core/cell.rs
1//! Shareable mutable containers.
2//!
3//! Rust memory safety is based on this rule: Given an object `T`, it is only possible to
4//! have one of the following:
5//!
6//! - Several immutable references (`&T`) to the object (also known as **aliasing**).
7//! - One mutable reference (`&mut T`) to the object (also known as **mutability**).
8//!
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
11//! mutate it.
12//!
13//! Shareable mutable containers exist to permit mutability in a controlled manner, even in the
14//! presence of aliasing. [`Cell<T>`], [`RefCell<T>`], and [`OnceCell<T>`] allow doing this in
15//! a single-threaded way—they do not implement [`Sync`]. (If you need to do aliasing and
16//! mutation among multiple threads, [`Mutex<T>`], [`RwLock<T>`], [`OnceLock<T>`] or [`atomic`]
17//! types are the correct data structures to do so).
18//!
19//! Values of the `Cell<T>`, `RefCell<T>`, and `OnceCell<T>` types may be mutated through shared
20//! references (i.e. the common `&T` type), whereas most Rust types can only be mutated through
21//! unique (`&mut T`) references. We say these cell types provide 'interior mutability'
22//! (mutable via `&T`), in contrast with typical Rust types that exhibit 'inherited mutability'
23//! (mutable only via `&mut T`).
24//!
25//! Cell types come in four flavors: `Cell<T>`, `RefCell<T>`, `OnceCell<T>`, and `LazyCell<T>`.
26//! Each provides a different way of providing safe interior mutability.
27//!
28//! ## `Cell<T>`
29//!
30//! [`Cell<T>`] implements interior mutability by moving values in and out of the cell. That is, an
31//! `&mut T` to the inner value can never be obtained, and the value itself cannot be directly
32//! obtained without replacing it with something else. Both of these rules ensure that there is
33//! never more than one reference pointing to the inner value. This type provides the following
34//! methods:
35//!
36//! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current
37//! interior value by duplicating it.
38//! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current
39//! interior value with [`Default::default()`] and returns the replaced value.
40//! - All types have:
41//! - [`replace`](Cell::replace): replaces the current interior value and returns the replaced
42//! value.
43//! - [`into_inner`](Cell::into_inner): this method consumes the `Cell<T>` and returns the
44//! interior value.
45//! - [`set`](Cell::set): this method replaces the interior value, dropping the replaced value.
46//!
47//! `Cell<T>` is typically used for more simple types where copying or moving values isn't too
48//! resource intensive (e.g. numbers), and should usually be preferred over other cell types when
49//! possible. For larger and non-copy types, `RefCell` provides some advantages.
50//!
51//! ## `RefCell<T>`
52//!
53//! [`RefCell<T>`] uses Rust's lifetimes to implement "dynamic borrowing", a process whereby one can
54//! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are
55//! tracked at _runtime_, unlike Rust's native reference types which are entirely tracked
56//! statically, at compile time.
57//!
58//! An immutable reference to a `RefCell`'s inner value (`&T`) can be obtained with
59//! [`borrow`](`RefCell::borrow`), and a mutable borrow (`&mut T`) can be obtained with
60//! [`borrow_mut`](`RefCell::borrow_mut`). When these functions are called, they first verify that
61//! Rust's borrow rules will be satisfied: any number of immutable borrows are allowed or a
62//! single mutable borrow is allowed, but never both. If a borrow is attempted that would violate
63//! these rules, the thread will panic.
64//!
65//! The corresponding [`Sync`] version of `RefCell<T>` is [`RwLock<T>`].
66//!
67//! ## `OnceCell<T>`
68//!
69//! [`OnceCell<T>`] is somewhat of a hybrid of `Cell` and `RefCell` that works for values that
70//! typically only need to be set once. This means that a reference `&T` can be obtained without
71//! moving or copying the inner value (unlike `Cell`) but also without runtime checks (unlike
72//! `RefCell`). However, its value can also not be updated once set unless you have a mutable
73//! reference to the `OnceCell`.
74//!
75//! `OnceCell` provides the following methods:
76//!
77//! - [`get`](OnceCell::get): obtain a reference to the inner value
78//! - [`set`](OnceCell::set): set the inner value if it is unset (returns a `Result`)
79//! - [`get_or_init`](OnceCell::get_or_init): return the inner value, initializing it if needed
80//! - [`get_mut`](OnceCell::get_mut): provide a mutable reference to the inner value, only available
81//! if you have a mutable reference to the cell itself.
82//!
83//! The corresponding [`Sync`] version of `OnceCell<T>` is [`OnceLock<T>`].
84//!
85//! ## `LazyCell<T, F>`
86//!
87//! A common pattern with OnceCell is, for a given OnceCell, to use the same function on every
88//! call to [`OnceCell::get_or_init`] with that cell. This is what is offered by [`LazyCell`],
89//! which pairs cells of `T` with functions of `F`, and always calls `F` before it yields `&T`.
90//! This happens implicitly by simply attempting to dereference the LazyCell to get its contents,
91//! so its use is much more transparent with a place which has been initialized by a constant.
92//!
93//! More complicated patterns that don't fit this description can be built on `OnceCell<T>` instead.
94//!
95//! `LazyCell` works by providing an implementation of `impl Deref` that calls the function,
96//! so you can just use it by dereference (e.g. `*lazy_cell` or `lazy_cell.deref()`).
97//!
98//! The corresponding [`Sync`] version of `LazyCell<T, F>` is [`LazyLock<T, F>`].
99//!
100//! # When to choose interior mutability
101//!
102//! The more common inherited mutability, where one must have unique access to mutate a value, is
103//! one of the key language elements that enables Rust to reason strongly about pointer aliasing,
104//! statically preventing crash bugs. Because of that, inherited mutability is preferred, and
105//! interior mutability is something of a last resort. Since cell types enable mutation where it
106//! would otherwise be disallowed though, there are occasions when interior mutability might be
107//! appropriate, or even *must* be used, e.g.
108//!
109//! * Introducing mutability 'inside' of something immutable
110//! * Implementation details of logically-immutable methods.
111//! * Mutating implementations of [`Clone`].
112//!
113//! ## Introducing mutability 'inside' of something immutable
114//!
115//! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can
116//! be cloned and shared between multiple parties. Because the contained values may be
117//! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be
118//! impossible to mutate data inside of these smart pointers at all.
119//!
120//! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce
121//! mutability:
122//!
123//! ```
124//! use std::cell::{RefCell, RefMut};
125//! use std::collections::HashMap;
126//! use std::rc::Rc;
127//!
128//! fn main() {
129//! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new()));
130//! // Create a new block to limit the scope of the dynamic borrow
131//! {
132//! let mut map: RefMut<'_, _> = shared_map.borrow_mut();
133//! map.insert("africa", 92388);
134//! map.insert("kyoto", 11837);
135//! map.insert("piccadilly", 11826);
136//! map.insert("marbles", 38);
137//! }
138//!
139//! // Note that if we had not let the previous borrow of the cache fall out
140//! // of scope then the subsequent borrow would cause a dynamic thread panic.
141//! // This is the major hazard of using `RefCell`.
142//! let total: i32 = shared_map.borrow().values().sum();
143//! println!("{total}");
144//! }
145//! ```
146//!
147//! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded
148//! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a
149//! multi-threaded situation.
150//!
151//! ## Implementation details of logically-immutable methods
152//!
153//! Occasionally it may be desirable not to expose in an API that there is mutation happening
154//! "under the hood". This may be because logically the operation is immutable, but e.g., caching
155//! forces the implementation to perform mutation; or because you must employ mutation to implement
156//! a trait method that was originally defined to take `&self`.
157//!
158//! ```
159//! # #![allow(dead_code)]
160//! use std::cell::OnceCell;
161//!
162//! struct Graph {
163//! edges: Vec<(i32, i32)>,
164//! span_tree_cache: OnceCell<Vec<(i32, i32)>>
165//! }
166//!
167//! impl Graph {
168//! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> {
169//! self.span_tree_cache
170//! .get_or_init(|| self.calc_span_tree())
171//! .clone()
172//! }
173//!
