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 /// Creates a new `Cell<T>` containing the given value.
319 fn from(t: T) -> Cell<T> {
325 /// Creates a new `Cell` containing the given value.
330 /// use std::cell::Cell;
332 /// let c = Cell::new(5);
334 #[stable(feature = "rust1", since = "1.0.0")]
335 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
337 pub const fn new(value: T) -> Cell<T> {
338 Cell { value: UnsafeCell::new(value) }
341 /// Sets the contained value.
346 /// use std::cell::Cell;
348 /// let c = Cell::new(5);
353 #[stable(feature = "rust1", since = "1.0.0")]
354 pub fn set(&self, val: T) {
355 let old = self.replace(val);
359 /// Swaps the values of two `Cell`s.
360 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
365 /// use std::cell::Cell;
367 /// let c1 = Cell::new(5i32);
368 /// let c2 = Cell::new(10i32);
370 /// assert_eq!(10, c1.get());
371 /// assert_eq!(5, c2.get());
374 #[stable(feature = "move_cell", since = "1.17.0")]
375 pub fn swap(&self, other: &Self) {
376 if ptr::eq(self, other) {
379 // SAFETY: This can be risky if called from separate threads, but `Cell`
380 // is `!Sync` so this won't happen. This also won't invalidate any
381 // pointers since `Cell` makes sure nothing else will be pointing into
382 // either of these `Cell`s.
384 ptr::swap(self.value.get(), other.value.get());
388 /// Replaces the contained value with `val`, and returns the old contained value.
393 /// use std::cell::Cell;
395 /// let cell = Cell::new(5);
396 /// assert_eq!(cell.get(), 5);
397 /// assert_eq!(cell.replace(10), 5);
398 /// assert_eq!(cell.get(), 10);
400 #[stable(feature = "move_cell", since = "1.17.0")]
401 pub fn replace(&self, val: T) -> T {
402 // SAFETY: This can cause data races if called from a separate thread,
403 // but `Cell` is `!Sync` so this won't happen.
404 mem::replace(unsafe { &mut *self.value.get() }, val)
407 /// Unwraps the value.
412 /// use std::cell::Cell;
414 /// let c = Cell::new(5);
415 /// let five = c.into_inner();
417 /// assert_eq!(five, 5);
419 #[stable(feature = "move_cell", since = "1.17.0")]
420 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
421 pub const fn into_inner(self) -> T {
422 self.value.into_inner()
426 impl<T: Copy> Cell<T> {
427 /// Returns a copy of the contained value.
432 /// use std::cell::Cell;
434 /// let c = Cell::new(5);
436 /// let five = c.get();
439 #[stable(feature = "rust1", since = "1.0.0")]
440 pub fn get(&self) -> T {
441 // SAFETY: This can cause data races if called from a separate thread,
442 // but `Cell` is `!Sync` so this won't happen.
443 unsafe { *self.value.get() }
446 /// Updates the contained value using a function and returns the new value.
451 /// #![feature(cell_update)]
453 /// use std::cell::Cell;
455 /// let c = Cell::new(5);
456 /// let new = c.update(|x| x + 1);
458 /// assert_eq!(new, 6);
459 /// assert_eq!(c.get(), 6);
462 #[unstable(feature = "cell_update", issue = "50186")]
463 pub fn update<F>(&self, f: F) -> T
467 let old = self.get();
474 impl<T: ?Sized> Cell<T> {
475 /// Returns a raw pointer to the underlying data in this cell.
480 /// use std::cell::Cell;
482 /// let c = Cell::new(5);
484 /// let ptr = c.as_ptr();
487 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
488 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
489 pub const fn as_ptr(&self) -> *mut T {
493 /// Returns a mutable reference to the underlying data.
495 /// This call borrows `Cell` mutably (at compile-time) which guarantees
496 /// that we possess the only reference.
498 /// However be cautious: this method expects `self` to be mutable, which is
499 /// generally not the case when using a `Cell`. If you require interior
500 /// mutability by reference, consider using `RefCell` which provides
501 /// run-time checked mutable borrows through its [`borrow_mut`] method.
503 /// [`borrow_mut`]: RefCell::borrow_mut()
508 /// use std::cell::Cell;
510 /// let mut c = Cell::new(5);
511 /// *c.get_mut() += 1;
513 /// assert_eq!(c.get(), 6);
516 #[stable(feature = "cell_get_mut", since = "1.11.0")]
517 pub fn get_mut(&mut self) -> &mut T {
521 /// Returns a `&Cell<T>` from a `&mut T`
526 /// use std::cell::Cell;
528 /// let slice: &mut [i32] = &mut [1, 2, 3];
529 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
530 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
532 /// assert_eq!(slice_cell.len(), 3);
535 #[stable(feature = "as_cell", since = "1.37.0")]
536 pub fn from_mut(t: &mut T) -> &Cell<T> {
537 // SAFETY: `&mut` ensures unique access.
538 unsafe { &*(t as *mut T as *const Cell<T>) }
542 impl<T: Default> Cell<T> {
543 /// Takes the value of the cell, leaving `Default::default()` in its place.
