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 mutability 'inside' of something immutable
  40 //! * Implementation details of logically-immutable methods.
  41 //! * Mutating implementations of `Clone`.
  42 //!
  43 //! ## Introducing mutability 'inside' of something immutable
  44 //!
  45 //! Many 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 with `&`, not `&mut`. Without cells it would be
  48 //! impossible to mutate data inside of these smart pointers 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 `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
  69 //! multi-threaded 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 //! # #![allow(dead_code)]
  80 //! use std::cell::RefCell;
  81 //!
  82 //! struct Graph {
  83 //!     edges: Vec<(i32, i32)>,
  84 //!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
  85 //! }
  86 //!
  87 //! impl Graph {
  88 //!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
  89 //!         // Create a new scope to contain the lifetime of the
  90 //!         // dynamic borrow
  91 //!         {
  92 //!             // Take a reference to the inside of cache cell
  93 //!             let mut cache = self.span_tree_cache.borrow_mut();
  94 //!             if cache.is_some() {
  95 //!                 return cache.as_ref().unwrap().clone();
  96 //!             }
  97 //!
  98 //!             let span_tree = self.calc_span_tree();
  99 //!             *cache = Some(span_tree);
 100 //!         }
 101 //!
 102 //!         // Recursive call to return the just-cached value.
 103 //!         // Note that if we had not let the previous borrow
 104 //!         // of the cache fall out of scope then the subsequent
 105 //!         // recursive borrow would cause a dynamic thread panic.
 106 //!         // This is the major hazard of using `RefCell`.
 107 //!         self.minimum_spanning_tree()
 108 //!     }
 109 //! #   fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
 110 //! }
 111 //! ```
 112 //!
 113 //! ## Mutating implementations of `Clone`
 114 //!
 115 //! This is simply a special - but common - case of the previous: hiding mutability for operations
 116 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
 117 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
 118 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
 119 //! `Cell<T>`.
 120 //!
 121 //! ```
 122 //! use std::cell::Cell;
 123 //!
 124 //! struct Rc<T> {
 125 //!     ptr: *mut RcBox<T>
 126 //! }
 127 //!
 128 //! struct RcBox<T> {
 129 //! # #[allow(dead_code)]
 130 //!     value: T,
 131 //!     refcount: Cell<usize>
 132 //! }
 133 //!
 134 //! impl<T> Clone for Rc<T> {
 135 //!     fn clone(&self) -> Rc<T> {
 136 //!         unsafe {
 137 //!             (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
 138 //!             Rc { ptr: self.ptr }
 139 //!         }
 140 //!     }
 141 //! }
 142 //! ```
 143 //!
 144
 145 #![stable(feature = "rust1", since = "1.0.0")]
 146
 147 use clone::Clone;
 148 use cmp::{PartialEq, Eq, PartialOrd, Ord, Ordering};
 149 use convert::From;
 150 use default::Default;
 151 use fmt::{self, Debug, Display};
 152 use marker::{Copy, PhantomData, Send, Sync, Sized, Unsize};
 153 use ops::{Deref, DerefMut, Drop, FnOnce, CoerceUnsized};
 154 use option::Option;
 155 use option::Option::{None, Some};
 156 use result::Result;
 157 use result::Result::{Ok, Err};
 158
 159 /// A mutable memory location that admits only `Copy` data.
 160 ///
 161 /// See the [module-level documentation](index.html) for more.
 162 #[stable(feature = "rust1", since = "1.0.0")]
 163 pub struct Cell<T> {
 164     value: UnsafeCell<T>,
 165 }
 166
 167 impl<T:Copy> Cell<T> {
 168     /// Creates a new `Cell` containing the given value.
 169     ///
 170     /// # Examples
 171     ///
 172     /// ```
 173     /// use std::cell::Cell;
 174     ///
 175     /// let c = Cell::new(5);
 176     /// ```
 177     #[stable(feature = "rust1", since = "1.0.0")]
 178     #[inline]
 179     pub const fn new(value: T) -> Cell<T> {
 180         Cell {
 181             value: UnsafeCell::new(value),
 182         }
 183     }
 184
 185     /// Returns a copy of the contained value.
