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 // ignore-tidy-undocumented-unsafe
192 #![stable(feature = "rust1", since = "1.0.0")]
194 use crate::cmp
::Ordering
;
195 use crate::fmt
::{self, Debug, Display}
;
196 use crate::marker
::Unsize
;
198 use crate::ops
::{Deref, DerefMut, CoerceUnsized}
;
201 /// A mutable memory location.
205 /// In this example, you can see that `Cell<T>` enables mutation inside an
206 /// immutable struct. In other words, it enables "interior mutability".
209 /// use std::cell::Cell;
211 /// struct SomeStruct {
212 /// regular_field: u8,
213 /// special_field: Cell<u8>,
216 /// let my_struct = SomeStruct {
217 /// regular_field: 0,
218 /// special_field: Cell::new(1),
221 /// let new_value = 100;
223 /// // ERROR: `my_struct` is immutable
224 /// // my_struct.regular_field = new_value;
226 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
227 /// // which can always be mutated
228 /// my_struct.special_field.set(new_value);
229 /// assert_eq!(my_struct.special_field.get(), new_value);
232 /// See the [module-level documentation](index.html) for more.
233 #[stable(feature = "rust1", since = "1.0.0")]
235 pub struct Cell
<T
: ?Sized
> {
236 value
: UnsafeCell
<T
>,
239 #[stable(feature = "rust1", since = "1.0.0")]
240 unsafe impl<T
: ?Sized
> Send
for Cell
<T
> where T
: Send {}
242 #[stable(feature = "rust1", since = "1.0.0")]
243 impl<T
: ?Sized
> !Sync
for Cell
<T
> {}
245 #[stable(feature = "rust1", since = "1.0.0")]
246 impl<T
:Copy
> Clone
for Cell
<T
> {
248 fn clone(&self) -> Cell
<T
> {
249 Cell
::new(self.get())
253 #[stable(feature = "rust1", since = "1.0.0")]
254 impl<T
: Default
> Default
for Cell
<T
> {
255 /// Creates a `Cell<T>`, with the `Default` value for T.
257 fn default() -> Cell
<T
> {
258 Cell
::new(Default
::default())
262 #[stable(feature = "rust1", since = "1.0.0")]
263 impl<T
: PartialEq
+ Copy
> PartialEq
for Cell
<T
> {
265 fn eq(&self, other
: &Cell
<T
>) -> bool
{
266 self.get() == other
.get()
270 #[stable(feature = "cell_eq", since = "1.2.0")]
271 impl<T
: Eq
+ Copy
> Eq
for Cell
<T
> {}
273 #[stable(feature = "cell_ord", since = "1.10.0")]
274 impl<T
: PartialOrd
+ Copy
> PartialOrd
for Cell
<T
> {
276 fn partial_cmp(&self, other
: &Cell
<T
>) -> Option
<Ordering
> {
277 self.get().partial_cmp(&other
.get())
281 fn lt(&self, other
: &Cell
<T
>) -> bool
{
282 self.get() < other
.get()
286 fn le(&self, other
: &Cell
<T
>) -> bool
{
287 self.get() <= other
.get()
291 fn gt(&self, other
: &Cell
<T
>) -> bool
{
292 self.get() > other
.get()
296 fn ge(&self, other
: &Cell
<T
>) -> bool
{
297 self.get() >= other
.get()
301 #[stable(feature = "cell_ord", since = "1.10.0")]
302 impl<T
: Ord
+ Copy
> Ord
for Cell
<T
> {
304 fn cmp(&self, other
: &Cell
<T
>) -> Ordering
{
305 self.get().cmp(&other
.get())
309 #[stable(feature = "cell_from", since = "1.12.0")]
310 impl<T
> From
<T
> for Cell
<T
> {
311 fn from(t
: T
) -> Cell
<T
> {
317 /// Creates a new `Cell` containing the given value.
322 /// use std::cell::Cell;
324 /// let c = Cell::new(5);
326 #[stable(feature = "rust1", since = "1.0.0")]
327 #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_cell_new", since = "1.32.0"))]
329 pub const fn new(value
: T
) -> Cell
<T
> {
331 value
: UnsafeCell
::new(value
),
335 /// Sets the contained value.
340 /// use std::cell::Cell;
342 /// let c = Cell::new(5);
347 #[stable(feature = "rust1", since = "1.0.0")]
348 pub fn set(&self, val
: T
) {
349 let old
= self.replace(val
);
353 /// Swaps the values of two Cells.
