rc.rs

// 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)]

//! Single-threaded reference-counting pointers.
//!
//! The type [`Rc<T>`][`Rc`] provides shared ownership of a value of type `T`,
//! allocated in the heap. Invoking [`clone()`][clone] on [`Rc`] produces a new
//! pointer to the same value in the heap. When the last [`Rc`] pointer to a
//! given value is destroyed, the pointed-to value is also destroyed.
//!
//! Shared references in Rust disallow mutation by default, and [`Rc`]
//! is no exception: you cannot obtain a mutable reference to
//! something inside an [`Rc`]. If you need mutability, put a [`Cell`]
//! or [`RefCell`] inside the [`Rc`]; see [an example of mutability
//! inside an Rc][mutability].
//!
//! [`Rc`] uses non-atomic reference counting. This means that overhead is very
//! low, but an [`Rc`] cannot be sent between threads, and consequently [`Rc`]
//! does not implement [`Send`][send]. As a result, the Rust compiler
//! will check *at compile time* that you are not sending [`Rc`]s between
//! threads. If you need multi-threaded, atomic reference counting, use
//! [`sync::Arc`][arc].
//!
//! The [`downgrade()`][downgrade] method can be used to create a non-owning
//! [`Weak`] pointer. A [`Weak`] pointer can be [`upgrade`][upgrade]d
//! to an [`Rc`], but this will return [`None`] if the value has
//! already been dropped.
//!
//! A cycle between [`Rc`] pointers will never be deallocated. For this reason,
//! [`Weak`] is used to break cycles. For example, a tree could have strong
//! [`Rc`] pointers from parent nodes to children, and [`Weak`] pointers from
//! children back to their parents.
//!
//! `Rc<T>` automatically dereferences to `T` (via the [`Deref`] trait),
//! so you can call `T`'s methods on a value of type [`Rc<T>`][`Rc`]. To avoid name
//! clashes with `T`'s methods, the methods of [`Rc<T>`][`Rc`] itself are [associated
//! functions][assoc], called using function-like syntax:
//!
//! ```
//! use std::rc::Rc;
//! let my_rc = Rc::new(());
//!
//! Rc::downgrade(&my_rc);
//! ```
//!
//! [`Weak<T>`][`Weak`] does not auto-dereference to `T`, because the value may have
//! already been destroyed.
//!
//! # 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`] allows us to share an `Owner` between multiple `Gadget`s,
//! and have the `Owner` remain allocated as long as any `Gadget` points at it.
//!
//! ```
//! 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: "Gadget Man".to_string(),
//!         }
//!     );
//!
//!     // Create `Gadget`s belonging to `gadget_owner`. Cloning the `Rc<Owner>`
//!     // value gives us a new pointer to the same `Owner` value, incrementing
//!     // the reference count in the process.
//!     let gadget1 = Gadget {
//!         id: 1,
//!         owner: gadget_owner.clone(),
//!     };
//!     let gadget2 = Gadget {
//!         id: 2,
//!         owner: gadget_owner.clone(),
//!     };
//!
//!     // Dispose of our local variable `gadget_owner`.
//!     drop(gadget_owner);
//!
//!     // Despite dropping `gadget_owner`, we're still able to print out the name
//!     // of the `Owner` of the `Gadget`s. This is because we've only dropped a
//!     // single `Rc<Owner>`, not the `Owner` it points to. As long as there are
//!     // other `Rc<Owner>` values pointing at the same `Owner`, it will remain
//!     // allocated. The field projection `gadget1.owner.name` works because
//!     // `Rc<Owner>` automatically dereferences to `Owner`.
//!     println!("Gadget {} owned by {}", gadget1.id, gadget1.owner.name);
//!     println!("Gadget {} owned by {}", gadget2.id, gadget2.owner.name);
//!
//!     // At the end of the function, `gadget1` and `gadget2` are 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` to `Gadget`, we will run into problems. An [`Rc`] pointer from `Owner`
//! to `Gadget` introduces a cycle between the values. This means that their
//! reference counts can never reach 0, and the values will remain allocated
//! forever: a memory leak. In order to get around this, we can use [`Weak`]
//! pointers.
//!
//! Rust actually makes it somewhat difficult to produce this loop in the first
//! place. In order to end up with two values that point at each other, one of
//! them needs to be mutable. This is difficult because [`Rc`] enforces
//! memory safety by only giving out shared references to the value it wraps,
//! and these don't allow direct mutation. We need to wrap the part of the
//! value 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.
//!
//! ```
//! 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 that we've put the `Owner`'s
//!     // vector of `Gadget`s 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![]),
//!         }
//!     );
//!
//!     // Create `Gadget`s 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 `Gadget`s to their `Owner`.
//!     {
//!         let mut gadgets = gadget_owner.gadgets.borrow_mut();
//!         gadgets.push(Rc::downgrade(&gadget1));
//!         gadgets.push(Rc::downgrade(&gadget2));
//!
//!         // `RefCell` dynamic borrow ends here.
//!     }
//!
//!     // Iterate over our `Gadget`s, printing their details out.
//!     for gadget_weak in gadget_owner.gadgets.borrow().iter() {
//!
//!         // `gadget_weak` is a `Weak<Gadget>`. Since `Weak` pointers can't
//!         // guarantee the value is still allocated, we need to call
//!         // `upgrade`, which returns an `Option<Rc<Gadget>>`.
//!         //
//!         // In this case we know the value still exists, so we simply
//!         // `unwrap` the `Option`. In a more complicated program, you might
//!         // need graceful error handling for a `None` result.
//!
//!         let gadget = gadget_weak.upgrade().unwrap();
//!         println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
//!     }
//!
//!     // At the end of the function, `gadget_owner`, `gadget1`, and `gadget2`
//!     // are destroyed. There are now no strong (`Rc`) pointers to the
//!     // gadgets, so they are destroyed. This zeroes the reference count on
//!     // Gadget Man, so he gets destroyed as well.
//! }
//! ```
//!
//! [`Rc`]: struct.Rc.html
//! [`Weak`]: struct.Weak.html
//! [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
//! [`Cell`]: ../../std/cell/struct.Cell.html
//! [`RefCell`]: ../../std/cell/struct.RefCell.html
//! [send]: ../../std/marker/trait.Send.html
//! [arc]: ../../std/sync/struct.Arc.html
//! [`Deref`]: ../../std/ops/trait.Deref.html
//! [downgrade]: struct.Rc.html#method.downgrade
//! [upgrade]: struct.Weak.html#method.upgrade
//! [`None`]: ../../std/option/enum.Option.html#variant.None
//! [assoc]: ../../book/method-syntax.html#associated-functions
//! [mutability]: ../../std/cell/index.html#introducing-mutability-inside-of-something-immutable

