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>` implements interior
  19 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
  20 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
  21 //! methods to retrieve and change the current interior value:
  22 //!
  23 //!  - For types that implement `Copy`, the `get` method retrieves the current interior value.
  24 //!  - For types that implement `Default`, the `take` method replaces the current interior value
  25 //!    with `Default::default()` and returns the replaced value.
  26 //!  - For all types, the `replace` method replaces the current interior value and returns the
  27 //!    replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
  28 //!    value. Additionally, the `set` method replaces the interior value, dropping the replaced
  29 //!    value.
  30 //!
  31 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
  32 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
  33 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
  34 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
  35 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
  36 //! panic.
  37 //!
  38 //! # When to choose interior mutability
  39 //!
  40 //! The more common inherited mutability, where one must have unique access to mutate a value, is
  41 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
  42 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
  43 //! interior mutability is something of a last resort. Since cell types enable mutation where it
  44 //! would otherwise be disallowed though, there are occasions when interior mutability might be
  45 //! appropriate, or even *must* be used, e.g.
  46 //!
  47 //! * Introducing mutability 'inside' of something immutable
  48 //! * Implementation details of logically-immutable methods.
  49 //! * Mutating implementations of `Clone`.
  50 //!
  51 //! ## Introducing mutability 'inside' of something immutable
  52 //!
  53 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
  54 //! cloned and shared between multiple parties. Because the contained values may be
  55 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
  56 //! impossible to mutate data inside of these smart pointers at all.
  57 //!
  58 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
  59 //! mutability:
  60 //!
  61 //! ```
  62 //! use std::collections::HashMap;
  63 //! use std::cell::RefCell;
  64 //! use std::rc::Rc;
  65 //!
  66 //! fn main() {
  67 //!     let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
  68 //!     shared_map.borrow_mut().insert("africa", 92388);
  69 //!     shared_map.borrow_mut().insert("kyoto", 11837);
  70 //!     shared_map.borrow_mut().insert("piccadilly", 11826);
  71 //!     shared_map.borrow_mut().insert("marbles", 38);
  72 //! }
  73 //! ```
  74 //!
  75 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
  76 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
  77 //! multi-threaded situation.
  78 //!
  79 //! ## Implementation details of logically-immutable methods
  80 //!
  81 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
  82 //! "under the hood". This may be because logically the operation is immutable, but e.g. caching
  83 //! forces the implementation to perform mutation; or because you must employ mutation to implement
  84 //! a trait method that was originally defined to take `&self`.
  85 //!
  86 //! ```
  87 //! # #![allow(dead_code)]
  88 //! use std::cell::RefCell;
  89 //!
  90 //! struct Graph {
  91 //!     edges: Vec<(i32, i32)>,
  92 //!     span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
  93 //! }
  94 //!
  95 //! impl Graph {
  96 //!     fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
  97 //!         // Create a new scope to contain the lifetime of the
  98 //!         // dynamic borrow
  99 //!         {
 100 //!             // Take a reference to the inside of cache cell
 101 //!             let mut cache = self.span_tree_cache.borrow_mut();
 102 //!             if cache.is_some() {
 103 //!                 return cache.as_ref().unwrap().clone();
 104 //!             }
 105 //!
 106 //!             let span_tree = self.calc_span_tree();
 107 //!             *cache = Some(span_tree);
 108 //!         }
 109 //!
 110 //!         // Recursive call to return the just-cached value.
 111 //!         // Note that if we had not let the previous borrow
 112 //!         // of the cache fall out of scope then the subsequent
 113 //!         // recursive borrow would cause a dynamic thread panic.
 114 //!         // This is the major hazard of using `RefCell`.
 115 //!         self.minimum_spanning_tree()
 116 //!     }
 117 //! #   fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
 118 //! }
 119 //! ```
 120 //!
 121 //! ## Mutating implementations of `Clone`
 122 //!
 123 //! This is simply a special - but common - case of the previous: hiding mutability for operations
 124 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
 125 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
 126 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
 127 //! `Cell<T>`.
 128 //!
 129 //! ```
 130 //! #![feature(core_intrinsics)]
 131 //! #![feature(shared)]
 132 //! use std::cell::Cell;
 133 //! use std::ptr::Shared;
 134 //! use std::intrinsics::abort;
 135 //!
 136 //! struct Rc<T: ?Sized> {
 137 //!     ptr: Shared<RcBox<T>>
 138 //! }
 139 //!
 140 //! struct RcBox<T: ?Sized> {
 141 //!     strong: Cell<usize>,
 142 //!     refcount: Cell<usize>,
 143 //!     value: T,
 144 //! }
 145 //!
