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 mutability 'inside' of something immutable
40 //! * Implementation details of logically-immutable methods.
41 //! * Mutating implementations of `Clone`.
43 //! ## Introducing mutability 'inside' of something immutable
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.
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 `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
69 //! multi-threaded situation.
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 //! # #![allow(dead_code)]
80 //! use std::cell::RefCell;
83 //! edges: Vec<(i32, i32)>,
84 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
88 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
89 //! // Create a new scope to contain the lifetime of the
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();
98 //! let span_tree = self.calc_span_tree();
99 //! *cache = Some(span_tree);
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()
109 //! # fn calc_span_tree(&self) -> Vec<(i32, i32)> { vec![] }
113 //! ## Mutating implementations of `Clone`
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
122 //! use std::cell::Cell;
125 //! ptr: *mut RcBox<T>
128 //! struct RcBox<T> {
129 //! # #[allow(dead_code)]
131 //! refcount: Cell<usize>
134 //! impl<T> Clone for Rc<T> {
135 //! fn clone(&self) -> Rc<T> {
137 //! (*self.ptr).refcount.set((*self.ptr).refcount.get() + 1);
138 //! Rc { ptr: self.ptr }
145 #![stable(feature = "rust1", since = "1.0.0")]
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};
153 use option::Option::{None, Some};
155 /// A mutable memory location that admits only `Copy` data.
157 /// See the [module-level documentation](index.html) for more.
158 #[stable(feature = "rust1", since = "1.0.0")]
160 value: UnsafeCell<T>,
163 impl<T:Copy> Cell<T> {
164 /// Creates a new `Cell` containing the given value.
169 /// use std::cell::Cell;
171 /// let c = Cell::new(5);
173 #[stable(feature = "rust1", since = "1.0.0")]
175 pub const fn new(value: T) -> Cell<T> {
177 value: UnsafeCell::new(value),
181 /// Returns a copy of the contained value.
186 /// use std::cell::Cell;
188 /// let c = Cell::new(5);
190 /// let five = c.get();
193 #[stable(feature = "rust1", since = "1.0.0")]
194 pub fn get(&self) -> T {
195 unsafe{ *self.value.get() }
198 /// Sets the contained value.
203 /// use std::cell::Cell;
205 /// let c = Cell::new(5);
210 #[stable(feature = "rust1", since = "1.0.0")]
211 pub fn set(&self, value: T) {
213 *self.value.get() = value;
217 /// Returns a reference to the underlying `UnsafeCell`.
222 /// #![feature(as_unsafe_cell)]
224 /// use std::cell::Cell;
226 /// let c = Cell::new(5);
228 /// let uc = c.as_unsafe_cell();
231 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
232 pub fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
237 #[stable(feature = "rust1", since = "1.0.0")]
238 unsafe impl<T> Send for Cell<T> where T: Send {}
240 #[stable(feature = "rust1", since = "1.0.0")]
241 impl<T> !Sync for Cell<T> {}
243 #[stable(feature = "rust1", since = "1.0.0")]
244 impl<T:Copy> Clone for Cell<T> {
246 fn clone(&self) -> Cell<T> {
247 Cell::new(self.get())
251 #[stable(feature = "rust1", since = "1.0.0")]
252 impl<T:Default + Copy> Default for Cell<T> {
254 fn default() -> Cell<T> {
255 Cell::new(Default::default())
259 #[stable(feature = "rust1", since = "1.0.0")]
260 impl<T:PartialEq + Copy> PartialEq for Cell<T> {
262 fn eq(&self, other: &Cell<T>) -> bool {
263 self.get() == other.get()
267 #[stable(feature = "cell_eq", since = "1.2.0")]
268 impl<T:Eq + Copy> Eq for Cell<T> {}
270 /// A mutable memory location with dynamically checked borrow rules
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>,
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`.
285 /// The cell is currently being written to, there is an active `borrow_mut`.
287 /// There are no outstanding borrows on this cell.
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;
298 /// Creates a new `RefCell` containing `value`.
