src/libcore/cell.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Shareable mutable containers.
  12 //!
  13 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
  14 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
  15 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
  16 //! with typical Rust types that exhibit 'inherited mutability'.
  17 //!
  18 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` provides `get` and `set`
  19 //! methods that change the interior value with a single method call. `Cell<T>` though is only
  20 //! compatible with types that implement `Copy`. For other types, one must use the `RefCell<T>`
  21 //! type, acquiring a write lock before mutating.
  22 //!
  23 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
  24 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
  25 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
  26 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
  27 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
  28 //! panic.
  29 //!
  30 //! # When to choose interior mutability
  31 //!
  32 //! The more common inherited mutability, where one must have unique access to mutate a value, is
  33 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
  34 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
  35 //! interior mutability is something of a last resort. Since cell types enable mutation where it
  36 //! would otherwise be disallowed though, there are occasions when interior mutability might be
  37 //! appropriate, or even *must* be used, e.g.
  38 //!
  39 //! * Introducing inherited mutability roots to shared types.
  40 //! * Implementation details of logically-immutable methods.
  41 //! * Mutating implementations of `Clone`.
  42 //!
  43 //! ## Introducing inherited mutability roots to shared types
  44 //!
  45 //! Shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
  46 //! cloned and shared between multiple parties. Because the contained values may be
  47 //! multiply-aliased, they can only be borrowed as shared references, not mutable references.
  48 //! Without cells it would be impossible to mutate data inside of shared boxes at all!
  49 //!
  50 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
  51 //! mutability:
  52 //!
  53 //! ```
  54 //! use std::collections::HashMap;
  55 //! use std::cell::RefCell;
  56 //! use std::rc::Rc;
  57 //!
  58 //! fn main() {
  59 //!     let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
  60 //!     shared_map.borrow_mut().insert("africa", 92388);
  61 //!     shared_map.borrow_mut().insert("kyoto", 11837);
  62 //!     shared_map.borrow_mut().insert("piccadilly", 11826);
  63 //!     shared_map.borrow_mut().insert("marbles", 38);
  64 //! }
  65 //! ```
  66 //!
  67 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
  68 //! scenarios. Consider using `Mutex<T>` if you need shared mutability in a multi-threaded
  69 //! situation.
  70 //!
  71 //! ## Implementation details of logically-immutable methods
  72 //!
  73 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
  74 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
  75 //! forces the implementation to perform mutation; or because you must employ mutation to implement
  76 //! a trait method that was originally defined to take `&self`.
  77 //!
  78 //! ```
  79 //! use std::cell::RefCell;
  80 //!
  81 //! struct Graph {
  82 //!     edges: Vec<(i32, i32)>,
  83 //!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
  84 //! }
  85 //!
  86 //! impl Graph {
  87 //!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
  88 //!         // Create a new scope to contain the lifetime of the
  89 //!         // dynamic borrow
  90 //!         {
  91 //!             // Take a reference to the inside of cache cell
  92 //!             let mut cache = self.span_tree_cache.borrow_mut();
  93 //!             if cache.is_some() {
  94 //!                 return cache.as_ref().unwrap().clone();
  95 //!             }
  96 //!
  97 //!             let span_tree = self.calc_span_tree();
  98 //!             *cache = Some(span_tree);
  99 //!         }
 100 //!
 101 //!         // Recursive call to return the just-cached value.
 102 //!         // Note that if we had not let the previous borrow
 103 //!         // of the cache fall out of scope then the subsequent
 104 //!         // recursive borrow would cause a dynamic thread panic.
 105 //!         // This is the major hazard of using `RefCell`.
 106 //!         self.minimum_spanning_tree()
 107 //!     }
 108 //! #   fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
 109 //! }
 110 //! ```
 111 //!
 112 //! ## Mutating implementations of `Clone`
 113 //!
 114 //! This is simply a special - but common - case of the previous: hiding mutability for operations
 115 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
 116 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
 117 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
 118 //! `Cell<T>`.
 119 //!
 120 //! ```
 121 //! use std::cell::Cell;
 122 //!
 123 //! struct Rc<T> {
 124 //!     ptr: *mut RcBox<T>
 125 //! }
 126 //!
