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: This can be risky if called from separate threads, but `Cell`
370 // is `!Sync` so this won't happen. This also won't invalidate any
371 // pointers since `Cell` makes sure nothing else will be pointing into
372 // either of these `Cell`s.
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 // SAFETY: This can cause data races if called from a separate thread,
393 // but `Cell` is `!Sync` so this won't happen.
394 mem::replace(unsafe { &mut *self.value.get() }, val)
397 /// Unwraps the value.
402 /// use std::cell::Cell;
404 /// let c = Cell::new(5);
405 /// let five = c.into_inner();
407 /// assert_eq!(five, 5);
409 #[stable(feature = "move_cell", since = "1.17.0")]
410 pub fn into_inner(self) -> T {
411 self.value.into_inner()
415 impl<T: Copy> Cell<T> {
416 /// Returns a copy of the contained value.
421 /// use std::cell::Cell;
423 /// let c = Cell::new(5);
425 /// let five = c.get();
428 #[stable(feature = "rust1", since = "1.0.0")]
429 pub fn get(&self) -> T {
430 // SAFETY: This can cause data races if called from a separate thread,
431 // but `Cell` is `!Sync` so this won't happen.
432 unsafe { *self.value.get() }
435 /// Updates the contained value using a function and returns the new value.
440 /// #![feature(cell_update)]
442 /// use std::cell::Cell;
444 /// let c = Cell::new(5);
445 /// let new = c.update(|x| x + 1);
447 /// assert_eq!(new, 6);
448 /// assert_eq!(c.get(), 6);
451 #[unstable(feature = "cell_update", issue = "50186")]
452 pub fn update<F>(&self, f: F) -> T
456 let old = self.get();
463 impl<T: ?Sized> Cell<T> {
464 /// Returns a raw pointer to the underlying data in this cell.
469 /// use std::cell::Cell;
471 /// let c = Cell::new(5);
473 /// let ptr = c.as_ptr();
476 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
477 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
478 pub const fn as_ptr(&self) -> *mut T {
482 /// Returns a mutable reference to the underlying data.
484 /// This call borrows `Cell` mutably (at compile-time) which guarantees
485 /// that we possess the only reference.
490 /// use std::cell::Cell;
492 /// let mut c = Cell::new(5);
493 /// *c.get_mut() += 1;
495 /// assert_eq!(c.get(), 6);
498 #[stable(feature = "cell_get_mut", since = "1.11.0")]
499 pub fn get_mut(&mut self) -> &mut T {
500 // SAFETY: This can cause data races if called from a separate thread,
501 // but `Cell` is `!Sync` so this won't happen, and `&mut` guarantees
503 unsafe { &mut *self.value.get() }
506 /// Returns a `&Cell<T>` from a `&mut T`
511 /// use std::cell::Cell;
513 /// let slice: &mut [i32] = &mut [1, 2, 3];
514 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
515 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
517 /// assert_eq!(slice_cell.len(), 3);
520 #[stable(feature = "as_cell", since = "1.37.0")]
521 pub fn from_mut(t: &mut T) -> &Cell<T> {
522 // SAFETY: `&mut` ensures unique access.
523 unsafe { &*(t as *mut T as *const Cell<T>) }
527 impl<T: Default> Cell<T> {
528 /// Takes the value of the cell, leaving `Default::default()` in its place.
533 /// use std::cell::Cell;
535 /// let c = Cell::new(5);
536 /// let five = c.take();
538 /// assert_eq!(five, 5);
539 /// assert_eq!(c.into_inner(), 0);
541 #[stable(feature = "move_cell", since = "1.17.0")]
542 pub fn take(&self) -> T {
543 self.replace(Default::default())
547 #[unstable(feature = "coerce_unsized", issue = "27732")]
548 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
551 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
556 /// use std::cell::Cell;
558 /// let slice: &mut [i32] = &mut [1, 2, 3];
559 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
560 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
562 /// assert_eq!(slice_cell.len(), 3);
564 #[stable(feature = "as_cell", since = "1.37.0")]
565 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
566 // SAFETY: `Cell<T>` has the same memory layout as `T`.
567 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
571 /// A mutable memory location with dynamically checked borrow rules
573 /// See the [module-level documentation](index.html) for more.
