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};
 149 use default::Default;
 150 use marker::{Copy, Send, Sync, Sized, Unsize};
 151 use ops::{Deref, DerefMut, Drop, FnOnce, CoerceUnsized};
 152 use option::Option;
 153 use option::Option::{None, Some};
 154
 155 /// A mutable memory location that admits only `Copy` data.
 156 ///
 157 /// See the [module-level documentation](index.html) for more.
 158 #[stable(feature = "rust1", since = "1.0.0")]
 159 pub struct Cell<T> {
 160     value: UnsafeCell<T>,
 161 }
 162
 163 impl<T:Copy> Cell<T> {
 164     /// Creates a new `Cell` containing the given value.
 165     ///
 166     /// # Examples
 167     ///
 168     /// ```
 169     /// use std::cell::Cell;
 170     ///
 171     /// let c = Cell::new(5);
 172     /// ```
 173     #[stable(feature = "rust1", since = "1.0.0")]
 174     #[inline]
 175     pub const fn new(value: T) -> Cell<T> {
 176         Cell {
 177             value: UnsafeCell::new(value),
 178         }
 179     }
 180
 181     /// Returns a copy of the contained value.
 182     ///
 183     /// # Examples
 184     ///
 185     /// ```
 186     /// use std::cell::Cell;
 187     ///
 188     /// let c = Cell::new(5);
 189     ///
 190     /// let five = c.get();
 191     /// ```
 192     #[inline]
 193     #[stable(feature = "rust1", since = "1.0.0")]
 194     pub fn get(&self) -> T {
 195         unsafe{ *self.value.get() }
 196     }
 197
 198     /// Sets the contained value.
 199     ///
 200     /// # Examples
 201     ///
 202     /// ```
 203     /// use std::cell::Cell;
 204     ///
 205     /// let c = Cell::new(5);
 206     ///
 207     /// c.set(10);
 208     /// ```
 209     #[inline]
 210     #[stable(feature = "rust1", since = "1.0.0")]
 211     pub fn set(&self, value: T) {
 212         unsafe {
 213             *self.value.get() = value;
 214         }
 215     }
 216
 217     /// Returns a reference to the underlying `UnsafeCell`.
 218     ///
 219     /// # Examples
 220     ///
 221     /// ```
 222     /// #![feature(as_unsafe_cell)]
 223     ///
 224     /// use std::cell::Cell;
 225     ///
 226     /// let c = Cell::new(5);
 227     ///
 228     /// let uc = c.as_unsafe_cell();
 229     /// ```
 230     #[inline]
 231     #[unstable(feature = "as_unsafe_cell", issue = "27708")]
 232     pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
 233         &self.value
 234     }
 235 }
 236
 237 #[stable(feature = "rust1", since = "1.0.0")]
 238 unsafe impl<T> Send for Cell<T> where T: Send {}
 239
 240 #[stable(feature = "rust1", since = "1.0.0")]
 241 impl<T> !Sync for Cell<T> {}
 242
 243 #[stable(feature = "rust1", since = "1.0.0")]
 244 impl<T:Copy> Clone for Cell<T> {
 245     #[inline]
 246     fn clone(&self) -> Cell<T> {
 247         Cell::new(self.get())
 248     }
 249 }
 250
 251 #[stable(feature = "rust1", since = "1.0.0")]
 252 impl<T:Default + Copy> Default for Cell<T> {
 253     #[inline]
 254     fn default() -> Cell<T> {
 255         Cell::new(Default::default())
 256     }
 257 }
 258
 259 #[stable(feature = "rust1", since = "1.0.0")]
 260 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
 261     #[inline]
 262     fn eq(&self, other: &Cell<T>) -> bool {
 263         self.get() == other.get()
 264     }
 265 }
 266
 267 #[stable(feature = "cell_eq", since = "1.2.0")]
 268 impl<T:Eq + Copy> Eq for Cell<T> {}
 269
 270 /// A mutable memory location with dynamically checked borrow rules
 271 ///
 272 /// See the [module-level documentation](index.html) for more.
