1 //! Shareable mutable containers.
3 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
4 //! have one of the following:
6 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
7 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
9 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
10 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
13 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
14 //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded
15 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
16 //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to
17 //! use [`Mutex`](../../std/sync/struct.Mutex.html),
18 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
19 //! [`atomic`](../../core/sync/atomic/index.html) types.
21 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
22 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
23 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
24 //! with typical Rust types that exhibit 'inherited mutability'.
26 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
27 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
28 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
29 //! methods to retrieve and change the current interior value:
31 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
32 //! - For types that implement `Default`, the `take` method replaces the current interior value
33 //! with `Default::default()` and returns the replaced value.
34 //! - For all types, the `replace` method replaces the current interior value and returns the
35 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
36 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
39 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
40 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
41 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
42 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
43 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
46 //! # When to choose interior mutability
48 //! The more common inherited mutability, where one must have unique access to mutate a value, is
49 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
50 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
51 //! interior mutability is something of a last resort. Since cell types enable mutation where it
52 //! would otherwise be disallowed though, there are occasions when interior mutability might be
53 //! appropriate, or even *must* be used, e.g.
55 //! * Introducing mutability 'inside' of something immutable
56 //! * Implementation details of logically-immutable methods.
57 //! * Mutating implementations of `Clone`.
59 //! ## Introducing mutability 'inside' of something immutable
61 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
62 //! cloned and shared between multiple parties. Because the contained values may be
63 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
64 //! impossible to mutate data inside of these smart pointers at all.
66 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
70 //! use std::cell::{RefCell, RefMut};
71 //! use std::collections::HashMap;
75 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
76 //! // Create a new block to limit the scope of the dynamic borrow
78 //! let mut map: RefMut<_> = shared_map.borrow_mut();
79 //! map.insert("africa", 92388);
80 //! map.insert("kyoto", 11837);
81 //! map.insert("piccadilly", 11826);
82 //! map.insert("marbles", 38);
85 //! // Note that if we had not let the previous borrow of the cache fall out
86 //! // of scope then the subsequent borrow would cause a dynamic thread panic.
87 //! // This is the major hazard of using `RefCell`.
88 //! let total: i32 = shared_map.borrow().values().sum();
89 //! println!("{}", total);
93 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
94 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
95 //! multi-threaded situation.
97 //! ## Implementation details of logically-immutable methods
99 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
100 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
101 //! forces the implementation to perform mutation; or because you must employ mutation to implement
102 //! a trait method that was originally defined to take `&self`.
105 //! # #![allow(dead_code)]
106 //! use std::cell::RefCell;
109 //! edges: Vec<(i32, i32)>,
110 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
114 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
115 //! self.span_tree_cache.borrow_mut()
116 //! .get_or_insert_with(|| self.calc_span_tree())
120 //! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
121 //! // Expensive computation goes here
127 //! ## Mutating implementations of `Clone`
129 //! This is simply a special - but common - case of the previous: hiding mutability for operations
130 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
131 //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the
132 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
136 //! #![feature(core_intrinsics)]
137 //! use std::cell::Cell;
138 //! use std::ptr::NonNull;
139 //! use std::intrinsics::abort;
140 //! use std::marker::PhantomData;
142 //! struct Rc<T: ?Sized> {
143 //! ptr: NonNull<RcBox<T>>,
144 //! phantom: PhantomData<RcBox<T>>,
147 //! struct RcBox<T: ?Sized> {
148 //! strong: Cell<usize>,
149 //! refcount: Cell<usize>,
153 //! impl<T: ?Sized> Clone for Rc<T> {
154 //! fn clone(&self) -> Rc<T> {
155 //! self.inc_strong();
158 //! phantom: PhantomData,
163 //! trait RcBoxPtr<T: ?Sized> {
165 //! fn inner(&self) -> &RcBox<T>;
167 //! fn strong(&self) -> usize {
168 //! self.inner().strong.get()
171 //! fn inc_strong(&self) {
174 //! .set(self.strong()
176 //! .unwrap_or_else(|| unsafe { abort() }));
180 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
181 //! fn inner(&self) -> &RcBox<T> {
183 //! self.ptr.as_ref()
190 #![stable(feature = "rust1", since = "1.0.0")]
192 use crate::cmp::Ordering;
193 use crate::fmt::{self, Debug, Display};
194 use crate::marker::Unsize;
196 use crate::ops::{CoerceUnsized, Deref, DerefMut};
199 /// A mutable memory location.
203 /// In this example, you can see that `Cell<T>` enables mutation inside an
204 /// immutable struct. In other words, it enables "interior mutability".