174//! fn calc_span_tree(&self) -> Vec<(i32, i32)> {
175//! // Expensive computation goes here
176//! vec![]
177//! }
178//! }
179//! ```
180//!
181//! ## Mutating implementations of `Clone`
182//!
183//! This is simply a special - but common - case of the previous: hiding mutability for operations
184//! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the
185//! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that
186//! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its
187//! reference counts within a `Cell<T>`.
188//!
189//! ```
190//! use std::cell::Cell;
191//! use std::ptr::NonNull;
192//! use std::process::abort;
193//! use std::marker::PhantomData;
194//!
195//! struct Rc<T: ?Sized> {
196//! ptr: NonNull<RcInner<T>>,
197//! phantom: PhantomData<RcInner<T>>,
198//! }
199//!
200//! struct RcInner<T: ?Sized> {
201//! strong: Cell<usize>,
202//! refcount: Cell<usize>,
203//! value: T,
204//! }
205//!
206//! impl<T: ?Sized> Clone for Rc<T> {
207//! fn clone(&self) -> Rc<T> {
208//! self.inc_strong();
209//! Rc {
210//! ptr: self.ptr,
211//! phantom: PhantomData,
212//! }
213//! }
214//! }
215//!
216//! trait RcInnerPtr<T: ?Sized> {
217//!
218//! fn inner(&self) -> &RcInner<T>;
219//!
220//! fn strong(&self) -> usize {
221//! self.inner().strong.get()
222//! }
223//!
224//! fn inc_strong(&self) {
225//! self.inner()
226//! .strong
227//! .set(self.strong()
228//! .checked_add(1)
229//! .unwrap_or_else(|| abort() ));
230//! }
231//! }
232//!
233//! impl<T: ?Sized> RcInnerPtr<T> for Rc<T> {
234//! fn inner(&self) -> &RcInner<T> {
235//! unsafe {
236//! self.ptr.as_ref()
237//! }
238//! }
239//! }
240//! ```
241//!
242//! [`Arc<T>`]: ../../std/sync/struct.Arc.html
243//! [`Rc<T>`]: ../../std/rc/struct.Rc.html
244//! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html
245//! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html
246//! [`OnceLock<T>`]: ../../std/sync/struct.OnceLock.html
247//! [`LazyLock<T, F>`]: ../../std/sync/struct.LazyLock.html
248//! [`Sync`]: ../../std/marker/trait.Sync.html
249//! [`atomic`]: crate::sync::atomic
250
251#![stable(feature = "rust1", since = "1.0.0")]
252
253use crate::cmp::Ordering;
254use crate::fmt::{self, Debug, Display};
255use crate::marker::{PhantomData, PointerLike, Unsize};
256use crate::mem;
257use crate::ops::{CoerceUnsized, Deref, DerefMut, DerefPure, DispatchFromDyn};
258use crate::panic::const_panic;
259use crate::pin::PinCoerceUnsized;
260use crate::ptr::{self, NonNull};
261
262mod lazy;
263mod once;
264
265#[stable(feature = "lazy_cell", since = "1.80.0")]
266pub use lazy::LazyCell;
267#[stable(feature = "once_cell", since = "1.70.0")]
268pub use once::OnceCell;
269
270/// A mutable memory location.
271///
272/// # Memory layout
273///
274/// `Cell<T>` has the same [memory layout and caveats as
275/// `UnsafeCell<T>`](UnsafeCell#memory-layout). In particular, this means that
276/// `Cell<T>` has the same in-memory representation as its inner type `T`.
277///
278/// # Examples
279///
280/// In this example, you can see that `Cell<T>` enables mutation inside an
281/// immutable struct. In other words, it enables "interior mutability".
282///
283/// ```
284/// use std::cell::Cell;
285///
286/// struct SomeStruct {
287/// regular_field: u8,
288/// special_field: Cell<u8>,
289/// }
290///
291/// let my_struct = SomeStruct {
292/// regular_field: 0,
293/// special_field: Cell::new(1),
294/// };
295///
296/// let new_value = 100;
297///
298/// // ERROR: `my_struct` is immutable
299/// // my_struct.regular_field = new_value;
300///
301/// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`,
302/// // which can always be mutated
303/// my_struct.special_field.set(new_value);
304/// assert_eq!(my_struct.special_field.get(), new_value);
305/// ```
306///
307/// See the [module-level documentation](self) for more.
308#[rustc_diagnostic_item = "Cell"]
309#[stable(feature = "rust1", since = "1.0.0")]
310#[repr(transparent)]
311#[rustc_pub_transparent]
312pub struct Cell<T: ?Sized> {
313 value: UnsafeCell<T>,
314}
315
316#[stable(feature = "rust1", since = "1.0.0")]
317unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {}
318
319// Note that this negative impl isn't strictly necessary for correctness,
320// as `Cell` wraps `UnsafeCell`, which is itself `!Sync`.
321// However, given how important `Cell`'s `!Sync`-ness is,
322// having an explicit negative impl is nice for documentation purposes
323// and results in nicer error messages.
324#[stable(feature = "rust1", since = "1.0.0")]
325impl<T: ?Sized> !Sync for Cell<T> {}
326
327#[stable(feature = "rust1", since = "1.0.0")]
328impl<T: Copy> Clone for Cell<T> {
329 #[inline]
330 fn clone(&self) -> Cell<T> {
331 Cell::new(self.get())
332 }
333}
334
335#[stable(feature = "rust1", since = "1.0.0")]
336impl<T: Default> Default for Cell<T> {
337 /// Creates a `Cell<T>`, with the `Default` value for T.
338 #[inline]
339 fn default() -> Cell<T> {
340 Cell::new(Default::default())
341 }
342}
343
344#[stable(feature = "rust1", since = "1.0.0")]
345impl<T: PartialEq + Copy> PartialEq for Cell<T> {
346 #[inline]
347 fn eq(&self, other: &Cell<T>) -> bool {
348 self.get() == other.get()
349 }
350}
351
352#[stable(feature = "cell_eq", since = "1.2.0")]
353impl<T: Eq + Copy> Eq for Cell<T> {}
354
355#[stable(feature = "cell_ord", since = "1.10.0")]
356impl<T: PartialOrd + Copy> PartialOrd for Cell<T> {
357 #[inline]
358 fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> {
359 self.get().partial_cmp(&other.get())
360 }
361
362 #[inline]
363 fn lt(&self, other: &Cell<T>) -> bool {
364 self.get() < other.get()
365 }
366
367 #[inline]
368 fn le(&self, other: &Cell<T>) -> bool {
369 self.get() <= other.get()
370 }
371
372 #[inline]
373 fn gt(&self, other: &Cell<T>) -> bool {
374 self.get() > other.get()
375 }
376
377 #[inline]
378 fn ge(&self, other: &Cell<T>) -> bool {
379 self.get() >= other.get()
380 }
381}
382
383#[stable(feature = "cell_ord", since = "1.10.0")]
384impl<T: Ord + Copy> Ord for Cell<T> {
385 #[inline]
386 fn cmp(&self, other: &Cell<T>) -> Ordering {
387 self.get().cmp(&other.get())
388 }
389}
390
391#[stable(feature = "cell_from", since = "1.12.0")]
392impl<T> From<T> for Cell<T> {
393 /// Creates a new `Cell<T>` containing the given value.
394 fn from(t: T) -> Cell<T> {
395 Cell::new(t)
396 }
397}
398
399impl<T> Cell<T> {
400 /// Creates a new `Cell` containing the given value.
401 ///
402 /// # Examples
403 ///
404 /// ```
405 /// use std::cell::Cell;
406 ///
407 /// let c = Cell::new(5);
408 /// ```
409 #[stable(feature = "rust1", since = "1.0.0")]
410 #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")]
411 #[inline]
412 pub const fn new(value: T) -> Cell<T> {
413 Cell { value: UnsafeCell::new(value) }
414 }
415
416 /// Sets the contained value.
