1 //! Shareable mutable containers.
3 //! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
4 //! have one of the following:
6 //! - Having several immutable references (`&T`) to the object (also known as **aliasing**).
7 //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**).
9 //! This is enforced by the Rust compiler. However, there are situations where this rule is not
10 //! flexible enough. Sometimes it is required to have multiple references to an object and yet
13 //! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
14 //! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded
15 //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement
16 //! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to
17 //! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types.
19 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
20 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
21 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
22 //! with typical Rust types that exhibit 'inherited mutability'.
24 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
25 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
26 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
27 //! methods to retrieve and change the current interior value:
29 //! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
31 //! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
32 //! interior value with [`Default::default()`] and returns the replaced value.
33 //! - For all types, the [`replace`](Cell::replace) method replaces the current interior value and
34 //! returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the
35 //! `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method
36 //! replaces the interior value, dropping the replaced value.
38 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
39 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
40 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
41 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
42 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
45 //! # When to choose interior mutability
47 //! The more common inherited mutability, where one must have unique access to mutate a value, is
48 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
49 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
50 //! interior mutability is something of a last resort. Since cell types enable mutation where it
51 //! would otherwise be disallowed though, there are occasions when interior mutability might be
52 //! appropriate, or even *must* be used, e.g.
54 //! * Introducing mutability 'inside' of something immutable
55 //! * Implementation details of logically-immutable methods.
56 //! * Mutating implementations of [`Clone`].
58 //! ## Introducing mutability 'inside' of something immutable
60 //! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
61 //! be cloned and shared between multiple parties. Because the contained values may be
62 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
63 //! impossible to mutate data inside of these smart pointers at all.
65 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
69 //! use std::cell::{RefCell, RefMut};
70 //! use std::collections::HashMap;
74 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
75 //! // Create a new block to limit the scope of the dynamic borrow
77 //! let mut map: RefMut<_> = shared_map.borrow_mut();
78 //! map.insert("africa", 92388);
79 //! map.insert("kyoto", 11837);
80 //! map.insert("piccadilly", 11826);
81 //! map.insert("marbles", 38);
84 //! // Note that if we had not let the previous borrow of the cache fall out
85 //! // of scope then the subsequent borrow would cause a dynamic thread panic.
86 //! // This is the major hazard of using `RefCell`.
87 //! let total: i32 = shared_map.borrow().values().sum();
88 //! println!("{}", total);
92 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
93 //! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
94 //! multi-threaded situation.
96 //! ## Implementation details of logically-immutable methods
98 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
99 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
100 //! forces the implementation to perform mutation; or because you must employ mutation to implement
101 //! a trait method that was originally defined to take `&self`.
104 //! # #![allow(dead_code)]
105 //! use std::cell::RefCell;
108 //! edges: Vec<(i32, i32)>,
109 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
113 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
114 //! self.span_tree_cache.borrow_mut()
115 //! .get_or_insert_with(|| self.calc_span_tree())
119 //! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
120 //! // Expensive computation goes here
126 //! ## Mutating implementations of `Clone`
128 //! This is simply a special - but common - case of the previous: hiding mutability for operations
129 //! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
130 //! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
131 //! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
132 //! reference counts within a `Cell<T>`.
135 //! use std::cell::Cell;
136 //! use std::ptr::NonNull;
137 //! use std::process::abort;
138 //! use std::marker::PhantomData;
140 //! struct Rc<T: ?Sized> {
141 //! ptr: NonNull<RcBox<T>>,
142 //! phantom: PhantomData<RcBox<T>>,
145 //! struct RcBox<T: ?Sized> {
146 //! strong: Cell<usize>,
147 //! refcount: Cell<usize>,
151 //! impl<T: ?Sized> Clone for Rc<T> {
152 //! fn clone(&self) -> Rc<T> {
153 //! self.inc_strong();
156 //! phantom: PhantomData,
161 //! trait RcBoxPtr<T: ?Sized> {
163 //! fn inner(&self) -> &RcBox<T>;
165 //! fn strong(&self) -> usize {
166 //! self.inner().strong.get()
169 //! fn inc_strong(&self) {
172 //! .set(self.strong()
174 //! .unwrap_or_else(|| abort() ));
178 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
179 //! fn inner(&self) -> &RcBox<T> {
181 //! self.ptr.as_ref()
187 //! [`Arc<T>`]: ../../std/sync/struct.Arc.html
188 //! [`Rc<T>`]: ../../std/rc/struct.Rc.html
189 //! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
190 //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
191 //! [`atomic`]: crate::sync::atomic
193 #![stable(feature = "rust1", since = "1.0.0")]
195 use crate::cmp::Ordering;
196 use crate::fmt::{self, Debug, Display};
197 use crate::marker::Unsize;
199 use crate::ops::{CoerceUnsized, Deref, DerefMut};
202 /// A mutable memory location.
206 /// In this example, you can see that `Cell<T>` enables mutation inside an
207 /// immutable struct. In other words, it enables "interior mutability".
210 /// use std::cell::Cell;
212 /// struct SomeStruct {
213 /// regular_field: u8,
214 /// special_field: Cell<u8>,
217 /// let my_struct = SomeStruct {
218 /// regular_field: 0,
219 /// special_field: Cell::new(1),
222 /// let new_value = 100;
224 /// // ERROR: `my_struct` is immutable
225 /// // my_struct.regular_field = new_value;
227 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
228 /// // which can always be mutated
229 /// my_struct.special_field.set(new_value);
230 /// assert_eq!(my_struct.special_field.get(), new_value);
233 /// See the [module-level documentation](self) for more.
