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.
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.
11 //! Shareable mutable containers.
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'.
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.
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
30 //! # When to choose interior mutability
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.
39 //! * Introducing inherited mutability roots to shared types.
40 //! * Implementation details of logically-immutable methods.
41 //! * Mutating implementations of `Clone`.
43 //! ## Introducing inherited mutability roots to shared types
45 //! Shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
46 //! cloned and shared between multiple parties. Because the contained values may be
47 //! multiply-aliased, they can only be borrowed as shared references, not mutable references.
48 //! Without cells it would be impossible to mutate data inside of shared boxes at all!
50 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
54 //! use std::collections::HashMap;
55 //! use std::cell::RefCell;
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);
67 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
68 //! scenarios. Consider using `Mutex<T>` if you need shared mutability in a multi-threaded
71 //! ## Implementation details of logically-immutable methods
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`.
79 //! use std::cell::RefCell;
82 //! edges: Vec<(i32, i32)>,
83 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
87 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
88 //! // Create a new scope to contain the lifetime of the
91 //! // Take a reference to the inside of cache cell
92 //! let mut cache = self.span_tree_cache.borrow_mut();
93 //! if cache.is_some() {
94 //! return cache.as_ref().unwrap().clone();
97 //! let span_tree = self.calc_span_tree();
98 //! *cache = Some(span_tree);
101 //! // Recursive call to return the just-cached value.
102 //! // Note that if we had not let the previous borrow
103 //! // of the cache fall out of scope then the subsequent
104 //! // recursive borrow would cause a dynamic thread panic.
105 //! // This is the major hazard of using `RefCell`.
106 //! self.minimum_spanning_tree()
108 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
112 //! ## Mutating implementations of `Clone`
114 //! This is simply a special - but common - case of the previous: hiding mutability for operations
115 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
116 //! is declared to take `&self`, not `&mut self`. Therefore any mutation that happens in the
117 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
121 //! use std::cell::Cell;
124 //! ptr: *mut RcBox<T>
127 //! struct RcBox<T> {
129 //! refcount: Cell<usize>
132 //! impl<T> Clone for Rc<T> {
133 //! fn clone(&self) -> Rc<T> {
135 //! (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
136 //! Rc { ptr: self.ptr }
143 #![stable(feature = "rust1", since = "1.0.0")]
146 use cmp::{PartialEq, Eq};
147 use default::Default;
148 use marker::{Copy, Send, Sync, Sized};
149 use ops::{Deref, DerefMut, Drop, FnOnce};
151 use option::Option::{None, Some};
153 /// A mutable memory location that admits only `Copy` data.
155 /// See the [module-level documentation](index.html) for more.
156 #[stable(feature = "rust1", since = "1.0.0")]
158 value: UnsafeCell<T>,
161 impl<T:Copy> Cell<T> {
162 /// Creates a new `Cell` containing the given value.
167 /// use std::cell::Cell;
169 /// let c = Cell::new(5);
171 #[stable(feature = "rust1", since = "1.0.0")]
173 pub const fn new(value: T) -> Cell<T> {
175 value: UnsafeCell::new(value),
179 /// Returns a copy of the contained value.
184 /// use std::cell::Cell;
186 /// let c = Cell::new(5);
188 /// let five = c.get();
191 #[stable(feature = "rust1", since = "1.0.0")]
192 pub fn get(&self) -> T {
193 unsafe{ *self.value.get() }
196 /// Sets the contained value.
201 /// use std::cell::Cell;
203 /// let c = Cell::new(5);
208 #[stable(feature = "rust1", since = "1.0.0")]
209 pub fn set(&self, value: T) {
211 *self.value.get() = value;
215 /// Returns a reference to the underlying `UnsafeCell`.
219 /// This function is `unsafe` because `UnsafeCell`'s field is public.
