add libcore from 2016-04-11 nightly

1 files changed, 861 insertions, 0 deletions

diff --git a/libcore/cell.rs b/libcore/cell.rs
new file mode 100644
index 0000000..a1c7a29
--- /dev/null
+++ b/libcore/cell.rs
@@ -0,0 +1,861 @@
+// 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>` provides `get` and `set`
+//! methods that change the interior value with a single method call. `Cell<T>` though is only
+//! compatible with types that implement `Copy`. For other types, one must use the `RefCell<T>`
+//! type, acquiring a write lock before mutating.
+//!
+//! `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>`.
+//!
+//! ```
+//! use std::cell::Cell;
+//!
+//! struct Rc<T> {
+//!     ptr: *mut RcBox<T>
+//! }
+//!
+//! struct RcBox<T> {
+//! # #[allow(dead_code)]
+//!     value: T,
+//!     refcount: Cell<usize>
+//! }
+//!
+//! impl<T> Clone for Rc<T> {
+//!     fn clone(&self) -> Rc<T> {
+//!         unsafe {
+//!             (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
+//!             Rc { ptr: self.ptr }
+//!         }
+//!     }
+//! }
+//! ```
+//!
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use clone::Clone;
+use cmp::{PartialEq, Eq};
+use default::Default;
+use marker::{Copy, Send, Sync, Sized, Unsize};
+use ops::{Deref, DerefMut, Drop, FnOnce, CoerceUnsized};
+use option::Option;
+use option::Option::{None, Some};
+
+/// A mutable memory location that admits only `Copy` data.
+///
+/// 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> {
+    /// 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 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() }
+    }
+
+    /// 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, value: T) {
+        unsafe {
+            *self.value.get() = value;
+        }
+    }
+
+    /// Returns a reference to the underlying `UnsafeCell`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(as_unsafe_cell)]
+    ///
+    /// use std::cell::Cell;
+    ///
+    /// let c = Cell::new(5);
+    ///
+    /// let uc = c.as_unsafe_cell();
+    /// ```
+    #[inline]
+    #[unstable(feature = "as_unsafe_cell", issue = "27708")]
+    pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
+        &self.value
+    }
+}
+
+#[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 + Copy> Default for Cell<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> {}
+
+/// 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 enumeration of values returned from the `state` method on a `RefCell<T>`.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[unstable(feature = "borrow_state", issue = "27733")]
+pub enum BorrowState {
+    /// The cell is currently being read, there is at least one active `borrow`.
+    Reading,
+    /// The cell is currently being written to, there is an active `borrow_mut`.
+    Writing,
+    /// There are no outstanding borrows on this cell.
+    Unused,
+}
+
+// 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);
+        unsafe { self.value.into_inner() }
+    }
+}
+
+impl<T: ?Sized> RefCell<T> {
+    /// Query the current state of this `RefCell`
+    ///
+    /// The returned value can be dispatched on to determine if a call to
+    /// `borrow` or `borrow_mut` would succeed.
+    #[unstable(feature = "borrow_state", issue = "27733")]
+    #[inline]
+    pub fn borrow_state(&self) -> BorrowState {
+        match self.borrow.get() {
+            WRITING => BorrowState::Writing,
+            UNUSED => BorrowState::Unused,
+            _ => BorrowState::Reading,
+        }
+    }
+
+    /// 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.
+    ///
+    /// # 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> {
+        match BorrowRef::new(&self.borrow) {
+            Some(b) => Ref {
+                value: unsafe { &*self.value.get() },
+                borrow: b,
+            },
+            None => panic!("RefCell<T> already mutably borrowed"),
+        }
+    }
+
+    /// 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.
+    ///
+    /// # 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> {
+        match BorrowRefMut::new(&self.borrow) {
+            Some(b) => RefMut {
+                value: unsafe { &mut *self.value.get() },
+                borrow: b,
+            },
+            None => panic!("RefCell<T> already borrowed"),
+        }
+    }
+
+    /// Returns a reference to the underlying `UnsafeCell`.
+    ///
+    /// This can be used to circumvent `RefCell`'s safety checks.
+    ///
+    /// This function is `unsafe` because `UnsafeCell`'s field is public.
+    #[inline]
+    #[unstable(feature = "as_unsafe_cell", issue = "27708")]
+    pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
+        &self.value
+    }
+}
+
+#[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> {
+    #[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> {}
+
+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 != WRITING && borrow != UNUSED);
+        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`.
+    #[unstable(feature = "cell_extras",
+               reason = "likely to be moved to a method, pending language changes",
+               issue = "27746")]
+    #[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`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// 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,
+        }
+    }
+
+    /// Make a new `Ref` for an optional component of the borrowed data, e.g. an
+    /// enum variant.
+    ///
+    /// The `RefCell` is already immutably borrowed, so this cannot fail.
+    ///
+    /// This is an associated function that needs to be used as
+    /// `Ref::filter_map(...)`.  A method would interfere with methods of the
+    /// same name on the contents of a `RefCell` used through `Deref`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # #![feature(cell_extras)]
+    /// use std::cell::{RefCell, Ref};
+    ///
+    /// let c = RefCell::new(Ok(5));
+    /// let b1: Ref<Result<u32, ()>> = c.borrow();
+    /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
+    /// assert_eq!(*b2, 5)
+    /// ```
+    #[unstable(feature = "cell_extras", reason = "recently added",
+               issue = "27746")]
+    #[rustc_deprecated(since = "1.8.0", reason = "can be built on `Ref::map`: \
+        https://crates.io/crates/ref_filter_map")]
+    #[inline]
+    pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
+        where F: FnOnce(&T) -> Option<&U>
+    {
+        f(orig.value).map(move |new| Ref {
+            value: new,
+            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> {}
+
+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`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// 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
+    {
+        RefMut {
+            value: f(orig.value),
+            borrow: orig.borrow,
+        }
+    }
+
+    /// Make a new `RefMut` for an optional 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::filter_map(...)`.  A method would interfere with methods of the
+    /// same name on the contents of a `RefCell` used through `Deref`.
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # #![feature(cell_extras)]
+    /// use std::cell::{RefCell, RefMut};
+    ///
+    /// let c = RefCell::new(Ok(5));
+    /// {
+    ///     let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
+    ///     let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| {
+    ///         o.as_mut().ok()
+    ///     }).unwrap();
+    ///     assert_eq!(*b2, 5);
+    ///     *b2 = 42;
+    /// }
+    /// assert_eq!(*c.borrow(), Ok(42));
+    /// ```
+    #[unstable(feature = "cell_extras", reason = "recently added",
+               issue = "27746")]
+    #[rustc_deprecated(since = "1.8.0", reason = "can be built on `RefMut::map`: \
+        https://crates.io/crates/ref_filter_map")]
+    #[inline]
+    pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
+        where F: FnOnce(&mut T) -> Option<&mut U>
+    {
+        let RefMut { value, borrow } = orig;
+        f(value).map(move |new| RefMut {
+            value: new,
+            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> {}
+
+/// 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.
+///
+/// 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.
+    ///
+    /// # Safety
+    ///
+    /// This function is unsafe because this thread or another thread may currently be
+    /// inspecting the inner value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::cell::UnsafeCell;
+    ///
+    /// let uc = UnsafeCell::new(5);
+    ///
+    /// let five = unsafe { uc.into_inner() };
+    /// ```
+    #[inline]
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub unsafe fn into_inner(self) -> T {
+        self.value
+    }
+}
+
+impl<T: ?Sized> UnsafeCell<T> {
+    /// Gets a mutable pointer to the wrapped value.
+    ///
+    /// # 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
+    }
+}