234 #[stable(feature = "rust1", since = "1.0.0")]
236 pub struct Cell<T: ?Sized> {
237 value: UnsafeCell<T>,
240 #[stable(feature = "rust1", since = "1.0.0")]
241 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
243 #[stable(feature = "rust1", since = "1.0.0")]
244 impl<T: ?Sized> !Sync for Cell<T> {}
246 #[stable(feature = "rust1", since = "1.0.0")]
247 impl<T: Copy> Clone for Cell<T> {
249 fn clone(&self) -> Cell<T> {
250 Cell::new(self.get())
254 #[stable(feature = "rust1", since = "1.0.0")]
255 impl<T: Default> Default for Cell<T> {
256 /// Creates a `Cell<T>`, with the `Default` value for T.
258 fn default() -> Cell<T> {
259 Cell::new(Default::default())
263 #[stable(feature = "rust1", since = "1.0.0")]
264 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
266 fn eq(&self, other: &Cell<T>) -> bool {
267 self.get() == other.get()
271 #[stable(feature = "cell_eq", since = "1.2.0")]
272 impl<T: Eq + Copy> Eq for Cell<T> {}
274 #[stable(feature = "cell_ord", since = "1.10.0")]
275 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
277 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
278 self.get().partial_cmp(&other.get())
282 fn lt(&self, other: &Cell<T>) -> bool {
283 self.get() < other.get()
287 fn le(&self, other: &Cell<T>) -> bool {
288 self.get() <= other.get()
292 fn gt(&self, other: &Cell<T>) -> bool {
293 self.get() > other.get()
297 fn ge(&self, other: &Cell<T>) -> bool {
298 self.get() >= other.get()
302 #[stable(feature = "cell_ord", since = "1.10.0")]
303 impl<T: Ord + Copy> Ord for Cell<T> {
305 fn cmp(&self, other: &Cell<T>) -> Ordering {
306 self.get().cmp(&other.get())
310 #[stable(feature = "cell_from", since = "1.12.0")]
311 impl<T> From<T> for Cell<T> {
312 fn from(t: T) -> Cell<T> {
318 /// Creates a new `Cell` containing the given value.
323 /// use std::cell::Cell;
325 /// let c = Cell::new(5);
327 #[stable(feature = "rust1", since = "1.0.0")]
328 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
330 pub const fn new(value: T) -> Cell<T> {
331 Cell { value: UnsafeCell::new(value) }
334 /// Sets the contained value.
339 /// use std::cell::Cell;
341 /// let c = Cell::new(5);
346 #[stable(feature = "rust1", since = "1.0.0")]
347 pub fn set(&self, val: T) {
348 let old = self.replace(val);
352 /// Swaps the values of two Cells.
353 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
358 /// use std::cell::Cell;
360 /// let c1 = Cell::new(5i32);
361 /// let c2 = Cell::new(10i32);
363 /// assert_eq!(10, c1.get());
364 /// assert_eq!(5, c2.get());
367 #[stable(feature = "move_cell", since = "1.17.0")]
368 pub fn swap(&self, other: &Self) {
369 if ptr::eq(self, other) {
372 // SAFETY: This can be risky if called from separate threads, but `Cell`
373 // is `!Sync` so this won't happen. This also won't invalidate any
374 // pointers since `Cell` makes sure nothing else will be pointing into
375 // either of these `Cell`s.
377 ptr::swap(self.value.get(), other.value.get());
381 /// Replaces the contained value with `val`, and returns the old contained value.
386 /// use std::cell::Cell;
388 /// let cell = Cell::new(5);
389 /// assert_eq!(cell.get(), 5);
390 /// assert_eq!(cell.replace(10), 5);
391 /// assert_eq!(cell.get(), 10);
393 #[stable(feature = "move_cell", since = "1.17.0")]
394 pub fn replace(&self, val: T) -> T {
395 // SAFETY: This can cause data races if called from a separate thread,
396 // but `Cell` is `!Sync` so this won't happen.
397 mem::replace(unsafe { &mut *self.value.get() }, val)
400 /// Unwraps the value.
405 /// use std::cell::Cell;
407 /// let c = Cell::new(5);
408 /// let five = c.into_inner();
410 /// assert_eq!(five, 5);
412 #[stable(feature = "move_cell", since = "1.17.0")]
413 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
414 pub const fn into_inner(self) -> T {
415 self.value.into_inner()
419 impl<T: Copy> Cell<T> {
420 /// Returns a copy of the contained value.
425 /// use std::cell::Cell;
427 /// let c = Cell::new(5);
429 /// let five = c.get();
432 #[stable(feature = "rust1", since = "1.0.0")]
433 pub fn get(&self) -> T {
434 // SAFETY: This can cause data races if called from a separate thread,
435 // but `Cell` is `!Sync` so this won't happen.
436 unsafe { *self.value.get() }
439 /// Updates the contained value using a function and returns the new value.
444 /// #![feature(cell_update)]
446 /// use std::cell::Cell;
448 /// let c = Cell::new(5);
449 /// let new = c.update(|x| x + 1);
451 /// assert_eq!(new, 6);
452 /// assert_eq!(c.get(), 6);
455 #[unstable(feature = "cell_update", issue = "50186")]
456 pub fn update<F>(&self, f: F) -> T
460 let old = self.get();
467 impl<T: ?Sized> Cell<T> {
468 /// Returns a raw pointer to the underlying data in this cell.
473 /// use std::cell::Cell;
475 /// let c = Cell::new(5);
477 /// let ptr = c.as_ptr();
480 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
481 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
482 pub const fn as_ptr(&self) -> *mut T {
486 /// Returns a mutable reference to the underlying data.
488 /// This call borrows `Cell` mutably (at compile-time) which guarantees
489 /// that we possess the only reference.
494 /// use std::cell::Cell;
496 /// let mut c = Cell::new(5);
497 /// *c.get_mut() += 1;
499 /// assert_eq!(c.get(), 6);
502 #[stable(feature = "cell_get_mut", since = "1.11.0")]
503 pub fn get_mut(&mut self) -> &mut T {
507 /// Returns a `&Cell<T>` from a `&mut T`
512 /// use std::cell::Cell;
514 /// let slice: &mut [i32] = &mut [1, 2, 3];
515 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
516 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
518 /// assert_eq!(slice_cell.len(), 3);
521 #[stable(feature = "as_cell", since = "1.37.0")]
522 pub fn from_mut(t: &mut T) -> &Cell<T> {
523 // SAFETY: `&mut` ensures unique access.
