Rename alloc::arc to alloc::sync, to match std::sync

1 files changed, 1936 insertions, 0 deletions

diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
new file mode 100644
index 00000000000..2abd9c85c57
--- /dev/null
+++ b/src/liballoc/sync.rs
@@ -0,0 +1,1936 @@
+// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![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::sync::atomic;
+use core::sync::atomic::Ordering::{Acquire, Relaxed, Release, SeqCst};
+use core::borrow;
+use core::fmt;
+use core::cmp::Ordering;
+use core::intrinsics::abort;
+use core::mem::{self, align_of_val, size_of_val};
+use core::ops::Deref;
+use core::ops::CoerceUnsized;
+use core::ptr::{self, NonNull};
+use core::marker::{Unsize, PhantomData};
+use core::hash::{Hash, Hasher};
+use core::{isize, usize};
+use core::convert::From;
+
+use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
+use boxed::Box;
+use string::String;
+use vec::Vec;
+
+/// 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 sentinel value that is used for the pointer of `Weak::new()`.
+const WEAK_EMPTY: usize = 1;
+
+/// 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 pointer to the same value in the heap. When the last `Arc`
+/// pointer to a given value is destroyed, the pointed-to value is
+/// also destroyed.
+///
+/// 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 values 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 has already been
+/// dropped.
+///
+/// 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 and b both point to the same memory location as foo.
+/// ```
+///
+/// The [`Arc::clone(&from)`] syntax is the most idiomatic because it conveys more explicitly
+/// the meaning of the code. In the example above, this syntax makes it easier to see that
+/// this code is creating a new reference rather than copying the whole content of foo.
+///
+/// ## `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][assoc], 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 value may have
+/// already been destroyed.
+///
+/// [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
+/// [assoc]: ../../book/first-edition/method-syntax.html#associated-functions
+/// [`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
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Arc<T: ?Sized> {
+    ptr: NonNull<ArcInner<T>>,
+    phantom: PhantomData<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> {}
+
+/// `Weak` is a version of [`Arc`] that holds a non-owning reference to the
+/// managed value. The value 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 inner value from being dropped, and `Weak` itself makes no
+/// guarantees about the value still being present and may return [`None`]
+/// when [`upgrade`]d.
+///
+/// A `Weak` pointer is useful for keeping a temporary reference to the value
+/// within [`Arc`] without extending its lifetime. 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 actually not truly "non-null". A `Weak::new()` will set this
+    // to a sentinel value, instead of needing to allocate some space in the
+    // heap.
+    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> {}
+
+#[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,
+        };
+        Arc { ptr: Box::into_raw_non_null(x), phantom: PhantomData }
+    }
+
+    /// Returns the contained 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: ?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(10);
+    /// let x_ptr = Arc::into_raw(x);
+    /// assert_eq!(unsafe { *x_ptr }, 10);
+    /// ```
+    #[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(10);
+    /// 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, 10);
+    ///
+    ///     // 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 {
+        // Align the unsized value to the end of the ArcInner.
+        // Because it is ?Sized, it will always be the last field in memory.
+        let align = align_of_val(&*ptr);
+        let layout = Layout::new::<ArcInner<()>>();
+        let offset = (layout.size() + layout.padding_needed_for(align)) as isize;
+
+        // 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));
+
+        Arc {
+            ptr: NonNull::new_unchecked(arc_ptr),
+            phantom: PhantomData,
+        }
+    }
+
+    /// Creates a new [`Weak`][weak] pointer to this value.
+    ///
+    /// [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(_) => return Weak { ptr: this.ptr },
+                Err(old) => cur = old,
+            }
+        }
+    }
+
+    /// Gets the number of [`Weak`][weak] pointers to this value.
+    ///
+    /// [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 value.
+    ///
+    /// # 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 value (not
+    /// just values that compare as equal).
