer can be [`upgrade`][upgrade]d
    /// to an `Arc`, but this will return [`None`] if the value stored in the allocation has
    /// already been dropped. In other words, `Weak` pointers do not keep the value
    /// inside the allocation alive; however, they *do* keep the allocation
    /// (the backing store for the value) alive.
    ///
    /// A cycle between `Arc` pointers will never be deallocated. For this reason,
    /// [`Weak`][weak] is used to break cycles. For example, a tree could have
    /// strong `Arc` pointers from parent nodes to children, and [`Weak`][weak]
    /// pointers from children back to their parents.
    ///
    /// # Cloning references
    ///
    /// Creating a new reference from an existing reference counted pointer is done using the
    /// `Clone` trait implemented for [`Arc<T>`][arc] and [`Weak<T>`][weak].
    ///
    /// ```
    /// use std::sync::Arc;
    /// let foo = Arc::new(vec![1.0, 2.0, 3.0]);
    /// // The two syntaxes below are equivalent.
    /// let a = foo.clone();
    /// let b = Arc::clone(&foo);
    /// // a, b, and foo are all Arcs that point to the same memory location
    /// ```
    ///
    /// ## `Deref` behavior
    ///
    /// `Arc<T>` automatically dereferences to `T` (via the [`Deref`][deref] trait),
    /// so you can call `T`'s methods on a value of type `Arc<T>`. To avoid name
    /// clashes with `T`'s methods, the methods of `Arc<T>` itself are associated
    /// functions, called using function-like syntax:
    ///
    /// ```
    /// use std::sync::Arc;
    /// let my_arc = Arc::new(());
    ///
    /// Arc::downgrade(&my_arc);
    /// ```
    ///
    /// [`Weak<T>`][weak] does not auto-dereference to `T`, because the inner value may have
    /// already been dropped.
    ///
    /// [arc]: struct.Arc.html
    /// [weak]: struct.Weak.html
    /// [`Rc<T>`]: ../../std/rc/struct.Rc.html
    /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
    /// [mutex]: ../../std/sync/struct.Mutex.html
    /// [rwlock]: ../../std/sync/struct.RwLock.html
    /// [atomic]: ../../std/sync/atomic/index.html
    /// [`Send`]: ../../std/marker/trait.Send.html
    /// [`Sync`]: ../../std/marker/trait.Sync.html
    /// [deref]: ../../std/ops/trait.Deref.html
    /// [downgrade]: struct.Arc.html#method.downgrade
    /// [upgrade]: struct.Weak.html#method.upgrade
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [`RefCell<T>`]: ../../std/cell/struct.RefCell.html
    /// [`std::sync`]: ../../std/sync/index.html
    /// [`Arc::clone(&from)`]: #method.clone
    ///
    /// # Examples
    ///
    /// Sharing some immutable data between threads:
    ///
    // Note that we **do not** run these tests here. The windows builders get super
    // unhappy if a thread outlives the main thread and then exits at the same time
    // (something deadlocks) so we just avoid this entirely by not running these
    // tests.
    /// ```no_run
    /// use std::sync::Arc;
    /// use std::thread;
    ///
    /// let five = Arc::new(5);
    ///
    /// for _ in 0..10 {
    ///     let five = Arc::clone(&five);
    ///
    ///     thread::spawn(move || {
    ///         println!("{:?}", five);
    ///     });
    /// }
    /// ```
    ///
    /// Sharing a mutable [`AtomicUsize`]:
    ///
    /// [`AtomicUsize`]: ../../std/sync/atomic/struct.AtomicUsize.html
    ///
    /// ```no_run
    /// use std::sync::Arc;
    /// use std::sync::atomic::{AtomicUsize, Ordering};
    /// use std::thread;
    ///
    /// let val = Arc::new(AtomicUsize::new(5));
    ///
    /// for _ in 0..10 {
    ///     let val = Arc::clone(&val);
    ///
    ///     thread::spawn(move || {
    ///         let v = val.fetch_add(1, Ordering::SeqCst);
    ///         println!("{:?}", v);
    ///     });
    /// }
    /// ```
    ///
    /// See the [`rc` documentation][rc_examples] for more examples of reference
    /// counting in general.
    ///
    /// [rc_examples]: ../../std/rc/index.html#examples
    #[cfg_attr(not(test), lang = "arc")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Arc<T: ?Sized> {
        ptr: NonNull<ArcInner<T>>,
        phantom: PhantomData<ArcInner<T>>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Arc<U>> for Arc<T> {}

    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Arc<U>> for Arc<T> {}

    impl<T: ?Sized> Arc<T> {
        fn from_inner(ptr: NonNull<ArcInner<T>>) -> Self {
            Self {
                ptr,
                phantom: PhantomData,
            }
        }

        unsafe fn from_ptr(ptr: *mut ArcInner<T>) -> Self {
            Self::from_inner(NonNull::new_unchecked(ptr))
        }
    }

    /// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
    /// managed allocation. The allocation is accessed by calling [`upgrade`] on the `Weak`
    /// pointer, which returns an [`Option`]`<`[`Arc`]`<T>>`.
    ///
    /// Since a `Weak` reference does not count towards ownership, it will not
    /// prevent the value stored in the allocation from being dropped, and `Weak` itself makes no
    /// guarantees about the value still being present. Thus it may return [`None`]
    /// when [`upgrade`]d. Note however that a `Weak` reference *does* prevent the allocation
    /// itself (the backing store) from being deallocated.
    ///
    /// A `Weak` pointer is useful for keeping a temporary reference to the allocation
    /// managed by [`Arc`] without preventing its inner value from being dropped. It is also used to
    /// prevent circular references between [`Arc`] pointers, since mutual owning references
    /// would never allow either [`Arc`] to be dropped. For example, a tree could
    /// have strong [`Arc`] pointers from parent nodes to children, and `Weak`
    /// pointers from children back to their parents.
    ///
    /// The typical way to obtain a `Weak` pointer is to call [`Arc::downgrade`].
    ///
    /// [`Arc`]: struct.Arc.html
    /// [`Arc::downgrade`]: struct.Arc.html#method.downgrade
    /// [`upgrade`]: struct.Weak.html#method.upgrade
    /// [`Option`]: ../../std/option/enum.Option.html
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    #[stable(feature = "arc_weak", since = "1.4.0")]
    pub struct Weak<T: ?Sized> {
        // This is a `NonNull` to allow optimizing the size of this type in enums,
        // but it is not necessarily a valid pointer.
        // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
        // to allocate space on the heap.  That's not a value a real pointer
        // will ever have because RcBox has alignment at least 2.
        ptr: NonNull<ArcInner<T>>,
    }

    #[stable(feature = "arc_weak", since = "1.4.0")]
    unsafe impl<T: ?Sized + Sync + Send> Send for Weak<T> {}
    #[stable(feature = "arc_weak", since = "1.4.0")]
    unsafe impl<T: ?Sized + Sync + Send> Sync for Weak<T> {}

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<Weak<U>> for Weak<T> {}

    #[stable(feature = "arc_weak", since = "1.4.0")]
    impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "(Weak)")
        }
    }

    struct ArcInner<T: ?Sized> {
        strong: atomic::AtomicUsize,

        // the value usize::MAX acts as a sentinel for temporarily "locking" the
        // ability to upgrade weak pointers or downgrade strong ones; this is used
        // to avoid races in `make_mut` and `get_mut`.
        weak: atomic::AtomicUsize,

        data: T,
    }

    unsafe impl<T: ?Sized + Sync + Send> Send for ArcInner<T> {}
    unsafe impl<T: ?Sized + Sync + Send> Sync for ArcInner<T> {}

    impl<T> Arc<T> {
        /// Constructs a new `Arc<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(data: T) -> Arc<T> {
            // Start the weak pointer count as 1 which is the weak pointer that's
            // held by all the strong pointers (kinda), see std/rc.rs for more info
            let x: Box<_> = box ArcInner {
                strong: atomic::AtomicUsize::new(1),
                weak: atomic::AtomicUsize::new(1),
                data,
            };
            Self::from_inner(Box::into_raw_non_null(x))
        }

