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`](../../std/sync/struct.Mutex.html),
18 //! [`RwLock`](../../std/sync/struct.RwLock.html) or
19 //! [`atomic`](../../core/sync/atomic/index.html) types.
21 //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e.
22 //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`)
23 //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast
24 //! with typical Rust types that exhibit 'inherited mutability'.
26 //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior
27 //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values,
28 //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides
29 //! methods to retrieve and change the current interior value:
31 //! - For types that implement `Copy`, the `get` method retrieves the current interior value.
32 //! - For types that implement `Default`, the `take` method replaces the current interior value
33 //! with `Default::default()` and returns the replaced value.
34 //! - For all types, the `replace` method replaces the current interior value and returns the
35 //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior
36 //! value. Additionally, the `set` method replaces the interior value, dropping the replaced
39 //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can
40 //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
41 //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked
42 //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt
43 //! to borrow a value that is already mutably borrowed; when this happens it results in thread
46 //! # When to choose interior mutability
48 //! The more common inherited mutability, where one must have unique access to mutate a value, is
49 //! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
50 //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
51 //! interior mutability is something of a last resort. Since cell types enable mutation where it
52 //! would otherwise be disallowed though, there are occasions when interior mutability might be
53 //! appropriate, or even *must* be used, e.g.
55 //! * Introducing mutability 'inside' of something immutable
56 //! * Implementation details of logically-immutable methods.
57 //! * Mutating implementations of `Clone`.
59 //! ## Introducing mutability 'inside' of something immutable
61 //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be
62 //! cloned and shared between multiple parties. Because the contained values may be
63 //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
64 //! impossible to mutate data inside of these smart pointers at all.
66 //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
70 //! use std::cell::{RefCell, RefMut};
71 //! use std::collections::HashMap;
75 //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
76 //! // Create a new block to limit the scope of the dynamic borrow
78 //! let mut map: RefMut<_> = shared_map.borrow_mut();
79 //! map.insert("africa", 92388);
80 //! map.insert("kyoto", 11837);
81 //! map.insert("piccadilly", 11826);
82 //! map.insert("marbles", 38);
85 //! // Note that if we had not let the previous borrow of the cache fall out
86 //! // of scope then the subsequent borrow would cause a dynamic thread panic.
87 //! // This is the major hazard of using `RefCell`.
88 //! let total: i32 = shared_map.borrow().values().sum();
89 //! println!("{}", total);
93 //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
94 //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a
95 //! multi-threaded situation.
97 //! ## Implementation details of logically-immutable methods
99 //! Occasionally it may be desirable not to expose in an API that there is mutation happening
100 //! "under the hood". This may be because logically the operation is immutable, but e.g., caching
101 //! forces the implementation to perform mutation; or because you must employ mutation to implement
102 //! a trait method that was originally defined to take `&self`.
105 //! # #![allow(dead_code)]
106 //! use std::cell::RefCell;
109 //! edges: Vec<(i32, i32)>,
110 //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>>
114 //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
115 //! self.span_tree_cache.borrow_mut()
116 //! .get_or_insert_with(|| self.calc_span_tree())
120 //! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
121 //! // Expensive computation goes here
127 //! ## Mutating implementations of `Clone`
129 //! This is simply a special - but common - case of the previous: hiding mutability for operations
130 //! that appear to be immutable. The `clone` method is expected to not change the source value, and
131 //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the
132 //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a
136 //! #![feature(core_intrinsics)]
137 //! use std::cell::Cell;
138 //! use std::ptr::NonNull;
139 //! use std::intrinsics::abort;
141 //! struct Rc<T: ?Sized> {
142 //! ptr: NonNull<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();
154 //! Rc { ptr: self.ptr }
158 //! trait RcBoxPtr<T: ?Sized> {
160 //! fn inner(&self) -> &RcBox<T>;
162 //! fn strong(&self) -> usize {
163 //! self.inner().strong.get()
166 //! fn inc_strong(&self) {
169 //! .set(self.strong()
171 //! .unwrap_or_else(|| unsafe { abort() }));
175 //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> {
176 //! fn inner(&self) -> &RcBox<T> {
178 //! self.ptr.as_ref()
185 #![stable(feature = "rust1", since = "1.0.0")]
187 use crate::cmp
::Ordering
;
188 use crate::fmt
::{self, Debug, Display}
;
189 use crate::marker
::Unsize
;
191 use crate::ops
::{Deref, DerefMut, CoerceUnsized}
;
194 /// A mutable memory location.
198 /// In this example, you can see that `Cell<T>` enables mutation inside an
199 /// immutable struct. In other words, it enables "interior mutability".
202 /// use std::cell::Cell;
204 /// struct SomeStruct {
205 /// regular_field: u8,
206 /// special_field: Cell<u8>,
209 /// let my_struct = SomeStruct {
210 /// regular_field: 0,
211 /// special_field: Cell::new(1),
214 /// let new_value = 100;
216 /// // ERROR: `my_struct` is immutable
217 /// // my_struct.regular_field = new_value;
219 /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
220 /// // which can always be mutated
221 /// my_struct.special_field.set(new_value);
222 /// assert_eq!(my_struct.special_field.get(), new_value);
225 /// See the [module-level documentation](index.html) for more.
