liballoc

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+// Copyright 2013-2014 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.
+
+#![allow(deprecated)]
+
+//! Thread-local reference-counted boxes (the `Rc<T>` type).
+//!
+//! The `Rc<T>` type provides shared ownership of an immutable value.
+//! Destruction is deterministic, and will occur as soon as the last owner is
+//! gone. It is marked as non-sendable because it avoids the overhead of atomic
+//! reference counting.
+//!
+//! The `downgrade` method can be used to create a non-owning `Weak<T>` pointer
+//! to the box. A `Weak<T>` pointer can be upgraded to an `Rc<T>` pointer, but
+//! will return `None` if the value has already been dropped.
+//!
+//! For example, a tree with parent pointers can be represented by putting the
+//! nodes behind strong `Rc<T>` pointers, and then storing the parent pointers
+//! as `Weak<T>` pointers.
+//!
+//! # Examples
+//!
+//! Consider a scenario where a set of `Gadget`s are owned by a given `Owner`.
+//! We want to have our `Gadget`s point to their `Owner`. We can't do this with
+//! unique ownership, because more than one gadget may belong to the same
+//! `Owner`. `Rc<T>` allows us to share an `Owner` between multiple `Gadget`s,
+//! and have the `Owner` remain allocated as long as any `Gadget` points at it.
+//!
+//! ```rust
+//! use std::rc::Rc;
+//!
+//! struct Owner {
+//!     name: String
+//!     // ...other fields
+//! }
+//!
+//! struct Gadget {
+//!     id: i32,
+//!     owner: Rc<Owner>
+//!     // ...other fields
+//! }
+//!
+//! fn main() {
+//!     // Create a reference counted Owner.
+//!     let gadget_owner : Rc<Owner> = Rc::new(
+//!         Owner { name: String::from("Gadget Man") }
+//!     );
+//!
+//!     // Create Gadgets belonging to gadget_owner. To increment the reference
+//!     // count we clone the `Rc<T>` object.
+//!     let gadget1 = Gadget { id: 1, owner: gadget_owner.clone() };
+//!     let gadget2 = Gadget { id: 2, owner: gadget_owner.clone() };
+//!
+//!     drop(gadget_owner);
+//!
+//!     // Despite dropping gadget_owner, we're still able to print out the name
+//!     // of the Owner of the Gadgets. This is because we've only dropped the
+//!     // reference count object, not the Owner it wraps. As long as there are
+//!     // other `Rc<T>` objects pointing at the same Owner, it will remain
+//!     // allocated. Notice that the `Rc<T>` wrapper around Gadget.owner gets
+//!     // automatically dereferenced for us.
+//!     println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
+//!     println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
+//!
+//!     // At the end of the method, gadget1 and gadget2 get destroyed, and with
+//!     // them the last counted references to our Owner. Gadget Man now gets
+//!     // destroyed as well.
+//! }
+//! ```
+//!
+//! If our requirements change, and we also need to be able to traverse from
+//! Owner → Gadget, we will run into problems: an `Rc<T>` pointer from Owner
+//! → Gadget introduces a cycle between the objects. This means that their
+//! reference counts can never reach 0, and the objects will remain allocated: a
+//! memory leak. In order to get around this, we can use `Weak<T>` pointers.
+//! These pointers don't contribute to the total count.
+//!
+//! Rust actually makes it somewhat difficult to produce this loop in the first
+//! place: in order to end up with two objects that point at each other, one of
+//! them needs to be mutable. This is problematic because `Rc<T>` enforces
+//! memory safety by only giving out shared references to the object it wraps,
+//! and these don't allow direct mutation. We need to wrap the part of the
+//! object we wish to mutate in a `RefCell`, which provides *interior
+//! mutability*: a method to achieve mutability through a shared reference.
+//! `RefCell` enforces Rust's borrowing rules at runtime.  Read the `Cell`
+//! documentation for more details on interior mutability.
+//!
