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 // Note that this negative impl isn't strictly necessary for correctness,
244 // as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
245 // However, given how important `Cell`'s `!Sync`-ness is,
246 // having an explicit negative impl is nice for documentation purposes
247 // and results in nicer error messages.
248 #[stable(feature = "rust1", since = "1.0.0")]
249 impl<T: ?Sized> !Sync for Cell<T> {}
251 #[stable(feature = "rust1", since = "1.0.0")]
252 impl<T: Copy> Clone for Cell<T> {
254 fn clone(&self) -> Cell<T> {
255 Cell::new(self.get())
259 #[stable(feature = "rust1", since = "1.0.0")]
260 impl<T: Default> Default for Cell<T> {
261 /// Creates a `Cell<T>`, with the `Default` value for T.
263 fn default() -> Cell<T> {
264 Cell::new(Default::default())
268 #[stable(feature = "rust1", since = "1.0.0")]
269 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
271 fn eq(&self, other: &Cell<T>) -> bool {
272 self.get() == other.get()
276 #[stable(feature = "cell_eq", since = "1.2.0")]
277 impl<T: Eq + Copy> Eq for Cell<T> {}
279 #[stable(feature = "cell_ord", since = "1.10.0")]
280 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
282 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
283 self.get().partial_cmp(&other.get())
287 fn lt(&self, other: &Cell<T>) -> bool {
288 self.get() < other.get()
292 fn le(&self, other: &Cell<T>) -> bool {
293 self.get() <= other.get()
297 fn gt(&self, other: &Cell<T>) -> bool {
298 self.get() > other.get()
302 fn ge(&self, other: &Cell<T>) -> bool {
303 self.get() >= other.get()
307 #[stable(feature = "cell_ord", since = "1.10.0")]
308 impl<T: Ord + Copy> Ord for Cell<T> {
310 fn cmp(&self, other: &Cell<T>) -> Ordering {
311 self.get().cmp(&other.get())
315 #[stable(feature = "cell_from", since = "1.12.0")]
316 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
317 impl<T> const From<T> for Cell<T> {
318 fn from(t: T) -> Cell<T> {
324 /// Creates a new `Cell` containing the given value.
329 /// use std::cell::Cell;
331 /// let c = Cell::new(5);
333 #[stable(feature = "rust1", since = "1.0.0")]
334 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
336 pub const fn new(value: T) -> Cell<T> {
337 Cell { value: UnsafeCell::new(value) }
340 /// Sets the contained value.
345 /// use std::cell::Cell;
347 /// let c = Cell::new(5);
352 #[stable(feature = "rust1", since = "1.0.0")]
353 pub fn set(&self, val: T) {
354 let old = self.replace(val);
358 /// Swaps the values of two `Cell`s.
359 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
364 /// use std::cell::Cell;
366 /// let c1 = Cell::new(5i32);
367 /// let c2 = Cell::new(10i32);
369 /// assert_eq!(10, c1.get());
370 /// assert_eq!(5, c2.get());
373 #[stable(feature = "move_cell", since = "1.17.0")]
374 pub fn swap(&self, other: &Self) {
375 if ptr::eq(self, other) {
378 // SAFETY: This can be risky if called from separate threads, but `Cell`
379 // is `!Sync` so this won't happen. This also won't invalidate any
380 // pointers since `Cell` makes sure nothing else will be pointing into
381 // either of these `Cell`s.
383 ptr::swap(self.value.get(), other.value.get());
387 /// Replaces the contained value with `val`, and returns the old contained value.
392 /// use std::cell::Cell;
394 /// let cell = Cell::new(5);
395 /// assert_eq!(cell.get(), 5);
396 /// assert_eq!(cell.replace(10), 5);
397 /// assert_eq!(cell.get(), 10);
399 #[stable(feature = "move_cell", since = "1.17.0")]
400 pub fn replace(&self, val: T) -> T {
401 // SAFETY: This can cause data races if called from a separate thread,
402 // but `Cell` is `!Sync` so this won't happen.
403 mem::replace(unsafe { &mut *self.value.get() }, val)
406 /// Unwraps the value.
411 /// use std::cell::Cell;
413 /// let c = Cell::new(5);
414 /// let five = c.into_inner();
416 /// assert_eq!(five, 5);
418 #[stable(feature = "move_cell", since = "1.17.0")]
419 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
420 pub const fn into_inner(self) -> T {
421 self.value.into_inner()
425 impl<T: Copy> Cell<T> {
426 /// Returns a copy of the contained value.
431 /// use std::cell::Cell;
433 /// let c = Cell::new(5);
435 /// let five = c.get();
438 #[stable(feature = "rust1", since = "1.0.0")]
439 pub fn get(&self) -> T {
440 // SAFETY: This can cause data races if called from a separate thread,
441 // but `Cell` is `!Sync` so this won't happen.
442 unsafe { *self.value.get() }
445 /// Updates the contained value using a function and returns the new value.
450 /// #![feature(cell_update)]
452 /// use std::cell::Cell;
454 /// let c = Cell::new(5);
455 /// let new = c.update(|x| x + 1);
457 /// assert_eq!(new, 6);
458 /// assert_eq!(c.get(), 6);
461 #[unstable(feature = "cell_update", issue = "50186")]
462 pub fn update<F>(&self, f: F) -> T
466 let old = self.get();
473 impl<T: ?Sized> Cell<T> {
474 /// Returns a raw pointer to the underlying data in this cell.