#![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::{Hash, Hasher};
use core::intrinsics::{abort, assume};
use core::marker;
use core::marker::Unsize;
use core::mem::{self, align_of_val, forget, size_of, size_of_val, uninitialized};
use core::ops::Deref;
use core::ops::CoerceUnsized;
use core::ptr::{self, Shared};
use core::convert::From;

use heap::deallocate;
use raw_vec::RawVec;

struct RcBox<T: ?Sized> {
    strong: Cell<usize>,
    weak: Cell<usize>,
    value: T,
}


/// A single-threaded reference-counting pointer.
///
/// See the [module-level documentation](./index.html) for more details.
///
/// The inherent methods of `Rc` are all associated functions, which means
/// that you have to call them as e.g. [`Rc::get_mut(&value)`][get_mut] instead of
/// `value.get_mut()`. This avoids conflicts with methods of the inner
/// type `T`.
///
/// [get_mut]: #method.get_mut
#[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,
                })),
            }
        }
    }

    /// Returns the contained value, if the `Rc` has exactly one strong reference.
    ///
    /// Otherwise, an [`Err`][result] is returned with the same `Rc` that was
    /// passed in.
    ///
    /// This will succeed even if there are outstanding weak references.
    ///
    /// [result]: ../../std/result/enum.Result.html
    ///
    /// # 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).unwrap_err(), 4);
    /// ```
    #[inline]
    #[stable(feature = "rc_unique", since = "1.4.0")]
    pub fn try_unwrap(this: Self) -> Result<T, Self> {
        if Rc::strong_count(&this) == 1 {
            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 whether [`Rc::try_unwrap`][try_unwrap] would return
    /// [`Ok`].
    ///
    /// [try_unwrap]: struct.Rc.html#method.try_unwrap
    /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
    #[unstable(feature = "rc_would_unwrap",
               reason = "just added for niche usecase",
               issue = "28356")]
    #[rustc_deprecated(since = "1.15.0", reason = "too niche; use `strong_count` instead")]
    pub fn would_unwrap(this: &Self) -> bool {
        Rc::strong_count(&this) == 1
    }