354 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
359 /// use std::cell::Cell;
361 /// let c1 = Cell::new(5i32);
362 /// let c2 = Cell::new(10i32);
364 /// assert_eq!(10, c1.get());
365 /// assert_eq!(5, c2.get());
368 #[stable(feature = "move_cell", since = "1.17.0")]
369 pub fn swap(&self, other
: &Self) {
370 if ptr
::eq(self, other
) {
374 ptr
::swap(self.value
.get(), other
.value
.get());
378 /// Replaces the contained value, and returns it.
383 /// use std::cell::Cell;
385 /// let cell = Cell::new(5);
386 /// assert_eq!(cell.get(), 5);
387 /// assert_eq!(cell.replace(10), 5);
388 /// assert_eq!(cell.get(), 10);
390 #[stable(feature = "move_cell", since = "1.17.0")]
391 pub fn replace(&self, val
: T
) -> T
{
392 mem
::replace(unsafe { &mut *self.value.get() }
, val
)
395 /// Unwraps the value.
400 /// use std::cell::Cell;
402 /// let c = Cell::new(5);
403 /// let five = c.into_inner();
405 /// assert_eq!(five, 5);
407 #[stable(feature = "move_cell", since = "1.17.0")]
408 pub fn into_inner(self) -> T
{
409 self.value
.into_inner()
413 impl<T
:Copy
> Cell
<T
> {
414 /// Returns a copy of the contained value.
419 /// use std::cell::Cell;
421 /// let c = Cell::new(5);
423 /// let five = c.get();
426 #[stable(feature = "rust1", since = "1.0.0")]
427 pub fn get(&self) -> T
{
428 unsafe{ *self.value.get() }
431 /// Updates the contained value using a function and returns the new value.
436 /// #![feature(cell_update)]
438 /// use std::cell::Cell;
440 /// let c = Cell::new(5);
441 /// let new = c.update(|x| x + 1);
443 /// assert_eq!(new, 6);
444 /// assert_eq!(c.get(), 6);
447 #[unstable(feature = "cell_update", issue = "50186")]
448 pub fn update
<F
>(&self, f
: F
) -> T
452 let old
= self.get();
459 impl<T
: ?Sized
> Cell
<T
> {
460 /// Returns a raw pointer to the underlying data in this cell.
465 /// use std::cell::Cell;
467 /// let c = Cell::new(5);
469 /// let ptr = c.as_ptr();
472 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
473 #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0"))]
474 pub const fn as_ptr(&self) -> *mut T
{
478 /// Returns a mutable reference to the underlying data.
480 /// This call borrows `Cell` mutably (at compile-time) which guarantees
481 /// that we possess the only reference.
486 /// use std::cell::Cell;
488 /// let mut c = Cell::new(5);
489 /// *c.get_mut() += 1;
491 /// assert_eq!(c.get(), 6);
494 #[stable(feature = "cell_get_mut", since = "1.11.0")]
495 pub fn get_mut(&mut self) -> &mut T
{
497 &mut *self.value
.get()
501 /// Returns a `&Cell<T>` from a `&mut T`
506 /// use std::cell::Cell;
508 /// let slice: &mut [i32] = &mut [1, 2, 3];
509 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
510 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
512 /// assert_eq!(slice_cell.len(), 3);
515 #[stable(feature = "as_cell", since = "1.37.0")]
516 pub fn from_mut(t
: &mut T
) -> &Cell
<T
> {
518 &*(t
as *mut T
as *const Cell
<T
>)
523 impl<T
: Default
> Cell
<T
> {
524 /// Takes the value of the cell, leaving `Default::default()` in its place.
529 /// use std::cell::Cell;
531 /// let c = Cell::new(5);
532 /// let five = c.take();
534 /// assert_eq!(five, 5);
535 /// assert_eq!(c.into_inner(), 0);
537 #[stable(feature = "move_cell", since = "1.17.0")]
538 pub fn take(&self) -> T
{
539 self.replace(Default
::default())
543 #[unstable(feature = "coerce_unsized", issue = "27732")]
544 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<Cell
<U
>> for Cell
<T
> {}
547 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
552 /// use std::cell::Cell;
554 /// let slice: &mut [i32] = &mut [1, 2, 3];
555 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
556 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
558 /// assert_eq!(slice_cell.len(), 3);
560 #[stable(feature = "as_cell", since = "1.37.0")]
561 pub fn as_slice_of_cells(&self) -> &[Cell
<T
>] {
563 &*(self as *const Cell
<[T
]> as *const [Cell
<T
>])
568 /// A mutable memory location with dynamically checked borrow rules
570 /// See the [module-level documentation](index.html) for more.