417 ///
418 /// # Examples
419 ///
420 /// ```
421 /// use std::cell::Cell;
422 ///
423 /// let c = Cell::new(5);
424 ///
425 /// c.set(10);
426 /// ```
427 #[inline]
428 #[stable(feature = "rust1", since = "1.0.0")]
429 pub fn set(&self, val: T) {
430 self.replace(val);
431 }
432
433 /// Swaps the values of two `Cell`s.
434 ///
435 /// The difference with `std::mem::swap` is that this function doesn't
436 /// require a `&mut` reference.
437 ///
438 /// # Panics
439 ///
440 /// This function will panic if `self` and `other` are different `Cell`s that partially overlap.
441 /// (Using just standard library methods, it is impossible to create such partially overlapping `Cell`s.
442 /// However, unsafe code is allowed to e.g. create two `&Cell<[i32; 2]>` that partially overlap.)
443 ///
444 /// # Examples
445 ///
446 /// ```
447 /// use std::cell::Cell;
448 ///
449 /// let c1 = Cell::new(5i32);
450 /// let c2 = Cell::new(10i32);
451 /// c1.swap(&c2);
452 /// assert_eq!(10, c1.get());
453 /// assert_eq!(5, c2.get());
454 /// ```
455 #[inline]
456 #[stable(feature = "move_cell", since = "1.17.0")]
457 pub fn swap(&self, other: &Self) {
458 // This function documents that it *will* panic, and intrinsics::is_nonoverlapping doesn't
459 // do the check in const, so trying to use it here would be inviting unnecessary fragility.
460 fn is_nonoverlapping<T>(src: *const T, dst: *const T) -> bool {
461 let src_usize = src.addr();
462 let dst_usize = dst.addr();
463 let diff = src_usize.abs_diff(dst_usize);
464 diff >= size_of::<T>()
465 }
466
467 if ptr::eq(self, other) {
468 // Swapping wouldn't change anything.
469 return;
470 }
471 if !is_nonoverlapping(self, other) {
472 // See <https://github.com/rust-lang/rust/issues/80778> for why we need to stop here.
473 panic!("`Cell::swap` on overlapping non-identical `Cell`s");
474 }
475 // SAFETY: This can be risky if called from separate threads, but `Cell`
476 // is `!Sync` so this won't happen. This also won't invalidate any
477 // pointers since `Cell` makes sure nothing else will be pointing into
478 // either of these `Cell`s. We also excluded shenanigans like partially overlapping `Cell`s,
479 // so `swap` will just properly copy two full values of type `T` back and forth.
480 unsafe {
481 mem::swap(&mut *self.value.get(), &mut *other.value.get());
482 }
483 }
484
485 /// Replaces the contained value with `val`, and returns the old contained value.
486 ///
487 /// # Examples
488 ///
489 /// ```
490 /// use std::cell::Cell;
491 ///
492 /// let cell = Cell::new(5);
493 /// assert_eq!(cell.get(), 5);
494 /// assert_eq!(cell.replace(10), 5);
495 /// assert_eq!(cell.get(), 10);
496 /// ```
497 #[inline]
498 #[stable(feature = "move_cell", since = "1.17.0")]
499 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
500 #[rustc_confusables("swap")]
501 pub const fn replace(&self, val: T) -> T {
502 // SAFETY: This can cause data races if called from a separate thread,
503 // but `Cell` is `!Sync` so this won't happen.
504 mem::replace(unsafe { &mut *self.value.get() }, val)
505 }
506
507 /// Unwraps the value, consuming the cell.
508 ///
509 /// # Examples
510 ///
511 /// ```
512 /// use std::cell::Cell;
513 ///
514 /// let c = Cell::new(5);
515 /// let five = c.into_inner();
516 ///
517 /// assert_eq!(five, 5);
518 /// ```
519 #[stable(feature = "move_cell", since = "1.17.0")]
520 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
521 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
522 pub const fn into_inner(self) -> T {
523 self.value.into_inner()
524 }
525}
526
527impl<T: Copy> Cell<T> {
528 /// Returns a copy of the contained value.
529 ///
530 /// # Examples
531 ///
532 /// ```
533 /// use std::cell::Cell;
534 ///
535 /// let c = Cell::new(5);
536 ///
537 /// let five = c.get();
538 /// ```
539 #[inline]
540 #[stable(feature = "rust1", since = "1.0.0")]
541 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
542 pub const fn get(&self) -> T {
543 // SAFETY: This can cause data races if called from a separate thread,
544 // but `Cell` is `!Sync` so this won't happen.
545 unsafe { *self.value.get() }
546 }
547
548 /// Updates the contained value using a function.
549 ///
550 /// # Examples
551 ///
552 /// ```
553 /// use std::cell::Cell;
554 ///
555 /// let c = Cell::new(5);
556 /// c.update(|x| x + 1);
557 /// assert_eq!(c.get(), 6);
558 /// ```
559 #[inline]
560 #[stable(feature = "cell_update", since = "1.88.0")]
561 pub fn update(&self, f: impl FnOnce(T) -> T) {
562 let old = self.get();
563 self.set(f(old));
564 }
565}
566
567impl<T: ?Sized> Cell<T> {
568 /// Returns a raw pointer to the underlying data in this cell.
569 ///
570 /// # Examples
571 ///
572 /// ```
573 /// use std::cell::Cell;
574 ///
575 /// let c = Cell::new(5);
576 ///
577 /// let ptr = c.as_ptr();
578 /// ```
579 #[inline]
580 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
581 #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
582 #[rustc_as_ptr]
583 #[rustc_never_returns_null_ptr]
584 pub const fn as_ptr(&self) -> *mut T {
585 self.value.get()
586 }
587
588 /// Returns a mutable reference to the underlying data.
589 ///
590 /// This call borrows `Cell` mutably (at compile-time) which guarantees
591 /// that we possess the only reference.
592 ///
593 /// However be cautious: this method expects `self` to be mutable, which is
594 /// generally not the case when using a `Cell`. If you require interior
595 /// mutability by reference, consider using `RefCell` which provides
596 /// run-time checked mutable borrows through its [`borrow_mut`] method.
597 ///
598 /// [`borrow_mut`]: RefCell::borrow_mut()
599 ///
600 /// # Examples
601 ///
602 /// ```
603 /// use std::cell::Cell;
604 ///
605 /// let mut c = Cell::new(5);
606 /// *c.get_mut() += 1;
607 ///
608 /// assert_eq!(c.get(), 6);
609 /// ```
610 #[inline]
611 #[stable(feature = "cell_get_mut", since = "1.11.0")]
612 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
613 pub const fn get_mut(&mut self) -> &mut T {
614 self.value.get_mut()
615 }
616
617 /// Returns a `&Cell<T>` from a `&mut T`
618 ///
619 /// # Examples
620 ///
621 /// ```
622 /// use std::cell::Cell;
623 ///
624 /// let slice: &mut [i32] = &mut [1, 2, 3];
625 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
626 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
627 ///
628 /// assert_eq!(slice_cell.len(), 3);
629 /// ```
630 #[inline]
631 #[stable(feature = "as_cell", since = "1.37.0")]
632 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
633 pub const fn from_mut(t: &mut T) -> &Cell<T> {
634 // SAFETY: `&mut` ensures unique access.
635 unsafe { &*(t as *mut T as *const Cell<T>) }
636 }
637}
638
639impl<T: Default> Cell<T> {
640 /// Takes the value of the cell, leaving `Default::default()` in its place.
641 ///
642 /// # Examples
643 ///
644 /// ```
645 /// use std::cell::Cell;
646 ///
647 /// let c = Cell::new(5);
648 /// let five = c.take();
649 ///
650 /// assert_eq!(five, 5);
651 /// assert_eq!(c.into_inner(), 0);
652 /// ```
653 #[stable(feature = "move_cell", since = "1.17.0")]
654 pub fn take(&self) -> T {
655 self.replace(Default::default())
656 }
657}
658
659#[unstable(feature = "coerce_unsized", issue = "18598")]
660impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
661
662// Allow types that wrap `Cell` to also implement `DispatchFromDyn`
663// and become dyn-compatible method receivers.