524 unsafe { &*(t as *mut T as *const Cell<T>) }
528 impl<T: Default> Cell<T> {
529 /// Takes the value of the cell, leaving `Default::default()` in its place.
534 /// use std::cell::Cell;
536 /// let c = Cell::new(5);
537 /// let five = c.take();
539 /// assert_eq!(five, 5);
540 /// assert_eq!(c.into_inner(), 0);
542 #[stable(feature = "move_cell", since = "1.17.0")]
543 pub fn take(&self) -> T {
544 self.replace(Default::default())
548 #[unstable(feature = "coerce_unsized", issue = "27732")]
549 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
552 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
557 /// use std::cell::Cell;
559 /// let slice: &mut [i32] = &mut [1, 2, 3];
560 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
561 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
563 /// assert_eq!(slice_cell.len(), 3);
565 #[stable(feature = "as_cell", since = "1.37.0")]
566 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
567 // SAFETY: `Cell<T>` has the same memory layout as `T`.
568 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
572 /// A mutable memory location with dynamically checked borrow rules
574 /// See the [module-level documentation](self) for more.
575 #[stable(feature = "rust1", since = "1.0.0")]
576 pub struct RefCell<T: ?Sized> {
577 borrow: Cell<BorrowFlag>,
578 // Stores the location of the earliest currently active borrow.
579 // This gets updated whenver we go from having zero borrows
580 // to having a single borrow. When a borrow occurs, this gets included
581 // in the generated `BorroeError/`BorrowMutError`
582 #[cfg(feature = "debug_refcell")]
583 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
584 value: UnsafeCell<T>,
587 /// An error returned by [`RefCell::try_borrow`].
588 #[stable(feature = "try_borrow", since = "1.13.0")]
589 pub struct BorrowError {
591 #[cfg(feature = "debug_refcell")]
592 location: &'static crate::panic::Location<'static>,
595 #[stable(feature = "try_borrow", since = "1.13.0")]
596 impl Debug for BorrowError {
597 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
598 let mut builder = f.debug_struct("BorrowError");
600 #[cfg(feature = "debug_refcell")]
601 builder.field("location", self.location);
607 #[stable(feature = "try_borrow", since = "1.13.0")]
608 impl Display for BorrowError {
609 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
610 Display::fmt("already mutably borrowed", f)
614 /// An error returned by [`RefCell::try_borrow_mut`].
615 #[stable(feature = "try_borrow", since = "1.13.0")]
616 pub struct BorrowMutError {
618 #[cfg(feature = "debug_refcell")]
619 location: &'static crate::panic::Location<'static>,
622 #[stable(feature = "try_borrow", since = "1.13.0")]
623 impl Debug for BorrowMutError {
624 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
625 let mut builder = f.debug_struct("BorrowMutError");
627 #[cfg(feature = "debug_refcell")]
628 builder.field("location", self.location);
634 #[stable(feature = "try_borrow", since = "1.13.0")]
635 impl Display for BorrowMutError {
636 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
637 Display::fmt("already borrowed", f)
641 // Positive values represent the number of `Ref` active. Negative values
642 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
643 // active at a time if they refer to distinct, nonoverlapping components of a
644 // `RefCell` (e.g., different ranges of a slice).
646 // `Ref` and `RefMut` are both two words in size, and so there will likely never
647 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
648 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
649 // However, this is not a guarantee, as a pathological program could repeatedly
650 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
651 // explicitly check for overflow and underflow in order to avoid unsafety, or at
652 // least behave correctly in the event that overflow or underflow happens (e.g.,
653 // see BorrowRef::new).
654 type BorrowFlag = isize;
655 const UNUSED: BorrowFlag = 0;
658 fn is_writing(x: BorrowFlag) -> bool {
663 fn is_reading(x: BorrowFlag) -> bool {
668 /// Creates a new `RefCell` containing `value`.
673 /// use std::cell::RefCell;
675 /// let c = RefCell::new(5);
677 #[stable(feature = "rust1", since = "1.0.0")]
678 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
680 pub const fn new(value: T) -> RefCell<T> {
682 value: UnsafeCell::new(value),
683 borrow: Cell::new(UNUSED),
684 #[cfg(feature = "debug_refcell")]
685 borrowed_at: Cell::new(None),
689 /// Consumes the `RefCell`, returning the wrapped value.
694 /// use std::cell::RefCell;
696 /// let c = RefCell::new(5);
698 /// let five = c.into_inner();
700 #[stable(feature = "rust1", since = "1.0.0")]
701 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
703 pub const fn into_inner(self) -> T {
704 // Since this function takes `self` (the `RefCell`) by value, the
705 // compiler statically verifies that it is not currently borrowed.
706 self.value.into_inner()
709 /// Replaces the wrapped value with a new one, returning the old value,
710 /// without deinitializing either one.
712 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
716 /// Panics if the value is currently borrowed.
721 /// use std::cell::RefCell;
722 /// let cell = RefCell::new(5);
723 /// let old_value = cell.replace(6);
724 /// assert_eq!(old_value, 5);
725 /// assert_eq!(cell, RefCell::new(6));
728 #[stable(feature = "refcell_replace", since = "1.24.0")]
730 pub fn replace(&self, t: T) -> T {
731 mem::replace(&mut *self.borrow_mut(), t)
734 /// Replaces the wrapped value with a new one computed from `f`, returning
735 /// the old value, without deinitializing either one.
739 /// Panics if the value is currently borrowed.