 273 #[stable(feature = "rust1", since = "1.0.0")]
 274 pub struct RefCell<T: ?Sized> {
 275     borrow: Cell<BorrowFlag>,
 276     value: UnsafeCell<T>,
 277 }
 278
 279 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
 280 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 281 #[unstable(feature = "borrow_state", issue = "27733")]
 282 pub enum BorrowState {
 283     /// The cell is currently being read, there is at least one active `borrow`.
 284     Reading,
 285     /// The cell is currently being written to, there is an active `borrow_mut`.
 286     Writing,
 287     /// There are no outstanding borrows on this cell.
 288     Unused,
 289 }
 290
 291 // Values [1, MAX-1] represent the number of `Ref` active
 292 // (will not outgrow its range since `usize` is the size of the address space)
 293 type BorrowFlag = usize;
 294 const UNUSED: BorrowFlag = 0;
 295 const WRITING: BorrowFlag = !0;
 296
 297 impl<T> RefCell<T> {
 298     /// Creates a new `RefCell` containing `value`.
 299     ///
 300     /// # Examples
 301     ///
 302     /// ```
 303     /// use std::cell::RefCell;
 304     ///
 305     /// let c = RefCell::new(5);
 306     /// ```
 307     #[stable(feature = "rust1", since = "1.0.0")]
 308     #[inline]
 309     pub const fn new(value: T) -> RefCell<T> {
 310         RefCell {
 311             value: UnsafeCell::new(value),
 312             borrow: Cell::new(UNUSED),
 313         }
 314     }
 315
 316     /// Consumes the `RefCell`, returning the wrapped value.
 317     ///
 318     /// # Examples
 319     ///
 320     /// ```
 321     /// use std::cell::RefCell;
 322     ///
 323     /// let c = RefCell::new(5);
 324     ///
 325     /// let five = c.into_inner();
 326     /// ```
 327     #[stable(feature = "rust1", since = "1.0.0")]
 328     #[inline]
 329     pub fn into_inner(self) -> T {
 330         // Since this function takes `self` (the `RefCell`) by value, the
 331         // compiler statically verifies that it is not currently borrowed.
 332         // Therefore the following assertion is just a `debug_assert!`.
 333         debug_assert!(self.borrow.get() == UNUSED);
 334         unsafe { self.value.into_inner() }
 335     }
 336 }
 337
 338 impl<T: ?Sized> RefCell<T> {
 339     /// Query the current state of this `RefCell`
 340     ///
 341     /// The returned value can be dispatched on to determine if a call to
 342     /// `borrow` or `borrow_mut` would succeed.
 343     #[unstable(feature = "borrow_state", issue = "27733")]
 344     #[inline]
 345     pub fn borrow_state(&self) -> BorrowState {
 346         match self.borrow.get() {
 347             WRITING => BorrowState::Writing,
 348             UNUSED => BorrowState::Unused,
 349             _ => BorrowState::Reading,
 350         }
 351     }
 352
 353     /// Immutably borrows the wrapped value.
 354     ///
 355     /// The borrow lasts until the returned `Ref` exits scope. Multiple
 356     /// immutable borrows can be taken out at the same time.
 357     ///
 358     /// # Panics
 359     ///
 360     /// Panics if the value is currently mutably borrowed.
 361     ///
 362     /// # Examples
 363     ///
 364     /// ```
 365     /// use std::cell::RefCell;
 366     ///
 367     /// let c = RefCell::new(5);
 368     ///
 369     /// let borrowed_five = c.borrow();
 370     /// let borrowed_five2 = c.borrow();
 371     /// ```
 372     ///
 373     /// An example of panic:
 374     ///
 375     /// ```
 376     /// use std::cell::RefCell;
 377     /// use std::thread;
 378     ///
 379     /// let result = thread::spawn(move || {
 380     ///    let c = RefCell::new(5);
 381     ///    let m = c.borrow_mut();
 382     ///
 383     ///    let b = c.borrow(); // this causes a panic
 384     /// }).join();
 385     ///
 386     /// assert!(result.is_err());
 387     /// ```
 388     #[stable(feature = "rust1", since = "1.0.0")]
 389     #[inline]
 390     pub fn borrow(&self) -> Ref<T> {
 391         match BorrowRef::new(&self.borrow) {
 392             Some(b) => Ref {
 393                 value: unsafe { &*self.value.get() },
 394                 borrow: b,
 395             },
 396             None => panic!("RefCell<T> already mutably borrowed"),
 397         }
 398     }
 399
 400     /// Mutably borrows the wrapped value.