664// Note that currently `Cell` itself cannot be a method receiver
665// because it does not implement Deref.
666// In other words:
667// `self: Cell<&Self>` won't work
668// `self: CellWrapper<Self>` becomes possible
669#[unstable(feature = "dispatch_from_dyn", issue = "none")]
670impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
671
672#[unstable(feature = "pointer_like_trait", issue = "none")]
673impl<T: PointerLike> PointerLike for Cell<T> {}
674
675impl<T> Cell<[T]> {
676 /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
677 ///
678 /// # Examples
679 ///
680 /// ```
681 /// use std::cell::Cell;
682 ///
683 /// let slice: &mut [i32] = &mut [1, 2, 3];
684 /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
685 /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
686 ///
687 /// assert_eq!(slice_cell.len(), 3);
688 /// ```
689 #[stable(feature = "as_cell", since = "1.37.0")]
690 #[rustc_const_stable(feature = "const_cell", since = "1.88.0")]
691 pub const fn as_slice_of_cells(&self) -> &[Cell<T>] {
692 // SAFETY: `Cell<T>` has the same memory layout as `T`.
693 unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
694 }
695}
696
697impl<T, const N: usize> Cell<[T; N]> {
698 /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
699 ///
700 /// # Examples
701 ///
702 /// ```
703 /// #![feature(as_array_of_cells)]
704 /// use std::cell::Cell;
705 ///
706 /// let mut array: [i32; 3] = [1, 2, 3];
707 /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
708 /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
709 /// ```
710 #[unstable(feature = "as_array_of_cells", issue = "88248")]
711 pub const fn as_array_of_cells(&self) -> &[Cell<T>; N] {
712 // SAFETY: `Cell<T>` has the same memory layout as `T`.
713 unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
714 }
715}
716
717/// A mutable memory location with dynamically checked borrow rules
718///
719/// See the [module-level documentation](self) for more.
720#[rustc_diagnostic_item = "RefCell"]
721#[stable(feature = "rust1", since = "1.0.0")]
722pub struct RefCell<T: ?Sized> {
723 borrow: Cell<BorrowCounter>,
724 // Stores the location of the earliest currently active borrow.
725 // This gets updated whenever we go from having zero borrows
726 // to having a single borrow. When a borrow occurs, this gets included
727 // in the generated `BorrowError`/`BorrowMutError`
728 #[cfg(feature = "debug_refcell")]
729 borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
730 value: UnsafeCell<T>,
731}
732
733/// An error returned by [`RefCell::try_borrow`].
734#[stable(feature = "try_borrow", since = "1.13.0")]
735#[non_exhaustive]
736#[derive(Debug)]
737pub struct BorrowError {
738 #[cfg(feature = "debug_refcell")]
739 location: &'static crate::panic::Location<'static>,
740}
741
742#[stable(feature = "try_borrow", since = "1.13.0")]
743impl Display for BorrowError {
744 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
745 #[cfg(feature = "debug_refcell")]
746 let res = write!(
747 f,
748 "RefCell already mutably borrowed; a previous borrow was at {}",
749 self.location
750 );
751
752 #[cfg(not(feature = "debug_refcell"))]
753 let res = Display::fmt("RefCell already mutably borrowed", f);
754
755 res
756 }
757}
758
759/// An error returned by [`RefCell::try_borrow_mut`].
760#[stable(feature = "try_borrow", since = "1.13.0")]
761#[non_exhaustive]
762#[derive(Debug)]
763pub struct BorrowMutError {
764 #[cfg(feature = "debug_refcell")]
765 location: &'static crate::panic::Location<'static>,
766}
767
768#[stable(feature = "try_borrow", since = "1.13.0")]
769impl Display for BorrowMutError {
770 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
771 #[cfg(feature = "debug_refcell")]
772 let res = write!(f, "RefCell already borrowed; a previous borrow was at {}", self.location);
773
774 #[cfg(not(feature = "debug_refcell"))]
775 let res = Display::fmt("RefCell already borrowed", f);
776
777 res
778 }
779}
780
781// This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
782#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
783#[track_caller]
784#[cold]
785const fn panic_already_borrowed(err: BorrowMutError) -> ! {
786 const_panic!(
787 "RefCell already borrowed",
788 "{err}",
789 err: BorrowMutError = err,
790 )
791}
792
793// This ensures the panicking code is outlined from `borrow` for `RefCell`.
794#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
795#[track_caller]
796#[cold]
797const fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
798 const_panic!(
799 "RefCell already mutably borrowed",
800 "{err}",
801 err: BorrowError = err,
802 )
803}
804
805// Positive values represent the number of `Ref` active. Negative values
806// represent the number of `RefMut` active. Multiple `RefMut`s can only be
807// active at a time if they refer to distinct, nonoverlapping components of a
808// `RefCell` (e.g., different ranges of a slice).
809//
810// `Ref` and `RefMut` are both two words in size, and so there will likely never
811// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
812// range. Thus, a `BorrowCounter` will probably never overflow or underflow.
813// However, this is not a guarantee, as a pathological program could repeatedly
814// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
815// explicitly check for overflow and underflow in order to avoid unsafety, or at
816// least behave correctly in the event that overflow or underflow happens (e.g.,
817// see BorrowRef::new).
818type BorrowCounter = isize;
819const UNUSED: BorrowCounter = 0;
820
821#[inline(always)]
822const fn is_writing(x: BorrowCounter) -> bool {
823 x < UNUSED
824}
825
826#[inline(always)]
827const fn is_reading(x: BorrowCounter) -> bool {
828 x > UNUSED
829}
830
831impl<T> RefCell<T> {
832 /// Creates a new `RefCell` containing `value`.
833 ///
834 /// # Examples
835 ///
836 /// ```
837 /// use std::cell::RefCell;
838 ///
839 /// let c = RefCell::new(5);
840 /// ```
841 #[stable(feature = "rust1", since = "1.0.0")]
842 #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
843 #[inline]
844 pub const fn new(value: T) -> RefCell<T> {
845 RefCell {
846 value: UnsafeCell::new(value),
847 borrow: Cell::new(UNUSED),
848 #[cfg(feature = "debug_refcell")]
849 borrowed_at: Cell::new(None),
850 }
851 }
852
853 /// Consumes the `RefCell`, returning the wrapped value.
854 ///
855 /// # Examples
856 ///
857 /// ```
858 /// use std::cell::RefCell;
859 ///
860 /// let c = RefCell::new(5);
861 ///
862 /// let five = c.into_inner();
863 /// ```
864 #[stable(feature = "rust1", since = "1.0.0")]
865 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
866 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
867 #[inline]
868 pub const fn into_inner(self) -> T {
869 // Since this function takes `self` (the `RefCell`) by value, the
870 // compiler statically verifies that it is not currently borrowed.
871 self.value.into_inner()
872 }
873
874 /// Replaces the wrapped value with a new one, returning the old value,
875 /// without deinitializing either one.
876 ///
877 /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
878 ///
879 /// # Panics
880 ///
881 /// Panics if the value is currently borrowed.
882 ///
883 /// # Examples
884 ///
885 /// ```
886 /// use std::cell::RefCell;
887 /// let cell = RefCell::new(5);
888 /// let old_value = cell.replace(6);
889 /// assert_eq!(old_value, 5);
890 /// assert_eq!(cell, RefCell::new(6));
891 /// ```
892 #[inline]
893 #[stable(feature = "refcell_replace", since = "1.24.0")]
894 #[track_caller]
895 #[rustc_confusables("swap")]
896 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
897 pub const fn replace(&self, t: T) -> T {
898 mem::replace(&mut self.borrow_mut(), t)
899 }
900
901 /// Replaces the wrapped value with a new one computed from `f`, returning
902 /// the old value, without deinitializing either one.
903 ///
904 /// # Panics
905 ///
906 /// Panics if the value is currently borrowed.
