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<T>`], [`RwLock<T>`] or [`atomic`] types.
19 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
20 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
21 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
22 //! with typical Rust types that exhibit 'inherited mutability'.
24 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
25 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
26 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
27 //! methods to retrieve and change the current interior value:
29 //! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
31 //! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
32 //! interior value with [`Default::default()`] and returns the replaced value.
33 //! - For all types, the [`replace`](Cell::replace) method replaces the current interior value and
34 //! returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the
35 //! `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method
36 //! replaces the interior value, dropping the replaced value.
38 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
39 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
40 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
41 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
42 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
45 //! # When to choose interior mutability
47 //! The more common inherited mutability, where one must have unique access to mutate a value, is
48 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
49 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
50 //! interior mutability is something of a last resort. Since cell types enable mutation where it
51 //! would otherwise be disallowed though, there are occasions when interior mutability might be
52 //! appropriate, or even *must* be used, e.g.
54 //! * Introducing mutability 'inside' of something immutable
55 //! * Implementation details of logically-immutable methods.
56 //! * Mutating implementations of [`Clone`].
58 //! ## Introducing mutability 'inside' of something immutable
60 //! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
61 //! be cloned and shared between multiple parties. Because the contained values may be
62 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
63 //! impossible to mutate data inside of these smart pointers at all.
65 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
69 //! use std::cell::{RefCell, RefMut};
70 //! use std::collections::HashMap;
74 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
75 //! // Create a new block to limit the scope of the dynamic borrow
77 //! let mut map: RefMut<_> = shared_map.borrow_mut();
78 //! map.insert("africa", 92388);
79 //! map.insert("kyoto", 11837);
80 //! map.insert("piccadilly", 11826);
81 //! map.insert("marbles", 38);
84 //! // Note that if we had not let the previous borrow of the cache fall out
85 //! // of scope then the subsequent borrow would cause a dynamic thread panic.
86 //! // This is the major hazard of using `RefCell`.
87 //! let total: i32 = shared_map.borrow().values().sum();
88 //! println!("{}", total);
92 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
93 //! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
94 //! multi-threaded situation.
96 //! ## Implementation details of logically-immutable methods
98 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
99 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
100 //! forces the implementation to perform mutation; or because you must employ mutation to implement
101 //! a trait method that was originally defined to take `&self`.
104 //! # #![allow(dead_code)]
105 //! use std::cell::RefCell;
108 //! edges: Vec<(i32, i32)>,
109 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
113 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
114 //! self.span_tree_cache.borrow_mut()
115 //! .get_or_insert_with(|| self.calc_span_tree())
119 //! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
120 //! // Expensive computation goes here
126 //! ## Mutating implementations of `Clone`
128 //! This is simply a special - but common - case of the previous: hiding mutability for operations
129 //! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
130 //! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
131 //! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
132 //! reference counts within a `Cell<T>`.
135 //! use std::cell::Cell;
136 //! use std::ptr::NonNull;
137 //! use std::process::abort;
138 //! use std::marker::PhantomData;
140 //! struct Rc<T: ?Sized> {
141 //! ptr: NonNull<RcBox<T>>,
142 //! phantom: PhantomData<RcBox<T>>,
145 //! struct RcBox<T: ?Sized> {
146 //! strong: Cell<usize>,
147 //! refcount: Cell<usize>,
151 //! impl<T: ?Sized> Clone for Rc<T> {
152 //! fn clone(&self) -> Rc<T> {
153 //! self.inc_strong();
156 //! phantom: PhantomData,
161 //! trait RcBoxPtr<T: ?Sized> {
163 //! fn inner(&self) -> &RcBox<T>;
165 //! fn strong(&self) -> usize {
166 //! self.inner().strong.get()
169 //! fn inc_strong(&self) {
172 //! .set(self.strong()
174 //! .unwrap_or_else(|| abort() ));
178 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
179 //! fn inner(&self) -> &RcBox<T> {
181 //! self.ptr.as_ref()
187 //! [`Arc<T>`]: ../../std/sync/struct.Arc.html
188 //! [`Rc<T>`]: ../../std/rc/struct.Rc.html
189 //! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
190 //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
191 //! [`atomic`]: crate::sync::atomic
193 #![stable(feature = "rust1", since = "1.0.0")]
195 use crate::cmp::Ordering;
196 use crate::fmt::{self, Debug, Display};
197 use crate::marker::Unsize;
199 use crate::ops::{CoerceUnsized, Deref, DerefMut};
202 /// A mutable memory location.
206 /// In this example, you can see that `Cell<T>` enables mutation inside an
207 /// immutable struct. In other words, it enables "interior mutability".
210 /// use std::cell::Cell;
212 /// struct SomeStruct {
213 /// regular_field: u8,
214 /// special_field: Cell<u8>,
217 /// let my_struct = SomeStruct {
218 /// regular_field: 0,
219 /// special_field: Cell::new(1),
222 /// let new_value = 100;
224 /// // ERROR: `my_struct` is immutable
225 /// // my_struct.regular_field = new_value;
227 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
228 /// // which can always be mutated
229 /// my_struct.special_field.set(new_value);
230 /// assert_eq!(my_struct.special_field.get(), new_value);
233 /// See the [module-level documentation](self) for more.
234 #[stable(feature = "rust1", since = "1.0.0")]
236 pub struct Cell<T: ?Sized> {
237 value: UnsafeCell<T>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
243 #[stable(feature = "rust1", since = "1.0.0")]
244 impl<T: ?Sized> !Sync for Cell<T> {}
246 #[stable(feature = "rust1", since = "1.0.0")]
247 impl<T: Copy> Clone for Cell<T> {
249 fn clone(&self) -> Cell<T> {
250 Cell::new(self.get())
254 #[stable(feature = "rust1", since = "1.0.0")]
255 impl<T: Default> Default for Cell<T> {
256 /// Creates a `Cell<T>`, with the `Default` value for T.
