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::{PhantomData, Unsize};
199 use crate::ops::{CoerceUnsized, Deref, DerefMut};
200 use crate::ptr::{self, NonNull};
205 #[unstable(feature = "once_cell", issue = "74465")]
206 pub use lazy::LazyCell;
207 #[unstable(feature = "once_cell", issue = "74465")]
208 pub use once::OnceCell;
210 /// A mutable memory location.
214 /// In this example, you can see that `Cell<T>` enables mutation inside an
215 /// immutable struct. In other words, it enables "interior mutability".
218 /// use std::cell::Cell;
220 /// struct SomeStruct {
221 /// regular_field: u8,
222 /// special_field: Cell<u8>,
225 /// let my_struct = SomeStruct {
226 /// regular_field: 0,
227 /// special_field: Cell::new(1),
230 /// let new_value = 100;
232 /// // ERROR: `my_struct` is immutable
233 /// // my_struct.regular_field = new_value;
235 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
236 /// // which can always be mutated
237 /// my_struct.special_field.set(new_value);
238 /// assert_eq!(my_struct.special_field.get(), new_value);
241 /// See the [module-level documentation](self) for more.
242 #[stable(feature = "rust1", since = "1.0.0")]
244 pub struct Cell<T: ?Sized> {
245 value: UnsafeCell<T>,
248 #[stable(feature = "rust1", since = "1.0.0")]
249 unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
251 // Note that this negative impl isn't strictly necessary for correctness,
252 // as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
253 // However, given how important `Cell`'s `!Sync`-ness is,
254 // having an explicit negative impl is nice for documentation purposes
255 // and results in nicer error messages.
256 #[stable(feature = "rust1", since = "1.0.0")]
257 impl<T: ?Sized> !Sync for Cell<T> {}
259 #[stable(feature = "rust1", since = "1.0.0")]
260 impl<T: Copy> Clone for Cell<T> {
262 fn clone(&self) -> Cell<T> {
263 Cell::new(self.get())
267 #[stable(feature = "rust1", since = "1.0.0")]
268 impl<T: Default> Default for Cell<T> {
269 /// Creates a `Cell<T>`, with the `Default` value for T.
271 fn default() -> Cell<T> {
272 Cell::new(Default::default())
276 #[stable(feature = "rust1", since = "1.0.0")]
277 impl<T: PartialEq + Copy> PartialEq for Cell<T> {
279 fn eq(&self, other: &Cell<T>) -> bool {
280 self.get() == other.get()
284 #[stable(feature = "cell_eq", since = "1.2.0")]
285 impl<T: Eq + Copy> Eq for Cell<T> {}
287 #[stable(feature = "cell_ord", since = "1.10.0")]
288 impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
290 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
291 self.get().partial_cmp(&other.get())
295 fn lt(&self, other: &Cell<T>) -> bool {
296 self.get() < other.get()
300 fn le(&self, other: &Cell<T>) -> bool {
301 self.get() <= other.get()
305 fn gt(&self, other: &Cell<T>) -> bool {
306 self.get() > other.get()
310 fn ge(&self, other: &Cell<T>) -> bool {
311 self.get() >= other.get()
315 #[stable(feature = "cell_ord", since = "1.10.0")]
316 impl<T: Ord + Copy> Ord for Cell<T> {
318 fn cmp(&self, other: &Cell<T>) -> Ordering {
319 self.get().cmp(&other.get())
323 #[stable(feature = "cell_from", since = "1.12.0")]
324 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
325 impl<T> const From<T> for Cell<T> {
326 /// Creates a new `Cell<T>` containing the given value.
327 fn from(t: T) -> Cell<T> {
333 /// Creates a new `Cell` containing the given value.
338 /// use std::cell::Cell;
340 /// let c = Cell::new(5);
342 #[stable(feature = "rust1", since = "1.0.0")]
343 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
345 pub const fn new(value: T) -> Cell<T> {
346 Cell { value: UnsafeCell::new(value) }
349 /// Sets the contained value.
354 /// use std::cell::Cell;
356 /// let c = Cell::new(5);
361 #[stable(feature = "rust1", since = "1.0.0")]
362 pub fn set(&self, val: T) {
363 let old = self.replace(val);
367 /// Swaps the values of two `Cell`s.
368 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
373 /// use std::cell::Cell;
375 /// let c1 = Cell::new(5i32);
376 /// let c2 = Cell::new(10i32);
378 /// assert_eq!(10, c1.get());
379 /// assert_eq!(5, c2.get());
382 #[stable(feature = "move_cell", since = "1.17.0")]
383 pub fn swap(&self, other: &Self) {
384 if ptr::eq(self, other) {
387 // SAFETY: This can be risky if called from separate threads, but `Cell`
388 // is `!Sync` so this won't happen. This also won't invalidate any
389 // pointers since `Cell` makes sure nothing else will be pointing into
390 // either of these `Cell`s.
392 ptr::swap(self.value.get(), other.value.get());
396 /// Replaces the contained value with `val`, and returns the old contained value.
401 /// use std::cell::Cell;
403 /// let cell = Cell::new(5);
404 /// assert_eq!(cell.get(), 5);
405 /// assert_eq!(cell.replace(10), 5);
406 /// assert_eq!(cell.get(), 10);
409 #[stable(feature = "move_cell", since = "1.17.0")]
410 pub fn replace(&self, val: T) -> T {
411 // SAFETY: This can cause data races if called from a separate thread,
412 // but `Cell` is `!Sync` so this won't happen.
413 mem::replace(unsafe { &mut *self.value.get() }, val)
416 /// Unwraps the value.
421 /// use std::cell::Cell;
423 /// let c = Cell::new(5);
424 /// let five = c.into_inner();
426 /// assert_eq!(five, 5);
428 #[stable(feature = "move_cell", since = "1.17.0")]
429 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
430 pub const fn into_inner(self) -> T {
431 self.value.into_inner()
435 impl<T: Copy> Cell<T> {
436 /// Returns a copy of the contained value.