        /// Constructs a new `Arc` with uninitialized contents.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(new_uninit)]
        /// #![feature(get_mut_unchecked)]
        ///
        /// use std::sync::Arc;
        ///
        /// let mut five = Arc::<u32>::new_uninit();
        ///
        /// let five = unsafe {
        ///     // Deferred initialization:
        ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
        ///
        ///     five.assume_init()
        /// };
        ///
        /// assert_eq!(*five, 5)
        /// ```
        #[unstable(feature = "new_uninit", issue = "63291")]
        pub fn new_uninit() -> Arc<mem::MaybeUninit<T>> {
            unsafe {
                Arc::from_ptr(Arc::allocate_for_layout(
                    Layout::new::<T>(),
                    |mem| mem as *mut ArcInner<mem::MaybeUninit<T>>,
                ))
            }
        }

        /// Constructs a new `Arc` with uninitialized contents, with the memory
        /// being filled with `0` bytes.
        ///
        /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and incorrect usage
        /// of this method.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(new_uninit)]
        ///
        /// use std::sync::Arc;
        ///
        /// let zero = Arc::<u32>::new_zeroed();
        /// let zero = unsafe { zero.assume_init() };
        ///
        /// assert_eq!(*zero, 0)
        /// ```
        ///
        /// [zeroed]: ../../std/mem/union.MaybeUninit.html#method.zeroed
        #[unstable(feature = "new_uninit", issue = "63291")]
        pub fn new_zeroed() -> Arc<mem::MaybeUninit<T>> {
            unsafe {
                let mut uninit = Self::new_uninit();
                ptr::write_bytes::<T>(Arc::get_mut_unchecked(&mut uninit).as_mut_ptr(), 0, 1);
                uninit
            }
        }

        /// Constructs a new `Pin<Arc<T>>`. If `T` does not implement `Unpin`, then
        /// `data` will be pinned in memory and unable to be moved.
        #[stable(feature = "pin", since = "1.33.0")]
        pub fn pin(data: T) -> Pin<Arc<T>> {
            unsafe { Pin::new_unchecked(Arc::new(data)) }
        }

        /// Returns the inner value, if the `Arc` has exactly one strong reference.
        ///
        /// Otherwise, an [`Err`][result] is returned with the same `Arc` that was
        /// passed in.
        ///
        /// This will succeed even if there are outstanding weak references.
        ///
        /// [result]: ../../std/result/enum.Result.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let x = Arc::new(3);
        /// assert_eq!(Arc::try_unwrap(x), Ok(3));
        ///
        /// let x = Arc::new(4);
        /// let _y = Arc::clone(&x);
        /// assert_eq!(*Arc::try_unwrap(x).unwrap_err(), 4);
        /// ```
        #[inline]
        #[stable(feature = "arc_unique", since = "1.4.0")]
        pub fn try_unwrap(this: Self) -> Result<T, Self> {
            // See `drop` for why all these atomics are like this
            if this.inner().strong.compare_exchange(1, 0, Release, Relaxed).is_err() {
                return Err(this);
            }

            atomic::fence(Acquire);

            unsafe {
                let elem = ptr::read(&this.ptr.as_ref().data);

                // Make a weak pointer to clean up the implicit strong-weak reference
                let _weak = Weak { ptr: this.ptr };
                mem::forget(this);

                Ok(elem)
            }
        }
    }

    impl<T> Arc<[T]> {
        /// Constructs a new reference-counted slice with uninitialized contents.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(new_uninit)]
        /// #![feature(get_mut_unchecked)]
        ///
        /// use std::sync::Arc;
        ///
        /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
        ///
        /// let values = unsafe {
        ///     // Deferred initialization:
        ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
        ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
        ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
        ///
        ///     values.assume_init()
        /// };
        ///
        /// assert_eq!(*values, [1, 2, 3])
        /// ```
        #[unstable(feature = "new_uninit", issue = "63291")]
        pub fn new_uninit_slice(len: usize) -> Arc<[mem::MaybeUninit<T>]> {
            unsafe {
                Arc::from_ptr(Arc::allocate_for_slice(len))
            }
        }
    }

    impl<T> Arc<mem::MaybeUninit<T>> {
        /// Converts to `Arc<T>`.
        ///
        /// # Safety
        ///
        /// As with [`MaybeUninit::assume_init`],
        /// it is up to the caller to guarantee that the inner value
        /// really is in an initialized state.
        /// Calling this when the content is not yet fully initialized
        /// causes immediate undefined behavior.
        ///
        /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(new_uninit)]
        /// #![feature(get_mut_unchecked)]
        ///
        /// use std::sync::Arc;
        ///
        /// let mut five = Arc::<u32>::new_uninit();
        ///
        /// let five = unsafe {
        ///     // Deferred initialization:
        ///     Arc::get_mut_unchecked(&mut five).as_mut_ptr().write(5);
        ///
        ///     five.assume_init()
        /// };
        ///
        /// assert_eq!(*five, 5)
        /// ```
        #[unstable(feature = "new_uninit", issue = "63291")]
        #[inline]
        pub unsafe fn assume_init(self) -> Arc<T> {
            Arc::from_inner(mem::ManuallyDrop::new(self).ptr.cast())
        }
    }

    impl<T> Arc<[mem::MaybeUninit<T>]> {
        /// Converts to `Arc<[T]>`.
        ///
        /// # Safety
        ///
        /// As with [`MaybeUninit::assume_init`],
        /// it is up to the caller to guarantee that the inner value
        /// really is in an initialized state.
        /// Calling this when the content is not yet fully initialized
        /// causes immediate undefined behavior.
        ///
        /// [`MaybeUninit::assume_init`]: ../../std/mem/union.MaybeUninit.html#method.assume_init
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(new_uninit)]
        /// #![feature(get_mut_unchecked)]
        ///
        /// use std::sync::Arc;
        ///
        /// let mut values = Arc::<[u32]>::new_uninit_slice(3);
        ///
        /// let values = unsafe {
        ///     // Deferred initialization:
        ///     Arc::get_mut_unchecked(&mut values)[0].as_mut_ptr().write(1);
        ///     Arc::get_mut_unchecked(&mut values)[1].as_mut_ptr().write(2);
        ///     Arc::get_mut_unchecked(&mut values)[2].as_mut_ptr().write(3);
        ///
        ///     values.assume_init()
        /// };
        ///
        /// assert_eq!(*values, [1, 2, 3])
        /// ```
        #[unstable(feature = "new_uninit", issue = "63291")]
        #[inline]
        pub unsafe fn assume_init(self) -> Arc<[T]> {
            Arc::from_ptr(mem::ManuallyDrop::new(self).ptr.as_ptr() as _)
        }
    }

    impl<T: ?Sized> Arc<T> {
        /// Consumes the `Arc`, returning the wrapped pointer.
        ///
        /// To avoid a memory leak the pointer must be converted back to an `Arc` using
        /// [`Arc::from_raw`][from_raw].
        ///
        /// [from_raw]: struct.Arc.html#method.from_raw
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let x = Arc::new("hello".to_owned());
        /// let x_ptr = Arc::into_raw(x);
        /// assert_eq!(unsafe { &*x_ptr }, "hello");
        /// ```
        #[stable(feature = "rc_raw", since = "1.17.0")]
        pub fn into_raw(this: Self) -> *const T {
            let ptr: *const T = &*this;
            mem::forget(this);
            ptr
        }

        /// Constructs an `Arc` from a raw pointer.
        ///
        /// The raw pointer must have been previously returned by a call to a
        /// [`Arc::into_raw`][into_raw].
        ///
        /// This function is unsafe because improper use may lead to memory problems. For example, a
        /// double-free may occur if the function is called twice on the same raw pointer.
        ///
        /// [into_raw]: struct.Arc.html#method.into_raw
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let x = Arc::new("hello".to_owned());
        /// let x_ptr = Arc::into_raw(x);
        ///
        /// unsafe {
        ///     // Convert back to an `Arc` to prevent leak.
        ///     let x = Arc::from_raw(x_ptr);
        ///     assert_eq!(&*x, "hello");
        ///
        ///     // Further calls to `Arc::from_raw(x_ptr)` would be memory-unsafe.
        /// }
        ///
        /// // The memory was freed when `x` went out of scope above, so `x_ptr` is now dangling!
        /// ```
        #[stable(feature = "rc_raw", since = "1.17.0")]
        pub unsafe fn from_raw(ptr: *const T) -> Self {
            let offset = data_offset(ptr);