303 /// use std::cell::RefCell;
305 /// let c = RefCell::new(5);
307 #[stable(feature = "rust1", since = "1.0.0")]
309 pub const fn new(value: T) -> RefCell<T> {
311 value: UnsafeCell::new(value),
312 borrow: Cell::new(UNUSED),
316 /// Consumes the `RefCell`, returning the wrapped value.
321 /// use std::cell::RefCell;
323 /// let c = RefCell::new(5);
325 /// let five = c.into_inner();
327 #[stable(feature = "rust1", since = "1.0.0")]
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() }
338 impl<T: ?Sized> RefCell<T> {
339 /// Query the current state of this `RefCell`
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")]
345 pub fn borrow_state(&self) -> BorrowState {
346 match self.borrow.get() {
347 WRITING => BorrowState::Writing,
348 UNUSED => BorrowState::Unused,
349 _ => BorrowState::Reading,
353 /// Immutably borrows the wrapped value.
355 /// The borrow lasts until the returned `Ref` exits scope. Multiple
356 /// immutable borrows can be taken out at the same time.
360 /// Panics if the value is currently mutably borrowed.
365 /// use std::cell::RefCell;
367 /// let c = RefCell::new(5);
369 /// let borrowed_five = c.borrow();
370 /// let borrowed_five2 = c.borrow();
373 /// An example of panic:
376 /// use std::cell::RefCell;
379 /// let result = thread::spawn(move || {
380 /// let c = RefCell::new(5);
381 /// let m = c.borrow_mut();
383 /// let b = c.borrow(); // this causes a panic
386 /// assert!(result.is_err());
388 #[stable(feature = "rust1", since = "1.0.0")]
390 pub fn borrow(&self) -> Ref<T> {
391 match BorrowRef::new(&self.borrow) {
393 _value: unsafe { &*self.value.get() },
396 None => panic!("RefCell<T> already mutably borrowed"),
400 /// Mutably borrows the wrapped value.
402 /// The borrow lasts until the returned `RefMut` exits scope. The value
403 /// cannot be borrowed while this borrow is active.
407 /// Panics if the value is currently borrowed.
412 /// use std::cell::RefCell;
414 /// let c = RefCell::new(5);
416 /// *c.borrow_mut() = 7;
418 /// assert_eq!(*c.borrow(), 7);
421 /// An example of panic:
424 /// use std::cell::RefCell;
427 /// let result = thread::spawn(move || {
428 /// let c = RefCell::new(5);
429 /// let m = c.borrow();
431 /// let b = c.borrow_mut(); // this causes a panic
434 /// assert!(result.is_err());
436 #[stable(feature = "rust1", since = "1.0.0")]
438 pub fn borrow_mut(&self) -> RefMut<T> {
439 match BorrowRefMut::new(&self.borrow) {
441 _value: unsafe { &mut *self.value.get() },
444 None => panic!("RefCell<T> already borrowed"),
448 /// Returns a reference to the underlying `UnsafeCell`.
450 /// This can be used to circumvent `RefCell`'s safety checks.
452 /// This function is `unsafe` because `UnsafeCell`'s field is public.
454 #[unstable(feature = "as_unsafe_cell", issue = "27708")]
455 pub unsafe fn as_unsafe_cell(&self) -> &UnsafeCell<T> {
460 #[stable(feature = "rust1", since = "1.0.0")]
461 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
463 #[stable(feature = "rust1", since = "1.0.0")]
464 impl<T: ?Sized> !Sync for RefCell<T> {}
466 #[stable(feature = "rust1", since = "1.0.0")]
467 impl<T: Clone> Clone for RefCell<T> {
469 fn clone(&self) -> RefCell<T> {
470 RefCell::new(self.borrow().clone())
474 #[stable(feature = "rust1", since = "1.0.0")]
475 impl<T:Default> Default for RefCell<T> {
477 fn default() -> RefCell<T> {
478 RefCell::new(Default::default())
482 #[stable(feature = "rust1", since = "1.0.0")]
483 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
485 fn eq(&self, other: &RefCell<T>) -> bool {
486 *self.borrow() == *other.borrow()
490 #[stable(feature = "cell_eq", since = "1.2.0")]
491 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
493 struct BorrowRef<'b> {
494 _borrow: &'b Cell<BorrowFlag>,
497 impl<'b> BorrowRef<'b> {
499 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
504 Some(BorrowRef { _borrow: borrow })
510 impl<'b> Drop for BorrowRef<'b> {
513 let borrow = self._borrow.get();
514 debug_assert!(borrow != WRITING && borrow != UNUSED);
515 self._borrow.set(borrow - 1);
519 impl<'b> Clone for BorrowRef<'b> {
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 }
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>`.