 186     ///
 187     /// # Examples
 188     ///
 189     /// ```
 190     /// use std::cell::Cell;
 191     ///
 192     /// let c = Cell::new(5);
 193     ///
 194     /// let five = c.get();
 195     /// ```
 196     #[inline]
 197     #[stable(feature = "rust1", since = "1.0.0")]
 198     pub fn get(&self) -> T {
 199         unsafe{ *self.value.get() }
 200     }
 201
 202     /// Sets the contained value.
 203     ///
 204     /// # Examples
 205     ///
 206     /// ```
 207     /// use std::cell::Cell;
 208     ///
 209     /// let c = Cell::new(5);
 210     ///
 211     /// c.set(10);
 212     /// ```
 213     #[inline]
 214     #[stable(feature = "rust1", since = "1.0.0")]
 215     pub fn set(&self, value: T) {
 216         unsafe {
 217             *self.value.get() = value;
 218         }
 219     }
 220
 221     /// Returns a reference to the underlying `UnsafeCell`.
 222     ///
 223     /// # Examples
 224     ///
 225     /// ```
 226     /// #![feature(as_unsafe_cell)]
 227     ///
 228     /// use std::cell::Cell;
 229     ///
 230     /// let c = Cell::new(5);
 231     ///
 232     /// let uc = c.as_unsafe_cell();
 233     /// ```
 234     #[inline]
 235     #[unstable(feature = "as_unsafe_cell", issue = "27708")]
 236     #[rustc_deprecated(since = "1.12.0", reason = "renamed to as_ptr")]
 237     pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
 238         &self.value
 239     }
 240
 241     /// Returns a raw pointer to the underlying data in this cell.
 242     ///
 243     /// # Examples
 244     ///
 245     /// ```
 246     /// use std::cell::Cell;
 247     ///
 248     /// let c = Cell::new(5);
 249     ///
 250     /// let ptr = c.as_ptr();
 251     /// ```
 252     #[inline]
 253     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 254     pub fn as_ptr(&self) -> *mut T {
 255         self.value.get()
 256     }
 257
 258     /// Returns a mutable reference to the underlying data.
 259     ///
 260     /// This call borrows `Cell` mutably (at compile-time) which guarantees
 261     /// that we possess the only reference.
 262     ///
 263     /// # Examples
 264     ///
 265     /// ```
 266     /// use std::cell::Cell;
 267     ///
 268     /// let mut c = Cell::new(5);
 269     /// *c.get_mut() += 1;
 270     ///
 271     /// assert_eq!(c.get(), 6);
 272     /// ```
 273     #[inline]
 274     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 275     pub fn get_mut(&mut self) -> &mut T {
 276         unsafe {
 277             &mut *self.value.get()
 278         }
 279     }
 280 }
 281
 282 #[stable(feature = "rust1", since = "1.0.0")]
 283 unsafe impl<T> Send for Cell<T> where T: Send {}
 284
 285 #[stable(feature = "rust1", since = "1.0.0")]
 286 impl<T> !Sync for Cell<T> {}
 287
 288 #[stable(feature = "rust1", since = "1.0.0")]
 289 impl<T:Copy> Clone for Cell<T> {
 290     #[inline]
 291     fn clone(&self) -> Cell<T> {
 292         Cell::new(self.get())
 293     }
 294 }
 295
 296 #[stable(feature = "rust1", since = "1.0.0")]
 297 impl<T:Default + Copy> Default for Cell<T> {
 298     #[inline]
 299     fn default() -> Cell<T> {
 300         Cell::new(Default::default())
 301     }
 302 }
 303
 304 #[stable(feature = "rust1", since = "1.0.0")]
 305 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 306     #[inline]
 307     fn eq(&self, other: &Cell<T>) -> bool {
 308         self.get() == other.get()
 309     }
 310 }
 311
 312 #[stable(feature = "cell_eq", since = "1.2.0")]
 313 impl<T:Eq + Copy> Eq for Cell<T> {}
 314
 315 #[stable(feature = "cell_ord", since = "1.10.0")]
 316 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
 317     #[inline]
 318     fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
 319         self.get().partial_cmp(&other.get())
 320     }
 321
 322     #[inline]
 323     fn lt(&self, other: &Cell<T>) -> bool {
 324         self.get() < other.get()
 325     }
 326
 327     #[inline]
 328     fn le(&self, other: &Cell<T>) -> bool {
 329         self.get() <= other.get()
 330     }
 331
 332     #[inline]
 333     fn gt(&self, other: &Cell<T>) -> bool {
 334         self.get() > other.get()
 335     }
 336
 337     #[inline]
 338     fn ge(&self, other: &Cell<T>) -> bool {
 339         self.get() >= other.get()
 340     }
 341 }
 342
 343 #[stable(feature = "cell_ord", since = "1.10.0")]
 344 impl<T:Ord + Copy> Ord for Cell<T> {
 345     #[inline]
 346     fn cmp(&self, other: &Cell<T>) -> Ordering {
 347         self.get().cmp(&other.get())
 348     }
 349 }
 350
 351 #[stable(feature = "cell_from", since = "1.12.0")]
 352 impl<T: Copy> From<T> for Cell<T> {
 353     fn from(t: T) -> Cell<T> {
 354         Cell::new(t)
 355     }
 356 }
 357
 358 /// A mutable memory location with dynamically checked borrow rules
 359 ///
 360 /// See the [module-level documentation](index.html) for more.