226 #[stable(feature = "rust1", since = "1.0.0")]
228 pub struct Cell
<T
: ?Sized
> {
229 value
: UnsafeCell
<T
>,
232 impl<T
:Copy
> Cell
<T
> {
233 /// Returns a copy of the contained value.
238 /// use std::cell::Cell;
240 /// let c = Cell::new(5);
242 /// let five = c.get();
245 #[stable(feature = "rust1", since = "1.0.0")]
246 pub fn get(&self) -> T
{
247 unsafe{ *self.value.get() }
250 /// Updates the contained value using a function and returns the new value.
255 /// #![feature(cell_update)]
257 /// use std::cell::Cell;
259 /// let c = Cell::new(5);
260 /// let new = c.update(|x| x + 1);
262 /// assert_eq!(new, 6);
263 /// assert_eq!(c.get(), 6);
266 #[unstable(feature = "cell_update", issue = "50186")]
267 pub fn update
<F
>(&self, f
: F
) -> T
271 let old
= self.get();
278 #[stable(feature = "rust1", since = "1.0.0")]
279 unsafe impl<T
: ?Sized
> Send
for Cell
<T
> where T
: Send {}
281 #[stable(feature = "rust1", since = "1.0.0")]
282 impl<T
: ?Sized
> !Sync
for Cell
<T
> {}
284 #[stable(feature = "rust1", since = "1.0.0")]
285 impl<T
:Copy
> Clone
for Cell
<T
> {
287 fn clone(&self) -> Cell
<T
> {
288 Cell
::new(self.get())
292 #[stable(feature = "rust1", since = "1.0.0")]
293 impl<T
: Default
> Default
for Cell
<T
> {
294 /// Creates a `Cell<T>`, with the `Default` value for T.
296 fn default() -> Cell
<T
> {
297 Cell
::new(Default
::default())
301 #[stable(feature = "rust1", since = "1.0.0")]
302 impl<T
: PartialEq
+ Copy
> PartialEq
for Cell
<T
> {
304 fn eq(&self, other
: &Cell
<T
>) -> bool
{
305 self.get() == other
.get()
309 #[stable(feature = "cell_eq", since = "1.2.0")]
310 impl<T
: Eq
+ Copy
> Eq
for Cell
<T
> {}
312 #[stable(feature = "cell_ord", since = "1.10.0")]
313 impl<T
: PartialOrd
+ Copy
> PartialOrd
for Cell
<T
> {
315 fn partial_cmp(&self, other
: &Cell
<T
>) -> Option
<Ordering
> {
316 self.get().partial_cmp(&other
.get())
320 fn lt(&self, other
: &Cell
<T
>) -> bool
{
321 self.get() < other
.get()
325 fn le(&self, other
: &Cell
<T
>) -> bool
{
326 self.get() <= other
.get()
330 fn gt(&self, other
: &Cell
<T
>) -> bool
{
331 self.get() > other
.get()
335 fn ge(&self, other
: &Cell
<T
>) -> bool
{
336 self.get() >= other
.get()
340 #[stable(feature = "cell_ord", since = "1.10.0")]
341 impl<T
: Ord
+ Copy
> Ord
for Cell
<T
> {
343 fn cmp(&self, other
: &Cell
<T
>) -> Ordering
{
344 self.get().cmp(&other
.get())
348 #[stable(feature = "cell_from", since = "1.12.0")]
349 impl<T
> From
<T
> for Cell
<T
> {
350 fn from(t
: T
) -> Cell
<T
> {
356 /// Creates a new `Cell` containing the given value.
361 /// use std::cell::Cell;
363 /// let c = Cell::new(5);
365 #[stable(feature = "rust1", since = "1.0.0")]
367 pub const fn new(value
: T
) -> Cell
<T
> {
369 value
: UnsafeCell
::new(value
),
373 /// Sets the contained value.
378 /// use std::cell::Cell;
380 /// let c = Cell::new(5);
385 #[stable(feature = "rust1", since = "1.0.0")]
386 pub fn set(&self, val
: T
) {
387 let old
= self.replace(val
);
391 /// Swaps the values of two Cells.
392 /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference.
397 /// use std::cell::Cell;
399 /// let c1 = Cell::new(5i32);
400 /// let c2 = Cell::new(10i32);
402 /// assert_eq!(10, c1.get());
403 /// assert_eq!(5, c2.get());
406 #[stable(feature = "move_cell", since = "1.17.0")]
407 pub fn swap(&self, other
: &Self) {
408 if ptr
::eq(self, other
) {
412 ptr
::swap(self.value
.get(), other
.value
.get());
416 /// Replaces the contained value, and returns it.
421 /// use std::cell::Cell;
423 /// let cell = Cell::new(5);
424 /// assert_eq!(cell.get(), 5);
425 /// assert_eq!(cell.replace(10), 5);
426 /// assert_eq!(cell.get(), 10);
428 #[stable(feature = "move_cell", since = "1.17.0")]
429 pub fn replace(&self, val
: T
) -> T
{
430 mem
::replace(unsafe { &mut *self.value.get() }
, val
)
433 /// Unwraps the value.
