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 /// # Examples
 192 ///
 193 /// Here you can see how using `Cell<T>` allows to use mutable field inside
 194 /// immutable struct (which is also called 'interior mutability').
 195 ///
 196 /// ```
 197 /// use std::cell::Cell;
 198 ///
 199 /// struct SomeStruct {
 200 ///     regular_field: u8,
 201 ///     special_field: Cell<u8>,
 202 /// }
 203 ///
 204 /// let my_struct = SomeStruct {
 205 ///     regular_field: 0,
 206 ///     special_field: Cell::new(1),
 207 /// };
 208 ///
 209 /// let new_value = 100;
 210 ///
 211 /// // ERROR, because my_struct is immutable
 212 /// // my_struct.regular_field = new_value;
 213 ///
 214 /// // WORKS, although `my_struct` is immutable, field `special_field` is mutable because it is Cell
 215 /// my_struct.special_field.set(new_value);
 216 /// assert_eq!(my_struct.special_field.get(), new_value);
 217 /// ```
 218 ///
 219 /// See the [module-level documentation](index.html) for more.
 220 #[stable(feature = "rust1", since = "1.0.0")]
 221 pub struct Cell<T> {
 222     value: UnsafeCell<T>,
 223 }
 224
 225 impl<T:Copy> Cell<T> {
 226     /// Returns a copy of the contained value.
 227     ///
 228     /// # Examples
 229     ///
 230     /// ```
 231     /// use std::cell::Cell;
 232     ///
 233     /// let c = Cell::new(5);
 234     ///
 235     /// let five = c.get();
 236     /// ```
 237     #[inline]
 238     #[stable(feature = "rust1", since = "1.0.0")]
 239     pub fn get(&self) -> T {
 240         unsafe{ *self.value.get() }
 241     }
 242 }
 243
 244 #[stable(feature = "rust1", since = "1.0.0")]
 245 unsafe impl<T> Send for Cell<T> where T: Send {}
 246
 247 #[stable(feature = "rust1", since = "1.0.0")]
 248 impl<T> !Sync for Cell<T> {}
 249
 250 #[stable(feature = "rust1", since = "1.0.0")]
 251 impl<T:Copy> Clone for Cell<T> {
 252     #[inline]
 253     fn clone(&self) -> Cell<T> {
 254         Cell::new(self.get())
 255     }
 256 }
 257
 258 #[stable(feature = "rust1", since = "1.0.0")]
 259 impl<T:Default> Default for Cell<T> {
 260     /// Creates a `Cell<T>`, with the `Default` value for T.
 261     #[inline]
 262     fn default() -> Cell<T> {
 263         Cell::new(Default::default())
 264     }
 265 }
 266
 267 #[stable(feature = "rust1", since = "1.0.0")]
 268 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 269     #[inline]
 270     fn eq(&self, other: &Cell<T>) -> bool {
 271         self.get() == other.get()
 272     }
 273 }
 274
 275 #[stable(feature = "cell_eq", since = "1.2.0")]
 276 impl<T:Eq + Copy> Eq for Cell<T> {}
 277
 278 #[stable(feature = "cell_ord", since = "1.10.0")]
 279 impl<T:PartialOrd + Copy> PartialOrd for Cell<T> {
 280     #[inline]
 281     fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
 282         self.get().partial_cmp(&other.get())
 283     }
 284
 285     #[inline]
 286     fn lt(&self, other: &Cell<T>) -> bool {
 287         self.get() < other.get()
 288     }
 289
 290     #[inline]
 291     fn le(&self, other: &Cell<T>) -> bool {
 292         self.get() <= other.get()
 293     }
 294
 295     #[inline]
 296     fn gt(&self, other: &Cell<T>) -> bool {
 297         self.get() > other.get()
 298     }
 299
 300     #[inline]
 301     fn ge(&self, other: &Cell<T>) -> bool {
 302         self.get() >= other.get()
 303     }
 304 }
 305
 306 #[stable(feature = "cell_ord", since = "1.10.0")]
 307 impl<T:Ord + Copy> Ord for Cell<T> {
 308     #[inline]
 309     fn cmp(&self, other: &Cell<T>) -> Ordering {
 310         self.get().cmp(&other.get())
 311     }
 312 }
 313
 314 #[stable(feature = "cell_from", since = "1.12.0")]
 315 impl<T> From<T> for Cell<T> {
 316     fn from(t: T) -> Cell<T> {
 317         Cell::new(t)
 318     }
 319 }
 320
 321 impl<T> Cell<T> {
 322     /// Creates a new `Cell` containing the given value.