            // Reverse the offset to find the original ArcInner.
            let fake_ptr = ptr as *mut ArcInner<T>;
            let arc_ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));

            Self::from_ptr(arc_ptr)
        }

        /// Consumes the `Arc`, returning the wrapped pointer as `NonNull<T>`.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(rc_into_raw_non_null)]
        ///
        /// use std::sync::Arc;
        ///
        /// let x = Arc::new("hello".to_owned());
        /// let ptr = Arc::into_raw_non_null(x);
        /// let deref = unsafe { ptr.as_ref() };
        /// assert_eq!(deref, "hello");
        /// ```
        #[unstable(feature = "rc_into_raw_non_null", issue = "47336")]
        #[inline]
        pub fn into_raw_non_null(this: Self) -> NonNull<T> {
            // safe because Arc guarantees its pointer is non-null
            unsafe { NonNull::new_unchecked(Arc::into_raw(this) as *mut _) }
        }

        /// Creates a new [`Weak`][weak] pointer to this allocation.
        ///
        /// [weak]: struct.Weak.html
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// let weak_five = Arc::downgrade(&five);
        /// ```
        #[stable(feature = "arc_weak", since = "1.4.0")]
        pub fn downgrade(this: &Self) -> Weak<T> {
            // This Relaxed is OK because we're checking the value in the CAS
            // below.
            let mut cur = this.inner().weak.load(Relaxed);

            loop {
                // check if the weak counter is currently "locked"; if so, spin.
                if cur == usize::MAX {
                    cur = this.inner().weak.load(Relaxed);
                    continue;
                }

                // NOTE: this code currently ignores the possibility of overflow
                // into usize::MAX; in general both Rc and Arc need to be adjusted
                // to deal with overflow.

                // Unlike with Clone(), we need this to be an Acquire read to
                // synchronize with the write coming from `is_unique`, so that the
                // events prior to that write happen before this read.
                match this.inner().weak.compare_exchange_weak(cur, cur + 1, Acquire, Relaxed) {
                    Ok(_) => {
                        // Make sure we do not create a dangling Weak
                        debug_assert!(!is_dangling(this.ptr));
                        return Weak { ptr: this.ptr };
                    }
                    Err(old) => cur = old,
                }
            }
        }

        /// Gets the number of [`Weak`][weak] pointers to this allocation.
        ///
        /// [weak]: struct.Weak.html
        ///
        /// # Safety
        ///
        /// This method by itself is safe, but using it correctly requires extra care.
        /// Another thread can change the weak count at any time,
        /// including potentially between calling this method and acting on the result.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        /// let _weak_five = Arc::downgrade(&five);
        ///
        /// // This assertion is deterministic because we haven't shared
        /// // the `Arc` or `Weak` between threads.
        /// assert_eq!(1, Arc::weak_count(&five));
        /// ```
        #[inline]
        #[stable(feature = "arc_counts", since = "1.15.0")]
        pub fn weak_count(this: &Self) -> usize {
            let cnt = this.inner().weak.load(SeqCst);
            // If the weak count is currently locked, the value of the
            // count was 0 just before taking the lock.
            if cnt == usize::MAX { 0 } else { cnt - 1 }
        }

        /// Gets the number of strong (`Arc`) pointers to this allocation.
        ///
        /// # Safety
        ///
        /// This method by itself is safe, but using it correctly requires extra care.
        /// Another thread can change the strong count at any time,
        /// including potentially between calling this method and acting on the result.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        /// let _also_five = Arc::clone(&five);
        ///
        /// // This assertion is deterministic because we haven't shared
        /// // the `Arc` between threads.
        /// assert_eq!(2, Arc::strong_count(&five));
        /// ```
        #[inline]
        #[stable(feature = "arc_counts", since = "1.15.0")]
        pub fn strong_count(this: &Self) -> usize {
            this.inner().strong.load(SeqCst)
        }

        #[inline]
        fn inner(&self) -> &ArcInner<T> {
            // This unsafety is ok because while this arc is alive we're guaranteed
            // that the inner pointer is valid. Furthermore, we know that the
            // `ArcInner` structure itself is `Sync` because the inner data is
            // `Sync` as well, so we're ok loaning out an immutable pointer to these
            // contents.
            unsafe { self.ptr.as_ref() }
        }

        // Non-inlined part of `drop`.
        #[inline(never)]
        unsafe fn drop_slow(&mut self) {
            // Destroy the data at this time, even though we may not free the box
            // allocation itself (there may still be weak pointers lying around).
            ptr::drop_in_place(&mut self.ptr.as_mut().data);

            if self.inner().weak.fetch_sub(1, Release) == 1 {
                atomic::fence(Acquire);
                Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
            }
        }

        #[inline]
        #[stable(feature = "ptr_eq", since = "1.17.0")]
        /// Returns `true` if the two `Arc`s point to the same allocation
        /// (in a vein similar to [`ptr::eq`]).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        /// let same_five = Arc::clone(&five);
        /// let other_five = Arc::new(5);
        ///
        /// assert!(Arc::ptr_eq(&five, &same_five));
        /// assert!(!Arc::ptr_eq(&five, &other_five));
        /// ```
        ///
        /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
        pub fn ptr_eq(this: &Self, other: &Self) -> bool {
            this.ptr.as_ptr() == other.ptr.as_ptr()
        }
    }

    impl<T: ?Sized> Arc<T> {
        /// Allocates an `ArcInner<T>` with sufficient space for
        /// a possibly-unsized inner value where the value has the layout provided.
        ///
        /// The function `mem_to_arcinner` is called with the data pointer
        /// and must return back a (potentially fat)-pointer for the `ArcInner<T>`.
        unsafe fn allocate_for_layout(
            value_layout: Layout,
            mem_to_arcinner: impl FnOnce(*mut u8) -> *mut ArcInner<T>
        ) -> *mut ArcInner<T> {
            // Calculate layout using the given value layout.
            // Previously, layout was calculated on the expression
            // `&*(ptr as *const ArcInner<T>)`, but this created a misaligned
            // reference (see #54908).
            let layout = Layout::new::<ArcInner<()>>()
                .extend(value_layout).unwrap().0
                .pad_to_align();

            let mem = Global.alloc(layout)
                .unwrap_or_else(|_| handle_alloc_error(layout));

            // Initialize the ArcInner
            let inner = mem_to_arcinner(mem.as_ptr());
            debug_assert_eq!(Layout::for_value(&*inner), layout);

            ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
            ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));

            inner
        }

        /// Allocates an `ArcInner<T>` with sufficient space for an unsized inner value.
        unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
            // Allocate for the `ArcInner<T>` using the given value.
            Self::allocate_for_layout(
                Layout::for_value(&*ptr),
                |mem| set_data_ptr(ptr as *mut T, mem) as *mut ArcInner<T>,
            )
        }

        fn from_box(v: Box<T>) -> Arc<T> {
            unsafe {
                let box_unique = Box::into_unique(v);
                let bptr = box_unique.as_ptr();

                let value_size = size_of_val(&*bptr);
                let ptr = Self::allocate_for_ptr(bptr);

                // Copy value as bytes
                ptr::copy_nonoverlapping(
                    bptr as *const T as *const u8,
                    &mut (*ptr).data as *mut _ as *mut u8,
                    value_size);

                // Free the allocation without dropping its contents
                box_free(box_unique);

                Self::from_ptr(ptr)
            }
        }
    }

    impl<T> Arc<[T]> {
        /// Allocates an `ArcInner<[T]>` with the given length.
        unsafe fn allocate_for_slice(len: usize) -> *mut ArcInner<[T]> {
            Self::allocate_for_layout(
                Layout::array::<T>(len).unwrap(),
                |mem| ptr::slice_from_raw_parts_mut(mem as *mut T, len) as *mut ArcInner<[T]>,
            )
        }
    }