574 #[stable(feature = "rust1", since = "1.0.0")]
575 pub struct RefCell<T: ?Sized> {
576 borrow: Cell<BorrowFlag>,
577 value: UnsafeCell<T>,
580 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
581 #[stable(feature = "try_borrow", since = "1.13.0")]
582 pub struct BorrowError {
586 #[stable(feature = "try_borrow", since = "1.13.0")]
587 impl Debug for BorrowError {
588 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
589 f.debug_struct("BorrowError").finish()
593 #[stable(feature = "try_borrow", since = "1.13.0")]
594 impl Display for BorrowError {
595 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
596 Display::fmt("already mutably borrowed", f)
600 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
601 #[stable(feature = "try_borrow", since = "1.13.0")]
602 pub struct BorrowMutError {
606 #[stable(feature = "try_borrow", since = "1.13.0")]
607 impl Debug for BorrowMutError {
608 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
609 f.debug_struct("BorrowMutError").finish()
613 #[stable(feature = "try_borrow", since = "1.13.0")]
614 impl Display for BorrowMutError {
615 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
616 Display::fmt("already borrowed", f)
620 // Positive values represent the number of `Ref` active. Negative values
621 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
622 // active at a time if they refer to distinct, nonoverlapping components of a
623 // `RefCell` (e.g., different ranges of a slice).
625 // `Ref` and `RefMut` are both two words in size, and so there will likely never
626 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
627 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
628 // However, this is not a guarantee, as a pathological program could repeatedly
629 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
630 // explicitly check for overflow and underflow in order to avoid unsafety, or at
631 // least behave correctly in the event that overflow or underflow happens (e.g.,
632 // see BorrowRef::new).
633 type BorrowFlag = isize;
634 const UNUSED: BorrowFlag = 0;
637 fn is_writing(x: BorrowFlag) -> bool {
642 fn is_reading(x: BorrowFlag) -> bool {
647 /// Creates a new `RefCell` containing `value`.
652 /// use std::cell::RefCell;
654 /// let c = RefCell::new(5);
656 #[stable(feature = "rust1", since = "1.0.0")]
657 #[rustc_const_stable(feature = "const_refcell_new", since = "1.32.0")]
659 pub const fn new(value: T) -> RefCell<T> {
660 RefCell { value: UnsafeCell::new(value), 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) {
830 // SAFETY: `BorrowRef` ensures that there is only immutable access
831 // to the value while borrowed.
832 Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }),
833 None => Err(BorrowError { _private: () }),
837 /// Mutably borrows the wrapped value.
839 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
840 /// from it exit scope. The value cannot be borrowed while this borrow is
845 /// Panics if the value is currently borrowed. For a non-panicking variant, use
846 /// [`try_borrow_mut`](#method.try_borrow_mut).
851 /// use std::cell::RefCell;
853 /// let c = RefCell::new(5);
855 /// *c.borrow_mut() = 7;
857 /// assert_eq!(*c.borrow(), 7);
860 /// An example of panic:
863 /// use std::cell::RefCell;
866 /// let result = thread::spawn(move || {
867 /// let c = RefCell::new(5);
868 /// let m = c.borrow();
870 /// let b = c.borrow_mut(); // this causes a panic
873 /// assert!(result.is_err());
875 #[stable(feature = "rust1", since = "1.0.0")]
877 pub fn borrow_mut(&self) -> RefMut<'_, T> {
878 self.try_borrow_mut().expect("already borrowed")
881 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
883 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
884 /// from it exit scope. The value cannot be borrowed while this borrow is
887 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
892 /// use std::cell::RefCell;
894 /// let c = RefCell::new(5);
897 /// let m = c.borrow();
898 /// assert!(c.try_borrow_mut().is_err());
901 /// assert!(c.try_borrow_mut().is_ok());
903 #[stable(feature = "try_borrow", since = "1.13.0")]
905 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
906 match BorrowRefMut::new(&self.borrow) {
907 // SAFETY: `BorrowRef` guarantees unique access.
908 Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }),
909 None => Err(BorrowMutError { _private: () }),
913 /// Returns a raw pointer to the underlying data in this cell.
