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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Shareable mutable containers.
    //!
    //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
    //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
    //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
    //! with typical Rust types that exhibit 'inherited mutability'.
    //!
    //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
    //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
    //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
    //! methods to retrieve and change the current interior value:
    //!
    //!  - For types that implement `Copy`, the `get` method retrieves the current interior value.
    //!  - For types that implement `Default`, the `take` method replaces the current interior value
    //!    with `Default::default()` and returns the replaced value.
    //!  - For all types, the `replace` method replaces the current interior value and returns the
    //!    replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
    //!    value. Additionally, the `set` method replaces the interior value, dropping the replaced
    //!    value.
    //!
    //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
    //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
    //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
    //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
    //! to borrow a value that is already mutably borrowed; when this happens it results in thread
    //! panic.
    //!
    //! # When to choose interior mutability
    //!
    //! The more common inherited mutability, where one must have unique access to mutate a value, is
    //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
    //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
    //! interior mutability is something of a last resort. Since cell types enable mutation where it
    //! would otherwise be disallowed though, there are occasions when interior mutability might be
    //! appropriate, or even *must* be used, e.g.
    //!
    //! * Introducing mutability 'inside' of something immutable
    //! * Implementation details of logically-immutable methods.
    //! * Mutating implementations of `Clone`.
    //!
    //! ## Introducing mutability 'inside' of something immutable
    //!
    //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
    //! cloned and shared between multiple parties. Because the contained values may be
    //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
    //! impossible to mutate data inside of these smart pointers at all.
    //!
    //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
    //! mutability:
    //!
    //! ```
    //! use std::collections::HashMap;
    //! use std::cell::RefCell;
    //! use std::rc::Rc;
    //!
    //! fn main() {
    //!     let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
    //!     shared_map.borrow_mut().insert("africa", 92388);
    //!     shared_map.borrow_mut().insert("kyoto", 11837);
    //!     shared_map.borrow_mut().insert("piccadilly", 11826);
    //!     shared_map.borrow_mut().insert("marbles", 38);
    //! }
    //! ```
    //!
    //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
    //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
    //! multi-threaded situation.
    //!
    //! ## Implementation details of logically-immutable methods
    //!
    //! Occasionally it may be desirable not to expose in an API that there is mutation happening
    //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
    //! forces the implementation to perform mutation; or because you must employ mutation to implement
    //! a trait method that was originally defined to take `&self`.
    //!
    //! ```
    //! # #![allow(dead_code)]
    //! use std::cell::RefCell;
    //!
    //! struct Graph {
    //!     edges: Vec<(i32, i32)>,
    //!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
    //! }
    //!
    //! impl Graph {
    //!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
    //!         // Create a new scope to contain the lifetime of the
    //!         // dynamic borrow
    //!         {
    //!             // Take a reference to the inside of cache cell
    //!             let mut cache = self.span_tree_cache.borrow_mut();
    //!             if cache.is_some() {
    //!                 return cache.as_ref().unwrap().clone();
    //!             }
    //!
    //!             let span_tree = self.calc_span_tree();
    //!             *cache = Some(span_tree);
    //!         }
    //!
    //!         // Recursive call to return the just-cached value.
    //!         // Note that if we had not let the previous borrow
    //!         // of the cache fall out of scope then the subsequent
    //!         // recursive borrow would cause a dynamic thread panic.
    //!         // This is the major hazard of using `RefCell`.
    //!         self.minimum_spanning_tree()
    //!     }
    //! #   fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
    //! }
    //! ```
    //!
    //! ## Mutating implementations of `Clone`
    //!
    //! This is simply a special - but common - case of the previous: hiding mutability for operations
    //! that appear to be immutable. The `clone` method is expected to not change the source value, and
    //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
    //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
    //! `Cell<T>`.
    //!
    //! ```
    //! #![feature(core_intrinsics)]
    //! #![feature(shared)]
    //! use std::cell::Cell;
    //! use std::ptr::Shared;
    //! use std::intrinsics::abort;
    //!
    //! struct Rc<T: ?Sized> {
    //!     ptr: Shared<RcBox<T>>
    //! }
    //!
    //! struct RcBox<T: ?Sized> {
    //!     strong: Cell<usize>,
    //!     refcount: Cell<usize>,
    //!     value: T,
    //! }
    //!
    //! impl<T: ?Sized> Clone for Rc<T> {
    //!     fn clone(&self) -> Rc<T> {
    //!         self.inc_strong();
    //!         Rc { ptr: self.ptr }
    //!     }
    //! }
    //!
    //! trait RcBoxPtr<T: ?Sized> {
    //!
    //!     fn inner(&self) -> &RcBox<T>;
    //!
    //!     fn strong(&self) -> usize {
    //!         self.inner().strong.get()
    //!     }
    //!
    //!     fn inc_strong(&self) {
    //!         self.inner()
    //!             .strong
    //!             .set(self.strong()
    //!                      .checked_add(1)
    //!                      .unwrap_or_else(|| unsafe { abort() }));
    //!     }
    //! }
    //!
    //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
    //!    fn inner(&self) -> &RcBox<T> {
    //!        unsafe {
    //!            self.ptr.as_ref()
    //!        }
    //!    }
    //! }
    //! ```
    //!