 401     ///
 402     /// The borrow lasts until the returned `RefMut` exits scope. The value
 403     /// cannot be borrowed while this borrow is active.
 404     ///
 405     /// # Panics
 406     ///
 407     /// Panics if the value is currently borrowed.
 408     ///
 409     /// # Examples
 410     ///
 411     /// ```
 412     /// use std::cell::RefCell;
 413     ///
 414     /// let c = RefCell::new(5);
 415     ///
 416     /// *c.borrow_mut() = 7;
 417     ///
 418     /// assert_eq!(*c.borrow(), 7);
 419     /// ```
 420     ///
 421     /// An example of panic:
 422     ///
 423     /// ```
 424     /// use std::cell::RefCell;
 425     /// use std::thread;
 426     ///
 427     /// let result = thread::spawn(move || {
 428     ///    let c = RefCell::new(5);
 429     ///    let m = c.borrow();
 430     ///
 431     ///    let b = c.borrow_mut(); // this causes a panic
 432     /// }).join();
 433     ///
 434     /// assert!(result.is_err());
 435     /// ```
 436     #[stable(feature = "rust1", since = "1.0.0")]
 437     #[inline]
 438     pub fn borrow_mut(&self) -> RefMut<T> {
 439         match BorrowRefMut::new(&self.borrow) {
 440             Some(b) => RefMut {
 441                 value: unsafe { &mut *self.value.get() },
 442                 borrow: b,
 443             },
 444             None => panic!("RefCell<T> already borrowed"),
 445         }
 446     }
 447
 448     /// Returns a reference to the underlying `UnsafeCell`.
 449     ///
 450     /// This can be used to circumvent `RefCell`'s safety checks.
 451     ///
 452     /// This function is `unsafe` because `UnsafeCell`'s field is public.
 453     #[inline]
 454     #[unstable(feature = "as_unsafe_cell", issue = "27708")]
 455     pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
 456         &self.value
 457     }
 458 }
 459
 460 #[stable(feature = "rust1", since = "1.0.0")]
 461 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
 462
 463 #[stable(feature = "rust1", since = "1.0.0")]
 464 impl<T: ?Sized> !Sync for RefCell<T> {}
 465
 466 #[stable(feature = "rust1", since = "1.0.0")]
 467 impl<T: Clone> Clone for RefCell<T> {
 468     #[inline]
 469     fn clone(&self) -> RefCell<T> {
 470         RefCell::new(self.borrow().clone())
 471     }
 472 }
 473
 474 #[stable(feature = "rust1", since = "1.0.0")]
 475 impl<T:Default> Default for RefCell<T> {
 476     #[inline]
 477     fn default() -> RefCell<T> {
 478         RefCell::new(Default::default())
 479     }
 480 }
 481
 482 #[stable(feature = "rust1", since = "1.0.0")]
 483 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
 484     #[inline]
 485     fn eq(&self, other: &RefCell<T>) -> bool {
 486         *self.borrow() == *other.borrow()
 487     }
 488 }
 489
 490 #[stable(feature = "cell_eq", since = "1.2.0")]
 491 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
 492
 493 struct BorrowRef<'b> {
 494     borrow: &'b Cell<BorrowFlag>,
 495 }
 496
 497 impl<'b> BorrowRef<'b> {
 498     #[inline]
 499     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
 500         match borrow.get() {
 501             WRITING => None,
 502             b => {
 503                 borrow.set(b + 1);
 504                 Some(BorrowRef { borrow: borrow })
 505             },
 506         }
 507     }
 508 }
 509
 510 impl<'b> Drop for BorrowRef<'b> {
 511     #[inline]
 512     fn drop(&mut self) {
 513         let borrow = self.borrow.get();
 514         debug_assert!(borrow != WRITING && borrow != UNUSED);
 515         self.borrow.set(borrow - 1);
 516     }
 517 }
 518
 519 impl<'b> Clone for BorrowRef<'b> {
 520     #[inline]
 521     fn clone(&self) -> BorrowRef<'b> {
 522         // Since this Ref exists, we know the borrow flag
 523         // is not set to WRITING.
