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    #![allow(deprecated)]

    //! Single-threaded reference-counting pointers. 'Rc' stands for 'Reference
    //! Counted'.
    //!
    //! 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 generally 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, 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.
    //!
    //! # Cloning references
    //!
    //! Creating a new reference from an existing reference counted pointer is done using the
    //! `Clone` trait implemented for [`Rc<T>`][`Rc`] and [`Weak<T>`][`Weak`].
    //!
    //! ```
    //! use std::rc::Rc;
    //! let foo = Rc::new(vec![1.0, 2.0, 3.0]);
    //! // The two syntaxes below are equivalent.
    //! let a = foo.clone();
    //! let b = Rc::clone(&foo);
    //! // a and b both point to the same memory location as foo.
    //! ```
    //!
    //! The `Rc::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.
    //!
    //! # 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: Rc::clone(&gadget_owner),
    //!     };
    //!     let gadget2 = Gadget {
    //!         id: 2,
    //!         owner: Rc::clone(&gadget_owner),
    //!     };
    //!
    //!     // 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: Rc::clone(&gadget_owner),
    //!         }
    //!     );
    //!     let gadget2 = Rc::new(
    //!         Gadget {
    //!             id: 2,
    //!             owner: Rc::clone(&gadget_owner),
    //!         }
    //!     );
    //!
    //!     // 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
    //! [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::any::Any;
    use core::borrow;
    use core::cell::Cell;
    use core::cmp::Ordering;
    use core::fmt;
    use core::hash::{Hash, Hasher};
    use core::intrinsics::abort;
    use core::marker;
    use core::marker::{Unpin, Unsize, PhantomData};
    use core::mem::{self, align_of_val, forget, size_of_val};
    use core::ops::{Deref, Receiver};
    use core::ops::{CoerceUnsized, DispatchFromDyn};
    use core::pin::Pin;
    use core::ptr::{self, NonNull};
    use core::convert::From;
    use core::usize;

    use alloc::{Global, Alloc, Layout, box_free, handle_alloc_error};
    use string::String;
    use vec::Vec;

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

    /// A single-threaded reference-counting pointer. 'Rc' stands for 'Reference
    /// Counted'.
    ///
    /// 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(&mut value)`][get_mut] instead of
    /// `value.get_mut()`. This avoids conflicts with methods of the inner
    /// type `T`.
    ///
    /// [get_mut]: #method.get_mut
    #[cfg_attr(not(test), lang = "rc")]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Rc<T: ?Sized> {
        ptr: NonNull<RcBox<T>>,
        phantom: PhantomData<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> {}

    #[unstable(feature = "dispatch_from_dyn", issue = "0")]
    impl<T: ?Sized + Unsize<U>, U: ?Sized> DispatchFromDyn<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> {
            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: Box::into_raw_non_null(box RcBox {
                    strong: Cell::new(1),
                    weak: Cell::new(1),
                    value,
                }),
                phantom: PhantomData,
            }
        }

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

    impl<T: ?Sized> Rc<T> {
        /// 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
        ///
        /// ```
        /// use std::rc::Rc;
        ///
        /// let x = Rc::new(10);
        /// let x_ptr = Rc::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 `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
        ///
        /// ```
        /// 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!
        /// ```
        #[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 RcBox.
            // Because it is ?Sized, it will always be the last field in memory.
            let align = align_of_val(&*ptr);
            let layout = Layout::new::<RcBox<()>>();
            let offset = (layout.size() + layout.padding_needed_for(align)) as isize;

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

            Rc {
                ptr: NonNull::new_unchecked(rc_ptr),
                phantom: PhantomData,
            }
        }

        /// 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();
            // Make sure we do not create a dangling Weak
            debug_assert!(!is_dangling(this.ptr));
            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 = Rc::clone(&five);
        ///
        /// 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]
        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 = Rc::clone(&x);
        /// 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) {
                unsafe {
                    Some(&mut this.ptr.as_mut().value)
                }
            } else {
                None
            }
        }