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

    use cmp::Ordering;
    use fmt::{self, Debug, Display};
    use marker::Unsize;
    use mem;
    use ops::{Deref, DerefMut, CoerceUnsized};
    use ptr;

    /// A mutable memory location.
    ///
    /// # Examples
    ///
    /// Here you can see how using `Cell<T>` allows to use mutable field inside
    /// immutable struct (which is also called 'interior mutability').
    ///
    /// ```
    /// use std::cell::Cell;
    ///
    /// struct SomeStruct {
    ///     regular_field: u8,
    ///     special_field: Cell<u8>,
    /// }
    ///
    /// let my_struct = SomeStruct {
    ///     regular_field: 0,
    ///     special_field: Cell::new(1),
    /// };
    ///
    /// let new_value = 100;
    ///
    /// // ERROR, because my_struct is immutable
    /// // my_struct.regular_field = new_value;
    ///
    /// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
    /// my_struct.special_field.set(new_value);
    /// assert_eq!(my_struct.special_field.get(), new_value);
    /// ```
    ///
    /// See the [module-level documentation](index.html) for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Cell<T> {
        value: UnsafeCell<T>,
    }

    impl<T:Copy> Cell<T> {
        /// Returns a copy of the contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        ///
        /// let five = c.get();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get(&self) -> T {
            unsafe{ *self.value.get() }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    unsafe impl<T> Send for Cell<T> where T: Send {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> !Sync for Cell<T> {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T:Copy> Clone for Cell<T> {
        #[inline]
        fn clone(&self) -> Cell<T> {
            Cell::new(self.get())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T:Default> Default for Cell<T> {
        /// Creates a `Cell<T>`, with the `Default` value for T.
        #[inline]
        fn default() -> Cell<T> {
            Cell::new(Default::default())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T:PartialEq + Copy> PartialEq for Cell<T> {
        #[inline]
        fn eq(&self, other: &Cell<T>) -> bool {
            self.get() == other.get()
        }
    }

    #[stable(feature = "cell_eq", since = "1.2.0")]
    impl<T:Eq + Copy> Eq for Cell<T> {}

    #[stable(feature = "cell_ord", since = "1.10.0")]
    impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
        #[inline]
        fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
            self.get().partial_cmp(&other.get())
        }

        #[inline]
        fn lt(&self, other: &Cell<T>) -> bool {
            self.get() < other.get()
        }

        #[inline]
        fn le(&self, other: &Cell<T>) -> bool {
            self.get() <= other.get()
        }

        #[inline]
        fn gt(&self, other: &Cell<T>) -> bool {
            self.get() > other.get()
        }

        #[inline]
        fn ge(&self, other: &Cell<T>) -> bool {
            self.get() >= other.get()
        }
    }

    #[stable(feature = "cell_ord", since = "1.10.0")]
    impl<T:Ord + Copy> Ord for Cell<T> {
        #[inline]
        fn cmp(&self, other: &Cell<T>) -> Ordering {
            self.get().cmp(&other.get())
        }
    }