258 fn default() -> Cell<T> {
259 Cell::new(Default::default())
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
266 fn eq(&self, other: &Cell<T>) -> bool {
267 self.get() == other.get()
271 #[stable(feature = "cell_eq", since = "1.2.0")]
272 impl<T: Eq + Copy> Eq for Cell<T> {}
274 #[stable(feature = "cell_ord", since = "1.10.0")]
275 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
277 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
278 self.get().partial_cmp(&other.get())
282 fn lt(&self, other: &Cell<T>) -> bool {
283 self.get() < other.get()
287 fn le(&self, other: &Cell<T>) -> bool {
288 self.get() <= other.get()
292 fn gt(&self, other: &Cell<T>) -> bool {
293 self.get() > other.get()
297 fn ge(&self, other: &Cell<T>) -> bool {
298 self.get() >= other.get()
302 #[stable(feature = "cell_ord", since = "1.10.0")]
303 impl<T: Ord + Copy> Ord for Cell<T> {
305 fn cmp(&self, other: &Cell<T>) -> Ordering {
306 self.get().cmp(&other.get())
310 #[stable(feature = "cell_from", since = "1.12.0")]
311 impl<T> From<T> for Cell<T> {
312 fn from(t: T) -> Cell<T> {
318 /// Creates a new `Cell` containing the given value.
323 /// use std::cell::Cell;
325 /// let c = Cell::new(5);
327 #[stable(feature = "rust1", since = "1.0.0")]
328 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
330 pub const fn new(value: T) -> Cell<T> {
331 Cell { value: UnsafeCell::new(value) }
334 /// Sets the contained value.
339 /// use std::cell::Cell;
341 /// let c = Cell::new(5);
346 #[stable(feature = "rust1", since = "1.0.0")]
347 pub fn set(&self, val: T) {
348 let old = self.replace(val);
352 /// Swaps the values of two `Cell`s.
353 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
358 /// use std::cell::Cell;
360 /// let c1 = Cell::new(5i32);
361 /// let c2 = Cell::new(10i32);
363 /// assert_eq!(10, c1.get());
364 /// assert_eq!(5, c2.get());
367 #[stable(feature = "move_cell", since = "1.17.0")]
368 pub fn swap(&self, other: &Self) {
369 if ptr::eq(self, other) {
372 // SAFETY: This can be risky if called from separate threads, but `Cell`
373 // is `!Sync` so this won't happen. This also won't invalidate any
374 // pointers since `Cell` makes sure nothing else will be pointing into
375 // either of these `Cell`s.
377 ptr::swap(self.value.get(), other.value.get());
381 /// Replaces the contained value with `val`, and returns the old contained value.
386 /// use std::cell::Cell;
388 /// let cell = Cell::new(5);
389 /// assert_eq!(cell.get(), 5);
390 /// assert_eq!(cell.replace(10), 5);
391 /// assert_eq!(cell.get(), 10);
393 #[stable(feature = "move_cell", since = "1.17.0")]
394 pub fn replace(&self, val: T) -> T {
395 // SAFETY: This can cause data races if called from a separate thread,
396 // but `Cell` is `!Sync` so this won't happen.
397 mem::replace(unsafe { &mut *self.value.get() }, val)
400 /// Unwraps the value.
405 /// use std::cell::Cell;
407 /// let c = Cell::new(5);
408 /// let five = c.into_inner();
410 /// assert_eq!(five, 5);
412 #[stable(feature = "move_cell", since = "1.17.0")]
413 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
414 pub const fn into_inner(self) -> T {
415 self.value.into_inner()
419 impl<T: Copy> Cell<T> {
420 /// Returns a copy of the contained value.
425 /// use std::cell::Cell;
427 /// let c = Cell::new(5);
429 /// let five = c.get();
432 #[stable(feature = "rust1", since = "1.0.0")]
433 pub fn get(&self) -> T {
434 // SAFETY: This can cause data races if called from a separate thread,
435 // but `Cell` is `!Sync` so this won't happen.
436 unsafe { *self.value.get() }
439 /// Updates the contained value using a function and returns the new value.
444 /// #![feature(cell_update)]
446 /// use std::cell::Cell;
448 /// let c = Cell::new(5);
449 /// let new = c.update(|x| x + 1);
451 /// assert_eq!(new, 6);
452 /// assert_eq!(c.get(), 6);
455 #[unstable(feature = "cell_update", issue = "50186")]
456 pub fn update<F>(&self, f: F) -> T
460 let old = self.get();
467 impl<T: ?Sized> Cell<T> {
468 /// Returns a raw pointer to the underlying data in this cell.
473 /// use std::cell::Cell;
475 /// let c = Cell::new(5);
477 /// let ptr = c.as_ptr();
480 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
481 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
482 pub const fn as_ptr(&self) -> *mut T {
486 /// Returns a mutable reference to the underlying data.
488 /// This call borrows `Cell` mutably (at compile-time) which guarantees
489 /// that we possess the only reference.
491 /// However be cautious: this method expects `self` to be mutable, which is
492 /// generally not the case when using a `Cell`. If you require interior
493 /// mutability by reference, consider using `RefCell` which provides
494 /// run-time checked mutable borrows through its [`borrow_mut`] method.
496 /// [`borrow_mut`]: RefCell::borrow_mut()
501 /// use std::cell::Cell;
503 /// let mut c = Cell::new(5);
504 /// *c.get_mut() += 1;
506 /// assert_eq!(c.get(), 6);
509 #[stable(feature = "cell_get_mut", since = "1.11.0")]
510 pub fn get_mut(&mut self) -> &mut T {
514 /// Returns a `&Cell<T>` from a `&mut T`
519 /// use std::cell::Cell;
521 /// let slice: &mut [i32] = &mut [1, 2, 3];
522 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
523 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
525 /// assert_eq!(slice_cell.len(), 3);
528 #[stable(feature = "as_cell", since = "1.37.0")]
529 pub fn from_mut(t: &mut T) -> &Cell<T> {
530 // SAFETY: `&mut` ensures unique access.