    /// Consumes the `Rc`, returning the wrapped pointer.
    ///
    /// To avoid a memory leak the pointer must be converted back to an `Rc` using
    /// [`Rc::from_raw`][from_raw].
    ///
    /// [from_raw]: struct.Rc.html#method.from_raw
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(rc_raw)]
    ///
    /// use std::rc::Rc;
    ///
    /// let x = Rc::new(10);
    /// let x_ptr = Rc::into_raw(x);
    /// assert_eq!(unsafe { *x_ptr }, 10);
    /// ```
    #[unstable(feature = "rc_raw", issue = "37197")]
    pub fn into_raw(this: Self) -> *mut T {
        let ptr = unsafe { &mut (**this.ptr).value as *mut _ };
        mem::forget(this);
        ptr
    }

    /// Constructs an `Rc` from a raw pointer.
    ///
    /// The raw pointer must have been previously returned by a call to a
    /// [`Rc::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.Rc.html#method.into_raw
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(rc_raw)]
    ///
    /// use std::rc::Rc;
    ///
    /// let x = Rc::new(10);
    /// let x_ptr = Rc::into_raw(x);
    ///
    /// unsafe {
    ///     // Convert back to an `Rc` to prevent leak.
    ///     let x = Rc::from_raw(x_ptr);
    ///     assert_eq!(*x, 10);
    ///
    ///     // Further calls to `Rc::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!
    /// ```
    #[unstable(feature = "rc_raw", issue = "37197")]
    pub unsafe fn from_raw(ptr: *mut T) -> Self {
        // To find the corresponding pointer to the `RcBox` we need to subtract the offset of the
        // `value` field from the pointer.
        Rc { ptr: Shared::new((ptr as *mut u8).offset(-offset_of!(RcBox<T>, value)) as *mut _) }
    }
}

impl Rc<str> {
    /// Constructs a new `Rc<str>` from a string slice.
    #[doc(hidden)]
    #[unstable(feature = "rustc_private",
               reason = "for internal use in rustc",
               issue = "0")]
    pub fn __from_str(value: &str) -> Rc<str> {
        unsafe {
            // Allocate enough space for `RcBox<str>`.
            let aligned_len = 2 + (value.len() + size_of::<usize>() - 1) / size_of::<usize>();
            let vec = RawVec::<usize>::with_capacity(aligned_len);
            let ptr = vec.ptr();
            forget(vec);
            // Initialize fields of `RcBox<str>`.
            *ptr.offset(0) = 1; // strong: Cell::new(1)
            *ptr.offset(1) = 1; // weak: Cell::new(1)
            ptr::copy_nonoverlapping(value.as_ptr(), ptr.offset(2) as *mut u8, value.len());
            // Combine the allocation address and the string length into a fat pointer to `RcBox`.
            let rcbox_ptr: *mut RcBox<str> = mem::transmute([ptr as usize, value.len()]);
            assert!(aligned_len * size_of::<usize>() == size_of_val(&*rcbox_ptr));
            Rc { ptr: Shared::new(rcbox_ptr) }
        }
    }
}

impl<T: ?Sized> Rc<T> {
    /// Creates a new [`Weak`][weak] pointer to this value.
    ///
    /// [weak]: struct.Weak.html
    ///
    /// # 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 }
    }

    /// Gets the number of [`Weak`][weak] pointers to this value.
    ///
    /// [weak]: struct.Weak.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    /// let _weak_five = Rc::downgrade(&five);
    ///
    /// assert_eq!(1, Rc::weak_count(&five));
    /// ```
    #[inline]
    #[stable(feature = "rc_counts", since = "1.15.0")]
    pub fn weak_count(this: &Self) -> usize {
        this.weak() - 1
    }