+    ///
+    /// # 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));
+    /// ```
+    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 an unsized value
+    unsafe fn allocate_for_ptr(ptr: *const T) -> *mut ArcInner<T> {
+        // Create a fake ArcInner to find allocation size and alignment
+        let fake_ptr = ptr as *mut ArcInner<T>;
+
+        let layout = Layout::for_value(&*fake_ptr);
+
+        let mem = Global.alloc(layout)
+            .unwrap_or_else(|_| handle_alloc_error(layout));
+
+        // Initialize the real ArcInner
+        let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut ArcInner<T>;
+
+        ptr::write(&mut (*inner).strong, atomic::AtomicUsize::new(1));
+        ptr::write(&mut (*inner).weak, atomic::AtomicUsize::new(1));
+
+        inner
+    }
+
+    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);
+
+            Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
+        }
+    }
+}
+
+// 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 v_ptr = v as *const [T];
+        let ptr = Self::allocate_for_ptr(v_ptr);
+
+        ptr::copy_nonoverlapping(
+            v.as_ptr(),
+            &mut (*ptr).data as *mut [T] as *mut T,
+            v.len());
+
+        Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
+    }
+}
+
+// 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 {
+        // 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) {
+                use core::slice::from_raw_parts_mut;
+
+                unsafe {
+                    let slice = from_raw_parts_mut(self.elems, self.n_elems);
+                    ptr::drop_in_place(slice);
+
+                    Global.dealloc(self.mem.cast(), self.layout.clone());
+                }
+            }
+        }
+
+        unsafe {
+            let v_ptr = v as *const [T];
+            let ptr = Self::allocate_for_ptr(v_ptr);
+
+            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: elems,
+                layout: layout,
+                n_elems: 0,
+            };
+
+            for (i, item) in v.iter().enumerate() {
+                ptr::write(elems.offset(i as isize), item.clone());
+                guard.n_elems += 1;
+            }
+
+            // All clear. Forget the guard so it doesn't free the new ArcInner.
+            mem::forget(guard);
+
+            Arc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
+        }
+    }
+}
+
+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 inner value, increasing the
+    /// strong reference count.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// 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();
+            }
+        }
+
+        Arc { ptr: self.ptr, phantom: PhantomData }
+    }
+}
+
+#[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
+    }
+}
+
+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 value,
+    /// then `make_mut` will invoke [`clone`][clone] on the inner value to
+    /// ensure unique ownership. This is also referred to as clone-on-write.
+    ///
+    /// 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
+    ///
+    /// # 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 values.
+    /// 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 to the inner value, if there are
+    /// no other `Arc` or [`Weak`][weak] pointers to the same value.
+    ///
+    /// 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 it's shared.
+    ///
+    /// [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(&mut this.ptr.as_mut().data)
+            }
+        } else {
+            None
+        }
+    }
+
+    /// 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` prior to decrements of the `weak` count (via drop,
+        // which uses Release).
+        if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() {
+            // Due to the previous acquire read, this will observe any writes to
+            // `strong` that were due to upgrading weak pointers; only strong
+            // clones remain, which require that the strong count is > 1 anyway.
+            let unique = self.inner().strong.load(Relaxed) == 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`][weak], so we `drop` the inner value.
+    ///
+    /// [weak]: struct.Weak.html
+    ///
+    /// # 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!"
+    /// ```
+    #[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<Any + Send + Sync> {
+    #[inline]
+    #[unstable(feature = "rc_downcast", issue = "44608")]
+    /// Attempt to downcast the `Arc<Any + Send + Sync>` to a concrete type.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(rc_downcast)]
+    /// use std::any::Any;
+    /// use std::sync::Arc;
+    ///
+    /// fn print_if_string(value: Arc<Any + Send + Sync>) {
+    ///     if let Ok(string) = value.downcast::<String>() {
+    ///         println!("String ({}): {}", string.len(), string);
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     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 { ptr, phantom: PhantomData })
+        } 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> {
+        unsafe {
+            Weak {
+                ptr: NonNull::new_unchecked(WEAK_EMPTY as *mut _),
+            }
+        }
+    }
+}
+
+impl<T: ?Sized> Weak<T> {
+    /// Attempts to upgrade the `Weak` pointer to an [`Arc`], extending
+    /// the lifetime of the value if successful.