531 unsafe { &*(t as *mut T as *const Cell<T>) }
535 impl<T: Default> Cell<T> {
536 /// Takes the value of the cell, leaving `Default::default()` in its place.
541 /// use std::cell::Cell;
543 /// let c = Cell::new(5);
544 /// let five = c.take();
546 /// assert_eq!(five, 5);
547 /// assert_eq!(c.into_inner(), 0);
549 #[stable(feature = "move_cell", since = "1.17.0")]
550 pub fn take(&self) -> T {
551 self.replace(Default::default())
555 #[unstable(feature = "coerce_unsized", issue = "27732")]
556 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
559 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
564 /// use std::cell::Cell;
566 /// let slice: &mut [i32] = &mut [1, 2, 3];
567 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
568 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
570 /// assert_eq!(slice_cell.len(), 3);
572 #[stable(feature = "as_cell", since = "1.37.0")]
573 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
574 // SAFETY: `Cell<T>` has the same memory layout as `T`.
575 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
579 impl<T, const N: usize> Cell<[T; N]> {
580 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
585 /// #![feature(as_array_of_cells)]
586 /// use std::cell::Cell;
588 /// let mut array: [i32; 3] = [1, 2, 3];
589 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
590 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
592 #[unstable(feature = "as_array_of_cells", issue = "88248")]
593 pub fn as_array_of_cells(&self) -> &[Cell<T>; N] {
594 // SAFETY: `Cell<T>` has the same memory layout as `T`.
595 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
599 /// A mutable memory location with dynamically checked borrow rules
601 /// See the [module-level documentation](self) for more.
602 #[stable(feature = "rust1", since = "1.0.0")]
603 pub struct RefCell<T: ?Sized> {
604 borrow: Cell<BorrowFlag>,
605 // Stores the location of the earliest currently active borrow.
606 // This gets updated whenever we go from having zero borrows
607 // to having a single borrow. When a borrow occurs, this gets included
608 // in the generated `BorrowError/`BorrowMutError`
609 #[cfg(feature = "debug_refcell")]
610 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
611 value: UnsafeCell<T>,
614 /// An error returned by [`RefCell::try_borrow`].
615 #[stable(feature = "try_borrow", since = "1.13.0")]
617 pub struct BorrowError {
618 #[cfg(feature = "debug_refcell")]
619 location: &'static crate::panic::Location<'static>,
622 #[stable(feature = "try_borrow", since = "1.13.0")]
623 impl Debug for BorrowError {
624 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
625 let mut builder = f.debug_struct("BorrowError");
627 #[cfg(feature = "debug_refcell")]
628 builder.field("location", self.location);
634 #[stable(feature = "try_borrow", since = "1.13.0")]
635 impl Display for BorrowError {
636 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
637 Display::fmt("already mutably borrowed", f)
641 /// An error returned by [`RefCell::try_borrow_mut`].
642 #[stable(feature = "try_borrow", since = "1.13.0")]
644 pub struct BorrowMutError {
645 #[cfg(feature = "debug_refcell")]
646 location: &'static crate::panic::Location<'static>,
649 #[stable(feature = "try_borrow", since = "1.13.0")]
650 impl Debug for BorrowMutError {
651 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
652 let mut builder = f.debug_struct("BorrowMutError");
654 #[cfg(feature = "debug_refcell")]
655 builder.field("location", self.location);
661 #[stable(feature = "try_borrow", since = "1.13.0")]
662 impl Display for BorrowMutError {
663 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
664 Display::fmt("already borrowed", f)
668 // Positive values represent the number of `Ref` active. Negative values
669 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
670 // active at a time if they refer to distinct, nonoverlapping components of a
671 // `RefCell` (e.g., different ranges of a slice).
673 // `Ref` and `RefMut` are both two words in size, and so there will likely never
674 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
675 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
676 // However, this is not a guarantee, as a pathological program could repeatedly
677 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
678 // explicitly check for overflow and underflow in order to avoid unsafety, or at
679 // least behave correctly in the event that overflow or underflow happens (e.g.,
680 // see BorrowRef::new).
681 type BorrowFlag = isize;
682 const UNUSED: BorrowFlag = 0;
685 fn is_writing(x: BorrowFlag) -> bool {
690 fn is_reading(x: BorrowFlag) -> bool {
695 /// Creates a new `RefCell` containing `value`.
700 /// use std::cell::RefCell;
702 /// let c = RefCell::new(5);
704 #[stable(feature = "rust1", since = "1.0.0")]
705 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
707 pub const fn new(value: T) -> RefCell<T> {
709 value: UnsafeCell::new(value),
710 borrow: Cell::new(UNUSED),
711 #[cfg(feature = "debug_refcell")]
712 borrowed_at: Cell::new(None),
716 /// Consumes the `RefCell`, returning the wrapped value.
721 /// use std::cell::RefCell;
723 /// let c = RefCell::new(5);
725 /// let five = c.into_inner();
727 #[stable(feature = "rust1", since = "1.0.0")]
728 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
730 pub const fn into_inner(self) -> T {
731 // Since this function takes `self` (the `RefCell`) by value, the
732 // compiler statically verifies that it is not currently borrowed.
733 self.value.into_inner()
736 /// Replaces the wrapped value with a new one, returning the old value,
737 /// without deinitializing either one.
739 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
743 /// Panics if the value is currently borrowed.
748 /// use std::cell::RefCell;
749 /// let cell = RefCell::new(5);
750 /// let old_value = cell.replace(6);
751 /// assert_eq!(old_value, 5);
752 /// assert_eq!(cell, RefCell::new(6));
755 #[stable(feature = "refcell_replace", since = "1.24.0")]
757 pub fn replace(&self, t: T) -> T {
758 mem::replace(&mut *self.borrow_mut(), t)
761 /// Replaces the wrapped value with a new one computed from `f`, returning
762 /// the old value, without deinitializing either one.