    /// Sets the data pointer of a `?Sized` raw pointer.
    ///
    /// For a slice/trait object, this sets the `data` field and leaves the rest
    /// unchanged. For a sized raw pointer, this simply sets the pointer.
    unsafe fn set_data_ptr<T: ?Sized, U>(mut ptr: *mut T, data: *mut U) -> *mut T {
        ptr::write(&mut ptr as *mut _ as *mut *mut u8, data as *mut u8);
        ptr
    }

    impl<T> Arc<[T]> {
        /// Copy elements from slice into newly allocated Arc<[T]>
        ///
        /// Unsafe because the caller must either take ownership or bind `T: Copy`.
        unsafe fn copy_from_slice(v: &[T]) -> Arc<[T]> {
            let ptr = Self::allocate_for_slice(v.len());

            ptr::copy_nonoverlapping(
                v.as_ptr(),
                &mut (*ptr).data as *mut [T] as *mut T,
                v.len());

            Self::from_ptr(ptr)
        }

        /// Constructs an `Arc<[T]>` from an iterator known to be of a certain size.
        ///
        /// Behavior is undefined should the size be wrong.
        unsafe fn from_iter_exact(iter: impl iter::Iterator<Item = T>, len: usize) -> Arc<[T]> {
            // Panic guard while cloning T elements.
            // In the event of a panic, elements that have been written
            // into the new ArcInner will be dropped, then the memory freed.
            struct Guard<T> {
                mem: NonNull<u8>,
                elems: *mut T,
                layout: Layout,
                n_elems: usize,
            }

            impl<T> Drop for Guard<T> {
                fn drop(&mut self) {
                    unsafe {
                        let slice = from_raw_parts_mut(self.elems, self.n_elems);
                        ptr::drop_in_place(slice);

                        Global.dealloc(self.mem.cast(), self.layout);
                    }
                }
            }

            let ptr = Self::allocate_for_slice(len);

            let mem = ptr as *mut _ as *mut u8;
            let layout = Layout::for_value(&*ptr);

            // Pointer to first element
            let elems = &mut (*ptr).data as *mut [T] as *mut T;

            let mut guard = Guard {
                mem: NonNull::new_unchecked(mem),
                elems,
                layout,
                n_elems: 0,
            };

            for (i, item) in iter.enumerate() {
                ptr::write(elems.add(i), item);
                guard.n_elems += 1;
            }

            // All clear. Forget the guard so it doesn't free the new ArcInner.
            mem::forget(guard);

            Self::from_ptr(ptr)
        }
    }

    /// Specialization trait used for `From<&[T]>`.
    trait ArcFromSlice<T> {
        fn from_slice(slice: &[T]) -> Self;
    }

    impl<T: Clone> ArcFromSlice<T> for Arc<[T]> {
        #[inline]
        default fn from_slice(v: &[T]) -> Self {
            unsafe {
                Self::from_iter_exact(v.iter().cloned(), v.len())
            }
        }
    }

    impl<T: Copy> ArcFromSlice<T> for Arc<[T]> {
        #[inline]
        fn from_slice(v: &[T]) -> Self {
            unsafe { Arc::copy_from_slice(v) }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Clone for Arc<T> {
        /// Makes a clone of the `Arc` pointer.
        ///
        /// This creates another pointer to the same allocation, increasing the
        /// strong reference count.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// let _ = Arc::clone(&five);
        /// ```
        #[inline]
        fn clone(&self) -> Arc<T> {
            // Using a relaxed ordering is alright here, as knowledge of the
            // original reference prevents other threads from erroneously deleting
            // the object.
            //
            // As explained in the [Boost documentation][1], Increasing the
            // reference counter can always be done with memory_order_relaxed: New
            // references to an object can only be formed from an existing
            // reference, and passing an existing reference from one thread to
            // another must already provide any required synchronization.
            //
            // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
            let old_size = self.inner().strong.fetch_add(1, Relaxed);

            // However we need to guard against massive refcounts in case someone
            // is `mem::forget`ing Arcs. If we don't do this the count can overflow
            // and users will use-after free. We racily saturate to `isize::MAX` on
            // the assumption that there aren't ~2 billion threads incrementing
            // the reference count at once. This branch will never be taken in
            // any realistic program.
            //
            // We abort because such a program is incredibly degenerate, and we
            // don't care to support it.
            if old_size > MAX_REFCOUNT {
                unsafe {
                    abort();
                }
            }

            Self::from_inner(self.ptr)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Deref for Arc<T> {
        type Target = T;

        #[inline]
        fn deref(&self) -> &T {
            &self.inner().data
        }
    }

    #[unstable(feature = "receiver_trait", issue = "0")]
    impl<T: ?Sized> Receiver for Arc<T> {}

    impl<T: Clone> Arc<T> {
        /// Makes a mutable reference into the given `Arc`.
        ///
        /// If there are other `Arc` or [`Weak`][weak] pointers to the same allocation,
        /// then `make_mut` will create a new allocation and invoke [`clone`][clone] on the inner value
        /// to ensure unique ownership. This is also referred to as clone-on-write.
        ///
        /// Note that this differs from the behavior of [`Rc::make_mut`] which disassociates
        /// any remaining `Weak` pointers.
        ///
        /// See also [`get_mut`][get_mut], which will fail rather than cloning.
        ///
        /// [weak]: struct.Weak.html
        /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
        /// [get_mut]: struct.Arc.html#method.get_mut
        /// [`Rc::make_mut`]: ../rc/struct.Rc.html#method.make_mut
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let mut data = Arc::new(5);
        ///
        /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
        /// let mut other_data = Arc::clone(&data); // Won't clone inner data
        /// *Arc::make_mut(&mut data) += 1;         // Clones inner data
        /// *Arc::make_mut(&mut data) += 1;         // Won't clone anything
        /// *Arc::make_mut(&mut other_data) *= 2;   // Won't clone anything
        ///
        /// // Now `data` and `other_data` point to different allocations.
        /// assert_eq!(*data, 8);
        /// assert_eq!(*other_data, 12);
        /// ```
        #[inline]
        #[stable(feature = "arc_unique", since = "1.4.0")]
        pub fn make_mut(this: &mut Self) -> &mut T {
            // Note that we hold both a strong reference and a weak reference.
            // Thus, releasing our strong reference only will not, by itself, cause
            // the memory to be deallocated.
            //
            // Use Acquire to ensure that we see any writes to `weak` that happen
            // before release writes (i.e., decrements) to `strong`. Since we hold a
            // weak count, there's no chance the ArcInner itself could be
            // deallocated.
            if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {
                // Another strong pointer exists; clone
                *this = Arc::new((**this).clone());
            } else if this.inner().weak.load(Relaxed) != 1 {
                // Relaxed suffices in the above because this is fundamentally an
                // optimization: we are always racing with weak pointers being
                // dropped. Worst case, we end up allocated a new Arc unnecessarily.

                // We removed the last strong ref, but there are additional weak
                // refs remaining. We'll move the contents to a new Arc, and
                // invalidate the other weak refs.

                // Note that it is not possible for the read of `weak` to yield
                // usize::MAX (i.e., locked), since the weak count can only be
                // locked by a thread with a strong reference.