744 /// use std::cell::RefCell;
745 /// let cell = RefCell::new(5);
746 /// let old_value = cell.replace_with(|&mut old| old + 1);
747 /// assert_eq!(old_value, 5);
748 /// assert_eq!(cell, RefCell::new(6));
751 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
753 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
754 let mut_borrow = &mut *self.borrow_mut();
755 let replacement = f(mut_borrow);
756 mem::replace(mut_borrow, replacement)
759 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
760 /// without deinitializing either one.
762 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
766 /// Panics if the value in either `RefCell` is currently borrowed.
771 /// use std::cell::RefCell;
772 /// let c = RefCell::new(5);
773 /// let d = RefCell::new(6);
775 /// assert_eq!(c, RefCell::new(6));
776 /// assert_eq!(d, RefCell::new(5));
779 #[stable(feature = "refcell_swap", since = "1.24.0")]
780 pub fn swap(&self, other: &Self) {
781 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
785 impl<T: ?Sized> RefCell<T> {
786 /// Immutably borrows the wrapped value.
788 /// The borrow lasts until the returned `Ref` exits scope. Multiple
789 /// immutable borrows can be taken out at the same time.
793 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
794 /// [`try_borrow`](#method.try_borrow).
799 /// use std::cell::RefCell;
801 /// let c = RefCell::new(5);
803 /// let borrowed_five = c.borrow();
804 /// let borrowed_five2 = c.borrow();
807 /// An example of panic:
810 /// use std::cell::RefCell;
812 /// let c = RefCell::new(5);
814 /// let m = c.borrow_mut();
815 /// let b = c.borrow(); // this causes a panic
817 #[stable(feature = "rust1", since = "1.0.0")]
820 pub fn borrow(&self) -> Ref<'_, T> {
821 self.try_borrow().expect("already mutably borrowed")
824 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
827 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
828 /// taken out at the same time.
830 /// This is the non-panicking variant of [`borrow`](#method.borrow).
835 /// use std::cell::RefCell;
837 /// let c = RefCell::new(5);
840 /// let m = c.borrow_mut();
841 /// assert!(c.try_borrow().is_err());
845 /// let m = c.borrow();
846 /// assert!(c.try_borrow().is_ok());
849 #[stable(feature = "try_borrow", since = "1.13.0")]
851 #[cfg_attr(feature = "debug_refcell", track_caller)]
852 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
853 match BorrowRef::new(&self.borrow) {
855 #[cfg(feature = "debug_refcell")]
857 // `borrowed_at` is always the *first* active borrow
858 if b.borrow.get() == 1 {
859 self.borrowed_at.set(Some(crate::panic::Location::caller()));
863 // SAFETY: `BorrowRef` ensures that there is only immutable access
864 // to the value while borrowed.
865 Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b })
867 None => Err(BorrowError {
869 // If a borrow occured, then we must already have an outstanding borrow,
870 // so `borrowed_at` will be `Some`
871 #[cfg(feature = "debug_refcell")]
872 location: self.borrowed_at.get().unwrap(),
877 /// Mutably borrows the wrapped value.
879 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
880 /// from it exit scope. The value cannot be borrowed while this borrow is
885 /// Panics if the value is currently borrowed. For a non-panicking variant, use
886 /// [`try_borrow_mut`](#method.try_borrow_mut).
891 /// use std::cell::RefCell;
893 /// let c = RefCell::new("hello".to_owned());
895 /// *c.borrow_mut() = "bonjour".to_owned();
897 /// assert_eq!(&*c.borrow(), "bonjour");
900 /// An example of panic:
903 /// use std::cell::RefCell;
905 /// let c = RefCell::new(5);
906 /// let m = c.borrow();
908 /// let b = c.borrow_mut(); // this causes a panic
910 #[stable(feature = "rust1", since = "1.0.0")]
913 pub fn borrow_mut(&self) -> RefMut<'_, T> {
914 self.try_borrow_mut().expect("already borrowed")
917 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
919 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
920 /// from it exit scope. The value cannot be borrowed while this borrow is
923 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
928 /// use std::cell::RefCell;
930 /// let c = RefCell::new(5);
933 /// let m = c.borrow();
934 /// assert!(c.try_borrow_mut().is_err());
937 /// assert!(c.try_borrow_mut().is_ok());
939 #[stable(feature = "try_borrow", since = "1.13.0")]
941 #[cfg_attr(feature = "debug_refcell", track_caller)]
942 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
943 match BorrowRefMut::new(&self.borrow) {
945 #[cfg(feature = "debug_refcell")]
947 self.borrowed_at.set(Some(crate::panic::Location::caller()));
950 // SAFETY: `BorrowRef` guarantees unique access.
951 Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b })
953 None => Err(BorrowMutError {
955 // If a borrow occured, then we must already have an outstanding borrow,
956 // so `borrowed_at` will be `Some`
957 #[cfg(feature = "debug_refcell")]
958 location: self.borrowed_at.get().unwrap(),
963 /// Returns a raw pointer to the underlying data in this cell.
968 /// use std::cell::RefCell;
970 /// let c = RefCell::new(5);
972 /// let ptr = c.as_ptr();
975 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
976 pub fn as_ptr(&self) -> *mut T {
980 /// Returns a mutable reference to the underlying data.
982 /// This call borrows `RefCell` mutably (at compile-time) so there is no
983 /// need for dynamic checks.
985 /// However be cautious: this method expects `self` to be mutable, which is
986 /// generally not the case when using a `RefCell`. Take a look at the
987 /// [`borrow_mut`] method instead if `self` isn't mutable.
989 /// Also, please be aware that this method is only for special circumstances and is usually
990 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
992 /// [`borrow_mut`]: RefCell::borrow_mut()
997 /// use std::cell::RefCell;
999 /// let mut c = RefCell::new(5);
1000 /// *c.get_mut() += 1;
1002 /// assert_eq!(c, RefCell::new(6));
1005 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1006 pub fn get_mut(&mut self) -> &mut T {
1007 self.value.get_mut()
1010 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1012 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1013 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1014 /// if some `Ref` or `RefMut` borrows have been leaked.