907 ///
908 /// # Examples
909 ///
910 /// ```
911 /// use std::cell::RefCell;
912 /// let cell = RefCell::new(5);
913 /// let old_value = cell.replace_with(|&mut old| old + 1);
914 /// assert_eq!(old_value, 5);
915 /// assert_eq!(cell, RefCell::new(6));
916 /// ```
917 #[inline]
918 #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
919 #[track_caller]
920 pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
921 let mut_borrow = &mut *self.borrow_mut();
922 let replacement = f(mut_borrow);
923 mem::replace(mut_borrow, replacement)
924 }
925
926 /// Swaps the wrapped value of `self` with the wrapped value of `other`,
927 /// without deinitializing either one.
928 ///
929 /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
930 ///
931 /// # Panics
932 ///
933 /// Panics if the value in either `RefCell` is currently borrowed, or
934 /// if `self` and `other` point to the same `RefCell`.
935 ///
936 /// # Examples
937 ///
938 /// ```
939 /// use std::cell::RefCell;
940 /// let c = RefCell::new(5);
941 /// let d = RefCell::new(6);
942 /// c.swap(&d);
943 /// assert_eq!(c, RefCell::new(6));
944 /// assert_eq!(d, RefCell::new(5));
945 /// ```
946 #[inline]
947 #[stable(feature = "refcell_swap", since = "1.24.0")]
948 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
949 pub const fn swap(&self, other: &Self) {
950 mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
951 }
952}
953
954impl<T: ?Sized> RefCell<T> {
955 /// Immutably borrows the wrapped value.
956 ///
957 /// The borrow lasts until the returned `Ref` exits scope. Multiple
958 /// immutable borrows can be taken out at the same time.
959 ///
960 /// # Panics
961 ///
962 /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
963 /// [`try_borrow`](#method.try_borrow).
964 ///
965 /// # Examples
966 ///
967 /// ```
968 /// use std::cell::RefCell;
969 ///
970 /// let c = RefCell::new(5);
971 ///
972 /// let borrowed_five = c.borrow();
973 /// let borrowed_five2 = c.borrow();
974 /// ```
975 ///
976 /// An example of panic:
977 ///
978 /// ```should_panic
979 /// use std::cell::RefCell;
980 ///
981 /// let c = RefCell::new(5);
982 ///
983 /// let m = c.borrow_mut();
984 /// let b = c.borrow(); // this causes a panic
985 /// ```
986 #[stable(feature = "rust1", since = "1.0.0")]
987 #[inline]
988 #[track_caller]
989 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
990 pub const fn borrow(&self) -> Ref<'_, T> {
991 match self.try_borrow() {
992 Ok(b) => b,
993 Err(err) => panic_already_mutably_borrowed(err),
994 }
995 }
996
997 /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
998 /// borrowed.
999 ///
1000 /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
1001 /// taken out at the same time.
1002 ///
1003 /// This is the non-panicking variant of [`borrow`](#method.borrow).
1004 ///
1005 /// # Examples
1006 ///
1007 /// ```
1008 /// use std::cell::RefCell;
1009 ///
1010 /// let c = RefCell::new(5);
1011 ///
1012 /// {
1013 /// let m = c.borrow_mut();
1014 /// assert!(c.try_borrow().is_err());
1015 /// }
1016 ///
1017 /// {
1018 /// let m = c.borrow();
1019 /// assert!(c.try_borrow().is_ok());
1020 /// }
1021 /// ```
1022 #[stable(feature = "try_borrow", since = "1.13.0")]
1023 #[inline]
1024 #[cfg_attr(feature = "debug_refcell", track_caller)]
1025 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1026 pub const fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
1027 match BorrowRef::new(&self.borrow) {
1028 Some(b) => {
1029 #[cfg(feature = "debug_refcell")]
1030 {
1031 // `borrowed_at` is always the *first* active borrow
1032 if b.borrow.get() == 1 {
1033 self.borrowed_at.replace(Some(crate::panic::Location::caller()));
1034 }
1035 }
1036
1037 // SAFETY: `BorrowRef` ensures that there is only immutable access
1038 // to the value while borrowed.
1039 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1040 Ok(Ref { value, borrow: b })
1041 }
1042 None => Err(BorrowError {
1043 // If a borrow occurred, then we must already have an outstanding borrow,
1044 // so `borrowed_at` will be `Some`
1045 #[cfg(feature = "debug_refcell")]
1046 location: self.borrowed_at.get().unwrap(),
1047 }),
1048 }
1049 }
1050
1051 /// Mutably borrows the wrapped value.
1052 ///
1053 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1054 /// from it exit scope. The value cannot be borrowed while this borrow is
1055 /// active.
1056 ///
1057 /// # Panics
1058 ///
1059 /// Panics if the value is currently borrowed. For a non-panicking variant, use
1060 /// [`try_borrow_mut`](#method.try_borrow_mut).
1061 ///
1062 /// # Examples
1063 ///
1064 /// ```
1065 /// use std::cell::RefCell;
1066 ///
1067 /// let c = RefCell::new("hello".to_owned());
1068 ///
1069 /// *c.borrow_mut() = "bonjour".to_owned();
1070 ///
1071 /// assert_eq!(&*c.borrow(), "bonjour");
1072 /// ```
1073 ///
1074 /// An example of panic:
1075 ///
1076 /// ```should_panic
1077 /// use std::cell::RefCell;
1078 ///
1079 /// let c = RefCell::new(5);
1080 /// let m = c.borrow();
1081 ///
1082 /// let b = c.borrow_mut(); // this causes a panic
1083 /// ```
1084 #[stable(feature = "rust1", since = "1.0.0")]
1085 #[inline]
1086 #[track_caller]
1087 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1088 pub const fn borrow_mut(&self) -> RefMut<'_, T> {
1089 match self.try_borrow_mut() {
1090 Ok(b) => b,
1091 Err(err) => panic_already_borrowed(err),
1092 }
1093 }
1094
1095 /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
1096 ///
1097 /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1098 /// from it exit scope. The value cannot be borrowed while this borrow is
1099 /// active.
1100 ///
1101 /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
1102 ///
1103 /// # Examples
1104 ///
1105 /// ```
1106 /// use std::cell::RefCell;
1107 ///
1108 /// let c = RefCell::new(5);
1109 ///
1110 /// {
1111 /// let m = c.borrow();
1112 /// assert!(c.try_borrow_mut().is_err());
1113 /// }
1114 ///
1115 /// assert!(c.try_borrow_mut().is_ok());
1116 /// ```
1117 #[stable(feature = "try_borrow", since = "1.13.0")]
1118 #[inline]
1119 #[cfg_attr(feature = "debug_refcell", track_caller)]
1120 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1121 pub const fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
1122 match BorrowRefMut::new(&self.borrow) {
1123 Some(b) => {
1124 #[cfg(feature = "debug_refcell")]
1125 {
1126 self.borrowed_at.replace(Some(crate::panic::Location::caller()));
1127 }
1128
1129 // SAFETY: `BorrowRefMut` guarantees unique access.
1130 let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1131 Ok(RefMut { value, borrow: b, marker: PhantomData })
1132 }
1133 None => Err(BorrowMutError {
1134 // If a borrow occurred, then we must already have an outstanding borrow,
1135 // so `borrowed_at` will be `Some`
1136 #[cfg(feature = "debug_refcell")]
1137 location: self.borrowed_at.get().unwrap(),
1138 }),
1139 }
1140 }
1141
1142 /// Returns a raw pointer to the underlying data in this cell.
1143 ///
1144 /// # Examples
1145 ///
1146 /// ```
1147 /// use std::cell::RefCell;
1148 ///
1149 /// let c = RefCell::new(5);
1150 ///
1151 /// let ptr = c.as_ptr();
1152 /// ```
1153 #[inline]
1154 #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1155 #[rustc_as_ptr]
1156 #[rustc_never_returns_null_ptr]
1157 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1158 pub const fn as_ptr(&self) -> *mut T {
1159 self.value.get()
1160 }
1161
1162 /// Returns a mutable reference to the underlying data.