918 /// use std::cell::RefCell;
920 /// let c = RefCell::new(5);
922 /// let ptr = c.as_ptr();
925 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
926 pub fn as_ptr(&self) -> *mut T {
930 /// Returns a mutable reference to the underlying data.
932 /// This call borrows `RefCell` mutably (at compile-time) so there is no
933 /// need for dynamic checks.
935 /// However be cautious: this method expects `self` to be mutable, which is
936 /// generally not the case when using a `RefCell`. Take a look at the
937 /// [`borrow_mut`] method instead if `self` isn't mutable.
939 /// Also, please be aware that this method is only for special circumstances and is usually
940 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
942 /// [`borrow_mut`]: #method.borrow_mut
947 /// use std::cell::RefCell;
949 /// let mut c = RefCell::new(5);
950 /// *c.get_mut() += 1;
952 /// assert_eq!(c, RefCell::new(6));
955 #[stable(feature = "cell_get_mut", since = "1.11.0")]
956 pub fn get_mut(&mut self) -> &mut T {
957 // SAFETY: `&mut` guarantees unique access.
958 unsafe { &mut *self.value.get() }
961 /// Immutably borrows the wrapped value, returning an error if the value is
962 /// currently mutably borrowed.
966 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
967 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
968 /// borrowing the `RefCell` while the reference returned by this method
969 /// is alive is undefined behaviour.
974 /// use std::cell::RefCell;
976 /// let c = RefCell::new(5);
979 /// let m = c.borrow_mut();
980 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
984 /// let m = c.borrow();
985 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
988 #[stable(feature = "borrow_state", since = "1.37.0")]
990 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
991 if !is_writing(self.borrow.get()) {
992 Ok(&*self.value.get())
994 Err(BorrowError { _private: () })
999 #[stable(feature = "rust1", since = "1.0.0")]
1000 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1002 #[stable(feature = "rust1", since = "1.0.0")]
1003 impl<T: ?Sized> !Sync for RefCell<T> {}
1005 #[stable(feature = "rust1", since = "1.0.0")]
1006 impl<T: Clone> Clone for RefCell<T> {
1009 /// Panics if the value is currently mutably borrowed.
1011 fn clone(&self) -> RefCell<T> {
1012 RefCell::new(self.borrow().clone())
1016 #[stable(feature = "rust1", since = "1.0.0")]
1017 impl<T: Default> Default for RefCell<T> {
1018 /// Creates a `RefCell<T>`, with the `Default` value for T.
1020 fn default() -> RefCell<T> {
1021 RefCell::new(Default::default())
1025 #[stable(feature = "rust1", since = "1.0.0")]
1026 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1029 /// Panics if the value in either `RefCell` is currently borrowed.
1031 fn eq(&self, other: &RefCell<T>) -> bool {
1032 *self.borrow() == *other.borrow()
1036 #[stable(feature = "cell_eq", since = "1.2.0")]
1037 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1039 #[stable(feature = "cell_ord", since = "1.10.0")]
1040 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1043 /// Panics if the value in either `RefCell` is currently borrowed.
1045 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1046 self.borrow().partial_cmp(&*other.borrow())
1051 /// Panics if the value in either `RefCell` is currently borrowed.
1053 fn lt(&self, other: &RefCell<T>) -> bool {
1054 *self.borrow() < *other.borrow()
1059 /// Panics if the value in either `RefCell` is currently borrowed.
1061 fn le(&self, other: &RefCell<T>) -> bool {
1062 *self.borrow() <= *other.borrow()
1067 /// Panics if the value in either `RefCell` is currently borrowed.
1069 fn gt(&self, other: &RefCell<T>) -> bool {
1070 *self.borrow() > *other.borrow()
1075 /// Panics if the value in either `RefCell` is currently borrowed.
1077 fn ge(&self, other: &RefCell<T>) -> bool {
1078 *self.borrow() >= *other.borrow()
1082 #[stable(feature = "cell_ord", since = "1.10.0")]
1083 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1086 /// Panics if the value in either `RefCell` is currently borrowed.