+//! ```rust
+//! use std::rc::Rc;
+//! use std::rc::Weak;
+//! use std::cell::RefCell;
+//!
+//! struct Owner {
+//!     name: String,
+//!     gadgets: RefCell<Vec<Weak<Gadget>>>,
+//!     // ...other fields
+//! }
+//!
+//! struct Gadget {
+//!     id: i32,
+//!     owner: Rc<Owner>,
+//!     // ...other fields
+//! }
+//!
+//! fn main() {
+//!     // Create a reference counted Owner. Note the fact that we've put the
+//!     // Owner's vector of Gadgets inside a RefCell so that we can mutate it
+//!     // through a shared reference.
+//!     let gadget_owner : Rc<Owner> = Rc::new(
+//!         Owner {
+//!             name: "Gadget Man".to_string(),
+//!             gadgets: RefCell::new(Vec::new()),
+//!         }
+//!     );
+//!
+//!     // Create Gadgets belonging to gadget_owner as before.
+//!     let gadget1 = Rc::new(Gadget{id: 1, owner: gadget_owner.clone()});
+//!     let gadget2 = Rc::new(Gadget{id: 2, owner: gadget_owner.clone()});
+//!
+//!     // Add the Gadgets to their Owner. To do this we mutably borrow from
+//!     // the RefCell holding the Owner's Gadgets.
+//!     gadget_owner.gadgets.borrow_mut().push(Rc::downgrade(&gadget1));
+//!     gadget_owner.gadgets.borrow_mut().push(Rc::downgrade(&gadget2));
+//!
+//!     // Iterate over our Gadgets, printing their details out
+//!     for gadget_opt in gadget_owner.gadgets.borrow().iter() {
+//!
+//!         // gadget_opt is a Weak<Gadget>. Since weak pointers can't guarantee
+//!         // that their object is still allocated, we need to call upgrade()
+//!         // on them to turn them into a strong reference. This returns an
+//!         // Option, which contains a reference to our object if it still
+//!         // exists.
+//!         let gadget = gadget_opt.upgrade().unwrap();
+//!         println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
+//!     }
+//!
+//!     // At the end of the method, gadget_owner, gadget1 and gadget2 get
+//!     // destroyed. There are now no strong (`Rc<T>`) references to the gadgets.
+//!     // Once they get destroyed, the Gadgets get destroyed. This zeroes the
+//!     // reference count on Gadget Man, they get destroyed as well.
+//! }
+//! ```
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[cfg(not(test))]
+use boxed::Box;
+#[cfg(test)]
+use std::boxed::Box;
+
+use core::borrow;
+use core::cell::Cell;
+use core::cmp::Ordering;
+use core::fmt;
+use core::hash::{Hasher, Hash};
+use core::intrinsics::{assume, abort};
+use core::marker;
+use core::marker::Unsize;
+use core::mem::{self, align_of_val, size_of_val, forget, uninitialized};
+use core::ops::Deref;
+use core::ops::CoerceUnsized;
+use core::ptr::{self, Shared};
+use core::convert::From;
+
+use heap::deallocate;
+
+struct RcBox<T: ?Sized> {
+    strong: Cell<usize>,
+    weak: Cell<usize>,
+    value: T,
+}
+
+
+/// A reference-counted pointer type over an immutable value.
+///
+/// See the [module level documentation](./index.html) for more details.
+#[unsafe_no_drop_flag]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Rc<T: ?Sized> {
+    ptr: Shared<RcBox<T>>,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> !marker::Send for Rc<T> {}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> !marker::Sync for Rc<T> {}
+
+#[unstable(feature = "coerce_unsized", issue = "27732")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}
+
+impl<T> Rc<T> {
+    /// Constructs a new `Rc<T>`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new(value: T) -> Rc<T> {
+        unsafe {
+            Rc {
+                // there is an implicit weak pointer owned by all the strong
+                // pointers, which ensures that the weak destructor never frees
+                // the allocation while the strong destructor is running, even
+                // if the weak pointer is stored inside the strong one.