        #[inline]
        #[stable(feature = "ptr_eq", since = "1.17.0")]
        /// Returns true if the two `Rc`s point to the same value (not
        /// just values that compare as equal).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::rc::Rc;
        ///
        /// let five = Rc::new(5);
        /// let same_five = Rc::clone(&five);
        /// 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 {
            this.ptr.as_ptr() == other.ptr.as_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 = Rc::clone(&data);    // 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.as_ref().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.
            unsafe {
                &mut this.ptr.as_mut().value
            }
        }
    }

    impl Rc<dyn Any> {
        #[inline]
        #[stable(feature = "rc_downcast", since = "1.29.0")]
        /// Attempt to downcast the `Rc<dyn Any>` to a concrete type.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::any::Any;
        /// use std::rc::Rc;
        ///
        /// fn print_if_string(value: Rc<dyn Any>) {
        ///     if let Ok(string) = value.downcast::<String>() {
        ///         println!("String ({}): {}", string.len(), string);
        ///     }
        /// }
        ///
        /// fn main() {
        ///     let my_string = "Hello World".to_string();
        ///     print_if_string(Rc::new(my_string));
        ///     print_if_string(Rc::new(0i8));
        /// }
        /// ```
        pub fn downcast<T: Any>(self) -> Result<Rc<T>, Rc<dyn Any>> {
            if (*self).is::<T>() {
                let ptr = self.ptr.cast::<RcBox<T>>();
                forget(self);
                Ok(Rc { ptr, phantom: PhantomData })
            } else {
                Err(self)
            }
        }
    }

    impl<T: ?Sized> Rc<T> {
        // Allocates an `RcBox<T>` with sufficient space for an unsized value
        unsafe fn allocate_for_ptr(ptr: *const T) -> *mut RcBox<T> {
            // Calculate layout using the given value.
            // Previously, layout was calculated on the expression
            // `&*(ptr as *const RcBox<T>)`, but this created a misaligned
            // reference (see #54908).
            let layout = Layout::new::<RcBox<()>>()
                .extend(Layout::for_value(&*ptr)).unwrap().0
                .pad_to_align().unwrap();

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

            // Initialize the RcBox
            let inner = set_data_ptr(ptr as *mut T, mem.as_ptr() as *mut u8) as *mut RcBox<T>;
            debug_assert_eq!(Layout::for_value(&*inner), layout);

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

            inner
        }

        fn from_box(v: Box<T>) -> Rc<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).value as *mut _ as *mut u8,
                    value_size);

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

                Rc { 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> Rc<[T]> {
        // Copy elements from slice into newly allocated Rc<[T]>
        //
        // Unsafe because the caller must either take ownership or bind `T: Copy`
        unsafe fn copy_from_slice(v: &[T]) -> Rc<[T]> {
            let v_ptr = v as *const [T];
            let ptr = Self::allocate_for_ptr(v_ptr);

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

            Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
        }
    }

    trait RcFromSlice<T> {
        fn from_slice(slice: &[T]) -> Self;
    }

    impl<T: Clone> RcFromSlice<T> for Rc<[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 RcBox 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, 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).value 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.add(i), item.clone());
                    guard.n_elems += 1;
                }

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

                Rc { ptr: NonNull::new_unchecked(ptr), phantom: PhantomData }
            }
        }
    }

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

    #[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
        }
    }

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

    #[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`], so we `drop` the inner value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::rc::Rc;
        ///
        /// struct Foo;
        ///
        /// impl Drop for Foo {
        ///     fn drop(&mut self) {
        ///         println!("dropped!");
        ///     }
        /// }
        ///
        /// let foo  = Rc::new(Foo);
        /// let foo2 = Rc::clone(&foo);
        ///
        /// drop(foo);    // Doesn't print anything
        /// drop(foo2);   // Prints "dropped!"
        /// ```
        ///
        /// [`Weak`]: ../../std/rc/struct.Weak.html
        fn drop(&mut self) {
            unsafe {
                self.dec_strong();
                if self.strong() == 0 {
                    // destroy the contained object
                    ptr::drop_in_place(self.ptr.as_mut());

                    // remove the implicit "strong weak" pointer now that we've
                    // destroyed the contents.
                    self.dec_weak();

                    if self.weak() == 0 {
                        Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
                    }
                }
            }
        }
    }

    #[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);
        ///
        /// let _ = Rc::clone(&five);
        /// ```
        #[inline]
        fn clone(&self) -> Rc<T> {
            self.inc_strong();
            Rc { ptr: self.ptr, phantom: PhantomData }
        }
    }

    #[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")]
    trait RcEqIdent<T: ?Sized + PartialEq> {
        fn eq(&self, other: &Rc<T>) -> bool;
        fn ne(&self, other: &Rc<T>) -> bool;
    }

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized + Eq> RcEqIdent<T> for Rc<T> {
        #[inline]
        fn eq(&self, other: &Rc<T>) -> bool {
            Rc::ptr_eq(self, other) || **self == **other
        }