                // Materialize our own implicit weak pointer, so that it can clean
                // up the ArcInner as needed.
                let weak = Weak { ptr: this.ptr };

                // mark the data itself as already deallocated
                unsafe {
                    // there is no data race in the implicit write caused by `read`
                    // here (due to zeroing) because data is no longer accessed by
                    // other threads (due to there being no more strong refs at this
                    // point).
                    let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
                    mem::swap(this, &mut swap);
                    mem::forget(swap);
                }
            } else {
                // We were the sole reference of either kind; bump back up the
                // strong ref count.
                this.inner().strong.store(1, Release);
            }

            // As with `get_mut()`, the unsafety is ok because our reference was
            // either unique to begin with, or became one upon cloning the contents.
            unsafe {
                &mut this.ptr.as_mut().data
            }
        }
    }

    impl<T: ?Sized> Arc<T> {
        /// Returns a mutable reference into the given `Arc`, if there are
        /// no other `Arc` or [`Weak`][weak] pointers to the same allocation.
        ///
        /// Returns [`None`][option] otherwise, because it is not safe to
        /// mutate a shared value.
        ///
        /// See also [`make_mut`][make_mut], which will [`clone`][clone]
        /// the inner value when there are other pointers.
        ///
        /// [weak]: struct.Weak.html
        /// [option]: ../../std/option/enum.Option.html
        /// [make_mut]: struct.Arc.html#method.make_mut
        /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let mut x = Arc::new(3);
        /// *Arc::get_mut(&mut x).unwrap() = 4;
        /// assert_eq!(*x, 4);
        ///
        /// let _y = Arc::clone(&x);
        /// assert!(Arc::get_mut(&mut x).is_none());
        /// ```
        #[inline]
        #[stable(feature = "arc_unique", since = "1.4.0")]
        pub fn get_mut(this: &mut Self) -> Option<&mut T> {
            if this.is_unique() {
                // This unsafety is ok because we're guaranteed that the pointer
                // returned is the *only* pointer that will ever be returned to T. Our
                // reference count is guaranteed to be 1 at this point, and we required
                // the Arc itself to be `mut`, so we're returning the only possible
                // reference to the inner data.
                unsafe {
                    Some(Arc::get_mut_unchecked(this))
                }
            } else {
                None
            }
        }

        /// Returns a mutable reference into the given `Arc`,
        /// without any check.
        ///
        /// See also [`get_mut`], which is safe and does appropriate checks.
        ///
        /// [`get_mut`]: struct.Arc.html#method.get_mut
        ///
        /// # Safety
        ///
        /// Any other `Arc` or [`Weak`] pointers to the same allocation must not be dereferenced
        /// for the duration of the returned borrow.
        /// This is trivially the case if no such pointers exist,
        /// for example immediately after `Arc::new`.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(get_mut_unchecked)]
        ///
        /// use std::sync::Arc;
        ///
        /// let mut x = Arc::new(String::new());
        /// unsafe {
        ///     Arc::get_mut_unchecked(&mut x).push_str("foo")
        /// }
        /// assert_eq!(*x, "foo");
        /// ```
        #[inline]
        #[unstable(feature = "get_mut_unchecked", issue = "63292")]
        pub unsafe fn get_mut_unchecked(this: &mut Self) -> &mut T {
            &mut this.ptr.as_mut().data
        }

        /// Determine whether this is the unique reference (including weak refs) to
        /// the underlying data.
        ///
        /// Note that this requires locking the weak ref count.
        fn is_unique(&mut self) -> bool {
            // lock the weak pointer count if we appear to be the sole weak pointer
            // holder.
            //
            // The acquire label here ensures a happens-before relationship with any
            // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements
            // of the `weak` count (via `Weak::drop`, which uses release).  If the upgraded
            // weak ref was never dropped, the CAS here will fail so we do not care to synchronize.
            if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
                // This needs to be an `Acquire` to synchronize with the decrement of the `strong`
                // counter in `drop` -- the only access that happens when any but the last reference
                // is being dropped.
                let unique = self.inner().strong.load(Acquire) == 1;

                // The release write here synchronizes with a read in `downgrade`,
                // effectively preventing the above read of `strong` from happening
                // after the write.
                self.inner().weak.store(1, Release); // release the lock
                unique
            } else {
                false
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
        /// Drops the `Arc`.
        ///
        /// This will decrement the strong reference count. If the strong reference
        /// count reaches zero then the only other references (if any) are
        /// [`Weak`], so we `drop` the inner value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// struct Foo;
        ///
        /// impl Drop for Foo {
        ///     fn drop(&mut self) {
        ///         println!("dropped!");
        ///     }
        /// }
        ///
        /// let foo  = Arc::new(Foo);
        /// let foo2 = Arc::clone(&foo);
        ///
        /// drop(foo);    // Doesn't print anything
        /// drop(foo2);   // Prints "dropped!"
        /// ```
        ///
        /// [`Weak`]: ../../std/sync/struct.Weak.html
        #[inline]
        fn drop(&mut self) {
            // Because `fetch_sub` is already atomic, we do not need to synchronize
            // with other threads unless we are going to delete the object. This
            // same logic applies to the below `fetch_sub` to the `weak` count.
            if self.inner().strong.fetch_sub(1, Release) != 1 {
                return;
            }

            // This fence is needed to prevent reordering of use of the data and
            // deletion of the data.  Because it is marked `Release`, the decreasing
            // of the reference count synchronizes with this `Acquire` fence. This
            // means that use of the data happens before decreasing the reference
            // count, which happens before this fence, which happens before the
            // deletion of the data.
            //
            // As explained in the [Boost documentation][1],
            //
            // > It is important to enforce any possible access to the object in one
            // > thread (through an existing reference) to *happen before* deleting
            // > the object in a different thread. This is achieved by a "release"
            // > operation after dropping a reference (any access to the object
            // > through this reference must obviously happened before), and an
            // > "acquire" operation before deleting the object.
            //
            // In particular, while the contents of an Arc are usually immutable, it's
            // possible to have interior writes to something like a Mutex<T>. Since a
            // Mutex is not acquired when it is deleted, we can't rely on its
            // synchronization logic to make writes in thread A visible to a destructor
            // running in thread B.
            //
            // Also note that the Acquire fence here could probably be replaced with an
            // Acquire load, which could improve performance in highly-contended
            // situations. See [2].
            //
            // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
            // [2]: (https://github.com/rust-lang/rust/pull/41714)
            atomic::fence(Acquire);

            unsafe {
                self.drop_slow();
            }
        }
    }

    impl Arc<dyn Any + Send + Sync> {
        #[inline]
        #[stable(feature = "rc_downcast", since = "1.29.0")]
        /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        /// use std::sync::Arc;
        ///
        /// fn print_if_string(value: Arc<dyn Any + Send + Sync>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// let my_string = "Hello World".to_string();
        /// print_if_string(Arc::new(my_string));
        /// print_if_string(Arc::new(0i8));
        /// ```
        pub fn downcast<T>(self) -> Result<Arc<T>, Self>
        where
            T: Any + Send + Sync + 'static,
        {
            if (*self).is::<T>() {
                let ptr = self.ptr.cast::<ArcInner<T>>();
                mem::forget(self);
                Ok(Arc::from_inner(ptr))
            } else {
                Err(self)
            }
        }
    }

    impl<T> Weak<T> {
        /// Constructs a new `Weak<T>`, without allocating any memory.
        /// Calling [`upgrade`] on the return value always gives [`None`].
        ///
        /// [`upgrade`]: struct.Weak.html#method.upgrade
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Weak;
        ///
        /// let empty: Weak<i64> = Weak::new();
        /// assert!(empty.upgrade().is_none());
        /// ```
        #[stable(feature = "downgraded_weak", since = "1.10.0")]
        pub fn new() -> Weak<T> {
            Weak {
                ptr: NonNull::new(usize::MAX as *mut ArcInner<T>).expect("MAX is not 0"),
            }
        }

        /// Returns a raw pointer to the object `T` pointed to by this `Weak<T>`.
        ///
        /// The pointer is valid only if there are some strong references. The pointer may be dangling
        /// or even [`null`] otherwise.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(weak_into_raw)]
        ///
        /// use std::sync::Arc;
        /// use std::ptr;
        ///
        /// let strong = Arc::new("hello".to_owned());
        /// let weak = Arc::downgrade(&strong);
        /// // Both point to the same object
        /// assert!(ptr::eq(&*strong, weak.as_raw()));
        /// // The strong here keeps it alive, so we can still access the object.
        /// assert_eq!("hello", unsafe { &*weak.as_raw() });
        ///
        /// drop(strong);
        /// // But not any more. We can do weak.as_raw(), but accessing the pointer would lead to
        /// // undefined behaviour.
        /// // assert_eq!("hello", unsafe { &*weak.as_raw() });
        /// ```
        ///
        /// [`null`]: ../../std/ptr/fn.null.html
        #[unstable(feature = "weak_into_raw", issue = "60728")]
        pub fn as_raw(&self) -> *const T {
            match self.inner() {
                None => ptr::null(),
                Some(inner) => {
                    let offset = data_offset_sized::<T>();
                    let ptr = inner as *const ArcInner<T>;
                    // Note: while the pointer we create may already point to dropped value, the
                    // allocation still lives (it must hold the weak point as long as we are alive).
                    // Therefore, the offset is OK to do, it won't get out of the allocation.
                    let ptr = unsafe { (ptr as *const u8).offset(offset) };
                    ptr as *const T
                }
            }
        }