479 /// use std::cell::Cell;
481 /// let c = Cell::new(5);
483 /// let ptr = c.as_ptr();
486 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
487 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
488 pub const fn as_ptr(&self) -> *mut T {
492 /// Returns a mutable reference to the underlying data.
494 /// This call borrows `Cell` mutably (at compile-time) which guarantees
495 /// that we possess the only reference.
497 /// However be cautious: this method expects `self` to be mutable, which is
498 /// generally not the case when using a `Cell`. If you require interior
499 /// mutability by reference, consider using `RefCell` which provides
500 /// run-time checked mutable borrows through its [`borrow_mut`] method.
502 /// [`borrow_mut`]: RefCell::borrow_mut()
507 /// use std::cell::Cell;
509 /// let mut c = Cell::new(5);
510 /// *c.get_mut() += 1;
512 /// assert_eq!(c.get(), 6);
515 #[stable(feature = "cell_get_mut", since = "1.11.0")]
516 pub fn get_mut(&mut self) -> &mut T {
520 /// Returns a `&Cell<T>` from a `&mut T`
525 /// use std::cell::Cell;
527 /// let slice: &mut [i32] = &mut [1, 2, 3];
528 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
529 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
531 /// assert_eq!(slice_cell.len(), 3);
534 #[stable(feature = "as_cell", since = "1.37.0")]
535 pub fn from_mut(t: &mut T) -> &Cell<T> {
536 // SAFETY: `&mut` ensures unique access.
537 unsafe { &*(t as *mut T as *const Cell<T>) }
541 impl<T: Default> Cell<T> {
542 /// Takes the value of the cell, leaving `Default::default()` in its place.
547 /// use std::cell::Cell;
549 /// let c = Cell::new(5);
550 /// let five = c.take();
552 /// assert_eq!(five, 5);
553 /// assert_eq!(c.into_inner(), 0);
555 #[stable(feature = "move_cell", since = "1.17.0")]
556 pub fn take(&self) -> T {
557 self.replace(Default::default())
561 #[unstable(feature = "coerce_unsized", issue = "27732")]
562 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
565 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
570 /// use std::cell::Cell;
572 /// let slice: &mut [i32] = &mut [1, 2, 3];
573 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
574 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
576 /// assert_eq!(slice_cell.len(), 3);
578 #[stable(feature = "as_cell", since = "1.37.0")]
579 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
580 // SAFETY: `Cell<T>` has the same memory layout as `T`.
581 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
585 impl<T, const N: usize> Cell<[T; N]> {
586 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
591 /// #![feature(as_array_of_cells)]
592 /// use std::cell::Cell;
594 /// let mut array: [i32; 3] = [1, 2, 3];
595 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
596 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
598 #[unstable(feature = "as_array_of_cells", issue = "88248")]
599 pub fn as_array_of_cells(&self) -> &[Cell<T>; N] {
600 // SAFETY: `Cell<T>` has the same memory layout as `T`.
601 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
605 /// A mutable memory location with dynamically checked borrow rules
607 /// See the [module-level documentation](self) for more.
608 #[stable(feature = "rust1", since = "1.0.0")]
609 pub struct RefCell<T: ?Sized> {
610 borrow: Cell<BorrowFlag>,
611 // Stores the location of the earliest currently active borrow.
612 // This gets updated whenever we go from having zero borrows
613 // to having a single borrow. When a borrow occurs, this gets included
614 // in the generated `BorrowError/`BorrowMutError`
615 #[cfg(feature = "debug_refcell")]
616 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
617 value: UnsafeCell<T>,
620 /// An error returned by [`RefCell::try_borrow`].
621 #[stable(feature = "try_borrow", since = "1.13.0")]
623 pub struct BorrowError {
624 #[cfg(feature = "debug_refcell")]
625 location: &'static crate::panic::Location<'static>,
628 #[stable(feature = "try_borrow", since = "1.13.0")]
629 impl Debug for BorrowError {
630 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
631 let mut builder = f.debug_struct("BorrowError");
633 #[cfg(feature = "debug_refcell")]
634 builder.field("location", self.location);
640 #[stable(feature = "try_borrow", since = "1.13.0")]
641 impl Display for BorrowError {
642 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
643 Display::fmt("already mutably borrowed", f)
647 /// An error returned by [`RefCell::try_borrow_mut`].
648 #[stable(feature = "try_borrow", since = "1.13.0")]
650 pub struct BorrowMutError {
651 #[cfg(feature = "debug_refcell")]
652 location: &'static crate::panic::Location<'static>,
655 #[stable(feature = "try_borrow", since = "1.13.0")]
656 impl Debug for BorrowMutError {
657 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
658 let mut builder = f.debug_struct("BorrowMutError");
660 #[cfg(feature = "debug_refcell")]
661 builder.field("location", self.location);
667 #[stable(feature = "try_borrow", since = "1.13.0")]
668 impl Display for BorrowMutError {
669 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
670 Display::fmt("already borrowed", f)
674 // Positive values represent the number of `Ref` active. Negative values
675 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
676 // active at a time if they refer to distinct, nonoverlapping components of a
677 // `RefCell` (e.g., different ranges of a slice).