 323     ///
 324     /// # Examples
 325     ///
 326     /// ```
 327     /// use std::cell::Cell;
 328     ///
 329     /// let c = Cell::new(5);
 330     /// ```
 331     #[stable(feature = "rust1", since = "1.0.0")]
 332     #[inline]
 333     pub const fn new(value: T) -> Cell<T> {
 334         Cell {
 335             value: UnsafeCell::new(value),
 336         }
 337     }
 338
 339     /// Returns a raw pointer to the underlying data in this cell.
 340     ///
 341     /// # Examples
 342     ///
 343     /// ```
 344     /// use std::cell::Cell;
 345     ///
 346     /// let c = Cell::new(5);
 347     ///
 348     /// let ptr = c.as_ptr();
 349     /// ```
 350     #[inline]
 351     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 352     pub fn as_ptr(&self) -> *mut T {
 353         self.value.get()
 354     }
 355
 356     /// Returns a mutable reference to the underlying data.
 357     ///
 358     /// This call borrows `Cell` mutably (at compile-time) which guarantees
 359     /// that we possess the only reference.
 360     ///
 361     /// # Examples
 362     ///
 363     /// ```
 364     /// use std::cell::Cell;
 365     ///
 366     /// let mut c = Cell::new(5);
 367     /// *c.get_mut() += 1;
 368     ///
 369     /// assert_eq!(c.get(), 6);
 370     /// ```
 371     #[inline]
 372     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 373     pub fn get_mut(&mut self) -> &mut T {
 374         unsafe {
 375             &mut *self.value.get()
 376         }
 377     }
 378
 379     /// Sets the contained value.
 380     ///
 381     /// # Examples
 382     ///
 383     /// ```
 384     /// use std::cell::Cell;
 385     ///
 386     /// let c = Cell::new(5);
 387     ///
 388     /// c.set(10);
 389     /// ```
 390     #[inline]
 391     #[stable(feature = "rust1", since = "1.0.0")]
 392     pub fn set(&self, val: T) {
 393         let old = self.replace(val);
 394         drop(old);
 395     }
 396
 397     /// Swaps the values of two Cells.
 398     /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
 399     ///
 400     /// # Examples
 401     ///
 402     /// ```
 403     /// use std::cell::Cell;
 404     ///
 405     /// let c1 = Cell::new(5i32);
 406     /// let c2 = Cell::new(10i32);
 407     /// c1.swap(&c2);
 408     /// assert_eq!(10, c1.get());
 409     /// assert_eq!(5, c2.get());
 410     /// ```
 411     #[inline]
 412     #[stable(feature = "move_cell", since = "1.17.0")]
 413     pub fn swap(&self, other: &Self) {
 414         if ptr::eq(self, other) {
 415             return;
 416         }
 417         unsafe {
 418             ptr::swap(self.value.get(), other.value.get());
 419         }
 420     }
 421
 422     /// Replaces the contained value, and returns it.
 423     ///
 424     /// # Examples
 425     ///
 426     /// ```
 427     /// use std::cell::Cell;
 428     ///
 429     /// let cell = Cell::new(5);
 430     /// assert_eq!(cell.get(), 5);
 431     /// assert_eq!(cell.replace(10), 5);
 432     /// assert_eq!(cell.get(), 10);
 433     /// ```
 434     #[stable(feature = "move_cell", since = "1.17.0")]
 435     pub fn replace(&self, val: T) -> T {
 436         mem::replace(unsafe { &mut *self.value.get() }, val)
 437     }
 438
 439     /// Unwraps the value.
 440     ///
 441     /// # Examples
 442     ///
 443     /// ```
 444     /// use std::cell::Cell;
 445     ///
 446     /// let c = Cell::new(5);
 447     /// let five = c.into_inner();
 448     ///
 449     /// assert_eq!(five, 5);
 450     /// ```
 451     #[stable(feature = "move_cell", since = "1.17.0")]
 452     pub fn into_inner(self) -> T {
 453         self.value.into_inner()
 454     }
 455 }
 456
 457 impl<T: Default> Cell<T> {
 458     /// Takes the value of the cell, leaving `Default::default()` in its place.