 127 //! struct RcBox<T> {
 128 //!     value: T,
 129 //!     refcount: Cell<usize>
 130 //! }
 131 //!
 132 //! impl<T> Clone for Rc<T> {
 133 //!     fn clone(&self) -> Rc<T> {
 134 //!         unsafe {
 135 //!             (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
 136 //!             Rc { ptr: self.ptr }
 137 //!         }
 138 //!     }
 139 //! }
 140 //! ```
 141 //!
 142
 143 #![stable(feature = "rust1", since = "1.0.0")]
 144
 145 use clone::Clone;
 146 use cmp::{PartialEq, Eq};
 147 use default::Default;
 148 use marker::{Copy, Send, Sync, Sized};
 149 use ops::{Deref, DerefMut, Drop, FnOnce};
 150 use option::Option;
 151 use option::Option::{None, Some};
 152
 153 /// A mutable memory location that admits only `Copy` data.
 154 ///
 155 /// See the [module-level documentation](index.html) for more.
 156 #[stable(feature = "rust1", since = "1.0.0")]
 157 pub struct Cell<T> {
 158     value: UnsafeCell<T>,
 159 }
 160
 161 impl<T:Copy> Cell<T> {
 162     /// Creates a new `Cell` containing the given value.
 163     ///
 164     /// # Examples
 165     ///
 166     /// ```
 167     /// use std::cell::Cell;
 168     ///
 169     /// let c = Cell::new(5);
 170     /// ```
 171     #[stable(feature = "rust1", since = "1.0.0")]
 172     #[inline]
 173     pub const fn new(value: T) -> Cell<T> {
 174         Cell {
 175             value: UnsafeCell::new(value),
 176         }
 177     }
 178
 179     /// Returns a copy of the contained value.
 180     ///
 181     /// # Examples
 182     ///
 183     /// ```
 184     /// use std::cell::Cell;
 185     ///
 186     /// let c = Cell::new(5);
 187     ///
 188     /// let five = c.get();
 189     /// ```
 190     #[inline]
 191     #[stable(feature = "rust1", since = "1.0.0")]
 192     pub fn get(&self) -> T {
 193         unsafe{ *self.value.get() }
 194     }
 195
 196     /// Sets the contained value.
 197     ///
 198     /// # Examples
 199     ///
 200     /// ```
 201     /// use std::cell::Cell;
 202     ///
 203     /// let c = Cell::new(5);
 204     ///
 205     /// c.set(10);
 206     /// ```
 207     #[inline]
 208     #[stable(feature = "rust1", since = "1.0.0")]
 209     pub fn set(&self, value: T) {
 210         unsafe {
 211             *self.value.get() = value;
 212         }
 213     }
 214
 215     /// Returns a reference to the underlying `UnsafeCell`.
 216     ///
 217     /// # Unsafety
 218     ///
 219     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 220     ///
 221     /// # Examples
 222     ///
 223     /// ```
 224     /// # #![feature(as_unsafe_cell)]
 225     /// use std::cell::Cell;
 226     ///
 227     /// let c = Cell::new(5);
 228     ///
 229     /// let uc = unsafe { c.as_unsafe_cell() };
 230     /// ```
 231     #[inline]
 232     #[unstable(feature = "as_unsafe_cell")]
 233     pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
 234         &self.value
 235     }
 236 }
 237
 238 #[stable(feature = "rust1", since = "1.0.0")]
 239 unsafe impl<T> Send for Cell<T> where T: Send {}
 240
 241 #[stable(feature = "rust1", since = "1.0.0")]
 242 impl<T:Copy> Clone for Cell<T> {
 243     #[inline]
 244     fn clone(&self) -> Cell<T> {
 245         Cell::new(self.get())
 246     }
 247 }
 248
 249 #[stable(feature = "rust1", since = "1.0.0")]
 250 impl<T:Default + Copy> Default for Cell<T> {
 251     #[stable(feature = "rust1", since = "1.0.0")]
 252     #[inline]
 253     fn default() -> Cell<T> {
 254         Cell::new(Default::default())
 255     }
 256 }
 257
 258 #[stable(feature = "rust1", since = "1.0.0")]
 259 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 260     #[inline]
 261     fn eq(&self, other: &Cell<T>) -> bool {
 262         self.get() == other.get()
 263     }
 264 }
 265
 266 #[stable(feature = "cell_eq", since = "1.2.0")]
 267 impl<T:Eq + Copy> Eq for Cell<T> {}
 268
 269 /// A mutable memory location with dynamically checked borrow rules
 270 ///
 271 /// See the [module-level documentation](index.html) for more.