224 /// # #![feature(core)]
225 /// use std::cell::Cell;
227 /// let c = Cell::new(5);
229 /// let uc = unsafe { c.as_unsafe_cell() };
232 #[unstable(feature = "core")]
233 pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
238 #[stable(feature = "rust1", since = "1.0.0")]
239 unsafe impl<T> Send for Cell<T> where T: Send {}
241 #[stable(feature = "rust1", since = "1.0.0")]
242 impl<T:Copy> Clone for Cell<T> {
244 fn clone(&self) -> Cell<T> {
245 Cell::new(self.get())
249 #[stable(feature = "rust1", since = "1.0.0")]
250 impl<T:Default + Copy> Default for Cell<T> {
251 #[stable(feature = "rust1", since = "1.0.0")]
253 fn default() -> Cell<T> {
254 Cell::new(Default::default())
258 #[stable(feature = "rust1", since = "1.0.0")]
259 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
261 fn eq(&self, other: &Cell<T>) -> bool {
262 self.get() == other.get()
266 #[stable(feature = "cell_eq", since = "1.2.0")]
267 impl<T:Eq + Copy> Eq for Cell<T> {}
269 /// A mutable memory location with dynamically checked borrow rules
271 /// See the [module-level documentation](index.html) for more.
272 #[stable(feature = "rust1", since = "1.0.0")]
273 pub struct RefCell<T: ?Sized> {
274 borrow: Cell<BorrowFlag>,
275 value: UnsafeCell<T>,
278 /// An enumeration of values returned from the `state` method on a `RefCell<T>`.
279 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
280 #[unstable(feature = "std_misc")]
281 pub enum BorrowState {
282 /// The cell is currently being read, there is at least one active `borrow`.
284 /// The cell is currently being written to, there is an active `borrow_mut`.
286 /// There are no outstanding borrows on this cell.
290 // Values [1, MAX-1] represent the number of `Ref` active
291 // (will not outgrow its range since `usize` is the size of the address space)
292 type BorrowFlag = usize;
293 const UNUSED: BorrowFlag = 0;
294 const WRITING: BorrowFlag = !0;
297 /// Creates a new `RefCell` containing `value`.
302 /// use std::cell::RefCell;
304 /// let c = RefCell::new(5);
306 #[stable(feature = "rust1", since = "1.0.0")]
308 pub const fn new(value: T) -> RefCell<T> {
310 value: UnsafeCell::new(value),
311 borrow: Cell::new(UNUSED),
315 /// Consumes the `RefCell`, returning the wrapped value.
320 /// use std::cell::RefCell;
322 /// let c = RefCell::new(5);
324 /// let five = c.into_inner();
326 #[stable(feature = "rust1", since = "1.0.0")]
328 pub fn into_inner(self) -> T {
329 // Since this function takes `self` (the `RefCell`) by value, the
330 // compiler statically verifies that it is not currently borrowed.
331 // Therefore the following assertion is just a `debug_assert!`.
332 debug_assert!(self.borrow.get() == UNUSED);
333 unsafe { self.value.into_inner() }
337 impl<T: ?Sized> RefCell<T> {
338 /// Query the current state of this `RefCell`
340 /// The returned value can be dispatched on to determine if a call to
341 /// `borrow` or `borrow_mut` would succeed.
342 #[unstable(feature = "std_misc")]
344 pub fn borrow_state(&self) -> BorrowState {
345 match self.borrow.get() {
346 WRITING => BorrowState::Writing,
347 UNUSED => BorrowState::Unused,
348 _ => BorrowState::Reading,
352 /// Immutably borrows the wrapped value.
354 /// The borrow lasts until the returned `Ref` exits scope. Multiple
355 /// immutable borrows can be taken out at the same time.
359 /// Panics if the value is currently mutably borrowed.
364 /// use std::cell::RefCell;
366 /// let c = RefCell::new(5);
368 /// let borrowed_five = c.borrow();
369 /// let borrowed_five2 = c.borrow();
372 /// An example of panic:
375 /// use std::cell::RefCell;
378 /// let result = thread::spawn(move || {
379 /// let c = RefCell::new(5);
380 /// let m = c.borrow_mut();
382 /// let b = c.borrow(); // this causes a panic
385 /// assert!(result.is_err());
387 #[stable(feature = "rust1", since = "1.0.0")]
389 pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
390 match BorrowRef::new(&self.borrow) {
392 _value: unsafe { &*self.value.get() },
395 None => panic!("RefCell<T> already mutably borrowed"),
399 /// Mutably borrows the wrapped value.
