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 = "27732")]
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.
815 /// use std::cell::RefCell;
816 /// let c = RefCell::new(5);
817 /// let d = RefCell::new(6);
819 /// assert_eq!(c, RefCell::new(6));
820 /// assert_eq!(d, RefCell::new(5));
823 #[stable(feature = "refcell_swap", since = "1.24.0")]
824 pub fn swap(&self, other: &Self) {
825 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
829 impl<T: ?Sized> RefCell<T> {
830 /// Immutably borrows the wrapped value.
832 /// The borrow lasts until the returned `Ref` exits scope. Multiple
833 /// immutable borrows can be taken out at the same time.
837 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
838 /// [`try_borrow`](#method.try_borrow).
843 /// use std::cell::RefCell;
845 /// let c = RefCell::new(5);
847 /// let borrowed_five = c.borrow();
848 /// let borrowed_five2 = c.borrow();
851 /// An example of panic:
854 /// use std::cell::RefCell;
856 /// let c = RefCell::new(5);
858 /// let m = c.borrow_mut();
859 /// let b = c.borrow(); // this causes a panic
861 #[stable(feature = "rust1", since = "1.0.0")]
864 pub fn borrow(&self) -> Ref<'_, T> {
865 self.try_borrow().expect("already mutably borrowed")
868 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
871 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
872 /// taken out at the same time.
874 /// This is the non-panicking variant of [`borrow`](#method.borrow).
879 /// use std::cell::RefCell;
881 /// let c = RefCell::new(5);
884 /// let m = c.borrow_mut();
885 /// assert!(c.try_borrow().is_err());
889 /// let m = c.borrow();
890 /// assert!(c.try_borrow().is_ok());
893 #[stable(feature = "try_borrow", since = "1.13.0")]
895 #[cfg_attr(feature = "debug_refcell", track_caller)]
896 pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
897 match BorrowRef::new(&self.borrow) {
899 #[cfg(feature = "debug_refcell")]
901 // `borrowed_at` is always the *first* active borrow
902 if b.borrow.get() == 1 {
903 self.borrowed_at.set(Some(crate::panic::Location::caller()));
907 // SAFETY: `BorrowRef` ensures that there is only immutable access
908 // to the value while borrowed.
909 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
910 Ok(Ref { value, borrow: b })
912 None => Err(BorrowError {
913 // If a borrow occurred, then we must already have an outstanding borrow,
914 // so `borrowed_at` will be `Some`
915 #[cfg(feature = "debug_refcell")]
916 location: self.borrowed_at.get().unwrap(),
921 /// Mutably borrows the wrapped value.
923 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
924 /// from it exit scope. The value cannot be borrowed while this borrow is
929 /// Panics if the value is currently borrowed. For a non-panicking variant, use
930 /// [`try_borrow_mut`](#method.try_borrow_mut).
935 /// use std::cell::RefCell;
937 /// let c = RefCell::new("hello".to_owned());
939 /// *c.borrow_mut() = "bonjour".to_owned();
941 /// assert_eq!(&*c.borrow(), "bonjour");
944 /// An example of panic:
947 /// use std::cell::RefCell;
949 /// let c = RefCell::new(5);
950 /// let m = c.borrow();
952 /// let b = c.borrow_mut(); // this causes a panic
954 #[stable(feature = "rust1", since = "1.0.0")]
957 pub fn borrow_mut(&self) -> RefMut<'_, T> {
958 self.try_borrow_mut().expect("already borrowed")
961 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
963 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
964 /// from it exit scope. The value cannot be borrowed while this borrow is
967 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
972 /// use std::cell::RefCell;
974 /// let c = RefCell::new(5);
977 /// let m = c.borrow();
978 /// assert!(c.try_borrow_mut().is_err());
981 /// assert!(c.try_borrow_mut().is_ok());
983 #[stable(feature = "try_borrow", since = "1.13.0")]
985 #[cfg_attr(feature = "debug_refcell", track_caller)]
986 pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
987 match BorrowRefMut::new(&self.borrow) {
989 #[cfg(feature = "debug_refcell")]
991 self.borrowed_at.set(Some(crate::panic::Location::caller()));
994 // SAFETY: `BorrowRefMut` guarantees unique access.