 361 #[stable(feature = "rust1", since = "1.0.0")]
 362 pub struct RefCell<T: ?Sized> {
 363     borrow: Cell<BorrowFlag>,
 364     value: UnsafeCell<T>,
 365 }
 366
 367 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
 368 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 369 #[unstable(feature = "borrow_state", issue = "27733")]
 370 pub enum BorrowState {
 371     /// The cell is currently being read, there is at least one active `borrow`.
 372     Reading,
 373     /// The cell is currently being written to, there is an active `borrow_mut`.
 374     Writing,
 375     /// There are no outstanding borrows on this cell.
 376     Unused,
 377 }
 378
 379 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
 380 #[unstable(feature = "try_borrow", issue = "35070")]
 381 pub struct BorrowError<'a, T: 'a + ?Sized> {
 382     marker: PhantomData<&'a RefCell<T>>,
 383 }
 384
 385 #[unstable(feature = "try_borrow", issue = "35070")]
 386 impl<'a, T: ?Sized> Debug for BorrowError<'a, T> {
 387     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 388         f.debug_struct("BorrowError").finish()
 389     }
 390 }
 391
 392 #[unstable(feature = "try_borrow", issue = "35070")]
 393 impl<'a, T: ?Sized> Display for BorrowError<'a, T> {
 394     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 395         Display::fmt("already mutably borrowed", f)
 396     }
 397 }
 398
 399 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
 400 #[unstable(feature = "try_borrow", issue = "35070")]
 401 pub struct BorrowMutError<'a, T: 'a + ?Sized> {
 402     marker: PhantomData<&'a RefCell<T>>,
 403 }
 404
 405 #[unstable(feature = "try_borrow", issue = "35070")]
 406 impl<'a, T: ?Sized> Debug for BorrowMutError<'a, T> {
 407     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 408         f.debug_struct("BorrowMutError").finish()
 409     }
 410 }
 411
 412 #[unstable(feature = "try_borrow", issue = "35070")]
 413 impl<'a, T: ?Sized> Display for BorrowMutError<'a, T> {
 414     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 415         Display::fmt("already borrowed", f)
 416     }
 417 }
 418
 419 // Values [1, MAX-1] represent the number of `Ref` active
 420 // (will not outgrow its range since `usize` is the size of the address space)
 421 type BorrowFlag = usize;
 422 const UNUSED: BorrowFlag = 0;
 423 const WRITING: BorrowFlag = !0;
 424
 425 impl<T> RefCell<T> {
 426     /// Creates a new `RefCell` containing `value`.
 427     ///
 428     /// # Examples
 429     ///
 430     /// ```
 431     /// use std::cell::RefCell;
 432     ///
 433     /// let c = RefCell::new(5);
 434     /// ```
 435     #[stable(feature = "rust1", since = "1.0.0")]
 436     #[inline]
 437     pub const fn new(value: T) -> RefCell<T> {
 438         RefCell {
 439             value: UnsafeCell::new(value),
 440             borrow: Cell::new(UNUSED),
 441         }
 442     }
 443
 444     /// Consumes the `RefCell`, returning the wrapped value.