+    ///
+    /// Returns [`None`] if the 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 = if self.ptr.as_ptr() as *const u8 as usize == WEAK_EMPTY {
+            return None;
+        } else {
+            unsafe { self.ptr.as_ref() }
+        };
+
+        // 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 {
+                    // null checked above
+                    ptr: self.ptr,
+                    phantom: PhantomData,
+                }),
+                Err(old) => n = old,
+            }
+        }
+    }
+}
+
+#[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 value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::{Arc, Weak};
+    ///
+    /// let weak_five = Arc::downgrade(&Arc::new(5));
+    ///
+    /// Weak::clone(&weak_five);
+    /// ```
+    #[inline]
+    fn clone(&self) -> Weak<T> {
+        let inner = if self.ptr.as_ptr() as *const u8 as usize == WEAK_EMPTY {
+            return Weak { ptr: self.ptr };
+        } else {
+            unsafe { self.ptr.as_ref() }
+        };
+        // 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();
+            }
+        }
+
+        return 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`].
+    ///
+    /// [`upgrade`]: struct.Weak.html#method.upgrade
+    /// [`None`]: ../../std/option/enum.Option.html#variant.None
+    ///
+    /// # 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 self.ptr.as_ptr() as *const u8 as usize == WEAK_EMPTY {
+            return;
+        } else {
+            unsafe { self.ptr.as_ref() }
+        };
+
+        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")]
+impl<T: ?Sized + PartialEq> PartialEq for Arc<T> {
+    /// Equality for two `Arc`s.
+    ///
+    /// Two `Arc`s are equal if their inner values are equal.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// assert!(five == Arc::new(5));
+    /// ```
+    fn eq(&self, other: &Arc<T>) -> bool {
+        *(*self) == *(*other)
+    }
+
+    /// Inequality for two `Arc`s.
+    ///
+    /// Two `Arc`s are unequal if their inner values are unequal.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::sync::Arc;
+    ///
+    /// let five = Arc::new(5);
+    ///
+    /// assert!(five != Arc::new(6));
+    /// ```
+    fn ne(&self, other: &Arc<T>) -> bool {
+        *(*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<'a, T: Clone> From<&'a [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<'a> From<&'a 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
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::boxed::Box;
+    use std::clone::Clone;
+    use std::sync::mpsc::channel;
+    use std::mem::drop;
+    use std::ops::Drop;
+    use std::option::Option;
+    use std::option::Option::{None, Some};
+    use std::sync::atomic;
+    use std::sync::atomic::Ordering::{Acquire, SeqCst};
+    use std::thread;
+    use std::sync::Mutex;
+    use std::convert::From;
+
+    use super::{Arc, Weak};
+    use vec::Vec;
+
+    struct Canary(*mut atomic::AtomicUsize);
+
+    impl Drop for Canary {
+        fn drop(&mut self) {
+            unsafe {
+                match *self {
+                    Canary(c) => {
+                        (*c).fetch_add(1, SeqCst);
+                    }
+                }
+            }
+        }
+    }
+
+    #[test]
+    #[cfg_attr(target_os = "emscripten", ignore)]
+    fn manually_share_arc() {
+        let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+        let arc_v = Arc::new(v);
+
+        let (tx, rx) = channel();
+
+        let _t = thread::spawn(move || {
+            let arc_v: Arc<Vec<i32>> = rx.recv().unwrap();
+            assert_eq!((*arc_v)[3], 4);
+        });
+
+        tx.send(arc_v.clone()).unwrap();
+
+        assert_eq!((*arc_v)[2], 3);
+        assert_eq!((*arc_v)[4], 5);
+    }
+
+    #[test]
+    fn test_arc_get_mut() {
+        let mut x = Arc::new(3);
+        *Arc::get_mut(&mut x).unwrap() = 4;
+        assert_eq!(*x, 4);