995 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
996 Ok(RefMut { value, borrow: b, marker: PhantomData })
998 None => Err(BorrowMutError {
999 // If a borrow occurred, then we must already have an outstanding borrow,
1000 // so `borrowed_at` will be `Some`
1001 #[cfg(feature = "debug_refcell")]
1002 location: self.borrowed_at.get().unwrap(),
1007 /// Returns a raw pointer to the underlying data in this cell.
1012 /// use std::cell::RefCell;
1014 /// let c = RefCell::new(5);
1016 /// let ptr = c.as_ptr();
1019 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1020 pub fn as_ptr(&self) -> *mut T {
1024 /// Returns a mutable reference to the underlying data.
1026 /// Since this method borrows `RefCell` mutably, it is statically guaranteed
1027 /// that no borrows to the underlying data exist. The dynamic checks inherent
1028 /// in [`borrow_mut`] and most other methods of `RefCell` are therefor
1031 /// This method can only be called if `RefCell` can be mutably borrowed,
1032 /// which in general is only the case directly after the `RefCell` has
1033 /// been created. In these situations, skipping the aforementioned dynamic
1034 /// borrowing checks may yield better ergonomics and runtime-performance.
1036 /// In most situations where `RefCell` is used, it can't be borrowed mutably.
1037 /// Use [`borrow_mut`] to get mutable access to the underlying data then.
1039 /// [`borrow_mut`]: RefCell::borrow_mut()
1044 /// use std::cell::RefCell;
1046 /// let mut c = RefCell::new(5);
1047 /// *c.get_mut() += 1;
1049 /// assert_eq!(c, RefCell::new(6));
1052 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1053 pub fn get_mut(&mut self) -> &mut T {
1054 self.value.get_mut()
1057 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1059 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1060 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1061 /// if some `Ref` or `RefMut` borrows have been leaked.
1063 /// [`get_mut`]: RefCell::get_mut()
1068 /// #![feature(cell_leak)]
1069 /// use std::cell::RefCell;
1071 /// let mut c = RefCell::new(0);
1072 /// std::mem::forget(c.borrow_mut());
1074 /// assert!(c.try_borrow().is_err());
1076 /// assert!(c.try_borrow().is_ok());
1078 #[unstable(feature = "cell_leak", issue = "69099")]
1079 pub fn undo_leak(&mut self) -> &mut T {
1080 *self.borrow.get_mut() = UNUSED;
1084 /// Immutably borrows the wrapped value, returning an error if the value is
1085 /// currently mutably borrowed.
1089 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1090 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1091 /// borrowing the `RefCell` while the reference returned by this method
1092 /// is alive is undefined behaviour.
1097 /// use std::cell::RefCell;
1099 /// let c = RefCell::new(5);
1102 /// let m = c.borrow_mut();
1103 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1107 /// let m = c.borrow();
1108 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1111 #[stable(feature = "borrow_state", since = "1.37.0")]
1113 pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1114 if !is_writing(self.borrow.get()) {
1115 // SAFETY: We check that nobody is actively writing now, but it is
1116 // the caller's responsibility to ensure that nobody writes until
1117 // the returned reference is no longer in use.
1118 // Also, `self.value.get()` refers to the value owned by `self`
1119 // and is thus guaranteed to be valid for the lifetime of `self`.
1120 Ok(unsafe { &*self.value.get() })
1123 // If a borrow occurred, then we must already have an outstanding borrow,
1124 // so `borrowed_at` will be `Some`
1125 #[cfg(feature = "debug_refcell")]
1126 location: self.borrowed_at.get().unwrap(),
1132 impl<T: Default> RefCell<T> {
1133 /// Takes the wrapped value, leaving `Default::default()` in its place.
1137 /// Panics if the value is currently borrowed.
1142 /// use std::cell::RefCell;
1144 /// let c = RefCell::new(5);
1145 /// let five = c.take();
1147 /// assert_eq!(five, 5);
1148 /// assert_eq!(c.into_inner(), 0);
1150 #[stable(feature = "refcell_take", since = "1.50.0")]
1151 pub fn take(&self) -> T {
1152 self.replace(Default::default())
1156 #[stable(feature = "rust1", since = "1.0.0")]
1157 unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1159 #[stable(feature = "rust1", since = "1.0.0")]
1160 impl<T: ?Sized> !Sync for RefCell<T> {}
1162 #[stable(feature = "rust1", since = "1.0.0")]
1163 impl<T: Clone> Clone for RefCell<T> {
1166 /// Panics if the value is currently mutably borrowed.