    #[stable(feature = "cell_from", since = "1.12.0")]
    impl<T> From<T> for Cell<T> {
        fn from(t: T) -> Cell<T> {
            Cell::new(t)
        }
    }

    impl<T> Cell<T> {
        /// Creates a new `Cell` containing the given value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub const fn new(value: T) -> Cell<T> {
            Cell {
                value: UnsafeCell::new(value),
            }
        }

        /// Returns a raw pointer to the underlying data in this cell.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        ///
        /// let ptr = c.as_ptr();
        /// ```
        #[inline]
        #[stable(feature = "cell_as_ptr", since = "1.12.0")]
        pub fn as_ptr(&self) -> *mut T {
            self.value.get()
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// This call borrows `Cell` mutably (at compile-time) which guarantees
        /// that we possess the only reference.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let mut c = Cell::new(5);
        /// *c.get_mut() += 1;
        ///
        /// assert_eq!(c.get(), 6);
        /// ```
        #[inline]
        #[stable(feature = "cell_get_mut", since = "1.11.0")]
        pub fn get_mut(&mut self) -> &mut T {
            unsafe {
                &mut *self.value.get()
            }
        }

        /// Sets the contained value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        ///
        /// c.set(10);
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn set(&self, val: T) {
            let old = self.replace(val);
            drop(old);
        }

        /// Swaps the values of two Cells.
        /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c1 = Cell::new(5i32);
        /// let c2 = Cell::new(10i32);
        /// c1.swap(&c2);
        /// assert_eq!(10, c1.get());
        /// assert_eq!(5, c2.get());
        /// ```
        #[inline]
        #[stable(feature = "move_cell", since = "1.17.0")]
        pub fn swap(&self, other: &Self) {
            if ptr::eq(self, other) {
                return;
            }
            unsafe {
                ptr::swap(self.value.get(), other.value.get());
            }
        }

        /// Replaces the contained value, and returns it.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let cell = Cell::new(5);
        /// assert_eq!(cell.get(), 5);
        /// assert_eq!(cell.replace(10), 5);
        /// assert_eq!(cell.get(), 10);
        /// ```
        #[stable(feature = "move_cell", since = "1.17.0")]
        pub fn replace(&self, val: T) -> T {
            mem::replace(unsafe { &mut *self.value.get() }, val)
        }

        /// Unwraps the value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        /// let five = c.into_inner();
        ///
        /// assert_eq!(five, 5);
        /// ```
        #[stable(feature = "move_cell", since = "1.17.0")]
        pub fn into_inner(self) -> T {
            self.value.into_inner()
        }
    }

    impl<T: Default> Cell<T> {
        /// Takes the value of the cell, leaving `Default::default()` in its place.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::Cell;
        ///
        /// let c = Cell::new(5);
        /// let five = c.take();
        ///
        /// assert_eq!(five, 5);
        /// assert_eq!(c.into_inner(), 0);
        /// ```
        #[stable(feature = "move_cell", since = "1.17.0")]
        pub fn take(&self) -> T {
            self.replace(Default::default())
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}

    /// A mutable memory location with dynamically checked borrow rules
    ///
    /// See the [module-level documentation](index.html) for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RefCell<T: ?Sized> {
        borrow: Cell<BorrowFlag>,
        value: UnsafeCell<T>,
    }

    /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
    #[stable(feature = "try_borrow", since = "1.13.0")]
    pub struct BorrowError {
        _private: (),
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Debug for BorrowError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("BorrowError").finish()
        }
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Display for BorrowError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            Display::fmt("already mutably borrowed", f)
        }
    }

    /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
    #[stable(feature = "try_borrow", since = "1.13.0")]
    pub struct BorrowMutError {
        _private: (),
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Debug for BorrowMutError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("BorrowMutError").finish()
        }
    }

    #[stable(feature = "try_borrow", since = "1.13.0")]
    impl Display for BorrowMutError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            Display::fmt("already borrowed", f)
        }
    }

    // Values [1, MAX-1] represent the number of `Ref` active
    // (will not outgrow its range since `usize` is the size of the address space)
    type BorrowFlag = usize;
    const UNUSED: BorrowFlag = 0;
    const WRITING: BorrowFlag = !0;

    impl<T> RefCell<T> {
        /// Creates a new `RefCell` containing `value`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub const fn new(value: T) -> RefCell<T> {
            RefCell {
                value: UnsafeCell::new(value),
                borrow: Cell::new(UNUSED),
            }
        }