207 /// use std::cell::Cell;
209 /// struct SomeStruct {
210 /// regular_field: u8,
211 /// special_field: Cell<u8>,
214 /// let my_struct = SomeStruct {
215 /// regular_field: 0,
216 /// special_field: Cell::new(1),
219 /// let new_value = 100;
221 /// // ERROR: `my_struct` is immutable
222 /// // my_struct.regular_field = new_value;
224 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
225 /// // which can always be mutated
226 /// my_struct.special_field.set(new_value);
227 /// assert_eq!(my_struct.special_field.get(), new_value);
230 /// See the [module-level documentation](index.html) for more.
231 #[stable(feature = "rust1", since = "1.0.0")]
233 pub struct Cell<T: ?Sized> {
234 value: UnsafeCell<T>,
237 #[stable(feature = "rust1", since = "1.0.0")]
238 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
240 #[stable(feature = "rust1", since = "1.0.0")]
241 impl<T: ?Sized> !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> Default for Cell<T> {
253 /// Creates a `Cell<T>`, with the `Default` value for T.
255 fn default() -> Cell<T> {
256 Cell::new(Default::default())
260 #[stable(feature = "rust1", since = "1.0.0")]
261 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
263 fn eq(&self, other: &Cell<T>) -> bool {
264 self.get() == other.get()
268 #[stable(feature = "cell_eq", since = "1.2.0")]
269 impl<T: Eq + Copy> Eq for Cell<T> {}
271 #[stable(feature = "cell_ord", since = "1.10.0")]
272 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
274 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
275 self.get().partial_cmp(&other.get())
279 fn lt(&self, other: &Cell<T>) -> bool {
280 self.get() < other.get()
284 fn le(&self, other: &Cell<T>) -> bool {
285 self.get() <= other.get()
289 fn gt(&self, other: &Cell<T>) -> bool {
290 self.get() > other.get()
294 fn ge(&self, other: &Cell<T>) -> bool {
295 self.get() >= other.get()
299 #[stable(feature = "cell_ord", since = "1.10.0")]
300 impl<T: Ord + Copy> Ord for Cell<T> {
302 fn cmp(&self, other: &Cell<T>) -> Ordering {
303 self.get().cmp(&other.get())
307 #[stable(feature = "cell_from", since = "1.12.0")]
308 impl<T> From<T> for Cell<T> {
309 fn from(t: T) -> Cell<T> {
315 /// Creates a new `Cell` containing the given value.
320 /// use std::cell::Cell;
322 /// let c = Cell::new(5);
324 #[stable(feature = "rust1", since = "1.0.0")]
325 #[rustc_const_stable(feature = "const_cell_new", since = "1.32.0")]
327 pub const fn new(value: T) -> Cell<T> {
328 Cell { value: UnsafeCell::new(value) }
331 /// Sets the contained value.
336 /// use std::cell::Cell;
338 /// let c = Cell::new(5);
343 #[stable(feature = "rust1", since = "1.0.0")]
344 pub fn set(&self, val: T) {
345 let old = self.replace(val);
349 /// Swaps the values of two Cells.
350 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
355 /// use std::cell::Cell;
357 /// let c1 = Cell::new(5i32);
358 /// let c2 = Cell::new(10i32);
360 /// assert_eq!(10, c1.get());
361 /// assert_eq!(5, c2.get());
364 #[stable(feature = "move_cell", since = "1.17.0")]
365 pub fn swap(&self, other: &Self) {
366 if ptr::eq(self, other) {
369 // SAFETY: not threadsafe, but it's OK since we know `Cell` isn't threadsafe
371 ptr::swap(self.value.get(), other.value.get());
375 /// Replaces the contained value, and returns it.
380 /// use std::cell::Cell;
382 /// let cell = Cell::new(5);
383 /// assert_eq!(cell.get(), 5);
384 /// assert_eq!(cell.replace(10), 5);
385 /// assert_eq!(cell.get(), 10);
387 #[stable(feature = "move_cell", since = "1.17.0")]
388 pub fn replace(&self, val: T) -> T {
389 // SAFETY: not threadsafe, but it's OK since we know `Cell` isn't threadsafe
390 mem::replace(unsafe { &mut *self.value.get() }, val)
393 /// Unwraps the value.
398 /// use std::cell::Cell;
400 /// let c = Cell::new(5);
401 /// let five = c.into_inner();
403 /// assert_eq!(five, 5);
405 #[stable(feature = "move_cell", since = "1.17.0")]
406 pub fn into_inner(self) -> T {
407 self.value.into_inner()
411 impl<T: Copy> Cell<T> {
412 /// Returns a copy of the contained value.