1169 fn clone(&self) -> RefCell<T> {
1170 RefCell::new(self.borrow().clone())
1175 /// Panics if `other` is currently mutably borrowed.
1178 fn clone_from(&mut self, other: &Self) {
1179 self.get_mut().clone_from(&other.borrow())
1183 #[stable(feature = "rust1", since = "1.0.0")]
1184 impl<T: Default> Default for RefCell<T> {
1185 /// Creates a `RefCell<T>`, with the `Default` value for T.
1187 fn default() -> RefCell<T> {
1188 RefCell::new(Default::default())
1192 #[stable(feature = "rust1", since = "1.0.0")]
1193 impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1196 /// Panics if the value in either `RefCell` is currently borrowed.
1198 fn eq(&self, other: &RefCell<T>) -> bool {
1199 *self.borrow() == *other.borrow()
1203 #[stable(feature = "cell_eq", since = "1.2.0")]
1204 impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1206 #[stable(feature = "cell_ord", since = "1.10.0")]
1207 impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1210 /// Panics if the value in either `RefCell` is currently borrowed.
1212 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1213 self.borrow().partial_cmp(&*other.borrow())
1218 /// Panics if the value in either `RefCell` is currently borrowed.
1220 fn lt(&self, other: &RefCell<T>) -> bool {
1221 *self.borrow() < *other.borrow()
1226 /// Panics if the value in either `RefCell` is currently borrowed.
1228 fn le(&self, other: &RefCell<T>) -> bool {
1229 *self.borrow() <= *other.borrow()
1234 /// Panics if the value in either `RefCell` is currently borrowed.
1236 fn gt(&self, other: &RefCell<T>) -> bool {
1237 *self.borrow() > *other.borrow()
1242 /// Panics if the value in either `RefCell` is currently borrowed.
1244 fn ge(&self, other: &RefCell<T>) -> bool {
1245 *self.borrow() >= *other.borrow()
1249 #[stable(feature = "cell_ord", since = "1.10.0")]
1250 impl<T: ?Sized + Ord> Ord for RefCell<T> {
1253 /// Panics if the value in either `RefCell` is currently borrowed.
1255 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1256 self.borrow().cmp(&*other.borrow())
1260 #[stable(feature = "cell_from", since = "1.12.0")]
1261 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
1262 impl<T> const From<T> for RefCell<T> {
1263 /// Creates a new `RefCell<T>` containing the given value.
1264 fn from(t: T) -> RefCell<T> {
1269 #[unstable(feature = "coerce_unsized", issue = "27732")]
1270 impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1272 struct BorrowRef<'b> {
1273 borrow: &'b Cell<BorrowFlag>,
1276 impl<'b> BorrowRef<'b> {
1278 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1279 let b = borrow.get().wrapping_add(1);
1281 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1282 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1283 // due to Rust's reference aliasing rules
1284 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1285 // into isize::MIN (the max amount of writing borrows) so we can't allow
1286 // an additional read borrow because isize can't represent so many read borrows
1287 // (this can only happen if you mem::forget more than a small constant amount of
1288 // `Ref`s, which is not good practice)
1291 // Incrementing borrow can result in a reading value (> 0) in these cases:
1292 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1293 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1294 // is large enough to represent having one more read borrow
1296 Some(BorrowRef { borrow })
1301 impl Drop for BorrowRef<'_> {
1303 fn drop(&mut self) {
1304 let borrow = self.borrow.get();
1305 debug_assert!(is_reading(borrow));
1306 self.borrow.set(borrow - 1);
1310 impl Clone for BorrowRef<'_> {
1312 fn clone(&self) -> Self {
1313 // Since this Ref exists, we know the borrow flag
1314 // is a reading borrow.
1315 let borrow = self.borrow.get();
1316 debug_assert!(is_reading(borrow));
1317 // Prevent the borrow counter from overflowing into
1318 // a writing borrow.
1319 assert!(borrow != isize::MAX);
1320 self.borrow.set(borrow + 1);
1321 BorrowRef { borrow: self.borrow }
1325 /// Wraps a borrowed reference to a value in a `RefCell` box.
1326 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1328 /// See the [module-level documentation](self) for more.
1329 #[stable(feature = "rust1", since = "1.0.0")]
1330 #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1331 pub struct Ref<'b, T: ?Sized + 'b> {
1332 // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
1333 // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
1334 // `NonNull` is also covariant over `T`, just like we would have with `&T`.