    /// Gets the number of strong (`Rc`) pointers to this value.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    /// let _also_five = five.clone();
    ///
    /// assert_eq!(2, Rc::strong_count(&five));
    /// ```
    #[inline]
    #[stable(feature = "rc_counts", since = "1.15.0")]
    pub fn strong_count(this: &Self) -> usize {
        this.strong()
    }

    /// Returns true if there are no other `Rc` or [`Weak`][weak] pointers to
    /// this inner value.
    ///
    /// [weak]: struct.Weak.html
    #[inline]
    #[unstable(feature = "is_unique", reason = "uniqueness has unclear meaning",
               issue = "28356")]
    #[rustc_deprecated(since = "1.15.0",
                       reason = "too niche; use `strong_count` and `weak_count` instead")]
    pub fn is_unique(this: &Self) -> bool {
        Rc::weak_count(this) == 0 && Rc::strong_count(this) == 1
    }

    /// Returns a mutable reference to the inner value, if there are
    /// no other `Rc` or [`Weak`][weak] pointers to the same value.
    ///
    /// Returns [`None`] 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
    /// [`None`]: ../../std/option/enum.Option.html#variant.None
    /// [make_mut]: struct.Rc.html#method.make_mut
    /// [clone]: ../../std/clone/trait.Clone.html#tymethod.clone
    ///
    /// # 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
        }
    }

    #[inline]
    #[unstable(feature = "ptr_eq",
               reason = "newly added",
               issue = "36497")]
    /// Returns true if the two `Rc`s point to the same value (not
    /// just values that compare as equal).
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(ptr_eq)]
    ///
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    /// let same_five = five.clone();
    /// let other_five = Rc::new(5);
    ///
    /// assert!(Rc::ptr_eq(&five, &same_five));
    /// assert!(!Rc::ptr_eq(&five, &other_five));
    /// ```
    pub fn ptr_eq(this: &Self, other: &Self) -> bool {
        let this_ptr: *const RcBox<T> = *this.ptr;
        let other_ptr: *const RcBox<T> = *other.ptr;
        this_ptr == other_ptr
    }
}

impl<T: Clone> Rc<T> {
    /// Makes a mutable reference into the given `Rc`.
    ///
    /// If there are other `Rc` 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.Rc.html#method.get_mut
    ///
    /// # 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
    ///
    /// // Now `data` and `other_data` point to different values.
    /// 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")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for Rc<T> {
    /// Drops the `Rc`.
    ///
    /// 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::rc::Rc;
    ///
    /// struct Foo;
    ///
    /// impl Drop for Foo {
    ///     fn drop(&mut self) {
    ///         println!("dropped!");
    ///     }
    /// }
    ///
    /// let foo  = Rc::new(Foo);
    /// let foo2 = foo.clone();
    ///
    /// drop(foo);    // Doesn't print anything
    /// drop(foo2);   // Prints "dropped!"
    /// ```
    fn drop(&mut self) {
        unsafe {
            let ptr = *self.ptr;

            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` pointer.
    ///
    /// This creates another pointer to the same inner value, increasing the
    /// strong reference count.
    ///
    /// # 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();
    /// assert_eq!(*x, 0);
    /// ```
    #[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`s.
    ///
    /// Two `Rc`s are equal if their inner values are equal.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    ///
    /// assert!(five == Rc::new(5));
    /// ```
    #[inline(always)]
    fn eq(&self, other: &Rc<T>) -> bool {
        **self == **other
    }

    /// Inequality for two `Rc`s.
    ///
    /// Two `Rc`s are unequal if their inner values are unequal.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    ///
    /// assert!(five != Rc::new(6));
    /// ```
    #[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`s.
    ///
    /// The two are compared by calling `partial_cmp()` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    /// use std::cmp::Ordering;
    ///
    /// let five = Rc::new(5);
    ///
    /// assert_eq!(Some(Ordering::Less), five.partial_cmp(&Rc::new(6)));
    /// ```
    #[inline(always)]
    fn partial_cmp(&self, other: &Rc<T>) -> Option<Ordering> {
        (**self).partial_cmp(&**other)
    }