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 // FIXME #12808: strange name to try to avoid interfering with
538 // field accesses of the contained type via Deref
540 _borrow: BorrowRef<'b>,
543 #[stable(feature = "rust1", since = "1.0.0")]
544 impl<'b, T: ?Sized> Deref for Ref<'b, T> {
548 fn deref(&self) -> &T {
553 impl<'b, T: ?Sized> Ref<'b, T> {
556 /// The `RefCell` is already immutably borrowed, so this cannot fail.
558 /// This is an associated function that needs to be used as
559 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
560 /// with the widespread use of `r.borrow().clone()` to clone the contents of
562 #[unstable(feature = "cell_extras",
563 reason = "likely to be moved to a method, pending language changes",
566 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
569 _borrow: orig._borrow.clone(),
573 /// Make a new `Ref` for a component of the borrowed data.
575 /// The `RefCell` is already immutably borrowed, so this cannot fail.
577 /// This is an associated function that needs to be used as `Ref::map(...)`.
578 /// A method would interfere with methods of the same name on the contents
579 /// of a `RefCell` used through `Deref`.
584 /// use std::cell::{RefCell, Ref};
586 /// let c = RefCell::new((5, 'b'));
587 /// let b1: Ref<(u32, char)> = c.borrow();
588 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
589 /// assert_eq!(*b2, 5)
591 #[stable(feature = "cell_map", since = "1.8.0")]
593 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
594 where F: FnOnce(&T) -> &U
597 _value: f(orig._value),
598 _borrow: orig._borrow,
602 /// Make a new `Ref` for an optional component of the borrowed data, e.g. an
605 /// The `RefCell` is already immutably borrowed, so this cannot fail.
607 /// This is an associated function that needs to be used as
608 /// `Ref::filter_map(...)`. A method would interfere with methods of the
609 /// same name on the contents of a `RefCell` used through `Deref`.
614 /// # #![feature(cell_extras)]
615 /// use std::cell::{RefCell, Ref};
617 /// let c = RefCell::new(Ok(5));
618 /// let b1: Ref<Result<u32, ()>> = c.borrow();
619 /// let b2: Ref<u32> = Ref::filter_map(b1, |o| o.as_ref().ok()).unwrap();
620 /// assert_eq!(*b2, 5)
622 #[unstable(feature = "cell_extras", reason = "recently added",
624 #[rustc_deprecated(since = "1.8.0", reason = "can be built on `Ref::map`: \
625 https://crates.io/crates/ref_filter_map")]
627 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Option<Ref<'b, U>>
628 where F: FnOnce(&T) -> Option<&U>
630 f(orig._value).map(move |new| Ref {
632 _borrow: orig._borrow,
637 #[unstable(feature = "coerce_unsized", issue = "27732")]
638 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
640 impl<'b, T: ?Sized> RefMut<'b, T> {
641 /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
644 /// The `RefCell` is already mutably borrowed, so this cannot fail.
646 /// This is an associated function that needs to be used as
647 /// `RefMut::map(...)`. A method would interfere with methods of the same
648 /// name on the contents of a `RefCell` used through `Deref`.