571 #[stable(feature = "rust1", since = "1.0.0")]
572 pub struct RefCell
<T
: ?Sized
> {
573 borrow
: Cell
<BorrowFlag
>,
574 value
: UnsafeCell
<T
>,
577 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
578 #[stable(feature = "try_borrow", since = "1.13.0")]
579 pub struct BorrowError
{
583 #[stable(feature = "try_borrow", since = "1.13.0")]
584 impl Debug
for BorrowError
{
585 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
586 f
.debug_struct("BorrowError").finish()
590 #[stable(feature = "try_borrow", since = "1.13.0")]
591 impl Display
for BorrowError
{
592 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
593 Display
::fmt("already mutably borrowed", f
)
597 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
598 #[stable(feature = "try_borrow", since = "1.13.0")]
599 pub struct BorrowMutError
{
603 #[stable(feature = "try_borrow", since = "1.13.0")]
604 impl Debug
for BorrowMutError
{
605 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
606 f
.debug_struct("BorrowMutError").finish()
610 #[stable(feature = "try_borrow", since = "1.13.0")]
611 impl Display
for BorrowMutError
{
612 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
613 Display
::fmt("already borrowed", f
)
617 // Positive values represent the number of `Ref` active. Negative values
618 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
619 // active at a time if they refer to distinct, nonoverlapping components of a
620 // `RefCell` (e.g., different ranges of a slice).
622 // `Ref` and `RefMut` are both two words in size, and so there will likely never
623 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
624 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
625 // However, this is not a guarantee, as a pathological program could repeatedly
626 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
627 // explicitly check for overflow and underflow in order to avoid unsafety, or at
628 // least behave correctly in the event that overflow or underflow happens (e.g.,
629 // see BorrowRef::new).
630 type BorrowFlag
= isize;
631 const UNUSED
: BorrowFlag
= 0;
634 fn is_writing(x
: BorrowFlag
) -> bool
{
639 fn is_reading(x
: BorrowFlag
) -> bool
{
644 /// Creates a new `RefCell` containing `value`.
649 /// use std::cell::RefCell;
651 /// let c = RefCell::new(5);
653 #[stable(feature = "rust1", since = "1.0.0")]
654 #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_refcell_new", since = "1.32.0"))]
656 pub const fn new(value
: T
) -> RefCell
<T
> {
658 value
: UnsafeCell
::new(value
),
659 borrow
: Cell
::new(UNUSED
),
663 /// Consumes the `RefCell`, returning the wrapped value.
668 /// use std::cell::RefCell;
670 /// let c = RefCell::new(5);
672 /// let five = c.into_inner();
674 #[stable(feature = "rust1", since = "1.0.0")]
676 pub fn into_inner(self) -> T
{
677 // Since this function takes `self` (the `RefCell`) by value, the
678 // compiler statically verifies that it is not currently borrowed.
679 // Therefore the following assertion is just a `debug_assert!`.
680 debug_assert
!(self.borrow
.get() == UNUSED
);
681 self.value
.into_inner()
684 /// Replaces the wrapped value with a new one, returning the old value,
685 /// without deinitializing either one.
687 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
691 /// Panics if the value is currently borrowed.
696 /// use std::cell::RefCell;
697 /// let cell = RefCell::new(5);
698 /// let old_value = cell.replace(6);
699 /// assert_eq!(old_value, 5);
700 /// assert_eq!(cell, RefCell::new(6));
703 #[stable(feature = "refcell_replace", since="1.24.0")]
704 pub fn replace(&self, t
: T
) -> T
{
705 mem
::replace(&mut *self.borrow_mut(), t
)
708 /// Replaces the wrapped value with a new one computed from `f`, returning
709 /// the old value, without deinitializing either one.
713 /// Panics if the value is currently borrowed.
718 /// use std::cell::RefCell;
719 /// let cell = RefCell::new(5);
720 /// let old_value = cell.replace_with(|&mut old| old + 1);
721 /// assert_eq!(old_value, 5);
722 /// assert_eq!(cell, RefCell::new(6));
725 #[stable(feature = "refcell_replace_swap", since="1.35.0")]
726 pub fn replace_with
<F
: FnOnce(&mut T
) -> T
>(&self, f
: F
) -> T
{
727 let mut_borrow
= &mut *self.borrow_mut();
728 let replacement
= f(mut_borrow
);
729 mem
::replace(mut_borrow
, replacement
)
732 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
733 /// without deinitializing either one.
735 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
739 /// Panics if the value in either `RefCell` is currently borrowed.
744 /// use std::cell::RefCell;
745 /// let c = RefCell::new(5);
746 /// let d = RefCell::new(6);
748 /// assert_eq!(c, RefCell::new(6));
749 /// assert_eq!(d, RefCell::new(5));
752 #[stable(feature = "refcell_swap", since="1.24.0")]
753 pub fn swap(&self, other
: &Self) {
754 mem
::swap(&mut *self.borrow_mut(), &mut *other
.borrow_mut())
758 impl<T
: ?Sized
> RefCell
<T
> {
759 /// Immutably borrows the wrapped value.