441 /// use std::cell::Cell;
443 /// let c = Cell::new(5);
445 /// let five = c.get();
448 #[stable(feature = "rust1", since = "1.0.0")]
449 pub fn get(&self) -> T {
450 // SAFETY: This can cause data races if called from a separate thread,
451 // but `Cell` is `!Sync` so this won't happen.
452 unsafe { *self.value.get() }
455 /// Updates the contained value using a function and returns the new value.
460 /// #![feature(cell_update)]
462 /// use std::cell::Cell;
464 /// let c = Cell::new(5);
465 /// let new = c.update(|x| x + 1);
467 /// assert_eq!(new, 6);
468 /// assert_eq!(c.get(), 6);
471 #[unstable(feature = "cell_update", issue = "50186")]
472 pub fn update<F>(&self, f: F) -> T
476 let old = self.get();
483 impl<T: ?Sized> Cell<T> {
484 /// Returns a raw pointer to the underlying data in this cell.
489 /// use std::cell::Cell;
491 /// let c = Cell::new(5);
493 /// let ptr = c.as_ptr();
496 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
497 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
498 pub const fn as_ptr(&self) -> *mut T {
502 /// Returns a mutable reference to the underlying data.
504 /// This call borrows `Cell` mutably (at compile-time) which guarantees
505 /// that we possess the only reference.
507 /// However be cautious: this method expects `self` to be mutable, which is
508 /// generally not the case when using a `Cell`. If you require interior
509 /// mutability by reference, consider using `RefCell` which provides
510 /// run-time checked mutable borrows through its [`borrow_mut`] method.
512 /// [`borrow_mut`]: RefCell::borrow_mut()
517 /// use std::cell::Cell;
519 /// let mut c = Cell::new(5);
520 /// *c.get_mut() += 1;
522 /// assert_eq!(c.get(), 6);
525 #[stable(feature = "cell_get_mut", since = "1.11.0")]
526 pub fn get_mut(&mut self) -> &mut T {
530 /// Returns a `&Cell<T>` from a `&mut T`
535 /// use std::cell::Cell;
537 /// let slice: &mut [i32] = &mut [1, 2, 3];
538 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
539 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
541 /// assert_eq!(slice_cell.len(), 3);
544 #[stable(feature = "as_cell", since = "1.37.0")]
545 pub fn from_mut(t: &mut T) -> &Cell<T> {
546 // SAFETY: `&mut` ensures unique access.
547 unsafe { &*(t as *mut T as *const Cell<T>) }
551 impl<T: Default> Cell<T> {
552 /// Takes the value of the cell, leaving `Default::default()` in its place.
557 /// use std::cell::Cell;
559 /// let c = Cell::new(5);
560 /// let five = c.take();
562 /// assert_eq!(five, 5);
563 /// assert_eq!(c.into_inner(), 0);
565 #[stable(feature = "move_cell", since = "1.17.0")]
566 pub fn take(&self) -> T {
567 self.replace(Default::default())
571 #[unstable(feature = "coerce_unsized", issue = "18598")]
572 impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
575 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
580 /// use std::cell::Cell;
582 /// let slice: &mut [i32] = &mut [1, 2, 3];
583 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
584 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
586 /// assert_eq!(slice_cell.len(), 3);
588 #[stable(feature = "as_cell", since = "1.37.0")]
589 pub fn as_slice_of_cells(&self) -> &[Cell<T>] {
590 // SAFETY: `Cell<T>` has the same memory layout as `T`.
591 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
595 impl<T, const N: usize> Cell<[T; N]> {
596 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
601 /// #![feature(as_array_of_cells)]
602 /// use std::cell::Cell;
604 /// let mut array: [i32; 3] = [1, 2, 3];
605 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
606 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
608 #[unstable(feature = "as_array_of_cells", issue = "88248")]
609 pub fn as_array_of_cells(&self) -> &[Cell<T>; N] {
610 // SAFETY: `Cell<T>` has the same memory layout as `T`.
611 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
615 /// A mutable memory location with dynamically checked borrow rules
617 /// See the [module-level documentation](self) for more.
618 #[cfg_attr(not(test), rustc_diagnostic_item = "RefCell")]
619 #[stable(feature = "rust1", since = "1.0.0")]
620 pub struct RefCell<T: ?Sized> {
621 borrow: Cell<BorrowFlag>,
622 // Stores the location of the earliest currently active borrow.
623 // This gets updated whenever we go from having zero borrows
624 // to having a single borrow. When a borrow occurs, this gets included
625 // in the generated `BorrowError/`BorrowMutError`
626 #[cfg(feature = "debug_refcell")]
627 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
628 value: UnsafeCell<T>,
631 /// An error returned by [`RefCell::try_borrow`].
632 #[stable(feature = "try_borrow", since = "1.13.0")]
634 pub struct BorrowError {
635 #[cfg(feature = "debug_refcell")]
636 location: &'static crate::panic::Location<'static>,
639 #[stable(feature = "try_borrow", since = "1.13.0")]
640 impl Debug for BorrowError {
641 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
642 let mut builder = f.debug_struct("BorrowError");
644 #[cfg(feature = "debug_refcell")]
645 builder.field("location", self.location);
651 #[stable(feature = "try_borrow", since = "1.13.0")]
652 impl Display for BorrowError {
653 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
654 Display::fmt("already mutably borrowed", f)
658 /// An error returned by [`RefCell::try_borrow_mut`].
659 #[stable(feature = "try_borrow", since = "1.13.0")]
661 pub struct BorrowMutError {
662 #[cfg(feature = "debug_refcell")]
663 location: &'static crate::panic::Location<'static>,
666 #[stable(feature = "try_borrow", since = "1.13.0")]
667 impl Debug for BorrowMutError {
668 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
669 let mut builder = f.debug_struct("BorrowMutError");
671 #[cfg(feature = "debug_refcell")]
672 builder.field("location", self.location);
678 #[stable(feature = "try_borrow", since = "1.13.0")]
679 impl Display for BorrowMutError {
680 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
681 Display::fmt("already borrowed", f)
685 // Positive values represent the number of `Ref` active. Negative values
686 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
687 // active at a time if they refer to distinct, nonoverlapping components of a
688 // `RefCell` (e.g., different ranges of a slice).