+                ptr: Shared::new(Box::into_raw(box RcBox {
+                    strong: Cell::new(1),
+                    weak: Cell::new(1),
+                    value: value,
+                })),
+            }
+        }
+    }
+
+    /// Unwraps the contained value if the `Rc<T>` has exactly one strong reference.
+    ///
+    /// Otherwise, an `Err` is returned with the same `Rc<T>`.
+    ///
+    /// This will succeed even if there are outstanding weak references.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let x = Rc::new(3);
+    /// assert_eq!(Rc::try_unwrap(x), Ok(3));
+    ///
+    /// let x = Rc::new(4);
+    /// let _y = x.clone();
+    /// assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
+    /// ```
+    #[inline]
+    #[stable(feature = "rc_unique", since = "1.4.0")]
+    pub fn try_unwrap(this: Self) -> Result<T, Self> {
+        if Rc::would_unwrap(&this) {
+            unsafe {
+                let val = ptr::read(&*this); // copy the contained object
+
+                // Indicate to Weaks that they can't be promoted by decrememting
+                // the strong count, and then remove the implicit "strong weak"
+                // pointer while also handling drop logic by just crafting a
+                // fake Weak.
+                this.dec_strong();
+                let _weak = Weak { ptr: this.ptr };
+                forget(this);
+                Ok(val)
+            }
+        } else {
+            Err(this)
+        }
+    }
+
+    /// Checks if `Rc::try_unwrap` would return `Ok`.
+    #[unstable(feature = "rc_would_unwrap",
+               reason = "just added for niche usecase",
+               issue = "28356")]
+    pub fn would_unwrap(this: &Self) -> bool {
+        Rc::strong_count(&this) == 1
+    }
+}
+
+impl<T: ?Sized> Rc<T> {
+    /// Downgrades the `Rc<T>` to a `Weak<T>` reference.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// let weak_five = Rc::downgrade(&five);
+    /// ```
+    #[stable(feature = "rc_weak", since = "1.4.0")]
+    pub fn downgrade(this: &Self) -> Weak<T> {
+        this.inc_weak();
+        Weak { ptr: this.ptr }
+    }
+
+    /// Get the number of weak references to this value.
+    #[inline]
+    #[unstable(feature = "rc_counts", reason = "not clearly useful",
+               issue = "28356")]
+    pub fn weak_count(this: &Self) -> usize {
+        this.weak() - 1
+    }
+
+    /// Get the number of strong references to this value.
+    #[inline]
+    #[unstable(feature = "rc_counts", reason = "not clearly useful",
+               issue = "28356")]
+    pub fn strong_count(this: &Self) -> usize {
+        this.strong()
+    }
+
+    /// Returns true if there are no other `Rc` or `Weak<T>` values that share
+    /// the same inner value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(rc_counts)]
+    ///
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// assert!(Rc::is_unique(&five));
+    /// ```
+    #[inline]
+    #[unstable(feature = "rc_counts", reason = "uniqueness has unclear meaning",
+               issue = "28356")]
+    pub fn is_unique(this: &Self) -> bool {
+        Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
+    }
+
+    /// Returns a mutable reference to the contained value if the `Rc<T>` has
+    /// one strong reference and no weak references.
+    ///
+    /// Returns `None` if the `Rc<T>` is not unique.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let mut x = Rc::new(3);
+    /// *Rc::get_mut(&mut x).unwrap() = 4;
+    /// assert_eq!(*x, 4);
+    ///
+    /// let _y = x.clone();
+    /// assert!(Rc::get_mut(&mut x).is_none());
+    /// ```
+    #[inline]
+    #[stable(feature = "rc_unique", since = "1.4.0")]
+    pub fn get_mut(this: &mut Self) -> Option<&mut T> {
+        if Rc::is_unique(this) {
+            let inner = unsafe { &mut **this.ptr };
+            Some(&mut inner.value)
+        } else {
+            None
+        }
+    }
+}
+
+impl<T: Clone> Rc<T> {
+    /// Make a mutable reference into the given `Rc<T>` by cloning the inner
+    /// data if the `Rc<T>` doesn't have one strong reference and no weak
+    /// references.