761 /// The borrow lasts until the returned `Ref` exits scope. Multiple
762 /// immutable borrows can be taken out at the same time.
766 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
767 /// [`try_borrow`](#method.try_borrow).
772 /// use std::cell::RefCell;
774 /// let c = RefCell::new(5);
776 /// let borrowed_five = c.borrow();
777 /// let borrowed_five2 = c.borrow();
780 /// An example of panic:
783 /// use std::cell::RefCell;
786 /// let result = thread::spawn(move || {
787 /// let c = RefCell::new(5);
788 /// let m = c.borrow_mut();
790 /// let b = c.borrow(); // this causes a panic
793 /// assert!(result.is_err());
795 #[stable(feature = "rust1", since = "1.0.0")]
797 pub fn borrow(&self) -> Ref
<'_
, T
> {
798 self.try_borrow().expect("already mutably borrowed")
801 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
804 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
805 /// taken out at the same time.
807 /// This is the non-panicking variant of [`borrow`](#method.borrow).
812 /// use std::cell::RefCell;
814 /// let c = RefCell::new(5);
817 /// let m = c.borrow_mut();
818 /// assert!(c.try_borrow().is_err());
822 /// let m = c.borrow();
823 /// assert!(c.try_borrow().is_ok());
826 #[stable(feature = "try_borrow", since = "1.13.0")]
828 pub fn try_borrow(&self) -> Result
<Ref
<'_
, T
>, BorrowError
> {
829 match BorrowRef
::new(&self.borrow
) {
831 value
: unsafe { &*self.value.get() }
,
834 None
=> Err(BorrowError { _private: () }
),
838 /// Mutably borrows the wrapped value.
840 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
841 /// from it exit scope. The value cannot be borrowed while this borrow is
846 /// Panics if the value is currently borrowed. For a non-panicking variant, use
847 /// [`try_borrow_mut`](#method.try_borrow_mut).
852 /// use std::cell::RefCell;
854 /// let c = RefCell::new(5);
856 /// *c.borrow_mut() = 7;
858 /// assert_eq!(*c.borrow(), 7);
861 /// An example of panic:
864 /// use std::cell::RefCell;
867 /// let result = thread::spawn(move || {
868 /// let c = RefCell::new(5);
869 /// let m = c.borrow();
871 /// let b = c.borrow_mut(); // this causes a panic
874 /// assert!(result.is_err());
876 #[stable(feature = "rust1", since = "1.0.0")]
878 pub fn borrow_mut(&self) -> RefMut
<'_
, T
> {
879 self.try_borrow_mut().expect("already borrowed")
882 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
884 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
885 /// from it exit scope. The value cannot be borrowed while this borrow is
888 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
893 /// use std::cell::RefCell;
895 /// let c = RefCell::new(5);
898 /// let m = c.borrow();
899 /// assert!(c.try_borrow_mut().is_err());
902 /// assert!(c.try_borrow_mut().is_ok());
904 #[stable(feature = "try_borrow", since = "1.13.0")]
906 pub fn try_borrow_mut(&self) -> Result
<RefMut
<'_
, T
>, BorrowMutError
> {
907 match BorrowRefMut
::new(&self.borrow
) {
908 Some(b
) => Ok(RefMut
{
909 value
: unsafe { &mut *self.value.get() }
,
912 None
=> Err(BorrowMutError { _private: () }
),
916 /// Returns a raw pointer to the underlying data in this cell.
921 /// use std::cell::RefCell;
923 /// let c = RefCell::new(5);
925 /// let ptr = c.as_ptr();
928 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
929 pub fn as_ptr(&self) -> *mut T
{
933 /// Returns a mutable reference to the underlying data.
935 /// This call borrows `RefCell` mutably (at compile-time) so there is no
936 /// need for dynamic checks.
938 /// However be cautious: this method expects `self` to be mutable, which is
939 /// generally not the case when using a `RefCell`. Take a look at the
940 /// [`borrow_mut`] method instead if `self` isn't mutable.
942 /// Also, please be aware that this method is only for special circumstances and is usually
943 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
945 /// [`borrow_mut`]: #method.borrow_mut
950 /// use std::cell::RefCell;
952 /// let mut c = RefCell::new(5);
953 /// *c.get_mut() += 1;
955 /// assert_eq!(c, RefCell::new(6));
958 #[stable(feature = "cell_get_mut", since = "1.11.0")]
959 pub fn get_mut(&mut self) -> &mut T
{
961 &mut *self.value
.get()
965 /// Immutably borrows the wrapped value, returning an error if the value is
966 /// currently mutably borrowed.