 459     ///
 460     /// # Examples
 461     ///
 462     /// ```
 463     /// use std::cell::Cell;
 464     ///
 465     /// let c = Cell::new(5);
 466     /// let five = c.take();
 467     ///
 468     /// assert_eq!(five, 5);
 469     /// assert_eq!(c.into_inner(), 0);
 470     /// ```
 471     #[stable(feature = "move_cell", since = "1.17.0")]
 472     pub fn take(&self) -> T {
 473         self.replace(Default::default())
 474     }
 475 }
 476
 477 #[unstable(feature = "coerce_unsized", issue = "27732")]
 478 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
 479
 480 /// A mutable memory location with dynamically checked borrow rules
 481 ///
 482 /// See the [module-level documentation](index.html) for more.
 483 #[stable(feature = "rust1", since = "1.0.0")]
 484 pub struct RefCell<T: ?Sized> {
 485     borrow: Cell<BorrowFlag>,
 486     value: UnsafeCell<T>,
 487 }
 488
 489 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
 490 #[stable(feature = "try_borrow", since = "1.13.0")]
 491 pub struct BorrowError {
 492     _private: (),
 493 }
 494
 495 #[stable(feature = "try_borrow", since = "1.13.0")]
 496 impl Debug for BorrowError {
 497     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 498         f.debug_struct("BorrowError").finish()
 499     }
 500 }
 501
 502 #[stable(feature = "try_borrow", since = "1.13.0")]
 503 impl Display for BorrowError {
 504     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 505         Display::fmt("already mutably borrowed", f)
 506     }
 507 }
 508
 509 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
 510 #[stable(feature = "try_borrow", since = "1.13.0")]
 511 pub struct BorrowMutError {
 512     _private: (),
 513 }
 514
 515 #[stable(feature = "try_borrow", since = "1.13.0")]
 516 impl Debug for BorrowMutError {
 517     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 518         f.debug_struct("BorrowMutError").finish()
 519     }
 520 }
 521
 522 #[stable(feature = "try_borrow", since = "1.13.0")]
 523 impl Display for BorrowMutError {
 524     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 525         Display::fmt("already borrowed", f)
 526     }
 527 }
 528
 529 // Values [1, MAX-1] represent the number of `Ref` active
 530 // (will not outgrow its range since `usize` is the size of the address space)
 531 type BorrowFlag = usize;
 532 const UNUSED: BorrowFlag = 0;
 533 const WRITING: BorrowFlag = !0;
 534
 535 impl<T> RefCell<T> {
 536     /// Creates a new `RefCell` containing `value`.
 537     ///
 538     /// # Examples
 539     ///
 540     /// ```
 541     /// use std::cell::RefCell;
 542     ///
 543     /// let c = RefCell::new(5);
 544     /// ```
 545     #[stable(feature = "rust1", since = "1.0.0")]
 546     #[inline]
 547     pub const fn new(value: T) -> RefCell<T> {
 548         RefCell {
 549             value: UnsafeCell::new(value),
 550             borrow: Cell::new(UNUSED),
 551         }
 552     }
 553
 554     /// Consumes the `RefCell`, returning the wrapped value.
 555     ///
 556     /// # Examples
 557     ///
 558     /// ```
 559     /// use std::cell::RefCell;
 560     ///
 561     /// let c = RefCell::new(5);
 562     ///
 563     /// let five = c.into_inner();
 564     /// ```
 565     #[stable(feature = "rust1", since = "1.0.0")]
 566     #[inline]
 567     pub fn into_inner(self) -> T {
 568         // Since this function takes `self` (the `RefCell`) by value, the
 569         // compiler statically verifies that it is not currently borrowed.
 570         // Therefore the following assertion is just a `debug_assert!`.
 571         debug_assert!(self.borrow.get() == UNUSED);
 572         self.value.into_inner()
 573     }
 574
 575     /// Replaces the wrapped value with a new one, returning the old value,
 576     /// without deinitializing either one.
 577     ///
 578     /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
 579     ///
 580     /// # Panics
 581     ///
 582     /// Panics if the value is currently borrowed.
 583     ///
 584     /// # Examples
 585     ///
 586     /// ```
 587     /// use std::cell::RefCell;
 588     /// let cell = RefCell::new(5);
 589     /// let old_value = cell.replace(6);
 590     /// assert_eq!(old_value, 5);
 591     /// assert_eq!(cell, RefCell::new(6));
 592     /// ```
 593     #[inline]
 594     #[stable(feature = "refcell_replace", since="1.24.0")]
 595     pub fn replace(&self, t: T) -> T {
 596         mem::replace(&mut *self.borrow_mut(), t)
 597     }
 598
 599     /// Replaces the wrapped value with a new one computed from `f`, returning
 600     /// the old value, without deinitializing either one.