766 /// Panics if the value is currently borrowed.
771 /// use std::cell::RefCell;
772 /// let cell = RefCell::new(5);
773 /// let old_value = cell.replace_with(|&mut old| old + 1);
774 /// assert_eq!(old_value, 5);
775 /// assert_eq!(cell, RefCell::new(6));
778 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
780 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
781 let mut_borrow = &mut *self.borrow_mut();
782 let replacement = f(mut_borrow);
783 mem::replace(mut_borrow, replacement)
786 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
787 /// without deinitializing either one.
789 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
793 /// Panics if the value in either `RefCell` is currently borrowed.
798 /// use std::cell::RefCell;
799 /// let c = RefCell::new(5);
800 /// let d = RefCell::new(6);
802 /// assert_eq!(c, RefCell::new(6));
803 /// assert_eq!(d, RefCell::new(5));
806 #[stable(feature = "refcell_swap", since = "1.24.0")]
807 pub fn swap(&self, other: &Self) {
808 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
812 impl<T: ?Sized> RefCell<T> {
813 /// Immutably borrows the wrapped value.
815 /// The borrow lasts until the returned `Ref` exits scope. Multiple
816 /// immutable borrows can be taken out at the same time.
820 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
821 /// [`try_borrow`](#method.try_borrow).
826 /// use std::cell::RefCell;
828 /// let c = RefCell::new(5);
830 /// let borrowed_five = c.borrow();
831 /// let borrowed_five2 = c.borrow();
834 /// An example of panic:
837 /// use std::cell::RefCell;
839 /// let c = RefCell::new(5);
841 /// let m = c.borrow_mut();
842 /// let b = c.borrow(); // this causes a panic
844 #[stable(feature = "rust1", since = "1.0.0")]
847 pub fn borrow(&self) -> Ref<'_, T> {
848 self.try_borrow().expect("already mutably borrowed")
851 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
854 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
855 /// taken out at the same time.
857 /// This is the non-panicking variant of [`borrow`](#method.borrow).
862 /// use std::cell::RefCell;
864 /// let c = RefCell::new(5);
867 /// let m = c.borrow_mut();
868 /// assert!(c.try_borrow().is_err());
872 /// let m = c.borrow();
873 /// assert!(c.try_borrow().is_ok());
876 #[stable(feature = "try_borrow", since = "1.13.0")]
878 #[cfg_attr(feature = "debug_refcell", track_caller)]
879 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
880 match BorrowRef::new(&self.borrow) {
882 #[cfg(feature = "debug_refcell")]
884 // `borrowed_at` is always the *first* active borrow
885 if b.borrow.get() == 1 {
886 self.borrowed_at.set(Some(crate::panic::Location::caller()));
890 // SAFETY: `BorrowRef` ensures that there is only immutable access
891 // to the value while borrowed.
892 Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
894 None => Err(BorrowError {
895 // If a borrow occured, then we must already have an outstanding borrow,
896 // so `borrowed_at` will be `Some`
897 #[cfg(feature = "debug_refcell")]
898 location: self.borrowed_at.get().unwrap(),
903 /// Mutably borrows the wrapped value.
905 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
906 /// from it exit scope. The value cannot be borrowed while this borrow is
911 /// Panics if the value is currently borrowed. For a non-panicking variant, use
912 /// [`try_borrow_mut`](#method.try_borrow_mut).
917 /// use std::cell::RefCell;
919 /// let c = RefCell::new("hello".to_owned());
921 /// *c.borrow_mut() = "bonjour".to_owned();
923 /// assert_eq!(&*c.borrow(), "bonjour");
926 /// An example of panic:
929 /// use std::cell::RefCell;
931 /// let c = RefCell::new(5);
932 /// let m = c.borrow();
934 /// let b = c.borrow_mut(); // this causes a panic
936 #[stable(feature = "rust1", since = "1.0.0")]
939 pub fn borrow_mut(&self) -> RefMut<'_, T> {
940 self.try_borrow_mut().expect("already borrowed")
943 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
945 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
946 /// from it exit scope. The value cannot be borrowed while this borrow is
949 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
954 /// use std::cell::RefCell;
956 /// let c = RefCell::new(5);
959 /// let m = c.borrow();
960 /// assert!(c.try_borrow_mut().is_err());
963 /// assert!(c.try_borrow_mut().is_ok());
965 #[stable(feature = "try_borrow", since = "1.13.0")]
967 #[cfg_attr(feature = "debug_refcell", track_caller)]
968 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
969 match BorrowRefMut::new(&self.borrow) {
971 #[cfg(feature = "debug_refcell")]
973 self.borrowed_at.set(Some(crate::panic::Location::caller()));
976 // SAFETY: `BorrowRef` guarantees unique access.
977 Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
979 None => Err(BorrowMutError {
980 // If a borrow occured, then we must already have an outstanding borrow,
981 // so `borrowed_at` will be `Some`
982 #[cfg(feature = "debug_refcell")]
983 location: self.borrowed_at.get().unwrap(),
988 /// Returns a raw pointer to the underlying data in this cell.
993 /// use std::cell::RefCell;
995 /// let c = RefCell::new(5);
997 /// let ptr = c.as_ptr();
1000 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1001 pub fn as_ptr(&self) -> *mut T {
1005 /// Returns a mutable reference to the underlying data.
1007 /// This call borrows `RefCell` mutably (at compile-time) so there is no
1008 /// need for dynamic checks.
1010 /// However be cautious: this method expects `self` to be mutable, which is
1011 /// generally not the case when using a `RefCell`. Take a look at the
1012 /// [`borrow_mut`] method instead if `self` isn't mutable.
1014 /// Also, please be aware that this method is only for special circumstances and is usually
1015 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
1017 /// [`borrow_mut`]: RefCell::borrow_mut()
1022 /// use std::cell::RefCell;
1024 /// let mut c = RefCell::new(5);
1025 /// *c.get_mut() += 1;
1027 /// assert_eq!(c, RefCell::new(6));
1030 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1031 pub fn get_mut(&mut self) -> &mut T {
1032 self.value.get_mut()
1035 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1037 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1038 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1039 /// if some `Ref` or `RefMut` borrows have been leaked.