417 /// use std::cell::Cell;
419 /// let c = Cell::new(5);
421 /// let five = c.get();
424 #[stable(feature = "rust1", since = "1.0.0")]
425 pub fn get(&self) -> T {
426 // SAFETY: not threadsafe, but it's OK since we know `Cell` isn't threadsafe
427 unsafe { *self.value.get() }
430 /// Updates the contained value using a function and returns the new value.
435 /// #![feature(cell_update)]
437 /// use std::cell::Cell;
439 /// let c = Cell::new(5);
440 /// let new = c.update(|x| x + 1);
442 /// assert_eq!(new, 6);
443 /// assert_eq!(c.get(), 6);
446 #[unstable(feature = "cell_update", issue = "50186")]
447 pub fn update<F>(&self, f: F) -> T
451 let old = self.get();
458 impl<T: ?Sized> Cell<T> {
459 /// Returns a raw pointer to the underlying data in this cell.
464 /// use std::cell::Cell;
466 /// let c = Cell::new(5);
468 /// let ptr = c.as_ptr();
471 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
472 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
473 pub const fn as_ptr(&self) -> *mut T {
477 /// Returns a mutable reference to the underlying data.
479 /// This call borrows `Cell` mutably (at compile-time) which guarantees
480 /// that we possess the only reference.
485 /// use std::cell::Cell;
487 /// let mut c = Cell::new(5);
488 /// *c.get_mut() += 1;
490 /// assert_eq!(c.get(), 6);
493 #[stable(feature = "cell_get_mut", since = "1.11.0")]
494 pub fn get_mut(&mut self) -> &mut T {
495 // SAFETY: not threadsafe, but it's OK since we know `Cell` isn't threadsafe
496 unsafe { &mut *self.value.get() }
499 /// Returns a `&Cell<T>` from a `&mut T`
504 /// use std::cell::Cell;
506 /// let slice: &mut [i32] = &mut [1, 2, 3];
507 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
508 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
510 /// assert_eq!(slice_cell.len(), 3);
513 #[stable(feature = "as_cell", since = "1.37.0")]
514 pub fn from_mut(t: &mut T) -> &Cell<T> {
515 // SAFETY: `&mut` ensures unique access
516 unsafe { &*(t as *mut T as *const Cell<T>) }
520 impl<T: Default> Cell<T> {
521 /// Takes the value of the cell, leaving `Default::default()` in its place.
526 /// use std::cell::Cell;
528 /// let c = Cell::new(5);
529 /// let five = c.take();
531 /// assert_eq!(five, 5);
532 /// assert_eq!(c.into_inner(), 0);
534 #[stable(feature = "move_cell", since = "1.17.0")]
535 pub fn take(&self) -> T {
536 self.replace(Default::default())
540 #[unstable(feature = "coerce_unsized", issue = "27732")]
541 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
544 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
549 /// use std::cell::Cell;
551 /// let slice: &mut [i32] = &mut [1, 2, 3];
552 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
553 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
555 /// assert_eq!(slice_cell.len(), 3);
557 #[stable(feature = "as_cell", since = "1.37.0")]
558 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
559 // SAFETY: `Cell<T>` has the same memory layout as `T`
560 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
564 /// A mutable memory location with dynamically checked borrow rules
566 /// See the [module-level documentation](index.html) for more.
567 #[stable(feature = "rust1", since = "1.0.0")]
568 pub struct RefCell<T: ?Sized> {
569 borrow: Cell<BorrowFlag>,
570 value: UnsafeCell<T>,
573 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
574 #[stable(feature = "try_borrow", since = "1.13.0")]
575 pub struct BorrowError {
579 #[stable(feature = "try_borrow", since = "1.13.0")]
580 impl Debug for BorrowError {
581 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
582 f.debug_struct("BorrowError").finish()
586 #[stable(feature = "try_borrow", since = "1.13.0")]
587 impl Display for BorrowError {
588 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
589 Display::fmt("already mutably borrowed", f)
593 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
594 #[stable(feature = "try_borrow", since = "1.13.0")]
595 pub struct BorrowMutError {
599 #[stable(feature = "try_borrow", since = "1.13.0")]
600 impl Debug for BorrowMutError {
601 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
602 f.debug_struct("BorrowMutError").finish()
606 #[stable(feature = "try_borrow", since = "1.13.0")]
607 impl Display for BorrowMutError {
608 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
609 Display::fmt("already borrowed", f)
613 // Positive values represent the number of `Ref` active. Negative values
614 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
615 // active at a time if they refer to distinct, nonoverlapping components of a
616 // `RefCell` (e.g., different ranges of a slice).