 601     ///
 602     /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
 603     ///
 604     /// # Panics
 605     ///
 606     /// Panics if the value is currently borrowed.
 607     ///
 608     /// # Examples
 609     ///
 610     /// ```
 611     /// #![feature(refcell_replace_swap)]
 612     /// use std::cell::RefCell;
 613     /// let cell = RefCell::new(5);
 614     /// let old_value = cell.replace_with(|&mut old| old + 1);
 615     /// assert_eq!(old_value, 5);
 616     /// assert_eq!(cell, RefCell::new(6));
 617     /// ```
 618     #[inline]
 619     #[unstable(feature = "refcell_replace_swap", issue="43570")]
 620     pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
 621         let mut_borrow = &mut *self.borrow_mut();
 622         let replacement = f(mut_borrow);
 623         mem::replace(mut_borrow, replacement)
 624     }
 625
 626     /// Swaps the wrapped value of `self` with the wrapped value of `other`,
 627     /// without deinitializing either one.
 628     ///
 629     /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
 630     ///
 631     /// # Panics
 632     ///
 633     /// Panics if the value in either `RefCell` is currently borrowed.
 634     ///
 635     /// # Examples
 636     ///
 637     /// ```
 638     /// use std::cell::RefCell;
 639     /// let c = RefCell::new(5);
 640     /// let d = RefCell::new(6);
 641     /// c.swap(&d);
 642     /// assert_eq!(c, RefCell::new(6));
 643     /// assert_eq!(d, RefCell::new(5));
 644     /// ```
 645     #[inline]
 646     #[stable(feature = "refcell_swap", since="1.24.0")]
 647     pub fn swap(&self, other: &Self) {
 648         mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
 649     }
 650 }
 651
 652 impl<T: ?Sized> RefCell<T> {
 653     /// Immutably borrows the wrapped value.
 654     ///
 655     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 656     /// immutable borrows can be taken out at the same time.
 657     ///
 658     /// # Panics
 659     ///
 660     /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
 661     /// [`try_borrow`](#method.try_borrow).
 662     ///
 663     /// # Examples
 664     ///
 665     /// ```
 666     /// use std::cell::RefCell;
 667     ///
 668     /// let c = RefCell::new(5);
 669     ///
 670     /// let borrowed_five = c.borrow();
 671     /// let borrowed_five2 = c.borrow();
 672     /// ```
 673     ///
 674     /// An example of panic:
 675     ///
 676     /// ```
 677     /// use std::cell::RefCell;
 678     /// use std::thread;
 679     ///
 680     /// let result = thread::spawn(move || {
 681     ///    let c = RefCell::new(5);
 682     ///    let m = c.borrow_mut();
 683     ///
 684     ///    let b = c.borrow(); // this causes a panic
 685     /// }).join();
 686     ///
 687     /// assert!(result.is_err());
 688     /// ```
 689     #[stable(feature = "rust1", since = "1.0.0")]
 690     #[inline]
 691     pub fn borrow(&self) -> Ref<T> {
 692         self.try_borrow().expect("already mutably borrowed")
 693     }
 694
 695     /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
 696     /// borrowed.
 697     ///
 698     /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
 699     /// taken out at the same time.
 700     ///
 701     /// This is the non-panicking variant of [`borrow`](#method.borrow).
 702     ///
 703     /// # Examples
 704     ///
 705     /// ```
 706     /// use std::cell::RefCell;
 707     ///
 708     /// let c = RefCell::new(5);
 709     ///
 710     /// {
 711     ///     let m = c.borrow_mut();
 712     ///     assert!(c.try_borrow().is_err());
 713     /// }
 714     ///
 715     /// {
 716     ///     let m = c.borrow();
 717     ///     assert!(c.try_borrow().is_ok());
 718     /// }
 719     /// ```
 720     #[stable(feature = "try_borrow", since = "1.13.0")]
 721     #[inline]
 722     pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
 723         match BorrowRef::new(&self.borrow) {
 724             Some(b) => Ok(Ref {
 725                 value: unsafe { &*self.value.get() },
 726                 borrow: b,
 727             }),
 728             None => Err(BorrowError { _private: () }),
 729         }
 730     }
 731
 732     /// Mutably borrows the wrapped value.
 733     ///
 734     /// The borrow lasts until the returned `RefMut` exits scope. The value
 735     /// cannot be borrowed while this borrow is active.
 736     ///
 737     /// # Panics
 738     ///
 739     /// Panics if the value is currently borrowed. For a non-panicking variant, use
 740     /// [`try_borrow_mut`](#method.try_borrow_mut).