548 /// use std::cell::Cell;
550 /// let c = Cell::new(5);
551 /// let five = c.take();
553 /// assert_eq!(five, 5);
554 /// assert_eq!(c.into_inner(), 0);
556 #[stable(feature = "move_cell", since = "1.17.0")]
557 pub fn take(&self) -> T {
558 self.replace(Default::default())
562 #[unstable(feature = "coerce_unsized", issue = "27732")]
563 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
566 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
571 /// use std::cell::Cell;
573 /// let slice: &mut [i32] = &mut [1, 2, 3];
574 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
575 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
577 /// assert_eq!(slice_cell.len(), 3);
579 #[stable(feature = "as_cell", since = "1.37.0")]
580 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
581 // SAFETY: `Cell<T>` has the same memory layout as `T`.
582 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
586 impl<T, const N: usize> Cell<[T; N]> {
587 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
592 /// #![feature(as_array_of_cells)]
593 /// use std::cell::Cell;
595 /// let mut array: [i32; 3] = [1, 2, 3];
596 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
597 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
599 #[unstable(feature = "as_array_of_cells", issue = "88248")]
600 pub fn as_array_of_cells(&self) -> &[Cell<T>; N] {
601 // SAFETY: `Cell<T>` has the same memory layout as `T`.
602 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
606 /// A mutable memory location with dynamically checked borrow rules
608 /// See the [module-level documentation](self) for more.
609 #[stable(feature = "rust1", since = "1.0.0")]
610 pub struct RefCell<T: ?Sized> {
611 borrow: Cell<BorrowFlag>,
612 // Stores the location of the earliest currently active borrow.
613 // This gets updated whenever we go from having zero borrows
614 // to having a single borrow. When a borrow occurs, this gets included
615 // in the generated `BorrowError/`BorrowMutError`
616 #[cfg(feature = "debug_refcell")]
617 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
618 value: UnsafeCell<T>,
621 /// An error returned by [`RefCell::try_borrow`].
622 #[stable(feature = "try_borrow", since = "1.13.0")]
624 pub struct BorrowError {
625 #[cfg(feature = "debug_refcell")]
626 location: &'static crate::panic::Location<'static>,
629 #[stable(feature = "try_borrow", since = "1.13.0")]
630 impl Debug for BorrowError {
631 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
632 let mut builder = f.debug_struct("BorrowError");
634 #[cfg(feature = "debug_refcell")]
635 builder.field("location", self.location);
641 #[stable(feature = "try_borrow", since = "1.13.0")]
642 impl Display for BorrowError {
643 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
644 Display::fmt("already mutably borrowed", f)
648 /// An error returned by [`RefCell::try_borrow_mut`].
649 #[stable(feature = "try_borrow", since = "1.13.0")]
651 pub struct BorrowMutError {
652 #[cfg(feature = "debug_refcell")]
653 location: &'static crate::panic::Location<'static>,
656 #[stable(feature = "try_borrow", since = "1.13.0")]
657 impl Debug for BorrowMutError {
658 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
659 let mut builder = f.debug_struct("BorrowMutError");
661 #[cfg(feature = "debug_refcell")]
662 builder.field("location", self.location);
668 #[stable(feature = "try_borrow", since = "1.13.0")]
669 impl Display for BorrowMutError {
670 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
671 Display::fmt("already borrowed", f)
675 // Positive values represent the number of `Ref` active. Negative values
676 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
677 // active at a time if they refer to distinct, nonoverlapping components of a
678 // `RefCell` (e.g., different ranges of a slice).
680 // `Ref` and `RefMut` are both two words in size, and so there will likely never
681 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
682 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
683 // However, this is not a guarantee, as a pathological program could repeatedly
684 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
685 // explicitly check for overflow and underflow in order to avoid unsafety, or at
686 // least behave correctly in the event that overflow or underflow happens (e.g.,
687 // see BorrowRef::new).
688 type BorrowFlag = isize;
689 const UNUSED: BorrowFlag = 0;
692 fn is_writing(x: BorrowFlag) -> bool {
697 fn is_reading(x: BorrowFlag) -> bool {
702 /// Creates a new `RefCell` containing `value`.
707 /// use std::cell::RefCell;
709 /// let c = RefCell::new(5);
711 #[stable(feature = "rust1", since = "1.0.0")]
712 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
714 pub const fn new(value: T) -> RefCell<T> {
716 value: UnsafeCell::new(value),
717 borrow: Cell::new(UNUSED),
718 #[cfg(feature = "debug_refcell")]
719 borrowed_at: Cell::new(None),
723 /// Consumes the `RefCell`, returning the wrapped value.
728 /// use std::cell::RefCell;
730 /// let c = RefCell::new(5);
732 /// let five = c.into_inner();
734 #[stable(feature = "rust1", since = "1.0.0")]
735 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
737 pub const fn into_inner(self) -> T {
738 // Since this function takes `self` (the `RefCell`) by value, the
739 // compiler statically verifies that it is not currently borrowed.
740 self.value.into_inner()
743 /// Replaces the wrapped value with a new one, returning the old value,
744 /// without deinitializing either one.