1016 /// [`get_mut`]: RefCell::get_mut()
1021 /// #![feature(cell_leak)]
1022 /// use std::cell::RefCell;
1024 /// let mut c = RefCell::new(0);
1025 /// std::mem::forget(c.borrow_mut());
1027 /// assert!(c.try_borrow().is_err());
1029 /// assert!(c.try_borrow().is_ok());
1031 #[unstable(feature = "cell_leak", issue = "69099")]
1032 pub fn undo_leak(&mut self) -> &mut T {
1033 *self.borrow.get_mut() = UNUSED;
1037 /// Immutably borrows the wrapped value, returning an error if the value is
1038 /// currently mutably borrowed.
1042 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1043 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1044 /// borrowing the `RefCell` while the reference returned by this method
1045 /// is alive is undefined behaviour.
1050 /// use std::cell::RefCell;
1052 /// let c = RefCell::new(5);
1055 /// let m = c.borrow_mut();
1056 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1060 /// let m = c.borrow();
1061 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1064 #[stable(feature = "borrow_state", since = "1.37.0")]
1066 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1067 if !is_writing(self.borrow.get()) {
1068 // SAFETY: We check that nobody is actively writing now, but it is
1069 // the caller's responsibility to ensure that nobody writes until
1070 // the returned reference is no longer in use.
1071 // Also, `self.value.get()` refers to the value owned by `self`
1072 // and is thus guaranteed to be valid for the lifetime of `self`.
1073 Ok(unsafe { &*self.value.get() })
1077 // If a borrow occured, then we must already have an outstanding borrow,
1078 // so `borrowed_at` will be `Some`
1079 #[cfg(feature = "debug_refcell")]
1080 location: self.borrowed_at.get().unwrap(),
1086 impl<T: Default> RefCell<T> {
1087 /// Takes the wrapped value, leaving `Default::default()` in its place.
1091 /// Panics if the value is currently borrowed.
1096 /// use std::cell::RefCell;
1098 /// let c = RefCell::new(5);
1099 /// let five = c.take();
1101 /// assert_eq!(five, 5);
1102 /// assert_eq!(c.into_inner(), 0);
1104 #[stable(feature = "refcell_take", since = "1.50.0")]
1105 pub fn take(&self) -> T {
1106 self.replace(Default::default())
1110 #[stable(feature = "rust1", since = "1.0.0")]
1111 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1113 #[stable(feature = "rust1", since = "1.0.0")]
1114 impl<T: ?Sized> !Sync for RefCell<T> {}
1116 #[stable(feature = "rust1", since = "1.0.0")]
1117 impl<T: Clone> Clone for RefCell<T> {
1120 /// Panics if the value is currently mutably borrowed.
1123 fn clone(&self) -> RefCell<T> {
1124 RefCell::new(self.borrow().clone())
1129 /// Panics if `other` is currently mutably borrowed.
1132 fn clone_from(&mut self, other: &Self) {
1133 self.get_mut().clone_from(&other.borrow())
1137 #[stable(feature = "rust1", since = "1.0.0")]
1138 impl<T: Default> Default for RefCell<T> {
1139 /// Creates a `RefCell<T>`, with the `Default` value for T.
1141 fn default() -> RefCell<T> {
1142 RefCell::new(Default::default())
1146 #[stable(feature = "rust1", since = "1.0.0")]
1147 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1150 /// Panics if the value in either `RefCell` is currently borrowed.
1152 fn eq(&self, other: &RefCell<T>) -> bool {
1153 *self.borrow() == *other.borrow()
1157 #[stable(feature = "cell_eq", since = "1.2.0")]
1158 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1160 #[stable(feature = "cell_ord", since = "1.10.0")]
1161 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1164 /// Panics if the value in either `RefCell` is currently borrowed.
1166 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1167 self.borrow().partial_cmp(&*other.borrow())
1172 /// Panics if the value in either `RefCell` is currently borrowed.
1174 fn lt(&self, other: &RefCell<T>) -> bool {
1175 *self.borrow() < *other.borrow()
1180 /// Panics if the value in either `RefCell` is currently borrowed.
1182 fn le(&self, other: &RefCell<T>) -> bool {
1183 *self.borrow() <= *other.borrow()
1188 /// Panics if the value in either `RefCell` is currently borrowed.
1190 fn gt(&self, other: &RefCell<T>) -> bool {
1191 *self.borrow() > *other.borrow()
1196 /// Panics if the value in either `RefCell` is currently borrowed.
1198 fn ge(&self, other: &RefCell<T>) -> bool {
1199 *self.borrow() >= *other.borrow()
1203 #[stable(feature = "cell_ord", since = "1.10.0")]
1204 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1207 /// Panics if the value in either `RefCell` is currently borrowed.
1209 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1210 self.borrow().cmp(&*other.borrow())
1214 #[stable(feature = "cell_from", since = "1.12.0")]
1215 impl<T> From<T> for RefCell<T> {
1216 fn from(t: T) -> RefCell<T> {
1221 #[unstable(feature = "coerce_unsized", issue = "27732")]
1222 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1224 struct BorrowRef<'b> {
1225 borrow: &'b Cell<BorrowFlag>,
1228 impl<'b> BorrowRef<'b> {
1230 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1231 let b = borrow.get().wrapping_add(1);
1233 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1234 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1235 // due to Rust's reference aliasing rules
1236 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1237 // into isize::MIN (the max amount of writing borrows) so we can't allow
1238 // an additional read borrow because isize can't represent so many read borrows
1239 // (this can only happen if you mem::forget more than a small constant amount of
1240 // `Ref`s, which is not good practice)
1243 // Incrementing borrow can result in a reading value (> 0) in these cases:
1244 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1245 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1246 // is large enough to represent having one more read borrow
1248 Some(BorrowRef { borrow })
1253 impl Drop for BorrowRef<'_> {
1255 fn drop(&mut self) {
1256 let borrow = self.borrow.get();
1257 debug_assert!(is_reading(borrow));
1258 self.borrow.set(borrow - 1);
1262 impl Clone for BorrowRef<'_> {
1264 fn clone(&self) -> Self {
1265 // Since this Ref exists, we know the borrow flag
1266 // is a reading borrow.