        /// Consumes the `Weak<T>` and turns it into a raw pointer.
        ///
        /// This converts the weak pointer into a raw pointer, preserving the original weak count. It
        /// can be turned back into the `Weak<T>` with [`from_raw`].
        ///
        /// The same restrictions of accessing the target of the pointer as with
        /// [`as_raw`] apply.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(weak_into_raw)]
        ///
        /// use std::sync::{Arc, Weak};
        ///
        /// let strong = Arc::new("hello".to_owned());
        /// let weak = Arc::downgrade(&strong);
        /// let raw = weak.into_raw();
        ///
        /// assert_eq!(1, Arc::weak_count(&strong));
        /// assert_eq!("hello", unsafe { &*raw });
        ///
        /// drop(unsafe { Weak::from_raw(raw) });
        /// assert_eq!(0, Arc::weak_count(&strong));
        /// ```
        ///
        /// [`from_raw`]: struct.Weak.html#method.from_raw
        /// [`as_raw`]: struct.Weak.html#method.as_raw
        #[unstable(feature = "weak_into_raw", issue = "60728")]
        pub fn into_raw(self) -> *const T {
            let result = self.as_raw();
            mem::forget(self);
            result
        }

        /// Converts a raw pointer previously created by [`into_raw`] back into
        /// `Weak<T>`.
        ///
        /// This can be used to safely get a strong reference (by calling [`upgrade`]
        /// later) or to deallocate the weak count by dropping the `Weak<T>`.
        ///
        /// It takes ownership of one weak count (with the exception of pointers created by [`new`],
        /// as these don't have any corresponding weak count).
        ///
        /// # Safety
        ///
        /// The pointer must have originated from the [`into_raw`] (or [`as_raw'], provided there was
        /// a corresponding [`forget`] on the `Weak<T>`) and must still own its potential weak reference
        /// count.
        ///
        /// It is allowed for the strong count to be 0 at the time of calling this, but the weak count
        /// must be non-zero or the pointer must have originated from a dangling `Weak<T>` (one created
        /// by [`new`]).
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(weak_into_raw)]
        ///
        /// use std::sync::{Arc, Weak};
        ///
        /// let strong = Arc::new("hello".to_owned());
        ///
        /// let raw_1 = Arc::downgrade(&strong).into_raw();
        /// let raw_2 = Arc::downgrade(&strong).into_raw();
        ///
        /// assert_eq!(2, Arc::weak_count(&strong));
        ///
        /// assert_eq!("hello", &*unsafe { Weak::from_raw(raw_1) }.upgrade().unwrap());
        /// assert_eq!(1, Arc::weak_count(&strong));
        ///
        /// drop(strong);
        ///
        /// // Decrement the last weak count.
        /// assert!(unsafe { Weak::from_raw(raw_2) }.upgrade().is_none());
        /// ```
        ///
        /// [`as_raw`]: struct.Weak.html#method.as_raw
        /// [`new`]: struct.Weak.html#method.new
        /// [`into_raw`]: struct.Weak.html#method.into_raw
        /// [`upgrade`]: struct.Weak.html#method.upgrade
        /// [`Weak`]: struct.Weak.html
        /// [`Arc`]: struct.Arc.html
        /// [`forget`]: ../../std/mem/fn.forget.html
        #[unstable(feature = "weak_into_raw", issue = "60728")]
        pub unsafe fn from_raw(ptr: *const T) -> Self {
            if ptr.is_null() {
                Self::new()
            } else {
                // See Arc::from_raw for details
                let offset = data_offset(ptr);
                let fake_ptr = ptr as *mut ArcInner<T>;
                let ptr = set_data_ptr(fake_ptr, (ptr as *mut u8).offset(-offset));
                Weak {
                    ptr: NonNull::new(ptr).expect("Invalid pointer passed to from_raw"),
                }
            }
        }
    }

    impl<T: ?Sized> Weak<T> {
        /// Attempts to upgrade the `Weak` pointer to an [`Arc`], delaying
        /// dropping of the inner value if successful.
        ///
        /// Returns [`None`] if the inner value has since been dropped.
        ///
        /// [`Arc`]: struct.Arc.html
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// let weak_five = Arc::downgrade(&five);
        ///
        /// let strong_five: Option<Arc<_>> = weak_five.upgrade();
        /// assert!(strong_five.is_some());
        ///
        /// // Destroy all strong pointers.
        /// drop(strong_five);
        /// drop(five);
        ///
        /// assert!(weak_five.upgrade().is_none());
        /// ```
        #[stable(feature = "arc_weak", since = "1.4.0")]
        pub fn upgrade(&self) -> Option<Arc<T>> {
            // We use a CAS loop to increment the strong count instead of a
            // fetch_add because once the count hits 0 it must never be above 0.
            let inner = self.inner()?;

            // Relaxed load because any write of 0 that we can observe
            // leaves the field in a permanently zero state (so a
            // "stale" read of 0 is fine), and any other value is
            // confirmed via the CAS below.
            let mut n = inner.strong.load(Relaxed);

            loop {
                if n == 0 {
                    return None;
                }

                // See comments in `Arc::clone` for why we do this (for `mem::forget`).
                if n > MAX_REFCOUNT {
                    unsafe {
                        abort();
                    }
                }

                // Relaxed is valid for the same reason it is on Arc's Clone impl
                match inner.strong.compare_exchange_weak(n, n + 1, Relaxed, Relaxed) {
                    Ok(_) => return Some(Arc::from_inner(self.ptr)), // null checked above
                    Err(old) => n = old,
                }
            }
        }

        /// Gets the number of strong (`Arc`) pointers pointing to this allocation.
        ///
        /// If `self` was created using [`Weak::new`], this will return 0.
        ///
        /// [`Weak::new`]: #method.new
        #[unstable(feature = "weak_counts", issue = "57977")]
        pub fn strong_count(&self) -> usize {
            if let Some(inner) = self.inner() {
                inner.strong.load(SeqCst)
            } else {
                0
            }
        }

        /// Gets an approximation of the number of `Weak` pointers pointing to this
        /// allocation.
        ///
        /// If `self` was created using [`Weak::new`], this will return 0. If not,
        /// the returned value is at least 1, since `self` still points to the
        /// allocation.
        ///
        /// # Accuracy
        ///
        /// Due to implementation details, the returned value can be off by 1 in
        /// either direction when other threads are manipulating any `Arc`s or
        /// `Weak`s pointing to the same allocation.
        ///
        /// [`Weak::new`]: #method.new
        #[unstable(feature = "weak_counts", issue = "57977")]
        pub fn weak_count(&self) -> Option<usize> {
            // Due to the implicit weak pointer added when any strong pointers are
            // around, we cannot implement `weak_count` correctly since it
            // necessarily requires accessing the strong count and weak count in an
            // unsynchronized fashion. So this version is a bit racy.
            self.inner().map(|inner| {
                let strong = inner.strong.load(SeqCst);
                let weak = inner.weak.load(SeqCst);
                if strong == 0 {
                    // If the last `Arc` has *just* been dropped, it might not yet
                    // have removed the implicit weak count, so the value we get
                    // here might be 1 too high.
                    weak
                } else {
                    // As long as there's still at least 1 `Arc` around, subtract
                    // the implicit weak pointer.
                    // Note that the last `Arc` might get dropped between the 2
                    // loads we do above, removing the implicit weak pointer. This
                    // means that the value might be 1 too low here. In order to not
                    // return 0 here (which would happen if we're the only weak
                    // pointer), we guard against that specifically.
                    cmp::max(1, weak - 1)
                }
            })
        }