679 // `Ref` and `RefMut` are both two words in size, and so there will likely never
680 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
681 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
682 // However, this is not a guarantee, as a pathological program could repeatedly
683 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
684 // explicitly check for overflow and underflow in order to avoid unsafety, or at
685 // least behave correctly in the event that overflow or underflow happens (e.g.,
686 // see BorrowRef::new).
687 type BorrowFlag = isize;
688 const UNUSED: BorrowFlag = 0;
691 fn is_writing(x: BorrowFlag) -> bool {
696 fn is_reading(x: BorrowFlag) -> bool {
701 /// Creates a new `RefCell` containing `value`.
706 /// use std::cell::RefCell;
708 /// let c = RefCell::new(5);
710 #[stable(feature = "rust1", since = "1.0.0")]
711 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
713 pub const fn new(value: T) -> RefCell<T> {
715 value: UnsafeCell::new(value),
716 borrow: Cell::new(UNUSED),
717 #[cfg(feature = "debug_refcell")]
718 borrowed_at: Cell::new(None),
722 /// Consumes the `RefCell`, returning the wrapped value.
727 /// use std::cell::RefCell;
729 /// let c = RefCell::new(5);
731 /// let five = c.into_inner();
733 #[stable(feature = "rust1", since = "1.0.0")]
734 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
736 pub const fn into_inner(self) -> T {
737 // Since this function takes `self` (the `RefCell`) by value, the
738 // compiler statically verifies that it is not currently borrowed.
739 self.value.into_inner()
742 /// Replaces the wrapped value with a new one, returning the old value,
743 /// without deinitializing either one.
745 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
749 /// Panics if the value is currently borrowed.
754 /// use std::cell::RefCell;
755 /// let cell = RefCell::new(5);
756 /// let old_value = cell.replace(6);
757 /// assert_eq!(old_value, 5);
758 /// assert_eq!(cell, RefCell::new(6));
761 #[stable(feature = "refcell_replace", since = "1.24.0")]
763 pub fn replace(&self, t: T) -> T {
764 mem::replace(&mut *self.borrow_mut(), t)
767 /// Replaces the wrapped value with a new one computed from `f`, returning
768 /// the old value, without deinitializing either one.
772 /// Panics if the value is currently borrowed.
777 /// use std::cell::RefCell;
778 /// let cell = RefCell::new(5);
779 /// let old_value = cell.replace_with(|&mut old| old + 1);
780 /// assert_eq!(old_value, 5);
781 /// assert_eq!(cell, RefCell::new(6));
784 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
786 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
787 let mut_borrow = &mut *self.borrow_mut();
788 let replacement = f(mut_borrow);
789 mem::replace(mut_borrow, replacement)
792 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
793 /// without deinitializing either one.
795 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
799 /// Panics if the value in either `RefCell` is currently borrowed.
804 /// use std::cell::RefCell;
805 /// let c = RefCell::new(5);
806 /// let d = RefCell::new(6);
808 /// assert_eq!(c, RefCell::new(6));
809 /// assert_eq!(d, RefCell::new(5));
812 #[stable(feature = "refcell_swap", since = "1.24.0")]
813 pub fn swap(&self, other: &Self) {
814 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
818 impl<T: ?Sized> RefCell<T> {
819 /// Immutably borrows the wrapped value.
821 /// The borrow lasts until the returned `Ref` exits scope. Multiple
822 /// immutable borrows can be taken out at the same time.
826 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
827 /// [`try_borrow`](#method.try_borrow).
832 /// use std::cell::RefCell;
834 /// let c = RefCell::new(5);
836 /// let borrowed_five = c.borrow();
837 /// let borrowed_five2 = c.borrow();
840 /// An example of panic:
843 /// use std::cell::RefCell;
845 /// let c = RefCell::new(5);
847 /// let m = c.borrow_mut();
848 /// let b = c.borrow(); // this causes a panic
850 #[stable(feature = "rust1", since = "1.0.0")]
853 pub fn borrow(&self) -> Ref<'_, T> {
854 self.try_borrow().expect("already mutably borrowed")
857 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
860 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
861 /// taken out at the same time.
863 /// This is the non-panicking variant of [`borrow`](#method.borrow).
868 /// use std::cell::RefCell;
870 /// let c = RefCell::new(5);
873 /// let m = c.borrow_mut();
874 /// assert!(c.try_borrow().is_err());
878 /// let m = c.borrow();
879 /// assert!(c.try_borrow().is_ok());
882 #[stable(feature = "try_borrow", since = "1.13.0")]
884 #[cfg_attr(feature = "debug_refcell", track_caller)]
885 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
886 match BorrowRef::new(&self.borrow) {
888 #[cfg(feature = "debug_refcell")]
890 // `borrowed_at` is always the *first* active borrow
891 if b.borrow.get() == 1 {
892 self.borrowed_at.set(Some(crate::panic::Location::caller()));
896 // SAFETY: `BorrowRef` ensures that there is only immutable access
897 // to the value while borrowed.
898 Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
900 None => Err(BorrowError {
901 // If a borrow occurred, then we must already have an outstanding borrow,
902 // so `borrowed_at` will be `Some`
903 #[cfg(feature = "debug_refcell")]
904 location: self.borrowed_at.get().unwrap(),
909 /// Mutably borrows the wrapped value.