+    ///
+    /// This is also referred to as a copy-on-write.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let mut data = Rc::new(5);
+    ///
+    /// *Rc::make_mut(&mut data) += 1;             // Won't clone anything
+    /// let mut other_data = data.clone(); // Won't clone inner data
+    /// *Rc::make_mut(&mut data) += 1;             // Clones inner data
+    /// *Rc::make_mut(&mut data) += 1;             // Won't clone anything
+    /// *Rc::make_mut(&mut other_data) *= 2;       // Won't clone anything
+    ///
+    /// // Note: data and other_data now point to different numbers
+    /// assert_eq!(*data, 8);
+    /// assert_eq!(*other_data, 12);
+    ///
+    /// ```
+    #[inline]
+    #[stable(feature = "rc_unique", since = "1.4.0")]
+    pub fn make_mut(this: &mut Self) -> &mut T {
+        if Rc::strong_count(this) != 1 {
+            // Gotta clone the data, there are other Rcs
+            *this = Rc::new((**this).clone())
+        } else if Rc::weak_count(this) != 0 {
+            // Can just steal the data, all that's left is Weaks
+            unsafe {
+                let mut swap = Rc::new(ptr::read(&(**this.ptr).value));
+                mem::swap(this, &mut swap);
+                swap.dec_strong();
+                // Remove implicit strong-weak ref (no need to craft a fake
+                // Weak here -- we know other Weaks can clean up for us)
+                swap.dec_weak();
+                forget(swap);
+            }
+        }
+        // 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 `Rc<T>` itself to be `mut`, so we're returning the only possible
+        // reference to the inner value.
+        let inner = unsafe { &mut **this.ptr };
+        &mut inner.value
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Deref for Rc<T> {
+    type Target = T;
+
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        &self.inner().value
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Drop for Rc<T> {
+    /// Drops the `Rc<T>`.
+    ///
+    /// This will decrement the strong reference count. If the strong reference
+    /// count becomes zero and the only other references are `Weak<T>` ones,
+    /// `drop`s the inner value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// {
+    ///     let five = Rc::new(5);
+    ///
+    ///     // stuff
+    ///
+    ///     drop(five); // explicit drop
+    /// }
+    /// {
+    ///     let five = Rc::new(5);
+    ///
+    ///     // stuff
+    ///
+    /// } // implicit drop
+    /// ```
+    #[unsafe_destructor_blind_to_params]
+    fn drop(&mut self) {
+        unsafe {
+            let ptr = *self.ptr;
+            let thin = ptr as *const ();
+
+            if thin as usize != mem::POST_DROP_USIZE {
+                self.dec_strong();
+                if self.strong() == 0 {
+                    // destroy the contained object
+                    ptr::drop_in_place(&mut (*ptr).value);
+
+                    // remove the implicit "strong weak" pointer now that we've
+                    // destroyed the contents.
+                    self.dec_weak();
+
+                    if self.weak() == 0 {
+                        deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> Clone for Rc<T> {
+    /// Makes a clone of the `Rc<T>`.
+    ///
+    /// When you clone an `Rc<T>`, it will create another pointer to the data and
+    /// increase the strong reference counter.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five.clone();
+    /// ```
+    #[inline]
+    fn clone(&self) -> Rc<T> {
+        self.inc_strong();
+        Rc { ptr: self.ptr }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Default> Default for Rc<T> {
+    /// Creates a new `Rc<T>`, with the `Default` value for `T`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let x: Rc<i32> = Default::default();
+    /// ```
+    #[inline]
+    fn default() -> Rc<T> {
+        Rc::new(Default::default())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + PartialEq> PartialEq for Rc<T> {
+    /// Equality for two `Rc<T>`s.