438 /// use std::cell::Cell;
440 /// let c = Cell::new(5);
441 /// let five = c.into_inner();
443 /// assert_eq!(five, 5);
445 #[stable(feature = "move_cell", since = "1.17.0")]
446 pub fn into_inner(self) -> T
{
447 self.value
.into_inner()
451 impl<T
: ?Sized
> Cell
<T
> {
452 /// Returns a raw pointer to the underlying data in this cell.
457 /// use std::cell::Cell;
459 /// let c = Cell::new(5);
461 /// let ptr = c.as_ptr();
464 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
465 pub const fn as_ptr(&self) -> *mut T
{
469 /// Returns a mutable reference to the underlying data.
471 /// This call borrows `Cell` mutably (at compile-time) which guarantees
472 /// that we possess the only reference.
477 /// use std::cell::Cell;
479 /// let mut c = Cell::new(5);
480 /// *c.get_mut() += 1;
482 /// assert_eq!(c.get(), 6);
485 #[stable(feature = "cell_get_mut", since = "1.11.0")]
486 pub fn get_mut(&mut self) -> &mut T
{
488 &mut *self.value
.get()
492 /// Returns a `&Cell<T>` from a `&mut T`
497 /// use std::cell::Cell;
499 /// let slice: &mut [i32] = &mut [1, 2, 3];
500 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
501 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
503 /// assert_eq!(slice_cell.len(), 3);
506 #[stable(feature = "as_cell", since = "1.37.0")]
507 pub fn from_mut(t
: &mut T
) -> &Cell
<T
> {
509 &*(t
as *mut T
as *const Cell
<T
>)
514 impl<T
: Default
> Cell
<T
> {
515 /// Takes the value of the cell, leaving `Default::default()` in its place.
520 /// use std::cell::Cell;
522 /// let c = Cell::new(5);
523 /// let five = c.take();
525 /// assert_eq!(five, 5);
526 /// assert_eq!(c.into_inner(), 0);
528 #[stable(feature = "move_cell", since = "1.17.0")]
529 pub fn take(&self) -> T
{
530 self.replace(Default
::default())
534 #[unstable(feature = "coerce_unsized", issue = "27732")]
535 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<Cell
<U
>> for Cell
<T
> {}
538 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
543 /// use std::cell::Cell;
545 /// let slice: &mut [i32] = &mut [1, 2, 3];
546 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
547 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
549 /// assert_eq!(slice_cell.len(), 3);
551 #[stable(feature = "as_cell", since = "1.37.0")]
552 pub fn as_slice_of_cells(&self) -> &[Cell
<T
>] {
554 &*(self as *const Cell
<[T
]> as *const [Cell
<T
>])
559 /// A mutable memory location with dynamically checked borrow rules
561 /// See the [module-level documentation](index.html) for more.
562 #[stable(feature = "rust1", since = "1.0.0")]
563 pub struct RefCell
<T
: ?Sized
> {
564 borrow
: Cell
<BorrowFlag
>,
565 value
: UnsafeCell
<T
>,
568 /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow).
569 #[stable(feature = "try_borrow", since = "1.13.0")]
570 pub struct BorrowError
{
574 #[stable(feature = "try_borrow", since = "1.13.0")]
575 impl Debug
for BorrowError
{
576 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
577 f
.debug_struct("BorrowError").finish()
581 #[stable(feature = "try_borrow", since = "1.13.0")]
582 impl Display
for BorrowError
{
583 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
584 Display
::fmt("already mutably borrowed", f
)
588 /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut).
589 #[stable(feature = "try_borrow", since = "1.13.0")]
590 pub struct BorrowMutError
{
594 #[stable(feature = "try_borrow", since = "1.13.0")]
595 impl Debug
for BorrowMutError
{
596 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
597 f
.debug_struct("BorrowMutError").finish()
601 #[stable(feature = "try_borrow", since = "1.13.0")]
602 impl Display
for BorrowMutError
{
603 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
604 Display
::fmt("already borrowed", f
)
608 // Positive values represent the number of `Ref` active. Negative values
609 // represent the number of `RefMut` active. Multiple `RefMut`s can only be
610 // active at a time if they refer to distinct, nonoverlapping components of a
611 // `RefCell` (e.g., different ranges of a slice).
613 // `Ref` and `RefMut` are both two words in size, and so there will likely never
614 // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
615 // range. Thus, a `BorrowFlag` will probably never overflow or underflow.
616 // However, this is not a guarantee, as a pathological program could repeatedly
617 // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
618 // explicitly check for overflow and underflow in order to avoid unsafety, or at
619 // least behave correctly in the event that overflow or underflow happens (e.g.,
620 // see BorrowRef::new).
621 type BorrowFlag
= isize;
622 const UNUSED
: BorrowFlag
= 0;
625 fn is_writing(x
: BorrowFlag
) -> bool
{
630 fn is_reading(x
: BorrowFlag
) -> bool
{
635 /// Creates a new `RefCell` containing `value`.