 272 #[stable(feature = "rust1", since = "1.0.0")]
 273 pub struct RefCell<T: ?Sized> {
 274     borrow: Cell<BorrowFlag>,
 275     value: UnsafeCell<T>,
 276 }
 277
 278 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
 279 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 280 #[unstable(feature = "borrow_state")]
 281 pub enum BorrowState {
 282     /// The cell is currently being read, there is at least one active `borrow`.
 283     Reading,
 284     /// The cell is currently being written to, there is an active `borrow_mut`.
 285     Writing,
 286     /// There are no outstanding borrows on this cell.
 287     Unused,
 288 }
 289
 290 // Values [1, MAX-1] represent the number of `Ref` active
 291 // (will not outgrow its range since `usize` is the size of the address space)
 292 type BorrowFlag = usize;
 293 const UNUSED: BorrowFlag = 0;
 294 const WRITING: BorrowFlag = !0;
 295
 296 impl<T> RefCell<T> {
 297     /// Creates a new `RefCell` containing `value`.
 298     ///
 299     /// # Examples
 300     ///
 301     /// ```
 302     /// use std::cell::RefCell;
 303     ///
 304     /// let c = RefCell::new(5);
 305     /// ```
 306     #[stable(feature = "rust1", since = "1.0.0")]
 307     #[inline]
 308     pub const fn new(value: T) -> RefCell<T> {
 309         RefCell {
 310             value: UnsafeCell::new(value),
 311             borrow: Cell::new(UNUSED),
 312         }
 313     }
 314
 315     /// Consumes the `RefCell`, returning the wrapped value.
 316     ///
 317     /// # Examples
 318     ///
 319     /// ```
 320     /// use std::cell::RefCell;
 321     ///
 322     /// let c = RefCell::new(5);
 323     ///
 324     /// let five = c.into_inner();
 325     /// ```
 326     #[stable(feature = "rust1", since = "1.0.0")]
 327     #[inline]
 328     pub fn into_inner(self) -> T {
 329         // Since this function takes `self` (the `RefCell`) by value, the
 330         // compiler statically verifies that it is not currently borrowed.
 331         // Therefore the following assertion is just a `debug_assert!`.
 332         debug_assert!(self.borrow.get() == UNUSED);
 333         unsafe { self.value.into_inner() }
 334     }
 335 }
 336
 337 impl<T: ?Sized> RefCell<T> {
 338     /// Query the current state of this `RefCell`
 339     ///
 340     /// The returned value can be dispatched on to determine if a call to
 341     /// `borrow` or `borrow_mut` would succeed.
 342     #[unstable(feature = "borrow_state")]
 343     #[inline]
 344     pub fn borrow_state(&self) -> BorrowState {
 345         match self.borrow.get() {
 346             WRITING => BorrowState::Writing,
 347             UNUSED => BorrowState::Unused,
 348             _ => BorrowState::Reading,
 349         }
 350     }
 351
 352     /// Immutably borrows the wrapped value.
 353     ///
 354     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 355     /// immutable borrows can be taken out at the same time.
 356     ///
 357     /// # Panics
 358     ///
 359     /// Panics if the value is currently mutably borrowed.