 146 //! impl<T: ?Sized> Clone for Rc<T> {
 147 //!     fn clone(&self) -> Rc<T> {
 148 //!         self.inc_strong();
 149 //!         Rc { ptr: self.ptr }
 150 //!     }
 151 //! }
 152 //!
 153 //! trait RcBoxPtr<T: ?Sized> {
 154 //!
 155 //!     fn inner(&self) -> &RcBox<T>;
 156 //!
 157 //!     fn strong(&self) -> usize {
 158 //!         self.inner().strong.get()
 159 //!     }
 160 //!
 161 //!     fn inc_strong(&self) {
 162 //!         self.inner()
 163 //!             .strong
 164 //!             .set(self.strong()
 165 //!                      .checked_add(1)
 166 //!                      .unwrap_or_else(|| unsafe { abort() }));
 167 //!     }
 168 //! }
 169 //!
 170 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
 171 //!    fn inner(&self) -> &RcBox<T> {
 172 //!        unsafe {
 173 //!            self.ptr.as_ref()
 174 //!        }
 175 //!    }
 176 //! }
 177 //! ```
 178 //!
 179
 180 #![stable(feature = "rust1", since = "1.0.0")]
 181
 182 use cmp::Ordering;
 183 use fmt::{self, Debug, Display};
 184 use marker::Unsize;
 185 use mem;
 186 use ops::{Deref, DerefMut, CoerceUnsized};
 187 use ptr;
 188
 189 /// A mutable memory location.
 190 ///
 191 /// See the [module-level documentation](index.html) for more.
 192 #[stable(feature = "rust1", since = "1.0.0")]
 193 pub struct Cell<T> {
 194     value: UnsafeCell<T>,
 195 }
 196
 197 impl<T:Copy> Cell<T> {
 198     /// Returns a copy of the contained value.
 199     ///
 200     /// # Examples
 201     ///
 202     /// ```
 203     /// use std::cell::Cell;
 204     ///
 205     /// let c = Cell::new(5);
 206     ///
 207     /// let five = c.get();
 208     /// ```
 209     #[inline]
 210     #[stable(feature = "rust1", since = "1.0.0")]
 211     pub fn get(&self) -> T {
 212         unsafe{ *self.value.get() }
 213     }
 214 }
 215
 216 #[stable(feature = "rust1", since = "1.0.0")]
 217 unsafe impl<T> Send for Cell<T> where T: Send {}
 218
 219 #[stable(feature = "rust1", since = "1.0.0")]
 220 impl<T> !Sync for Cell<T> {}
 221
 222 #[stable(feature = "rust1", since = "1.0.0")]
 223 impl<T:Copy> Clone for Cell<T> {
 224     #[inline]
 225     fn clone(&self) -> Cell<T> {
 226         Cell::new(self.get())
 227     }
 228 }
 229
 230 #[stable(feature = "rust1", since = "1.0.0")]
 231 impl<T:Default> Default for Cell<T> {
 232     /// Creates a `Cell<T>`, with the `Default` value for T.
 233     #[inline]
 234     fn default() -> Cell<T> {
 235         Cell::new(Default::default())
 236     }
 237 }
 238
 239 #[stable(feature = "rust1", since = "1.0.0")]
 240 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 241     #[inline]
 242     fn eq(&self, other: &Cell<T>) -> bool {
 243         self.get() == other.get()
 244     }
 245 }
 246
 247 #[stable(feature = "cell_eq", since = "1.2.0")]
 248 impl<T:Eq + Copy> Eq for Cell<T> {}
 249
 250 #[stable(feature = "cell_ord", since = "1.10.0")]
 251 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
 252     #[inline]
 253     fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
 254         self.get().partial_cmp(&other.get())
 255     }
 256
 257     #[inline]
 258     fn lt(&self, other: &Cell<T>) -> bool {
 259         self.get() < other.get()
 260     }
 261
 262     #[inline]
 263     fn le(&self, other: &Cell<T>) -> bool {
 264         self.get() <= other.get()
 265     }
 266
 267     #[inline]
 268     fn gt(&self, other: &Cell<T>) -> bool {
 269         self.get() > other.get()
 270     }
 271
 272     #[inline]
 273     fn ge(&self, other: &Cell<T>) -> bool {
 274         self.get() >= other.get()
 275     }
 276 }
 277
 278 #[stable(feature = "cell_ord", since = "1.10.0")]
 279 impl<T:Ord + Copy> Ord for Cell<T> {
 280     #[inline]
 281     fn cmp(&self, other: &Cell<T>) -> Ordering {
 282         self.get().cmp(&other.get())
 283     }
 284 }
 285
 286 #[stable(feature = "cell_from", since = "1.12.0")]
 287 impl<T> From<T> for Cell<T> {
 288     fn from(t: T) -> Cell<T> {
 289         Cell::new(t)
 290     }
 291 }
 292
 293 impl<T> Cell<T> {
 294     /// Creates a new `Cell` containing the given value.