        /// Consumes the `RefCell`, returning the wrapped value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// let five = c.into_inner();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn into_inner(self) -> T {
            // Since this function takes `self` (the `RefCell`) by value, the
            // compiler statically verifies that it is not currently borrowed.
            // Therefore the following assertion is just a `debug_assert!`.
            debug_assert!(self.borrow.get() == UNUSED);
            self.value.into_inner()
        }

        /// Replaces the wrapped value with a new one, returning the old value,
        /// without deinitializing either one.
        ///
        /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
        ///
        /// # Panics
        ///
        /// Panics if the value is currently borrowed.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        /// let cell = RefCell::new(5);
        /// let old_value = cell.replace(6);
        /// assert_eq!(old_value, 5);
        /// assert_eq!(cell, RefCell::new(6));
        /// ```
        #[inline]
        #[stable(feature = "refcell_replace", since="1.24.0")]
        pub fn replace(&self, t: T) -> T {
            mem::replace(&mut *self.borrow_mut(), t)
        }

        /// Replaces the wrapped value with a new one computed from `f`, returning
        /// the old value, without deinitializing either one.
        ///
        /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
        ///
        /// # Panics
        ///
        /// Panics if the value is currently borrowed.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(refcell_replace_swap)]
        /// use std::cell::RefCell;
        /// let cell = RefCell::new(5);
        /// let old_value = cell.replace_with(|&mut old| old + 1);
        /// assert_eq!(old_value, 5);
        /// assert_eq!(cell, RefCell::new(6));
        /// ```
        #[inline]
        #[unstable(feature = "refcell_replace_swap", issue="43570")]
        pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
            let mut_borrow = &mut *self.borrow_mut();
            let replacement = f(mut_borrow);
            mem::replace(mut_borrow, replacement)
        }

        /// Swaps the wrapped value of `self` with the wrapped value of `other`,
        /// without deinitializing either one.
        ///
        /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
        ///
        /// # Panics
        ///
        /// Panics if the value in either `RefCell` is currently borrowed.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        /// let c = RefCell::new(5);
        /// let d = RefCell::new(6);
        /// c.swap(&d);
        /// assert_eq!(c, RefCell::new(6));
        /// assert_eq!(d, RefCell::new(5));
        /// ```
        #[inline]
        #[stable(feature = "refcell_swap", since="1.24.0")]
        pub fn swap(&self, other: &Self) {
            mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
        }
    }

    impl<T: ?Sized> RefCell<T> {
        /// Immutably borrows the wrapped value.
        ///
        /// The borrow lasts until the returned `Ref` exits scope. Multiple
        /// immutable borrows can be taken out at the same time.
        ///
        /// # Panics
        ///
        /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
        /// [`try_borrow`](#method.try_borrow).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// let borrowed_five = c.borrow();
        /// let borrowed_five2 = c.borrow();
        /// ```
        ///
        /// An example of panic:
        ///
        /// ```
        /// use std::cell::RefCell;
        /// use std::thread;
        ///
        /// let result = thread::spawn(move || {
        ///    let c = RefCell::new(5);
        ///    let m = c.borrow_mut();
        ///
        ///    let b = c.borrow(); // this causes a panic
        /// }).join();
        ///
        /// assert!(result.is_err());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn borrow(&self) -> Ref<T> {
            self.try_borrow().expect("already mutably borrowed")
        }

        /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
        /// borrowed.
        ///
        /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
        /// taken out at the same time.
        ///
        /// This is the non-panicking variant of [`borrow`](#method.borrow).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// {
        ///     let m = c.borrow_mut();
        ///     assert!(c.try_borrow().is_err());
        /// }
        ///
        /// {
        ///     let m = c.borrow();
        ///     assert!(c.try_borrow().is_ok());
        /// }
        /// ```
        #[stable(feature = "try_borrow", since = "1.13.0")]
        #[inline]
        pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
            match BorrowRef::new(&self.borrow) {
                Some(b) => Ok(Ref {
                    value: unsafe { &*self.value.get() },
                    borrow: b,
                }),
                None => Err(BorrowError { _private: () }),
            }
        }