911 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
912 /// from it exit scope. The value cannot be borrowed while this borrow is
917 /// Panics if the value is currently borrowed. For a non-panicking variant, use
918 /// [`try_borrow_mut`](#method.try_borrow_mut).
923 /// use std::cell::RefCell;
925 /// let c = RefCell::new("hello".to_owned());
927 /// *c.borrow_mut() = "bonjour".to_owned();
929 /// assert_eq!(&*c.borrow(), "bonjour");
932 /// An example of panic:
935 /// use std::cell::RefCell;
937 /// let c = RefCell::new(5);
938 /// let m = c.borrow();
940 /// let b = c.borrow_mut(); // this causes a panic
942 #[stable(feature = "rust1", since = "1.0.0")]
945 pub fn borrow_mut(&self) -> RefMut<'_, T> {
946 self.try_borrow_mut().expect("already borrowed")
949 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
951 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
952 /// from it exit scope. The value cannot be borrowed while this borrow is
955 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
960 /// use std::cell::RefCell;
962 /// let c = RefCell::new(5);
965 /// let m = c.borrow();
966 /// assert!(c.try_borrow_mut().is_err());
969 /// assert!(c.try_borrow_mut().is_ok());
971 #[stable(feature = "try_borrow", since = "1.13.0")]
973 #[cfg_attr(feature = "debug_refcell", track_caller)]
974 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
975 match BorrowRefMut::new(&self.borrow) {
977 #[cfg(feature = "debug_refcell")]
979 self.borrowed_at.set(Some(crate::panic::Location::caller()));
982 // SAFETY: `BorrowRef` guarantees unique access.
983 Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
985 None => Err(BorrowMutError {
986 // If a borrow occurred, then we must already have an outstanding borrow,
987 // so `borrowed_at` will be `Some`
988 #[cfg(feature = "debug_refcell")]
989 location: self.borrowed_at.get().unwrap(),
994 /// Returns a raw pointer to the underlying data in this cell.
999 /// use std::cell::RefCell;
1001 /// let c = RefCell::new(5);
1003 /// let ptr = c.as_ptr();
1006 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1007 pub fn as_ptr(&self) -> *mut T {
1011 /// Returns a mutable reference to the underlying data.
1013 /// This call borrows `RefCell` mutably (at compile-time) so there is no
1014 /// need for dynamic checks.
1016 /// However be cautious: this method expects `self` to be mutable, which is
1017 /// generally not the case when using a `RefCell`. Take a look at the
1018 /// [`borrow_mut`] method instead if `self` isn't mutable.
1020 /// Also, please be aware that this method is only for special circumstances and is usually
1021 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
1023 /// [`borrow_mut`]: RefCell::borrow_mut()
1028 /// use std::cell::RefCell;
1030 /// let mut c = RefCell::new(5);
1031 /// *c.get_mut() += 1;
1033 /// assert_eq!(c, RefCell::new(6));
1036 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1037 pub fn get_mut(&mut self) -> &mut T {
1038 self.value.get_mut()
1041 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1043 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1044 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1045 /// if some `Ref` or `RefMut` borrows have been leaked.
1047 /// [`get_mut`]: RefCell::get_mut()
1052 /// #![feature(cell_leak)]
1053 /// use std::cell::RefCell;
1055 /// let mut c = RefCell::new(0);
1056 /// std::mem::forget(c.borrow_mut());
1058 /// assert!(c.try_borrow().is_err());
1060 /// assert!(c.try_borrow().is_ok());
1062 #[unstable(feature = "cell_leak", issue = "69099")]
1063 pub fn undo_leak(&mut self) -> &mut T {
1064 *self.borrow.get_mut() = UNUSED;
1068 /// Immutably borrows the wrapped value, returning an error if the value is
1069 /// currently mutably borrowed.
1073 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1074 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1075 /// borrowing the `RefCell` while the reference returned by this method
1076 /// is alive is undefined behaviour.
1081 /// use std::cell::RefCell;
1083 /// let c = RefCell::new(5);
1086 /// let m = c.borrow_mut();
1087 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1091 /// let m = c.borrow();
1092 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1095 #[stable(feature = "borrow_state", since = "1.37.0")]
1097 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1098 if !is_writing(self.borrow.get()) {
1099 // SAFETY: We check that nobody is actively writing now, but it is
1100 // the caller's responsibility to ensure that nobody writes until
1101 // the returned reference is no longer in use.
1102 // Also, `self.value.get()` refers to the value owned by `self`
1103 // and is thus guaranteed to be valid for the lifetime of `self`.
1104 Ok(unsafe { &*self.value.get() })
1107 // If a borrow occurred, then we must already have an outstanding borrow,
1108 // so `borrowed_at` will be `Some`
1109 #[cfg(feature = "debug_refcell")]
1110 location: self.borrowed_at.get().unwrap(),
1116 impl<T: Default> RefCell<T> {
1117 /// Takes the wrapped value, leaving `Default::default()` in its place.
1121 /// Panics if the value is currently borrowed.
1126 /// use std::cell::RefCell;
1128 /// let c = RefCell::new(5);
1129 /// let five = c.take();
1131 /// assert_eq!(five, 5);
1132 /// assert_eq!(c.into_inner(), 0);
1134 #[stable(feature = "refcell_take", since = "1.50.0")]
1135 pub fn take(&self) -> T {
1136 self.replace(Default::default())
1140 #[stable(feature = "rust1", since = "1.0.0")]
1141 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1143 #[stable(feature = "rust1", since = "1.0.0")]
1144 impl<T: ?Sized> !Sync for RefCell<T> {}
1146 #[stable(feature = "rust1", since = "1.0.0")]
1147 impl<T: Clone> Clone for RefCell<T> {
1150 /// Panics if the value is currently mutably borrowed.