 741     ///
 742     /// # Examples
 743     ///
 744     /// ```
 745     /// use std::cell::RefCell;
 746     ///
 747     /// let c = RefCell::new(5);
 748     ///
 749     /// *c.borrow_mut() = 7;
 750     ///
 751     /// assert_eq!(*c.borrow(), 7);
 752     /// ```
 753     ///
 754     /// An example of panic:
 755     ///
 756     /// ```
 757     /// use std::cell::RefCell;
 758     /// use std::thread;
 759     ///
 760     /// let result = thread::spawn(move || {
 761     ///    let c = RefCell::new(5);
 762     ///    let m = c.borrow();
 763     ///
 764     ///    let b = c.borrow_mut(); // this causes a panic
 765     /// }).join();
 766     ///
 767     /// assert!(result.is_err());
 768     /// ```
 769     #[stable(feature = "rust1", since = "1.0.0")]
 770     #[inline]
 771     pub fn borrow_mut(&self) -> RefMut<T> {
 772         self.try_borrow_mut().expect("already borrowed")
 773     }
 774
 775     /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
 776     ///
 777     /// The borrow lasts until the returned `RefMut` exits scope. The value cannot be borrowed
 778     /// while this borrow is active.
 779     ///
 780     /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
 781     ///
 782     /// # Examples
 783     ///
 784     /// ```
 785     /// use std::cell::RefCell;
 786     ///
 787     /// let c = RefCell::new(5);
 788     ///
 789     /// {
 790     ///     let m = c.borrow();
 791     ///     assert!(c.try_borrow_mut().is_err());
 792     /// }
 793     ///
 794     /// assert!(c.try_borrow_mut().is_ok());
 795     /// ```
 796     #[stable(feature = "try_borrow", since = "1.13.0")]
 797     #[inline]
 798     pub fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
 799         match BorrowRefMut::new(&self.borrow) {
 800             Some(b) => Ok(RefMut {
 801                 value: unsafe { &mut *self.value.get() },
 802                 borrow: b,
 803             }),
 804             None => Err(BorrowMutError { _private: () }),
 805         }
 806     }
 807
 808     /// Returns a raw pointer to the underlying data in this cell.
 809     ///
 810     /// # Examples
 811     ///
 812     /// ```
 813     /// use std::cell::RefCell;
 814     ///
 815     /// let c = RefCell::new(5);
 816     ///
 817     /// let ptr = c.as_ptr();
 818     /// ```
 819     #[inline]
 820     #[stable(feature = "cell_as_ptr", since = "1.12.0")]
 821     pub fn as_ptr(&self) -> *mut T {
 822         self.value.get()
 823     }
 824
 825     /// Returns a mutable reference to the underlying data.
 826     ///
 827     /// This call borrows `RefCell` mutably (at compile-time) so there is no
 828     /// need for dynamic checks.
 829     ///
 830     /// However be cautious: this method expects `self` to be mutable, which is
 831     /// generally not the case when using a `RefCell`. Take a look at the
 832     /// [`borrow_mut`] method instead if `self` isn't mutable.
 833     ///
 834     /// Also, please be aware that this method is only for special circumstances and is usually
 835     /// not what you want. In case of doubt, use [`borrow_mut`] instead.
 836     ///
 837     /// [`borrow_mut`]: #method.borrow_mut
 838     ///
 839     /// # Examples
 840     ///
 841     /// ```
 842     /// use std::cell::RefCell;
 843     ///
 844     /// let mut c = RefCell::new(5);
 845     /// *c.get_mut() += 1;
 846     ///
 847     /// assert_eq!(c, RefCell::new(6));
 848     /// ```
 849     #[inline]
 850     #[stable(feature = "cell_get_mut", since = "1.11.0")]
 851     pub fn get_mut(&mut self) -> &mut T {
 852         unsafe {
 853             &mut *self.value.get()
 854         }
 855     }
 856 }
 857
 858 #[stable(feature = "rust1", since = "1.0.0")]
 859 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 860
 861 #[stable(feature = "rust1", since = "1.0.0")]
 862 impl<T: ?Sized> !Sync for RefCell<T> {}
 863
 864 #[stable(feature = "rust1", since = "1.0.0")]
 865 impl<T: Clone> Clone for RefCell<T> {
 866     #[inline]
 867     fn clone(&self) -> RefCell<T> {
 868         RefCell::new(self.borrow().clone())
 869     }
 870 }
 871
 872 #[stable(feature = "rust1", since = "1.0.0")]
 873 impl<T:Default> Default for RefCell<T> {
 874     /// Creates a `RefCell<T>`, with the `Default` value for T.