690 // `Ref` and `RefMut` are both two words in size, and so there will likely never
691 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
692 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
693 // However, this is not a guarantee, as a pathological program could repeatedly
694 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
695 // explicitly check for overflow and underflow in order to avoid unsafety, or at
696 // least behave correctly in the event that overflow or underflow happens (e.g.,
697 // see BorrowRef::new).
698 type BorrowFlag = isize;
699 const UNUSED: BorrowFlag = 0;
702 fn is_writing(x: BorrowFlag) -> bool {
707 fn is_reading(x: BorrowFlag) -> bool {
712 /// Creates a new `RefCell` containing `value`.
717 /// use std::cell::RefCell;
719 /// let c = RefCell::new(5);
721 #[stable(feature = "rust1", since = "1.0.0")]
722 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
724 pub const fn new(value: T) -> RefCell<T> {
726 value: UnsafeCell::new(value),
727 borrow: Cell::new(UNUSED),
728 #[cfg(feature = "debug_refcell")]
729 borrowed_at: Cell::new(None),
733 /// Consumes the `RefCell`, returning the wrapped value.
738 /// use std::cell::RefCell;
740 /// let c = RefCell::new(5);
742 /// let five = c.into_inner();
744 #[stable(feature = "rust1", since = "1.0.0")]
745 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
747 pub const fn into_inner(self) -> T {
748 // Since this function takes `self` (the `RefCell`) by value, the
749 // compiler statically verifies that it is not currently borrowed.
750 self.value.into_inner()
753 /// Replaces the wrapped value with a new one, returning the old value,
754 /// without deinitializing either one.
756 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
760 /// Panics if the value is currently borrowed.
765 /// use std::cell::RefCell;
766 /// let cell = RefCell::new(5);
767 /// let old_value = cell.replace(6);
768 /// assert_eq!(old_value, 5);
769 /// assert_eq!(cell, RefCell::new(6));
772 #[stable(feature = "refcell_replace", since = "1.24.0")]
774 pub fn replace(&self, t: T) -> T {
775 mem::replace(&mut *self.borrow_mut(), t)
778 /// Replaces the wrapped value with a new one computed from `f`, returning
779 /// the old value, without deinitializing either one.
783 /// Panics if the value is currently borrowed.
788 /// use std::cell::RefCell;
789 /// let cell = RefCell::new(5);
790 /// let old_value = cell.replace_with(|&mut old| old + 1);
791 /// assert_eq!(old_value, 5);
792 /// assert_eq!(cell, RefCell::new(6));
795 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
797 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
798 let mut_borrow = &mut *self.borrow_mut();
799 let replacement = f(mut_borrow);
800 mem::replace(mut_borrow, replacement)
803 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
804 /// without deinitializing either one.
806 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
810 /// Panics if the value in either `RefCell` is currently borrowed, or
811 /// if `self` and `other` point to the same `RefCell`.
816 /// use std::cell::RefCell;
817 /// let c = RefCell::new(5);
818 /// let d = RefCell::new(6);
820 /// assert_eq!(c, RefCell::new(6));
821 /// assert_eq!(d, RefCell::new(5));
824 #[stable(feature = "refcell_swap", since = "1.24.0")]
825 pub fn swap(&self, other: &Self) {
826 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
830 impl<T: ?Sized> RefCell<T> {
831 /// Immutably borrows the wrapped value.
833 /// The borrow lasts until the returned `Ref` exits scope. Multiple
834 /// immutable borrows can be taken out at the same time.
838 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
839 /// [`try_borrow`](#method.try_borrow).
844 /// use std::cell::RefCell;
846 /// let c = RefCell::new(5);
848 /// let borrowed_five = c.borrow();
849 /// let borrowed_five2 = c.borrow();
852 /// An example of panic:
855 /// use std::cell::RefCell;
857 /// let c = RefCell::new(5);
859 /// let m = c.borrow_mut();
860 /// let b = c.borrow(); // this causes a panic
862 #[stable(feature = "rust1", since = "1.0.0")]
865 pub fn borrow(&self) -> Ref<'_, T> {
866 self.try_borrow().expect("already mutably borrowed")
869 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
872 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
873 /// taken out at the same time.
875 /// This is the non-panicking variant of [`borrow`](#method.borrow).
880 /// use std::cell::RefCell;
882 /// let c = RefCell::new(5);
885 /// let m = c.borrow_mut();
886 /// assert!(c.try_borrow().is_err());
890 /// let m = c.borrow();
891 /// assert!(c.try_borrow().is_ok());
894 #[stable(feature = "try_borrow", since = "1.13.0")]
896 #[cfg_attr(feature = "debug_refcell", track_caller)]
897 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
898 match BorrowRef::new(&self.borrow) {
900 #[cfg(feature = "debug_refcell")]
902 // `borrowed_at` is always the *first* active borrow
903 if b.borrow.get() == 1 {
904 self.borrowed_at.set(Some(crate::panic::Location::caller()));
908 // SAFETY: `BorrowRef` ensures that there is only immutable access
909 // to the value while borrowed.
910 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
911 Ok(Ref { value, borrow: b })
913 None => Err(BorrowError {
914 // If a borrow occurred, then we must already have an outstanding borrow,
915 // so `borrowed_at` will be `Some`
916 #[cfg(feature = "debug_refcell")]
917 location: self.borrowed_at.get().unwrap(),
922 /// Mutably borrows the wrapped value.
924 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
925 /// from it exit scope. The value cannot be borrowed while this borrow is
930 /// Panics if the value is currently borrowed. For a non-panicking variant, use
931 /// [`try_borrow_mut`](#method.try_borrow_mut).