 445     ///
 446     /// # Examples
 447     ///
 448     /// ```
 449     /// use std::cell::RefCell;
 450     ///
 451     /// let c = RefCell::new(5);
 452     ///
 453     /// let five = c.into_inner();
 454     /// ```
 455     #[stable(feature = "rust1", since = "1.0.0")]
 456     #[inline]
 457     pub fn into_inner(self) -> T {
 458         // Since this function takes `self` (the `RefCell`) by value, the
 459         // compiler statically verifies that it is not currently borrowed.
 460         // Therefore the following assertion is just a `debug_assert!`.
 461         debug_assert!(self.borrow.get() == UNUSED);
 462         unsafe { self.value.into_inner() }
 463     }
 464 }
 465
 466 impl<T: ?Sized> RefCell<T> {
 467     /// Query the current state of this `RefCell`
 468     ///
 469     /// The returned value can be dispatched on to determine if a call to
 470     /// `borrow` or `borrow_mut` would succeed.
 471     ///
 472     /// # Examples
 473     ///
 474     /// ```
 475     /// #![feature(borrow_state)]
 476     ///
 477     /// use std::cell::{BorrowState, RefCell};
 478     ///
 479     /// let c = RefCell::new(5);
 480     ///
 481     /// match c.borrow_state() {
 482     ///     BorrowState::Writing => println!("Cannot be borrowed"),
 483     ///     BorrowState::Reading => println!("Cannot be borrowed mutably"),
 484     ///     BorrowState::Unused => println!("Can be borrowed (mutably as well)"),
 485     /// }
 486     /// ```
 487     #[unstable(feature = "borrow_state", issue = "27733")]
 488     #[inline]
 489     pub fn borrow_state(&self) -> BorrowState {
 490         match self.borrow.get() {
 491             WRITING => BorrowState::Writing,
 492             UNUSED => BorrowState::Unused,
 493             _ => BorrowState::Reading,
 494         }
 495     }
 496
 497     /// Immutably borrows the wrapped value.
 498     ///
 499     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 500     /// immutable borrows can be taken out at the same time.
 501     ///
 502     /// # Panics
 503     ///
 504     /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
 505     /// [`try_borrow`](#method.try_borrow).
 506     ///
 507     /// # Examples
 508     ///
 509     /// ```
 510     /// use std::cell::RefCell;
 511     ///
 512     /// let c = RefCell::new(5);
 513     ///
 514     /// let borrowed_five = c.borrow();
 515     /// let borrowed_five2 = c.borrow();
 516     /// ```
 517     ///
 518     /// An example of panic:
 519     ///
 520     /// ```
 521     /// use std::cell::RefCell;
 522     /// use std::thread;
 523     ///
 524     /// let result = thread::spawn(move || {
 525     ///    let c = RefCell::new(5);
 526     ///    let m = c.borrow_mut();
 527     ///
 528     ///    let b = c.borrow(); // this causes a panic
 529     /// }).join();
 530     ///
 531     /// assert!(result.is_err());
 532     /// ```
 533     #[stable(feature = "rust1", since = "1.0.0")]
 534     #[inline]
 535     pub fn borrow(&self) -> Ref<T> {
 536         self.try_borrow().expect("already mutably borrowed")
 537     }
 538
 539     /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
 540     /// borrowed.
 541     ///
 542     /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
 543     /// taken out at the same time.
 544     ///
 545     /// This is the non-panicking variant of [`borrow`](#method.borrow).
 546     ///
 547     /// # Examples
 548     ///
 549     /// ```
 550     /// #![feature(try_borrow)]
 551     ///
 552     /// use std::cell::RefCell;
 553     ///
 554     /// let c = RefCell::new(5);
 555     ///
 556     /// {
 557     ///     let m = c.borrow_mut();
 558     ///     assert!(c.try_borrow().is_err());
 559     /// }
 560     ///
 561     /// {
 562     ///     let m = c.borrow();
 563     ///     assert!(c.try_borrow().is_ok());
 564     /// }
 565     /// ```
 566     #[unstable(feature = "try_borrow", issue = "35070")]
 567     #[inline]
 568     pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError<T>> {
 569         match BorrowRef::new(&self.borrow) {
 570             Some(b) => Ok(Ref {
 571                 value: unsafe { &*self.value.get() },
 572                 borrow: b,
 573             }),
 574             None => Err(BorrowError { marker: PhantomData }),
 575         }
 576     }
 577
 578     /// Mutably borrows the wrapped value.
 579     ///
 580     /// The borrow lasts until the returned `RefMut` exits scope. The value
 581     /// cannot be borrowed while this borrow is active.