 360     ///
 361     /// # Examples
 362     ///
 363     /// ```
 364     /// use std::cell::RefCell;
 365     ///
 366     /// let c = RefCell::new(5);
 367     ///
 368     /// let borrowed_five = c.borrow();
 369     /// let borrowed_five2 = c.borrow();
 370     /// ```
 371     ///
 372     /// An example of panic:
 373     ///
 374     /// ```
 375     /// use std::cell::RefCell;
 376     /// use std::thread;
 377     ///
 378     /// let result = thread::spawn(move || {
 379     ///    let c = RefCell::new(5);
 380     ///    let m = c.borrow_mut();
 381     ///
 382     ///    let b = c.borrow(); // this causes a panic
 383     /// }).join();
 384     ///
 385     /// assert!(result.is_err());
 386     /// ```
 387     #[stable(feature = "rust1", since = "1.0.0")]
 388     #[inline]
 389     pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
 390         match BorrowRef::new(&self.borrow) {
 391             Some(b) => Ref {
 392                 _value: unsafe { &*self.value.get() },
 393                 _borrow: b,
 394             },
 395             None => panic!("RefCell<T> already mutably borrowed"),
 396         }
 397     }
 398
 399     /// Mutably borrows the wrapped value.
 400     ///
 401     /// The borrow lasts until the returned `RefMut` exits scope. The value
 402     /// cannot be borrowed while this borrow is active.
 403     ///
 404     /// # Panics
 405     ///
 406     /// Panics if the value is currently borrowed.
 407     ///
 408     /// # Examples
 409     ///
 410     /// ```
 411     /// use std::cell::RefCell;
 412     ///
 413     /// let c = RefCell::new(5);
 414     ///
 415     /// let borrowed_five = c.borrow_mut();
 416     /// ```
 417     ///
 418     /// An example of panic:
 419     ///
 420     /// ```
 421     /// use std::cell::RefCell;
 422     /// use std::thread;
 423     ///
 424     /// let result = thread::spawn(move || {
 425     ///    let c = RefCell::new(5);
 426     ///    let m = c.borrow();
 427     ///
 428     ///    let b = c.borrow_mut(); // this causes a panic
 429     /// }).join();
 430     ///
 431     /// assert!(result.is_err());
 432     /// ```
 433     #[stable(feature = "rust1", since = "1.0.0")]
 434     #[inline]
 435     pub fn borrow_mut<'a>(&'a self) -> RefMut<'a, T> {
 436         match BorrowRefMut::new(&self.borrow) {
 437             Some(b) => RefMut {
 438                 _value: unsafe { &mut *self.value.get() },
 439                 _borrow: b,
 440             },
 441             None => panic!("RefCell<T> already borrowed"),
 442         }
 443     }
 444
 445     /// Returns a reference to the underlying `UnsafeCell`.
 446     ///
 447     /// This can be used to circumvent `RefCell`'s safety checks.
 448     ///
 449     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 450     #[inline]
 451     #[unstable(feature = "as_unsafe_cell")]
 452     pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
 453         &self.value
 454     }
 455 }
 456
 457 #[stable(feature = "rust1", since = "1.0.0")]
 458 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 459
 460 #[stable(feature = "rust1", since = "1.0.0")]
 461 impl<T: Clone> Clone for RefCell<T> {
 462     #[inline]
 463     fn clone(&self) -> RefCell<T> {
 464         RefCell::new(self.borrow().clone())
 465     }
 466 }
 467
 468 #[stable(feature = "rust1", since = "1.0.0")]
 469 impl<T:Default> Default for RefCell<T> {
 470     #[stable(feature = "rust1", since = "1.0.0")]
 471     #[inline]
 472     fn default() -> RefCell<T> {
 473         RefCell::new(Default::default())
 474     }
 475 }
 476
 477 #[stable(feature = "rust1", since = "1.0.0")]
 478 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
 479     #[inline]
 480     fn eq(&self, other: &RefCell<T>) -> bool {
 481         *self.borrow() == *other.borrow()
 482     }
 483 }
 484
 485 #[stable(feature = "cell_eq", since = "1.2.0")]
 486 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 487
 488 struct BorrowRef<'b> {
 489     _borrow: &'b Cell<BorrowFlag>,
 490 }
 491
 492 impl<'b> BorrowRef<'b> {
 493     #[inline]
 494     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 495         match borrow.get() {
 496             WRITING => None,
 497             b => {
 498                 borrow.set(b + 1);
 499                 Some(BorrowRef { _borrow: borrow })
 500             },
 501         }
 502     }
 503 }
 504
 505 impl<'b> Drop for BorrowRef<'b> {
 506     #[inline]
 507     fn drop(&mut self) {
 508         let borrow = self._borrow.get();
 509         debug_assert!(borrow != WRITING && borrow != UNUSED);
 510         self._borrow.set(borrow - 1);
 511     }
 512 }
 513
 514 impl<'b> Clone for BorrowRef<'b> {
 515     #[inline]
 516     fn clone(&self) -> BorrowRef<'b> {
 517         // Since this Ref exists, we know the borrow flag
 518         // is not set to WRITING.