1336 borrow: BorrowRef<'b>,
1339 #[stable(feature = "rust1", since = "1.0.0")]
1340 impl<T: ?Sized> Deref for Ref<'_, T> {
1344 fn deref(&self) -> &T {
1345 // SAFETY: the value is accessible as long as we hold our borrow.
1346 unsafe { self.value.as_ref() }
1350 impl<'b, T: ?Sized> Ref<'b, T> {
1353 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1355 /// This is an associated function that needs to be used as
1356 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1357 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1359 #[stable(feature = "cell_extras", since = "1.15.0")]
1362 pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1363 Ref { value: orig.value, borrow: orig.borrow.clone() }
1366 /// Makes a new `Ref` for a component of the borrowed data.
1368 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1370 /// This is an associated function that needs to be used as `Ref::map(...)`.
1371 /// A method would interfere with methods of the same name on the contents
1372 /// of a `RefCell` used through `Deref`.
1377 /// use std::cell::{RefCell, Ref};
1379 /// let c = RefCell::new((5, 'b'));
1380 /// let b1: Ref<(u32, char)> = c.borrow();
1381 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1382 /// assert_eq!(*b2, 5)
1384 #[stable(feature = "cell_map", since = "1.8.0")]
1386 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1388 F: FnOnce(&T) -> &U,
1390 Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
1393 /// Makes a new `Ref` for an optional component of the borrowed data. The
1394 /// original guard is returned as an `Err(..)` if the closure returns
1397 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1399 /// This is an associated function that needs to be used as
1400 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1401 /// name on the contents of a `RefCell` used through `Deref`.
1406 /// use std::cell::{RefCell, Ref};
1408 /// let c = RefCell::new(vec![1, 2, 3]);
1409 /// let b1: Ref<Vec<u32>> = c.borrow();
1410 /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1411 /// assert_eq!(*b2.unwrap(), 2);
1413 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1415 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1417 F: FnOnce(&T) -> Option<&U>,
1420 Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
1425 /// Splits a `Ref` into multiple `Ref`s for different components of the
1428 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1430 /// This is an associated function that needs to be used as
1431 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1432 /// name on the contents of a `RefCell` used through `Deref`.
1437 /// use std::cell::{Ref, RefCell};
1439 /// let cell = RefCell::new([1, 2, 3, 4]);
1440 /// let borrow = cell.borrow();
1441 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1442 /// assert_eq!(*begin, [1, 2]);
1443 /// assert_eq!(*end, [3, 4]);
1445 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1447 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1449 F: FnOnce(&T) -> (&U, &V),
1451 let (a, b) = f(&*orig);
1452 let borrow = orig.borrow.clone();
1454 Ref { value: NonNull::from(a), borrow },
1455 Ref { value: NonNull::from(b), borrow: orig.borrow },
1459 /// Convert into a reference to the underlying data.
1461 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1462 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1463 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1464 /// have occurred in total.
1466 /// This is an associated function that needs to be used as
1467 /// `Ref::leak(...)`. A method would interfere with methods of the
1468 /// same name on the contents of a `RefCell` used through `Deref`.
1473 /// #![feature(cell_leak)]
1474 /// use std::cell::{RefCell, Ref};
1475 /// let cell = RefCell::new(0);
1477 /// let value = Ref::leak(cell.borrow());
1478 /// assert_eq!(*value, 0);
1480 /// assert!(cell.try_borrow().is_ok());
1481 /// assert!(cell.try_borrow_mut().is_err());
1483 #[unstable(feature = "cell_leak", issue = "69099")]
1484 pub fn leak(orig: Ref<'b, T>) -> &'b T {
1485 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1486 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1487 // unique reference to the borrowed RefCell. No further mutable references can be created
1488 // from the original cell.
1489 mem::forget(orig.borrow);
1490 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1491 unsafe { orig.value.as_ref() }
1495 #[unstable(feature = "coerce_unsized", issue = "27732")]
1496 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1498 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1499 impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1500 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1505 impl<'b, T: ?Sized> RefMut<'b, T> {
1506 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1509 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1511 /// This is an associated function that needs to be used as
1512 /// `RefMut::map(...)`. A method would interfere with methods of the same
1513 /// name on the contents of a `RefCell` used through `Deref`.