401 /// The borrow lasts until the returned `RefMut` exits scope. The value
402 /// cannot be borrowed while this borrow is active.
406 /// Panics if the value is currently borrowed.
411 /// use std::cell::RefCell;
413 /// let c = RefCell::new(5);
415 /// let borrowed_five = c.borrow_mut();
418 /// An example of panic:
421 /// use std::cell::RefCell;
424 /// let result = thread::spawn(move || {
425 /// let c = RefCell::new(5);
426 /// let m = c.borrow();
428 /// let b = c.borrow_mut(); // this causes a panic
431 /// assert!(result.is_err());
433 #[stable(feature = "rust1", since = "1.0.0")]
435 pub fn borrow_mut<'a>(&'a self) -> RefMut<'a, T> {
436 match BorrowRefMut::new(&self.borrow) {
438 _value: unsafe { &mut *self.value.get() },
441 None => panic!("RefCell<T> already borrowed"),
445 /// Returns a reference to the underlying `UnsafeCell`.
447 /// This can be used to circumvent `RefCell`'s safety checks.
449 /// This function is `unsafe` because `UnsafeCell`'s field is public.
451 #[unstable(feature = "core")]
452 pub unsafe fn as_unsafe_cell<'a>(&'a self) -> &'a UnsafeCell<T> {
457 #[stable(feature = "rust1", since = "1.0.0")]
458 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
460 #[stable(feature = "rust1", since = "1.0.0")]
461 impl<T: Clone> Clone for RefCell<T> {
463 fn clone(&self) -> RefCell<T> {
464 RefCell::new(self.borrow().clone())
468 #[stable(feature = "rust1", since = "1.0.0")]
469 impl<T:Default> Default for RefCell<T> {
470 #[stable(feature = "rust1", since = "1.0.0")]
472 fn default() -> RefCell<T> {
473 RefCell::new(Default::default())
477 #[stable(feature = "rust1", since = "1.0.0")]
478 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
480 fn eq(&self, other: &RefCell<T>) -> bool {
481 *self.borrow() == *other.borrow()
485 #[stable(feature = "cell_eq", since = "1.2.0")]
486 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
488 struct BorrowRef<'b> {
489 _borrow: &'b Cell<BorrowFlag>,
492 impl<'b> BorrowRef<'b> {
494 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
499 Some(BorrowRef { _borrow: borrow })
505 impl<'b> Drop for BorrowRef<'b> {
508 let borrow = self._borrow.get();
509 debug_assert!(borrow != WRITING && borrow != UNUSED);
510 self._borrow.set(borrow - 1);
514 impl<'b> Clone for BorrowRef<'b> {
516 fn clone(&self) -> BorrowRef<'b> {
517 // Since this Ref exists, we know the borrow flag
518 // is not set to WRITING.
519 let borrow = self._borrow.get();
520 debug_assert!(borrow != WRITING && borrow != UNUSED);
521 self._borrow.set(borrow + 1);
522 BorrowRef { _borrow: self._borrow }
526 /// Wraps a borrowed reference to a value in a `RefCell` box.
527 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
529 /// See the [module-level documentation](index.html) for more.
530 #[stable(feature = "rust1", since = "1.0.0")]
531 pub struct Ref<'b, T: ?Sized + 'b> {
532 // FIXME #12808: strange name to try to avoid interfering with
533 // field accesses of the contained type via Deref
535 _borrow: BorrowRef<'b>,
538 #[stable(feature = "rust1", since = "1.0.0")]
539 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
543 fn deref<'a>(&'a self) -> &'a T {
550 /// The `RefCell` is already immutably borrowed, so this cannot fail.