 875     #[inline]
 876     fn default() -> RefCell<T> {
 877         RefCell::new(Default::default())
 878     }
 879 }
 880
 881 #[stable(feature = "rust1", since = "1.0.0")]
 882 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
 883     #[inline]
 884     fn eq(&self, other: &RefCell<T>) -> bool {
 885         *self.borrow() == *other.borrow()
 886     }
 887 }
 888
 889 #[stable(feature = "cell_eq", since = "1.2.0")]
 890 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 891
 892 #[stable(feature = "cell_ord", since = "1.10.0")]
 893 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
 894     #[inline]
 895     fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
 896         self.borrow().partial_cmp(&*other.borrow())
 897     }
 898
 899     #[inline]
 900     fn lt(&self, other: &RefCell<T>) -> bool {
 901         *self.borrow() < *other.borrow()
 902     }
 903
 904     #[inline]
 905     fn le(&self, other: &RefCell<T>) -> bool {
 906         *self.borrow() <= *other.borrow()
 907     }
 908
 909     #[inline]
 910     fn gt(&self, other: &RefCell<T>) -> bool {
 911         *self.borrow() > *other.borrow()
 912     }
 913
 914     #[inline]
 915     fn ge(&self, other: &RefCell<T>) -> bool {
 916         *self.borrow() >= *other.borrow()
 917     }
 918 }
 919
 920 #[stable(feature = "cell_ord", since = "1.10.0")]
 921 impl<T: ?Sized + Ord> Ord for RefCell<T> {
 922     #[inline]
 923     fn cmp(&self, other: &RefCell<T>) -> Ordering {
 924         self.borrow().cmp(&*other.borrow())
 925     }
 926 }
 927
 928 #[stable(feature = "cell_from", since = "1.12.0")]
 929 impl<T> From<T> for RefCell<T> {
 930     fn from(t: T) -> RefCell<T> {
 931         RefCell::new(t)
 932     }
 933 }
 934
 935 #[unstable(feature = "coerce_unsized", issue = "27732")]
 936 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
 937
 938 struct BorrowRef<'b> {
 939     borrow: &'b Cell<BorrowFlag>,
 940 }
 941
 942 impl<'b> BorrowRef<'b> {
 943     #[inline]
 944     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 945         match borrow.get() {
 946             WRITING => None,
 947             b => {
 948                 borrow.set(b + 1);
 949                 Some(BorrowRef { borrow: borrow })
 950             },
 951         }
 952     }
 953 }
 954
 955 impl<'b> Drop for BorrowRef<'b> {
 956     #[inline]
 957     fn drop(&mut self) {
 958         let borrow = self.borrow.get();
 959         debug_assert!(borrow != WRITING && borrow != UNUSED);
 960         self.borrow.set(borrow - 1);
 961     }
 962 }
 963
 964 impl<'b> Clone for BorrowRef<'b> {
 965     #[inline]
 966     fn clone(&self) -> BorrowRef<'b> {
 967         // Since this Ref exists, we know the borrow flag
 968         // is not set to WRITING.
 969         let borrow = self.borrow.get();
 970         debug_assert!(borrow != UNUSED);
 971         // Prevent the borrow counter from overflowing.
 972         assert!(borrow != WRITING);
 973         self.borrow.set(borrow + 1);
 974         BorrowRef { borrow: self.borrow }
 975     }
 976 }
 977
 978 /// Wraps a borrowed reference to a value in a `RefCell` box.
 979 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
 980 ///
 981 /// See the [module-level documentation](index.html) for more.
 982 #[stable(feature = "rust1", since = "1.0.0")]
 983 pub struct Ref<'b, T: ?Sized + 'b> {
 984     value: &'b T,
 985     borrow: BorrowRef<'b>,
 986 }
 987
 988 #[stable(feature = "rust1", since = "1.0.0")]
 989 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 990     type Target = T;
 991
 992     #[inline]
 993     fn deref(&self) -> &T {
 994         self.value
 995     }
 996 }
 997
 998 impl<'b, T: ?Sized> Ref<'b, T> {
 999     /// Copies a `Ref`.
1000     ///
1001     /// The `RefCell` is already immutably borrowed, so this cannot fail.
1002     ///
1003     /// This is an associated function that needs to be used as
1004     /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
1005     /// with the widespread use of `r.borrow().clone()` to clone the contents of
1006     /// a `RefCell`.