640 /// use std::cell::RefCell;
642 /// let c = RefCell::new(5);
644 #[stable(feature = "rust1", since = "1.0.0")]
646 pub const fn new(value
: T
) -> RefCell
<T
> {
648 value
: UnsafeCell
::new(value
),
649 borrow
: Cell
::new(UNUSED
),
653 /// Consumes the `RefCell`, returning the wrapped value.
658 /// use std::cell::RefCell;
660 /// let c = RefCell::new(5);
662 /// let five = c.into_inner();
664 #[stable(feature = "rust1", since = "1.0.0")]
666 pub fn into_inner(self) -> T
{
667 // Since this function takes `self` (the `RefCell`) by value, the
668 // compiler statically verifies that it is not currently borrowed.
669 // Therefore the following assertion is just a `debug_assert!`.
670 debug_assert
!(self.borrow
.get() == UNUSED
);
671 self.value
.into_inner()
674 /// Replaces the wrapped value with a new one, returning the old value,
675 /// without deinitializing either one.
677 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
681 /// Panics if the value is currently borrowed.
686 /// use std::cell::RefCell;
687 /// let cell = RefCell::new(5);
688 /// let old_value = cell.replace(6);
689 /// assert_eq!(old_value, 5);
690 /// assert_eq!(cell, RefCell::new(6));
693 #[stable(feature = "refcell_replace", since="1.24.0")]
694 pub fn replace(&self, t
: T
) -> T
{
695 mem
::replace(&mut *self.borrow_mut(), t
)
698 /// Replaces the wrapped value with a new one computed from `f`, returning
699 /// the old value, without deinitializing either one.
703 /// Panics if the value is currently borrowed.
708 /// use std::cell::RefCell;
709 /// let cell = RefCell::new(5);
710 /// let old_value = cell.replace_with(|&mut old| old + 1);
711 /// assert_eq!(old_value, 5);
712 /// assert_eq!(cell, RefCell::new(6));
715 #[stable(feature = "refcell_replace_swap", since="1.35.0")]
716 pub fn replace_with
<F
: FnOnce(&mut T
) -> T
>(&self, f
: F
) -> T
{
717 let mut_borrow
= &mut *self.borrow_mut();
718 let replacement
= f(mut_borrow
);
719 mem
::replace(mut_borrow
, replacement
)
722 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
723 /// without deinitializing either one.
725 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
729 /// Panics if the value in either `RefCell` is currently borrowed.
734 /// use std::cell::RefCell;
735 /// let c = RefCell::new(5);
736 /// let d = RefCell::new(6);
738 /// assert_eq!(c, RefCell::new(6));
739 /// assert_eq!(d, RefCell::new(5));
742 #[stable(feature = "refcell_swap", since="1.24.0")]
743 pub fn swap(&self, other
: &Self) {
744 mem
::swap(&mut *self.borrow_mut(), &mut *other
.borrow_mut())
748 impl<T
: ?Sized
> RefCell
<T
> {
749 /// Immutably borrows the wrapped value.
751 /// The borrow lasts until the returned `Ref` exits scope. Multiple
752 /// immutable borrows can be taken out at the same time.
756 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
757 /// [`try_borrow`](#method.try_borrow).
762 /// use std::cell::RefCell;
764 /// let c = RefCell::new(5);
766 /// let borrowed_five = c.borrow();
767 /// let borrowed_five2 = c.borrow();
770 /// An example of panic:
773 /// use std::cell::RefCell;
776 /// let result = thread::spawn(move || {
777 /// let c = RefCell::new(5);
778 /// let m = c.borrow_mut();
780 /// let b = c.borrow(); // this causes a panic
783 /// assert!(result.is_err());
785 #[stable(feature = "rust1", since = "1.0.0")]
787 pub fn borrow(&self) -> Ref
<'_
, T
> {
788 self.try_borrow().expect("already mutably borrowed")
791 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
794 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
795 /// taken out at the same time.
797 /// This is the non-panicking variant of [`borrow`](#method.borrow).
802 /// use std::cell::RefCell;
804 /// let c = RefCell::new(5);
807 /// let m = c.borrow_mut();
808 /// assert!(c.try_borrow().is_err());
812 /// let m = c.borrow();
813 /// assert!(c.try_borrow().is_ok());
816 #[stable(feature = "try_borrow", since = "1.13.0")]
818 pub fn try_borrow(&self) -> Result
<Ref
<'_
, T
>, BorrowError
> {
819 match BorrowRef
::new(&self.borrow
) {
821 value
: unsafe { &*self.value.get() }
,
824 None
=> Err(BorrowError { _private: () }
),
828 /// Mutably borrows the wrapped value.
830 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
831 /// from it exit scope. The value cannot be borrowed while this borrow is
836 /// Panics if the value is currently borrowed. For a non-panicking variant, use
837 /// [`try_borrow_mut`](#method.try_borrow_mut).