 295     ///
 296     /// # Examples
 297     ///
 298     /// ```
 299     /// use std::cell::Cell;
 300     ///
 301     /// let c = Cell::new(5);
 302     /// ```
 303     #[stable(feature = "rust1", since = "1.0.0")]
 304     #[inline]
 305     pub const fn new(value: T) -> Cell<T> {
 306         Cell {
 307             value: UnsafeCell::new(value),
 308         }
 309     }
 310
 311     /// Returns a raw pointer to the underlying data in this cell.
 312     ///
 313     /// # Examples
 314     ///
 315     /// ```
 316     /// use std::cell::Cell;
 317     ///
 318     /// let c = Cell::new(5);
 319     ///
 320     /// let ptr = c.as_ptr();
 321     /// ```
 322     #[inline]
 323     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 324     pub fn as_ptr(&self) -> *mut T {
 325         self.value.get()
 326     }
 327
 328     /// Returns a mutable reference to the underlying data.
 329     ///
 330     /// This call borrows `Cell` mutably (at compile-time) which guarantees
 331     /// that we possess the only reference.
 332     ///
 333     /// # Examples
 334     ///
 335     /// ```
 336     /// use std::cell::Cell;
 337     ///
 338     /// let mut c = Cell::new(5);
 339     /// *c.get_mut() += 1;
 340     ///
 341     /// assert_eq!(c.get(), 6);
 342     /// ```
 343     #[inline]
 344     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 345     pub fn get_mut(&mut self) -> &mut T {
 346         unsafe {
 347             &mut *self.value.get()
 348         }
 349     }
 350
 351     /// Sets the contained value.
 352     ///
 353     /// # Examples
 354     ///
 355     /// ```
 356     /// use std::cell::Cell;
 357     ///
 358     /// let c = Cell::new(5);
 359     ///
 360     /// c.set(10);
 361     /// ```
 362     #[inline]
 363     #[stable(feature = "rust1", since = "1.0.0")]
 364     pub fn set(&self, val: T) {
 365         let old = self.replace(val);
 366         drop(old);
 367     }
 368
 369     /// Swaps the values of two Cells.
 370     /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
 371     ///
 372     /// # Examples
 373     ///
 374     /// ```
 375     /// use std::cell::Cell;
 376     ///
 377     /// let c1 = Cell::new(5i32);
 378     /// let c2 = Cell::new(10i32);
 379     /// c1.swap(&c2);
 380     /// assert_eq!(10, c1.get());
 381     /// assert_eq!(5, c2.get());
 382     /// ```
 383     #[inline]
 384     #[stable(feature = "move_cell", since = "1.17.0")]
 385     pub fn swap(&self, other: &Self) {
 386         if ptr::eq(self, other) {
 387             return;
 388         }
 389         unsafe {
 390             ptr::swap(self.value.get(), other.value.get());
 391         }
 392     }
 393
 394     /// Replaces the contained value, and returns it.
 395     ///
 396     /// # Examples
 397     ///
 398     /// ```
 399     /// use std::cell::Cell;
 400     ///
 401     /// let cell = Cell::new(5);
 402     /// assert_eq!(cell.get(), 5);
 403     /// assert_eq!(cell.replace(10), 5);
 404     /// assert_eq!(cell.get(), 10);
 405     /// ```
 406     #[stable(feature = "move_cell", since = "1.17.0")]
 407     pub fn replace(&self, val: T) -> T {
 408         mem::replace(unsafe { &mut *self.value.get() }, val)
 409     }
 410
 411     /// Unwraps the value.
 412     ///
 413     /// # Examples
 414     ///
 415     /// ```
 416     /// use std::cell::Cell;
 417     ///
 418     /// let c = Cell::new(5);
 419     /// let five = c.into_inner();
 420     ///
 421     /// assert_eq!(five, 5);
 422     /// ```
 423     #[stable(feature = "move_cell", since = "1.17.0")]
 424     pub fn into_inner(self) -> T {
 425         unsafe { self.value.into_inner() }
 426     }
 427 }
 428
 429 impl<T: Default> Cell<T> {
 430     /// Takes the value of the cell, leaving `Default::default()` in its place.
 431     ///
 432     /// # Examples
 433     ///
 434     /// ```
 435     /// use std::cell::Cell;
 436     ///
 437     /// let c = Cell::new(5);
 438     /// let five = c.take();
 439     ///
 440     /// assert_eq!(five, 5);
 441     /// assert_eq!(c.into_inner(), 0);
 442     /// ```
 443     #[stable(feature = "move_cell", since = "1.17.0")]
 444     pub fn take(&self) -> T {
 445         self.replace(Default::default())
 446     }
 447 }
 448
 449 #[unstable(feature = "coerce_unsized", issue = "27732")]
 450 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
 451
 452 /// A mutable memory location with dynamically checked borrow rules
 453 ///
 454 /// See the [module-level documentation](index.html) for more.