1518 /// use std::cell::{RefCell, RefMut};
1520 /// let c = RefCell::new((5, 'b'));
1522 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1523 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1524 /// assert_eq!(*b2, 5);
1527 /// assert_eq!(*c.borrow(), (42, 'b'));
1529 #[stable(feature = "cell_map", since = "1.8.0")]
1531 pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1533 F: FnOnce(&mut T) -> &mut U,
1535 let value = NonNull::from(f(&mut *orig));
1536 RefMut { value, borrow: orig.borrow, marker: PhantomData }
1539 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1540 /// original guard is returned as an `Err(..)` if the closure returns
1543 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1545 /// This is an associated function that needs to be used as
1546 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1547 /// same name on the contents of a `RefCell` used through `Deref`.
1552 /// use std::cell::{RefCell, RefMut};
1554 /// let c = RefCell::new(vec![1, 2, 3]);
1557 /// let b1: RefMut<Vec<u32>> = c.borrow_mut();
1558 /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1560 /// if let Ok(mut b2) = b2 {
1565 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1567 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1569 pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1571 F: FnOnce(&mut T) -> Option<&mut U>,
1573 // SAFETY: function holds onto an exclusive reference for the duration
1574 // of its call through `orig`, and the pointer is only de-referenced
1575 // inside of the function call never allowing the exclusive reference to
1577 match f(&mut *orig) {
1579 Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
1585 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1588 /// The underlying `RefCell` will remain mutably borrowed until both
1589 /// returned `RefMut`s go out of scope.
1591 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1593 /// This is an associated function that needs to be used as
1594 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1595 /// same name on the contents of a `RefCell` used through `Deref`.
1600 /// use std::cell::{RefCell, RefMut};
1602 /// let cell = RefCell::new([1, 2, 3, 4]);
1603 /// let borrow = cell.borrow_mut();
1604 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1605 /// assert_eq!(*begin, [1, 2]);
1606 /// assert_eq!(*end, [3, 4]);
1607 /// begin.copy_from_slice(&[4, 3]);
1608 /// end.copy_from_slice(&[2, 1]);
1610 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1612 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1613 mut orig: RefMut<'b, T>,
1615 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1617 F: FnOnce(&mut T) -> (&mut U, &mut V),
1619 let borrow = orig.borrow.clone();
1620 let (a, b) = f(&mut *orig);
1622 RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
1623 RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
1627 /// Convert into a mutable reference to the underlying data.
1629 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1630 /// mutably borrowed, making the returned reference the only to the interior.
1632 /// This is an associated function that needs to be used as
1633 /// `RefMut::leak(...)`. A method would interfere with methods of the
1634 /// same name on the contents of a `RefCell` used through `Deref`.
1639 /// #![feature(cell_leak)]
1640 /// use std::cell::{RefCell, RefMut};
1641 /// let cell = RefCell::new(0);
1643 /// let value = RefMut::leak(cell.borrow_mut());
1644 /// assert_eq!(*value, 0);
1647 /// assert!(cell.try_borrow_mut().is_err());
1649 #[unstable(feature = "cell_leak", issue = "69099")]
1650 pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
1651 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1652 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1653 // require a unique reference to the borrowed RefCell. No further references can be created
1654 // from the original cell within that lifetime, making the current borrow the only
1655 // reference for the remaining lifetime.
1656 mem::forget(orig.borrow);
1657 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1658 unsafe { orig.value.as_mut() }
1662 struct BorrowRefMut<'b> {
1663 borrow: &'b Cell<BorrowFlag>,
1666 impl Drop for BorrowRefMut<'_> {
1668 fn drop(&mut self) {
1669 let borrow = self.borrow.get();
1670 debug_assert!(is_writing(borrow));
1671 self.borrow.set(borrow + 1);
1675 impl<'b> BorrowRefMut<'b> {
1677 fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1678 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1679 // mutable reference, and so there must currently be no existing
1680 // references. Thus, while clone increments the mutable refcount, here
1681 // we explicitly only allow going from UNUSED to UNUSED - 1.
1682 match borrow.get() {
1684 borrow.set(UNUSED - 1);
1685 Some(BorrowRefMut { borrow })
1691 // Clones a `BorrowRefMut`.
1693 // This is only valid if each `BorrowRefMut` is used to track a mutable
1694 // reference to a distinct, nonoverlapping range of the original object.
1695 // This isn't in a Clone impl so that code doesn't call this implicitly.
1697 fn clone(&self) -> BorrowRefMut<'b> {
1698 let borrow = self.borrow.get();
1699 debug_assert!(is_writing(borrow));
1700 // Prevent the borrow counter from underflowing.