842 /// use std::cell::RefCell;
844 /// let c = RefCell::new(5);
846 /// *c.borrow_mut() = 7;
848 /// assert_eq!(*c.borrow(), 7);
851 /// An example of panic:
854 /// use std::cell::RefCell;
857 /// let result = thread::spawn(move || {
858 /// let c = RefCell::new(5);
859 /// let m = c.borrow();
861 /// let b = c.borrow_mut(); // this causes a panic
864 /// assert!(result.is_err());
866 #[stable(feature = "rust1", since = "1.0.0")]
868 pub fn borrow_mut(&self) -> RefMut
<'_
, T
> {
869 self.try_borrow_mut().expect("already borrowed")
872 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
874 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
875 /// from it exit scope. The value cannot be borrowed while this borrow is
878 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
883 /// use std::cell::RefCell;
885 /// let c = RefCell::new(5);
888 /// let m = c.borrow();
889 /// assert!(c.try_borrow_mut().is_err());
892 /// assert!(c.try_borrow_mut().is_ok());
894 #[stable(feature = "try_borrow", since = "1.13.0")]
896 pub fn try_borrow_mut(&self) -> Result
<RefMut
<'_
, T
>, BorrowMutError
> {
897 match BorrowRefMut
::new(&self.borrow
) {
898 Some(b
) => Ok(RefMut
{
899 value
: unsafe { &mut *self.value.get() }
,
902 None
=> Err(BorrowMutError { _private: () }
),
906 /// Returns a raw pointer to the underlying data in this cell.
911 /// use std::cell::RefCell;
913 /// let c = RefCell::new(5);
915 /// let ptr = c.as_ptr();
918 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
919 pub fn as_ptr(&self) -> *mut T
{
923 /// Returns a mutable reference to the underlying data.
925 /// This call borrows `RefCell` mutably (at compile-time) so there is no
926 /// need for dynamic checks.
928 /// However be cautious: this method expects `self` to be mutable, which is
929 /// generally not the case when using a `RefCell`. Take a look at the
930 /// [`borrow_mut`] method instead if `self` isn't mutable.
932 /// Also, please be aware that this method is only for special circumstances and is usually
933 /// not what you want. In case of doubt, use [`borrow_mut`] instead.
935 /// [`borrow_mut`]: #method.borrow_mut
940 /// use std::cell::RefCell;
942 /// let mut c = RefCell::new(5);
943 /// *c.get_mut() += 1;
945 /// assert_eq!(c, RefCell::new(6));
948 #[stable(feature = "cell_get_mut", since = "1.11.0")]
949 pub fn get_mut(&mut self) -> &mut T
{
951 &mut *self.value
.get()
955 /// Immutably borrows the wrapped value, returning an error if the value is
956 /// currently mutably borrowed.
960 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
961 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
962 /// borrowing the `RefCell` while the reference returned by this method
963 /// is alive is undefined behaviour.
968 /// use std::cell::RefCell;
970 /// let c = RefCell::new(5);
973 /// let m = c.borrow_mut();
974 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
978 /// let m = c.borrow();
979 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
982 #[stable(feature = "borrow_state", since = "1.37.0")]
984 pub unsafe fn try_borrow_unguarded(&self) -> Result
<&T
, BorrowError
> {
985 if !is_writing(self.borrow
.get()) {
986 Ok(&*self.value
.get())
988 Err(BorrowError { _private: () }
)
993 #[stable(feature = "rust1", since = "1.0.0")]
994 unsafe impl<T
: ?Sized
> Send
for RefCell
<T
> where T
: Send {}
996 #[stable(feature = "rust1", since = "1.0.0")]
997 impl<T
: ?Sized
> !Sync
for RefCell
<T
> {}
999 #[stable(feature = "rust1", since = "1.0.0")]
1000 impl<T
: Clone
> Clone
for RefCell
<T
> {
1003 /// Panics if the value is currently mutably borrowed.
1005 fn clone(&self) -> RefCell
<T
> {
1006 RefCell
::new(self.borrow().clone())
1010 #[stable(feature = "rust1", since = "1.0.0")]
1011 impl<T
: Default
> Default
for RefCell
<T
> {
1012 /// Creates a `RefCell<T>`, with the `Default` value for T.
1014 fn default() -> RefCell
<T
> {
1015 RefCell
::new(Default
::default())
1019 #[stable(feature = "rust1", since = "1.0.0")]
1020 impl<T
: ?Sized
+ PartialEq
> PartialEq
for RefCell
<T
> {
1023 /// Panics if the value in either `RefCell` is currently borrowed.
1025 fn eq(&self, other
: &RefCell
<T
>) -> bool
{
1026 *self.borrow() == *other
.borrow()
1030 #[stable(feature = "cell_eq", since = "1.2.0")]
1031 impl<T
: ?Sized
+ Eq
> Eq
for RefCell
<T
> {}
1033 #[stable(feature = "cell_ord", since = "1.10.0")]
1034 impl<T
: ?Sized
+ PartialOrd
> PartialOrd
for RefCell
<T
> {
1037 /// Panics if the value in either `RefCell` is currently borrowed.
1039 fn partial_cmp(&self, other
: &RefCell
<T
>) -> Option
<Ordering
> {
1040 self.borrow().partial_cmp(&*other
.borrow())
1045 /// Panics if the value in either `RefCell` is currently borrowed.
1047 fn lt(&self, other
: &RefCell
<T
>) -> bool
{
1048 *self.borrow() < *other
.borrow()
1053 /// Panics if the value in either `RefCell` is currently borrowed.