936 /// use std::cell::RefCell;
938 /// let c = RefCell::new("hello".to_owned());
940 /// *c.borrow_mut() = "bonjour".to_owned();
942 /// assert_eq!(&*c.borrow(), "bonjour");
945 /// An example of panic:
948 /// use std::cell::RefCell;
950 /// let c = RefCell::new(5);
951 /// let m = c.borrow();
953 /// let b = c.borrow_mut(); // this causes a panic
955 #[stable(feature = "rust1", since = "1.0.0")]
958 pub fn borrow_mut(&self) -> RefMut<'_, T> {
959 self.try_borrow_mut().expect("already borrowed")
962 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
964 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
965 /// from it exit scope. The value cannot be borrowed while this borrow is
968 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
973 /// use std::cell::RefCell;
975 /// let c = RefCell::new(5);
978 /// let m = c.borrow();
979 /// assert!(c.try_borrow_mut().is_err());
982 /// assert!(c.try_borrow_mut().is_ok());
984 #[stable(feature = "try_borrow", since = "1.13.0")]
986 #[cfg_attr(feature = "debug_refcell", track_caller)]
987 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
988 match BorrowRefMut::new(&self.borrow) {
990 #[cfg(feature = "debug_refcell")]
992 self.borrowed_at.set(Some(crate::panic::Location::caller()));
995 // SAFETY: `BorrowRefMut` guarantees unique access.
996 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
997 Ok(RefMut { value, borrow: b, marker: PhantomData })
999 None => Err(BorrowMutError {
1000 // If a borrow occurred, then we must already have an outstanding borrow,
1001 // so `borrowed_at` will be `Some`
1002 #[cfg(feature = "debug_refcell")]
1003 location: self.borrowed_at.get().unwrap(),
1008 /// Returns a raw pointer to the underlying data in this cell.
1013 /// use std::cell::RefCell;
1015 /// let c = RefCell::new(5);
1017 /// let ptr = c.as_ptr();
1020 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1021 pub fn as_ptr(&self) -> *mut T {
1025 /// Returns a mutable reference to the underlying data.
1027 /// Since this method borrows `RefCell` mutably, it is statically guaranteed
1028 /// that no borrows to the underlying data exist. The dynamic checks inherent
1029 /// in [`borrow_mut`] and most other methods of `RefCell` are therefore
1032 /// This method can only be called if `RefCell` can be mutably borrowed,
1033 /// which in general is only the case directly after the `RefCell` has
1034 /// been created. In these situations, skipping the aforementioned dynamic
1035 /// borrowing checks may yield better ergonomics and runtime-performance.
1037 /// In most situations where `RefCell` is used, it can't be borrowed mutably.
1038 /// Use [`borrow_mut`] to get mutable access to the underlying data then.
1040 /// [`borrow_mut`]: RefCell::borrow_mut()
1045 /// use std::cell::RefCell;
1047 /// let mut c = RefCell::new(5);
1048 /// *c.get_mut() += 1;
1050 /// assert_eq!(c, RefCell::new(6));
1053 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1054 pub fn get_mut(&mut self) -> &mut T {
1055 self.value.get_mut()
1058 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1060 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1061 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1062 /// if some `Ref` or `RefMut` borrows have been leaked.
1064 /// [`get_mut`]: RefCell::get_mut()
1069 /// #![feature(cell_leak)]
1070 /// use std::cell::RefCell;
1072 /// let mut c = RefCell::new(0);
1073 /// std::mem::forget(c.borrow_mut());
1075 /// assert!(c.try_borrow().is_err());
1077 /// assert!(c.try_borrow().is_ok());
1079 #[unstable(feature = "cell_leak", issue = "69099")]
1080 pub fn undo_leak(&mut self) -> &mut T {
1081 *self.borrow.get_mut() = UNUSED;
1085 /// Immutably borrows the wrapped value, returning an error if the value is
1086 /// currently mutably borrowed.
1090 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1091 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1092 /// borrowing the `RefCell` while the reference returned by this method
1093 /// is alive is undefined behaviour.
1098 /// use std::cell::RefCell;
1100 /// let c = RefCell::new(5);
1103 /// let m = c.borrow_mut();
1104 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1108 /// let m = c.borrow();
1109 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1112 #[stable(feature = "borrow_state", since = "1.37.0")]
1114 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1115 if !is_writing(self.borrow.get()) {
1116 // SAFETY: We check that nobody is actively writing now, but it is
1117 // the caller's responsibility to ensure that nobody writes until
1118 // the returned reference is no longer in use.
1119 // Also, `self.value.get()` refers to the value owned by `self`
1120 // and is thus guaranteed to be valid for the lifetime of `self`.
1121 Ok(unsafe { &*self.value.get() })
1124 // If a borrow occurred, then we must already have an outstanding borrow,
1125 // so `borrowed_at` will be `Some`
1126 #[cfg(feature = "debug_refcell")]
1127 location: self.borrowed_at.get().unwrap(),
1133 impl<T: Default> RefCell<T> {
1134 /// Takes the wrapped value, leaving `Default::default()` in its place.
1138 /// Panics if the value is currently borrowed.
1143 /// use std::cell::RefCell;
1145 /// let c = RefCell::new(5);
1146 /// let five = c.take();
1148 /// assert_eq!(five, 5);
1149 /// assert_eq!(c.into_inner(), 0);
1151 #[stable(feature = "refcell_take", since = "1.50.0")]
1152 pub fn take(&self) -> T {
1153 self.replace(Default::default())
1157 #[stable(feature = "rust1", since = "1.0.0")]
1158 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1160 #[stable(feature = "rust1", since = "1.0.0")]
1161 impl<T: ?Sized> !Sync for RefCell<T> {}
1163 #[stable(feature = "rust1", since = "1.0.0")]
1164 impl<T: Clone> Clone for RefCell<T> {
1167 /// Panics if the value is currently mutably borrowed.
1170 fn clone(&self) -> RefCell<T> {
1171 RefCell::new(self.borrow().clone())
1176 /// Panics if `other` is currently mutably borrowed.
1179 fn clone_from(&mut self, other: &Self) {
1180 self.get_mut().clone_from(&other.borrow())
1184 #[stable(feature = "rust1", since = "1.0.0")]
1185 impl<T: Default> Default for RefCell<T> {
1186 /// Creates a `RefCell<T>`, with the `Default` value for T.