 519         let borrow = self._borrow.get();
 520         debug_assert!(borrow != WRITING && borrow != UNUSED);
 521         self._borrow.set(borrow + 1);
 522         BorrowRef { _borrow: self._borrow }
 523     }
 524 }
 525
 526 /// Wraps a borrowed reference to a value in a `RefCell` box.
 527 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
 528 ///
 529 /// See the [module-level documentation](index.html) for more.
 530 #[stable(feature = "rust1", since = "1.0.0")]
 531 pub struct Ref<'b, T: ?Sized + 'b> {
 532     // FIXME #12808: strange name to try to avoid interfering with
 533     // field accesses of the contained type via Deref
 534     _value: &'b T,
 535     _borrow: BorrowRef<'b>,
 536 }
 537
 538 #[stable(feature = "rust1", since = "1.0.0")]
 539 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 540     type Target = T;
 541
 542     #[inline]
 543     fn deref<'a>(&'a self) -> &'a T {
 544         self._value
 545     }
 546 }
 547
 548 /// Copies a `Ref`.
 549 ///
 550 /// The `RefCell` is already immutably borrowed, so this cannot fail.
 551 ///
 552 /// A `Clone` implementation would interfere with the widespread
 553 /// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
 554 #[deprecated(since = "1.2.0", reason = "moved to a `Ref::clone` associated function")]
 555 #[unstable(feature = "core",
 556            reason = "likely to be moved to a method, pending language changes")]
 557 #[inline]
 558 pub fn clone_ref<'b, T:Clone>(orig: &Ref<'b, T>) -> Ref<'b, T> {
 559     Ref::clone(orig)
 560 }
 561
 562 impl<'b, T: ?Sized> Ref<'b, T> {
 563     /// Copies a `Ref`.
 564     ///
 565     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 566     ///
 567     /// This is an associated function that needs to be used as
 568     /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
 569     /// with the widespread use of `r.borrow().clone()` to clone the contents of
 570     /// a `RefCell`.
 571     #[unstable(feature = "cell_extras",
 572                reason = "likely to be moved to a method, pending language changes")]
 573     #[inline]
 574     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
 575         Ref {
 576             _value: orig._value,
 577             _borrow: orig._borrow.clone(),
 578         }
 579     }
 580
 581     /// Make a new `Ref` for a component of the borrowed data.
 582     ///
 583     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 584     ///
 585     /// This is an associated function that needs to be used as `Ref::map(...)`.
 586     /// A method would interfere with methods of the same name on the contents
 587     /// of a `RefCell` used through `Deref`.
 588     ///
 589     /// # Example
 590     ///
 591     /// ```
 592     /// # #![feature(cell_extras)]
 593     /// use std::cell::{RefCell, Ref};
 594     ///
 595     /// let c = RefCell::new((5, 'b'));
 596     /// let b1: Ref<(u32, char)> = c.borrow();
 597     /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
 598     /// assert_eq!(*b2, 5)
 599     /// ```
 600     #[unstable(feature = "cell_extras", reason = "recently added")]
 601     #[inline]
 602     pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
 603         where F: FnOnce(&T) -> &U
 604     {
 605         Ref {
 606             _value: f(orig._value),
 607             _borrow: orig._borrow,
 608         }
 609     }
 610
 611     /// Make a new `Ref` for a optional component of the borrowed data, e.g. an
 612     /// enum variant.
 613     ///
 614     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 615     ///
 616     /// This is an associated function that needs to be used as
 617     /// `Ref::filter_map(...)`.  A method would interfere with methods of the
 618     /// same name on the contents of a `RefCell` used through `Deref`.