970 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
971 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
972 /// borrowing the `RefCell` while the reference returned by this method
973 /// is alive is undefined behaviour.
978 /// use std::cell::RefCell;
980 /// let c = RefCell::new(5);
983 /// let m = c.borrow_mut();
984 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
988 /// let m = c.borrow();
989 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
992 #[stable(feature = "borrow_state", since = "1.37.0")]
994 pub unsafe fn try_borrow_unguarded(&self) -> Result
<&T
, BorrowError
> {
995 if !is_writing(self.borrow
.get()) {
996 Ok(&*self.value
.get())
998 Err(BorrowError { _private: () }
)
1003 #[stable(feature = "rust1", since = "1.0.0")]
1004 unsafe impl<T
: ?Sized
> Send
for RefCell
<T
> where T
: Send {}
1006 #[stable(feature = "rust1", since = "1.0.0")]
1007 impl<T
: ?Sized
> !Sync
for RefCell
<T
> {}
1009 #[stable(feature = "rust1", since = "1.0.0")]
1010 impl<T
: Clone
> Clone
for RefCell
<T
> {
1013 /// Panics if the value is currently mutably borrowed.
1015 fn clone(&self) -> RefCell
<T
> {
1016 RefCell
::new(self.borrow().clone())
1020 #[stable(feature = "rust1", since = "1.0.0")]
1021 impl<T
: Default
> Default
for RefCell
<T
> {
1022 /// Creates a `RefCell<T>`, with the `Default` value for T.
1024 fn default() -> RefCell
<T
> {
1025 RefCell
::new(Default
::default())
1029 #[stable(feature = "rust1", since = "1.0.0")]
1030 impl<T
: ?Sized
+ PartialEq
> PartialEq
for RefCell
<T
> {
1033 /// Panics if the value in either `RefCell` is currently borrowed.
1035 fn eq(&self, other
: &RefCell
<T
>) -> bool
{
1036 *self.borrow() == *other
.borrow()
1040 #[stable(feature = "cell_eq", since = "1.2.0")]
1041 impl<T
: ?Sized
+ Eq
> Eq
for RefCell
<T
> {}
1043 #[stable(feature = "cell_ord", since = "1.10.0")]
1044 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for RefCell
<T
> {
1047 /// Panics if the value in either `RefCell` is currently borrowed.
1049 fn partial_cmp(&self, other
: &RefCell
<T
>) -> Option
<Ordering
> {
1050 self.borrow().partial_cmp(&*other
.borrow())
1055 /// Panics if the value in either `RefCell` is currently borrowed.
1057 fn lt(&self, other
: &RefCell
<T
>) -> bool
{
1058 *self.borrow() < *other
.borrow()
1063 /// Panics if the value in either `RefCell` is currently borrowed.
1065 fn le(&self, other
: &RefCell
<T
>) -> bool
{
1066 *self.borrow() <= *other
.borrow()
1071 /// Panics if the value in either `RefCell` is currently borrowed.
1073 fn gt(&self, other
: &RefCell
<T
>) -> bool
{
1074 *self.borrow() > *other
.borrow()
1079 /// Panics if the value in either `RefCell` is currently borrowed.
1081 fn ge(&self, other
: &RefCell
<T
>) -> bool
{
1082 *self.borrow() >= *other
.borrow()
1086 #[stable(feature = "cell_ord", since = "1.10.0")]
1087 impl<T
: ?Sized
+ Ord
> Ord
for RefCell
<T
> {
1090 /// Panics if the value in either `RefCell` is currently borrowed.
1092 fn cmp(&self, other
: &RefCell
<T
>) -> Ordering
{
1093 self.borrow().cmp(&*other
.borrow())
1097 #[stable(feature = "cell_from", since = "1.12.0")]
1098 impl<T
> From
<T
> for RefCell
<T
> {
1099 fn from(t
: T
) -> RefCell
<T
> {
1104 #[unstable(feature = "coerce_unsized", issue = "27732")]
1105 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<RefCell
<U
>> for RefCell
<T
> {}
1107 struct BorrowRef
<'b
> {
1108 borrow
: &'b Cell
<BorrowFlag
>,
1111 impl<'b
> BorrowRef
<'b
> {
1113 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRef
<'b
>> {
1114 let b
= borrow
.get().wrapping_add(1);
1116 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1117 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1118 // due to Rust's reference aliasing rules
1119 // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
1120 // into isize::min_value() (the max amount of writing borrows) so we can't allow
1121 // an additional read borrow because isize can't represent so many read borrows
1122 // (this can only happen if you mem::forget more than a small constant amount of
1123 // `Ref`s, which is not good practice)
1126 // Incrementing borrow can result in a reading value (> 0) in these cases:
1127 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1128 // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
1129 // is large enough to represent having one more read borrow
1131 Some(BorrowRef { borrow }
)
1136 impl Drop
for BorrowRef
<'_
> {
1138 fn drop(&mut self) {
1139 let borrow
= self.borrow
.get();
1140 debug_assert
!(is_reading(borrow
));
1141 self.borrow
.set(borrow
- 1);
1145 impl Clone
for BorrowRef
<'_
> {
1147 fn clone(&self) -> Self {
1148 // Since this Ref exists, we know the borrow flag
1149 // is a reading borrow.