+        let y = x.clone();
+        assert!(Arc::get_mut(&mut x).is_none());
+        drop(y);
+        assert!(Arc::get_mut(&mut x).is_some());
+        let _w = Arc::downgrade(&x);
+        assert!(Arc::get_mut(&mut x).is_none());
+    }
+
+    #[test]
+    fn try_unwrap() {
+        let x = Arc::new(3);
+        assert_eq!(Arc::try_unwrap(x), Ok(3));
+        let x = Arc::new(4);
+        let _y = x.clone();
+        assert_eq!(Arc::try_unwrap(x), Err(Arc::new(4)));
+        let x = Arc::new(5);
+        let _w = Arc::downgrade(&x);
+        assert_eq!(Arc::try_unwrap(x), Ok(5));
+    }
+
+    #[test]
+    fn into_from_raw() {
+        let x = Arc::new(box "hello");
+        let y = x.clone();
+
+        let x_ptr = Arc::into_raw(x);
+        drop(y);
+        unsafe {
+            assert_eq!(**x_ptr, "hello");
+
+            let x = Arc::from_raw(x_ptr);
+            assert_eq!(**x, "hello");
+
+            assert_eq!(Arc::try_unwrap(x).map(|x| *x), Ok("hello"));
+        }
+    }
+
+    #[test]
+    fn test_into_from_raw_unsized() {
+        use std::fmt::Display;
+        use std::string::ToString;
+
+        let arc: Arc<str> = Arc::from("foo");
+
+        let ptr = Arc::into_raw(arc.clone());
+        let arc2 = unsafe { Arc::from_raw(ptr) };
+
+        assert_eq!(unsafe { &*ptr }, "foo");
+        assert_eq!(arc, arc2);
+
+        let arc: Arc<Display> = Arc::new(123);
+
+        let ptr = Arc::into_raw(arc.clone());
+        let arc2 = unsafe { Arc::from_raw(ptr) };
+
+        assert_eq!(unsafe { &*ptr }.to_string(), "123");
+        assert_eq!(arc2.to_string(), "123");
+    }
+
+    #[test]
+    fn test_cowarc_clone_make_mut() {
+        let mut cow0 = Arc::new(75);
+        let mut cow1 = cow0.clone();
+        let mut cow2 = cow1.clone();
+
+        assert!(75 == *Arc::make_mut(&mut cow0));
+        assert!(75 == *Arc::make_mut(&mut cow1));
+        assert!(75 == *Arc::make_mut(&mut cow2));
+
+        *Arc::make_mut(&mut cow0) += 1;
+        *Arc::make_mut(&mut cow1) += 2;
+        *Arc::make_mut(&mut cow2) += 3;
+
+        assert!(76 == *cow0);
+        assert!(77 == *cow1);
+        assert!(78 == *cow2);
+
+        // none should point to the same backing memory
+        assert!(*cow0 != *cow1);
+        assert!(*cow0 != *cow2);
+        assert!(*cow1 != *cow2);
+    }
+
+    #[test]
+    fn test_cowarc_clone_unique2() {
+        let mut cow0 = Arc::new(75);
+        let cow1 = cow0.clone();
+        let cow2 = cow1.clone();
+
+        assert!(75 == *cow0);
+        assert!(75 == *cow1);
+        assert!(75 == *cow2);
+
+        *Arc::make_mut(&mut cow0) += 1;
+        assert!(76 == *cow0);
+        assert!(75 == *cow1);
+        assert!(75 == *cow2);
+
+        // cow1 and cow2 should share the same contents
+        // cow0 should have a unique reference
+        assert!(*cow0 != *cow1);
+        assert!(*cow0 != *cow2);
+        assert!(*cow1 == *cow2);
+    }
+
+    #[test]
+    fn test_cowarc_clone_weak() {
+        let mut cow0 = Arc::new(75);
+        let cow1_weak = Arc::downgrade(&cow0);
+
+        assert!(75 == *cow0);
+        assert!(75 == *cow1_weak.upgrade().unwrap());
+
+        *Arc::make_mut(&mut cow0) += 1;
+
+        assert!(76 == *cow0);
+        assert!(cow1_weak.upgrade().is_none());
+    }
+
+    #[test]
+    fn test_live() {
+        let x = Arc::new(5);
+        let y = Arc::downgrade(&x);
+        assert!(y.upgrade().is_some());
+    }
+
+    #[test]
+    fn test_dead() {
+        let x = Arc::new(5);
+        let y = Arc::downgrade(&x);
+        drop(x);
+        assert!(y.upgrade().is_none());
+    }
+
+    #[test]
+    fn weak_self_cyclic() {
+        struct Cycle {
+            x: Mutex<Option<Weak<Cycle>>>,
+        }
+
+        let a = Arc::new(Cycle { x: Mutex::new(None) });
+        let b = Arc::downgrade(&a.clone());
+        *a.x.lock().unwrap() = Some(b);
+
+        // hopefully we don't double-free (or leak)...