 455 #[stable(feature = "rust1", since = "1.0.0")]
 456 pub struct RefCell<T: ?Sized> {
 457     borrow: Cell<BorrowFlag>,
 458     value: UnsafeCell<T>,
 459 }
 460
 461 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
 462 #[stable(feature = "try_borrow", since = "1.13.0")]
 463 pub struct BorrowError {
 464     _private: (),
 465 }
 466
 467 #[stable(feature = "try_borrow", since = "1.13.0")]
 468 impl Debug for BorrowError {
 469     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 470         f.debug_struct("BorrowError").finish()
 471     }
 472 }
 473
 474 #[stable(feature = "try_borrow", since = "1.13.0")]
 475 impl Display for BorrowError {
 476     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 477         Display::fmt("already mutably borrowed", f)
 478     }
 479 }
 480
 481 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
 482 #[stable(feature = "try_borrow", since = "1.13.0")]
 483 pub struct BorrowMutError {
 484     _private: (),
 485 }
 486
 487 #[stable(feature = "try_borrow", since = "1.13.0")]
 488 impl Debug for BorrowMutError {
 489     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 490         f.debug_struct("BorrowMutError").finish()
 491     }
 492 }
 493
 494 #[stable(feature = "try_borrow", since = "1.13.0")]
 495 impl Display for BorrowMutError {
 496     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 497         Display::fmt("already borrowed", f)
 498     }
 499 }
 500
 501 // Values [1, MAX-1] represent the number of `Ref` active
 502 // (will not outgrow its range since `usize` is the size of the address space)
 503 type BorrowFlag = usize;
 504 const UNUSED: BorrowFlag = 0;
 505 const WRITING: BorrowFlag = !0;
 506
 507 impl<T> RefCell<T> {
 508     /// Creates a new `RefCell` containing `value`.
 509     ///
 510     /// # Examples
 511     ///
 512     /// ```
 513     /// use std::cell::RefCell;
 514     ///
 515     /// let c = RefCell::new(5);
 516     /// ```
 517     #[stable(feature = "rust1", since = "1.0.0")]
 518     #[inline]
 519     pub const fn new(value: T) -> RefCell<T> {
 520         RefCell {
 521             value: UnsafeCell::new(value),
 522             borrow: Cell::new(UNUSED),
 523         }
 524     }
 525
 526     /// Consumes the `RefCell`, returning the wrapped value.
 527     ///
 528     /// # Examples
 529     ///
 530     /// ```
 531     /// use std::cell::RefCell;
 532     ///
 533     /// let c = RefCell::new(5);
 534     ///
 535     /// let five = c.into_inner();
 536     /// ```
 537     #[stable(feature = "rust1", since = "1.0.0")]
 538     #[inline]
 539     pub fn into_inner(self) -> T {
 540         // Since this function takes `self` (the `RefCell`) by value, the
 541         // compiler statically verifies that it is not currently borrowed.
 542         // Therefore the following assertion is just a `debug_assert!`.
 543         debug_assert!(self.borrow.get() == UNUSED);
 544         unsafe { self.value.into_inner() }
 545     }
 546 }
 547
 548 impl<T: ?Sized> RefCell<T> {
 549     /// Immutably borrows the wrapped value.
 550     ///
 551     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 552     /// immutable borrows can be taken out at the same time.
 553     ///
 554     /// # Panics
 555     ///
 556     /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
 557     /// [`try_borrow`](#method.try_borrow).
 558     ///
 559     /// # Examples
 560     ///
 561     /// ```
 562     /// use std::cell::RefCell;
 563     ///
 564     /// let c = RefCell::new(5);
 565     ///
 566     /// let borrowed_five = c.borrow();
 567     /// let borrowed_five2 = c.borrow();
 568     /// ```
 569     ///
 570     /// An example of panic:
 571     ///
 572     /// ```
 573     /// use std::cell::RefCell;
 574     /// use std::thread;
 575     ///
 576     /// let result = thread::spawn(move || {
 577     ///    let c = RefCell::new(5);
 578     ///    let m = c.borrow_mut();
 579     ///
 580     ///    let b = c.borrow(); // this causes a panic
 581     /// }).join();
 582     ///
 583     /// assert!(result.is_err());
 584     /// ```
 585     #[stable(feature = "rust1", since = "1.0.0")]
 586     #[inline]
 587     pub fn borrow(&self) -> Ref<T> {
 588         self.try_borrow().expect("already mutably borrowed")
 589     }
 590
 591     /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
 592     /// borrowed.