618 // `Ref` and `RefMut` are both two words in size, and so there will likely never
619 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
620 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
621 // However, this is not a guarantee, as a pathological program could repeatedly
622 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
623 // explicitly check for overflow and underflow in order to avoid unsafety, or at
624 // least behave correctly in the event that overflow or underflow happens (e.g.,
625 // see BorrowRef::new).
626 type BorrowFlag = isize;
627 const UNUSED: BorrowFlag = 0;
630 fn is_writing(x: BorrowFlag) -> bool {
635 fn is_reading(x: BorrowFlag) -> bool {
640 /// Creates a new `RefCell` containing `value`.
645 /// use std::cell::RefCell;
647 /// let c = RefCell::new(5);
649 #[stable(feature = "rust1", since = "1.0.0")]
650 #[rustc_const_stable(feature = "const_refcell_new", since = "1.32.0")]
652 pub const fn new(value: T) -> RefCell<T> {
653 RefCell { value: UnsafeCell::new(value), borrow: Cell::new(UNUSED) }
656 /// Consumes the `RefCell`, returning the wrapped value.
661 /// use std::cell::RefCell;
663 /// let c = RefCell::new(5);
665 /// let five = c.into_inner();
667 #[stable(feature = "rust1", since = "1.0.0")]
669 pub fn into_inner(self) -> T {
670 // Since this function takes `self` (the `RefCell`) by value, the
671 // compiler statically verifies that it is not currently borrowed.
672 // Therefore the following assertion is just a `debug_assert!`.
673 debug_assert!(self.borrow.get() == UNUSED);
674 self.value.into_inner()
677 /// Replaces the wrapped value with a new one, returning the old value,
678 /// without deinitializing either one.
680 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
684 /// Panics if the value is currently borrowed.
689 /// use std::cell::RefCell;
690 /// let cell = RefCell::new(5);
691 /// let old_value = cell.replace(6);
692 /// assert_eq!(old_value, 5);
693 /// assert_eq!(cell, RefCell::new(6));
696 #[stable(feature = "refcell_replace", since = "1.24.0")]
697 pub fn replace(&self, t: T) -> T {
698 mem::replace(&mut *self.borrow_mut(), t)
701 /// Replaces the wrapped value with a new one computed from `f`, returning
702 /// the old value, without deinitializing either one.
706 /// Panics if the value is currently borrowed.
711 /// use std::cell::RefCell;
712 /// let cell = RefCell::new(5);
713 /// let old_value = cell.replace_with(|&mut old| old + 1);
714 /// assert_eq!(old_value, 5);
715 /// assert_eq!(cell, RefCell::new(6));
718 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
719 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
720 let mut_borrow = &mut *self.borrow_mut();
721 let replacement = f(mut_borrow);
722 mem::replace(mut_borrow, replacement)
725 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
726 /// without deinitializing either one.
728 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
732 /// Panics if the value in either `RefCell` is currently borrowed.
737 /// use std::cell::RefCell;
738 /// let c = RefCell::new(5);
739 /// let d = RefCell::new(6);
741 /// assert_eq!(c, RefCell::new(6));
742 /// assert_eq!(d, RefCell::new(5));
745 #[stable(feature = "refcell_swap", since = "1.24.0")]
746 pub fn swap(&self, other: &Self) {
747 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
751 impl<T: ?Sized> RefCell<T> {
752 /// Immutably borrows the wrapped value.
754 /// The borrow lasts until the returned `Ref` exits scope. Multiple
755 /// immutable borrows can be taken out at the same time.
759 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
760 /// [`try_borrow`](#method.try_borrow).
765 /// use std::cell::RefCell;
767 /// let c = RefCell::new(5);
769 /// let borrowed_five = c.borrow();
770 /// let borrowed_five2 = c.borrow();
773 /// An example of panic:
776 /// use std::cell::RefCell;
779 /// let result = thread::spawn(move || {
780 /// let c = RefCell::new(5);
781 /// let m = c.borrow_mut();
783 /// let b = c.borrow(); // this causes a panic
786 /// assert!(result.is_err());
788 #[stable(feature = "rust1", since = "1.0.0")]
790 pub fn borrow(&self) -> Ref<'_, T> {
791 self.try_borrow().expect("already mutably borrowed")
794 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
797 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
798 /// taken out at the same time.
800 /// This is the non-panicking variant of [`borrow`](#method.borrow).
805 /// use std::cell::RefCell;
807 /// let c = RefCell::new(5);
810 /// let m = c.borrow_mut();
811 /// assert!(c.try_borrow().is_err());
815 /// let m = c.borrow();
816 /// assert!(c.try_borrow().is_ok());
819 #[stable(feature = "try_borrow", since = "1.13.0")]
821 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
822 match BorrowRef::new(&self.borrow) {
823 // SAFETY: `BorrowRef` ensures that there is only immutable access
824 // to the value while borrowed
825 Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }),
826 None => Err(BorrowError { _private: () }),
830 /// Mutably borrows the wrapped value.