1055 fn le(&self, other
: &RefCell
<T
>) -> bool
{
1056 *self.borrow() <= *other
.borrow()
1061 /// Panics if the value in either `RefCell` is currently borrowed.
1063 fn gt(&self, other
: &RefCell
<T
>) -> bool
{
1064 *self.borrow() > *other
.borrow()
1069 /// Panics if the value in either `RefCell` is currently borrowed.
1071 fn ge(&self, other
: &RefCell
<T
>) -> bool
{
1072 *self.borrow() >= *other
.borrow()
1076 #[stable(feature = "cell_ord", since = "1.10.0")]
1077 impl<T
: ?Sized
+ Ord
> Ord
for RefCell
<T
> {
1080 /// Panics if the value in either `RefCell` is currently borrowed.
1082 fn cmp(&self, other
: &RefCell
<T
>) -> Ordering
{
1083 self.borrow().cmp(&*other
.borrow())
1087 #[stable(feature = "cell_from", since = "1.12.0")]
1088 impl<T
> From
<T
> for RefCell
<T
> {
1089 fn from(t
: T
) -> RefCell
<T
> {
1094 #[unstable(feature = "coerce_unsized", issue = "27732")]
1095 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<RefCell
<U
>> for RefCell
<T
> {}
1097 struct BorrowRef
<'b
> {
1098 borrow
: &'b Cell
<BorrowFlag
>,
1101 impl<'b
> BorrowRef
<'b
> {
1103 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRef
<'b
>> {
1104 let b
= borrow
.get().wrapping_add(1);
1106 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1107 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1108 // due to Rust's reference aliasing rules
1109 // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed
1110 // into isize::min_value() (the max amount of writing borrows) so we can't allow
1111 // an additional read borrow because isize can't represent so many read borrows
1112 // (this can only happen if you mem::forget more than a small constant amount of
1113 // `Ref`s, which is not good practice)
1116 // Incrementing borrow can result in a reading value (> 0) in these cases:
1117 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1118 // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize
1119 // is large enough to represent having one more read borrow
1121 Some(BorrowRef { borrow }
)
1126 impl Drop
for BorrowRef
<'_
> {
1128 fn drop(&mut self) {
1129 let borrow
= self.borrow
.get();
1130 debug_assert
!(is_reading(borrow
));
1131 self.borrow
.set(borrow
- 1);
1135 impl Clone
for BorrowRef
<'_
> {
1137 fn clone(&self) -> Self {
1138 // Since this Ref exists, we know the borrow flag
1139 // is a reading borrow.
1140 let borrow
= self.borrow
.get();
1141 debug_assert
!(is_reading(borrow
));
1142 // Prevent the borrow counter from overflowing into
1143 // a writing borrow.
1144 assert
!(borrow
!= isize::max_value());
1145 self.borrow
.set(borrow
+ 1);
1146 BorrowRef { borrow: self.borrow }
1150 /// Wraps a borrowed reference to a value in a `RefCell` box.
1151 /// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1153 /// See the [module-level documentation](index.html) for more.
1154 #[stable(feature = "rust1", since = "1.0.0")]
1155 pub struct Ref
<'b
, T
: ?Sized
+ 'b
> {
1157 borrow
: BorrowRef
<'b
>,
1160 #[stable(feature = "rust1", since = "1.0.0")]
1161 impl<T
: ?Sized
> Deref
for Ref
<'_
, T
> {
1165 fn deref(&self) -> &T
{
1170 impl<'b
, T
: ?Sized
> Ref
<'b
, T
> {
1173 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1175 /// This is an associated function that needs to be used as
1176 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1177 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1179 #[stable(feature = "cell_extras", since = "1.15.0")]
1181 pub fn clone(orig
: &Ref
<'b
, T
>) -> Ref
<'b
, T
> {
1184 borrow
: orig
.borrow
.clone(),
1188 /// Makes a new `Ref` for a component of the borrowed data.
1190 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1192 /// This is an associated function that needs to be used as `Ref::map(...)`.
1193 /// A method would interfere with methods of the same name on the contents
1194 /// of a `RefCell` used through `Deref`.
1199 /// use std::cell::{RefCell, Ref};
1201 /// let c = RefCell::new((5, 'b'));
1202 /// let b1: Ref<(u32, char)> = c.borrow();
1203 /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0);
1204 /// assert_eq!(*b2, 5)
1206 #[stable(feature = "cell_map", since = "1.8.0")]
1208 pub fn map
<U
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> Ref
<'b
, U
>
1209 where F
: FnOnce(&T
) -> &U
1212 value
: f(orig
.value
),
1213 borrow
: orig
.borrow
,
1217 /// Splits a `Ref` into multiple `Ref`s for different components of the
1220 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1222 /// This is an associated function that needs to be used as
1223 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1224 /// name on the contents of a `RefCell` used through `Deref`.