1153 fn clone(&self) -> RefCell<T> {
1154 RefCell::new(self.borrow().clone())
1159 /// Panics if `other` is currently mutably borrowed.
1162 fn clone_from(&mut self, other: &Self) {
1163 self.get_mut().clone_from(&other.borrow())
1167 #[stable(feature = "rust1", since = "1.0.0")]
1168 impl<T: Default> Default for RefCell<T> {
1169 /// Creates a `RefCell<T>`, with the `Default` value for T.
1171 fn default() -> RefCell<T> {
1172 RefCell::new(Default::default())
1176 #[stable(feature = "rust1", since = "1.0.0")]
1177 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1180 /// Panics if the value in either `RefCell` is currently borrowed.
1182 fn eq(&self, other: &RefCell<T>) -> bool {
1183 *self.borrow() == *other.borrow()
1187 #[stable(feature = "cell_eq", since = "1.2.0")]
1188 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1190 #[stable(feature = "cell_ord", since = "1.10.0")]
1191 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1194 /// Panics if the value in either `RefCell` is currently borrowed.
1196 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1197 self.borrow().partial_cmp(&*other.borrow())
1202 /// Panics if the value in either `RefCell` is currently borrowed.
1204 fn lt(&self, other: &RefCell<T>) -> bool {
1205 *self.borrow() < *other.borrow()
1210 /// Panics if the value in either `RefCell` is currently borrowed.
1212 fn le(&self, other: &RefCell<T>) -> bool {
1213 *self.borrow() <= *other.borrow()
1218 /// Panics if the value in either `RefCell` is currently borrowed.
1220 fn gt(&self, other: &RefCell<T>) -> bool {
1221 *self.borrow() > *other.borrow()
1226 /// Panics if the value in either `RefCell` is currently borrowed.
1228 fn ge(&self, other: &RefCell<T>) -> bool {
1229 *self.borrow() >= *other.borrow()
1233 #[stable(feature = "cell_ord", since = "1.10.0")]
1234 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1237 /// Panics if the value in either `RefCell` is currently borrowed.
1239 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1240 self.borrow().cmp(&*other.borrow())
1244 #[stable(feature = "cell_from", since = "1.12.0")]
1245 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1246 impl<T> const From<T> for RefCell<T> {
1247 fn from(t: T) -> RefCell<T> {
1252 #[unstable(feature = "coerce_unsized", issue = "27732")]
1253 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1255 struct BorrowRef<'b> {
1256 borrow: &'b Cell<BorrowFlag>,
1259 impl<'b> BorrowRef<'b> {
1261 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1262 let b = borrow.get().wrapping_add(1);
1264 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1265 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1266 // due to Rust's reference aliasing rules
1267 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1268 // into isize::MIN (the max amount of writing borrows) so we can't allow
1269 // an additional read borrow because isize can't represent so many read borrows
1270 // (this can only happen if you mem::forget more than a small constant amount of
1271 // `Ref`s, which is not good practice)
1274 // Incrementing borrow can result in a reading value (> 0) in these cases:
1275 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1276 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1277 // is large enough to represent having one more read borrow
1279 Some(BorrowRef { borrow })
1284 impl Drop for BorrowRef<'_> {
1286 fn drop(&mut self) {
1287 let borrow = self.borrow.get();
1288 debug_assert!(is_reading(borrow));
1289 self.borrow.set(borrow - 1);
1293 impl Clone for BorrowRef<'_> {
1295 fn clone(&self) -> Self {
1296 // Since this Ref exists, we know the borrow flag
1297 // is a reading borrow.
1298 let borrow = self.borrow.get();
1299 debug_assert!(is_reading(borrow));
1300 // Prevent the borrow counter from overflowing into
1301 // a writing borrow.
1302 assert!(borrow != isize::MAX);
1303 self.borrow.set(borrow + 1);
1304 BorrowRef { borrow: self.borrow }
1308 /// Wraps a borrowed reference to a value in a `RefCell` box.
1309 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1311 /// See the [module-level documentation](self) for more.
1312 #[stable(feature = "rust1", since = "1.0.0")]
1315 must_not_suspend = "holding a Ref across suspend \
1316 points can cause BorrowErrors"
1318 pub struct Ref<'b, T: ?Sized + 'b> {
1320 borrow: BorrowRef<'b>,
1323 #[stable(feature = "rust1", since = "1.0.0")]
1324 impl<T: ?Sized> Deref for Ref<'_, T> {
1328 fn deref(&self) -> &T {
1333 impl<'b, T: ?Sized> Ref<'b, T> {
1336 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1338 /// This is an associated function that needs to be used as
1339 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1340 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1342 #[stable(feature = "cell_extras", since = "1.15.0")]
1345 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1346 Ref { value: orig.value, borrow: orig.borrow.clone() }
1349 /// Makes a new `Ref` for a component of the borrowed data.
1351 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1353 /// This is an associated function that needs to be used as `Ref::map(...)`.
1354 /// A method would interfere with methods of the same name on the contents
1355 /// of a `RefCell` used through `Deref`.