1150 let borrow
= self.borrow
.get();
1151 debug_assert
!(is_reading(borrow
));
1152 // Prevent the borrow counter from overflowing into
1153 // a writing borrow.
1154 assert
!(borrow
!= isize::max_value());
1155 self.borrow
.set(borrow
+ 1);
1156 BorrowRef { borrow: self.borrow }
1160 /// Wraps a borrowed reference to a value in a `RefCell` box.
1161 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1163 /// See the [module-level documentation](index.html) for more.
1164 #[stable(feature = "rust1", since = "1.0.0")]
1165 pub struct Ref
<'b
, T
: ?Sized
+ 'b
> {
1167 borrow
: BorrowRef
<'b
>,
1170 #[stable(feature = "rust1", since = "1.0.0")]
1171 impl<T
: ?Sized
> Deref
for Ref
<'_
, T
> {
1175 fn deref(&self) -> &T
{
1180 impl<'b
, T
: ?Sized
> Ref
<'b
, T
> {
1183 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1185 /// This is an associated function that needs to be used as
1186 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1187 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1189 #[stable(feature = "cell_extras", since = "1.15.0")]
1191 pub fn clone(orig
: &Ref
<'b
, T
>) -> Ref
<'b
, T
> {
1194 borrow
: orig
.borrow
.clone(),
1198 /// Makes a new `Ref` for a component of the borrowed data.
1200 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1202 /// This is an associated function that needs to be used as `Ref::map(...)`.
1203 /// A method would interfere with methods of the same name on the contents
1204 /// of a `RefCell` used through `Deref`.
1209 /// use std::cell::{RefCell, Ref};
1211 /// let c = RefCell::new((5, 'b'));
1212 /// let b1: Ref<(u32, char)> = c.borrow();
1213 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1214 /// assert_eq!(*b2, 5)
1216 #[stable(feature = "cell_map", since = "1.8.0")]
1218 pub fn map
<U
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> Ref
<'b
, U
>
1219 where F
: FnOnce(&T
) -> &U
1222 value
: f(orig
.value
),
1223 borrow
: orig
.borrow
,
1227 /// Splits a `Ref` into multiple `Ref`s for different components of the
1230 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1232 /// This is an associated function that needs to be used as
1233 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1234 /// name on the contents of a `RefCell` used through `Deref`.
1239 /// use std::cell::{Ref, RefCell};
1241 /// let cell = RefCell::new([1, 2, 3, 4]);
1242 /// let borrow = cell.borrow();
1243 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1244 /// assert_eq!(*begin, [1, 2]);
1245 /// assert_eq!(*end, [3, 4]);
1247 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1249 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> (Ref
<'b
, U
>, Ref
<'b
, V
>)
1250 where F
: FnOnce(&T
) -> (&U
, &V
)
1252 let (a
, b
) = f(orig
.value
);
1253 let borrow
= orig
.borrow
.clone();
1254 (Ref { value: a, borrow }
, Ref { value: b, borrow: orig.borrow }
)
1258 #[unstable(feature = "coerce_unsized", issue = "27732")]
1259 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Ref
<'b
, U
>> for Ref
<'b
, T
> {}
1261 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1262 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Ref
<'_
, T
> {
1263 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1268 impl<'b
, T
: ?Sized
> RefMut
<'b
, T
> {
1269 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1272 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1274 /// This is an associated function that needs to be used as
1275 /// `RefMut::map(...)`. A method would interfere with methods of the same
1276 /// name on the contents of a `RefCell` used through `Deref`.