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

    #[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.
        ///
        /// If `T` also implements `Eq`, two `Rc`s that point to the same value are
        /// always equal.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::rc::Rc;
        ///
        /// let five = Rc::new(5);
        ///
        /// assert!(five == Rc::new(5));
        /// ```
        #[inline]
        fn eq(&self, other: &Rc<T>) -> bool {
            RcEqIdent::eq(self, other)
        }

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

    #[stable(feature = "shared_from_slice", since = "1.21.0")]
    impl<'a, T: Clone> From<&'a [T]> for Rc<[T]> {
        #[inline]
        fn from(v: &[T]) -> Rc<[T]> {
            <Self as RcFromSlice<T>>::from_slice(v)
        }
    }

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

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

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

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

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

                rc
            }
        }
    }

    /// `Weak` is a version of [`Rc`] 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`]`<`[`Rc`]`<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 [`Rc`] without extending its lifetime. It is also used to prevent
    /// circular references between [`Rc`] pointers, since mutual owning references
    /// would never allow either [`Rc`] to be dropped. For example, a tree could
    /// have strong [`Rc`] 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 [`Rc::downgrade`].
    ///
    /// [`Rc`]: struct.Rc.html
    /// [`Rc::downgrade`]: struct.Rc.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 = "rc_weak", since = "1.4.0")]
    pub struct Weak<T: ?Sized> {
        // This is a `NonNull` to allow optimizing the size of this type in enums,
        // but it is not necessarily a valid pointer.
        // `Weak::new` sets this to `usize::MAX` so that it doesn’t need
        // to allocate space on the heap.  That's not a value a real pointer
        // will ever have because RcBox has alignment at least 2.
        ptr: NonNull<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> {}

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

    impl<T> Weak<T> {
        /// Constructs a new `Weak<T>`, without allocating any memory.
        /// Calling [`upgrade`] on the return value always gives [`None`].
        ///
        /// [`upgrade`]: #method.upgrade
        /// [`None`]: ../../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> {
            Weak {
                ptr: NonNull::new(usize::MAX as *mut RcBox<T>).expect("MAX is not 0"),
            }
        }
    }

    pub(crate) fn is_dangling<T: ?Sized>(ptr: NonNull<T>) -> bool {
        let address = ptr.as_ptr() as *mut () as usize;
        address == usize::MAX
    }

    impl<T: ?Sized> Weak<T> {
        /// Attempts to upgrade the `Weak` pointer to an [`Rc`], extending
        /// the lifetime of the value if successful.
        ///
        /// Returns [`None`] if the value has since been dropped.
        ///
        /// [`Rc`]: struct.Rc.html
        /// [`None`]: ../../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>> {
            let inner = self.inner()?;
            if inner.strong() == 0 {
                None
            } else {
                inner.inc_strong();
                Some(Rc { ptr: self.ptr, phantom: PhantomData })
            }
        }

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

        /// Returns true if the two `Weak`s point to the same value (not just values
        /// that compare as equal).
        ///
        /// # Notes
        ///
        /// Since this compares pointers it means that `Weak::new()` will equal each
        /// other, even though they don't point to any value.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(weak_ptr_eq)]
        /// use std::rc::{Rc, Weak};
        ///
        /// let first_rc = Rc::new(5);
        /// let first = Rc::downgrade(&first_rc);
        /// let second = Rc::downgrade(&first_rc);
        ///
        /// assert!(Weak::ptr_eq(&first, &second));
        ///
        /// let third_rc = Rc::new(5);
        /// let third = Rc::downgrade(&third_rc);
        ///
        /// assert!(!Weak::ptr_eq(&first, &third));
        /// ```
        ///
        /// Comparing `Weak::new`.
        ///
        /// ```
        /// #![feature(weak_ptr_eq)]
        /// use std::rc::{Rc, Weak};
        ///
        /// let first = Weak::new();
        /// let second = Weak::new();
        /// assert!(Weak::ptr_eq(&first, &second));
        ///
        /// let third_rc = Rc::new(());
        /// let third = Rc::downgrade(&third_rc);
        /// assert!(!Weak::ptr_eq(&first, &third));
        /// ```
        #[inline]
        #[unstable(feature = "weak_ptr_eq", issue = "55981")]
        pub fn ptr_eq(this: &Self, other: &Self) -> bool {
            this.ptr.as_ptr() == other.ptr.as_ptr()
        }
    }

    #[stable(feature = "rc_weak", since = "1.4.0")]
    impl<T: ?Sized> Drop for Weak<T> {
        /// Drops the `Weak` pointer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::rc::{Rc, Weak};
        ///
        /// 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::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 let Some(inner) = self.inner() {
                inner.dec_weak();
                // the weak count starts at 1, and will only go to zero if all
                // the strong pointers have disappeared.
                if inner.weak() == 0 {
                    unsafe {
                        Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()));
                    }
                }
            }
        }
    }