1701 assert!(borrow != isize::MIN);
1702 self.borrow.set(borrow - 1);
1703 BorrowRefMut { borrow: self.borrow }
1707 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1709 /// See the [module-level documentation](self) for more.
1710 #[stable(feature = "rust1", since = "1.0.0")]
1711 #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1712 pub struct RefMut<'b, T: ?Sized + 'b> {
1713 // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
1714 // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
1716 borrow: BorrowRefMut<'b>,
1717 // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
1718 marker: PhantomData<&'b mut T>,
1721 #[stable(feature = "rust1", since = "1.0.0")]
1722 impl<T: ?Sized> Deref for RefMut<'_, T> {
1726 fn deref(&self) -> &T {
1727 // SAFETY: the value is accessible as long as we hold our borrow.
1728 unsafe { self.value.as_ref() }
1732 #[stable(feature = "rust1", since = "1.0.0")]
1733 impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1735 fn deref_mut(&mut self) -> &mut T {
1736 // SAFETY: the value is accessible as long as we hold our borrow.
1737 unsafe { self.value.as_mut() }
1741 #[unstable(feature = "coerce_unsized", issue = "27732")]
1742 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1744 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1745 impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1746 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1751 /// The core primitive for interior mutability in Rust.
1753 /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1754 /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1755 /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior.
1756 /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1757 /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1759 /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally
1760 /// use `UnsafeCell` to wrap their data.
1762 /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1763 /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1764 /// aliasing `&mut`, not even with `UnsafeCell<T>`.
1766 /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1767 /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1770 /// [`.get()`]: `UnsafeCell::get`
1772 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1774 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then
1775 /// you must not access the data in any way that contradicts that reference for the remainder of
1776 /// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it
1777 /// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
1778 /// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut
1779 /// T` reference that is released to safe code, then you must not access the data within the
1780 /// `UnsafeCell` until that reference expires.
1782 /// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
1783 /// until the reference expires. As a special exception, given an `&T`, any part of it that is
1784 /// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
1785 /// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
1786 /// of what a reference points to, this means the memory an `&T` points to can be deallocted only if
1787 /// *every part of it* (including padding) is inside an `UnsafeCell`.
1789 /// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
1790 /// live memory and the compiler is allowed to insert spurious reads if it can prove that this
1791 /// memory has not yet been deallocated.
1793 /// - At all times, you must avoid data races. If multiple threads have access to
1794 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1795 /// accesses (or use atomics).
1797 /// To assist with proper design, the following scenarios are explicitly declared legal
1798 /// for single-threaded code:
1800 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1801 /// references, but not with a `&mut T`
1803 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1804 /// co-exist with it. A `&mut T` must always be unique.
1806 /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1807 /// `&UnsafeCell<T>` references alias the cell) is
1808 /// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1809 /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1810 /// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1811 /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1812 /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1813 /// may be aliased for the duration of that `&mut` borrow.
1814 /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1817 /// [`.get_mut()`]: `UnsafeCell::get_mut`
1821 /// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
1822 /// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
1823 /// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
1824 /// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
1825 /// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
1826 /// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
1827 /// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
1828 /// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
1829 /// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
1830 /// thus this can cause distortions in the type size in these cases.
1832 /// Note that the only valid way to obtain a `*mut T` pointer to the contents of a
1833 /// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference
1834 /// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
1835 /// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
1836 /// same memory layout, the following is not allowed and undefined behavior:
1839 /// # use std::cell::UnsafeCell;
1840 /// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
1841 /// let t = ptr as *const UnsafeCell<T> as *mut T;
1842 /// // This is undefined behavior, because the `*mut T` pointer
1843 /// // was not obtained through `.get()` nor `.raw_get()`:
1844 /// unsafe { &mut *t }
1848 /// Instead, do this:
1851 /// # use std::cell::UnsafeCell;
1852 /// // Safety: the caller must ensure that there are no references that
1853 /// // point to the *contents* of the `UnsafeCell`.
1854 /// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
1855 /// unsafe { &mut *ptr.get() }
1859 /// Converting in the other direction from a `&mut T`
1860 /// to an `&UnsafeCell<T>` is allowed:
1863 /// # use std::cell::UnsafeCell;
1864 /// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
1865 /// let t = ptr as *mut T as *const UnsafeCell<T>;
1866 /// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
1871 /// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
1872 /// [`.raw_get()`]: `UnsafeCell::raw_get`
1876 /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
1877 /// there being multiple references aliasing the cell:
1880 /// use std::cell::UnsafeCell;
1882 /// let x: UnsafeCell<i32> = 42.into();
1883 /// // Get multiple / concurrent / shared references to the same `x`.