552 /// A `Clone` implementation would interfere with the widespread
553 /// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
554 #[deprecated(since = "1.2.0", reason = "moved to a `Ref::clone` associated function")]
555 #[unstable(feature = "core",
556 reason = "likely to be moved to a method, pending language changes")]
558 pub fn clone_ref<'b, T:Clone>(orig: &Ref<'b, T>) -> Ref<'b, T> {
562 impl<'b, T: ?Sized> Ref<'b, T> {
565 /// The `RefCell` is already immutably borrowed, so this cannot fail.
567 /// This is an associated function that needs to be used as `Ref::clone(...)`.
568 /// A `Clone` implementation or a method would interfere with the widespread
569 /// use of `r.borrow().clone()` to clone the contents of a `RefCell`.
570 #[unstable(feature = "cell_extras",
571 reason = "likely to be moved to a method, pending language changes")]
573 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
576 _borrow: orig._borrow.clone(),
580 /// Make a new `Ref` for a component of the borrowed data.
582 /// The `RefCell` is already immutably borrowed, so this cannot fail.
584 /// This is an associated function that needs to be used as `Ref::map(...)`.
585 /// A method would interfere with methods of the same name on the contents of a `RefCell`
586 /// used through `Deref`.
591 /// # #![feature(cell_extras)]
592 /// use std::cell::{RefCell, Ref};
594 /// let c = RefCell::new((5, 'b'));
595 /// let b1: Ref<(u32, char)> = c.borrow();
596 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
597 /// assert_eq!(*b2, 5)
599 #[unstable(feature = "cell_extras", reason = "recently added")]
601 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
602 where F: FnOnce(&T) -> &U
605 _value: f(orig._value),
606 _borrow: orig._borrow,
610 /// Make a new `Ref` for a optional component of the borrowed data, e.g. an enum variant.
612 /// The `RefCell` is already immutably borrowed, so this cannot fail.
614 /// This is an associated function that needs to be used as `Ref::filter_map(...)`.
615 /// A method would interfere with methods of the same name on the contents of a `RefCell`
616 /// used through `Deref`.
621 /// # #![feature(cell_extras)]
622 /// use std::cell::{RefCell, Ref};
624 /// let c = RefCell::new(Ok(5));
625 /// let b1: Ref<Result<u32, ()>> = c.borrow();
626 /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
627 /// assert_eq!(*b2, 5)
629 #[unstable(feature = "cell_extras", reason = "recently added")]
631 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
632 where F: FnOnce(&T) -> Option<&U>
634 f(orig._value).map(move |new| Ref {
636 _borrow: orig._borrow,
641 impl<'b, T: ?Sized> RefMut<'b, T> {
642 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum variant.
644 /// The `RefCell` is already mutably borrowed, so this cannot fail.
646 /// This is an associated function that needs to be used as `RefMut::map(...)`.
647 /// A method would interfere with methods of the same name on the contents of a `RefCell`
648 /// used through `Deref`.
653 /// # #![feature(cell_extras)]
654 /// use std::cell::{RefCell, RefMut};
656 /// let c = RefCell::new((5, 'b'));
658 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
659 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
660 /// assert_eq!(*b2, 5);
663 /// assert_eq!(*c.borrow(), (42, 'b'));
665 #[unstable(feature = "cell_extras", reason = "recently added")]
667 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
668 where F: FnOnce(&mut T) -> &mut U
671 _value: f(orig._value),
672 _borrow: orig._borrow,
676 /// Make a new `RefMut` for a optional component of the borrowed data, e.g. an enum variant.
678 /// The `RefCell` is already mutably borrowed, so this cannot fail.
680 /// This is an associated function that needs to be used as `RefMut::filter_map(...)`.
681 /// A method would interfere with methods of the same name on the contents of a `RefCell`
682 /// used through `Deref`.