1088 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1089 self.borrow().cmp(&*other.borrow())
1093 #[stable(feature = "cell_from", since = "1.12.0")]
1094 impl<T> From<T> for RefCell<T> {
1095 fn from(t: T) -> RefCell<T> {
1100 #[unstable(feature = "coerce_unsized", issue = "27732")]
1101 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1103 struct BorrowRef<'b> {
1104 borrow: &'b Cell<BorrowFlag>,
1107 impl<'b> BorrowRef<'b> {
1109 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1110 let b = borrow.get().wrapping_add(1);
1112 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1113 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1114 // due to Rust's reference aliasing rules
1115 // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
1116 // into isize::min_value() (the max amount of writing borrows) so we can't allow
1117 // an additional read borrow because isize can't represent so many read borrows
1118 // (this can only happen if you mem::forget more than a small constant amount of
1119 // `Ref`s, which is not good practice)
1122 // Incrementing borrow can result in a reading value (> 0) in these cases:
1123 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1124 // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
1125 // is large enough to represent having one more read borrow
1127 Some(BorrowRef { borrow })
1132 impl Drop for BorrowRef<'_> {
1134 fn drop(&mut self) {
1135 let borrow = self.borrow.get();
1136 debug_assert!(is_reading(borrow));
1137 self.borrow.set(borrow - 1);
1141 impl Clone for BorrowRef<'_> {
1143 fn clone(&self) -> Self {
1144 // Since this Ref exists, we know the borrow flag
1145 // is a reading borrow.
1146 let borrow = self.borrow.get();
1147 debug_assert!(is_reading(borrow));
1148 // Prevent the borrow counter from overflowing into
1149 // a writing borrow.
1150 assert!(borrow != isize::max_value());
1151 self.borrow.set(borrow + 1);
1152 BorrowRef { borrow: self.borrow }
1156 /// Wraps a borrowed reference to a value in a `RefCell` box.
1157 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1159 /// See the [module-level documentation](index.html) for more.
1160 #[stable(feature = "rust1", since = "1.0.0")]
1161 pub struct Ref<'b, T: ?Sized + 'b> {
1163 borrow: BorrowRef<'b>,
1166 #[stable(feature = "rust1", since = "1.0.0")]
1167 impl<T: ?Sized> Deref for Ref<'_, T> {
1171 fn deref(&self) -> &T {
1176 impl<'b, T: ?Sized> Ref<'b, T> {
1179 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1181 /// This is an associated function that needs to be used as
1182 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1183 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1185 #[stable(feature = "cell_extras", since = "1.15.0")]
1187 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1188 Ref { value: orig.value, borrow: orig.borrow.clone() }
1191 /// Makes a new `Ref` for a component of the borrowed data.
1193 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1195 /// This is an associated function that needs to be used as `Ref::map(...)`.
1196 /// A method would interfere with methods of the same name on the contents
1197 /// of a `RefCell` used through `Deref`.
1202 /// use std::cell::{RefCell, Ref};
1204 /// let c = RefCell::new((5, 'b'));
1205 /// let b1: Ref<(u32, char)> = c.borrow();
1206 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1207 /// assert_eq!(*b2, 5)
1209 #[stable(feature = "cell_map", since = "1.8.0")]
1211 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1213 F: FnOnce(&T) -> &U,
1215 Ref { value: f(orig.value), borrow: orig.borrow }
1218 /// Splits a `Ref` into multiple `Ref`s for different components of the
1221 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1223 /// This is an associated function that needs to be used as
1224 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1225 /// name on the contents of a `RefCell` used through `Deref`.
1230 /// use std::cell::{Ref, RefCell};
1232 /// let cell = RefCell::new([1, 2, 3, 4]);
1233 /// let borrow = cell.borrow();
1234 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1235 /// assert_eq!(*begin, [1, 2]);
1236 /// assert_eq!(*end, [3, 4]);
1238 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1240 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1242 F: FnOnce(&T) -> (&U, &V),
1244 let (a, b) = f(orig.value);
1245 let borrow = orig.borrow.clone();
1246 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1250 #[unstable(feature = "coerce_unsized", issue = "27732")]
1251 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1253 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1254 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1255 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1260 impl<'b, T: ?Sized> RefMut<'b, T> {
1261 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1264 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1266 /// This is an associated function that needs to be used as
1267 /// `RefMut::map(...)`. A method would interfere with methods of the same
1268 /// name on the contents of a `RefCell` used through `Deref`.