1163 ///
1164 /// Since this method borrows `RefCell` mutably, it is statically guaranteed
1165 /// that no borrows to the underlying data exist. The dynamic checks inherent
1166 /// in [`borrow_mut`] and most other methods of `RefCell` are therefore
1167 /// unnecessary. Note that this method does not reset the borrowing state if borrows were previously leaked
1168 /// (e.g., via [`forget()`] on a [`Ref`] or [`RefMut`]). For that purpose,
1169 /// consider using the unstable [`undo_leak`] method.
1170 ///
1171 /// This method can only be called if `RefCell` can be mutably borrowed,
1172 /// which in general is only the case directly after the `RefCell` has
1173 /// been created. In these situations, skipping the aforementioned dynamic
1174 /// borrowing checks may yield better ergonomics and runtime-performance.
1175 ///
1176 /// In most situations where `RefCell` is used, it can't be borrowed mutably.
1177 /// Use [`borrow_mut`] to get mutable access to the underlying data then.
1178 ///
1179 /// [`borrow_mut`]: RefCell::borrow_mut()
1180 /// [`forget()`]: mem::forget
1181 /// [`undo_leak`]: RefCell::undo_leak()
1182 ///
1183 /// # Examples
1184 ///
1185 /// ```
1186 /// use std::cell::RefCell;
1187 ///
1188 /// let mut c = RefCell::new(5);
1189 /// *c.get_mut() += 1;
1190 ///
1191 /// assert_eq!(c, RefCell::new(6));
1192 /// ```
1193 #[inline]
1194 #[stable(feature = "cell_get_mut", since = "1.11.0")]
1195 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1196 pub const fn get_mut(&mut self) -> &mut T {
1197 self.value.get_mut()
1198 }
1199
1200 /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1201 ///
1202 /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1203 /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1204 /// if some `Ref` or `RefMut` borrows have been leaked.
1205 ///
1206 /// [`get_mut`]: RefCell::get_mut()
1207 ///
1208 /// # Examples
1209 ///
1210 /// ```
1211 /// #![feature(cell_leak)]
1212 /// use std::cell::RefCell;
1213 ///
1214 /// let mut c = RefCell::new(0);
1215 /// std::mem::forget(c.borrow_mut());
1216 ///
1217 /// assert!(c.try_borrow().is_err());
1218 /// c.undo_leak();
1219 /// assert!(c.try_borrow().is_ok());
1220 /// ```
1221 #[unstable(feature = "cell_leak", issue = "69099")]
1222 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1223 pub const fn undo_leak(&mut self) -> &mut T {
1224 *self.borrow.get_mut() = UNUSED;
1225 self.get_mut()
1226 }
1227
1228 /// Immutably borrows the wrapped value, returning an error if the value is
1229 /// currently mutably borrowed.
1230 ///
1231 /// # Safety
1232 ///
1233 /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1234 /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1235 /// borrowing the `RefCell` while the reference returned by this method
1236 /// is alive is undefined behavior.
1237 ///
1238 /// # Examples
1239 ///
1240 /// ```
1241 /// use std::cell::RefCell;
1242 ///
1243 /// let c = RefCell::new(5);
1244 ///
1245 /// {
1246 /// let m = c.borrow_mut();
1247 /// assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1248 /// }
1249 ///
1250 /// {
1251 /// let m = c.borrow();
1252 /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1253 /// }
1254 /// ```
1255 #[stable(feature = "borrow_state", since = "1.37.0")]
1256 #[inline]
1257 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1258 pub const unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1259 if !is_writing(self.borrow.get()) {
1260 // SAFETY: We check that nobody is actively writing now, but it is
1261 // the caller's responsibility to ensure that nobody writes until
1262 // the returned reference is no longer in use.
1263 // Also, `self.value.get()` refers to the value owned by `self`
1264 // and is thus guaranteed to be valid for the lifetime of `self`.
1265 Ok(unsafe { &*self.value.get() })
1266 } else {
1267 Err(BorrowError {
1268 // If a borrow occurred, then we must already have an outstanding borrow,
1269 // so `borrowed_at` will be `Some`
1270 #[cfg(feature = "debug_refcell")]
1271 location: self.borrowed_at.get().unwrap(),
1272 })
1273 }
1274 }
1275}
1276
1277impl<T: Default> RefCell<T> {
1278 /// Takes the wrapped value, leaving `Default::default()` in its place.
1279 ///
1280 /// # Panics
1281 ///
1282 /// Panics if the value is currently borrowed.
1283 ///
1284 /// # Examples
1285 ///
1286 /// ```
1287 /// use std::cell::RefCell;
1288 ///
1289 /// let c = RefCell::new(5);
1290 /// let five = c.take();
1291 ///
1292 /// assert_eq!(five, 5);
1293 /// assert_eq!(c.into_inner(), 0);
1294 /// ```
1295 #[stable(feature = "refcell_take", since = "1.50.0")]
1296 pub fn take(&self) -> T {
1297 self.replace(Default::default())
1298 }
1299}
1300
1301#[stable(feature = "rust1", since = "1.0.0")]
1302unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1303
1304#[stable(feature = "rust1", since = "1.0.0")]
1305impl<T: ?Sized> !Sync for RefCell<T> {}
1306
1307#[stable(feature = "rust1", since = "1.0.0")]
1308impl<T: Clone> Clone for RefCell<T> {
1309 /// # Panics
1310 ///
1311 /// Panics if the value is currently mutably borrowed.
1312 #[inline]
1313 #[track_caller]
1314 fn clone(&self) -> RefCell<T> {
1315 RefCell::new(self.borrow().clone())
1316 }
1317
1318 /// # Panics
1319 ///
1320 /// Panics if `source` is currently mutably borrowed.
1321 #[inline]
1322 #[track_caller]
1323 fn clone_from(&mut self, source: &Self) {
1324 self.get_mut().clone_from(&source.borrow())
1325 }
1326}
1327
1328#[stable(feature = "rust1", since = "1.0.0")]
1329impl<T: Default> Default for RefCell<T> {
1330 /// Creates a `RefCell<T>`, with the `Default` value for T.
1331 #[inline]
1332 fn default() -> RefCell<T> {
1333 RefCell::new(Default::default())
1334 }
1335}
1336
1337#[stable(feature = "rust1", since = "1.0.0")]
1338impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1339 /// # Panics
1340 ///
1341 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1342 #[inline]
1343 fn eq(&self, other: &RefCell<T>) -> bool {
1344 *self.borrow() == *other.borrow()
1345 }
1346}
1347
1348#[stable(feature = "cell_eq", since = "1.2.0")]
1349impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1350
1351#[stable(feature = "cell_ord", since = "1.10.0")]
1352impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1353 /// # Panics
1354 ///
1355 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1356 #[inline]
1357 fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1358 self.borrow().partial_cmp(&*other.borrow())
1359 }
1360
1361 /// # Panics
1362 ///
1363 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1364 #[inline]
1365 fn lt(&self, other: &RefCell<T>) -> bool {
1366 *self.borrow() < *other.borrow()
1367 }
1368
1369 /// # Panics
1370 ///
1371 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1372 #[inline]
1373 fn le(&self, other: &RefCell<T>) -> bool {
1374 *self.borrow() <= *other.borrow()
1375 }
1376
1377 /// # Panics
1378 ///
1379 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1380 #[inline]
1381 fn gt(&self, other: &RefCell<T>) -> bool {
1382 *self.borrow() > *other.borrow()
1383 }
1384
1385 /// # Panics
1386 ///
1387 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1388 #[inline]
1389 fn ge(&self, other: &RefCell<T>) -> bool {
1390 *self.borrow() >= *other.borrow()
1391 }
1392}
1393
1394#[stable(feature = "cell_ord", since = "1.10.0")]
1395impl<T: ?Sized + Ord> Ord for RefCell<T> {
1396 /// # Panics
1397 ///
1398 /// Panics if the value in either `RefCell` is currently mutably borrowed.
1399 #[inline]
1400 fn cmp(&self, other: &RefCell<T>) -> Ordering {
1401 self.borrow().cmp(&*other.borrow())
1402 }
1403}
1404
1405#[stable(feature = "cell_from", since = "1.12.0")]
1406impl<T> From<T> for RefCell<T> {
1407 /// Creates a new `RefCell<T>` containing the given value.