 593     ///
 594     /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
 595     /// taken out at the same time.
 596     ///
 597     /// This is the non-panicking variant of [`borrow`](#method.borrow).
 598     ///
 599     /// # Examples
 600     ///
 601     /// ```
 602     /// use std::cell::RefCell;
 603     ///
 604     /// let c = RefCell::new(5);
 605     ///
 606     /// {
 607     ///     let m = c.borrow_mut();
 608     ///     assert!(c.try_borrow().is_err());
 609     /// }
 610     ///
 611     /// {
 612     ///     let m = c.borrow();
 613     ///     assert!(c.try_borrow().is_ok());
 614     /// }
 615     /// ```
 616     #[stable(feature = "try_borrow", since = "1.13.0")]
 617     #[inline]
 618     pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
 619         match BorrowRef::new(&self.borrow) {
 620             Some(b) => Ok(Ref {
 621                 value: unsafe { &*self.value.get() },
 622                 borrow: b,
 623             }),
 624             None => Err(BorrowError { _private: () }),
 625         }
 626     }
 627
 628     /// Mutably borrows the wrapped value.
 629     ///
 630     /// The borrow lasts until the returned `RefMut` exits scope. The value
 631     /// cannot be borrowed while this borrow is active.
 632     ///
 633     /// # Panics
 634     ///
 635     /// Panics if the value is currently borrowed. For a non-panicking variant, use
 636     /// [`try_borrow_mut`](#method.try_borrow_mut).
 637     ///
 638     /// # Examples
 639     ///
 640     /// ```
 641     /// use std::cell::RefCell;
 642     ///
 643     /// let c = RefCell::new(5);
 644     ///
 645     /// *c.borrow_mut() = 7;
 646     ///
 647     /// assert_eq!(*c.borrow(), 7);
 648     /// ```
 649     ///
 650     /// An example of panic:
 651     ///
 652     /// ```
 653     /// use std::cell::RefCell;
 654     /// use std::thread;
 655     ///
 656     /// let result = thread::spawn(move || {
 657     ///    let c = RefCell::new(5);
 658     ///    let m = c.borrow();
 659     ///
 660     ///    let b = c.borrow_mut(); // this causes a panic
 661     /// }).join();
 662     ///
 663     /// assert!(result.is_err());
 664     /// ```
 665     #[stable(feature = "rust1", since = "1.0.0")]
 666     #[inline]
 667     pub fn borrow_mut(&self) -> RefMut<T> {
 668         self.try_borrow_mut().expect("already borrowed")
 669     }
 670
 671     /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
 672     ///
 673     /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
 674     /// while this borrow is active.
 675     ///
 676     /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
 677     ///
 678     /// # Examples
 679     ///
 680     /// ```
 681     /// use std::cell::RefCell;
 682     ///
 683     /// let c = RefCell::new(5);
 684     ///
 685     /// {
 686     ///     let m = c.borrow();
 687     ///     assert!(c.try_borrow_mut().is_err());
 688     /// }
 689     ///
 690     /// assert!(c.try_borrow_mut().is_ok());
 691     /// ```
 692     #[stable(feature = "try_borrow", since = "1.13.0")]
 693     #[inline]
 694     pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
 695         match BorrowRefMut::new(&self.borrow) {
 696             Some(b) => Ok(RefMut {
 697                 value: unsafe { &mut *self.value.get() },
 698                 borrow: b,
 699             }),
 700             None => Err(BorrowMutError { _private: () }),
 701         }
 702     }
 703
 704     /// Returns a raw pointer to the underlying data in this cell.
 705     ///
 706     /// # Examples
 707     ///
 708     /// ```
 709     /// use std::cell::RefCell;
 710     ///
 711     /// let c = RefCell::new(5);
 712     ///
 713     /// let ptr = c.as_ptr();
 714     /// ```
 715     #[inline]
 716     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 717     pub fn as_ptr(&self) -> *mut T {
 718         self.value.get()
 719     }
 720
 721     /// Returns a mutable reference to the underlying data.
 722     ///
 723     /// This call borrows `RefCell` mutably (at compile-time) so there is no
 724     /// need for dynamic checks.
 725     ///
 726     /// However be cautious: this method expects `self` to be mutable, which is
 727     /// generally not the case when using a `RefCell`. Take a look at the
 728     /// [`borrow_mut`] method instead if `self` isn't mutable.
 729     ///
 730     /// Also, please be aware that this method is only for special circumstances and is usually
 731     /// not you want. In case of doubt, use [`borrow_mut`] instead.