1273 /// use std::cell::{RefCell, RefMut};
1275 /// let c = RefCell::new((5, 'b'));
1277 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1278 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1279 /// assert_eq!(*b2, 5);
1282 /// assert_eq!(*c.borrow(), (42, 'b'));
1284 #[stable(feature = "cell_map", since = "1.8.0")]
1286 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1288 F: FnOnce(&mut T) -> &mut U,
1290 // FIXME(nll-rfc#40): fix borrow-check
1291 let RefMut { value, borrow } = orig;
1292 RefMut { value: f(value), borrow }
1295 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1298 /// The underlying `RefCell` will remain mutably borrowed until both
1299 /// returned `RefMut`s go out of scope.
1301 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1303 /// This is an associated function that needs to be used as
1304 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1305 /// same name on the contents of a `RefCell` used through `Deref`.
1310 /// use std::cell::{RefCell, RefMut};
1312 /// let cell = RefCell::new([1, 2, 3, 4]);
1313 /// let borrow = cell.borrow_mut();
1314 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1315 /// assert_eq!(*begin, [1, 2]);
1316 /// assert_eq!(*end, [3, 4]);
1317 /// begin.copy_from_slice(&[4, 3]);
1318 /// end.copy_from_slice(&[2, 1]);
1320 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1322 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1323 orig: RefMut<'b, T>,
1325 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1327 F: FnOnce(&mut T) -> (&mut U, &mut V),
1329 let (a, b) = f(orig.value);
1330 let borrow = orig.borrow.clone();
1331 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1335 struct BorrowRefMut<'b> {
1336 borrow: &'b Cell<BorrowFlag>,
1339 impl Drop for BorrowRefMut<'_> {
1341 fn drop(&mut self) {
1342 let borrow = self.borrow.get();
1343 debug_assert!(is_writing(borrow));
1344 self.borrow.set(borrow + 1);
1348 impl<'b> BorrowRefMut<'b> {
1350 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1351 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1352 // mutable reference, and so there must currently be no existing
1353 // references. Thus, while clone increments the mutable refcount, here
1354 // we explicitly only allow going from UNUSED to UNUSED - 1.
1355 match borrow.get() {
1357 borrow.set(UNUSED - 1);
1358 Some(BorrowRefMut { borrow })
1364 // Clones a `BorrowRefMut`.
1366 // This is only valid if each `BorrowRefMut` is used to track a mutable
1367 // reference to a distinct, nonoverlapping range of the original object.
1368 // This isn't in a Clone impl so that code doesn't call this implicitly.
1370 fn clone(&self) -> BorrowRefMut<'b> {
1371 let borrow = self.borrow.get();
1372 debug_assert!(is_writing(borrow));
1373 // Prevent the borrow counter from underflowing.
1374 assert!(borrow != isize::min_value());
1375 self.borrow.set(borrow - 1);
1376 BorrowRefMut { borrow: self.borrow }
1380 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1382 /// See the [module-level documentation](index.html) for more.
1383 #[stable(feature = "rust1", since = "1.0.0")]
1384 pub struct RefMut<'b, T: ?Sized + 'b> {
1386 borrow: BorrowRefMut<'b>,
1389 #[stable(feature = "rust1", since = "1.0.0")]
1390 impl<T: ?Sized> Deref for RefMut<'_, T> {
1394 fn deref(&self) -> &T {
1399 #[stable(feature = "rust1", since = "1.0.0")]
1400 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1402 fn deref_mut(&mut self) -> &mut T {
1407 #[unstable(feature = "coerce_unsized", issue = "27732")]
1408 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1410 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1411 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1412 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1417 /// The core primitive for interior mutability in Rust.
1419 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1420 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1421 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1422 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1424 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1425 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1426 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1427 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1428 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1429 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1431 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1432 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1434 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1436 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1437 /// reference) that is accessible by safe code (for example, because you returned it),
1438 /// then you must not access the data in any way that contradicts that reference for the
1439 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1440 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1441 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1442 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1443 /// safe code, then you must not access the data within the `UnsafeCell` until that
1444 /// reference expires.
1446 /// - At all times, you must avoid data races. If multiple threads have access to
1447 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1448 /// accesses (or use atomics).