1007     #[stable(feature = "cell_extras", since = "1.15.0")]
1008     #[inline]
1009     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1010         Ref {
1011             value: orig.value,
1012             borrow: orig.borrow.clone(),
1013         }
1014     }
1015
1016     /// Make a new `Ref` for a component of the borrowed data.
1017     ///
1018     /// The `RefCell` is already immutably borrowed, so this cannot fail.
1019     ///
1020     /// This is an associated function that needs to be used as `Ref::map(...)`.
1021     /// A method would interfere with methods of the same name on the contents
1022     /// of a `RefCell` used through `Deref`.
1023     ///
1024     /// # Examples
1025     ///
1026     /// ```
1027     /// use std::cell::{RefCell, Ref};
1028     ///
1029     /// let c = RefCell::new((5, 'b'));
1030     /// let b1: Ref<(u32, char)> = c.borrow();
1031     /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1032     /// assert_eq!(*b2, 5)
1033     /// ```
1034     #[stable(feature = "cell_map", since = "1.8.0")]
1035     #[inline]
1036     pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1037         where F: FnOnce(&T) -> &U
1038     {
1039         Ref {
1040             value: f(orig.value),
1041             borrow: orig.borrow,
1042         }
1043     }
1044 }
1045
1046 #[unstable(feature = "coerce_unsized", issue = "27732")]
1047 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1048
1049 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1050 impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
1051     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1052         self.value.fmt(f)
1053     }
1054 }
1055
1056 impl<'b, T: ?Sized> RefMut<'b, T> {
1057     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
1058     /// variant.
1059     ///
1060     /// The `RefCell` is already mutably borrowed, so this cannot fail.
1061     ///
1062     /// This is an associated function that needs to be used as
1063     /// `RefMut::map(...)`.  A method would interfere with methods of the same
1064     /// name on the contents of a `RefCell` used through `Deref`.
1065     ///
1066     /// # Examples
1067     ///
1068     /// ```
1069     /// use std::cell::{RefCell, RefMut};
1070     ///
1071     /// let c = RefCell::new((5, 'b'));
1072     /// {
1073     ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
1074     ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1075     ///     assert_eq!(*b2, 5);
1076     ///     *b2 = 42;
1077     /// }
1078     /// assert_eq!(*c.borrow(), (42, 'b'));
1079     /// ```
1080     #[stable(feature = "cell_map", since = "1.8.0")]
1081     #[inline]
1082     pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1083         where F: FnOnce(&mut T) -> &mut U
1084     {
1085         // FIXME(nll-rfc#40): fix borrow-check
1086         let RefMut { value, borrow } = orig;
1087         RefMut {
1088             value: f(value),
1089             borrow: borrow,
1090         }
1091     }
1092 }
1093
1094 struct BorrowRefMut<'b> {
1095     borrow: &'b Cell<BorrowFlag>,
1096 }
1097
1098 impl<'b> Drop for BorrowRefMut<'b> {
1099     #[inline]
1100     fn drop(&mut self) {
1101         let borrow = self.borrow.get();
1102         debug_assert!(borrow == WRITING);
1103         self.borrow.set(UNUSED);
1104     }
1105 }
1106
1107 impl<'b> BorrowRefMut<'b> {
1108     #[inline]
1109     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1110         match borrow.get() {
1111             UNUSED => {
1112                 borrow.set(WRITING);
1113                 Some(BorrowRefMut { borrow: borrow })
1114             },
1115             _ => None,
1116         }
1117     }
1118 }
1119
1120 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1121 ///
1122 /// See the [module-level documentation](index.html) for more.
1123 #[stable(feature = "rust1", since = "1.0.0")]
1124 pub struct RefMut<'b, T: ?Sized + 'b> {
1125     value: &'b mut T,
1126     borrow: BorrowRefMut<'b>,
1127 }
1128
1129 #[stable(feature = "rust1", since = "1.0.0")]
1130 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
1131     type Target = T;
1132
1133     #[inline]
1134     fn deref(&self) -> &T {
1135         self.value
1136     }
1137 }
1138
1139 #[stable(feature = "rust1", since = "1.0.0")]
1140 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
1141     #[inline]
1142     fn deref_mut(&mut self) -> &mut T {
1143         self.value
1144     }
1145 }
1146
1147 #[unstable(feature = "coerce_unsized", issue = "27732")]
1148 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1149
1150 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1151 impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
1152     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
1153         self.value.fmt(f)
1154     }
1155 }
1156
1157 /// The core primitive for interior mutability in Rust.