1408 fn from(t: T) -> RefCell<T> {
1409 RefCell::new(t)
1410 }
1411}
1412
1413#[unstable(feature = "coerce_unsized", issue = "18598")]
1414impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1415
1416struct BorrowRef<'b> {
1417 borrow: &'b Cell<BorrowCounter>,
1418}
1419
1420impl<'b> BorrowRef<'b> {
1421 #[inline]
1422 const fn new(borrow: &'b Cell<BorrowCounter>) -> Option<BorrowRef<'b>> {
1423 let b = borrow.get().wrapping_add(1);
1424 if !is_reading(b) {
1425 // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1426 // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1427 // due to Rust's reference aliasing rules
1428 // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1429 // into isize::MIN (the max amount of writing borrows) so we can't allow
1430 // an additional read borrow because isize can't represent so many read borrows
1431 // (this can only happen if you mem::forget more than a small constant amount of
1432 // `Ref`s, which is not good practice)
1433 None
1434 } else {
1435 // Incrementing borrow can result in a reading value (> 0) in these cases:
1436 // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1437 // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1438 // is large enough to represent having one more read borrow
1439 borrow.replace(b);
1440 Some(BorrowRef { borrow })
1441 }
1442 }
1443}
1444
1445#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1446impl const Drop for BorrowRef<'_> {
1447 #[inline]
1448 fn drop(&mut self) {
1449 let borrow = self.borrow.get();
1450 debug_assert!(is_reading(borrow));
1451 self.borrow.replace(borrow - 1);
1452 }
1453}
1454
1455#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1456impl const Clone for BorrowRef<'_> {
1457 #[inline]
1458 fn clone(&self) -> Self {
1459 // Since this Ref exists, we know the borrow flag
1460 // is a reading borrow.
1461 let borrow = self.borrow.get();
1462 debug_assert!(is_reading(borrow));
1463 // Prevent the borrow counter from overflowing into
1464 // a writing borrow.
1465 assert!(borrow != BorrowCounter::MAX);
1466 self.borrow.replace(borrow + 1);
1467 BorrowRef { borrow: self.borrow }
1468 }
1469}
1470
1471/// Wraps a borrowed reference to a value in a `RefCell` box.
1472/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1473///
1474/// See the [module-level documentation](self) for more.
1475#[stable(feature = "rust1", since = "1.0.0")]
1476#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1477#[rustc_diagnostic_item = "RefCellRef"]
1478pub struct Ref<'b, T: ?Sized + 'b> {
1479 // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
1480 // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
1481 // `NonNull` is also covariant over `T`, just like we would have with `&T`.
1482 value: NonNull<T>,
1483 borrow: BorrowRef<'b>,
1484}
1485
1486#[stable(feature = "rust1", since = "1.0.0")]
1487#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1488impl<T: ?Sized> const Deref for Ref<'_, T> {
1489 type Target = T;
1490
1491 #[inline]
1492 fn deref(&self) -> &T {
1493 // SAFETY: the value is accessible as long as we hold our borrow.
1494 unsafe { self.value.as_ref() }
1495 }
1496}
1497
1498#[unstable(feature = "deref_pure_trait", issue = "87121")]
1499unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}
1500
1501impl<'b, T: ?Sized> Ref<'b, T> {
1502 /// Copies a `Ref`.
1503 ///
1504 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1505 ///
1506 /// This is an associated function that needs to be used as
1507 /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1508 /// with the widespread use of `r.borrow().clone()` to clone the contents of
1509 /// a `RefCell`.
1510 #[stable(feature = "cell_extras", since = "1.15.0")]
1511 #[must_use]
1512 #[inline]
1513 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1514 pub const fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1515 Ref { value: orig.value, borrow: orig.borrow.clone() }
1516 }
1517
1518 /// Makes a new `Ref` for a component of the borrowed data.
1519 ///
1520 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1521 ///
1522 /// This is an associated function that needs to be used as `Ref::map(...)`.
1523 /// A method would interfere with methods of the same name on the contents
1524 /// of a `RefCell` used through `Deref`.
1525 ///
1526 /// # Examples
1527 ///
1528 /// ```
1529 /// use std::cell::{RefCell, Ref};
1530 ///
1531 /// let c = RefCell::new((5, 'b'));
1532 /// let b1: Ref<'_, (u32, char)> = c.borrow();
1533 /// let b2: Ref<'_, u32> = Ref::map(b1, |t| &t.0);
1534 /// assert_eq!(*b2, 5)
1535 /// ```
1536 #[stable(feature = "cell_map", since = "1.8.0")]
1537 #[inline]
1538 pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1539 where
1540 F: FnOnce(&T) -> &U,
1541 {
1542 Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
1543 }
1544
1545 /// Makes a new `Ref` for an optional component of the borrowed data. The
1546 /// original guard is returned as an `Err(..)` if the closure returns
1547 /// `None`.
1548 ///
1549 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1550 ///
1551 /// This is an associated function that needs to be used as
1552 /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1553 /// name on the contents of a `RefCell` used through `Deref`.
1554 ///
1555 /// # Examples
1556 ///
1557 /// ```
1558 /// use std::cell::{RefCell, Ref};
1559 ///
1560 /// let c = RefCell::new(vec![1, 2, 3]);
1561 /// let b1: Ref<'_, Vec<u32>> = c.borrow();
1562 /// let b2: Result<Ref<'_, u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1563 /// assert_eq!(*b2.unwrap(), 2);
1564 /// ```
1565 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1566 #[inline]
1567 pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1568 where
1569 F: FnOnce(&T) -> Option<&U>,
1570 {
1571 match f(&*orig) {
1572 Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
1573 None => Err(orig),
1574 }
1575 }
1576
1577 /// Splits a `Ref` into multiple `Ref`s for different components of the
1578 /// borrowed data.
1579 ///
1580 /// The `RefCell` is already immutably borrowed, so this cannot fail.
1581 ///
1582 /// This is an associated function that needs to be used as
1583 /// `Ref::map_split(...)`. A method would interfere with methods of the same
1584 /// name on the contents of a `RefCell` used through `Deref`.
1585 ///
1586 /// # Examples
1587 ///
1588 /// ```
1589 /// use std::cell::{Ref, RefCell};
1590 ///
1591 /// let cell = RefCell::new([1, 2, 3, 4]);
1592 /// let borrow = cell.borrow();
1593 /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1594 /// assert_eq!(*begin, [1, 2]);
1595 /// assert_eq!(*end, [3, 4]);
1596 /// ```
1597 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1598 #[inline]
1599 pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1600 where
1601 F: FnOnce(&T) -> (&U, &V),
1602 {
1603 let (a, b) = f(&*orig);
1604 let borrow = orig.borrow.clone();
1605 (
1606 Ref { value: NonNull::from(a), borrow },
1607 Ref { value: NonNull::from(b), borrow: orig.borrow },
1608 )
1609 }
1610
1611 /// Converts into a reference to the underlying data.
1612 ///
1613 /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1614 /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1615 /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1616 /// have occurred in total.
1617 ///
1618 /// This is an associated function that needs to be used as
1619 /// `Ref::leak(...)`. A method would interfere with methods of the
1620 /// same name on the contents of a `RefCell` used through `Deref`.
1621 ///
1622 /// # Examples
1623 ///
1624 /// ```
1625 /// #![feature(cell_leak)]
1626 /// use std::cell::{RefCell, Ref};
1627 /// let cell = RefCell::new(0);
1628 ///
1629 /// let value = Ref::leak(cell.borrow());
1630 /// assert_eq!(*value, 0);
1631 ///
1632 /// assert!(cell.try_borrow().is_ok());
1633 /// assert!(cell.try_borrow_mut().is_err());
1634 /// ```
1635 #[unstable(feature = "cell_leak", issue = "69099")]
1636 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1637 pub const fn leak(orig: Ref<'b, T>) -> &'b T {
1638 // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1639 // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1640 // unique reference to the borrowed RefCell. No further mutable references can be created
1641 // from the original cell.