+    ///
+    /// Two `Rc<T>`s are equal if their inner value are equal.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five == Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn eq(&self, other: &Rc<T>) -> bool {
+        **self == **other
+    }
+
+    /// Inequality for two `Rc<T>`s.
+    ///
+    /// Two `Rc<T>`s are unequal if their inner value are unequal.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five != Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn ne(&self, other: &Rc<T>) -> bool {
+        **self != **other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + Eq> Eq for Rc<T> {}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + PartialOrd> PartialOrd for Rc<T> {
+    /// Partial comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `partial_cmp()` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five.partial_cmp(&Rc::new(5));
+    /// ```
+    #[inline(always)]
+    fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
+        (**self).partial_cmp(&**other)
+    }
+
+    /// Less-than comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `<` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five < Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn lt(&self, other: &Rc<T>) -> bool {
+        **self < **other
+    }
+
+    /// 'Less-than or equal to' comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `<=` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five <= Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn le(&self, other: &Rc<T>) -> bool {
+        **self <= **other
+    }
+
+    /// Greater-than comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `>` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five > Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn gt(&self, other: &Rc<T>) -> bool {
+        **self > **other
+    }
+
+    /// 'Greater-than or equal to' comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `>=` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five >= Rc::new(5);
+    /// ```
+    #[inline(always)]
+    fn ge(&self, other: &Rc<T>) -> bool {
+        **self >= **other
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + Ord> Ord for Rc<T> {
+    /// Comparison for two `Rc<T>`s.
+    ///
+    /// The two are compared by calling `cmp()` on their inner values.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// five.partial_cmp(&Rc::new(5));
+    /// ```
+    #[inline]
+    fn cmp(&self, other: &Rc<T>) -> Ordering {
+        (**self).cmp(&**other)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + Hash> Hash for Rc<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        (**self).hash(state);
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized + fmt::Display> fmt::Display for Rc<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 Rc<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 Rc<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Pointer::fmt(&*self.ptr, f)
+    }
+}
+
+#[stable(feature = "from_for_ptrs", since = "1.6.0")]
+impl<T> From<T> for Rc<T> {
+    fn from(t: T) -> Self {
+        Rc::new(t)
+    }
+}
+
+/// A weak version of `Rc<T>`.
+///
+/// Weak references do not count when determining if the inner value should be
+/// dropped.
+///
+/// See the [module level documentation](./index.html) for more.
+#[unsafe_no_drop_flag]
+#[stable(feature = "rc_weak", since = "1.4.0")]
+pub struct Weak<T: ?Sized> {
+    ptr: Shared<RcBox<T>>,
+}
+
+#[stable(feature = "rc_weak", since = "1.4.0")]
+impl<T: ?Sized> !marker::Send for Weak<T> {}
+#[stable(feature = "rc_weak", since = "1.4.0")]
+impl<T: ?Sized> !marker::Sync for Weak<T> {}
+
+#[unstable(feature = "coerce_unsized", issue = "27732")]
+impl<T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Weak<U>> for Weak<T> {}
+
+impl<T: ?Sized> Weak<T> {
+    /// Upgrades a weak reference to a strong reference.
+    ///
+    /// Upgrades the `Weak<T>` reference to an `Rc<T>`, if possible.
+    ///
+    /// Returns `None` if there were no strong references and the data was
+    /// destroyed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let five = Rc::new(5);
+    ///
+    /// let weak_five = Rc::downgrade(&five);
+    ///
+    /// let strong_five: Option<Rc<_>> = weak_five.upgrade();
+    /// ```
+    #[stable(feature = "rc_weak", since = "1.4.0")]
+    pub fn upgrade(&self) -> Option<Rc<T>> {
+        if self.strong() == 0 {
+            None
+        } else {
+            self.inc_strong();
+            Some(Rc { ptr: self.ptr })
+        }
+    }
+}
+
+#[stable(feature = "rc_weak", since = "1.4.0")]
+impl<T: ?Sized> Drop for Weak<T> {
+    /// Drops the `Weak<T>`.