    #[stable(feature = "rc_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::rc::{Rc, Weak};
        ///
        /// let weak_five = Rc::downgrade(&Rc::new(5));
        ///
        /// let _ = Weak::clone(&weak_five);
        /// ```
        #[inline]
        fn clone(&self) -> Weak<T> {
            if let Some(inner) = self.inner() {
                inner.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>`, allocating memory for `T` without initializing
        /// it. Calling [`upgrade`] on the return value always gives [`None`].
        ///
        /// [`None`]: ../../std/option/enum.Option.html
        /// [`upgrade`]: ../../std/rc/struct.Weak.html#method.upgrade
        ///
        /// # 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 safely. 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) {
            // We want to abort on overflow instead of dropping the value.
            // The reference count will never be zero when this is called;
            // nevertheless, we insert an abort here to hint LLVM at
            // an otherwise missed optimization.
            if self.strong() == 0 || self.strong() == usize::max_value() {
                unsafe { abort(); }
            }
            self.inner().strong.set(self.strong() + 1);
        }

        #[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) {
            // We want to abort on overflow instead of dropping the value.
            // The reference count will never be zero when this is called;
            // nevertheless, we insert an abort here to hint LLVM at
            // an otherwise missed optimization.
            if self.weak() == 0 || self.weak() == usize::max_value() {
                unsafe { abort(); }
            }
            self.inner().weak.set(self.weak() + 1);
        }

        #[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 {
                self.ptr.as_ref()
            }
        }
    }

    impl<T: ?Sized> RcBoxPtr<T> for RcBox<T> {
        #[inline(always)]
        fn inner(&self) -> &RcBox<T> {
            self
        }
    }

    #[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 test_into_from_raw_unsized() {
            use std::fmt::Display;
            use std::string::ToString;

            let rc: Rc<str> = Rc::from("foo");

            let ptr = Rc::into_raw(rc.clone());
            let rc2 = unsafe { Rc::from_raw(ptr) };

            assert_eq!(unsafe { &*ptr }, "foo");
            assert_eq!(rc, rc2);

            let rc: Rc<dyn Display> = Rc::new(123);

            let ptr = Rc::into_raw(rc.clone());
            let rc2 = unsafe { Rc::from_raw(ptr) };

            assert_eq!(unsafe { &*ptr }.to_string(), "123");
            assert_eq!(rc2.to_string(), "123");
        }

        #[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));
        }

        #[test]
        fn test_from_str() {
            let r: Rc<str> = Rc::from("foo");

            assert_eq!(&r[..], "foo");
        }

        #[test]
        fn test_copy_from_slice() {
            let s: &[u32] = &[1, 2, 3];
            let r: Rc<[u32]> = Rc::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: Rc<[X]> = Rc::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: Rc<[Fail]> = Rc::from(s);
        }

        #[test]
        fn test_from_box() {
            let b: Box<u32> = box 123;
            let r: Rc<u32> = Rc::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: Rc<str> = Rc::from(s);

            assert_eq!(&r[..], "foo");
        }

        #[test]
        fn test_from_box_slice() {
            let s = vec![1, 2, 3].into_boxed_slice();
            let r: Rc<[u32]> = Rc::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<dyn Display> = box 123;
            let r: Rc<dyn Display> = Rc::from(b);

            assert_eq!(r.to_string(), "123");
        }

        #[test]
        fn test_from_box_trait_zero_sized() {
            use std::fmt::Debug;

            let b: Box<dyn Debug> = box ();
            let r: Rc<dyn Debug> = Rc::from(b);

            assert_eq!(format!("{:?}", r), "()");
        }

        #[test]
        fn test_from_vec() {
            let v = vec![1, 2, 3];
            let r: Rc<[u32]> = Rc::from(v);

            assert_eq!(&r[..], [1, 2, 3]);
        }

        #[test]
        fn test_downcast() {
            use std::any::Any;

            let r1: Rc<dyn Any> = Rc::new(i32::max_value());
            let r2: Rc<dyn Any> = Rc::new("abc");

            assert!(r1.clone().downcast::<u32>().is_err());

            let r1i32 = r1.downcast::<i32>();
            assert!(r1i32.is_ok());
            assert_eq!(r1i32.unwrap(), Rc::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(), Rc::new("abc"));
        }
    }

    #[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
        }
    }

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

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