        /// Mutably borrows the wrapped value.
        ///
        /// The borrow lasts until the returned `RefMut` exits scope. The value
        /// cannot be borrowed while this borrow is active.
        ///
        /// # Panics
        ///
        /// Panics if the value is currently borrowed. For a non-panicking variant, use
        /// [`try_borrow_mut`](#method.try_borrow_mut).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// *c.borrow_mut() = 7;
        ///
        /// assert_eq!(*c.borrow(), 7);
        /// ```
        ///
        /// An example of panic:
        ///
        /// ```
        /// use std::cell::RefCell;
        /// use std::thread;
        ///
        /// let result = thread::spawn(move || {
        ///    let c = RefCell::new(5);
        ///    let m = c.borrow();
        ///
        ///    let b = c.borrow_mut(); // this causes a panic
        /// }).join();
        ///
        /// assert!(result.is_err());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn borrow_mut(&self) -> RefMut<T> {
            self.try_borrow_mut().expect("already borrowed")
        }

        /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
        ///
        /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
        /// while this borrow is active.
        ///
        /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// {
        ///     let m = c.borrow();
        ///     assert!(c.try_borrow_mut().is_err());
        /// }
        ///
        /// assert!(c.try_borrow_mut().is_ok());
        /// ```
        #[stable(feature = "try_borrow", since = "1.13.0")]
        #[inline]
        pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
            match BorrowRefMut::new(&self.borrow) {
                Some(b) => Ok(RefMut {
                    value: unsafe { &mut *self.value.get() },
                    borrow: b,
                }),
                None => Err(BorrowMutError { _private: () }),
            }
        }

        /// Returns a raw pointer to the underlying data in this cell.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let c = RefCell::new(5);
        ///
        /// let ptr = c.as_ptr();
        /// ```
        #[inline]
        #[stable(feature = "cell_as_ptr", since = "1.12.0")]
        pub fn as_ptr(&self) -> *mut T {
            self.value.get()
        }

        /// Returns a mutable reference to the underlying data.
        ///
        /// This call borrows `RefCell` mutably (at compile-time) so there is no
        /// need for dynamic checks.
        ///
        /// However be cautious: this method expects `self` to be mutable, which is
        /// generally not the case when using a `RefCell`. Take a look at the
        /// [`borrow_mut`] method instead if `self` isn't mutable.
        ///
        /// Also, please be aware that this method is only for special circumstances and is usually
        /// not what you want. In case of doubt, use [`borrow_mut`] instead.
        ///
        /// [`borrow_mut`]: #method.borrow_mut
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::RefCell;
        ///
        /// let mut c = RefCell::new(5);
        /// *c.get_mut() += 1;
        ///
        /// assert_eq!(c, RefCell::new(6));
        /// ```
        #[inline]
        #[stable(feature = "cell_get_mut", since = "1.11.0")]
        pub fn get_mut(&mut self) -> &mut T {
            unsafe {
                &mut *self.value.get()
            }
        }
    }

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

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Clone> Clone for RefCell<T> {
        #[inline]
        fn clone(&self) -> RefCell<T> {
            RefCell::new(self.borrow().clone())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T:Default> Default for RefCell<T> {
        /// Creates a `RefCell<T>`, with the `Default` value for T.
        #[inline]
        fn default() -> RefCell<T> {
            RefCell::new(Default::default())
        }
    }

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

    #[stable(feature = "cell_eq", since = "1.2.0")]
    impl<T: ?Sized + Eq> Eq for RefCell<T> {}

    #[stable(feature = "cell_ord", since = "1.10.0")]
    impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
        #[inline]
        fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
            self.borrow().partial_cmp(&*other.borrow())
        }

        #[inline]
        fn lt(&self, other: &RefCell<T>) -> bool {
            *self.borrow() < *other.borrow()
        }

        #[inline]
        fn le(&self, other: &RefCell<T>) -> bool {
            *self.borrow() <= *other.borrow()
        }

        #[inline]
        fn gt(&self, other: &RefCell<T>) -> bool {
            *self.borrow() > *other.borrow()
        }

        #[inline]
        fn ge(&self, other: &RefCell<T>) -> bool {
            *self.borrow() >= *other.borrow()
        }
    }