 582     ///
 583     /// # Panics
 584     ///
 585     /// Panics if the value is currently borrowed. For a non-panicking variant, use
 586     /// [`try_borrow_mut`](#method.try_borrow_mut).
 587     ///
 588     /// # Examples
 589     ///
 590     /// ```
 591     /// use std::cell::RefCell;
 592     ///
 593     /// let c = RefCell::new(5);
 594     ///
 595     /// *c.borrow_mut() = 7;
 596     ///
 597     /// assert_eq!(*c.borrow(), 7);
 598     /// ```
 599     ///
 600     /// An example of panic:
 601     ///
 602     /// ```
 603     /// use std::cell::RefCell;
 604     /// use std::thread;
 605     ///
 606     /// let result = thread::spawn(move || {
 607     ///    let c = RefCell::new(5);
 608     ///    let m = c.borrow();
 609     ///
 610     ///    let b = c.borrow_mut(); // this causes a panic
 611     /// }).join();
 612     ///
 613     /// assert!(result.is_err());
 614     /// ```
 615     #[stable(feature = "rust1", since = "1.0.0")]
 616     #[inline]
 617     pub fn borrow_mut(&self) -> RefMut<T> {
 618         self.try_borrow_mut().expect("already borrowed")
 619     }
 620
 621     /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
 622     ///
 623     /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
 624     /// while this borrow is active.
 625     ///
 626     /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
 627     ///
 628     /// # Examples
 629     ///
 630     /// ```
 631     /// #![feature(try_borrow)]
 632     ///
 633     /// use std::cell::RefCell;
 634     ///
 635     /// let c = RefCell::new(5);
 636     ///
 637     /// {
 638     ///     let m = c.borrow();
 639     ///     assert!(c.try_borrow_mut().is_err());
 640     /// }
 641     ///
 642     /// assert!(c.try_borrow_mut().is_ok());
 643     /// ```
 644     #[unstable(feature = "try_borrow", issue = "35070")]
 645     #[inline]
 646     pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError<T>> {
 647         match BorrowRefMut::new(&self.borrow) {
 648             Some(b) => Ok(RefMut {
 649                 value: unsafe { &mut *self.value.get() },
 650                 borrow: b,
 651             }),
 652             None => Err(BorrowMutError { marker: PhantomData }),
 653         }
 654     }
 655
 656     /// Returns a reference to the underlying `UnsafeCell`.
 657     ///
 658     /// This can be used to circumvent `RefCell`'s safety checks.
 659     ///
 660     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 661     ///
 662     /// # Examples
 663     ///
 664     /// ```
 665     /// #![feature(as_unsafe_cell)]
 666     ///
 667     /// use std::cell::RefCell;
 668     ///
 669     /// let c = RefCell::new(5);
 670     /// let c = unsafe { c.as_unsafe_cell() };
 671     /// ```
 672     #[inline]
 673     #[unstable(feature = "as_unsafe_cell", issue = "27708")]
 674     #[rustc_deprecated(since = "1.12.0", reason = "renamed to as_ptr")]
 675     pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
 676         &self.value
 677     }
 678
 679     /// Returns a raw pointer to the underlying data in this cell.
 680     ///
 681     /// # Examples
 682     ///
 683     /// ```
 684     /// use std::cell::RefCell;
 685     ///
 686     /// let c = RefCell::new(5);
 687     ///
 688     /// let ptr = c.as_ptr();
 689     /// ```
 690     #[inline]
 691     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 692     pub fn as_ptr(&self) -> *mut T {
 693         self.value.get()
 694     }
 695
 696     /// Returns a mutable reference to the underlying data.
 697     ///
 698     /// This call borrows `RefCell` mutably (at compile-time) so there is no
 699     /// need for dynamic checks.