1188 fn default() -> RefCell<T> {
1189 RefCell::new(Default::default())
1193 #[stable(feature = "rust1", since = "1.0.0")]
1194 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1197 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1199 fn eq(&self, other: &RefCell<T>) -> bool {
1200 *self.borrow() == *other.borrow()
1204 #[stable(feature = "cell_eq", since = "1.2.0")]
1205 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1207 #[stable(feature = "cell_ord", since = "1.10.0")]
1208 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1211 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1213 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1214 self.borrow().partial_cmp(&*other.borrow())
1219 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1221 fn lt(&self, other: &RefCell<T>) -> bool {
1222 *self.borrow() < *other.borrow()
1227 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1229 fn le(&self, other: &RefCell<T>) -> bool {
1230 *self.borrow() <= *other.borrow()
1235 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1237 fn gt(&self, other: &RefCell<T>) -> bool {
1238 *self.borrow() > *other.borrow()
1243 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1245 fn ge(&self, other: &RefCell<T>) -> bool {
1246 *self.borrow() >= *other.borrow()
1250 #[stable(feature = "cell_ord", since = "1.10.0")]
1251 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1254 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1256 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1257 self.borrow().cmp(&*other.borrow())
1261 #[stable(feature = "cell_from", since = "1.12.0")]
1262 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1263 impl<T> const From<T> for RefCell<T> {
1264 /// Creates a new `RefCell<T>` containing the given value.
1265 fn from(t: T) -> RefCell<T> {
1270 #[unstable(feature = "coerce_unsized", issue = "18598")]
1271 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1273 struct BorrowRef<'b> {
1274 borrow: &'b Cell<BorrowFlag>,
1277 impl<'b> BorrowRef<'b> {
1279 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1280 let b = borrow.get().wrapping_add(1);
1282 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1283 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1284 // due to Rust's reference aliasing rules
1285 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1286 // into isize::MIN (the max amount of writing borrows) so we can't allow
1287 // an additional read borrow because isize can't represent so many read borrows
1288 // (this can only happen if you mem::forget more than a small constant amount of
1289 // `Ref`s, which is not good practice)
1292 // Incrementing borrow can result in a reading value (> 0) in these cases:
1293 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1294 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1295 // is large enough to represent having one more read borrow
1297 Some(BorrowRef { borrow })
1302 impl Drop for BorrowRef<'_> {
1304 fn drop(&mut self) {
1305 let borrow = self.borrow.get();
1306 debug_assert!(is_reading(borrow));
1307 self.borrow.set(borrow - 1);
1311 impl Clone for BorrowRef<'_> {
1313 fn clone(&self) -> Self {
1314 // Since this Ref exists, we know the borrow flag
1315 // is a reading borrow.
1316 let borrow = self.borrow.get();
1317 debug_assert!(is_reading(borrow));
1318 // Prevent the borrow counter from overflowing into
1319 // a writing borrow.
1320 assert!(borrow != isize::MAX);
1321 self.borrow.set(borrow + 1);
1322 BorrowRef { borrow: self.borrow }
1326 /// Wraps a borrowed reference to a value in a `RefCell` box.
1327 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1329 /// See the [module-level documentation](self) for more.
1330 #[stable(feature = "rust1", since = "1.0.0")]
1331 #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1332 pub struct Ref<'b, T: ?Sized + 'b> {
1333 // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
1334 // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
1335 // `NonNull` is also covariant over `T`, just like we would have with `&T`.
1337 borrow: BorrowRef<'b>,
1340 #[stable(feature = "rust1", since = "1.0.0")]
1341 impl<T: ?Sized> Deref for Ref<'_, T> {
1345 fn deref(&self) -> &T {
1346 // SAFETY: the value is accessible as long as we hold our borrow.
1347 unsafe { self.value.as_ref() }
1351 impl<'b, T: ?Sized> Ref<'b, T> {
1354 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1356 /// This is an associated function that needs to be used as
1357 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1358 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1360 #[stable(feature = "cell_extras", since = "1.15.0")]
1363 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1364 Ref { value: orig.value, borrow: orig.borrow.clone() }
1367 /// Makes a new `Ref` for a component of the borrowed data.
1369 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1371 /// This is an associated function that needs to be used as `Ref::map(...)`.
1372 /// A method would interfere with methods of the same name on the contents
1373 /// of a `RefCell` used through `Deref`.
1378 /// use std::cell::{RefCell, Ref};
1380 /// let c = RefCell::new((5, 'b'));
1381 /// let b1: Ref<(u32, char)> = c.borrow();
1382 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1383 /// assert_eq!(*b2, 5)
1385 #[stable(feature = "cell_map", since = "1.8.0")]
1387 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1389 F: FnOnce(&T) -> &U,
1391 Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
1394 /// Makes a new `Ref` for an optional component of the borrowed data. The
1395 /// original guard is returned as an `Err(..)` if the closure returns
1398 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1400 /// This is an associated function that needs to be used as
1401 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1402 /// name on the contents of a `RefCell` used through `Deref`.
1407 /// use std::cell::{RefCell, Ref};
1409 /// let c = RefCell::new(vec![1, 2, 3]);
1410 /// let b1: Ref<Vec<u32>> = c.borrow();
1411 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1412 /// assert_eq!(*b2.unwrap(), 2);
1414 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1416 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1418 F: FnOnce(&T) -> Option<&U>,
1421 Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
1426 /// Splits a `Ref` into multiple `Ref`s for different components of the
1429 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1431 /// This is an associated function that needs to be used as
1432 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1433 /// name on the contents of a `RefCell` used through `Deref`.
1438 /// use std::cell::{Ref, RefCell};
1440 /// let cell = RefCell::new([1, 2, 3, 4]);
1441 /// let borrow = cell.borrow();
1442 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1443 /// assert_eq!(*begin, [1, 2]);
1444 /// assert_eq!(*end, [3, 4]);
1446 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1448 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1450 F: FnOnce(&T) -> (&U, &V),
1452 let (a, b) = f(&*orig);
1453 let borrow = orig.borrow.clone();
1455 Ref { value: NonNull::from(a), borrow },
1456 Ref { value: NonNull::from(b), borrow: orig.borrow },
1460 /// Convert into a reference to the underlying data.
1462 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1463 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1464 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1465 /// have occurred in total.