746 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
750 /// Panics if the value is currently borrowed.
755 /// use std::cell::RefCell;
756 /// let cell = RefCell::new(5);
757 /// let old_value = cell.replace(6);
758 /// assert_eq!(old_value, 5);
759 /// assert_eq!(cell, RefCell::new(6));
762 #[stable(feature = "refcell_replace", since = "1.24.0")]
764 pub fn replace(&self, t: T) -> T {
765 mem::replace(&mut *self.borrow_mut(), t)
768 /// Replaces the wrapped value with a new one computed from `f`, returning
769 /// the old value, without deinitializing either one.
773 /// Panics if the value is currently borrowed.
778 /// use std::cell::RefCell;
779 /// let cell = RefCell::new(5);
780 /// let old_value = cell.replace_with(|&mut old| old + 1);
781 /// assert_eq!(old_value, 5);
782 /// assert_eq!(cell, RefCell::new(6));
785 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
787 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
788 let mut_borrow = &mut *self.borrow_mut();
789 let replacement = f(mut_borrow);
790 mem::replace(mut_borrow, replacement)
793 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
794 /// without deinitializing either one.
796 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
800 /// Panics if the value in either `RefCell` is currently borrowed.
805 /// use std::cell::RefCell;
806 /// let c = RefCell::new(5);
807 /// let d = RefCell::new(6);
809 /// assert_eq!(c, RefCell::new(6));
810 /// assert_eq!(d, RefCell::new(5));
813 #[stable(feature = "refcell_swap", since = "1.24.0")]
814 pub fn swap(&self, other: &Self) {
815 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
819 impl<T: ?Sized> RefCell<T> {
820 /// Immutably borrows the wrapped value.
822 /// The borrow lasts until the returned `Ref` exits scope. Multiple
823 /// immutable borrows can be taken out at the same time.
827 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
828 /// [`try_borrow`](#method.try_borrow).
833 /// use std::cell::RefCell;
835 /// let c = RefCell::new(5);
837 /// let borrowed_five = c.borrow();
838 /// let borrowed_five2 = c.borrow();
841 /// An example of panic:
844 /// use std::cell::RefCell;
846 /// let c = RefCell::new(5);
848 /// let m = c.borrow_mut();
849 /// let b = c.borrow(); // this causes a panic
851 #[stable(feature = "rust1", since = "1.0.0")]
854 pub fn borrow(&self) -> Ref<'_, T> {
855 self.try_borrow().expect("already mutably borrowed")
858 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
861 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
862 /// taken out at the same time.
864 /// This is the non-panicking variant of [`borrow`](#method.borrow).
869 /// use std::cell::RefCell;
871 /// let c = RefCell::new(5);
874 /// let m = c.borrow_mut();
875 /// assert!(c.try_borrow().is_err());
879 /// let m = c.borrow();
880 /// assert!(c.try_borrow().is_ok());
883 #[stable(feature = "try_borrow", since = "1.13.0")]
885 #[cfg_attr(feature = "debug_refcell", track_caller)]
886 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
887 match BorrowRef::new(&self.borrow) {
889 #[cfg(feature = "debug_refcell")]
891 // `borrowed_at` is always the *first* active borrow
892 if b.borrow.get() == 1 {
893 self.borrowed_at.set(Some(crate::panic::Location::caller()));
897 // SAFETY: `BorrowRef` ensures that there is only immutable access
898 // to the value while borrowed.
899 Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
901 None => Err(BorrowError {
902 // If a borrow occurred, then we must already have an outstanding borrow,
903 // so `borrowed_at` will be `Some`
904 #[cfg(feature = "debug_refcell")]
905 location: self.borrowed_at.get().unwrap(),
910 /// Mutably borrows the wrapped value.
912 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
913 /// from it exit scope. The value cannot be borrowed while this borrow is
918 /// Panics if the value is currently borrowed. For a non-panicking variant, use
919 /// [`try_borrow_mut`](#method.try_borrow_mut).
924 /// use std::cell::RefCell;
926 /// let c = RefCell::new("hello".to_owned());
928 /// *c.borrow_mut() = "bonjour".to_owned();
930 /// assert_eq!(&*c.borrow(), "bonjour");
933 /// An example of panic:
936 /// use std::cell::RefCell;
938 /// let c = RefCell::new(5);
939 /// let m = c.borrow();
941 /// let b = c.borrow_mut(); // this causes a panic
943 #[stable(feature = "rust1", since = "1.0.0")]
946 pub fn borrow_mut(&self) -> RefMut<'_, T> {
947 self.try_borrow_mut().expect("already borrowed")
950 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
952 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
953 /// from it exit scope. The value cannot be borrowed while this borrow is
956 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
961 /// use std::cell::RefCell;
963 /// let c = RefCell::new(5);
966 /// let m = c.borrow();
967 /// assert!(c.try_borrow_mut().is_err());
970 /// assert!(c.try_borrow_mut().is_ok());
972 #[stable(feature = "try_borrow", since = "1.13.0")]
974 #[cfg_attr(feature = "debug_refcell", track_caller)]
975 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
976 match BorrowRefMut::new(&self.borrow) {
978 #[cfg(feature = "debug_refcell")]
980 self.borrowed_at.set(Some(crate::panic::Location::caller()));
983 // SAFETY: `BorrowRef` guarantees unique access.