 732     ///
 733     /// [`borrow_mut`]: #method.borrow_mut
 734     ///
 735     /// # Examples
 736     ///
 737     /// ```
 738     /// use std::cell::RefCell;
 739     ///
 740     /// let mut c = RefCell::new(5);
 741     /// *c.get_mut() += 1;
 742     ///
 743     /// assert_eq!(c, RefCell::new(6));
 744     /// ```
 745     #[inline]
 746     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 747     pub fn get_mut(&mut self) -> &mut T {
 748         unsafe {
 749             &mut *self.value.get()
 750         }
 751     }
 752 }
 753
 754 #[stable(feature = "rust1", since = "1.0.0")]
 755 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 756
 757 #[stable(feature = "rust1", since = "1.0.0")]
 758 impl<T: ?Sized> !Sync for RefCell<T> {}
 759
 760 #[stable(feature = "rust1", since = "1.0.0")]
 761 impl<T: Clone> Clone for RefCell<T> {
 762     #[inline]
 763     fn clone(&self) -> RefCell<T> {
 764         RefCell::new(self.borrow().clone())
 765     }
 766 }
 767
 768 #[stable(feature = "rust1", since = "1.0.0")]
 769 impl<T:Default> Default for RefCell<T> {
 770     /// Creates a `RefCell<T>`, with the `Default` value for T.
 771     #[inline]
 772     fn default() -> RefCell<T> {
 773         RefCell::new(Default::default())
 774     }
 775 }
 776
 777 #[stable(feature = "rust1", since = "1.0.0")]
 778 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
 779     #[inline]
 780     fn eq(&self, other: &RefCell<T>) -> bool {
 781         *self.borrow() == *other.borrow()
 782     }
 783 }
 784
 785 #[stable(feature = "cell_eq", since = "1.2.0")]
 786 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 787
 788 #[stable(feature = "cell_ord", since = "1.10.0")]
 789 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
 790     #[inline]
 791     fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
 792         self.borrow().partial_cmp(&*other.borrow())
 793     }
 794
 795     #[inline]
 796     fn lt(&self, other: &RefCell<T>) -> bool {
 797         *self.borrow() < *other.borrow()
 798     }
 799
 800     #[inline]
 801     fn le(&self, other: &RefCell<T>) -> bool {
 802         *self.borrow() <= *other.borrow()
 803     }
 804
 805     #[inline]
 806     fn gt(&self, other: &RefCell<T>) -> bool {
 807         *self.borrow() > *other.borrow()
 808     }
 809
 810     #[inline]
 811     fn ge(&self, other: &RefCell<T>) -> bool {
 812         *self.borrow() >= *other.borrow()
 813     }
 814 }
 815
 816 #[stable(feature = "cell_ord", since = "1.10.0")]
 817 impl<T: ?Sized + Ord> Ord for RefCell<T> {
 818     #[inline]
 819     fn cmp(&self, other: &RefCell<T>) -> Ordering {
 820         self.borrow().cmp(&*other.borrow())
 821     }
 822 }
 823
 824 #[stable(feature = "cell_from", since = "1.12.0")]
 825 impl<T> From<T> for RefCell<T> {
 826     fn from(t: T) -> RefCell<T> {
 827         RefCell::new(t)
 828     }
 829 }
 830
 831 #[unstable(feature = "coerce_unsized", issue = "27732")]
 832 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
 833
 834 struct BorrowRef<'b> {
 835     borrow: &'b Cell<BorrowFlag>,
 836 }
 837
 838 impl<'b> BorrowRef<'b> {
 839     #[inline]
 840     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 841         match borrow.get() {
 842             WRITING => None,
 843             b => {
 844                 borrow.set(b + 1);
 845                 Some(BorrowRef { borrow: borrow })
 846             },
 847         }
 848     }
 849 }
 850
 851 impl<'b> Drop for BorrowRef<'b> {
 852     #[inline]
 853     fn drop(&mut self) {
 854         let borrow = self.borrow.get();
 855         debug_assert!(borrow != WRITING && borrow != UNUSED);
 856         self.borrow.set(borrow - 1);
 857     }
 858 }
 859
 860 impl<'b> Clone for BorrowRef<'b> {
 861     #[inline]
 862     fn clone(&self) -> BorrowRef<'b> {
 863         // Since this Ref exists, we know the borrow flag
 864         // is not set to WRITING.
 865         let borrow = self.borrow.get();
 866         debug_assert!(borrow != UNUSED);
 867         // Prevent the borrow counter from overflowing.
 868         assert!(borrow != WRITING);
 869         self.borrow.set(borrow + 1);
 870         BorrowRef { borrow: self.borrow }
 871     }
 872 }
 873
 874 /// Wraps a borrowed reference to a value in a `RefCell` box.