832 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
833 /// from it exit scope. The value cannot be borrowed while this borrow is
838 /// Panics if the value is currently borrowed. For a non-panicking variant, use
839 /// [`try_borrow_mut`](#method.try_borrow_mut).
844 /// use std::cell::RefCell;
846 /// let c = RefCell::new(5);
848 /// *c.borrow_mut() = 7;
850 /// assert_eq!(*c.borrow(), 7);
853 /// An example of panic:
856 /// use std::cell::RefCell;
859 /// let result = thread::spawn(move || {
860 /// let c = RefCell::new(5);
861 /// let m = c.borrow();
863 /// let b = c.borrow_mut(); // this causes a panic
866 /// assert!(result.is_err());
868 #[stable(feature = "rust1", since = "1.0.0")]
870 pub fn borrow_mut(&self) -> RefMut<'_, T> {
871 self.try_borrow_mut().expect("already borrowed")
874 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
876 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
877 /// from it exit scope. The value cannot be borrowed while this borrow is
880 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
885 /// use std::cell::RefCell;
887 /// let c = RefCell::new(5);
890 /// let m = c.borrow();
891 /// assert!(c.try_borrow_mut().is_err());
894 /// assert!(c.try_borrow_mut().is_ok());
896 #[stable(feature = "try_borrow", since = "1.13.0")]
898 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
899 match BorrowRefMut::new(&self.borrow) {
900 // SAFETY: `BorrowRef` gurantees unique access
901 Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }),
902 None => Err(BorrowMutError { _private: () }),
906 /// Returns a raw pointer to the underlying data in this cell.
911 /// use std::cell::RefCell;
913 /// let c = RefCell::new(5);
915 /// let ptr = c.as_ptr();
918 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
919 pub fn as_ptr(&self) -> *mut T {
923 /// Returns a mutable reference to the underlying data.
925 /// This call borrows `RefCell` mutably (at compile-time) so there is no
926 /// need for dynamic checks.
928 /// However be cautious: this method expects `self` to be mutable, which is
929 /// generally not the case when using a `RefCell`. Take a look at the
930 /// [`borrow_mut`] method instead if `self` isn't mutable.
932 /// Also, please be aware that this method is only for special circumstances and is usually
933 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
935 /// [`borrow_mut`]: #method.borrow_mut
940 /// use std::cell::RefCell;
942 /// let mut c = RefCell::new(5);
943 /// *c.get_mut() += 1;
945 /// assert_eq!(c, RefCell::new(6));
948 #[stable(feature = "cell_get_mut", since = "1.11.0")]
949 pub fn get_mut(&mut self) -> &mut T {
950 // SAFETY: `&mut` guarantees unique access
951 unsafe { &mut *self.value.get() }
954 /// Immutably borrows the wrapped value, returning an error if the value is
955 /// currently mutably borrowed.
959 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
960 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
961 /// borrowing the `RefCell` while the reference returned by this method
962 /// is alive is undefined behaviour.
967 /// use std::cell::RefCell;
969 /// let c = RefCell::new(5);
972 /// let m = c.borrow_mut();
973 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
977 /// let m = c.borrow();
978 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
981 #[stable(feature = "borrow_state", since = "1.37.0")]
983 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
984 if !is_writing(self.borrow.get()) {
985 Ok(&*self.value.get())
987 Err(BorrowError { _private: () })
992 #[stable(feature = "rust1", since = "1.0.0")]
993 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
995 #[stable(feature = "rust1", since = "1.0.0")]
996 impl<T: ?Sized> !Sync for RefCell<T> {}
998 #[stable(feature = "rust1", since = "1.0.0")]
999 impl<T: Clone> Clone for RefCell<T> {
1002 /// Panics if the value is currently mutably borrowed.
1004 fn clone(&self) -> RefCell<T> {
1005 RefCell::new(self.borrow().clone())
1009 #[stable(feature = "rust1", since = "1.0.0")]
1010 impl<T: Default> Default for RefCell<T> {
1011 /// Creates a `RefCell<T>`, with the `Default` value for T.
1013 fn default() -> RefCell<T> {
1014 RefCell::new(Default::default())
1018 #[stable(feature = "rust1", since = "1.0.0")]
1019 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1022 /// Panics if the value in either `RefCell` is currently borrowed.