+    ///
+    /// This will decrement the weak reference count.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// {
+    ///     let five = Rc::new(5);
+    ///     let weak_five = Rc::downgrade(&five);
+    ///
+    ///     // stuff
+    ///
+    ///     drop(weak_five); // explicit drop
+    /// }
+    /// {
+    ///     let five = Rc::new(5);
+    ///     let weak_five = Rc::downgrade(&five);
+    ///
+    ///     // stuff
+    ///
+    /// } // implicit drop
+    /// ```
+    fn drop(&mut self) {
+        unsafe {
+            let ptr = *self.ptr;
+            let thin = ptr as *const ();
+
+            if thin as usize != mem::POST_DROP_USIZE {
+                self.dec_weak();
+                // the weak count starts at 1, and will only go to zero if all
+                // the strong pointers have disappeared.
+                if self.weak() == 0 {
+                    deallocate(ptr as *mut u8, size_of_val(&*ptr), align_of_val(&*ptr))
+                }
+            }
+        }
+    }
+}
+
+#[stable(feature = "rc_weak", since = "1.4.0")]
+impl<T: ?Sized> Clone for Weak<T> {
+    /// Makes a clone of the `Weak<T>`.
+    ///
+    /// This increases the weak reference count.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::rc::Rc;
+    ///
+    /// let weak_five = Rc::downgrade(&Rc::new(5));
+    ///
+    /// weak_five.clone();
+    /// ```
+    #[inline]
+    fn clone(&self) -> Weak<T> {
+        self.inc_weak();
+        Weak { ptr: self.ptr }
+    }
+}
+
+#[stable(feature = "rc_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)")
+    }
+}
+
+impl<T> Weak<T> {
+    /// Constructs a new `Weak<T>` without an accompanying instance of T.
+    ///
+    /// This allocates memory for T, but does not initialize it. Calling
+    /// Weak<T>::upgrade() on the return value always gives None.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(downgraded_weak)]
+    ///
+    /// use std::rc::Weak;
+    ///
+    /// let empty: Weak<i64> = Weak::new();
+    /// ```
+    #[unstable(feature = "downgraded_weak",
+               reason = "recently added",
+               issue="30425")]
+    pub fn new() -> Weak<T> {
+        unsafe {
+            Weak {
+                ptr: Shared::new(Box::into_raw(box RcBox {
+                    strong: Cell::new(0),
+                    weak: Cell::new(1),
+                    value: uninitialized(),
+                })),
+            }
+        }
+    }
+}
+
+// NOTE: We checked_add here to deal with mem::forget safety. In particular
+// if you mem::forget Rcs (or Weaks), the ref-count can overflow, and then
+// you can free the allocation while outstanding Rcs (or Weaks) exist.
+// We abort because this is such a degenerate scenario that we don't care about
+// what happens -- no real program should ever experience this.
+//
+// This should have negligible overhead since you don't actually need to
+// clone these much in Rust thanks to ownership and move-semantics.
+
+#[doc(hidden)]
+trait RcBoxPtr<T: ?Sized> {
+    fn inner(&self) -> &RcBox<T>;
+
+    #[inline]
+    fn strong(&self) -> usize {
+        self.inner().strong.get()
+    }
+
+    #[inline]
+    fn inc_strong(&self) {
+        self.inner().strong.set(self.strong().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
+    }
+
+    #[inline]
+    fn dec_strong(&self) {
+        self.inner().strong.set(self.strong() - 1);
+    }
+
+    #[inline]
+    fn weak(&self) -> usize {
+        self.inner().weak.get()
+    }
+
+    #[inline]
+    fn inc_weak(&self) {
+        self.inner().weak.set(self.weak().checked_add(1).unwrap_or_else(|| unsafe { abort() }));
+    }
+
+    #[inline]
+    fn dec_weak(&self) {
+        self.inner().weak.set(self.weak() - 1);
+    }
+}
+
+impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
+    #[inline(always)]
+    fn inner(&self) -> &RcBox<T> {
+        unsafe {
+            // Safe to assume this here, as if it weren't true, we'd be breaking
+            // the contract anyway.