1884 /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
1887 /// // SAFETY: within this scope there are no other references to `x`'s contents,
1888 /// // so ours is effectively unique.
1889 /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
1890 /// *p1_exclusive += 27; // |
1891 /// } // <---------- cannot go beyond this point -------------------+
1894 /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
1895 /// // so we can have multiple shared accesses concurrently.
1896 /// let p2_shared: &i32 = &*p2.get();
1897 /// assert_eq!(*p2_shared, 42 + 27);
1898 /// let p1_shared: &i32 = &*p1.get();
1899 /// assert_eq!(*p1_shared, *p2_shared);
1903 /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
1904 /// implies exclusive access to its `T`:
1907 /// #![forbid(unsafe_code)] // with exclusive accesses,
1908 /// // `UnsafeCell` is a transparent no-op wrapper,
1909 /// // so no need for `unsafe` here.
1910 /// use std::cell::UnsafeCell;
1912 /// let mut x: UnsafeCell<i32> = 42.into();
1914 /// // Get a compile-time-checked unique reference to `x`.
1915 /// let p_unique: &mut UnsafeCell<i32> = &mut x;
1916 /// // With an exclusive reference, we can mutate the contents for free.
1917 /// *p_unique.get_mut() = 0;
1918 /// // Or, equivalently:
1919 /// x = UnsafeCell::new(0);
1921 /// // When we own the value, we can extract the contents for free.
1922 /// let contents: i32 = x.into_inner();
1923 /// assert_eq!(contents, 0);
1925 #[lang = "unsafe_cell"]
1926 #[stable(feature = "rust1", since = "1.0.0")]
1927 #[repr(transparent)]
1928 pub struct UnsafeCell<T: ?Sized> {
1932 #[stable(feature = "rust1", since = "1.0.0")]
1933 impl<T: ?Sized> !Sync for UnsafeCell<T> {}
1935 impl<T> UnsafeCell<T> {
1936 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1939 /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
1944 /// use std::cell::UnsafeCell;
1946 /// let uc = UnsafeCell::new(5);
1948 #[stable(feature = "rust1", since = "1.0.0")]
1949 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
1951 pub const fn new(value: T) -> UnsafeCell<T> {
1952 UnsafeCell { value }
1955 /// Unwraps the value.
1960 /// use std::cell::UnsafeCell;
1962 /// let uc = UnsafeCell::new(5);
1964 /// let five = uc.into_inner();
1967 #[stable(feature = "rust1", since = "1.0.0")]
1968 #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")]
1969 pub const fn into_inner(self) -> T {
1974 impl<T: ?Sized> UnsafeCell<T> {
1975 /// Gets a mutable pointer to the wrapped value.
1977 /// This can be cast to a pointer of any kind.
1978 /// Ensure that the access is unique (no active references, mutable or not)
1979 /// when casting to `&mut T`, and ensure that there are no mutations
1980 /// or mutable aliases going on when casting to `&T`
1985 /// use std::cell::UnsafeCell;
1987 /// let uc = UnsafeCell::new(5);
1989 /// let five = uc.get();
1992 #[stable(feature = "rust1", since = "1.0.0")]
1993 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
1994 pub const fn get(&self) -> *mut T {
1995 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1996 // #[repr(transparent)]. This exploits libstd's special status, there is
1997 // no guarantee for user code that this will work in future versions of the compiler!
1998 self as *const UnsafeCell<T> as *const T as *mut T
2001 /// Returns a mutable reference to the underlying data.
2003 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
2004 /// guarantees that we possess the only reference.
2009 /// use std::cell::UnsafeCell;
2011 /// let mut c = UnsafeCell::new(5);
2012 /// *c.get_mut() += 1;
2014 /// assert_eq!(*c.get_mut(), 6);
2017 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
2018 #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")]
2019 pub const fn get_mut(&mut self) -> &mut T {
2023 /// Gets a mutable pointer to the wrapped value.
2024 /// The difference from [`get`] is that this function accepts a raw pointer,
2025 /// which is useful to avoid the creation of temporary references.