        /// Returns `None` when the pointer is dangling and there is no allocated `ArcInner`,
        /// (i.e., when this `Weak` was created by `Weak::new`).
        #[inline]
        fn inner(&self) -> Option<&ArcInner<T>> {
            if is_dangling(self.ptr) {
                None
            } else {
                Some(unsafe { self.ptr.as_ref() })
            }
        }

        /// Returns `true` if the two `Weak`s point to the same allocation (similar to
        /// [`ptr::eq`]), or if both don't point to any allocation
        /// (because they were created with `Weak::new()`).
        ///
        /// # Notes
        ///
        /// Since this compares pointers it means that `Weak::new()` will equal each
        /// other, even though they don't point to any allocation.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let first_rc = Arc::new(5);
        /// let first = Arc::downgrade(&first_rc);
        /// let second = Arc::downgrade(&first_rc);
        ///
        /// assert!(first.ptr_eq(&second));
        ///
        /// let third_rc = Arc::new(5);
        /// let third = Arc::downgrade(&third_rc);
        ///
        /// assert!(!first.ptr_eq(&third));
        /// ```
        ///
        /// Comparing `Weak::new`.
        ///
        /// ```
        /// use std::sync::{Arc, Weak};
        ///
        /// let first = Weak::new();
        /// let second = Weak::new();
        /// assert!(first.ptr_eq(&second));
        ///
        /// let third_rc = Arc::new(());
        /// let third = Arc::downgrade(&third_rc);
        /// assert!(!first.ptr_eq(&third));
        /// ```
        ///
        /// [`ptr::eq`]: ../../std/ptr/fn.eq.html
        #[inline]
        #[stable(feature = "weak_ptr_eq", since = "1.39.0")]
        pub fn ptr_eq(&self, other: &Self) -> bool {
            self.ptr.as_ptr() == other.ptr.as_ptr()
        }
    }

    #[stable(feature = "arc_weak", since = "1.4.0")]
    impl<T: ?Sized> Clone for Weak<T> {
        /// Makes a clone of the `Weak` pointer that points to the same allocation.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::{Arc, Weak};
        ///
        /// let weak_five = Arc::downgrade(&Arc::new(5));
        ///
        /// let _ = Weak::clone(&weak_five);
        /// ```
        #[inline]
        fn clone(&self) -> Weak<T> {
            let inner = if let Some(inner) = self.inner() {
                inner
            } else {
                return Weak { ptr: self.ptr };
            };
            // See comments in Arc::clone() for why this is relaxed.  This can use a
            // fetch_add (ignoring the lock) because the weak count is only locked
            // where are *no other* weak pointers in existence. (So we can't be
            // running this code in that case).
            let old_size = inner.weak.fetch_add(1, Relaxed);

            // See comments in Arc::clone() for why we do this (for mem::forget).
            if old_size > MAX_REFCOUNT {
                unsafe {
                    abort();
                }
            }

            Weak { ptr: self.ptr }
        }
    }

    #[stable(feature = "downgraded_weak", since = "1.10.0")]
    impl<T> Default for Weak<T> {
        /// Constructs a new `Weak<T>`, without allocating memory.
        /// Calling [`upgrade`] on the return value always
        /// gives [`None`].
        ///
        /// [`None`]: ../../std/option/enum.Option.html#variant.None
        /// [`upgrade`]: ../../std/sync/struct.Weak.html#method.upgrade
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Weak;
        ///
        /// let empty: Weak<i64> = Default::default();
        /// assert!(empty.upgrade().is_none());
        /// ```
        fn default() -> Weak<T> {
            Weak::new()
        }
    }

    #[stable(feature = "arc_weak", since = "1.4.0")]
    impl<T: ?Sized> Drop for Weak<T> {
        /// Drops the `Weak` pointer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::{Arc, Weak};
        ///
        /// struct Foo;
        ///
        /// impl Drop for Foo {
        ///     fn drop(&mut self) {
        ///         println!("dropped!");
        ///     }
        /// }
        ///
        /// let foo = Arc::new(Foo);
        /// let weak_foo = Arc::downgrade(&foo);
        /// let other_weak_foo = Weak::clone(&weak_foo);
        ///
        /// drop(weak_foo);   // Doesn't print anything
        /// drop(foo);        // Prints "dropped!"
        ///
        /// assert!(other_weak_foo.upgrade().is_none());
        /// ```
        fn drop(&mut self) {
            // If we find out that we were the last weak pointer, then its time to
            // deallocate the data entirely. See the discussion in Arc::drop() about
            // the memory orderings
            //
            // It's not necessary to check for the locked state here, because the
            // weak count can only be locked if there was precisely one weak ref,
            // meaning that drop could only subsequently run ON that remaining weak
            // ref, which can only happen after the lock is released.
            let inner = if let Some(inner) = self.inner() {
                inner
            } else {
                return
            };

            if inner.weak.fetch_sub(1, Release) == 1 {
                atomic::fence(Acquire);
                unsafe {
                    Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
                }
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    trait ArcEqIdent<T: ?Sized + PartialEq> {
        fn eq(&self, other: &Arc<T>) -> bool;
        fn ne(&self, other: &Arc<T>) -> bool;
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialEq> ArcEqIdent<T> for Arc<T> {
        #[inline]
        default fn eq(&self, other: &Arc<T>) -> bool {
            **self == **other
        }
        #[inline]
        default fn ne(&self, other: &Arc<T>) -> bool {
            **self != **other
        }
    }

    /// We're doing this specialization here, and not as a more general optimization on `&T`, because it
    /// would otherwise add a cost to all equality checks on refs. We assume that `Arc`s are used to
    /// store large values, that are slow to clone, but also heavy to check for equality, causing this
    /// cost to pay off more easily. It's also more likely to have two `Arc` clones, that point to
    /// the same value, than two `&T`s.
    ///
    /// We can only do this when `T: Eq` as a `PartialEq` might be deliberately irreflexive.
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> ArcEqIdent<T> for Arc<T> {
        #[inline]
        fn eq(&self, other: &Arc<T>) -> bool {
            Arc::ptr_eq(self, other) || **self == **other
        }

        #[inline]
        fn ne(&self, other: &Arc<T>) -> bool {
            !Arc::ptr_eq(self, other) && **self != **other
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
        /// Equality for two `Arc`s.
        ///
        /// Two `Arc`s are equal if their inner values are equal, even if they are
        /// stored in different allocation.
        ///
        /// If `T` also implements `Eq` (implying reflexivity of equality),
        /// two `Arc`s that point to the same allocation are always equal.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five == Arc::new(5));
        /// ```
        #[inline]
        fn eq(&self, other: &Arc<T>) -> bool {
            ArcEqIdent::eq(self, other)
        }

        /// Inequality for two `Arc`s.
        ///
        /// Two `Arc`s are unequal if their inner values are unequal.
        ///
        /// If `T` also implements `Eq` (implying reflexivity of equality),
        /// two `Arc`s that point to the same value are never unequal.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five != Arc::new(6));
        /// ```
        #[inline]
        fn ne(&self, other: &Arc<T>) -> bool {
            ArcEqIdent::ne(self, other)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + PartialOrd> PartialOrd for Arc<T> {
        /// Partial comparison for two `Arc`s.
        ///
        /// The two are compared by calling `partial_cmp()` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        /// use std::cmp::Ordering;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Arc::new(6)));
        /// ```
        fn partial_cmp(&self, other: &Arc<T>) -> Option<Ordering> {
            (**self).partial_cmp(&**other)
        }

        /// Less-than comparison for two `Arc`s.
        ///
        /// The two are compared by calling `<` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five < Arc::new(6));
        /// ```
        fn lt(&self, other: &Arc<T>) -> bool {
            *(*self) < *(*other)
        }

        /// 'Less than or equal to' comparison for two `Arc`s.
        ///
        /// The two are compared by calling `<=` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five <= Arc::new(5));
        /// ```
        fn le(&self, other: &Arc<T>) -> bool {
            *(*self) <= *(*other)
        }