1467 /// This is an associated function that needs to be used as
1468 /// `Ref::leak(...)`. A method would interfere with methods of the
1469 /// same name on the contents of a `RefCell` used through `Deref`.
1474 /// #![feature(cell_leak)]
1475 /// use std::cell::{RefCell, Ref};
1476 /// let cell = RefCell::new(0);
1478 /// let value = Ref::leak(cell.borrow());
1479 /// assert_eq!(*value, 0);
1481 /// assert!(cell.try_borrow().is_ok());
1482 /// assert!(cell.try_borrow_mut().is_err());
1484 #[unstable(feature = "cell_leak", issue = "69099")]
1485 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1486 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1487 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1488 // unique reference to the borrowed RefCell. No further mutable references can be created
1489 // from the original cell.
1490 mem::forget(orig.borrow);
1491 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1492 unsafe { orig.value.as_ref() }
1496 #[unstable(feature = "coerce_unsized", issue = "18598")]
1497 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1499 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1500 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1501 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1506 impl<'b, T: ?Sized> RefMut<'b, T> {
1507 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1510 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1512 /// This is an associated function that needs to be used as
1513 /// `RefMut::map(...)`. A method would interfere with methods of the same
1514 /// name on the contents of a `RefCell` used through `Deref`.
1519 /// use std::cell::{RefCell, RefMut};
1521 /// let c = RefCell::new((5, 'b'));
1523 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1524 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1525 /// assert_eq!(*b2, 5);
1528 /// assert_eq!(*c.borrow(), (42, 'b'));
1530 #[stable(feature = "cell_map", since = "1.8.0")]
1532 pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1534 F: FnOnce(&mut T) -> &mut U,
1536 let value = NonNull::from(f(&mut *orig));
1537 RefMut { value, borrow: orig.borrow, marker: PhantomData }
1540 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1541 /// original guard is returned as an `Err(..)` if the closure returns
1544 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1546 /// This is an associated function that needs to be used as
1547 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1548 /// same name on the contents of a `RefCell` used through `Deref`.
1553 /// use std::cell::{RefCell, RefMut};
1555 /// let c = RefCell::new(vec![1, 2, 3]);
1558 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1559 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1561 /// if let Ok(mut b2) = b2 {
1566 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1568 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1570 pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1572 F: FnOnce(&mut T) -> Option<&mut U>,
1574 // SAFETY: function holds onto an exclusive reference for the duration
1575 // of its call through `orig`, and the pointer is only de-referenced
1576 // inside of the function call never allowing the exclusive reference to
1578 match f(&mut *orig) {
1580 Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
1586 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1589 /// The underlying `RefCell` will remain mutably borrowed until both
1590 /// returned `RefMut`s go out of scope.
1592 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1594 /// This is an associated function that needs to be used as
1595 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1596 /// same name on the contents of a `RefCell` used through `Deref`.
1601 /// use std::cell::{RefCell, RefMut};
1603 /// let cell = RefCell::new([1, 2, 3, 4]);
1604 /// let borrow = cell.borrow_mut();
1605 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1606 /// assert_eq!(*begin, [1, 2]);
1607 /// assert_eq!(*end, [3, 4]);
1608 /// begin.copy_from_slice(&[4, 3]);
1609 /// end.copy_from_slice(&[2, 1]);
1611 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1613 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1614 mut orig: RefMut<'b, T>,
1616 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1618 F: FnOnce(&mut T) -> (&mut U, &mut V),
1620 let borrow = orig.borrow.clone();
1621 let (a, b) = f(&mut *orig);
1623 RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
1624 RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
1628 /// Convert into a mutable reference to the underlying data.
1630 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1631 /// mutably borrowed, making the returned reference the only to the interior.
1633 /// This is an associated function that needs to be used as
1634 /// `RefMut::leak(...)`. A method would interfere with methods of the
1635 /// same name on the contents of a `RefCell` used through `Deref`.
1640 /// #![feature(cell_leak)]
1641 /// use std::cell::{RefCell, RefMut};
1642 /// let cell = RefCell::new(0);
1644 /// let value = RefMut::leak(cell.borrow_mut());
1645 /// assert_eq!(*value, 0);
1648 /// assert!(cell.try_borrow_mut().is_err());
1650 #[unstable(feature = "cell_leak", issue = "69099")]
1651 pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
1652 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1653 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1654 // require a unique reference to the borrowed RefCell. No further references can be created
1655 // from the original cell within that lifetime, making the current borrow the only
1656 // reference for the remaining lifetime.
1657 mem::forget(orig.borrow);
1658 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1659 unsafe { orig.value.as_mut() }
1663 struct BorrowRefMut<'b> {
1664 borrow: &'b Cell<BorrowFlag>,
1667 impl Drop for BorrowRefMut<'_> {
1669 fn drop(&mut self) {
1670 let borrow = self.borrow.get();
1671 debug_assert!(is_writing(borrow));
1672 self.borrow.set(borrow + 1);
1676 impl<'b> BorrowRefMut<'b> {
1678 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1679 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1680 // mutable reference, and so there must currently be no existing
1681 // references. Thus, while clone increments the mutable refcount, here
1682 // we explicitly only allow going from UNUSED to UNUSED - 1.
1683 match borrow.get() {
1685 borrow.set(UNUSED - 1);
1686 Some(BorrowRefMut { borrow })
1692 // Clones a `BorrowRefMut`.
1694 // This is only valid if each `BorrowRefMut` is used to track a mutable
1695 // reference to a distinct, nonoverlapping range of the original object.
1696 // This isn't in a Clone impl so that code doesn't call this implicitly.
1698 fn clone(&self) -> BorrowRefMut<'b> {
1699 let borrow = self.borrow.get();
1700 debug_assert!(is_writing(borrow));
1701 // Prevent the borrow counter from underflowing.
1702 assert!(borrow != isize::MIN);
1703 self.borrow.set(borrow - 1);
1704 BorrowRefMut { borrow: self.borrow }
1708 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1710 /// See the [module-level documentation](self) for more.