1229 /// use std::cell::{Ref, RefCell};
1231 /// let cell = RefCell::new([1, 2, 3, 4]);
1232 /// let borrow = cell.borrow();
1233 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1234 /// assert_eq!(*begin, [1, 2]);
1235 /// assert_eq!(*end, [3, 4]);
1237 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1239 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(orig
: Ref
<'b
, T
>, f
: F
) -> (Ref
<'b
, U
>, Ref
<'b
, V
>)
1240 where F
: FnOnce(&T
) -> (&U
, &V
)
1242 let (a
, b
) = f(orig
.value
);
1243 let borrow
= orig
.borrow
.clone();
1244 (Ref { value: a, borrow }
, Ref { value: b, borrow: orig.borrow }
)
1248 #[unstable(feature = "coerce_unsized", issue = "27732")]
1249 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<Ref
<'b
, U
>> for Ref
<'b
, T
> {}
1251 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1252 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for Ref
<'_
, T
> {
1253 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1258 impl<'b
, T
: ?Sized
> RefMut
<'b
, T
> {
1259 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1262 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1264 /// This is an associated function that needs to be used as
1265 /// `RefMut::map(...)`. A method would interfere with methods of the same
1266 /// name on the contents of a `RefCell` used through `Deref`.
1271 /// use std::cell::{RefCell, RefMut};
1273 /// let c = RefCell::new((5, 'b'));
1275 /// let b1: RefMut<(u32, char)> = c.borrow_mut();
1276 /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0);
1277 /// assert_eq!(*b2, 5);
1280 /// assert_eq!(*c.borrow(), (42, 'b'));
1282 #[stable(feature = "cell_map", since = "1.8.0")]
1284 pub fn map
<U
: ?Sized
, F
>(orig
: RefMut
<'b
, T
>, f
: F
) -> RefMut
<'b
, U
>
1285 where F
: FnOnce(&mut T
) -> &mut U
1287 // FIXME(nll-rfc#40): fix borrow-check
1288 let RefMut { value, borrow }
= orig
;
1295 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1298 /// The underlying `RefCell` will remain mutably borrowed until both
1299 /// returned `RefMut`s go out of scope.
1301 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1303 /// This is an associated function that needs to be used as
1304 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1305 /// same name on the contents of a `RefCell` used through `Deref`.
1310 /// use std::cell::{RefCell, RefMut};
1312 /// let cell = RefCell::new([1, 2, 3, 4]);
1313 /// let borrow = cell.borrow_mut();
1314 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1315 /// assert_eq!(*begin, [1, 2]);
1316 /// assert_eq!(*end, [3, 4]);
1317 /// begin.copy_from_slice(&[4, 3]);
1318 /// end.copy_from_slice(&[2, 1]);
1320 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1322 pub fn map_split
<U
: ?Sized
, V
: ?Sized
, F
>(
1323 orig
: RefMut
<'b
, T
>, f
: F
1324 ) -> (RefMut
<'b
, U
>, RefMut
<'b
, V
>)
1325 where F
: FnOnce(&mut T
) -> (&mut U
, &mut V
)
1327 let (a
, b
) = f(orig
.value
);
1328 let borrow
= orig
.borrow
.clone();
1329 (RefMut { value: a, borrow }
, RefMut { value: b, borrow: orig.borrow }
)
1333 struct BorrowRefMut
<'b
> {
1334 borrow
: &'b Cell
<BorrowFlag
>,
1337 impl Drop
for BorrowRefMut
<'_
> {
1339 fn drop(&mut self) {
1340 let borrow
= self.borrow
.get();
1341 debug_assert
!(is_writing(borrow
));
1342 self.borrow
.set(borrow
+ 1);
1346 impl<'b
> BorrowRefMut
<'b
> {
1348 fn new(borrow
: &'b Cell
<BorrowFlag
>) -> Option
<BorrowRefMut
<'b
>> {
1349 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1350 // mutable reference, and so there must currently be no existing
1351 // references. Thus, while clone increments the mutable refcount, here
1352 // we explicitly only allow going from UNUSED to UNUSED - 1.
1353 match borrow
.get() {
1355 borrow
.set(UNUSED
- 1);
1356 Some(BorrowRefMut { borrow }
)
1362 // Clones a `BorrowRefMut`.
1364 // This is only valid if each `BorrowRefMut` is used to track a mutable
1365 // reference to a distinct, nonoverlapping range of the original object.
1366 // This isn't in a Clone impl so that code doesn't call this implicitly.
1368 fn clone(&self) -> BorrowRefMut
<'b
> {
1369 let borrow
= self.borrow
.get();
1370 debug_assert
!(is_writing(borrow
));
1371 // Prevent the borrow counter from underflowing.
1372 assert
!(borrow
!= isize::min_value());
1373 self.borrow
.set(borrow
- 1);
1374 BorrowRefMut { borrow: self.borrow }
1378 /// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1380 /// See the [module-level documentation](index.html) for more.