1450 /// To assist with proper design, the following scenarios are explicitly declared legal
1451 /// for single-threaded code:
1453 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1454 /// references, but not with a `&mut T`
1456 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1457 /// co-exist with it. A `&mut T` must always be unique.
1459 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1460 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1461 /// to have multiple `&mut UnsafeCell<T>` aliases.
1466 /// use std::cell::UnsafeCell;
1468 /// # #[allow(dead_code)]
1469 /// struct NotThreadSafe<T> {
1470 /// value: UnsafeCell<T>,
1473 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1475 #[lang = "unsafe_cell"]
1476 #[stable(feature = "rust1", since = "1.0.0")]
1477 #[repr(transparent)]
1478 pub struct UnsafeCell<T: ?Sized> {
1482 #[stable(feature = "rust1", since = "1.0.0")]
1483 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1485 impl<T> UnsafeCell<T> {
1486 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1489 /// All access to the inner value through methods is `unsafe`.
1494 /// use std::cell::UnsafeCell;
1496 /// let uc = UnsafeCell::new(5);
1498 #[stable(feature = "rust1", since = "1.0.0")]
1499 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1501 pub const fn new(value: T) -> UnsafeCell<T> {
1502 UnsafeCell { value }
1505 /// Unwraps the value.
1510 /// use std::cell::UnsafeCell;
1512 /// let uc = UnsafeCell::new(5);
1514 /// let five = uc.into_inner();
1517 #[stable(feature = "rust1", since = "1.0.0")]
1518 pub fn into_inner(self) -> T {
1523 impl<T: ?Sized> UnsafeCell<T> {
1524 /// Gets a mutable pointer to the wrapped value.
1526 /// This can be cast to a pointer of any kind.
1527 /// Ensure that the access is unique (no active references, mutable or not)
1528 /// when casting to `&mut T`, and ensure that there are no mutations
1529 /// or mutable aliases going on when casting to `&T`
1534 /// use std::cell::UnsafeCell;
1536 /// let uc = UnsafeCell::new(5);
1538 /// let five = uc.get();
1541 #[stable(feature = "rust1", since = "1.0.0")]
1542 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1543 pub const fn get(&self) -> *mut T {
1544 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1545 // #[repr(transparent)]. This exploits libstd's special status, there is
1546 // no guarantee for user code that this will work in future versions of the compiler!
1547 self as *const UnsafeCell<T> as *const T as *mut T
1550 /// Gets a mutable pointer to the wrapped value.
1551 /// The difference to [`get`] is that this function accepts a raw pointer,
1552 /// which is useful to avoid the creation of temporary references.
1554 /// The result can be cast to a pointer of any kind.
1555 /// Ensure that the access is unique (no active references, mutable or not)
1556 /// when casting to `&mut T`, and ensure that there are no mutations
1557 /// or mutable aliases going on when casting to `&T`.
1559 /// [`get`]: #method.get
1563 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1564 /// calling `get` would require creating a reference to uninitialized data:
1567 /// #![feature(unsafe_cell_raw_get)]
1568 /// use std::cell::UnsafeCell;
1569 /// use std::mem::MaybeUninit;
1571 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1572 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1573 /// let uc = unsafe { m.assume_init() };
1575 /// assert_eq!(uc.into_inner(), 5);
1578 #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
1579 pub const fn raw_get(this: *const Self) -> *mut T {
1580 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1581 // #[repr(transparent)]. This exploits libstd's special status, there is
1582 // no guarantee for user code that this will work in future versions of the compiler!
1583 this as *const T as *mut T
1587 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1588 impl<T: Default> Default for UnsafeCell<T> {
1589 /// Creates an `UnsafeCell`, with the `Default` value for T.
1590 fn default() -> UnsafeCell<T> {
1591 UnsafeCell::new(Default::default())
1595 #[stable(feature = "cell_from", since = "1.12.0")]
1596 impl<T> From<T> for UnsafeCell<T> {
1597 fn from(t: T) -> UnsafeCell<T> {
1602 #[unstable(feature = "coerce_unsized", issue = "27732")]
1603 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1606 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1607 let _: UnsafeCell<&dyn Send> = a;
1608 let _: Cell<&dyn Send> = b;
1609 let _: RefCell<&dyn Send> = c;