1024 fn eq(&self, other: &RefCell<T>) -> bool {
1025 *self.borrow() == *other.borrow()
1029 #[stable(feature = "cell_eq", since = "1.2.0")]
1030 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1032 #[stable(feature = "cell_ord", since = "1.10.0")]
1033 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1036 /// Panics if the value in either `RefCell` is currently borrowed.
1038 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1039 self.borrow().partial_cmp(&*other.borrow())
1044 /// Panics if the value in either `RefCell` is currently borrowed.
1046 fn lt(&self, other: &RefCell<T>) -> bool {
1047 *self.borrow() < *other.borrow()
1052 /// Panics if the value in either `RefCell` is currently borrowed.
1054 fn le(&self, other: &RefCell<T>) -> bool {
1055 *self.borrow() <= *other.borrow()
1060 /// Panics if the value in either `RefCell` is currently borrowed.
1062 fn gt(&self, other: &RefCell<T>) -> bool {
1063 *self.borrow() > *other.borrow()
1068 /// Panics if the value in either `RefCell` is currently borrowed.
1070 fn ge(&self, other: &RefCell<T>) -> bool {
1071 *self.borrow() >= *other.borrow()
1075 #[stable(feature = "cell_ord", since = "1.10.0")]
1076 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1079 /// Panics if the value in either `RefCell` is currently borrowed.
1081 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1082 self.borrow().cmp(&*other.borrow())
1086 #[stable(feature = "cell_from", since = "1.12.0")]
1087 impl<T> From<T> for RefCell<T> {
1088 fn from(t: T) -> RefCell<T> {
1093 #[unstable(feature = "coerce_unsized", issue = "27732")]
1094 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1096 struct BorrowRef<'b> {
1097 borrow: &'b Cell<BorrowFlag>,
1100 impl<'b> BorrowRef<'b> {
1102 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1103 let b = borrow.get().wrapping_add(1);
1105 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1106 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1107 // due to Rust's reference aliasing rules
1108 // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
1109 // into isize::min_value() (the max amount of writing borrows) so we can't allow
1110 // an additional read borrow because isize can't represent so many read borrows
1111 // (this can only happen if you mem::forget more than a small constant amount of
1112 // `Ref`s, which is not good practice)
1115 // Incrementing borrow can result in a reading value (> 0) in these cases:
1116 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1117 // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
1118 // is large enough to represent having one more read borrow
1120 Some(BorrowRef { borrow })
1125 impl Drop for BorrowRef<'_> {
1127 fn drop(&mut self) {
1128 let borrow = self.borrow.get();
1129 debug_assert!(is_reading(borrow));
1130 self.borrow.set(borrow - 1);
1134 impl Clone for BorrowRef<'_> {
1136 fn clone(&self) -> Self {
1137 // Since this Ref exists, we know the borrow flag
1138 // is a reading borrow.
1139 let borrow = self.borrow.get();
1140 debug_assert!(is_reading(borrow));
1141 // Prevent the borrow counter from overflowing into
1142 // a writing borrow.
1143 assert!(borrow != isize::max_value());
1144 self.borrow.set(borrow + 1);
1145 BorrowRef { borrow: self.borrow }
1149 /// Wraps a borrowed reference to a value in a `RefCell` box.
1150 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1152 /// See the [module-level documentation](index.html) for more.
1153 #[stable(feature = "rust1", since = "1.0.0")]
1154 pub struct Ref<'b, T: ?Sized + 'b> {
1156 borrow: BorrowRef<'b>,
1159 #[stable(feature = "rust1", since = "1.0.0")]
1160 impl<T: ?Sized> Deref for Ref<'_, T> {
1164 fn deref(&self) -> &T {
1169 impl<'b, T: ?Sized> Ref<'b, T> {
1172 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1174 /// This is an associated function that needs to be used as
1175 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1176 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1178 #[stable(feature = "cell_extras", since = "1.15.0")]
1180 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1181 Ref { value: orig.value, borrow: orig.borrow.clone() }
1184 /// Makes a new `Ref` for a component of the borrowed data.
1186 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1188 /// This is an associated function that needs to be used as `Ref::map(...)`.
1189 /// A method would interfere with methods of the same name on the contents
1190 /// of a `RefCell` used through `Deref`.
1195 /// use std::cell::{RefCell, Ref};
1197 /// let c = RefCell::new((5, 'b'));
1198 /// let b1: Ref<(u32, char)> = c.borrow();
1199 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1200 /// assert_eq!(*b2, 5)
1202 #[stable(feature = "cell_map", since = "1.8.0")]
1204 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1206 F: FnOnce(&T) -> &U,
1208 Ref { value: f(orig.value), borrow: orig.borrow }
1211 /// Splits a `Ref` into multiple `Ref`s for different components of the
1214 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1216 /// This is an associated function that needs to be used as
1217 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1218 /// name on the contents of a `RefCell` used through `Deref`.