984 Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
986 None => Err(BorrowMutError {
987 // If a borrow occurred, then we must already have an outstanding borrow,
988 // so `borrowed_at` will be `Some`
989 #[cfg(feature = "debug_refcell")]
990 location: self.borrowed_at.get().unwrap(),
995 /// Returns a raw pointer to the underlying data in this cell.
1000 /// use std::cell::RefCell;
1002 /// let c = RefCell::new(5);
1004 /// let ptr = c.as_ptr();
1007 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1008 pub fn as_ptr(&self) -> *mut T {
1012 /// Returns a mutable reference to the underlying data.
1014 /// This call borrows `RefCell` mutably (at compile-time) so there is no
1015 /// need for dynamic checks.
1017 /// However be cautious: this method expects `self` to be mutable, which is
1018 /// generally not the case when using a `RefCell`. Take a look at the
1019 /// [`borrow_mut`] method instead if `self` isn't mutable.
1021 /// Also, please be aware that this method is only for special circumstances and is usually
1022 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
1024 /// [`borrow_mut`]: RefCell::borrow_mut()
1029 /// use std::cell::RefCell;
1031 /// let mut c = RefCell::new(5);
1032 /// *c.get_mut() += 1;
1034 /// assert_eq!(c, RefCell::new(6));
1037 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1038 pub fn get_mut(&mut self) -> &mut T {
1039 self.value.get_mut()
1042 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1044 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1045 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1046 /// if some `Ref` or `RefMut` borrows have been leaked.
1048 /// [`get_mut`]: RefCell::get_mut()
1053 /// #![feature(cell_leak)]
1054 /// use std::cell::RefCell;
1056 /// let mut c = RefCell::new(0);
1057 /// std::mem::forget(c.borrow_mut());
1059 /// assert!(c.try_borrow().is_err());
1061 /// assert!(c.try_borrow().is_ok());
1063 #[unstable(feature = "cell_leak", issue = "69099")]
1064 pub fn undo_leak(&mut self) -> &mut T {
1065 *self.borrow.get_mut() = UNUSED;
1069 /// Immutably borrows the wrapped value, returning an error if the value is
1070 /// currently mutably borrowed.
1074 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1075 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1076 /// borrowing the `RefCell` while the reference returned by this method
1077 /// is alive is undefined behaviour.
1082 /// use std::cell::RefCell;
1084 /// let c = RefCell::new(5);
1087 /// let m = c.borrow_mut();
1088 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1092 /// let m = c.borrow();
1093 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1096 #[stable(feature = "borrow_state", since = "1.37.0")]
1098 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1099 if !is_writing(self.borrow.get()) {
1100 // SAFETY: We check that nobody is actively writing now, but it is
1101 // the caller's responsibility to ensure that nobody writes until
1102 // the returned reference is no longer in use.
1103 // Also, `self.value.get()` refers to the value owned by `self`
1104 // and is thus guaranteed to be valid for the lifetime of `self`.
1105 Ok(unsafe { &*self.value.get() })
1108 // If a borrow occurred, then we must already have an outstanding borrow,
1109 // so `borrowed_at` will be `Some`
1110 #[cfg(feature = "debug_refcell")]
1111 location: self.borrowed_at.get().unwrap(),
1117 impl<T: Default> RefCell<T> {
1118 /// Takes the wrapped value, leaving `Default::default()` in its place.
1122 /// Panics if the value is currently borrowed.
1127 /// use std::cell::RefCell;
1129 /// let c = RefCell::new(5);
1130 /// let five = c.take();
1132 /// assert_eq!(five, 5);
1133 /// assert_eq!(c.into_inner(), 0);
1135 #[stable(feature = "refcell_take", since = "1.50.0")]
1136 pub fn take(&self) -> T {
1137 self.replace(Default::default())
1141 #[stable(feature = "rust1", since = "1.0.0")]
1142 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1144 #[stable(feature = "rust1", since = "1.0.0")]
1145 impl<T: ?Sized> !Sync for RefCell<T> {}
1147 #[stable(feature = "rust1", since = "1.0.0")]
1148 impl<T: Clone> Clone for RefCell<T> {
1151 /// Panics if the value is currently mutably borrowed.
1154 fn clone(&self) -> RefCell<T> {
1155 RefCell::new(self.borrow().clone())
1160 /// Panics if `other` is currently mutably borrowed.
1163 fn clone_from(&mut self, other: &Self) {
1164 self.get_mut().clone_from(&other.borrow())
1168 #[stable(feature = "rust1", since = "1.0.0")]
1169 impl<T: Default> Default for RefCell<T> {
1170 /// Creates a `RefCell<T>`, with the `Default` value for T.
1172 fn default() -> RefCell<T> {
1173 RefCell::new(Default::default())
1177 #[stable(feature = "rust1", since = "1.0.0")]
1178 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1181 /// Panics if the value in either `RefCell` is currently borrowed.