+    }
+
+    #[test]
+    fn drop_arc() {
+        let mut canary = atomic::AtomicUsize::new(0);
+        let x = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
+        drop(x);
+        assert!(canary.load(Acquire) == 1);
+    }
+
+    #[test]
+    fn drop_arc_weak() {
+        let mut canary = atomic::AtomicUsize::new(0);
+        let arc = Arc::new(Canary(&mut canary as *mut atomic::AtomicUsize));
+        let arc_weak = Arc::downgrade(&arc);
+        assert!(canary.load(Acquire) == 0);
+        drop(arc);
+        assert!(canary.load(Acquire) == 1);
+        drop(arc_weak);
+    }
+
+    #[test]
+    fn test_strong_count() {
+        let a = Arc::new(0);
+        assert!(Arc::strong_count(&a) == 1);
+        let w = Arc::downgrade(&a);
+        assert!(Arc::strong_count(&a) == 1);
+        let b = w.upgrade().expect("");
+        assert!(Arc::strong_count(&b) == 2);
+        assert!(Arc::strong_count(&a) == 2);
+        drop(w);
+        drop(a);
+        assert!(Arc::strong_count(&b) == 1);
+        let c = b.clone();
+        assert!(Arc::strong_count(&b) == 2);
+        assert!(Arc::strong_count(&c) == 2);
+    }
+
+    #[test]
+    fn test_weak_count() {
+        let a = Arc::new(0);
+        assert!(Arc::strong_count(&a) == 1);
+        assert!(Arc::weak_count(&a) == 0);
+        let w = Arc::downgrade(&a);
+        assert!(Arc::strong_count(&a) == 1);
+        assert!(Arc::weak_count(&a) == 1);
+        let x = w.clone();
+        assert!(Arc::weak_count(&a) == 2);
+        drop(w);
+        drop(x);
+        assert!(Arc::strong_count(&a) == 1);
+        assert!(Arc::weak_count(&a) == 0);
+        let c = a.clone();
+        assert!(Arc::strong_count(&a) == 2);
+        assert!(Arc::weak_count(&a) == 0);
+        let d = Arc::downgrade(&c);
+        assert!(Arc::weak_count(&c) == 1);
+        assert!(Arc::strong_count(&c) == 2);
+
+        drop(a);
+        drop(c);
+        drop(d);
+    }
+
+    #[test]
+    fn show_arc() {
+        let a = Arc::new(5);
+        assert_eq!(format!("{:?}", a), "5");
+    }
+
+    // Make sure deriving works with Arc<T>
+    #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
+    struct Foo {
+        inner: Arc<i32>,
+    }
+
+    #[test]
+    fn test_unsized() {
+        let x: Arc<[i32]> = Arc::new([1, 2, 3]);
+        assert_eq!(format!("{:?}", x), "[1, 2, 3]");
+        let y = Arc::downgrade(&x.clone());
+        drop(x);
+        assert!(y.upgrade().is_none());
+    }
+
+    #[test]
+    fn test_from_owned() {
+        let foo = 123;
+        let foo_arc = Arc::from(foo);
+        assert!(123 == *foo_arc);
+    }
+
+    #[test]
+    fn test_new_weak() {
+        let foo: Weak<usize> = Weak::new();
+        assert!(foo.upgrade().is_none());
+    }
+
+    #[test]
+    fn test_ptr_eq() {
+        let five = Arc::new(5);
+        let same_five = five.clone();
+        let other_five = Arc::new(5);
+
+        assert!(Arc::ptr_eq(&five, &same_five));
+        assert!(!Arc::ptr_eq(&five, &other_five));
+    }
+
+    #[test]
+    #[cfg_attr(target_os = "emscripten", ignore)]
+    fn test_weak_count_locked() {
+        let mut a = Arc::new(atomic::AtomicBool::new(false));
+        let a2 = a.clone();
+        let t = thread::spawn(move || {
+            for _i in 0..1000000 {
+                Arc::get_mut(&mut a);
+            }