1041 /// [`get_mut`]: RefCell::get_mut()
1046 /// #![feature(cell_leak)]
1047 /// use std::cell::RefCell;
1049 /// let mut c = RefCell::new(0);
1050 /// std::mem::forget(c.borrow_mut());
1052 /// assert!(c.try_borrow().is_err());
1054 /// assert!(c.try_borrow().is_ok());
1056 #[unstable(feature = "cell_leak", issue = "69099")]
1057 pub fn undo_leak(&mut self) -> &mut T {
1058 *self.borrow.get_mut() = UNUSED;
1062 /// Immutably borrows the wrapped value, returning an error if the value is
1063 /// currently mutably borrowed.
1067 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1068 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1069 /// borrowing the `RefCell` while the reference returned by this method
1070 /// is alive is undefined behaviour.
1075 /// use std::cell::RefCell;
1077 /// let c = RefCell::new(5);
1080 /// let m = c.borrow_mut();
1081 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1085 /// let m = c.borrow();
1086 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1089 #[stable(feature = "borrow_state", since = "1.37.0")]
1091 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1092 if !is_writing(self.borrow.get()) {
1093 // SAFETY: We check that nobody is actively writing now, but it is
1094 // the caller's responsibility to ensure that nobody writes until
1095 // the returned reference is no longer in use.
1096 // Also, `self.value.get()` refers to the value owned by `self`
1097 // and is thus guaranteed to be valid for the lifetime of `self`.
1098 Ok(unsafe { &*self.value.get() })
1101 // If a borrow occured, then we must already have an outstanding borrow,
1102 // so `borrowed_at` will be `Some`
1103 #[cfg(feature = "debug_refcell")]
1104 location: self.borrowed_at.get().unwrap(),
1110 impl<T: Default> RefCell<T> {
1111 /// Takes the wrapped value, leaving `Default::default()` in its place.
1115 /// Panics if the value is currently borrowed.
1120 /// use std::cell::RefCell;
1122 /// let c = RefCell::new(5);
1123 /// let five = c.take();
1125 /// assert_eq!(five, 5);
1126 /// assert_eq!(c.into_inner(), 0);
1128 #[stable(feature = "refcell_take", since = "1.50.0")]
1129 pub fn take(&self) -> T {
1130 self.replace(Default::default())
1134 #[stable(feature = "rust1", since = "1.0.0")]
1135 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1137 #[stable(feature = "rust1", since = "1.0.0")]
1138 impl<T: ?Sized> !Sync for RefCell<T> {}
1140 #[stable(feature = "rust1", since = "1.0.0")]
1141 impl<T: Clone> Clone for RefCell<T> {
1144 /// Panics if the value is currently mutably borrowed.
1147 fn clone(&self) -> RefCell<T> {
1148 RefCell::new(self.borrow().clone())
1153 /// Panics if `other` is currently mutably borrowed.
1156 fn clone_from(&mut self, other: &Self) {
1157 self.get_mut().clone_from(&other.borrow())
1161 #[stable(feature = "rust1", since = "1.0.0")]
1162 impl<T: Default> Default for RefCell<T> {
1163 /// Creates a `RefCell<T>`, with the `Default` value for T.
1165 fn default() -> RefCell<T> {
1166 RefCell::new(Default::default())
1170 #[stable(feature = "rust1", since = "1.0.0")]
1171 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1174 /// Panics if the value in either `RefCell` is currently borrowed.
1176 fn eq(&self, other: &RefCell<T>) -> bool {
1177 *self.borrow() == *other.borrow()
1181 #[stable(feature = "cell_eq", since = "1.2.0")]
1182 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1184 #[stable(feature = "cell_ord", since = "1.10.0")]
1185 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1188 /// Panics if the value in either `RefCell` is currently borrowed.
1190 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1191 self.borrow().partial_cmp(&*other.borrow())
1196 /// Panics if the value in either `RefCell` is currently borrowed.
1198 fn lt(&self, other: &RefCell<T>) -> bool {
1199 *self.borrow() < *other.borrow()
1204 /// Panics if the value in either `RefCell` is currently borrowed.
1206 fn le(&self, other: &RefCell<T>) -> bool {
1207 *self.borrow() <= *other.borrow()
1212 /// Panics if the value in either `RefCell` is currently borrowed.
1214 fn gt(&self, other: &RefCell<T>) -> bool {
1215 *self.borrow() > *other.borrow()
1220 /// Panics if the value in either `RefCell` is currently borrowed.
1222 fn ge(&self, other: &RefCell<T>) -> bool {
1223 *self.borrow() >= *other.borrow()
1227 #[stable(feature = "cell_ord", since = "1.10.0")]
1228 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1231 /// Panics if the value in either `RefCell` is currently borrowed.
1233 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1234 self.borrow().cmp(&*other.borrow())
1238 #[stable(feature = "cell_from", since = "1.12.0")]
1239 impl<T> From<T> for RefCell<T> {
1240 fn from(t: T) -> RefCell<T> {
1245 #[unstable(feature = "coerce_unsized", issue = "27732")]
1246 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1248 struct BorrowRef<'b> {
1249 borrow: &'b Cell<BorrowFlag>,
1252 impl<'b> BorrowRef<'b> {
1254 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1255 let b = borrow.get().wrapping_add(1);
1257 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1258 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1259 // due to Rust's reference aliasing rules
1260 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1261 // into isize::MIN (the max amount of writing borrows) so we can't allow
1262 // an additional read borrow because isize can't represent so many read borrows
1263 // (this can only happen if you mem::forget more than a small constant amount of
1264 // `Ref`s, which is not good practice)
1267 // Incrementing borrow can result in a reading value (> 0) in these cases:
1268 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1269 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1270 // is large enough to represent having one more read borrow
1272 Some(BorrowRef { borrow })
1277 impl Drop for BorrowRef<'_> {
1279 fn drop(&mut self) {
1280 let borrow = self.borrow.get();
1281 debug_assert!(is_reading(borrow));
1282 self.borrow.set(borrow - 1);
1286 impl Clone for BorrowRef<'_> {
1288 fn clone(&self) -> Self {
1289 // Since this Ref exists, we know the borrow flag
1290 // is a reading borrow.