1711 #[stable(feature = "rust1", since = "1.0.0")]
1712 #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1713 pub struct RefMut<'b, T: ?Sized + 'b> {
1714 // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
1715 // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
1717 borrow: BorrowRefMut<'b>,
1718 // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
1719 marker: PhantomData<&'b mut T>,
1722 #[stable(feature = "rust1", since = "1.0.0")]
1723 impl<T: ?Sized> Deref for RefMut<'_, T> {
1727 fn deref(&self) -> &T {
1728 // SAFETY: the value is accessible as long as we hold our borrow.
1729 unsafe { self.value.as_ref() }
1733 #[stable(feature = "rust1", since = "1.0.0")]
1734 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1736 fn deref_mut(&mut self) -> &mut T {
1737 // SAFETY: the value is accessible as long as we hold our borrow.
1738 unsafe { self.value.as_mut() }
1742 #[unstable(feature = "coerce_unsized", issue = "18598")]
1743 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1745 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1746 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1747 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1752 /// The core primitive for interior mutability in Rust.
1754 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1755 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1756 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1757 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1758 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1760 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1761 /// use `UnsafeCell` to wrap their data.
1763 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1764 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1765 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1767 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1768 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1771 /// [`.get()`]: `UnsafeCell::get`
1773 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1775 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then
1776 /// you must not access the data in any way that contradicts that reference for the remainder of
1777 /// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it
1778 /// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
1779 /// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut
1780 /// T` reference that is released to safe code, then you must not access the data within the
1781 /// `UnsafeCell` until that reference expires.
1783 /// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
1784 /// until the reference expires. As a special exception, given an `&T`, any part of it that is
1785 /// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
1786 /// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
1787 /// of what a reference points to, this means the memory an `&T` points to can be deallocated only if
1788 /// *every part of it* (including padding) is inside an `UnsafeCell`.
1790 /// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
1791 /// live memory and the compiler is allowed to insert spurious reads if it can prove that this
1792 /// memory has not yet been deallocated.
1794 /// - At all times, you must avoid data races. If multiple threads have access to
1795 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1796 /// accesses (or use atomics).
1798 /// To assist with proper design, the following scenarios are explicitly declared legal
1799 /// for single-threaded code:
1801 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1802 /// references, but not with a `&mut T`
1804 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1805 /// co-exist with it. A `&mut T` must always be unique.
1807 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1808 /// `&UnsafeCell<T>` references alias the cell) is
1809 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1810 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1811 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1812 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1813 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1814 /// may be aliased for the duration of that `&mut` borrow.
1815 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1818 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1822 /// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
1823 /// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
1824 /// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
1825 /// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
1826 /// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
1827 /// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
1828 /// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
1829 /// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
1830 /// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
1831 /// thus this can cause distortions in the type size in these cases.
1833 /// Note that the only valid way to obtain a `*mut T` pointer to the contents of a
1834 /// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference
1835 /// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
1836 /// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
1837 /// same memory layout, the following is not allowed and undefined behavior:
1840 /// # use std::cell::UnsafeCell;
1841 /// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
1842 /// let t = ptr as *const UnsafeCell<T> as *mut T;
1843 /// // This is undefined behavior, because the `*mut T` pointer
1844 /// // was not obtained through `.get()` nor `.raw_get()`:
1845 /// unsafe { &mut *t }
1849 /// Instead, do this:
1852 /// # use std::cell::UnsafeCell;
1853 /// // Safety: the caller must ensure that there are no references that
1854 /// // point to the *contents* of the `UnsafeCell`.
1855 /// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
1856 /// unsafe { &mut *ptr.get() }
1860 /// Converting in the other direction from a `&mut T`
1861 /// to an `&UnsafeCell<T>` is allowed:
1864 /// # use std::cell::UnsafeCell;
1865 /// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
1866 /// let t = ptr as *mut T as *const UnsafeCell<T>;
1867 /// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
1872 /// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
1873 /// [`.raw_get()`]: `UnsafeCell::raw_get`
1877 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1878 /// there being multiple references aliasing the cell:
1881 /// use std::cell::UnsafeCell;
1883 /// let x: UnsafeCell<i32> = 42.into();
1884 /// // Get multiple / concurrent / shared references to the same `x`.
1885 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1888 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1889 /// // so ours is effectively unique.
1890 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1891 /// *p1_exclusive += 27; // |
1892 /// } // <---------- cannot go beyond this point -------------------+
1895 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1896 /// // so we can have multiple shared accesses concurrently.
1897 /// let p2_shared: &i32 = &*p2.get();
1898 /// assert_eq!(*p2_shared, 42 + 27);
1899 /// let p1_shared: &i32 = &*p1.get();
1900 /// assert_eq!(*p1_shared, *p2_shared);
1904 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1905 /// implies exclusive access to its `T`:
1908 /// #![forbid(unsafe_code)] // with exclusive accesses,
1909 /// // `UnsafeCell` is a transparent no-op wrapper,
1910 /// // so no need for `unsafe` here.
1911 /// use std::cell::UnsafeCell;
1913 /// let mut x: UnsafeCell<i32> = 42.into();
1915 /// // Get a compile-time-checked unique reference to `x`.
1916 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1917 /// // With an exclusive reference, we can mutate the contents for free.
1918 /// *p_unique.get_mut() = 0;
1919 /// // Or, equivalently:
1920 /// x = UnsafeCell::new(0);
1922 /// // When we own the value, we can extract the contents for free.
1923 /// let contents: i32 = x.into_inner();
1924 /// assert_eq!(contents, 0);
1926 #[lang = "unsafe_cell"]
1927 #[stable(feature = "rust1", since = "1.0.0")]
1928 #[repr(transparent)]
1929 pub struct UnsafeCell<T: ?Sized> {
1933 #[stable(feature = "rust1", since = "1.0.0")]
1934 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1936 impl<T> UnsafeCell<T> {
1937 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1940 /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
1945 /// use std::cell::UnsafeCell;
1947 /// let uc = UnsafeCell::new(5);
1949 #[stable(feature = "rust1", since = "1.0.0")]
1950 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1952 pub const fn new(value: T) -> UnsafeCell<T> {
1953 UnsafeCell { value }
1956 /// Unwraps the value.