+            a.store(true, SeqCst);
+        });
+
+        while !a2.load(SeqCst) {
+            let n = Arc::weak_count(&a2);
+            assert!(n < 2, "bad weak count: {}", n);
+        }
+        t.join().unwrap();
+    }
+
+    #[test]
+    fn test_from_str() {
+        let r: Arc<str> = Arc::from("foo");
+
+        assert_eq!(&r[..], "foo");
+    }
+
+    #[test]
+    fn test_copy_from_slice() {
+        let s: &[u32] = &[1, 2, 3];
+        let r: Arc<[u32]> = Arc::from(s);
+
+        assert_eq!(&r[..], [1, 2, 3]);
+    }
+
+    #[test]
+    fn test_clone_from_slice() {
+        #[derive(Clone, Debug, Eq, PartialEq)]
+        struct X(u32);
+
+        let s: &[X] = &[X(1), X(2), X(3)];
+        let r: Arc<[X]> = Arc::from(s);
+
+        assert_eq!(&r[..], s);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_clone_from_slice_panic() {
+        use std::string::{String, ToString};
+
+        struct Fail(u32, String);
+
+        impl Clone for Fail {
+            fn clone(&self) -> Fail {
+                if self.0 == 2 {
+                    panic!();
+                }
+                Fail(self.0, self.1.clone())
+            }
+        }
+
+        let s: &[Fail] = &[
+            Fail(0, "foo".to_string()),
+            Fail(1, "bar".to_string()),
+            Fail(2, "baz".to_string()),
+        ];
+
+        // Should panic, but not cause memory corruption
+        let _r: Arc<[Fail]> = Arc::from(s);
+    }
+
+    #[test]
+    fn test_from_box() {
+        let b: Box<u32> = box 123;
+        let r: Arc<u32> = Arc::from(b);
+
+        assert_eq!(*r, 123);
+    }
+
+    #[test]
+    fn test_from_box_str() {
+        use std::string::String;
+
+        let s = String::from("foo").into_boxed_str();
+        let r: Arc<str> = Arc::from(s);
+
+        assert_eq!(&r[..], "foo");
+    }
+
+    #[test]
+    fn test_from_box_slice() {
+        let s = vec![1, 2, 3].into_boxed_slice();
+        let r: Arc<[u32]> = Arc::from(s);
+
+        assert_eq!(&r[..], [1, 2, 3]);
+    }
+
+    #[test]
+    fn test_from_box_trait() {
+        use std::fmt::Display;
+        use std::string::ToString;
+
+        let b: Box<Display> = box 123;
+        let r: Arc<Display> = Arc::from(b);
+
+        assert_eq!(r.to_string(), "123");
+    }
+
+    #[test]
+    fn test_from_box_trait_zero_sized() {
+        use std::fmt::Debug;
+
+        let b: Box<Debug> = box ();
+        let r: Arc<Debug> = Arc::from(b);
+
+        assert_eq!(format!("{:?}", r), "()");
+    }
+
+    #[test]
+    fn test_from_vec() {
+        let v = vec![1, 2, 3];
+        let r: Arc<[u32]> = Arc::from(v);
+
+        assert_eq!(&r[..], [1, 2, 3]);
+    }
+
+    #[test]
+    fn test_downcast() {
+        use std::any::Any;
+
+        let r1: Arc<Any + Send + Sync> = Arc::new(i32::max_value());
+        let r2: Arc<Any + Send + Sync> = Arc::new("abc");
+
+        assert!(r1.clone().downcast::<u32>().is_err());
+
+        let r1i32 = r1.downcast::<i32>();
+        assert!(r1i32.is_ok());
+        assert_eq!(r1i32.unwrap(), Arc::new(i32::max_value()));
+
+        assert!(r2.clone().downcast::<i32>().is_err());
+
+        let r2str = r2.downcast::<&'static str>();
+        assert!(r2str.is_ok());
+        assert_eq!(r2str.unwrap(), Arc::new("abc"));
+    }
+}
+
+#[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
+    }
+}