2027 /// The result can be cast to a pointer of any kind.
2028 /// Ensure that the access is unique (no active references, mutable or not)
2029 /// when casting to `&mut T`, and ensure that there are no mutations
2030 /// or mutable aliases going on when casting to `&T`.
2032 /// [`get`]: UnsafeCell::get()
2036 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
2037 /// calling `get` would require creating a reference to uninitialized data:
2040 /// use std::cell::UnsafeCell;
2041 /// use std::mem::MaybeUninit;
2043 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
2044 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
2045 /// let uc = unsafe { m.assume_init() };
2047 /// assert_eq!(uc.into_inner(), 5);
2050 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2051 #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2052 pub const fn raw_get(this: *const Self) -> *mut T {
2053 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2054 // #[repr(transparent)]. This exploits libstd's special status, there is
2055 // no guarantee for user code that this will work in future versions of the compiler!
2056 this as *const T as *mut T
2060 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
2061 impl<T: Default> Default for UnsafeCell<T> {
2062 /// Creates an `UnsafeCell`, with the `Default` value for T.
2063 fn default() -> UnsafeCell<T> {
2064 UnsafeCell::new(Default::default())
2068 #[stable(feature = "cell_from", since = "1.12.0")]
2069 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2070 impl<T> const From<T> for UnsafeCell<T> {
2071 /// Creates a new `UnsafeCell<T>` containing the given value.
2072 fn from(t: T) -> UnsafeCell<T> {
2077 #[unstable(feature = "coerce_unsized", issue = "27732")]
2078 impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
2080 /// [`UnsafeCell`], but [`Sync`].
2082 /// This is just an `UnsafeCell`, except it implements `Sync`
2083 /// if `T` implements `Sync`.
2085 /// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
2086 /// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
2087 /// shared between threads, if that's intentional.
2088 /// Providing proper synchronization is still the task of the user,
2089 /// making this type just as unsafe to use.
2091 /// See [`UnsafeCell`] for details.
2092 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2093 #[repr(transparent)]
2094 pub struct SyncUnsafeCell<T: ?Sized> {
2095 value: UnsafeCell<T>,
2098 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2099 unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
2101 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2102 impl<T> SyncUnsafeCell<T> {
2103 /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
2105 pub const fn new(value: T) -> Self {
2106 Self { value: UnsafeCell { value } }
2109 /// Unwraps the value.
2111 pub const fn into_inner(self) -> T {
2112 self.value.into_inner()
2116 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2117 impl<T: ?Sized> SyncUnsafeCell<T> {
2118 /// Gets a mutable pointer to the wrapped value.
2120 /// This can be cast to a pointer of any kind.
2121 /// Ensure that the access is unique (no active references, mutable or not)
2122 /// when casting to `&mut T`, and ensure that there are no mutations
2123 /// or mutable aliases going on when casting to `&T`
2125 pub const fn get(&self) -> *mut T {
2129 /// Returns a mutable reference to the underlying data.
2131 /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
2132 /// guarantees that we possess the only reference.
2134 pub const fn get_mut(&mut self) -> &mut T {
2135 self.value.get_mut()
2138 /// Gets a mutable pointer to the wrapped value.
2140 /// See [`UnsafeCell::get`] for details.
2142 pub const fn raw_get(this: *const Self) -> *mut T {
2143 // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
2144 // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
2145 // See UnsafeCell::raw_get.
2146 this as *const T as *mut T
2150 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2151 impl<T: Default> Default for SyncUnsafeCell<T> {
2152 /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
2153 fn default() -> SyncUnsafeCell<T> {
2154 SyncUnsafeCell::new(Default::default())
2158 #[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2159 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2160 impl<T> const From<T> for SyncUnsafeCell<T> {
2161 /// Creates a new `SyncUnsafeCell<T>` containing the given value.
2162 fn from(t: T) -> SyncUnsafeCell<T> {
2163 SyncUnsafeCell::new(t)
2167 #[unstable(feature = "coerce_unsized", issue = "27732")]
2168 //#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2169 impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2172 fn assert_coerce_unsized(
2173 a: UnsafeCell<&i32>,
2174 b: SyncUnsafeCell<&i32>,
2178 let _: UnsafeCell<&dyn Send> = a;
2179 let _: SyncUnsafeCell<&dyn Send> = b;
2180 let _: Cell<&dyn Send> = c;
2181 let _: RefCell<&dyn Send> = d;