1183 fn eq(&self, other: &RefCell<T>) -> bool {
1184 *self.borrow() == *other.borrow()
1188 #[stable(feature = "cell_eq", since = "1.2.0")]
1189 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1191 #[stable(feature = "cell_ord", since = "1.10.0")]
1192 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1195 /// Panics if the value in either `RefCell` is currently borrowed.
1197 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1198 self.borrow().partial_cmp(&*other.borrow())
1203 /// Panics if the value in either `RefCell` is currently borrowed.
1205 fn lt(&self, other: &RefCell<T>) -> bool {
1206 *self.borrow() < *other.borrow()
1211 /// Panics if the value in either `RefCell` is currently borrowed.
1213 fn le(&self, other: &RefCell<T>) -> bool {
1214 *self.borrow() <= *other.borrow()
1219 /// Panics if the value in either `RefCell` is currently borrowed.
1221 fn gt(&self, other: &RefCell<T>) -> bool {
1222 *self.borrow() > *other.borrow()
1227 /// Panics if the value in either `RefCell` is currently borrowed.
1229 fn ge(&self, other: &RefCell<T>) -> bool {
1230 *self.borrow() >= *other.borrow()
1234 #[stable(feature = "cell_ord", since = "1.10.0")]
1235 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1238 /// Panics if the value in either `RefCell` is currently borrowed.
1240 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1241 self.borrow().cmp(&*other.borrow())
1245 #[stable(feature = "cell_from", since = "1.12.0")]
1246 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1247 impl<T> const From<T> for RefCell<T> {
1248 /// Creates a new `RefCell<T>` containing the given value.
1249 fn from(t: T) -> RefCell<T> {
1254 #[unstable(feature = "coerce_unsized", issue = "27732")]
1255 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1257 struct BorrowRef<'b> {
1258 borrow: &'b Cell<BorrowFlag>,
1261 impl<'b> BorrowRef<'b> {
1263 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1264 let b = borrow.get().wrapping_add(1);
1266 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1267 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1268 // due to Rust's reference aliasing rules
1269 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1270 // into isize::MIN (the max amount of writing borrows) so we can't allow
1271 // an additional read borrow because isize can't represent so many read borrows
1272 // (this can only happen if you mem::forget more than a small constant amount of
1273 // `Ref`s, which is not good practice)
1276 // Incrementing borrow can result in a reading value (> 0) in these cases:
1277 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1278 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1279 // is large enough to represent having one more read borrow
1281 Some(BorrowRef { borrow })
1286 impl Drop for BorrowRef<'_> {
1288 fn drop(&mut self) {
1289 let borrow = self.borrow.get();
1290 debug_assert!(is_reading(borrow));
1291 self.borrow.set(borrow - 1);
1295 impl Clone for BorrowRef<'_> {
1297 fn clone(&self) -> Self {
1298 // Since this Ref exists, we know the borrow flag
1299 // is a reading borrow.
1300 let borrow = self.borrow.get();
1301 debug_assert!(is_reading(borrow));
1302 // Prevent the borrow counter from overflowing into
1303 // a writing borrow.
1304 assert!(borrow != isize::MAX);
1305 self.borrow.set(borrow + 1);
1306 BorrowRef { borrow: self.borrow }
1310 /// Wraps a borrowed reference to a value in a `RefCell` box.
1311 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1313 /// See the [module-level documentation](self) for more.
1314 #[stable(feature = "rust1", since = "1.0.0")]
1315 #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1316 pub struct Ref<'b, T: ?Sized + 'b> {
1318 borrow: BorrowRef<'b>,
1321 #[stable(feature = "rust1", since = "1.0.0")]
1322 impl<T: ?Sized> Deref for Ref<'_, T> {
1326 fn deref(&self) -> &T {
1331 impl<'b, T: ?Sized> Ref<'b, T> {
1334 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1336 /// This is an associated function that needs to be used as
1337 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1338 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1340 #[stable(feature = "cell_extras", since = "1.15.0")]
1343 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1344 Ref { value: orig.value, borrow: orig.borrow.clone() }
1347 /// Makes a new `Ref` for a component of the borrowed data.
1349 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1351 /// This is an associated function that needs to be used as `Ref::map(...)`.
1352 /// A method would interfere with methods of the same name on the contents
1353 /// of a `RefCell` used through `Deref`.
1358 /// use std::cell::{RefCell, Ref};
1360 /// let c = RefCell::new((5, 'b'));
1361 /// let b1: Ref<(u32, char)> = c.borrow();
1362 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1363 /// assert_eq!(*b2, 5)
1365 #[stable(feature = "cell_map", since = "1.8.0")]
1367 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1369 F: FnOnce(&T) -> &U,
1371 Ref { value: f(orig.value), borrow: orig.borrow }
1374 /// Makes a new `Ref` for an optional component of the borrowed data. The
1375 /// original guard is returned as an `Err(..)` if the closure returns
1378 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1380 /// This is an associated function that needs to be used as
1381 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1382 /// name on the contents of a `RefCell` used through `Deref`.