    #[stable(feature = "cell_ord", since = "1.10.0")]
    impl<T: ?Sized + Ord> Ord for RefCell<T> {
        #[inline]
        fn cmp(&self, other: &RefCell<T>) -> Ordering {
            self.borrow().cmp(&*other.borrow())
        }
    }

    #[stable(feature = "cell_from", since = "1.12.0")]
    impl<T> From<T> for RefCell<T> {
        fn from(t: T) -> RefCell<T> {
            RefCell::new(t)
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}

    struct BorrowRef<'b> {
        borrow: &'b Cell<BorrowFlag>,
    }

    impl<'b> BorrowRef<'b> {
        #[inline]
        fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
            match borrow.get() {
                WRITING => None,
                b => {
                    borrow.set(b + 1);
                    Some(BorrowRef { borrow: borrow })
                },
            }
        }
    }

    impl<'b> Drop for BorrowRef<'b> {
        #[inline]
        fn drop(&mut self) {
            let borrow = self.borrow.get();
            debug_assert!(borrow != WRITING && borrow != UNUSED);
            self.borrow.set(borrow - 1);
        }
    }

    impl<'b> Clone for BorrowRef<'b> {
        #[inline]
        fn clone(&self) -> BorrowRef<'b> {
            // Since this Ref exists, we know the borrow flag
            // is not set to WRITING.
            let borrow = self.borrow.get();
            debug_assert!(borrow != UNUSED);
            // Prevent the borrow counter from overflowing.
            assert!(borrow != WRITING);
            self.borrow.set(borrow + 1);
            BorrowRef { borrow: self.borrow }
        }
    }

    /// Wraps a borrowed reference to a value in a `RefCell` box.
    /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
    ///
    /// See the [module-level documentation](index.html) for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Ref<'b, T: ?Sized + 'b> {
        value: &'b T,
        borrow: BorrowRef<'b>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'b, T: ?Sized> Deref for Ref<'b, T> {
        type Target = T;

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

    impl<'b, T: ?Sized> Ref<'b, T> {
        /// Copies a `Ref`.
        ///
        /// The `RefCell` is already immutably borrowed, so this cannot fail.
        ///
        /// This is an associated function that needs to be used as
        /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
        /// with the widespread use of `r.borrow().clone()` to clone the contents of
        /// a `RefCell`.
        #[stable(feature = "cell_extras", since = "1.15.0")]
        #[inline]
        pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
            Ref {
                value: orig.value,
                borrow: orig.borrow.clone(),
            }
        }

        /// Make a new `Ref` for a component of the borrowed data.
        ///
        /// The `RefCell` is already immutably borrowed, so this cannot fail.
        ///
        /// This is an associated function that needs to be used as `Ref::map(...)`.
        /// A method would interfere with methods of the same name on the contents
        /// of a `RefCell` used through `Deref`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::{RefCell, Ref};
        ///
        /// let c = RefCell::new((5, 'b'));
        /// let b1: Ref<(u32, char)> = c.borrow();
        /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
        /// assert_eq!(*b2, 5)
        /// ```
        #[stable(feature = "cell_map", since = "1.8.0")]
        #[inline]
        pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
            where F: FnOnce(&T) -> &U
        {
            Ref {
                value: f(orig.value),
                borrow: orig.borrow,
            }
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}

    #[stable(feature = "std_guard_impls", since = "1.20.0")]
    impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.value.fmt(f)
        }
    }

    impl<'b, T: ?Sized> RefMut<'b, T> {
        /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
        /// variant.
        ///
        /// The `RefCell` is already mutably borrowed, so this cannot fail.
        ///
        /// This is an associated function that needs to be used as
        /// `RefMut::map(...)`.  A method would interfere with methods of the same
        /// name on the contents of a `RefCell` used through `Deref`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::{RefCell, RefMut};
        ///
        /// let c = RefCell::new((5, 'b'));
        /// {
        ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
        ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
        ///     assert_eq!(*b2, 5);
        ///     *b2 = 42;
        /// }
        /// assert_eq!(*c.borrow(), (42, 'b'));
        /// ```
        #[stable(feature = "cell_map", since = "1.8.0")]
        #[inline]
        pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
            where F: FnOnce(&mut T) -> &mut U
        {
            // FIXME(nll-rfc#40): fix borrow-check
            let RefMut { value, borrow } = orig;
            RefMut {
                value: f(value),
                borrow: borrow,
            }
        }
    }