 875 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
 876 ///
 877 /// See the [module-level documentation](index.html) for more.
 878 #[stable(feature = "rust1", since = "1.0.0")]
 879 pub struct Ref<'b, T: ?Sized + 'b> {
 880     value: &'b T,
 881     borrow: BorrowRef<'b>,
 882 }
 883
 884 #[stable(feature = "rust1", since = "1.0.0")]
 885 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 886     type Target = T;
 887
 888     #[inline]
 889     fn deref(&self) -> &T {
 890         self.value
 891     }
 892 }
 893
 894 impl<'b, T: ?Sized> Ref<'b, T> {
 895     /// Copies a `Ref`.
 896     ///
 897     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 898     ///
 899     /// This is an associated function that needs to be used as
 900     /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
 901     /// with the widespread use of `r.borrow().clone()` to clone the contents of
 902     /// a `RefCell`.
 903     #[stable(feature = "cell_extras", since = "1.15.0")]
 904     #[inline]
 905     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
 906         Ref {
 907             value: orig.value,
 908             borrow: orig.borrow.clone(),
 909         }
 910     }
 911
 912     /// Make a new `Ref` for a component of the borrowed data.
 913     ///
 914     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 915     ///
 916     /// This is an associated function that needs to be used as `Ref::map(...)`.
 917     /// A method would interfere with methods of the same name on the contents
 918     /// of a `RefCell` used through `Deref`.
 919     ///
 920     /// # Example
 921     ///
 922     /// ```
 923     /// use std::cell::{RefCell, Ref};
 924     ///
 925     /// let c = RefCell::new((5, 'b'));
 926     /// let b1: Ref<(u32, char)> = c.borrow();
 927     /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
 928     /// assert_eq!(*b2, 5)
 929     /// ```
 930     #[stable(feature = "cell_map", since = "1.8.0")]
 931     #[inline]
 932     pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
 933         where F: FnOnce(&T) -> &U
 934     {
 935         Ref {
 936             value: f(orig.value),
 937             borrow: orig.borrow,
 938         }
 939     }
 940 }
 941
 942 #[unstable(feature = "coerce_unsized", issue = "27732")]
 943 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
 944
 945 #[stable(feature = "std_guard_impls", since = "1.20.0")]
 946 impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
 947     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 948         self.value.fmt(f)
 949     }
 950 }
 951
 952 impl<'b, T: ?Sized> RefMut<'b, T> {
 953     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
 954     /// variant.
 955     ///
 956     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 957     ///
 958     /// This is an associated function that needs to be used as
 959     /// `RefMut::map(...)`.  A method would interfere with methods of the same
 960     /// name on the contents of a `RefCell` used through `Deref`.
 961     ///
 962     /// # Example
 963     ///
 964     /// ```
 965     /// use std::cell::{RefCell, RefMut};
 966     ///
 967     /// let c = RefCell::new((5, 'b'));
 968     /// {
 969     ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
 970     ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
 971     ///     assert_eq!(*b2, 5);
 972     ///     *b2 = 42;
 973     /// }
 974     /// assert_eq!(*c.borrow(), (42, 'b'));
 975     /// ```
 976     #[stable(feature = "cell_map", since = "1.8.0")]
 977     #[inline]
 978     pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
 979         where F: FnOnce(&mut T) -> &mut U
 980     {
 981         RefMut {
 982             value: f(orig.value),
 983             borrow: orig.borrow,
 984         }
 985     }
 986 }
 987
 988 struct BorrowRefMut<'b> {
 989     borrow: &'b Cell<BorrowFlag>,
 990 }
 991
 992 impl<'b> Drop for BorrowRefMut<'b> {
 993     #[inline]
 994     fn drop(&mut self) {
 995         let borrow = self.borrow.get();
 996         debug_assert!(borrow == WRITING);
 997         self.borrow.set(UNUSED);
 998     }
 999 }
1000
1001 impl<'b> BorrowRefMut<'b> {
1002     #[inline]
1003     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1004         match borrow.get() {
1005             UNUSED => {
1006                 borrow.set(WRITING);
1007                 Some(BorrowRefMut { borrow: borrow })
1008             },
1009             _ => None,
1010         }
1011     }
1012 }
1013
1014 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1015 ///
1016 /// See the [module-level documentation](index.html) for more.