1381 #[stable(feature = "rust1", since = "1.0.0")]
1382 pub struct RefMut
<'b
, T
: ?Sized
+ 'b
> {
1384 borrow
: BorrowRefMut
<'b
>,
1387 #[stable(feature = "rust1", since = "1.0.0")]
1388 impl<T
: ?Sized
> Deref
for RefMut
<'_
, T
> {
1392 fn deref(&self) -> &T
{
1397 #[stable(feature = "rust1", since = "1.0.0")]
1398 impl<T
: ?Sized
> DerefMut
for RefMut
<'_
, T
> {
1400 fn deref_mut(&mut self) -> &mut T
{
1405 #[unstable(feature = "coerce_unsized", issue = "27732")]
1406 impl<'b
, T
: ?Sized
+ Unsize
<U
>, U
: ?Sized
> CoerceUnsized
<RefMut
<'b
, U
>> for RefMut
<'b
, T
> {}
1408 #[stable(feature = "std_guard_impls", since = "1.20.0")]
1409 impl<T
: ?Sized
+ fmt
::Display
> fmt
::Display
for RefMut
<'_
, T
> {
1410 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1415 /// The core primitive for interior mutability in Rust.
1417 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the
1418 /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'.
1419 /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered
1420 /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior.
1422 /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are
1423 /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably
1424 /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language
1425 /// feature to work around the restriction that `&T` may not be mutated. All other types that
1426 /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their
1427 /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`.
1429 /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to
1430 /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly.
1432 /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1434 /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T`
1435 /// reference) that is accessible by safe code (for example, because you returned it),
1436 /// then you must not access the data in any way that contradicts that reference for the
1437 /// remainder of `'a`. For example, this means that if you take the `*mut T` from an
1438 /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable
1439 /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's
1440 /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to
1441 /// safe code, then you must not access the data within the `UnsafeCell` until that
1442 /// reference expires.
1444 /// - At all times, you must avoid data races. If multiple threads have access to
1445 /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other
1446 /// accesses (or use atomics).
1448 /// To assist with proper design, the following scenarios are explicitly declared legal
1449 /// for single-threaded code:
1451 /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1452 /// references, but not with a `&mut T`
1454 /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1455 /// co-exist with it. A `&mut T` must always be unique.
1457 /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is
1458 /// ok (provided you enforce the invariants some other way), it is still undefined behavior
1459 /// to have multiple `&mut UnsafeCell<T>` aliases.
1464 /// use std::cell::UnsafeCell;
1466 /// # #[allow(dead_code)]
1467 /// struct NotThreadSafe<T> {
1468 /// value: UnsafeCell<T>,
1471 /// unsafe impl<T> Sync for NotThreadSafe<T> {}
1473 #[lang = "unsafe_cell"]
1474 #[stable(feature = "rust1", since = "1.0.0")]
1475 #[repr(transparent)]
1476 pub struct UnsafeCell
<T
: ?Sized
> {
1480 #[stable(feature = "rust1", since = "1.0.0")]
1481 impl<T
: ?Sized
> !Sync
for UnsafeCell
<T
> {}
1483 impl<T
> UnsafeCell
<T
> {
1484 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
1487 /// All access to the inner value through methods is `unsafe`.
1492 /// use std::cell::UnsafeCell;
1494 /// let uc = UnsafeCell::new(5);
1496 #[stable(feature = "rust1", since = "1.0.0")]
1498 pub const fn new(value
: T
) -> UnsafeCell
<T
> {
1499 UnsafeCell { value }
1502 /// Unwraps the value.
1507 /// use std::cell::UnsafeCell;
1509 /// let uc = UnsafeCell::new(5);
1511 /// let five = uc.into_inner();
1514 #[stable(feature = "rust1", since = "1.0.0")]
1515 pub fn into_inner(self) -> T
{
1520 impl<T
: ?Sized
> UnsafeCell
<T
> {
1521 /// Gets a mutable pointer to the wrapped value.
1523 /// This can be cast to a pointer of any kind.
1524 /// Ensure that the access is unique (no active references, mutable or not)
1525 /// when casting to `&mut T`, and ensure that there are no mutations
1526 /// or mutable aliases going on when casting to `&T`
1531 /// use std::cell::UnsafeCell;
1533 /// let uc = UnsafeCell::new(5);
1535 /// let five = uc.get();
1538 #[stable(feature = "rust1", since = "1.0.0")]
1539 pub const fn get(&self) -> *mut T
{
1540 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
1541 // #[repr(transparent)]
1542 self as *const UnsafeCell
<T
> as *const T
as *mut T
1546 #[stable(feature = "unsafe_cell_default", since = "1.10.0")]
1547 impl<T
: Default
> Default
for UnsafeCell
<T
> {
1548 /// Creates an `UnsafeCell`, with the `Default` value for T.
1549 fn default() -> UnsafeCell
<T
> {
1550 UnsafeCell
::new(Default
::default())
1554 #[stable(feature = "cell_from", since = "1.12.0")]
1555 impl<T
> From
<T
> for UnsafeCell
<T
> {
1556 fn from(t
: T
) -> UnsafeCell
<T
> {
1561 #[unstable(feature = "coerce_unsized", issue = "27732")]
1562 impl<T
: CoerceUnsized
<U
>, U
> CoerceUnsized
<UnsafeCell
<U
>> for UnsafeCell
<T
> {}
1565 fn assert_coerce_unsized(a
: UnsafeCell
<&i32>, b
: Cell
<&i32>, c
: RefCell
<&i32>) {
1566 let _
: UnsafeCell
<&dyn Send
> = a
;
1567 let _
: Cell
<&dyn Send
> = b
;
1568 let _
: RefCell
<&dyn Send
> = c
;