+            // This allows the null check to be elided in the destructor if we
+            // manipulated the reference count in the same function.
+            assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
+            &(**self.ptr)
+        }
+    }
+}
+
+impl<T: ?Sized> RcBoxPtr<T> for Weak<T> {
+    #[inline(always)]
+    fn inner(&self) -> &RcBox<T> {
+        unsafe {
+            // Safe to assume this here, as if it weren't true, we'd be breaking
+            // the contract anyway.
+            // This allows the null check to be elided in the destructor if we
+            // manipulated the reference count in the same function.
+            assume(!(*(&self.ptr as *const _ as *const *const ())).is_null());
+            &(**self.ptr)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Rc, Weak};
+    use std::boxed::Box;
+    use std::cell::RefCell;
+    use std::option::Option;
+    use std::option::Option::{Some, None};
+    use std::result::Result::{Err, Ok};
+    use std::mem::drop;
+    use std::clone::Clone;
+    use std::convert::From;
+
+    #[test]
+    fn test_clone() {
+        let x = Rc::new(RefCell::new(5));
+        let y = x.clone();
+        *x.borrow_mut() = 20;
+        assert_eq!(*y.borrow(), 20);
+    }
+
+    #[test]
+    fn test_simple() {
+        let x = Rc::new(5);
+        assert_eq!(*x, 5);
+    }
+
+    #[test]
+    fn test_simple_clone() {
+        let x = Rc::new(5);
+        let y = x.clone();
+        assert_eq!(*x, 5);
+        assert_eq!(*y, 5);
+    }
+
+    #[test]
+    fn test_destructor() {
+        let x: Rc<Box<_>> = Rc::new(box 5);
+        assert_eq!(**x, 5);
+    }
+
+    #[test]
+    fn test_live() {
+        let x = Rc::new(5);
+        let y = Rc::downgrade(&x);
+        assert!(y.upgrade().is_some());
+    }
+
+    #[test]
+    fn test_dead() {
+        let x = Rc::new(5);
+        let y = Rc::downgrade(&x);
+        drop(x);
+        assert!(y.upgrade().is_none());
+    }
+
+    #[test]
+    fn weak_self_cyclic() {
+        struct Cycle {
+            x: RefCell<Option<Weak<Cycle>>>,
+        }
+
+        let a = Rc::new(Cycle { x: RefCell::new(None) });
+        let b = Rc::downgrade(&a.clone());
+        *a.x.borrow_mut() = Some(b);
+
+        // hopefully we don't double-free (or leak)...
+    }
+
+    #[test]
+    fn is_unique() {
+        let x = Rc::new(3);
+        assert!(Rc::is_unique(&x));
+        let y = x.clone();
+        assert!(!Rc::is_unique(&x));
+        drop(y);
+        assert!(Rc::is_unique(&x));
+        let w = Rc::downgrade(&x);
+        assert!(!Rc::is_unique(&x));
+        drop(w);
+        assert!(Rc::is_unique(&x));
+    }
+
+    #[test]
+    fn test_strong_count() {
+        let a = Rc::new(0);
+        assert!(Rc::strong_count(&a) == 1);
+        let w = Rc::downgrade(&a);
+        assert!(Rc::strong_count(&a) == 1);
+        let b = w.upgrade().expect("upgrade of live rc failed");
+        assert!(Rc::strong_count(&b) == 2);
+        assert!(Rc::strong_count(&a) == 2);
+        drop(w);
+        drop(a);
+        assert!(Rc::strong_count(&b) == 1);
+        let c = b.clone();
+        assert!(Rc::strong_count(&b) == 2);
+        assert!(Rc::strong_count(&c) == 2);