1267 let borrow = self.borrow.get();
1268 debug_assert!(is_reading(borrow));
1269 // Prevent the borrow counter from overflowing into
1270 // a writing borrow.
1271 assert!(borrow != isize::MAX);
1272 self.borrow.set(borrow + 1);
1273 BorrowRef { borrow: self.borrow }
1277 /// Wraps a borrowed reference to a value in a `RefCell` box.
1278 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1280 /// See the [module-level documentation](self) for more.
1281 #[stable(feature = "rust1", since = "1.0.0")]
1282 pub struct Ref<'b, T: ?Sized + 'b> {
1284 borrow: BorrowRef<'b>,
1287 #[stable(feature = "rust1", since = "1.0.0")]
1288 impl<T: ?Sized> Deref for Ref<'_, T> {
1292 fn deref(&self) -> &T {
1297 impl<'b, T: ?Sized> Ref<'b, T> {
1300 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1302 /// This is an associated function that needs to be used as
1303 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1304 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1306 #[stable(feature = "cell_extras", since = "1.15.0")]
1308 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1309 Ref { value: orig.value, borrow: orig.borrow.clone() }
1312 /// Makes a new `Ref` for a component of the borrowed data.
1314 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1316 /// This is an associated function that needs to be used as `Ref::map(...)`.
1317 /// A method would interfere with methods of the same name on the contents
1318 /// of a `RefCell` used through `Deref`.
1323 /// use std::cell::{RefCell, Ref};
1325 /// let c = RefCell::new((5, 'b'));
1326 /// let b1: Ref<(u32, char)> = c.borrow();
1327 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1328 /// assert_eq!(*b2, 5)
1330 #[stable(feature = "cell_map", since = "1.8.0")]
1332 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1334 F: FnOnce(&T) -> &U,
1336 Ref { value: f(orig.value), borrow: orig.borrow }
1339 /// Makes a new `Ref` for an optional component of the borrowed data. The
1340 /// original guard is returned as an `Err(..)` if the closure returns
1343 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1345 /// This is an associated function that needs to be used as
1346 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1347 /// name on the contents of a `RefCell` used through `Deref`.
1352 /// #![feature(cell_filter_map)]
1354 /// use std::cell::{RefCell, Ref};
1356 /// let c = RefCell::new(vec![1, 2, 3]);
1357 /// let b1: Ref<Vec<u32>> = c.borrow();
1358 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1359 /// assert_eq!(*b2.unwrap(), 2);
1361 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1363 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1365 F: FnOnce(&T) -> Option<&U>,
1367 match f(orig.value) {
1368 Some(value) => Ok(Ref { value, borrow: orig.borrow }),
1373 /// Splits a `Ref` into multiple `Ref`s for different components of the
1376 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1378 /// This is an associated function that needs to be used as
1379 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1380 /// name on the contents of a `RefCell` used through `Deref`.
1385 /// use std::cell::{Ref, RefCell};
1387 /// let cell = RefCell::new([1, 2, 3, 4]);
1388 /// let borrow = cell.borrow();
1389 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1390 /// assert_eq!(*begin, [1, 2]);
1391 /// assert_eq!(*end, [3, 4]);
1393 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1395 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1397 F: FnOnce(&T) -> (&U, &V),
1399 let (a, b) = f(orig.value);
1400 let borrow = orig.borrow.clone();
1401 (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow })
1404 /// Convert into a reference to the underlying data.
1406 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1407 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1408 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1409 /// have occurred in total.
1411 /// This is an associated function that needs to be used as
1412 /// `Ref::leak(...)`. A method would interfere with methods of the
1413 /// same name on the contents of a `RefCell` used through `Deref`.
1418 /// #![feature(cell_leak)]
1419 /// use std::cell::{RefCell, Ref};
1420 /// let cell = RefCell::new(0);
1422 /// let value = Ref::leak(cell.borrow());
1423 /// assert_eq!(*value, 0);
1425 /// assert!(cell.try_borrow().is_ok());
1426 /// assert!(cell.try_borrow_mut().is_err());
1428 #[unstable(feature = "cell_leak", issue = "69099")]
1429 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1430 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1431 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1432 // unique reference to the borrowed RefCell. No further mutable references can be created
1433 // from the original cell.
1434 mem::forget(orig.borrow);
1439 #[unstable(feature = "coerce_unsized", issue = "27732")]
1440 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1442 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1443 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1444 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1449 impl<'b, T: ?Sized> RefMut<'b, T> {
1450 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1453 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1455 /// This is an associated function that needs to be used as
1456 /// `RefMut::map(...)`. A method would interfere with methods of the same
1457 /// name on the contents of a `RefCell` used through `Deref`.
1462 /// use std::cell::{RefCell, RefMut};
1464 /// let c = RefCell::new((5, 'b'));
1466 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1467 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1468 /// assert_eq!(*b2, 5);
1471 /// assert_eq!(*c.borrow(), (42, 'b'));
1473 #[stable(feature = "cell_map", since = "1.8.0")]
1475 pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1477 F: FnOnce(&mut T) -> &mut U,
1479 // FIXME(nll-rfc#40): fix borrow-check
1480 let RefMut { value, borrow } = orig;
1481 RefMut { value: f(value), borrow }
1484 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1485 /// original guard is returned as an `Err(..)` if the closure returns
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::filter_map(...)`. A method would interfere with methods of the
1492 /// same name on the contents of a `RefCell` used through `Deref`.