1360 /// use std::cell::{RefCell, Ref};
1362 /// let c = RefCell::new((5, 'b'));
1363 /// let b1: Ref<(u32, char)> = c.borrow();
1364 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1365 /// assert_eq!(*b2, 5)
1367 #[stable(feature = "cell_map", since = "1.8.0")]
1369 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1371 F: FnOnce(&T) -> &U,
1373 Ref { value: f(orig.value), borrow: orig.borrow }
1376 /// Makes a new `Ref` for an optional component of the borrowed data. The
1377 /// original guard is returned as an `Err(..)` if the closure returns
1380 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1382 /// This is an associated function that needs to be used as
1383 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1384 /// name on the contents of a `RefCell` used through `Deref`.
1389 /// #![feature(cell_filter_map)]
1391 /// use std::cell::{RefCell, Ref};
1393 /// let c = RefCell::new(vec![1, 2, 3]);
1394 /// let b1: Ref<Vec<u32>> = c.borrow();
1395 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1396 /// assert_eq!(*b2.unwrap(), 2);
1398 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1400 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1402 F: FnOnce(&T) -> Option<&U>,
1404 match f(orig.value) {
1405 Some(value) => Ok(Ref { value, borrow: orig.borrow }),
1410 /// Splits a `Ref` into multiple `Ref`s for different components of the
1413 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1415 /// This is an associated function that needs to be used as
1416 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1417 /// name on the contents of a `RefCell` used through `Deref`.
1422 /// use std::cell::{Ref, RefCell};
1424 /// let cell = RefCell::new([1, 2, 3, 4]);
1425 /// let borrow = cell.borrow();
1426 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1427 /// assert_eq!(*begin, [1, 2]);
1428 /// assert_eq!(*end, [3, 4]);
1430 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1432 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1434 F: FnOnce(&T) -> (&U, &V),
1436 let (a, b) = f(orig.value);
1437 let borrow = orig.borrow.clone();
1438 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1441 /// Convert into a reference to the underlying data.
1443 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1444 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1445 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1446 /// have occurred in total.
1448 /// This is an associated function that needs to be used as
1449 /// `Ref::leak(...)`. A method would interfere with methods of the
1450 /// same name on the contents of a `RefCell` used through `Deref`.
1455 /// #![feature(cell_leak)]
1456 /// use std::cell::{RefCell, Ref};
1457 /// let cell = RefCell::new(0);
1459 /// let value = Ref::leak(cell.borrow());
1460 /// assert_eq!(*value, 0);
1462 /// assert!(cell.try_borrow().is_ok());
1463 /// assert!(cell.try_borrow_mut().is_err());
1465 #[unstable(feature = "cell_leak", issue = "69099")]
1466 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1467 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1468 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1469 // unique reference to the borrowed RefCell. No further mutable references can be created
1470 // from the original cell.
1471 mem::forget(orig.borrow);
1476 #[unstable(feature = "coerce_unsized", issue = "27732")]
1477 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1479 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1480 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1481 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1486 impl<'b, T: ?Sized> RefMut<'b, T> {
1487 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1490 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1492 /// This is an associated function that needs to be used as
1493 /// `RefMut::map(...)`. A method would interfere with methods of the same
1494 /// name on the contents of a `RefCell` used through `Deref`.
1499 /// use std::cell::{RefCell, RefMut};
1501 /// let c = RefCell::new((5, 'b'));
1503 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1504 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1505 /// assert_eq!(*b2, 5);
1508 /// assert_eq!(*c.borrow(), (42, 'b'));
1510 #[stable(feature = "cell_map", since = "1.8.0")]
1512 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1514 F: FnOnce(&mut T) -> &mut U,
1516 // FIXME(nll-rfc#40): fix borrow-check
1517 let RefMut { value, borrow } = orig;
1518 RefMut { value: f(value), borrow }
1521 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1522 /// original guard is returned as an `Err(..)` if the closure returns
1525 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1527 /// This is an associated function that needs to be used as
1528 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1529 /// same name on the contents of a `RefCell` used through `Deref`.
1534 /// #![feature(cell_filter_map)]
1536 /// use std::cell::{RefCell, RefMut};
1538 /// let c = RefCell::new(vec![1, 2, 3]);
1541 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1542 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1544 /// if let Ok(mut b2) = b2 {
1549 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1551 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1553 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1555 F: FnOnce(&mut T) -> Option<&mut U>,
1557 // FIXME(nll-rfc#40): fix borrow-check
1558 let RefMut { value, borrow } = orig;
1559 let value = value as *mut T;
1560 // SAFETY: function holds onto an exclusive reference for the duration
1561 // of its call through `orig`, and the pointer is only de-referenced
1562 // inside of the function call never allowing the exclusive reference to
1564 match f(unsafe { &mut *value }) {
1565 Some(value) => Ok(RefMut { value, borrow }),
1567 // SAFETY: same as above.
1568 Err(RefMut { value: unsafe { &mut *value }, borrow })
1573 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1576 /// The underlying `RefCell` will remain mutably borrowed until both
1577 /// returned `RefMut`s go out of scope.
1579 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1581 /// This is an associated function that needs to be used as
1582 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1583 /// same name on the contents of a `RefCell` used through `Deref`.