 524         let borrow = self.borrow.get();
 525         debug_assert!(borrow != WRITING && borrow != UNUSED);
 526         self.borrow.set(borrow + 1);
 527         BorrowRef { borrow: self.borrow }
 528     }
 529 }
 530
 531 /// Wraps a borrowed reference to a value in a `RefCell` box.
 532 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
 533 ///
 534 /// See the [module-level documentation](index.html) for more.
 535 #[stable(feature = "rust1", since = "1.0.0")]
 536 pub struct Ref<'b, T: ?Sized + 'b> {
 537     value: &'b T,
 538     borrow: BorrowRef<'b>,
 539 }
 540
 541 #[stable(feature = "rust1", since = "1.0.0")]
 542 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
 543     type Target = T;
 544
 545     #[inline]
 546     fn deref(&self) -> &T {
 547         self.value
 548     }
 549 }
 550
 551 impl<'b, T: ?Sized> Ref<'b, T> {
 552     /// Copies a `Ref`.
 553     ///
 554     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 555     ///
 556     /// This is an associated function that needs to be used as
 557     /// `Ref::clone(...)`.  A `Clone` implementation or a method would interfere
 558     /// with the widespread use of `r.borrow().clone()` to clone the contents of
 559     /// a `RefCell`.
 560     #[unstable(feature = "cell_extras",
 561                reason = "likely to be moved to a method, pending language changes",
 562                issue = "27746")]
 563     #[inline]
 564     pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
 565         Ref {
 566             value: orig.value,
 567             borrow: orig.borrow.clone(),
 568         }
 569     }
 570
 571     /// Make a new `Ref` for a component of the borrowed data.
 572     ///
 573     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 574     ///
 575     /// This is an associated function that needs to be used as `Ref::map(...)`.
 576     /// A method would interfere with methods of the same name on the contents
 577     /// of a `RefCell` used through `Deref`.
 578     ///
 579     /// # Example
 580     ///
 581     /// ```
 582     /// use std::cell::{RefCell, Ref};
 583     ///
 584     /// let c = RefCell::new((5, 'b'));
 585     /// let b1: Ref<(u32, char)> = c.borrow();
 586     /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
 587     /// assert_eq!(*b2, 5)
 588     /// ```
 589     #[stable(feature = "cell_map", since = "1.8.0")]
 590     #[inline]
 591     pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
 592         where F: FnOnce(&T) -> &U
 593     {
 594         Ref {
 595             value: f(orig.value),
 596             borrow: orig.borrow,
 597         }
 598     }
 599
 600     /// Make a new `Ref` for an optional component of the borrowed data, e.g. an
 601     /// enum variant.
 602     ///
 603     /// The `RefCell` is already immutably borrowed, so this cannot fail.
 604     ///
 605     /// This is an associated function that needs to be used as
 606     /// `Ref::filter_map(...)`.  A method would interfere with methods of the
 607     /// same name on the contents of a `RefCell` used through `Deref`.