1017 #[stable(feature = "rust1", since = "1.0.0")]
1018 pub struct RefMut<'b, T: ?Sized + 'b> {
1019     value: &'b mut T,
1020     borrow: BorrowRefMut<'b>,
1021 }
1022
1023 #[stable(feature = "rust1", since = "1.0.0")]
1024 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
1025     type Target = T;
1026
1027     #[inline]
1028     fn deref(&self) -> &T {
1029         self.value
1030     }
1031 }
1032
1033 #[stable(feature = "rust1", since = "1.0.0")]
1034 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
1035     #[inline]
1036     fn deref_mut(&mut self) -> &mut T {
1037         self.value
1038     }
1039 }
1040
1041 #[unstable(feature = "coerce_unsized", issue = "27732")]
1042 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1043
1044 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1045 impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
1046     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1047         self.value.fmt(f)
1048     }
1049 }
1050
1051 /// The core primitive for interior mutability in Rust.
1052 ///
1053 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1054 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1055 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1056 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1057 ///
1058 /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
1059 /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
1060 /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
1061 /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
1062 /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
1063 /// to ensure that no two mutable references obtained this way are active at the same time, and
1064 /// that there are no active mutable references or mutations when an immutable reference is obtained
1065 /// from the cell. This is often done via runtime checks.
1066 ///
1067 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
1068 /// okay (provided you enforce the invariants some other way); it is still undefined behavior
1069 /// to have multiple `&mut UnsafeCell<T>` aliases.
1070 ///
1071 ///
1072 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
1073 ///
1074 /// # Examples
1075 ///
1076 /// ```
1077 /// use std::cell::UnsafeCell;
1078 /// use std::marker::Sync;
1079 ///
1080 /// # #[allow(dead_code)]
1081 /// struct NotThreadSafe<T> {
1082 ///     value: UnsafeCell<T>,
1083 /// }
1084 ///
1085 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1086 /// ```
1087 #[lang = "unsafe_cell"]
1088 #[stable(feature = "rust1", since = "1.0.0")]
1089 pub struct UnsafeCell<T: ?Sized> {
1090     value: T,
1091 }
1092
1093 #[stable(feature = "rust1", since = "1.0.0")]
1094 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1095
1096 impl<T> UnsafeCell<T> {
1097     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1098     /// value.
1099     ///
1100     /// All access to the inner value through methods is `unsafe`.
1101     ///
1102     /// # Examples
1103     ///
1104     /// ```
1105     /// use std::cell::UnsafeCell;
1106     ///
1107     /// let uc = UnsafeCell::new(5);
1108     /// ```
1109     #[stable(feature = "rust1", since = "1.0.0")]
1110     #[inline]
1111     pub const fn new(value: T) -> UnsafeCell<T> {
1112         UnsafeCell { value: value }
1113     }
1114
1115     /// Unwraps the value.
1116     ///
1117     /// # Safety
1118     ///
1119     /// This function is unsafe because this thread or another thread may currently be
1120     /// inspecting the inner value.
1121     ///
1122     /// # Examples
1123     ///
1124     /// ```
1125     /// use std::cell::UnsafeCell;
1126     ///
1127     /// let uc = UnsafeCell::new(5);
1128     ///
1129     /// let five = unsafe { uc.into_inner() };
1130     /// ```
1131     #[inline]
1132     #[stable(feature = "rust1", since = "1.0.0")]
1133     pub unsafe fn into_inner(self) -> T {
1134         self.value
1135     }
1136 }
1137
1138 impl<T: ?Sized> UnsafeCell<T> {
1139     /// Gets a mutable pointer to the wrapped value.
1140     ///
1141     /// This can be cast to a pointer of any kind.
1142     /// Ensure that the access is unique when casting to
1143     /// `&mut T`, and ensure that there are no mutations or mutable
1144     /// aliases going on when casting to `&T`
1145     ///
1146     /// # Examples
1147     ///
1148     /// ```
1149     /// use std::cell::UnsafeCell;
1150     ///
1151     /// let uc = UnsafeCell::new(5);
1152     ///
1153     /// let five = uc.get();
1154     /// ```
1155     #[inline]
1156     #[stable(feature = "rust1", since = "1.0.0")]
1157     pub fn get(&self) -> *mut T {
1158         &self.value as *const T as *mut T
1159     }
1160 }
1161
1162 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1163 impl<T: Default> Default for UnsafeCell<T> {
1164     /// Creates an `UnsafeCell`, with the `Default` value for T.
1165     fn default() -> UnsafeCell<T> {
1166         UnsafeCell::new(Default::default())
1167     }
1168 }
1169
1170 #[stable(feature = "cell_from", since = "1.12.0")]
1171 impl<T> From<T> for UnsafeCell<T> {
1172     fn from(t: T) -> UnsafeCell<T> {
1173         UnsafeCell::new(t)
1174     }
1175 }
1176
1177 #[unstable(feature = "coerce_unsized", issue = "27732")]
1178 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1179
1180 #[allow(unused)]
1181 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1182     let _: UnsafeCell<&Send> = a;
1183     let _: Cell<&Send> = b;
1184     let _: RefCell<&Send> = c;
1185 }