1158 ///
1159 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1160 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1161 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1162 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1163 ///
1164 /// The compiler makes optimizations based on the knowledge that `&T` is not mutably aliased or
1165 /// mutated, and that `&mut T` is unique. When building abstractions like `Cell`, `RefCell`,
1166 /// `Mutex`, etc, you need to turn these optimizations off. `UnsafeCell` is the only legal way
1167 /// to do this. When `UnsafeCell<T>` is immutably aliased, it is still safe to obtain a mutable
1168 /// reference to its interior and/or to mutate it. However, it is up to the abstraction designer
1169 /// to ensure that no two mutable references obtained this way are active at the same time, and
1170 /// that there are no active mutable references or mutations when an immutable reference is obtained
1171 /// from the cell. This is often done via runtime checks.
1172 ///
1173 /// Note that while mutating or mutably aliasing the contents of an `& UnsafeCell<T>` is
1174 /// okay (provided you enforce the invariants some other way); it is still undefined behavior
1175 /// to have multiple `&mut UnsafeCell<T>` aliases.
1176 ///
1177 ///
1178 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
1179 ///
1180 /// # Examples
1181 ///
1182 /// ```
1183 /// use std::cell::UnsafeCell;
1184 /// use std::marker::Sync;
1185 ///
1186 /// # #[allow(dead_code)]
1187 /// struct NotThreadSafe<T> {
1188 ///     value: UnsafeCell<T>,
1189 /// }
1190 ///
1191 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1192 /// ```
1193 #[lang = "unsafe_cell"]
1194 #[stable(feature = "rust1", since = "1.0.0")]
1195 pub struct UnsafeCell<T: ?Sized> {
1196     value: T,
1197 }
1198
1199 #[stable(feature = "rust1", since = "1.0.0")]
1200 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1201
1202 impl<T> UnsafeCell<T> {
1203     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1204     /// value.
1205     ///
1206     /// All access to the inner value through methods is `unsafe`.
1207     ///
1208     /// # Examples
1209     ///
1210     /// ```
1211     /// use std::cell::UnsafeCell;
1212     ///
1213     /// let uc = UnsafeCell::new(5);
1214     /// ```
1215     #[stable(feature = "rust1", since = "1.0.0")]
1216     #[inline]
1217     pub const fn new(value: T) -> UnsafeCell<T> {
1218         UnsafeCell { value: value }
1219     }
1220
1221     /// Unwraps the value.
1222     ///
1223     /// # Examples
1224     ///
1225     /// ```
1226     /// use std::cell::UnsafeCell;
1227     ///
1228     /// let uc = UnsafeCell::new(5);
1229     ///
1230     /// let five = uc.into_inner();
1231     /// ```
1232     #[inline]
1233     #[stable(feature = "rust1", since = "1.0.0")]
1234     pub fn into_inner(self) -> T {
1235         self.value
1236     }
1237 }
1238
1239 impl<T: ?Sized> UnsafeCell<T> {
1240     /// Gets a mutable pointer to the wrapped value.
1241     ///
1242     /// This can be cast to a pointer of any kind.
1243     /// Ensure that the access is unique when casting to
1244     /// `&mut T`, and ensure that there are no mutations or mutable
1245     /// aliases going on when casting to `&T`
1246     ///
1247     /// # Examples
1248     ///
1249     /// ```
1250     /// use std::cell::UnsafeCell;
1251     ///
1252     /// let uc = UnsafeCell::new(5);
1253     ///
1254     /// let five = uc.get();
1255     /// ```
1256     #[inline]
1257     #[stable(feature = "rust1", since = "1.0.0")]
1258     pub fn get(&self) -> *mut T {
1259         &self.value as *const T as *mut T
1260     }
1261 }
1262
1263 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1264 impl<T: Default> Default for UnsafeCell<T> {
1265     /// Creates an `UnsafeCell`, with the `Default` value for T.
1266     fn default() -> UnsafeCell<T> {
1267         UnsafeCell::new(Default::default())
1268     }
1269 }
1270
1271 #[stable(feature = "cell_from", since = "1.12.0")]
1272 impl<T> From<T> for UnsafeCell<T> {
1273     fn from(t: T) -> UnsafeCell<T> {
1274         UnsafeCell::new(t)
1275     }
1276 }
1277
1278 #[unstable(feature = "coerce_unsized", issue = "27732")]
1279 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1280
1281 #[allow(unused)]
1282 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1283     let _: UnsafeCell<&Send> = a;
1284     let _: Cell<&Send> = b;
1285     let _: RefCell<&Send> = c;
1286 }