 608     ///
 609     /// # Example
 610     ///
 611     /// ```
 612     /// # #![feature(cell_extras)]
 613     /// use std::cell::{RefCell, Ref};
 614     ///
 615     /// let c = RefCell::new(Ok(5));
 616     /// let b1: Ref<Result<u32, ()>> = c.borrow();
 617     /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
 618     /// assert_eq!(*b2, 5)
 619     /// ```
 620     #[unstable(feature = "cell_extras", reason = "recently added",
 621                issue = "27746")]
 622     #[rustc_deprecated(since = "1.8.0", reason = "can be built on `Ref::map`: \
 623         https://crates.io/crates/ref_filter_map")]
 624     #[inline]
 625     pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
 626         where F: FnOnce(&T) -> Option<&U>
 627     {
 628         f(orig.value).map(move |new| Ref {
 629             value: new,
 630             borrow: orig.borrow,
 631         })
 632     }
 633 }
 634
 635 #[unstable(feature = "coerce_unsized", issue = "27732")]
 636 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
 637
 638 impl<'b, T: ?Sized> RefMut<'b, T> {
 639     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
 640     /// variant.
 641     ///
 642     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 643     ///
 644     /// This is an associated function that needs to be used as
 645     /// `RefMut::map(...)`.  A method would interfere with methods of the same
 646     /// name on the contents of a `RefCell` used through `Deref`.
 647     ///
 648     /// # Example
 649     ///
 650     /// ```
 651     /// use std::cell::{RefCell, RefMut};
 652     ///
 653     /// let c = RefCell::new((5, 'b'));
 654     /// {
 655     ///     let b1: RefMut<(u32, char)> = c.borrow_mut();
 656     ///     let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
 657     ///     assert_eq!(*b2, 5);
 658     ///     *b2 = 42;
 659     /// }
 660     /// assert_eq!(*c.borrow(), (42, 'b'));
 661     /// ```
 662     #[stable(feature = "cell_map", since = "1.8.0")]
 663     #[inline]
 664     pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
 665         where F: FnOnce(&mut T) -> &mut U
 666     {
 667         RefMut {
 668             value: f(orig.value),
 669             borrow: orig.borrow,
 670         }
 671     }
 672
 673     /// Make a new `RefMut` for an optional component of the borrowed data, e.g.
 674     /// an enum variant.
 675     ///
 676     /// The `RefCell` is already mutably borrowed, so this cannot fail.
 677     ///
 678     /// This is an associated function that needs to be used as
 679     /// `RefMut::filter_map(...)`.  A method would interfere with methods of the
 680     /// same name on the contents of a `RefCell` used through `Deref`.
 681     ///
 682     /// # Example
 683     ///
 684     /// ```
 685     /// # #![feature(cell_extras)]
 686     /// use std::cell::{RefCell, RefMut};
 687     ///
 688     /// let c = RefCell::new(Ok(5));
 689     /// {
 690     ///     let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
 691     ///     let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| {
 692     ///         o.as_mut().ok()
 693     ///     }).unwrap();
 694     ///     assert_eq!(*b2, 5);
 695     ///     *b2 = 42;
 696     /// }
 697     /// assert_eq!(*c.borrow(), Ok(42));
 698     /// ```
 699     #[unstable(feature = "cell_extras", reason = "recently added",
 700                issue = "27746")]
 701     #[rustc_deprecated(since = "1.8.0", reason = "can be built on `RefMut::map`: \
 702         https://crates.io/crates/ref_filter_map")]
 703     #[inline]
 704     pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
 705         where F: FnOnce(&mut T) -> Option<&mut U>
 706     {
 707         let RefMut { value, borrow } = orig;
 708         f(value).map(move |new| RefMut {
 709             value: new,
 710             borrow: borrow,
 711         })
 712     }
 713 }
 714
 715 struct BorrowRefMut<'b> {
 716     borrow: &'b Cell<BorrowFlag>,
 717 }
 718
 719 impl<'b> Drop for BorrowRefMut<'b> {
 720     #[inline]
 721     fn drop(&mut self) {
 722         let borrow = self.borrow.get();
 723         debug_assert!(borrow == WRITING);
 724         self.borrow.set(UNUSED);
 725     }
 726 }
 727
 728 impl<'b> BorrowRefMut<'b> {
 729     #[inline]
 730     fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
 731         match borrow.get() {
 732             UNUSED => {
 733                 borrow.set(WRITING);
 734                 Some(BorrowRefMut { borrow: borrow })
 735             },
 736             _ => None,
 737         }
 738     }
 739 }
 740
 741 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
 742 ///
 743 /// See the [module-level documentation](index.html) for more.