1291 let borrow = self.borrow.get();
1292 debug_assert!(is_reading(borrow));
1293 // Prevent the borrow counter from overflowing into
1294 // a writing borrow.
1295 assert!(borrow != isize::MAX);
1296 self.borrow.set(borrow + 1);
1297 BorrowRef { borrow: self.borrow }
1301 /// Wraps a borrowed reference to a value in a `RefCell` box.
1302 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1304 /// See the [module-level documentation](self) for more.
1305 #[stable(feature = "rust1", since = "1.0.0")]
1308 must_not_suspend = "holding a Ref across suspend \
1309 points can cause BorrowErrors"
1311 pub struct Ref<'b, T: ?Sized + 'b> {
1313 borrow: BorrowRef<'b>,
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 impl<T: ?Sized> Deref for Ref<'_, T> {
1321 fn deref(&self) -> &T {
1326 impl<'b, T: ?Sized> Ref<'b, T> {
1329 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1331 /// This is an associated function that needs to be used as
1332 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1333 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1335 #[stable(feature = "cell_extras", since = "1.15.0")]
1337 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1338 Ref { value: orig.value, borrow: orig.borrow.clone() }
1341 /// Makes a new `Ref` for a component of the borrowed data.
1343 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1345 /// This is an associated function that needs to be used as `Ref::map(...)`.
1346 /// A method would interfere with methods of the same name on the contents
1347 /// of a `RefCell` used through `Deref`.
1352 /// use std::cell::{RefCell, Ref};
1354 /// let c = RefCell::new((5, 'b'));
1355 /// let b1: Ref<(u32, char)> = c.borrow();
1356 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1357 /// assert_eq!(*b2, 5)
1359 #[stable(feature = "cell_map", since = "1.8.0")]
1361 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1363 F: FnOnce(&T) -> &U,
1365 Ref { value: f(orig.value), borrow: orig.borrow }
1368 /// Makes a new `Ref` for an optional component of the borrowed data. The
1369 /// original guard is returned as an `Err(..)` if the closure returns
1372 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1374 /// This is an associated function that needs to be used as
1375 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1376 /// name on the contents of a `RefCell` used through `Deref`.
1381 /// #![feature(cell_filter_map)]
1383 /// use std::cell::{RefCell, Ref};
1385 /// let c = RefCell::new(vec![1, 2, 3]);
1386 /// let b1: Ref<Vec<u32>> = c.borrow();
1387 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1388 /// assert_eq!(*b2.unwrap(), 2);
1390 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1392 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1394 F: FnOnce(&T) -> Option<&U>,
1396 match f(orig.value) {
1397 Some(value) => Ok(Ref { value, borrow: orig.borrow }),
1402 /// Splits a `Ref` into multiple `Ref`s for different components of the
1405 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1407 /// This is an associated function that needs to be used as
1408 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1409 /// name on the contents of a `RefCell` used through `Deref`.
1414 /// use std::cell::{Ref, RefCell};
1416 /// let cell = RefCell::new([1, 2, 3, 4]);
1417 /// let borrow = cell.borrow();
1418 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1419 /// assert_eq!(*begin, [1, 2]);
1420 /// assert_eq!(*end, [3, 4]);
1422 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1424 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1426 F: FnOnce(&T) -> (&U, &V),
1428 let (a, b) = f(orig.value);
1429 let borrow = orig.borrow.clone();
1430 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1433 /// Convert into a reference to the underlying data.
1435 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1436 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1437 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1438 /// have occurred in total.
1440 /// This is an associated function that needs to be used as
1441 /// `Ref::leak(...)`. A method would interfere with methods of the
1442 /// same name on the contents of a `RefCell` used through `Deref`.
1447 /// #![feature(cell_leak)]
1448 /// use std::cell::{RefCell, Ref};
1449 /// let cell = RefCell::new(0);
1451 /// let value = Ref::leak(cell.borrow());
1452 /// assert_eq!(*value, 0);
1454 /// assert!(cell.try_borrow().is_ok());
1455 /// assert!(cell.try_borrow_mut().is_err());
1457 #[unstable(feature = "cell_leak", issue = "69099")]
1458 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1459 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1460 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1461 // unique reference to the borrowed RefCell. No further mutable references can be created
1462 // from the original cell.
1463 mem::forget(orig.borrow);
1468 #[unstable(feature = "coerce_unsized", issue = "27732")]
1469 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1471 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1472 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1473 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1478 impl<'b, T: ?Sized> RefMut<'b, T> {
1479 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1482 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1484 /// This is an associated function that needs to be used as
1485 /// `RefMut::map(...)`. A method would interfere with methods of the same
1486 /// name on the contents of a `RefCell` used through `Deref`.
1491 /// use std::cell::{RefCell, RefMut};
1493 /// let c = RefCell::new((5, 'b'));
1495 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1496 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1497 /// assert_eq!(*b2, 5);
1500 /// assert_eq!(*c.borrow(), (42, 'b'));
1502 #[stable(feature = "cell_map", since = "1.8.0")]
1504 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1506 F: FnOnce(&mut T) -> &mut U,
1508 // FIXME(nll-rfc#40): fix borrow-check
1509 let RefMut { value, borrow } = orig;
1510 RefMut { value: f(value), borrow }
1513 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1514 /// original guard is returned as an `Err(..)` if the closure returns
1517 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1519 /// This is an associated function that needs to be used as
1520 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1521 /// same name on the contents of a `RefCell` used through `Deref`.