 619     ///
 620     /// # Example
 621     ///
 622     /// ```
 623     /// # #![feature(cell_extras)]
 624     /// use std::cell::{RefCell, Ref};
 625     ///
 626     /// let c = RefCell::new(Ok(5));
 627     /// let b1: Ref<Result<u32, ()>> = c.borrow();
 628     /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
 629     /// assert_eq!(*b2, 5)
 630     /// ```
 631     #[unstable(feature = "cell_extras", reason = "recently added")]
 632     #[inline]
 633     pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
 634         where F: FnOnce(&T) -> Option<&U>
 635     {
 636         f(orig._value).map(move |new| Ref {
 637             _value: new,
 638             _borrow: orig._borrow,
 639         })
 640     }
 641 }
 642
 643 impl<'b, T: ?Sized> RefMut<'b, T> {
 644     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
 645     /// variant.
 646     ///
 647     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 648     ///
 649     /// This is an associated function that needs to be used as
 650     /// `RefMut::map(...)`.  A method would interfere with methods of the same
 651     /// name on the contents of a `RefCell` used through `Deref`.
 652     ///
 653     /// # Example
 654     ///
 655     /// ```
 656     /// # #![feature(cell_extras)]
 657     /// use std::cell::{RefCell, RefMut};
 658     ///
 659     /// let c = RefCell::new((5, 'b'));
 660     /// {
 661     ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
 662     ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
 663     ///     assert_eq!(*b2, 5);
 664     ///     *b2 = 42;
 665     /// }
 666     /// assert_eq!(*c.borrow(), (42, 'b'));
 667     /// ```
 668     #[unstable(feature = "cell_extras", reason = "recently added")]
 669     #[inline]
 670     pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
 671         where F: FnOnce(&mut T) -> &mut U
 672     {
 673         RefMut {
 674             _value: f(orig._value),
 675             _borrow: orig._borrow,
 676         }
 677     }
 678
 679     /// Make a new `RefMut` for a optional component of the borrowed data, e.g.
 680     /// an enum variant.
 681     ///
 682     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 683     ///
 684     /// This is an associated function that needs to be used as
 685     /// `RefMut::filter_map(...)`.  A method would interfere with methods of the
 686     /// same name on the contents of a `RefCell` used through `Deref`.
 687     ///
 688     /// # Example
 689     ///
 690     /// ```
 691     /// # #![feature(cell_extras)]
 692     /// use std::cell::{RefCell, RefMut};
 693     ///
 694     /// let c = RefCell::new(Ok(5));
 695     /// {
 696     ///     let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
 697     ///     let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| {
 698     ///         o.as_mut().ok()
 699     ///     }).unwrap();
 700     ///     assert_eq!(*b2, 5);
 701     ///     *b2 = 42;
 702     /// }
 703     /// assert_eq!(*c.borrow(), Ok(42));
 704     /// ```
 705     #[unstable(feature = "cell_extras", reason = "recently added")]
 706     #[inline]
 707     pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
 708         where F: FnOnce(&mut T) -> Option<&mut U>
 709     {
 710         let RefMut { _value, _borrow } = orig;
 711         f(_value).map(move |new| RefMut {
 712             _value: new,
 713             _borrow: _borrow,
 714         })
 715     }
 716 }
 717
 718 struct BorrowRefMut<'b> {
 719     _borrow: &'b Cell<BorrowFlag>,
 720 }
 721
 722 impl<'b> Drop for BorrowRefMut<'b> {
 723     #[inline]
 724     fn drop(&mut self) {
 725         let borrow = self._borrow.get();
 726         debug_assert!(borrow == WRITING);
 727         self._borrow.set(UNUSED);
 728     }
 729 }
 730
 731 impl<'b> BorrowRefMut<'b> {
 732     #[inline]
 733     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
 734         match borrow.get() {
 735             UNUSED => {
 736                 borrow.set(WRITING);
 737                 Some(BorrowRefMut { _borrow: borrow })
 738             },
 739             _ => None,
 740         }
 741     }
 742 }
 743
 744 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
 745 ///
 746 /// See the [module-level documentation](index.html) for more.