1387 /// #![feature(cell_filter_map)]
1389 /// use std::cell::{RefCell, Ref};
1391 /// let c = RefCell::new(vec![1, 2, 3]);
1392 /// let b1: Ref<Vec<u32>> = c.borrow();
1393 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1394 /// assert_eq!(*b2.unwrap(), 2);
1396 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1398 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1400 F: FnOnce(&T) -> Option<&U>,
1402 match f(orig.value) {
1403 Some(value) => Ok(Ref { value, borrow: orig.borrow }),
1408 /// Splits a `Ref` into multiple `Ref`s for different components of the
1411 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1413 /// This is an associated function that needs to be used as
1414 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1415 /// name on the contents of a `RefCell` used through `Deref`.
1420 /// use std::cell::{Ref, RefCell};
1422 /// let cell = RefCell::new([1, 2, 3, 4]);
1423 /// let borrow = cell.borrow();
1424 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1425 /// assert_eq!(*begin, [1, 2]);
1426 /// assert_eq!(*end, [3, 4]);
1428 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1430 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1432 F: FnOnce(&T) -> (&U, &V),
1434 let (a, b) = f(orig.value);
1435 let borrow = orig.borrow.clone();
1436 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1439 /// Convert into a reference to the underlying data.
1441 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1442 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1443 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1444 /// have occurred in total.
1446 /// This is an associated function that needs to be used as
1447 /// `Ref::leak(...)`. A method would interfere with methods of the
1448 /// same name on the contents of a `RefCell` used through `Deref`.
1453 /// #![feature(cell_leak)]
1454 /// use std::cell::{RefCell, Ref};
1455 /// let cell = RefCell::new(0);
1457 /// let value = Ref::leak(cell.borrow());
1458 /// assert_eq!(*value, 0);
1460 /// assert!(cell.try_borrow().is_ok());
1461 /// assert!(cell.try_borrow_mut().is_err());
1463 #[unstable(feature = "cell_leak", issue = "69099")]
1464 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1465 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1466 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1467 // unique reference to the borrowed RefCell. No further mutable references can be created
1468 // from the original cell.
1469 mem::forget(orig.borrow);
1474 #[unstable(feature = "coerce_unsized", issue = "27732")]
1475 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1477 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1478 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1479 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1484 impl<'b, T: ?Sized> RefMut<'b, T> {
1485 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1488 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1490 /// This is an associated function that needs to be used as
1491 /// `RefMut::map(...)`. A method would interfere with methods of the same
1492 /// name on the contents of a `RefCell` used through `Deref`.
1497 /// use std::cell::{RefCell, RefMut};
1499 /// let c = RefCell::new((5, 'b'));
1501 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1502 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1503 /// assert_eq!(*b2, 5);
1506 /// assert_eq!(*c.borrow(), (42, 'b'));
1508 #[stable(feature = "cell_map", since = "1.8.0")]
1510 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1512 F: FnOnce(&mut T) -> &mut U,
1514 // FIXME(nll-rfc#40): fix borrow-check
1515 let RefMut { value, borrow } = orig;
1516 RefMut { value: f(value), borrow }
1519 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1520 /// original guard is returned as an `Err(..)` if the closure returns
1523 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1525 /// This is an associated function that needs to be used as
1526 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1527 /// same name on the contents of a `RefCell` used through `Deref`.
1532 /// #![feature(cell_filter_map)]
1534 /// use std::cell::{RefCell, RefMut};
1536 /// let c = RefCell::new(vec![1, 2, 3]);
1539 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1540 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1542 /// if let Ok(mut b2) = b2 {
1547 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1549 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1551 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1553 F: FnOnce(&mut T) -> Option<&mut U>,
1555 // FIXME(nll-rfc#40): fix borrow-check
1556 let RefMut { value, borrow } = orig;
1557 let value = value as *mut T;
1558 // SAFETY: function holds onto an exclusive reference for the duration
1559 // of its call through `orig`, and the pointer is only de-referenced
1560 // inside of the function call never allowing the exclusive reference to
1562 match f(unsafe { &mut *value }) {
1563 Some(value) => Ok(RefMut { value, borrow }),
1565 // SAFETY: same as above.
1566 Err(RefMut { value: unsafe { &mut *value }, borrow })
1571 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1574 /// The underlying `RefCell` will remain mutably borrowed until both
1575 /// returned `RefMut`s go out of scope.
1577 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1579 /// This is an associated function that needs to be used as
1580 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1581 /// same name on the contents of a `RefCell` used through `Deref`.
1586 /// use std::cell::{RefCell, RefMut};
1588 /// let cell = RefCell::new([1, 2, 3, 4]);
1589 /// let borrow = cell.borrow_mut();
1590 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1591 /// assert_eq!(*begin, [1, 2]);
1592 /// assert_eq!(*end, [3, 4]);
1593 /// begin.copy_from_slice(&[4, 3]);
1594 /// end.copy_from_slice(&[2, 1]);
1596 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1598 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1599 orig: RefMut<'b, T>,
1601 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1603 F: FnOnce(&mut T) -> (&mut U, &mut V),
1605 let (a, b) = f(orig.value);
1606 let borrow = orig.borrow.clone();
1607 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1610 /// Convert into a mutable reference to the underlying data.