1281 /// use std::cell::{RefCell, RefMut};
1283 /// let c = RefCell::new((5, 'b'));
1285 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1286 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1287 /// assert_eq!(*b2, 5);
1290 /// assert_eq!(*c.borrow(), (42, 'b'));
1292 #[stable(feature = "cell_map", since = "1.8.0")]
1294 pub fn map
<U
: ?Sized
, F
>(orig
: RefMut
<'b
, T
>, f
: F
) -> RefMut
<'b
, U
>
1295 where F
: FnOnce(&mut T
) -> &mut U
1297 // FIXME(nll-rfc#40): fix borrow-check
1298 let RefMut { value, borrow }
= orig
;
1305 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1308 /// The underlying `RefCell` will remain mutably borrowed until both
1309 /// returned `RefMut`s go out of scope.
1311 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1313 /// This is an associated function that needs to be used as
1314 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1315 /// same name on the contents of a `RefCell` used through `Deref`.
1320 /// use std::cell::{RefCell, RefMut};
1322 /// let cell = RefCell::new([1, 2, 3, 4]);
1323 /// let borrow = cell.borrow_mut();
1324 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1325 /// assert_eq!(*begin, [1, 2]);
1326 /// assert_eq!(*end, [3, 4]);
1327 /// begin.copy_from_slice(&[4, 3]);
1328 /// end.copy_from_slice(&[2, 1]);
1330 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1332 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(
1333 orig
: RefMut
<'b
, T
>, f
: F
1334 ) -> (RefMut
<'b
, U
>, RefMut
<'b
, V
>)
1335 where F
: FnOnce(&mut T
) -> (&mut U
, &mut V
)
1337 let (a
, b
) = f(orig
.value
);
1338 let borrow
= orig
.borrow
.clone();
1339 (RefMut { value: a, borrow }
, RefMut { value: b, borrow: orig.borrow }
)
1343 struct BorrowRefMut
<'b
> {
1344 borrow
: &'b Cell
<BorrowFlag
>,
1347 impl Drop
for BorrowRefMut
<'_
> {
1349 fn drop(&mut self) {
1350 let borrow
= self.borrow
.get();
1351 debug_assert
!(is_writing(borrow
));
1352 self.borrow
.set(borrow
+ 1);
1356 impl<'b
> BorrowRefMut
<'b
> {
1358 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRefMut
<'b
>> {
1359 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1360 // mutable reference, and so there must currently be no existing
1361 // references. Thus, while clone increments the mutable refcount, here
1362 // we explicitly only allow going from UNUSED to UNUSED - 1.
1363 match borrow
.get() {
1365 borrow
.set(UNUSED
- 1);
1366 Some(BorrowRefMut { borrow }
)
1372 // Clones a `BorrowRefMut`.
1374 // This is only valid if each `BorrowRefMut` is used to track a mutable
1375 // reference to a distinct, nonoverlapping range of the original object.
1376 // This isn't in a Clone impl so that code doesn't call this implicitly.
1378 fn clone(&self) -> BorrowRefMut
<'b
> {
1379 let borrow
= self.borrow
.get();
1380 debug_assert
!(is_writing(borrow
));
1381 // Prevent the borrow counter from underflowing.
1382 assert
!(borrow
!= isize::min_value());
1383 self.borrow
.set(borrow
- 1);
1384 BorrowRefMut { borrow: self.borrow }
1388 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1390 /// See the [module-level documentation](index.html) for more.
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 pub struct RefMut
<'b
, T
: ?Sized
+ 'b
> {
1394 borrow
: BorrowRefMut
<'b
>,
1397 #[stable(feature = "rust1", since = "1.0.0")]
1398 impl<T
: ?Sized
> Deref
for RefMut
<'_
, T
> {
1402 fn deref(&self) -> &T
{
1407 #[stable(feature = "rust1", since = "1.0.0")]
1408 impl<T
: ?Sized
> DerefMut
for RefMut
<'_
, T
> {
1410 fn deref_mut(&mut self) -> &mut T
{
1415 #[unstable(feature = "coerce_unsized", issue = "27732")]
1416 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<RefMut
<'b
, U
>> for RefMut
<'b
, T
> {}
1418 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1419 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for RefMut
<'_
, T
> {
1420 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1425 /// The core primitive for interior mutability in Rust.
1427 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1428 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1429 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1430 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1432 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1433 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1434 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1435 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1436 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1437 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1439 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1440 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1442 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1444 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1445 /// reference) that is accessible by safe code (for example, because you returned it),
1446 /// then you must not access the data in any way that contradicts that reference for the
1447 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1448 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1449 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1450 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1451 /// safe code, then you must not access the data within the `UnsafeCell` until that
1452 /// reference expires.
1454 /// - At all times, you must avoid data races. If multiple threads have access to
1455 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1456 /// accesses (or use atomics).
1458 /// To assist with proper design, the following scenarios are explicitly declared legal
1459 /// for single-threaded code:
1461 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1462 /// references, but not with a `&mut T`
1464 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1465 /// co-exist with it. A `&mut T` must always be unique.