 747 #[stable(feature = "rust1", since = "1.0.0")]
 748 pub struct RefMut<'b, T: ?Sized + 'b> {
 749     // FIXME #12808: strange name to try to avoid interfering with
 750     // field accesses of the contained type via Deref
 751     _value: &'b mut T,
 752     _borrow: BorrowRefMut<'b>,
 753 }
 754
 755 #[stable(feature = "rust1", since = "1.0.0")]
 756 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
 757     type Target = T;
 758
 759     #[inline]
 760     fn deref<'a>(&'a self) -> &'a T {
 761         self._value
 762     }
 763 }
 764
 765 #[stable(feature = "rust1", since = "1.0.0")]
 766 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
 767     #[inline]
 768     fn deref_mut<'a>(&'a mut self) -> &'a mut T {
 769         self._value
 770     }
 771 }
 772
 773 /// The core primitive for interior mutability in Rust.
 774 ///
 775 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
 776 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
 777 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
 778 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
 779 ///
 780 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
 781 ///
 782 /// # Examples
 783 ///
 784 /// ```
 785 /// use std::cell::UnsafeCell;
 786 /// use std::marker::Sync;
 787 ///
 788 /// struct NotThreadSafe<T> {
 789 ///     value: UnsafeCell<T>,
 790 /// }
 791 ///
 792 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
 793 /// ```
 794 ///
 795 /// **NOTE:** `UnsafeCell<T>`'s fields are public to allow static initializers. It is not
 796 /// recommended to access its fields directly, `get` should be used instead.
 797 #[lang = "unsafe_cell"]
 798 #[stable(feature = "rust1", since = "1.0.0")]
 799 pub struct UnsafeCell<T: ?Sized> {
 800     /// Wrapped value
 801     ///
 802     /// This field should not be accessed directly, it is made public for static
 803     /// initializers.
 804     #[deprecated(since = "1.2.0", reason = "use `get` to access the wrapped \
 805         value or `new` to initialize `UnsafeCell` in statics")]
 806     #[unstable(feature = "core")]
 807     pub value: T,
 808 }
 809
 810 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
 811
 812 impl<T> UnsafeCell<T> {
 813     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
 814     /// value.
 815     ///
 816     /// All access to the inner value through methods is `unsafe`, and it is highly discouraged to
 817     /// access the fields directly.
 818     ///
 819     /// # Examples
 820     ///
 821     /// ```
 822     /// use std::cell::UnsafeCell;
 823     ///
 824     /// let uc = UnsafeCell::new(5);
 825     /// ```
 826     #[stable(feature = "rust1", since = "1.0.0")]
 827     #[inline]
 828     pub const fn new(value: T) -> UnsafeCell<T> {
 829         #![allow(deprecated)]
 830         UnsafeCell { value: value }
 831     }
 832
 833     /// Unwraps the value.
 834     ///
 835     /// # Unsafety
 836     ///
 837     /// This function is unsafe because this thread or another thread may currently be
 838     /// inspecting the inner value.
 839     ///
 840     /// # Examples
 841     ///
 842     /// ```
 843     /// use std::cell::UnsafeCell;
 844     ///
 845     /// let uc = UnsafeCell::new(5);
 846     ///
 847     /// let five = unsafe { uc.into_inner() };
 848     /// ```
 849     #[inline]
 850     #[stable(feature = "rust1", since = "1.0.0")]
 851     pub unsafe fn into_inner(self) -> T {
 852         #![allow(deprecated)]
 853         self.value
 854     }
 855 }
 856
 857 impl<T: ?Sized> UnsafeCell<T> {
 858     /// Gets a mutable pointer to the wrapped value.
 859     ///
 860     /// # Examples
 861     ///
 862     /// ```
 863     /// use std::cell::UnsafeCell;
 864     ///
 865     /// let uc = UnsafeCell::new(5);
 866     ///
 867     /// let five = uc.get();
 868     /// ```
 869     #[inline]
 870     #[stable(feature = "rust1", since = "1.0.0")]
 871     pub fn get(&self) -> *mut T {
 872         // FIXME(#23542) Replace with type ascription.
 873         #![allow(trivial_casts)]
 874         #![allow(deprecated)]
 875         &self.value as *const T as *mut T
 876     }
 877 }