1526 /// #![feature(cell_filter_map)]
1528 /// use std::cell::{RefCell, RefMut};
1530 /// let c = RefCell::new(vec![1, 2, 3]);
1533 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1534 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1536 /// if let Ok(mut b2) = b2 {
1541 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1543 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1545 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1547 F: FnOnce(&mut T) -> Option<&mut U>,
1549 // FIXME(nll-rfc#40): fix borrow-check
1550 let RefMut { value, borrow } = orig;
1551 let value = value as *mut T;
1552 // SAFETY: function holds onto an exclusive reference for the duration
1553 // of its call through `orig`, and the pointer is only de-referenced
1554 // inside of the function call never allowing the exclusive reference to
1556 match f(unsafe { &mut *value }) {
1557 Some(value) => Ok(RefMut { value, borrow }),
1559 // SAFETY: same as above.
1560 Err(RefMut { value: unsafe { &mut *value }, borrow })
1565 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1568 /// The underlying `RefCell` will remain mutably borrowed until both
1569 /// returned `RefMut`s go out of scope.
1571 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1573 /// This is an associated function that needs to be used as
1574 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1575 /// same name on the contents of a `RefCell` used through `Deref`.
1580 /// use std::cell::{RefCell, RefMut};
1582 /// let cell = RefCell::new([1, 2, 3, 4]);
1583 /// let borrow = cell.borrow_mut();
1584 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1585 /// assert_eq!(*begin, [1, 2]);
1586 /// assert_eq!(*end, [3, 4]);
1587 /// begin.copy_from_slice(&[4, 3]);
1588 /// end.copy_from_slice(&[2, 1]);
1590 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1592 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1593 orig: RefMut<'b, T>,
1595 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1597 F: FnOnce(&mut T) -> (&mut U, &mut V),
1599 let (a, b) = f(orig.value);
1600 let borrow = orig.borrow.clone();
1601 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1604 /// Convert into a mutable reference to the underlying data.
1606 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1607 /// mutably borrowed, making the returned reference the only to the interior.
1609 /// This is an associated function that needs to be used as
1610 /// `RefMut::leak(...)`. A method would interfere with methods of the
1611 /// same name on the contents of a `RefCell` used through `Deref`.
1616 /// #![feature(cell_leak)]
1617 /// use std::cell::{RefCell, RefMut};
1618 /// let cell = RefCell::new(0);
1620 /// let value = RefMut::leak(cell.borrow_mut());
1621 /// assert_eq!(*value, 0);
1624 /// assert!(cell.try_borrow_mut().is_err());
1626 #[unstable(feature = "cell_leak", issue = "69099")]
1627 pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
1628 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1629 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1630 // require a unique reference to the borrowed RefCell. No further references can be created
1631 // from the original cell within that lifetime, making the current borrow the only
1632 // reference for the remaining lifetime.
1633 mem::forget(orig.borrow);
1638 struct BorrowRefMut<'b> {
1639 borrow: &'b Cell<BorrowFlag>,
1642 impl Drop for BorrowRefMut<'_> {
1644 fn drop(&mut self) {
1645 let borrow = self.borrow.get();
1646 debug_assert!(is_writing(borrow));
1647 self.borrow.set(borrow + 1);
1651 impl<'b> BorrowRefMut<'b> {
1653 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1654 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1655 // mutable reference, and so there must currently be no existing
1656 // references. Thus, while clone increments the mutable refcount, here
1657 // we explicitly only allow going from UNUSED to UNUSED - 1.
1658 match borrow.get() {
1660 borrow.set(UNUSED - 1);
1661 Some(BorrowRefMut { borrow })
1667 // Clones a `BorrowRefMut`.
1669 // This is only valid if each `BorrowRefMut` is used to track a mutable
1670 // reference to a distinct, nonoverlapping range of the original object.
1671 // This isn't in a Clone impl so that code doesn't call this implicitly.
1673 fn clone(&self) -> BorrowRefMut<'b> {
1674 let borrow = self.borrow.get();
1675 debug_assert!(is_writing(borrow));
1676 // Prevent the borrow counter from underflowing.
1677 assert!(borrow != isize::MIN);
1678 self.borrow.set(borrow - 1);
1679 BorrowRefMut { borrow: self.borrow }
1683 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1685 /// See the [module-level documentation](self) for more.
1686 #[stable(feature = "rust1", since = "1.0.0")]
1689 must_not_suspend = "holding a RefMut across suspend \
1690 points can cause BorrowErrors"
1692 pub struct RefMut<'b, T: ?Sized + 'b> {
1694 borrow: BorrowRefMut<'b>,
1697 #[stable(feature = "rust1", since = "1.0.0")]
1698 impl<T: ?Sized> Deref for RefMut<'_, T> {
1702 fn deref(&self) -> &T {
1707 #[stable(feature = "rust1", since = "1.0.0")]
1708 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1710 fn deref_mut(&mut self) -> &mut T {
1715 #[unstable(feature = "coerce_unsized", issue = "27732")]
1716 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1718 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1719 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1720 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1725 /// The core primitive for interior mutability in Rust.
1727 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1728 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1729 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1730 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1731 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1733 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1734 /// use `UnsafeCell` to wrap their data.
1736 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1737 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1738 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1740 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1741 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1744 /// [`.get()`]: `UnsafeCell::get`
1746 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1748 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1749 /// reference) that is accessible by safe code (for example, because you returned it),
1750 /// then you must not access the data in any way that contradicts that reference for the
1751 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1752 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1753 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1754 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1755 /// safe code, then you must not access the data within the `UnsafeCell` until that
1756 /// reference expires.
1758 /// - At all times, you must avoid data races. If multiple threads have access to
1759 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1760 /// accesses (or use atomics).
1762 /// To assist with proper design, the following scenarios are explicitly declared legal
1763 /// for single-threaded code:
1765 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1766 /// references, but not with a `&mut T`
1768 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1769 /// co-exist with it. A `&mut T` must always be unique.