1961 /// use std::cell::UnsafeCell;
1963 /// let uc = UnsafeCell::new(5);
1965 /// let five = uc.into_inner();
1968 #[stable(feature = "rust1", since = "1.0.0")]
1969 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1970 pub const fn into_inner(self) -> T {
1975 impl<T: ?Sized> UnsafeCell<T> {
1976 /// Gets a mutable pointer to the wrapped value.
1978 /// This can be cast to a pointer of any kind.
1979 /// Ensure that the access is unique (no active references, mutable or not)
1980 /// when casting to `&mut T`, and ensure that there are no mutations
1981 /// or mutable aliases going on when casting to `&T`
1986 /// use std::cell::UnsafeCell;
1988 /// let uc = UnsafeCell::new(5);
1990 /// let five = uc.get();
1993 #[stable(feature = "rust1", since = "1.0.0")]
1994 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1995 pub const fn get(&self) -> *mut T {
1996 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1997 // #[repr(transparent)]. This exploits std's special status, there is
1998 // no guarantee for user code that this will work in future versions of the compiler!
1999 self as *const UnsafeCell<T> as *const T as *mut T
2002 /// Returns a mutable reference to the underlying data.
2004 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
2005 /// guarantees that we possess the only reference.
2010 /// use std::cell::UnsafeCell;
2012 /// let mut c = UnsafeCell::new(5);
2013 /// *c.get_mut() += 1;
2015 /// assert_eq!(*c.get_mut(), 6);
2018 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
2019 #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
2020 pub const fn get_mut(&mut self) -> &mut T {
2024 /// Gets a mutable pointer to the wrapped value.
2025 /// The difference from [`get`] is that this function accepts a raw pointer,
2026 /// which is useful to avoid the creation of temporary references.
2028 /// The result can be cast to a pointer of any kind.
2029 /// Ensure that the access is unique (no active references, mutable or not)
2030 /// when casting to `&mut T`, and ensure that there are no mutations
2031 /// or mutable aliases going on when casting to `&T`.
2033 /// [`get`]: UnsafeCell::get()
2037 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
2038 /// calling `get` would require creating a reference to uninitialized data:
2041 /// use std::cell::UnsafeCell;
2042 /// use std::mem::MaybeUninit;
2044 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
2045 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
2046 /// let uc = unsafe { m.assume_init() };
2048 /// assert_eq!(uc.into_inner(), 5);
2051 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2052 #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2053 pub const fn raw_get(this: *const Self) -> *mut T {
2054 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2055 // #[repr(transparent)]. This exploits std's special status, there is
2056 // no guarantee for user code that this will work in future versions of the compiler!
2057 this as *const T as *mut T
2061 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
2062 impl<T: Default> Default for UnsafeCell<T> {
2063 /// Creates an `UnsafeCell`, with the `Default` value for T.
2064 fn default() -> UnsafeCell<T> {
2065 UnsafeCell::new(Default::default())
2069 #[stable(feature = "cell_from", since = "1.12.0")]
2070 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2071 impl<T> const From<T> for UnsafeCell<T> {
2072 /// Creates a new `UnsafeCell<T>` containing the given value.
2073 fn from(t: T) -> UnsafeCell<T> {
2078 #[unstable(feature = "coerce_unsized", issue = "18598")]
2079 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
2081 /// [`UnsafeCell`], but [`Sync`].
2083 /// This is just an `UnsafeCell`, except it implements `Sync`
2084 /// if `T` implements `Sync`.
2086 /// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
2087 /// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
2088 /// shared between threads, if that's intentional.
2089 /// Providing proper synchronization is still the task of the user,
2090 /// making this type just as unsafe to use.
2092 /// See [`UnsafeCell`] for details.
2093 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2094 #[repr(transparent)]
2095 pub struct SyncUnsafeCell<T: ?Sized> {
2096 value: UnsafeCell<T>,
2099 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2100 unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
2102 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2103 impl<T> SyncUnsafeCell<T> {
2104 /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
2106 pub const fn new(value: T) -> Self {
2107 Self { value: UnsafeCell { value } }
2110 /// Unwraps the value.
2112 pub const fn into_inner(self) -> T {
2113 self.value.into_inner()
2117 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2118 impl<T: ?Sized> SyncUnsafeCell<T> {
2119 /// Gets a mutable pointer to the wrapped value.
2121 /// This can be cast to a pointer of any kind.
2122 /// Ensure that the access is unique (no active references, mutable or not)
2123 /// when casting to `&mut T`, and ensure that there are no mutations
2124 /// or mutable aliases going on when casting to `&T`
2126 pub const fn get(&self) -> *mut T {
2130 /// Returns a mutable reference to the underlying data.
2132 /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
2133 /// guarantees that we possess the only reference.
2135 pub const fn get_mut(&mut self) -> &mut T {
2136 self.value.get_mut()
2139 /// Gets a mutable pointer to the wrapped value.
2141 /// See [`UnsafeCell::get`] for details.
2143 pub const fn raw_get(this: *const Self) -> *mut T {
2144 // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
2145 // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
2146 // See UnsafeCell::raw_get.
2147 this as *const T as *mut T
2151 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2152 impl<T: Default> Default for SyncUnsafeCell<T> {
2153 /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
2154 fn default() -> SyncUnsafeCell<T> {
2155 SyncUnsafeCell::new(Default::default())
2159 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2160 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2161 impl<T> const From<T> for SyncUnsafeCell<T> {
2162 /// Creates a new `SyncUnsafeCell<T>` containing the given value.
2163 fn from(t: T) -> SyncUnsafeCell<T> {
2164 SyncUnsafeCell::new(t)
2168 #[unstable(feature = "coerce_unsized", issue = "18598")]
2169 //#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2170 impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2173 fn assert_coerce_unsized(
2174 a: UnsafeCell<&i32>,
2175 b: SyncUnsafeCell<&i32>,
2179 let _: UnsafeCell<&dyn Send> = a;
2180 let _: SyncUnsafeCell<&dyn Send> = b;
2181 let _: Cell<&dyn Send> = c;
2182 let _: RefCell<&dyn Send> = d;