 700     ///
 701     /// # Examples
 702     ///
 703     /// ```
 704     /// use std::cell::RefCell;
 705     ///
 706     /// let mut c = RefCell::new(5);
 707     /// *c.get_mut() += 1;
 708     ///
 709     /// assert_eq!(c, RefCell::new(6));
 710     /// ```
 711     #[inline]
 712     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 713     pub fn get_mut(&mut self) -> &mut T {
 714         unsafe {
 715             &mut *self.value.get()
 716         }
 717     }
 718 }
 719
 720 #[stable(feature = "rust1", since = "1.0.0")]
 721 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 722
 723 #[stable(feature = "rust1", since = "1.0.0")]
 724 impl<T: ?Sized> !Sync for RefCell<T> {}
 725
 726 #[stable(feature = "rust1", since = "1.0.0")]
 727 impl<T: Clone> Clone for RefCell<T> {
 728     #[inline]
 729     fn clone(&self) -> RefCell<T> {
 730         RefCell::new(self.borrow().clone())
 731     }
 732 }
 733
 734 #[stable(feature = "rust1", since = "1.0.0")]
 735 impl<T:Default> Default for RefCell<T> {
 736     #[inline]
 737     fn default() -> RefCell<T> {
 738         RefCell::new(Default::default())
 739     }
 740 }
 741
 742 #[stable(feature = "rust1", since = "1.0.0")]
 743 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
 744     #[inline]
 745     fn eq(&self, other: &RefCell<T>) -> bool {
 746         *self.borrow() == *other.borrow()
 747     }
 748 }
 749
 750 #[stable(feature = "cell_eq", since = "1.2.0")]
 751 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 752
 753 #[stable(feature = "cell_ord", since = "1.10.0")]
 754 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
 755     #[inline]
 756     fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
 757         self.borrow().partial_cmp(&*other.borrow())
 758     }
 759
 760     #[inline]
 761     fn lt(&self, other: &RefCell<T>) -> bool {
 762         *self.borrow() < *other.borrow()
 763     }
 764
 765     #[inline]
 766     fn le(&self, other: &RefCell<T>) -> bool {
 767         *self.borrow() <= *other.borrow()
 768     }
 769
 770     #[inline]
 771     fn gt(&self, other: &RefCell<T>) -> bool {
 772         *self.borrow() > *other.borrow()
 773     }
 774
 775     #[inline]
 776     fn ge(&self, other: &RefCell<T>) -> bool {
 777         *self.borrow() >= *other.borrow()
 778     }
 779 }
 780
 781 #[stable(feature = "cell_ord", since = "1.10.0")]
 782 impl<T: ?Sized + Ord> Ord for RefCell<T> {
 783     #[inline]
 784     fn cmp(&self, other: &RefCell<T>) -> Ordering {
 785         self.borrow().cmp(&*other.borrow())
 786     }
 787 }
 788
 789 #[stable(feature = "cell_from", since = "1.12.0")]
 790 impl<T> From<T> for RefCell<T> {
 791     fn from(t: T) -> RefCell<T> {
 792         RefCell::new(t)
 793     }
 794 }
 795
 796 struct BorrowRef<'b> {
 797     borrow: &'b Cell<BorrowFlag>,
 798 }
 799
 800 impl<'b> BorrowRef<'b> {
 801     #[inline]
 802     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 803         match borrow.get() {
 804             WRITING => None,
 805             b => {
 806                 borrow.set(b + 1);
 807                 Some(BorrowRef { borrow: borrow })
 808             },
 809         }
 810     }
 811 }
 812
 813 impl<'b> Drop for BorrowRef<'b> {
 814     #[inline]
 815     fn drop(&mut self) {
 816         let borrow = self.borrow.get();
 817         debug_assert!(borrow != WRITING && borrow != UNUSED);
 818         self.borrow.set(borrow - 1);
 819     }
 820 }
 821
 822 impl<'b> Clone for BorrowRef<'b> {
 823     #[inline]
 824     fn clone(&self) -> BorrowRef<'b> {
 825         // Since this Ref exists, we know the borrow flag
 826         // is not set to WRITING.
 827         let borrow = self.borrow.get();
 828         debug_assert!(borrow != UNUSED);
 829         // Prevent the borrow counter from overflowing.
 830         assert!(borrow != WRITING);
 831         self.borrow.set(borrow + 1);
 832         BorrowRef { borrow: self.borrow }
 833     }
 834 }
 835
 836 /// Wraps a borrowed reference to a value in a `RefCell` box.
 837 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
 838 ///
 839 /// See the [module-level documentation](index.html) for more.