1223 /// use std::cell::{Ref, RefCell};
1225 /// let cell = RefCell::new([1, 2, 3, 4]);
1226 /// let borrow = cell.borrow();
1227 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1228 /// assert_eq!(*begin, [1, 2]);
1229 /// assert_eq!(*end, [3, 4]);
1231 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1233 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1235 F: FnOnce(&T) -> (&U, &V),
1237 let (a, b) = f(orig.value);
1238 let borrow = orig.borrow.clone();
1239 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1243 #[unstable(feature = "coerce_unsized", issue = "27732")]
1244 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1246 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1247 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1248 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1253 impl<'b, T: ?Sized> RefMut<'b, T> {
1254 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1257 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1259 /// This is an associated function that needs to be used as
1260 /// `RefMut::map(...)`. A method would interfere with methods of the same
1261 /// name on the contents of a `RefCell` used through `Deref`.
1266 /// use std::cell::{RefCell, RefMut};
1268 /// let c = RefCell::new((5, 'b'));
1270 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1271 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1272 /// assert_eq!(*b2, 5);
1275 /// assert_eq!(*c.borrow(), (42, 'b'));
1277 #[stable(feature = "cell_map", since = "1.8.0")]
1279 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1281 F: FnOnce(&mut T) -> &mut U,
1283 // FIXME(nll-rfc#40): fix borrow-check
1284 let RefMut { value, borrow } = orig;
1285 RefMut { value: f(value), borrow }
1288 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1291 /// The underlying `RefCell` will remain mutably borrowed until both
1292 /// returned `RefMut`s go out of scope.
1294 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1296 /// This is an associated function that needs to be used as
1297 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1298 /// same name on the contents of a `RefCell` used through `Deref`.
1303 /// use std::cell::{RefCell, RefMut};
1305 /// let cell = RefCell::new([1, 2, 3, 4]);
1306 /// let borrow = cell.borrow_mut();
1307 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1308 /// assert_eq!(*begin, [1, 2]);
1309 /// assert_eq!(*end, [3, 4]);
1310 /// begin.copy_from_slice(&[4, 3]);
1311 /// end.copy_from_slice(&[2, 1]);
1313 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1315 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1316 orig: RefMut<'b, T>,
1318 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1320 F: FnOnce(&mut T) -> (&mut U, &mut V),
1322 let (a, b) = f(orig.value);
1323 let borrow = orig.borrow.clone();
1324 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1328 struct BorrowRefMut<'b> {
1329 borrow: &'b Cell<BorrowFlag>,
1332 impl Drop for BorrowRefMut<'_> {
1334 fn drop(&mut self) {
1335 let borrow = self.borrow.get();
1336 debug_assert!(is_writing(borrow));
1337 self.borrow.set(borrow + 1);
1341 impl<'b> BorrowRefMut<'b> {
1343 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1344 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1345 // mutable reference, and so there must currently be no existing
1346 // references. Thus, while clone increments the mutable refcount, here
1347 // we explicitly only allow going from UNUSED to UNUSED - 1.
1348 match borrow.get() {
1350 borrow.set(UNUSED - 1);
1351 Some(BorrowRefMut { borrow })
1357 // Clones a `BorrowRefMut`.
1359 // This is only valid if each `BorrowRefMut` is used to track a mutable
1360 // reference to a distinct, nonoverlapping range of the original object.
1361 // This isn't in a Clone impl so that code doesn't call this implicitly.
1363 fn clone(&self) -> BorrowRefMut<'b> {
1364 let borrow = self.borrow.get();
1365 debug_assert!(is_writing(borrow));
1366 // Prevent the borrow counter from underflowing.
1367 assert!(borrow != isize::min_value());
1368 self.borrow.set(borrow - 1);
1369 BorrowRefMut { borrow: self.borrow }
1373 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1375 /// See the [module-level documentation](index.html) for more.
1376 #[stable(feature = "rust1", since = "1.0.0")]
1377 pub struct RefMut<'b, T: ?Sized + 'b> {
1379 borrow: BorrowRefMut<'b>,
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<T: ?Sized> Deref for RefMut<'_, T> {
1387 fn deref(&self) -> &T {
1392 #[stable(feature = "rust1", since = "1.0.0")]
1393 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1395 fn deref_mut(&mut self) -> &mut T {
1400 #[unstable(feature = "coerce_unsized", issue = "27732")]
1401 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1403 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1404 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1405 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1410 /// The core primitive for interior mutability in Rust.