 744 #[stable(feature = "rust1", since = "1.0.0")]
 745 pub struct RefMut<'b, T: ?Sized + 'b> {
 746     value: &'b mut T,
 747     borrow: BorrowRefMut<'b>,
 748 }
 749
 750 #[stable(feature = "rust1", since = "1.0.0")]
 751 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
 752     type Target = T;
 753
 754     #[inline]
 755     fn deref(&self) -> &T {
 756         self.value
 757     }
 758 }
 759
 760 #[stable(feature = "rust1", since = "1.0.0")]
 761 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
 762     #[inline]
 763     fn deref_mut(&mut self) -> &mut T {
 764         self.value
 765     }
 766 }
 767
 768 #[unstable(feature = "coerce_unsized", issue = "27732")]
 769 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
 770
 771 /// The core primitive for interior mutability in Rust.
 772 ///
 773 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
 774 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
 775 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
 776 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
 777 ///
 778 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
 779 ///
 780 /// # Examples
 781 ///
 782 /// ```
 783 /// use std::cell::UnsafeCell;
 784 /// use std::marker::Sync;
 785 ///
 786 /// # #[allow(dead_code)]
 787 /// struct NotThreadSafe<T> {
 788 ///     value: UnsafeCell<T>,
 789 /// }
 790 ///
 791 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
 792 /// ```
 793 #[lang = "unsafe_cell"]
 794 #[stable(feature = "rust1", since = "1.0.0")]
 795 pub struct UnsafeCell<T: ?Sized> {
 796     value: T,
 797 }
 798
 799 #[stable(feature = "rust1", since = "1.0.0")]
 800 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
 801
 802 impl<T> UnsafeCell<T> {
 803     /// Constructs a new instance of `UnsafeCell` which will wrap the specified
 804     /// value.
 805     ///
 806     /// All access to the inner value through methods is `unsafe`.
 807     ///
 808     /// # Examples
 809     ///
 810     /// ```
 811     /// use std::cell::UnsafeCell;
 812     ///
 813     /// let uc = UnsafeCell::new(5);
 814     /// ```
 815     #[stable(feature = "rust1", since = "1.0.0")]
 816     #[inline]
 817     pub const fn new(value: T) -> UnsafeCell<T> {
 818         UnsafeCell { value: value }
 819     }
 820
 821     /// Unwraps the value.
 822     ///
 823     /// # Safety
 824     ///
 825     /// This function is unsafe because this thread or another thread may currently be
 826     /// inspecting the inner value.
 827     ///
 828     /// # Examples
 829     ///
 830     /// ```
 831     /// use std::cell::UnsafeCell;
 832     ///
 833     /// let uc = UnsafeCell::new(5);
 834     ///
 835     /// let five = unsafe { uc.into_inner() };
 836     /// ```
 837     #[inline]
 838     #[stable(feature = "rust1", since = "1.0.0")]
 839     pub unsafe fn into_inner(self) -> T {
 840         self.value
 841     }
 842 }
 843
 844 impl<T: ?Sized> UnsafeCell<T> {
 845     /// Gets a mutable pointer to the wrapped value.
 846     ///
 847     /// # Examples
 848     ///
 849     /// ```
 850     /// use std::cell::UnsafeCell;
 851     ///
 852     /// let uc = UnsafeCell::new(5);
 853     ///
 854     /// let five = uc.get();
 855     /// ```
 856     #[inline]
 857     #[stable(feature = "rust1", since = "1.0.0")]
 858     pub fn get(&self) -> *mut T {
 859         &self.value as *const T as *mut T
 860     }
 861 }