1497 /// #![feature(cell_filter_map)]
1499 /// use std::cell::{RefCell, RefMut};
1501 /// let c = RefCell::new(vec![1, 2, 3]);
1504 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1505 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1507 /// if let Ok(mut b2) = b2 {
1512 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1514 #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")]
1516 pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1518 F: FnOnce(&mut T) -> Option<&mut U>,
1520 // FIXME(nll-rfc#40): fix borrow-check
1521 let RefMut { value, borrow } = orig;
1522 let value = value as *mut T;
1523 // SAFETY: function holds onto an exclusive reference for the duration
1524 // of its call through `orig`, and the pointer is only de-referenced
1525 // inside of the function call never allowing the exclusive reference to
1527 match f(unsafe { &mut *value }) {
1528 Some(value) => Ok(RefMut { value, borrow }),
1530 // SAFETY: same as above.
1531 Err(RefMut { value: unsafe { &mut *value }, borrow })
1536 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1539 /// The underlying `RefCell` will remain mutably borrowed until both
1540 /// returned `RefMut`s go out of scope.
1542 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1544 /// This is an associated function that needs to be used as
1545 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1546 /// same name on the contents of a `RefCell` used through `Deref`.
1551 /// use std::cell::{RefCell, RefMut};
1553 /// let cell = RefCell::new([1, 2, 3, 4]);
1554 /// let borrow = cell.borrow_mut();
1555 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1556 /// assert_eq!(*begin, [1, 2]);
1557 /// assert_eq!(*end, [3, 4]);
1558 /// begin.copy_from_slice(&[4, 3]);
1559 /// end.copy_from_slice(&[2, 1]);
1561 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1563 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1564 orig: RefMut<'b, T>,
1566 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1568 F: FnOnce(&mut T) -> (&mut U, &mut V),
1570 let (a, b) = f(orig.value);
1571 let borrow = orig.borrow.clone();
1572 (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow })
1575 /// Convert into a mutable reference to the underlying data.
1577 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1578 /// mutably borrowed, making the returned reference the only to the interior.
1580 /// This is an associated function that needs to be used as
1581 /// `RefMut::leak(...)`. A method would interfere with methods of the
1582 /// same name on the contents of a `RefCell` used through `Deref`.
1587 /// #![feature(cell_leak)]
1588 /// use std::cell::{RefCell, RefMut};
1589 /// let cell = RefCell::new(0);
1591 /// let value = RefMut::leak(cell.borrow_mut());
1592 /// assert_eq!(*value, 0);
1595 /// assert!(cell.try_borrow_mut().is_err());
1597 #[unstable(feature = "cell_leak", issue = "69099")]
1598 pub fn leak(orig: RefMut<'b, T>) -> &'b mut T {
1599 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1600 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1601 // require a unique reference to the borrowed RefCell. No further references can be created
1602 // from the original cell within that lifetime, making the current borrow the only
1603 // reference for the remaining lifetime.
1604 mem::forget(orig.borrow);
1609 struct BorrowRefMut<'b> {
1610 borrow: &'b Cell<BorrowFlag>,
1613 impl Drop for BorrowRefMut<'_> {
1615 fn drop(&mut self) {
1616 let borrow = self.borrow.get();
1617 debug_assert!(is_writing(borrow));
1618 self.borrow.set(borrow + 1);
1622 impl<'b> BorrowRefMut<'b> {
1624 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1625 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1626 // mutable reference, and so there must currently be no existing
1627 // references. Thus, while clone increments the mutable refcount, here
1628 // we explicitly only allow going from UNUSED to UNUSED - 1.
1629 match borrow.get() {
1631 borrow.set(UNUSED - 1);
1632 Some(BorrowRefMut { borrow })
1638 // Clones a `BorrowRefMut`.
1640 // This is only valid if each `BorrowRefMut` is used to track a mutable
1641 // reference to a distinct, nonoverlapping range of the original object.
1642 // This isn't in a Clone impl so that code doesn't call this implicitly.
1644 fn clone(&self) -> BorrowRefMut<'b> {
1645 let borrow = self.borrow.get();
1646 debug_assert!(is_writing(borrow));
1647 // Prevent the borrow counter from underflowing.
1648 assert!(borrow != isize::MIN);
1649 self.borrow.set(borrow - 1);
1650 BorrowRefMut { borrow: self.borrow }
1654 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1656 /// See the [module-level documentation](self) for more.
1657 #[stable(feature = "rust1", since = "1.0.0")]
1658 pub struct RefMut<'b, T: ?Sized + 'b> {
1660 borrow: BorrowRefMut<'b>,
1663 #[stable(feature = "rust1", since = "1.0.0")]
1664 impl<T: ?Sized> Deref for RefMut<'_, T> {
1668 fn deref(&self) -> &T {
1673 #[stable(feature = "rust1", since = "1.0.0")]
1674 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1676 fn deref_mut(&mut self) -> &mut T {
1681 #[unstable(feature = "coerce_unsized", issue = "27732")]
1682 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1684 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1685 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1686 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1691 /// The core primitive for interior mutability in Rust.
1693 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1694 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1695 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1696 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1697 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1699 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1700 /// use `UnsafeCell` to wrap their data.
1702 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1703 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1704 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1706 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1707 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1710 /// [`.get()`]: `UnsafeCell::get`
1712 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1714 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1715 /// reference) that is accessible by safe code (for example, because you returned it),
1716 /// then you must not access the data in any way that contradicts that reference for the
1717 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1718 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1719 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1720 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1721 /// safe code, then you must not access the data within the `UnsafeCell` until that
1722 /// reference expires.
1724 /// - At all times, you must avoid data races. If multiple threads have access to
1725 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1726 /// accesses (or use atomics).
1728 /// To assist with proper design, the following scenarios are explicitly declared legal
1729 /// for single-threaded code:
1731 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1732 /// references, but not with a `&mut T`
1734 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1735 /// co-exist with it. A `&mut T` must always be unique.
1737 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1738 /// `&UnsafeCell<T>` references alias the cell) is
1739 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1740 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1741 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1742 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1743 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1744 /// may be aliased for the duration of that `&mut` borrow.