1771 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1772 /// `&UnsafeCell<T>` references alias the cell) is
1773 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1774 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1775 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1776 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1777 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1778 /// may be aliased for the duration of that `&mut` borrow.
1779 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1782 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1786 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1787 /// there being multiple references aliasing the cell:
1790 /// use std::cell::UnsafeCell;
1792 /// let x: UnsafeCell<i32> = 42.into();
1793 /// // Get multiple / concurrent / shared references to the same `x`.
1794 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1797 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1798 /// // so ours is effectively unique.
1799 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1800 /// *p1_exclusive += 27; // |
1801 /// } // <---------- cannot go beyond this point -------------------+
1804 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1805 /// // so we can have multiple shared accesses concurrently.
1806 /// let p2_shared: &i32 = &*p2.get();
1807 /// assert_eq!(*p2_shared, 42 + 27);
1808 /// let p1_shared: &i32 = &*p1.get();
1809 /// assert_eq!(*p1_shared, *p2_shared);
1813 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1814 /// implies exclusive access to its `T`:
1817 /// #![forbid(unsafe_code)] // with exclusive accesses,
1818 /// // `UnsafeCell` is a transparent no-op wrapper,
1819 /// // so no need for `unsafe` here.
1820 /// use std::cell::UnsafeCell;
1822 /// let mut x: UnsafeCell<i32> = 42.into();
1824 /// // Get a compile-time-checked unique reference to `x`.
1825 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1826 /// // With an exclusive reference, we can mutate the contents for free.
1827 /// *p_unique.get_mut() = 0;
1828 /// // Or, equivalently:
1829 /// x = UnsafeCell::new(0);
1831 /// // When we own the value, we can extract the contents for free.
1832 /// let contents: i32 = x.into_inner();
1833 /// assert_eq!(contents, 0);
1835 #[lang = "unsafe_cell"]
1836 #[stable(feature = "rust1", since = "1.0.0")]
1837 #[repr(transparent)]
1838 #[repr(no_niche)] // rust-lang/rust#68303.
1839 pub struct UnsafeCell<T: ?Sized> {
1843 #[stable(feature = "rust1", since = "1.0.0")]
1844 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1846 impl<T> UnsafeCell<T> {
1847 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1850 /// All access to the inner value through methods is `unsafe`.
1855 /// use std::cell::UnsafeCell;
1857 /// let uc = UnsafeCell::new(5);
1859 #[stable(feature = "rust1", since = "1.0.0")]
1860 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1862 pub const fn new(value: T) -> UnsafeCell<T> {
1863 UnsafeCell { value }
1866 /// Unwraps the value.
1871 /// use std::cell::UnsafeCell;
1873 /// let uc = UnsafeCell::new(5);
1875 /// let five = uc.into_inner();
1878 #[stable(feature = "rust1", since = "1.0.0")]
1879 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1880 pub const fn into_inner(self) -> T {
1885 impl<T: ?Sized> UnsafeCell<T> {
1886 /// Gets a mutable pointer to the wrapped value.
1888 /// This can be cast to a pointer of any kind.
1889 /// Ensure that the access is unique (no active references, mutable or not)
1890 /// when casting to `&mut T`, and ensure that there are no mutations
1891 /// or mutable aliases going on when casting to `&T`
1896 /// use std::cell::UnsafeCell;
1898 /// let uc = UnsafeCell::new(5);
1900 /// let five = uc.get();
1903 #[stable(feature = "rust1", since = "1.0.0")]
1904 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1905 pub const fn get(&self) -> *mut T {
1906 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1907 // #[repr(transparent)]. This exploits libstd's special status, there is
1908 // no guarantee for user code that this will work in future versions of the compiler!
1909 self as *const UnsafeCell<T> as *const T as *mut T
1912 /// Returns a mutable reference to the underlying data.
1914 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
1915 /// guarantees that we possess the only reference.
1920 /// use std::cell::UnsafeCell;
1922 /// let mut c = UnsafeCell::new(5);
1923 /// *c.get_mut() += 1;
1925 /// assert_eq!(*c.get_mut(), 6);
1928 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
1929 #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
1930 pub const fn get_mut(&mut self) -> &mut T {
1934 /// Gets a mutable pointer to the wrapped value.
1935 /// The difference from [`get`] is that this function accepts a raw pointer,
1936 /// which is useful to avoid the creation of temporary references.
1938 /// The result can be cast to a pointer of any kind.
1939 /// Ensure that the access is unique (no active references, mutable or not)
1940 /// when casting to `&mut T`, and ensure that there are no mutations
1941 /// or mutable aliases going on when casting to `&T`.
1943 /// [`get`]: UnsafeCell::get()
1947 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1948 /// calling `get` would require creating a reference to uninitialized data:
1951 /// use std::cell::UnsafeCell;
1952 /// use std::mem::MaybeUninit;
1954 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1955 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1956 /// let uc = unsafe { m.assume_init() };
1958 /// assert_eq!(uc.into_inner(), 5);
1961 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
1962 pub const fn raw_get(this: *const Self) -> *mut T {
1963 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1964 // #[repr(transparent)]. This exploits libstd's special status, there is
1965 // no guarantee for user code that this will work in future versions of the compiler!
1966 this as *const T as *mut T
1970 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1971 impl<T: Default> Default for UnsafeCell<T> {
1972 /// Creates an `UnsafeCell`, with the `Default` value for T.
1973 fn default() -> UnsafeCell<T> {
1974 UnsafeCell::new(Default::default())
1978 #[stable(feature = "cell_from", since = "1.12.0")]
1979 impl<T> From<T> for UnsafeCell<T> {
1980 fn from(t: T) -> UnsafeCell<T> {
1985 #[unstable(feature = "coerce_unsized", issue = "27732")]
1986 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1989 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1990 let _: UnsafeCell<&dyn Send> = a;
1991 let _: Cell<&dyn Send> = b;
1992 let _: RefCell<&dyn Send> = c;