 840 #[stable(feature = "rust1", since = "1.0.0")]
 841 pub struct Ref<'b, T: ?Sized + 'b> {
 842     value: &'b T,
 843     borrow: BorrowRef<'b>,
 844 }
 845
 846 #[stable(feature = "rust1", since = "1.0.0")]
 847 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 848     type Target = T;
 849
 850     #[inline]
 851     fn deref(&self) -> &T {
 852         self.value
 853     }
 854 }
 855
 856 impl<'b, T: ?Sized> Ref<'b, T> {
 857     /// Copies a `Ref`.
 858     ///
 859     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 860     ///
 861     /// This is an associated function that needs to be used as
 862     /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
 863     /// with the widespread use of `r.borrow().clone()` to clone the contents of
 864     /// a `RefCell`.
 865     #[unstable(feature = "cell_extras",
 866                reason = "likely to be moved to a method, pending language changes",
 867                issue = "27746")]
 868     #[inline]
 869     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
 870         Ref {
 871             value: orig.value,
 872             borrow: orig.borrow.clone(),
 873         }
 874     }
 875
 876     /// Make a new `Ref` for a component of the borrowed data.
 877     ///
 878     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 879     ///
 880     /// This is an associated function that needs to be used as `Ref::map(...)`.
 881     /// A method would interfere with methods of the same name on the contents
 882     /// of a `RefCell` used through `Deref`.
 883     ///
 884     /// # Example
 885     ///
 886     /// ```
 887     /// use std::cell::{RefCell, Ref};
 888     ///
 889     /// let c = RefCell::new((5, 'b'));
 890     /// let b1: Ref<(u32, char)> = c.borrow();
 891     /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
 892     /// assert_eq!(*b2, 5)
 893     /// ```
 894     #[stable(feature = "cell_map", since = "1.8.0")]
 895     #[inline]
 896     pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
 897         where F: FnOnce(&T) -> &U
 898     {
 899         Ref {
 900             value: f(orig.value),
 901             borrow: orig.borrow,
 902         }
 903     }
 904 }
 905
 906 #[unstable(feature = "coerce_unsized", issue = "27732")]
 907 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
 908
 909 impl<'b, T: ?Sized> RefMut<'b, T> {
 910     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
 911     /// variant.
 912     ///
 913     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 914     ///
 915     /// This is an associated function that needs to be used as
 916     /// `RefMut::map(...)`.  A method would interfere with methods of the same
 917     /// name on the contents of a `RefCell` used through `Deref`.
 918     ///
 919     /// # Example
 920     ///
 921     /// ```
 922     /// use std::cell::{RefCell, RefMut};
 923     ///
 924     /// let c = RefCell::new((5, 'b'));
 925     /// {
 926     ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
 927     ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
 928     ///     assert_eq!(*b2, 5);
 929     ///     *b2 = 42;
 930     /// }
 931     /// assert_eq!(*c.borrow(), (42, 'b'));
 932     /// ```
 933     #[stable(feature = "cell_map", since = "1.8.0")]
 934     #[inline]
 935     pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
 936         where F: FnOnce(&mut T) -> &mut U
 937     {
 938         RefMut {
 939             value: f(orig.value),
 940             borrow: orig.borrow,
 941         }
 942     }
 943 }
 944
 945 struct BorrowRefMut<'b> {
 946     borrow: &'b Cell<BorrowFlag>,
 947 }
 948
 949 impl<'b> Drop for BorrowRefMut<'b> {
 950     #[inline]
 951     fn drop(&mut self) {
 952         let borrow = self.borrow.get();
 953         debug_assert!(borrow == WRITING);
 954         self.borrow.set(UNUSED);
 955     }
 956 }
 957
 958 impl<'b> BorrowRefMut<'b> {
 959     #[inline]
 960     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
 961         match borrow.get() {
 962             UNUSED => {
 963                 borrow.set(WRITING);
 964                 Some(BorrowRefMut { borrow: borrow })
 965             },
 966             _ => None,
 967         }
 968     }
 969 }
 970
 971 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
 972 ///
 973 /// See the [module-level documentation](index.html) for more.
 974 #[stable(feature = "rust1", since = "1.0.0")]
 975 pub struct RefMut<'b, T: ?Sized + 'b> {
 976     value: &'b mut T,
 977     borrow: BorrowRefMut<'b>,
 978 }
 979
 980 #[stable(feature = "rust1", since = "1.0.0")]
 981 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
 982     type Target = T;
 983
 984     #[inline]
 985     fn deref(&self) -> &T {
 986         self.value
 987     }
 988 }
 989
 990 #[stable(feature = "rust1", since = "1.0.0")]
 991 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
 992     #[inline]
 993     fn deref_mut(&mut self) -> &mut T {
 994         self.value
 995     }
 996 }
 997
 998 #[unstable(feature = "coerce_unsized", issue = "27732")]
 999 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1000
1001 /// The core primitive for interior mutability in Rust.