1612 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1613 /// mutably borrowed, making the returned reference the only to the interior.
1615 /// This is an associated function that needs to be used as
1616 /// `RefMut::leak(...)`. A method would interfere with methods of the
1617 /// same name on the contents of a `RefCell` used through `Deref`.
1622 /// #![feature(cell_leak)]
1623 /// use std::cell::{RefCell, RefMut};
1624 /// let cell = RefCell::new(0);
1626 /// let value = RefMut::leak(cell.borrow_mut());
1627 /// assert_eq!(*value, 0);
1630 /// assert!(cell.try_borrow_mut().is_err());
1632 #[unstable(feature = "cell_leak", issue = "69099")]
1633 pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
1634 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1635 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1636 // require a unique reference to the borrowed RefCell. No further references can be created
1637 // from the original cell within that lifetime, making the current borrow the only
1638 // reference for the remaining lifetime.
1639 mem::forget(orig.borrow);
1644 struct BorrowRefMut<'b> {
1645 borrow: &'b Cell<BorrowFlag>,
1648 impl Drop for BorrowRefMut<'_> {
1650 fn drop(&mut self) {
1651 let borrow = self.borrow.get();
1652 debug_assert!(is_writing(borrow));
1653 self.borrow.set(borrow + 1);
1657 impl<'b> BorrowRefMut<'b> {
1659 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1660 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1661 // mutable reference, and so there must currently be no existing
1662 // references. Thus, while clone increments the mutable refcount, here
1663 // we explicitly only allow going from UNUSED to UNUSED - 1.
1664 match borrow.get() {
1666 borrow.set(UNUSED - 1);
1667 Some(BorrowRefMut { borrow })
1673 // Clones a `BorrowRefMut`.
1675 // This is only valid if each `BorrowRefMut` is used to track a mutable
1676 // reference to a distinct, nonoverlapping range of the original object.
1677 // This isn't in a Clone impl so that code doesn't call this implicitly.
1679 fn clone(&self) -> BorrowRefMut<'b> {
1680 let borrow = self.borrow.get();
1681 debug_assert!(is_writing(borrow));
1682 // Prevent the borrow counter from underflowing.
1683 assert!(borrow != isize::MIN);
1684 self.borrow.set(borrow - 1);
1685 BorrowRefMut { borrow: self.borrow }
1689 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1691 /// See the [module-level documentation](self) for more.
1692 #[stable(feature = "rust1", since = "1.0.0")]
1693 #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1694 pub struct RefMut<'b, T: ?Sized + 'b> {
1696 borrow: BorrowRefMut<'b>,
1699 #[stable(feature = "rust1", since = "1.0.0")]
1700 impl<T: ?Sized> Deref for RefMut<'_, T> {
1704 fn deref(&self) -> &T {
1709 #[stable(feature = "rust1", since = "1.0.0")]
1710 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1712 fn deref_mut(&mut self) -> &mut T {
1717 #[unstable(feature = "coerce_unsized", issue = "27732")]
1718 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1720 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1721 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1722 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1727 /// The core primitive for interior mutability in Rust.
1729 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1730 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1731 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1732 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1733 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1735 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1736 /// use `UnsafeCell` to wrap their data.
1738 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1739 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1740 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1742 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1743 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1746 /// [`.get()`]: `UnsafeCell::get`
1748 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1750 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1751 /// reference) that is accessible by safe code (for example, because you returned it),
1752 /// then you must not access the data in any way that contradicts that reference for the
1753 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1754 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1755 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1756 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1757 /// safe code, then you must not access the data within the `UnsafeCell` until that
1758 /// reference expires.
1760 /// - At all times, you must avoid data races. If multiple threads have access to
1761 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1762 /// accesses (or use atomics).
1764 /// To assist with proper design, the following scenarios are explicitly declared legal
1765 /// for single-threaded code:
1767 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1768 /// references, but not with a `&mut T`
1770 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1771 /// co-exist with it. A `&mut T` must always be unique.
1773 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1774 /// `&UnsafeCell<T>` references alias the cell) is
1775 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1776 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1777 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1778 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1779 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1780 /// may be aliased for the duration of that `&mut` borrow.
1781 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1784 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1788 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1789 /// there being multiple references aliasing the cell:
1792 /// use std::cell::UnsafeCell;
1794 /// let x: UnsafeCell<i32> = 42.into();
1795 /// // Get multiple / concurrent / shared references to the same `x`.
1796 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1799 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1800 /// // so ours is effectively unique.
1801 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1802 /// *p1_exclusive += 27; // |
1803 /// } // <---------- cannot go beyond this point -------------------+
1806 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1807 /// // so we can have multiple shared accesses concurrently.