1412 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1413 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1414 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1415 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1417 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1418 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1419 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1420 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1421 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1422 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1424 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1425 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1427 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1429 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1430 /// reference) that is accessible by safe code (for example, because you returned it),
1431 /// then you must not access the data in any way that contradicts that reference for the
1432 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1433 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1434 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1435 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1436 /// safe code, then you must not access the data within the `UnsafeCell` until that
1437 /// reference expires.
1439 /// - At all times, you must avoid data races. If multiple threads have access to
1440 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1441 /// accesses (or use atomics).
1443 /// To assist with proper design, the following scenarios are explicitly declared legal
1444 /// for single-threaded code:
1446 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1447 /// references, but not with a `&mut T`
1449 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1450 /// co-exist with it. A `&mut T` must always be unique.
1452 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1453 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1454 /// to have multiple `&mut UnsafeCell<T>` aliases.
1459 /// use std::cell::UnsafeCell;
1461 /// # #[allow(dead_code)]
1462 /// struct NotThreadSafe<T> {
1463 /// value: UnsafeCell<T>,
1466 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1468 #[lang = "unsafe_cell"]
1469 #[stable(feature = "rust1", since = "1.0.0")]
1470 #[repr(transparent)]
1471 pub struct UnsafeCell<T: ?Sized> {
1475 #[stable(feature = "rust1", since = "1.0.0")]
1476 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1478 impl<T> UnsafeCell<T> {
1479 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1482 /// All access to the inner value through methods is `unsafe`.
1487 /// use std::cell::UnsafeCell;
1489 /// let uc = UnsafeCell::new(5);
1491 #[stable(feature = "rust1", since = "1.0.0")]
1492 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1494 pub const fn new(value: T) -> UnsafeCell<T> {
1495 UnsafeCell { value }
1498 /// Unwraps the value.
1503 /// use std::cell::UnsafeCell;
1505 /// let uc = UnsafeCell::new(5);
1507 /// let five = uc.into_inner();
1510 #[stable(feature = "rust1", since = "1.0.0")]
1511 pub fn into_inner(self) -> T {
1516 impl<T: ?Sized> UnsafeCell<T> {
1517 /// Gets a mutable pointer to the wrapped value.
1519 /// This can be cast to a pointer of any kind.
1520 /// Ensure that the access is unique (no active references, mutable or not)
1521 /// when casting to `&mut T`, and ensure that there are no mutations
1522 /// or mutable aliases going on when casting to `&T`
1527 /// use std::cell::UnsafeCell;
1529 /// let uc = UnsafeCell::new(5);
1531 /// let five = uc.get();
1534 #[stable(feature = "rust1", since = "1.0.0")]
1535 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1536 pub const fn get(&self) -> *mut T {
1537 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1538 // #[repr(transparent)]. This exploits libstd's special status, there is
1539 // no guarantee for user code that this will work in future versions of the compiler!
1540 self as *const UnsafeCell<T> as *const T as *mut T
1543 /// Gets a mutable pointer to the wrapped value.
1544 /// The difference to [`get`] is that this function accepts a raw pointer,
1545 /// which is useful to avoid the creation of temporary references.
1547 /// The result can be cast to a pointer of any kind.
1548 /// Ensure that the access is unique (no active references, mutable or not)
1549 /// when casting to `&mut T`, and ensure that there are no mutations
1550 /// or mutable aliases going on when casting to `&T`.
1552 /// [`get`]: #method.get
1556 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1557 /// calling `get` would require creating a reference to uninitialized data:
1560 /// #![feature(unsafe_cell_raw_get)]
1561 /// use std::cell::UnsafeCell;
1562 /// use std::mem::MaybeUninit;
1564 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1565 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1566 /// let uc = unsafe { m.assume_init() };
1568 /// assert_eq!(uc.into_inner(), 5);
1571 #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
1572 pub const fn raw_get(this: *const Self) -> *mut T {
1573 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1574 // #[repr(transparent)]. This exploits libstd's special status, there is
1575 // no guarantee for user code that this will work in future versions of the compiler!
1576 this as *const T as *mut T
1580 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1581 impl<T: Default> Default for UnsafeCell<T> {
1582 /// Creates an `UnsafeCell`, with the `Default` value for T.
1583 fn default() -> UnsafeCell<T> {
1584 UnsafeCell::new(Default::default())
1588 #[stable(feature = "cell_from", since = "1.12.0")]
1589 impl<T> From<T> for UnsafeCell<T> {
1590 fn from(t: T) -> UnsafeCell<T> {
1595 #[unstable(feature = "coerce_unsized", issue = "27732")]
1596 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1599 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1600 let _: UnsafeCell<&dyn Send> = a;
1601 let _: Cell<&dyn Send> = b;
1602 let _: RefCell<&dyn Send> = c;