        /// Greater-than comparison for two `Arc`s.
        ///
        /// The two are compared by calling `>` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five > Arc::new(4));
        /// ```
        fn gt(&self, other: &Arc<T>) -> bool {
            *(*self) > *(*other)
        }

        /// 'Greater than or equal to' comparison for two `Arc`s.
        ///
        /// The two are compared by calling `>=` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert!(five >= Arc::new(5));
        /// ```
        fn ge(&self, other: &Arc<T>) -> bool {
            *(*self) >= *(*other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Ord> Ord for Arc<T> {
        /// Comparison for two `Arc`s.
        ///
        /// The two are compared by calling `cmp()` on their inner values.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        /// use std::cmp::Ordering;
        ///
        /// let five = Arc::new(5);
        ///
        /// assert_eq!(Ordering::Less, five.cmp(&Arc::new(6)));
        /// ```
        fn cmp(&self, other: &Arc<T>) -> Ordering {
            (**self).cmp(&**other)
        }
    }
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> Eq for Arc<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + fmt::Display> fmt::Display for Arc<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + fmt::Debug> fmt::Debug for Arc<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Debug::fmt(&**self, f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> fmt::Pointer for Arc<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Pointer::fmt(&(&**self as *const T), f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Default> Default for Arc<T> {
        /// Creates a new `Arc<T>`, with the `Default` value for `T`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::sync::Arc;
        ///
        /// let x: Arc<i32> = Default::default();
        /// assert_eq!(*x, 0);
        /// ```
        fn default() -> Arc<T> {
            Arc::new(Default::default())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Hash> Hash for Arc<T> {
        fn hash<H: Hasher>(&self, state: &mut H) {
            (**self).hash(state)
        }
    }

    #[stable(feature = "from_for_ptrs", since = "1.6.0")]
    impl<T> From<T> for Arc<T> {
        fn from(t: T) -> Self {
            Arc::new(t)
        }
    }

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl<T: Clone> From<&[T]> for Arc<[T]> {
        #[inline]
        fn from(v: &[T]) -> Arc<[T]> {
            <Self as ArcFromSlice<T>>::from_slice(v)
        }
    }

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl From<&str> for Arc<str> {
        #[inline]
        fn from(v: &str) -> Arc<str> {
            let arc = Arc::<[u8]>::from(v.as_bytes());
            unsafe { Arc::from_raw(Arc::into_raw(arc) as *const str) }
        }
    }

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl From<String> for Arc<str> {
        #[inline]
        fn from(v: String) -> Arc<str> {
            Arc::from(&v[..])
        }
    }

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl<T: ?Sized> From<Box<T>> for Arc<T> {
        #[inline]
        fn from(v: Box<T>) -> Arc<T> {
            Arc::from_box(v)
        }
    }

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl<T> From<Vec<T>> for Arc<[T]> {
        #[inline]
        fn from(mut v: Vec<T>) -> Arc<[T]> {
            unsafe {
                let arc = Arc::copy_from_slice(&v);

                // Allow the Vec to free its memory, but not destroy its contents
                v.set_len(0);

                arc
            }
        }
    }

    #[unstable(feature = "boxed_slice_try_from", issue = "0")]
    impl<T, const N: usize> TryFrom<Arc<[T]>> for Arc<[T; N]>
    where
        [T; N]: LengthAtMost32,
    {
        type Error = Arc<[T]>;

        fn try_from(boxed_slice: Arc<[T]>) -> Result<Self, Self::Error> {
            if boxed_slice.len() == N {
                Ok(unsafe { Arc::from_raw(Arc::into_raw(boxed_slice) as *mut [T; N]) })
            } else {
                Err(boxed_slice)
            }
        }
    }

    #[stable(feature = "shared_from_iter", since = "1.37.0")]
    impl<T> iter::FromIterator<T> for Arc<[T]> {
        /// Takes each element in the `Iterator` and collects it into an `Arc<[T]>`.
        ///
        /// # Performance characteristics
        ///
        /// ## The general case
        ///
        /// In the general case, collecting into `Arc<[T]>` is done by first
        /// collecting into a `Vec<T>`. That is, when writing the following:
        ///
        /// ```rust
        /// # use std::sync::Arc;
        /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0).collect();
        /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
        /// ```
        ///
        /// this behaves as if we wrote:
        ///
        /// ```rust
        /// # use std::sync::Arc;
        /// let evens: Arc<[u8]> = (0..10).filter(|&x| x % 2 == 0)
        ///     .collect::<Vec<_>>() // The first set of allocations happens here.
        ///     .into(); // A second allocation for `Arc<[T]>` happens here.
        /// # assert_eq!(&*evens, &[0, 2, 4, 6, 8]);
        /// ```
        ///
        /// This will allocate as many times as needed for constructing the `Vec<T>`
        /// and then it will allocate once for turning the `Vec<T>` into the `Arc<[T]>`.
        ///
        /// ## Iterators of known length
        ///
        /// When your `Iterator` implements `TrustedLen` and is of an exact size,
        /// a single allocation will be made for the `Arc<[T]>`. For example:
        ///
        /// ```rust
        /// # use std::sync::Arc;
        /// let evens: Arc<[u8]> = (0..10).collect(); // Just a single allocation happens here.
        /// # assert_eq!(&*evens, &*(0..10).collect::<Vec<_>>());
        /// ```
        fn from_iter<I: iter::IntoIterator<Item = T>>(iter: I) -> Self {
            ArcFromIter::from_iter(iter.into_iter())
        }
    }

    /// Specialization trait used for collecting into `Arc<[T]>`.
    trait ArcFromIter<T, I> {
        fn from_iter(iter: I) -> Self;
    }

    impl<T, I: Iterator<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
        default fn from_iter(iter: I) -> Self {
            iter.collect::<Vec<T>>().into()
        }
    }

    impl<T, I: iter::TrustedLen<Item = T>> ArcFromIter<T, I> for Arc<[T]> {
        default fn from_iter(iter: I) -> Self {
            // This is the case for a `TrustedLen` iterator.
            let (low, high) = iter.size_hint();
            if let Some(high) = high {
                debug_assert_eq!(
                    low, high,
                    "TrustedLen iterator's size hint is not exact: {:?}",
                    (low, high)
                );

                unsafe {
                    // SAFETY: We need to ensure that the iterator has an exact length and we have.
                    Arc::from_iter_exact(iter, low)
                }
            } else {
                // Fall back to normal implementation.
                iter.collect::<Vec<T>>().into()
            }
        }
    }

    impl<'a, T: 'a + Clone> ArcFromIter<&'a T, slice::Iter<'a, T>> for Arc<[T]> {
        fn from_iter(iter: slice::Iter<'a, T>) -> Self {
            // Delegate to `impl<T: Clone> From<&[T]> for Arc<[T]>`.
            //
            // In the case that `T: Copy`, we get to use `ptr::copy_nonoverlapping`
            // which is even more performant.
            //
            // In the fall-back case we have `T: Clone`. This is still better
            // than the `TrustedLen` implementation as slices have a known length
            // and so we get to avoid calling `size_hint` and avoid the branching.
            iter.as_slice().into()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> borrow::Borrow<T> for Arc<T> {
        fn borrow(&self) -> &T {
            &**self
        }
    }

    #[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
    impl<T: ?Sized> AsRef<T> for Arc<T> {
        fn as_ref(&self) -> &T {
            &**self
        }
    }

    #[stable(feature = "pin", since = "1.33.0")]
    impl<T: ?Sized> Unpin for Arc<T> { }

    /// Computes the offset of the data field within `ArcInner`.
    unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
        // Align the unsized value to the end of the `ArcInner`.
        // Because it is `?Sized`, it will always be the last field in memory.
        data_offset_align(align_of_val(&*ptr))
    }

    /// Computes the offset of the data field within `ArcInner`.
    ///
    /// Unlike [`data_offset`], this doesn't need the pointer, but it works only on `T: Sized`.
    fn data_offset_sized<T>() -> isize {
        data_offset_align(align_of::<T>())
    }

    #[inline]
    fn data_offset_align(align: usize) -> isize {
        let layout = Layout::new::<ArcInner<()>>();
        (layout.size() + layout.padding_needed_for(align)) as isize
    }