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

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

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

    /// 'Greater than or equal to' comparison for two `Rc`s.
    ///
    /// The two are compared by calling `>=` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    ///
    /// assert!(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`s.
    ///
    /// The two are compared by calling `cmp()` on their inner values.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    /// use std::cmp::Ordering;
    ///
    /// let five = Rc::new(5);
    ///
    /// assert_eq!(Ordering::Less, five.cmp(&Rc::new(6)));
    /// ```
    #[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`][rc].
///
/// `Weak` pointers do not count towards determining if the inner value
/// should be dropped.
///
/// The typical way to obtain a `Weak` pointer is to call
/// [`Rc::downgrade`][downgrade].
///
/// See the [module-level documentation](./index.html) for more details.
///
/// [rc]: struct.Rc.html
/// [downgrade]: struct.Rc.html#method.downgrade
#[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> Weak<T> {
    /// Constructs a new `Weak<T>`, without an accompanying instance of `T`.
    ///
    /// This allocates memory for `T`, but does not initialize it. Calling
    /// [`upgrade`][upgrade] on the return value always gives
    /// [`None`][option].
    ///
    /// [upgrade]: struct.Weak.html#method.upgrade
    /// [option]: ../../std/option/enum.Option.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::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: Shared::new(Box::into_raw(box RcBox {
                    strong: Cell::new(0),
                    weak: Cell::new(1),
                    value: uninitialized(),
                })),
            }
        }
    }
}

impl<T: ?Sized> Weak<T> {
    /// Upgrades the `Weak` pointer to an [`Rc`][rc], if possible.
    ///
    /// Returns [`None`][option] if the strong count has reached zero and the
    /// inner value was destroyed.
    ///
    /// [rc]: struct.Rc.html
    /// [option]: ../../std/option/enum.Option.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// let five = Rc::new(5);
    ///
    /// let weak_five = Rc::downgrade(&five);
    ///
    /// let strong_five: Option<Rc<_>> = 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 = "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` pointer.
    ///
    /// This will decrement the weak reference count.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Rc;
    ///
    /// struct Foo;
    ///
    /// impl Drop for Foo {
    ///     fn drop(&mut self) {
    ///         println!("dropped!");
    ///     }
    /// }
    ///
    /// let foo = Rc::new(Foo);
    /// let weak_foo = Rc::downgrade(&foo);
    /// let other_weak_foo = weak_foo.clone();
    ///
    /// drop(weak_foo);   // Doesn't print anything
    /// drop(foo);        // Prints "dropped!"
    ///
    /// assert!(other_weak_foo.upgrade().is_none());
    /// ```
    fn drop(&mut self) {
        unsafe {
            let ptr = *self.ptr;

            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` pointer.
    ///
    /// This creates another pointer to the same inner value, increasing 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)")
    }
}

#[stable(feature = "downgraded_weak", since = "1.10.0")]
impl<T> Default for Weak<T> {
    /// Constructs a new `Weak<T>`, without an accompanying instance of `T`.
    ///
    /// This allocates memory for `T`, but does not initialize it. Calling
    /// [`upgrade`][upgrade] on the return value always gives
    /// [`None`][option].
    ///
    /// [upgrade]: struct.Weak.html#method.upgrade
    /// [option]: ../../std/option/enum.Option.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::rc::Weak;
    ///
    /// let empty: Weak<i64> = Default::default();
    /// assert!(empty.upgrade().is_none());
    /// ```
    fn default() -> Weak<T> {
        Weak::new()
    }
}

// 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::{None, Some};
    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 into_from_raw() {
        let x = Rc::new(box "hello");
        let y = x.clone();

        let x_ptr = Rc::into_raw(x);
        drop(y);
        unsafe {
            assert_eq!(**x_ptr, "hello");

            let x = Rc::from_raw(x_ptr);
            assert_eq!(**x, "hello");

            assert_eq!(Rc::try_unwrap(x).map(|x| *x), Ok("hello"));
        }
    }

    #[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());
    }

    #[test]
    fn test_ptr_eq() {
        let five = Rc::new(5);
        let same_five = five.clone();
        let other_five = Rc::new(5);

        assert!(Rc::ptr_eq(&five, &same_five));
        assert!(!Rc::ptr_eq(&five, &other_five));
    }
}

#[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
    }
}