1467 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1468 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1469 /// to have multiple `&mut UnsafeCell<T>` aliases.
1474 /// use std::cell::UnsafeCell;
1476 /// # #[allow(dead_code)]
1477 /// struct NotThreadSafe<T> {
1478 /// value: UnsafeCell<T>,
1481 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1483 #[lang = "unsafe_cell"]
1484 #[stable(feature = "rust1", since = "1.0.0")]
1485 #[repr(transparent)]
1486 pub struct UnsafeCell
<T
: ?Sized
> {
1490 #[stable(feature = "rust1", since = "1.0.0")]
1491 impl<T
: ?Sized
> !Sync
for UnsafeCell
<T
> {}
1493 impl<T
> UnsafeCell
<T
> {
1494 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1497 /// All access to the inner value through methods is `unsafe`.
1502 /// use std::cell::UnsafeCell;
1504 /// let uc = UnsafeCell::new(5);
1506 #[stable(feature = "rust1", since = "1.0.0")]
1509 rustc_const_stable(feature
= "const_unsafe_cell_new", since
= "1.32.0"),
1512 pub const fn new(value
: T
) -> UnsafeCell
<T
> {
1513 UnsafeCell { value }
1516 /// Unwraps the value.
1521 /// use std::cell::UnsafeCell;
1523 /// let uc = UnsafeCell::new(5);
1525 /// let five = uc.into_inner();
1528 #[stable(feature = "rust1", since = "1.0.0")]
1529 pub fn into_inner(self) -> T
{
1534 impl<T
: ?Sized
> UnsafeCell
<T
> {
1535 /// Gets a mutable pointer to the wrapped value.
1537 /// This can be cast to a pointer of any kind.
1538 /// Ensure that the access is unique (no active references, mutable or not)
1539 /// when casting to `&mut T`, and ensure that there are no mutations
1540 /// or mutable aliases going on when casting to `&T`
1545 /// use std::cell::UnsafeCell;
1547 /// let uc = UnsafeCell::new(5);
1549 /// let five = uc.get();
1552 #[stable(feature = "rust1", since = "1.0.0")]
1555 rustc_const_stable(feature
= "const_unsafecell_get", since
= "1.32.0"),
1557 pub const fn get(&self) -> *mut T
{
1558 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1559 // #[repr(transparent)]. This exploits libstd's special status, there is
1560 // no guarantee for user code that this will work in future versions of the compiler!
1561 self as *const UnsafeCell
<T
> as *const T
as *mut T
1564 /// Gets a mutable pointer to the wrapped value.
1565 /// The difference to [`get`] is that this function accepts a raw pointer,
1566 /// which is useful to avoid the creation of temporary references.
1568 /// The result can be cast to a pointer of any kind.
1569 /// Ensure that the access is unique (no active references, mutable or not)
1570 /// when casting to `&mut T`, and ensure that there are no mutations
1571 /// or mutable aliases going on when casting to `&T`.
1573 /// [`get`]: #method.get
1577 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1578 /// calling `get` would require creating a reference to uninitialized data:
1581 /// #![feature(unsafe_cell_raw_get)]
1582 /// use std::cell::UnsafeCell;
1583 /// use std::mem::MaybeUninit;
1585 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1586 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1587 /// let uc = unsafe { m.assume_init() };
1589 /// assert_eq!(uc.into_inner(), 5);
1592 #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
1593 pub const fn raw_get(this
: *const Self) -> *mut T
{
1594 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1595 // #[repr(transparent)]. This exploits libstd's special status, there is
1596 // no guarantee for user code that this will work in future versions of the compiler!
1597 this
as *const T
as *mut T
1601 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1602 impl<T
: Default
> Default
for UnsafeCell
<T
> {
1603 /// Creates an `UnsafeCell`, with the `Default` value for T.
1604 fn default() -> UnsafeCell
<T
> {
1605 UnsafeCell
::new(Default
::default())
1609 #[stable(feature = "cell_from", since = "1.12.0")]
1610 impl<T
> From
<T
> for UnsafeCell
<T
> {
1611 fn from(t
: T
) -> UnsafeCell
<T
> {
1616 #[unstable(feature = "coerce_unsized", issue = "27732")]
1617 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<UnsafeCell
<U
>> for UnsafeCell
<T
> {}
1620 fn assert_coerce_unsized(a
: UnsafeCell
<&i32>, b
: Cell
<&i32>, c
: RefCell
<&i32>) {
1621 let _
: UnsafeCell
<&dyn Send
> = a
;
1622 let _
: Cell
<&dyn Send
> = b
;
1623 let _
: RefCell
<&dyn Send
> = c
;