1808 /// let p2_shared: &i32 = &*p2.get();
1809 /// assert_eq!(*p2_shared, 42 + 27);
1810 /// let p1_shared: &i32 = &*p1.get();
1811 /// assert_eq!(*p1_shared, *p2_shared);
1815 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1816 /// implies exclusive access to its `T`:
1819 /// #![forbid(unsafe_code)] // with exclusive accesses,
1820 /// // `UnsafeCell` is a transparent no-op wrapper,
1821 /// // so no need for `unsafe` here.
1822 /// use std::cell::UnsafeCell;
1824 /// let mut x: UnsafeCell<i32> = 42.into();
1826 /// // Get a compile-time-checked unique reference to `x`.
1827 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1828 /// // With an exclusive reference, we can mutate the contents for free.
1829 /// *p_unique.get_mut() = 0;
1830 /// // Or, equivalently:
1831 /// x = UnsafeCell::new(0);
1833 /// // When we own the value, we can extract the contents for free.
1834 /// let contents: i32 = x.into_inner();
1835 /// assert_eq!(contents, 0);
1837 #[lang = "unsafe_cell"]
1838 #[stable(feature = "rust1", since = "1.0.0")]
1839 #[repr(transparent)]
1840 #[repr(no_niche)] // rust-lang/rust#68303.
1841 pub struct UnsafeCell<T: ?Sized> {
1845 #[stable(feature = "rust1", since = "1.0.0")]
1846 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1848 impl<T> UnsafeCell<T> {
1849 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1852 /// All access to the inner value through methods is `unsafe`.
1857 /// use std::cell::UnsafeCell;
1859 /// let uc = UnsafeCell::new(5);
1861 #[stable(feature = "rust1", since = "1.0.0")]
1862 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1864 pub const fn new(value: T) -> UnsafeCell<T> {
1865 UnsafeCell { value }
1868 /// Unwraps the value.
1873 /// use std::cell::UnsafeCell;
1875 /// let uc = UnsafeCell::new(5);
1877 /// let five = uc.into_inner();
1880 #[stable(feature = "rust1", since = "1.0.0")]
1881 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1882 pub const fn into_inner(self) -> T {
1887 impl<T: ?Sized> UnsafeCell<T> {
1888 /// Gets a mutable pointer to the wrapped value.
1890 /// This can be cast to a pointer of any kind.
1891 /// Ensure that the access is unique (no active references, mutable or not)
1892 /// when casting to `&mut T`, and ensure that there are no mutations
1893 /// or mutable aliases going on when casting to `&T`
1898 /// use std::cell::UnsafeCell;
1900 /// let uc = UnsafeCell::new(5);
1902 /// let five = uc.get();
1905 #[stable(feature = "rust1", since = "1.0.0")]
1906 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1907 pub const fn get(&self) -> *mut T {
1908 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1909 // #[repr(transparent)]. This exploits libstd's special status, there is
1910 // no guarantee for user code that this will work in future versions of the compiler!
1911 self as *const UnsafeCell<T> as *const T as *mut T
1914 /// Returns a mutable reference to the underlying data.
1916 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
1917 /// guarantees that we possess the only reference.
1922 /// use std::cell::UnsafeCell;
1924 /// let mut c = UnsafeCell::new(5);
1925 /// *c.get_mut() += 1;
1927 /// assert_eq!(*c.get_mut(), 6);
1930 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
1931 #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
1932 pub const fn get_mut(&mut self) -> &mut T {
1936 /// Gets a mutable pointer to the wrapped value.
1937 /// The difference from [`get`] is that this function accepts a raw pointer,
1938 /// which is useful to avoid the creation of temporary references.
1940 /// The result can be cast to a pointer of any kind.
1941 /// Ensure that the access is unique (no active references, mutable or not)
1942 /// when casting to `&mut T`, and ensure that there are no mutations
1943 /// or mutable aliases going on when casting to `&T`.
1945 /// [`get`]: UnsafeCell::get()
1949 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1950 /// calling `get` would require creating a reference to uninitialized data:
1953 /// use std::cell::UnsafeCell;
1954 /// use std::mem::MaybeUninit;
1956 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1957 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1958 /// let uc = unsafe { m.assume_init() };
1960 /// assert_eq!(uc.into_inner(), 5);
1963 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
1964 #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
1965 pub const fn raw_get(this: *const Self) -> *mut T {
1966 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1967 // #[repr(transparent)]. This exploits libstd's special status, there is
1968 // no guarantee for user code that this will work in future versions of the compiler!
1969 this as *const T as *mut T
1973 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1974 impl<T: Default> Default for UnsafeCell<T> {
1975 /// Creates an `UnsafeCell`, with the `Default` value for T.
1976 fn default() -> UnsafeCell<T> {
1977 UnsafeCell::new(Default::default())
1981 #[stable(feature = "cell_from", since = "1.12.0")]
1982 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1983 impl<T> const From<T> for UnsafeCell<T> {
1984 /// Creates a new `UnsafeCell<T>` containing the given value.
1985 fn from(t: T) -> UnsafeCell<T> {
1990 #[unstable(feature = "coerce_unsized", issue = "27732")]
1991 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1994 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1995 let _: UnsafeCell<&dyn Send> = a;
1996 let _: Cell<&dyn Send> = b;
1997 let _: RefCell<&dyn Send> = c;