1745 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1748 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1752 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1753 /// there being multiple references aliasing the cell:
1756 /// use std::cell::UnsafeCell;
1758 /// let x: UnsafeCell<i32> = 42.into();
1759 /// // Get multiple / concurrent / shared references to the same `x`.
1760 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1763 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1764 /// // so ours is effectively unique.
1765 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1766 /// *p1_exclusive += 27; // |
1767 /// } // <---------- cannot go beyond this point -------------------+
1770 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1771 /// // so we can have multiple shared accesses concurrently.
1772 /// let p2_shared: &i32 = &*p2.get();
1773 /// assert_eq!(*p2_shared, 42 + 27);
1774 /// let p1_shared: &i32 = &*p1.get();
1775 /// assert_eq!(*p1_shared, *p2_shared);
1779 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1780 /// implies exclusive access to its `T`:
1783 /// #![forbid(unsafe_code)] // with exclusive accesses,
1784 /// // `UnsafeCell` is a transparent no-op wrapper,
1785 /// // so no need for `unsafe` here.
1786 /// use std::cell::UnsafeCell;
1788 /// let mut x: UnsafeCell<i32> = 42.into();
1790 /// // Get a compile-time-checked unique reference to `x`.
1791 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1792 /// // With an exclusive reference, we can mutate the contents for free.
1793 /// *p_unique.get_mut() = 0;
1794 /// // Or, equivalently:
1795 /// x = UnsafeCell::new(0);
1797 /// // When we own the value, we can extract the contents for free.
1798 /// let contents: i32 = x.into_inner();
1799 /// assert_eq!(contents, 0);
1801 #[lang = "unsafe_cell"]
1802 #[stable(feature = "rust1", since = "1.0.0")]
1803 #[repr(transparent)]
1804 #[repr(no_niche)] // rust-lang/rust#68303.
1805 pub struct UnsafeCell<T: ?Sized> {
1809 #[stable(feature = "rust1", since = "1.0.0")]
1810 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1812 impl<T> UnsafeCell<T> {
1813 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1816 /// All access to the inner value through methods is `unsafe`.
1821 /// use std::cell::UnsafeCell;
1823 /// let uc = UnsafeCell::new(5);
1825 #[stable(feature = "rust1", since = "1.0.0")]
1826 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1828 pub const fn new(value: T) -> UnsafeCell<T> {
1829 UnsafeCell { value }
1832 /// Unwraps the value.
1837 /// use std::cell::UnsafeCell;
1839 /// let uc = UnsafeCell::new(5);
1841 /// let five = uc.into_inner();
1844 #[stable(feature = "rust1", since = "1.0.0")]
1845 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1846 pub const fn into_inner(self) -> T {
1851 impl<T: ?Sized> UnsafeCell<T> {
1852 /// Gets a mutable pointer to the wrapped value.
1854 /// This can be cast to a pointer of any kind.
1855 /// Ensure that the access is unique (no active references, mutable or not)
1856 /// when casting to `&mut T`, and ensure that there are no mutations
1857 /// or mutable aliases going on when casting to `&T`
1862 /// use std::cell::UnsafeCell;
1864 /// let uc = UnsafeCell::new(5);
1866 /// let five = uc.get();
1869 #[stable(feature = "rust1", since = "1.0.0")]
1870 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1871 pub const fn get(&self) -> *mut T {
1872 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1873 // #[repr(transparent)]. This exploits libstd's special status, there is
1874 // no guarantee for user code that this will work in future versions of the compiler!
1875 self as *const UnsafeCell<T> as *const T as *mut T
1878 /// Returns a mutable reference to the underlying data.
1880 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
1881 /// guarantees that we possess the only reference.
1886 /// use std::cell::UnsafeCell;
1888 /// let mut c = UnsafeCell::new(5);
1889 /// *c.get_mut() += 1;
1891 /// assert_eq!(*c.get_mut(), 6);
1894 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
1895 pub fn get_mut(&mut self) -> &mut T {
1899 /// Gets a mutable pointer to the wrapped value.
1900 /// The difference to [`get`] is that this function accepts a raw pointer,
1901 /// which is useful to avoid the creation of temporary references.
1903 /// The result can be cast to a pointer of any kind.
1904 /// Ensure that the access is unique (no active references, mutable or not)
1905 /// when casting to `&mut T`, and ensure that there are no mutations
1906 /// or mutable aliases going on when casting to `&T`.
1908 /// [`get`]: UnsafeCell::get()
1912 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
1913 /// calling `get` would require creating a reference to uninitialized data:
1916 /// #![feature(unsafe_cell_raw_get)]
1917 /// use std::cell::UnsafeCell;
1918 /// use std::mem::MaybeUninit;
1920 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
1921 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
1922 /// let uc = unsafe { m.assume_init() };
1924 /// assert_eq!(uc.into_inner(), 5);
1927 #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")]
1928 pub const fn raw_get(this: *const Self) -> *mut T {
1929 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1930 // #[repr(transparent)]. This exploits libstd's special status, there is
1931 // no guarantee for user code that this will work in future versions of the compiler!
1932 this as *const T as *mut T
1936 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1937 impl<T: Default> Default for UnsafeCell<T> {
1938 /// Creates an `UnsafeCell`, with the `Default` value for T.
1939 fn default() -> UnsafeCell<T> {
1940 UnsafeCell::new(Default::default())
1944 #[stable(feature = "cell_from", since = "1.12.0")]
1945 impl<T> From<T> for UnsafeCell<T> {
1946 fn from(t: T) -> UnsafeCell<T> {
1951 #[unstable(feature = "coerce_unsized", issue = "27732")]
1952 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
1955 fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) {
1956 let _: UnsafeCell<&dyn Send> = a;
1957 let _: Cell<&dyn Send> = b;
1958 let _: RefCell<&dyn Send> = c;