1588 /// use std::cell::{RefCell, RefMut};
1590 /// let cell = RefCell::new([1, 2, 3, 4]);
1591 /// let borrow = cell.borrow_mut();
1592 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1593 /// assert_eq!(*begin, [1, 2]);
1594 /// assert_eq!(*end, [3, 4]);
1595 /// begin.copy_from_slice(&[4, 3]);
1596 /// end.copy_from_slice(&[2, 1]);
1598 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1600 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1601 orig: RefMut<'b, T>,
1603 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1605 F: FnOnce(&mut T) -> (&mut U, &mut V),
1607 let (a, b) = f(orig.value);
1608 let borrow = orig.borrow.clone();
1609 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1612 /// Convert into a mutable reference to the underlying data.
1614 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1615 /// mutably borrowed, making the returned reference the only to the interior.
1617 /// This is an associated function that needs to be used as
1618 /// `RefMut::leak(...)`. A method would interfere with methods of the
1619 /// same name on the contents of a `RefCell` used through `Deref`.
1624 /// #![feature(cell_leak)]
1625 /// use std::cell::{RefCell, RefMut};
1626 /// let cell = RefCell::new(0);
1628 /// let value = RefMut::leak(cell.borrow_mut());
1629 /// assert_eq!(*value, 0);
1632 /// assert!(cell.try_borrow_mut().is_err());
1634 #[unstable(feature = "cell_leak", issue = "69099")]
1635 pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
1636 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1637 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1638 // require a unique reference to the borrowed RefCell. No further references can be created
1639 // from the original cell within that lifetime, making the current borrow the only
1640 // reference for the remaining lifetime.
1641 mem::forget(orig.borrow);
1646 struct BorrowRefMut<'b> {
1647 borrow: &'b Cell<BorrowFlag>,
1650 impl Drop for BorrowRefMut<'_> {
1652 fn drop(&mut self) {
1653 let borrow = self.borrow.get();
1654 debug_assert!(is_writing(borrow));
1655 self.borrow.set(borrow + 1);
1659 impl<'b> BorrowRefMut<'b> {
1661 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1662 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1663 // mutable reference, and so there must currently be no existing
1664 // references. Thus, while clone increments the mutable refcount, here
1665 // we explicitly only allow going from UNUSED to UNUSED - 1.
1666 match borrow.get() {
1668 borrow.set(UNUSED - 1);
1669 Some(BorrowRefMut { borrow })
1675 // Clones a `BorrowRefMut`.
1677 // This is only valid if each `BorrowRefMut` is used to track a mutable
1678 // reference to a distinct, nonoverlapping range of the original object.
1679 // This isn't in a Clone impl so that code doesn't call this implicitly.
1681 fn clone(&self) -> BorrowRefMut<'b> {
1682 let borrow = self.borrow.get();
1683 debug_assert!(is_writing(borrow));
1684 // Prevent the borrow counter from underflowing.
1685 assert!(borrow != isize::MIN);
1686 self.borrow.set(borrow - 1);
1687 BorrowRefMut { borrow: self.borrow }
1691 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1693 /// See the [module-level documentation](self) for more.
1694 #[stable(feature = "rust1", since = "1.0.0")]
1697 must_not_suspend = "holding a RefMut across suspend \
1698 points can cause BorrowErrors"
1700 pub struct RefMut<'b, T: ?Sized + 'b> {
1702 borrow: BorrowRefMut<'b>,
1705 #[stable(feature = "rust1", since = "1.0.0")]
1706 impl<T: ?Sized> Deref for RefMut<'_, T> {
1710 fn deref(&self) -> &T {
1715 #[stable(feature = "rust1", since = "1.0.0")]
1716 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1718 fn deref_mut(&mut self) -> &mut T {
1723 #[unstable(feature = "coerce_unsized", issue = "27732")]
1724 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1726 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1727 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1728 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1733 /// The core primitive for interior mutability in Rust.
1735 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1736 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1737 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1738 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1739 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1741 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1742 /// use `UnsafeCell` to wrap their data.
1744 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1745 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1746 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1748 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1749 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1752 /// [`.get()`]: `UnsafeCell::get`
1754 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1756 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1757 /// reference) that is accessible by safe code (for example, because you returned it),
1758 /// then you must not access the data in any way that contradicts that reference for the
1759 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1760 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1761 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1762 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1763 /// safe code, then you must not access the data within the `UnsafeCell` until that
1764 /// reference expires.
1766 /// - At all times, you must avoid data races. If multiple threads have access to
1767 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1768 /// accesses (or use atomics).
1770 /// To assist with proper design, the following scenarios are explicitly declared legal
1771 /// for single-threaded code:
1773 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1774 /// references, but not with a `&mut T`
1776 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1777 /// co-exist with it. A `&mut T` must always be unique.
1779 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1780 /// `&UnsafeCell<T>` references alias the cell) is
1781 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1782 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1783 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1784 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1785 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1786 /// may be aliased for the duration of that `&mut` borrow.
1787 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1790 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1794 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1795 /// there being multiple references aliasing the cell:
1798 /// use std::cell::UnsafeCell;
1800 /// let x: UnsafeCell<i32> = 42.into();
1801 /// // Get multiple / concurrent / shared references to the same `x`.
1802 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1805 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1806 /// // so ours is effectively unique.
1807 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1808 /// *p1_exclusive += 27; // |
1809 /// } // <---------- cannot go beyond this point -------------------+
1812 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1813 /// // so we can have multiple shared accesses concurrently.