687 /// # #![feature(cell_extras)]
688 /// use std::cell::{RefCell, RefMut};
690 /// let c = RefCell::new(Ok(5));
692 /// let b1: RefMut<Result<u32, ()>> = c.borrow_mut();
693 /// let mut b2: RefMut<u32> = RefMut::filter_map(b1, |o| o.as_mut().ok()).unwrap();
694 /// assert_eq!(*b2, 5);
697 /// assert_eq!(*c.borrow(), Ok(42));
699 #[unstable(feature = "cell_extras", reason = "recently added")]
701 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
702 where F: FnOnce(&mut T) -> Option<&mut U>
704 let RefMut { _value, _borrow } = orig;
705 f(_value).map(move |new| RefMut {
712 struct BorrowRefMut<'b> {
713 _borrow: &'b Cell<BorrowFlag>,
716 impl<'b> Drop for BorrowRefMut<'b> {
719 let borrow = self._borrow.get();
720 debug_assert!(borrow == WRITING);
721 self._borrow.set(UNUSED);
725 impl<'b> BorrowRefMut<'b> {
727 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
731 Some(BorrowRefMut { _borrow: borrow })
738 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
740 /// See the [module-level documentation](index.html) for more.
741 #[stable(feature = "rust1", since = "1.0.0")]
742 pub struct RefMut<'b, T: ?Sized + 'b> {
743 // FIXME #12808: strange name to try to avoid interfering with
744 // field accesses of the contained type via Deref
746 _borrow: BorrowRefMut<'b>,
749 #[stable(feature = "rust1", since = "1.0.0")]
750 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
754 fn deref<'a>(&'a self) -> &'a T {
759 #[stable(feature = "rust1", since = "1.0.0")]
760 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
762 fn deref_mut<'a>(&'a mut self) -> &'a mut T {
767 /// The core primitive for interior mutability in Rust.
769 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
770 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
771 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
772 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
774 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
779 /// use std::cell::UnsafeCell;
780 /// use std::marker::Sync;
782 /// struct NotThreadSafe<T> {
783 /// value: UnsafeCell<T>,
786 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
789 /// **NOTE:** `UnsafeCell<T>`'s fields are public to allow static initializers. It is not
790 /// recommended to access its fields directly, `get` should be used instead.
791 #[lang = "unsafe_cell"]
792 #[stable(feature = "rust1", since = "1.0.0")]
793 pub struct UnsafeCell<T: ?Sized> {
796 /// This field should not be accessed directly, it is made public for static
798 #[deprecated(since = "1.2.0", reason = "use `get` to access the wrapped \
799 value or `new` to initialize `UnsafeCell` in statics")]
800 #[unstable(feature = "core")]
804 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
806 impl<T> UnsafeCell<T> {
807 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
810 /// All access to the inner value through methods is `unsafe`, and it is highly discouraged to
811 /// access the fields directly.
816 /// use std::cell::UnsafeCell;
818 /// let uc = UnsafeCell::new(5);
820 #[stable(feature = "rust1", since = "1.0.0")]
822 pub const fn new(value: T) -> UnsafeCell<T> {
823 #![allow(deprecated)]
824 UnsafeCell { value: value }
827 /// Unwraps the value.
831 /// This function is unsafe because this thread or another thread may currently be
832 /// inspecting the inner value.
837 /// use std::cell::UnsafeCell;
839 /// let uc = UnsafeCell::new(5);
841 /// let five = unsafe { uc.into_inner() };
844 #[stable(feature = "rust1", since = "1.0.0")]
845 pub unsafe fn into_inner(self) -> T {
846 #![allow(deprecated)]
851 impl<T: ?Sized> UnsafeCell<T> {
852 /// Gets a mutable pointer to the wrapped value.
857 /// use std::cell::UnsafeCell;
859 /// let uc = UnsafeCell::new(5);
861 /// let five = uc.get();
864 #[stable(feature = "rust1", since = "1.0.0")]
865 pub fn get(&self) -> *mut T {
866 // FIXME(#23542) Replace with type ascription.
867 #![allow(trivial_casts)]
868 #![allow(deprecated)]
869 &self.value as *const T as *mut T