1002 ///
1003 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1004 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1005 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1006 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1007 ///
1008 /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
1009 /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
1010 /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
1011 /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
1012 /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
1013 /// to ensure that no two mutable references obtained this way are active at the same time, and
1014 /// that there are no active mutable references or mutations when an immutable reference is obtained
1015 /// from the cell. This is often done via runtime checks.
1016 ///
1017 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
1018 /// okay (provided you enforce the invariants some other way); it is still undefined behavior
1019 /// to have multiple `&mut UnsafeCell<T>` aliases.
1020 ///
1021 ///
1022 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
1023 ///
1024 /// # Examples
1025 ///
1026 /// ```
1027 /// use std::cell::UnsafeCell;
1028 /// use std::marker::Sync;
1029 ///
1030 /// # #[allow(dead_code)]
1031 /// struct NotThreadSafe<T> {
1032 ///     value: UnsafeCell<T>,
1033 /// }
1034 ///
1035 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1036 /// ```
1037 #[lang = "unsafe_cell"]
1038 #[stable(feature = "rust1", since = "1.0.0")]
1039 pub struct UnsafeCell<T: ?Sized> {
1040     value: T,
1041 }
1042
1043 #[stable(feature = "rust1", since = "1.0.0")]
1044 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1045
1046 impl<T> UnsafeCell<T> {
1047     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1048     /// value.
1049     ///
1050     /// All access to the inner value through methods is `unsafe`.
1051     ///
1052     /// # Examples
1053     ///
1054     /// ```
1055     /// use std::cell::UnsafeCell;
1056     ///
1057     /// let uc = UnsafeCell::new(5);
1058     /// ```
1059     #[stable(feature = "rust1", since = "1.0.0")]
1060     #[inline]
1061     pub const fn new(value: T) -> UnsafeCell<T> {
1062         UnsafeCell { value: value }
1063     }
1064
1065     /// Unwraps the value.
1066     ///
1067     /// # Safety
1068     ///
1069     /// This function is unsafe because this thread or another thread may currently be
1070     /// inspecting the inner value.
1071     ///
1072     /// # Examples
1073     ///
1074     /// ```
1075     /// use std::cell::UnsafeCell;
1076     ///
1077     /// let uc = UnsafeCell::new(5);
1078     ///
1079     /// let five = unsafe { uc.into_inner() };
1080     /// ```
1081     #[inline]
1082     #[stable(feature = "rust1", since = "1.0.0")]
1083     pub unsafe fn into_inner(self) -> T {
1084         self.value
1085     }
1086 }
1087
1088 impl<T: ?Sized> UnsafeCell<T> {
1089     /// Gets a mutable pointer to the wrapped value.
1090     ///
1091     /// This can be cast to a pointer of any kind.
1092     /// Ensure that the access is unique when casting to
1093     /// `&mut T`, and ensure that there are no mutations or mutable
1094     /// aliases going on when casting to `&T`
1095     ///
1096     /// # Examples
1097     ///
1098     /// ```
1099     /// use std::cell::UnsafeCell;
1100     ///
1101     /// let uc = UnsafeCell::new(5);
1102     ///
1103     /// let five = uc.get();
1104     /// ```
1105     #[inline]
1106     #[stable(feature = "rust1", since = "1.0.0")]
1107     pub fn get(&self) -> *mut T {
1108         &self.value as *const T as *mut T
1109     }
1110 }
1111
1112 #[stable(feature = "unsafe_cell_default", since = "1.9.0")]
1113 impl<T: Default> Default for UnsafeCell<T> {
1114     fn default() -> UnsafeCell<T> {
1115         UnsafeCell::new(Default::default())
1116     }
1117 }
1118
1119 #[stable(feature = "cell_from", since = "1.12.0")]
1120 impl<T> From<T> for UnsafeCell<T> {
1121     fn from(t: T) -> UnsafeCell<T> {
1122         UnsafeCell::new(t)
1123     }
1124 }