+    }
+
+    #[test]
+    fn test_weak_count() {
+        let a = Rc::new(0);
+        assert!(Rc::strong_count(&a) == 1);
+        assert!(Rc::weak_count(&a) == 0);
+        let w = Rc::downgrade(&a);
+        assert!(Rc::strong_count(&a) == 1);
+        assert!(Rc::weak_count(&a) == 1);
+        drop(w);
+        assert!(Rc::strong_count(&a) == 1);
+        assert!(Rc::weak_count(&a) == 0);
+        let c = a.clone();
+        assert!(Rc::strong_count(&a) == 2);
+        assert!(Rc::weak_count(&a) == 0);
+        drop(c);
+    }
+
+    #[test]
+    fn try_unwrap() {
+        let x = Rc::new(3);
+        assert_eq!(Rc::try_unwrap(x), Ok(3));
+        let x = Rc::new(4);
+        let _y = x.clone();
+        assert_eq!(Rc::try_unwrap(x), Err(Rc::new(4)));
+        let x = Rc::new(5);
+        let _w = Rc::downgrade(&x);
+        assert_eq!(Rc::try_unwrap(x), Ok(5));
+    }
+
+    #[test]
+    fn get_mut() {
+        let mut x = Rc::new(3);
+        *Rc::get_mut(&mut x).unwrap() = 4;
+        assert_eq!(*x, 4);
+        let y = x.clone();
+        assert!(Rc::get_mut(&mut x).is_none());
+        drop(y);
+        assert!(Rc::get_mut(&mut x).is_some());
+        let _w = Rc::downgrade(&x);
+        assert!(Rc::get_mut(&mut x).is_none());
+    }
+
+    #[test]
+    fn test_cowrc_clone_make_unique() {
+        let mut cow0 = Rc::new(75);
+        let mut cow1 = cow0.clone();
+        let mut cow2 = cow1.clone();
+
+        assert!(75 == *Rc::make_mut(&mut cow0));
+        assert!(75 == *Rc::make_mut(&mut cow1));
+        assert!(75 == *Rc::make_mut(&mut cow2));
+
+        *Rc::make_mut(&mut cow0) += 1;
+        *Rc::make_mut(&mut cow1) += 2;
+        *Rc::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_cowrc_clone_unique2() {
+        let mut cow0 = Rc::new(75);
+        let cow1 = cow0.clone();
+        let cow2 = cow1.clone();
+
+        assert!(75 == *cow0);
+        assert!(75 == *cow1);
+        assert!(75 == *cow2);
+
+        *Rc::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_cowrc_clone_weak() {
+        let mut cow0 = Rc::new(75);
+        let cow1_weak = Rc::downgrade(&cow0);
+
+        assert!(75 == *cow0);
+        assert!(75 == *cow1_weak.upgrade().unwrap());
+
+        *Rc::make_mut(&mut cow0) += 1;
+
+        assert!(76 == *cow0);
+        assert!(cow1_weak.upgrade().is_none());
+    }
+
+    #[test]
+    fn test_show() {
+        let foo = Rc::new(75);
+        assert_eq!(format!("{:?}", foo), "75");
+    }
+
+    #[test]
+    fn test_unsized() {
+        let foo: Rc<[i32]> = Rc::new([1, 2, 3]);
+        assert_eq!(foo, foo.clone());
+    }
+
+    #[test]
+    fn test_from_owned() {
+        let foo = 123;
+        let foo_rc = Rc::from(foo);
+        assert!(123 == *foo_rc);
+    }
+
+    #[test]
+    fn test_new_weak() {
+        let foo: Weak<usize> = Weak::new();
+        assert!(foo.upgrade().is_none());
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: ?Sized> borrow::Borrow<T> for Rc<T> {
+    fn borrow(&self) -> &T {
+        &**self
+    }
+}
+
+#[stable(since = "1.5.0", feature = "smart_ptr_as_ref")]
+impl<T: ?Sized> AsRef<T> for Rc<T> {
+    fn as_ref(&self) -> &T {
+        &**self
+    }
+}