    struct BorrowRefMut<'b> {
        borrow: &'b Cell<BorrowFlag>,
    }

    impl<'b> Drop for BorrowRefMut<'b> {
        #[inline]
        fn drop(&mut self) {
            let borrow = self.borrow.get();
            debug_assert!(borrow == WRITING);
            self.borrow.set(UNUSED);
        }
    }

    impl<'b> BorrowRefMut<'b> {
        #[inline]
        fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
            match borrow.get() {
                UNUSED => {
                    borrow.set(WRITING);
                    Some(BorrowRefMut { borrow: borrow })
                },
                _ => None,
            }
        }
    }

    /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
    ///
    /// See the [module-level documentation](index.html) for more.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct RefMut<'b, T: ?Sized + 'b> {
        value: &'b mut T,
        borrow: BorrowRefMut<'b>,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
        type Target = T;

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

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
        #[inline]
        fn deref_mut(&mut self) -> &mut T {
            self.value
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}

    #[stable(feature = "std_guard_impls", since = "1.20.0")]
    impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.value.fmt(f)
        }
    }

    /// The core primitive for interior mutability in Rust.
    ///
    /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
    /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
    /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
    /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
    ///
    /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
    /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
    /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
    /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
    /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
    /// to ensure that no two mutable references obtained this way are active at the same time, and
    /// that there are no active mutable references or mutations when an immutable reference is obtained
    /// from the cell. This is often done via runtime checks.
    ///
    /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
    /// okay (provided you enforce the invariants some other way); it is still undefined behavior
    /// to have multiple `&mut UnsafeCell<T>` aliases.
    ///
    ///
    /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::cell::UnsafeCell;
    /// use std::marker::Sync;
    ///
    /// # #[allow(dead_code)]
    /// struct NotThreadSafe<T> {
    ///     value: UnsafeCell<T>,
    /// }
    ///
    /// unsafe impl<T> Sync for NotThreadSafe<T> {}
    /// ```
    #[lang = "unsafe_cell"]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct UnsafeCell<T: ?Sized> {
        value: T,
    }

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

    impl<T> UnsafeCell<T> {
        /// Constructs a new instance of `UnsafeCell` which will wrap the specified
        /// value.
        ///
        /// All access to the inner value through methods is `unsafe`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::UnsafeCell;
        ///
        /// let uc = UnsafeCell::new(5);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub const fn new(value: T) -> UnsafeCell<T> {
            UnsafeCell { value: value }
        }

        /// Unwraps the value.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::UnsafeCell;
        ///
        /// let uc = UnsafeCell::new(5);
        ///
        /// let five = uc.into_inner();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> T {
            self.value
        }
    }

    impl<T: ?Sized> UnsafeCell<T> {
        /// Gets a mutable pointer to the wrapped value.
        ///
        /// This can be cast to a pointer of any kind.
        /// Ensure that the access is unique when casting to
        /// `&mut T`, and ensure that there are no mutations or mutable
        /// aliases going on when casting to `&T`
        ///
        /// # Examples
        ///
        /// ```
        /// use std::cell::UnsafeCell;
        ///
        /// let uc = UnsafeCell::new(5);
        ///
        /// let five = uc.get();
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get(&self) -> *mut T {
            &self.value as *const T as *mut T
        }
    }

    #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
    impl<T: Default> Default for UnsafeCell<T> {
        /// Creates an `UnsafeCell`, with the `Default` value for T.
        fn default() -> UnsafeCell<T> {
            UnsafeCell::new(Default::default())
        }
    }

    #[stable(feature = "cell_from", since = "1.12.0")]
    impl<T> From<T> for UnsafeCell<T> {
        fn from(t: T) -> UnsafeCell<T> {
            UnsafeCell::new(t)
        }
    }

    #[unstable(feature = "coerce_unsized", issue = "27732")]
    impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}

    #[allow(unused)]
    fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
        let _: UnsafeCell<&Send> = a;
        let _: Cell<&Send> = b;
        let _: RefCell<&Send> = c;
    }

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