1814 /// let p2_shared: &i32 = &*p2.get();
1815 /// assert_eq!(*p2_shared, 42 + 27);
1816 /// let p1_shared: &i32 = &*p1.get();
1817 /// assert_eq!(*p1_shared, *p2_shared);
1821 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1822 /// implies exclusive access to its `T`:
1825 /// #![forbid(unsafe_code)] // with exclusive accesses,
1826 /// // `UnsafeCell` is a transparent no-op wrapper,
1827 /// // so no need for `unsafe` here.
1828 /// use std::cell::UnsafeCell;
1830 /// let mut x: UnsafeCell<i32> = 42.into();
1832 /// // Get a compile-time-checked unique reference to `x`.
1833 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1834 /// // With an exclusive reference, we can mutate the contents for free.
1835 /// *p_unique.get_mut() = 0;
1836 /// // Or, equivalently:
1837 /// x = UnsafeCell::new(0);
1839 /// // When we own the value, we can extract the contents for free.
1840 /// let contents: i32 = x.into_inner();
1841 /// assert_eq!(contents, 0);
1843 #[lang = "unsafe_cell"]
1844 #[stable(feature = "rust1", since = "1.0.0")]
1845 #[repr(transparent)]
1846 #[repr(no_niche)] // rust-lang/rust#68303.
1847 pub struct UnsafeCell<T: ?Sized> {
1851 #[stable(feature = "rust1", since = "1.0.0")]
1852 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1854 impl<T> UnsafeCell<T> {
1855 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1858 /// All access to the inner value through methods is `unsafe`.
1863 /// use std::cell::UnsafeCell;
1865 /// let uc = UnsafeCell::new(5);
1867 #[stable(feature = "rust1", since = "1.0.0")]
1868 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1870 pub const fn new(value: T) -> UnsafeCell<T> {
1871 UnsafeCell { value }
1874 /// Unwraps the value.
1879 /// use std::cell::UnsafeCell;
1881 /// let uc = UnsafeCell::new(5);
1883 /// let five = uc.into_inner();
1886 #[stable(feature = "rust1", since = "1.0.0")]
1887 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1888 pub const fn into_inner(self) -> T {
1893 impl<T: ?Sized> UnsafeCell<T> {
1894 /// Gets a mutable pointer to the wrapped value.
1896 /// This can be cast to a pointer of any kind.
1897 /// Ensure that the access is unique (no active references, mutable or not)
1898 /// when casting to `&mut T`, and ensure that there are no mutations
1899 /// or mutable aliases going on when casting to `&T`
1904 /// use std::cell::UnsafeCell;
1906 /// let uc = UnsafeCell::new(5);
1908 /// let five = uc.get();
1911 #[stable(feature = "rust1", since = "1.0.0")]
1912 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1913 pub const fn get(&self) -> *mut T {
1914 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1915 // #[repr(transparent)]. This exploits libstd's special status, there is
1916 // no guarantee for user code that this will work in future versions of the compiler!
1917 self as *const UnsafeCell<T> as *const T as *mut T
1920 /// Returns a mutable reference to the underlying data.
1922 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
1923 /// guarantees that we possess the only reference.
1928 /// use std::cell::UnsafeCell;
1930 /// let mut c = UnsafeCell::new(5);
1931 /// *c.get_mut() += 1;
1933 /// assert_eq!(*c.get_mut(), 6);
1936 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
1937 #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
1938 pub const fn get_mut(&mut self) -> &mut T {
1942 /// Gets a mutable pointer to the wrapped value.
1943 /// The difference from [`get`] is that this function accepts a raw pointer,
1944 /// which is useful to avoid the creation of temporary references.
1946 /// The result can be cast to a pointer of any kind.
1947 /// Ensure that the access is unique (no active references, mutable or not)
1948 /// when casting to `&mut T`, and ensure that there are no mutations
1949 /// or mutable aliases going on when casting to `&T`.
1951 /// [`get`]: UnsafeCell::get()
1955 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1956 /// calling `get` would require creating a reference to uninitialized data:
1959 /// use std::cell::UnsafeCell;
1960 /// use std::mem::MaybeUninit;
1962 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1963 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1964 /// let uc = unsafe { m.assume_init() };
1966 /// assert_eq!(uc.into_inner(), 5);
1969 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
1970 pub const fn raw_get(this: *const Self) -> *mut T {
1971 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1972 // #[repr(transparent)]. This exploits libstd's special status, there is
1973 // no guarantee for user code that this will work in future versions of the compiler!
1974 this as *const T as *mut T
1978 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1979 impl<T: Default> Default for UnsafeCell<T> {
1980 /// Creates an `UnsafeCell`, with the `Default` value for T.
1981 fn default() -> UnsafeCell<T> {
1982 UnsafeCell::new(Default::default())
1986 #[stable(feature = "cell_from", since = "1.12.0")]
1987 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1988 impl<T> const From<T> for UnsafeCell<T> {
1989 fn from(t: T) -> UnsafeCell<T> {
1994 #[unstable(feature = "coerce_unsized", issue = "27732")]
1995 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1998 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1999 let _: UnsafeCell<&dyn Send> = a;
2000 let _: Cell<&dyn Send> = b;
2001 let _: RefCell<&dyn Send> = c;