653 /// use std::cell::{RefCell, RefMut};
655 /// let c = RefCell::new((5, 'b'));
657 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
658 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
659 /// assert_eq!(*b2, 5);
662 /// assert_eq!(*c.borrow(), (42, 'b'));
664 #[stable(feature = "cell_map", since = "1.8.0")]
666 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
667 where F: FnOnce(&mut T) -> &mut U
670 _value: f(orig._value),
671 _borrow: orig._borrow,
675 /// Make a new `RefMut` for an optional component of the borrowed data, e.g.
678 /// The `RefCell` is already mutably borrowed, so this cannot fail.
680 /// This is an associated function that needs to be used as
681 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
682 /// same name on the contents of a `RefCell` 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| {
696 /// assert_eq!(*b2, 5);
699 /// assert_eq!(*c.borrow(), Ok(42));
701 #[unstable(feature = "cell_extras", reason = "recently added",
703 #[rustc_deprecated(since = "1.8.0", reason = "can be built on `RefMut::map`: \
704 https://crates.io/crates/ref_filter_map")]
706 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Option<RefMut<'b, U>>
707 where F: FnOnce(&mut T) -> Option<&mut U>
709 let RefMut { _value, _borrow } = orig;
710 f(_value).map(move |new| RefMut {
717 struct BorrowRefMut<'b> {
718 _borrow: &'b Cell<BorrowFlag>,
721 impl<'b> Drop for BorrowRefMut<'b> {
724 let borrow = self._borrow.get();
725 debug_assert!(borrow == WRITING);
726 self._borrow.set(UNUSED);
730 impl<'b> BorrowRefMut<'b> {
732 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
736 Some(BorrowRefMut { _borrow: borrow })
743 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
745 /// See the [module-level documentation](index.html) for more.
746 #[stable(feature = "rust1", since = "1.0.0")]
747 pub struct RefMut<'b, T: ?Sized + 'b> {
748 // FIXME #12808: strange name to try to avoid interfering with
749 // field accesses of the contained type via Deref
751 _borrow: BorrowRefMut<'b>,
754 #[stable(feature = "rust1", since = "1.0.0")]
755 impl<'b, T: ?Sized> Deref for RefMut<'b, T> {
759 fn deref(&self) -> &T {
764 #[stable(feature = "rust1", since = "1.0.0")]
765 impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
767 fn deref_mut(&mut self) -> &mut T {
772 #[unstable(feature = "coerce_unsized", issue = "27732")]
773 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
775 /// The core primitive for interior mutability in Rust.
777 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
778 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
779 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
780 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
782 /// Types like `Cell<T>` and `RefCell<T>` use this type to wrap their internal data.
787 /// use std::cell::UnsafeCell;
788 /// use std::marker::Sync;
790 /// # #[allow(dead_code)]
791 /// struct NotThreadSafe<T> {
792 /// value: UnsafeCell<T>,
795 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
797 #[lang = "unsafe_cell"]
798 #[stable(feature = "rust1", since = "1.0.0")]
799 pub struct UnsafeCell<T: ?Sized> {
803 #[stable(feature = "rust1", since = "1.0.0")]
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`.
815 /// use std::cell::UnsafeCell;
817 /// let uc = UnsafeCell::new(5);
819 #[stable(feature = "rust1", since = "1.0.0")]
821 pub const fn new(value: T) -> UnsafeCell<T> {
822 UnsafeCell { value: value }
825 /// Unwraps the value.
829 /// This function is unsafe because this thread or another thread may currently be
830 /// inspecting the inner value.
835 /// use std::cell::UnsafeCell;
837 /// let uc = UnsafeCell::new(5);
839 /// let five = unsafe { uc.into_inner() };
842 #[stable(feature = "rust1", since = "1.0.0")]
843 pub unsafe fn into_inner(self) -> T {
848 impl<T: ?Sized> UnsafeCell<T> {
849 /// Gets a mutable pointer to the wrapped value.
854 /// use std::cell::UnsafeCell;
856 /// let uc = UnsafeCell::new(5);
858 /// let five = uc.get();
861 #[stable(feature = "rust1", since = "1.0.0")]
862 pub fn get(&self) -> *mut T {
863 &self.value as *const T as *mut T