1642 mem::forget(orig.borrow);
1643 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1644 unsafe { orig.value.as_ref() }
1645 }
1646}
1647
1648#[unstable(feature = "coerce_unsized", issue = "18598")]
1649impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1650
1651#[stable(feature = "std_guard_impls", since = "1.20.0")]
1652impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1653 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1654 (**self).fmt(f)
1655 }
1656}
1657
1658impl<'b, T: ?Sized> RefMut<'b, T> {
1659 /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1660 /// variant.
1661 ///
1662 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1663 ///
1664 /// This is an associated function that needs to be used as
1665 /// `RefMut::map(...)`. A method would interfere with methods of the same
1666 /// name on the contents of a `RefCell` used through `Deref`.
1667 ///
1668 /// # Examples
1669 ///
1670 /// ```
1671 /// use std::cell::{RefCell, RefMut};
1672 ///
1673 /// let c = RefCell::new((5, 'b'));
1674 /// {
1675 /// let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
1676 /// let mut b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
1677 /// assert_eq!(*b2, 5);
1678 /// *b2 = 42;
1679 /// }
1680 /// assert_eq!(*c.borrow(), (42, 'b'));
1681 /// ```
1682 #[stable(feature = "cell_map", since = "1.8.0")]
1683 #[inline]
1684 pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1685 where
1686 F: FnOnce(&mut T) -> &mut U,
1687 {
1688 let value = NonNull::from(f(&mut *orig));
1689 RefMut { value, borrow: orig.borrow, marker: PhantomData }
1690 }
1691
1692 /// Makes a new `RefMut` for an optional component of the borrowed data. The
1693 /// original guard is returned as an `Err(..)` if the closure returns
1694 /// `None`.
1695 ///
1696 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1697 ///
1698 /// This is an associated function that needs to be used as
1699 /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1700 /// same name on the contents of a `RefCell` used through `Deref`.
1701 ///
1702 /// # Examples
1703 ///
1704 /// ```
1705 /// use std::cell::{RefCell, RefMut};
1706 ///
1707 /// let c = RefCell::new(vec![1, 2, 3]);
1708 ///
1709 /// {
1710 /// let b1: RefMut<'_, Vec<u32>> = c.borrow_mut();
1711 /// let mut b2: Result<RefMut<'_, u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1712 ///
1713 /// if let Ok(mut b2) = b2 {
1714 /// *b2 += 2;
1715 /// }
1716 /// }
1717 ///
1718 /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1719 /// ```
1720 #[stable(feature = "cell_filter_map", since = "1.63.0")]
1721 #[inline]
1722 pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1723 where
1724 F: FnOnce(&mut T) -> Option<&mut U>,
1725 {
1726 // SAFETY: function holds onto an exclusive reference for the duration
1727 // of its call through `orig`, and the pointer is only de-referenced
1728 // inside of the function call never allowing the exclusive reference to
1729 // escape.
1730 match f(&mut *orig) {
1731 Some(value) => {
1732 Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
1733 }
1734 None => Err(orig),
1735 }
1736 }
1737
1738 /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1739 /// borrowed data.
1740 ///
1741 /// The underlying `RefCell` will remain mutably borrowed until both
1742 /// returned `RefMut`s go out of scope.
1743 ///
1744 /// The `RefCell` is already mutably borrowed, so this cannot fail.
1745 ///
1746 /// This is an associated function that needs to be used as
1747 /// `RefMut::map_split(...)`. A method would interfere with methods of the
1748 /// same name on the contents of a `RefCell` used through `Deref`.
1749 ///
1750 /// # Examples
1751 ///
1752 /// ```
1753 /// use std::cell::{RefCell, RefMut};
1754 ///
1755 /// let cell = RefCell::new([1, 2, 3, 4]);
1756 /// let borrow = cell.borrow_mut();
1757 /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1758 /// assert_eq!(*begin, [1, 2]);
1759 /// assert_eq!(*end, [3, 4]);
1760 /// begin.copy_from_slice(&[4, 3]);
1761 /// end.copy_from_slice(&[2, 1]);
1762 /// ```
1763 #[stable(feature = "refcell_map_split", since = "1.35.0")]
1764 #[inline]
1765 pub fn map_split<U: ?Sized, V: ?Sized, F>(
1766 mut orig: RefMut<'b, T>,
1767 f: F,
1768 ) -> (RefMut<'b, U>, RefMut<'b, V>)
1769 where
1770 F: FnOnce(&mut T) -> (&mut U, &mut V),
1771 {
1772 let borrow = orig.borrow.clone();
1773 let (a, b) = f(&mut *orig);
1774 (
1775 RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
1776 RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
1777 )
1778 }
1779
1780 /// Converts into a mutable reference to the underlying data.
1781 ///
1782 /// The underlying `RefCell` can not be borrowed from again and will always appear already
1783 /// mutably borrowed, making the returned reference the only to the interior.
1784 ///
1785 /// This is an associated function that needs to be used as
1786 /// `RefMut::leak(...)`. A method would interfere with methods of the
1787 /// same name on the contents of a `RefCell` used through `Deref`.
1788 ///
1789 /// # Examples
1790 ///
1791 /// ```
1792 /// #![feature(cell_leak)]
1793 /// use std::cell::{RefCell, RefMut};
1794 /// let cell = RefCell::new(0);
1795 ///
1796 /// let value = RefMut::leak(cell.borrow_mut());
1797 /// assert_eq!(*value, 0);
1798 /// *value = 1;
1799 ///
1800 /// assert!(cell.try_borrow_mut().is_err());
1801 /// ```
1802 #[unstable(feature = "cell_leak", issue = "69099")]
1803 #[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1804 pub const fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
1805 // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1806 // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1807 // require a unique reference to the borrowed RefCell. No further references can be created
1808 // from the original cell within that lifetime, making the current borrow the only
1809 // reference for the remaining lifetime.
1810 mem::forget(orig.borrow);
1811 // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1812 unsafe { orig.value.as_mut() }
1813 }
1814}
1815
1816struct BorrowRefMut<'b> {
1817 borrow: &'b Cell<BorrowCounter>,
1818}
1819
1820#[rustc_const_unstable(feature = "const_ref_cell", issue = "137844")]
1821impl const Drop for BorrowRefMut<'_> {
1822 #[inline]
1823 fn drop(&mut self) {
1824 let borrow = self.borrow.get();
1825 debug_assert!(is_writing(borrow));
1826 self.borrow.replace(borrow + 1);
1827 }
1828}
1829
1830impl<'b> BorrowRefMut<'b> {
1831 #[inline]
1832 const fn new(borrow: &'b Cell<BorrowCounter>) -> Option<BorrowRefMut<'b>> {
1833 // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1834 // mutable reference, and so there must currently be no existing
1835 // references. Thus, while clone increments the mutable refcount, here
1836 // we explicitly only allow going from UNUSED to UNUSED - 1.
1837 match borrow.get() {
1838 UNUSED => {
1839 borrow.replace(UNUSED - 1);
1840 Some(BorrowRefMut { borrow })
1841 }
1842 _ => None,
1843 }
1844 }
1845
1846 // Clones a `BorrowRefMut`.
1847 //
1848 // This is only valid if each `BorrowRefMut` is used to track a mutable
1849 // reference to a distinct, nonoverlapping range of the original object.
1850 // This isn't in a Clone impl so that code doesn't call this implicitly.
1851 #[inline]
1852 fn clone(&self) -> BorrowRefMut<'b> {
1853 let borrow = self.borrow.get();
1854 debug_assert!(is_writing(borrow));
1855 // Prevent the borrow counter from underflowing.
1856 assert!(borrow != BorrowCounter::MIN);
1857 self.borrow.set(borrow - 1);
1858 BorrowRefMut { borrow: self.borrow }
1859 }
1860}
1861
1862/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1863///
1864/// See the [module-level documentation](self) for more.
1865#[stable(feature = "rust1", since = "1.0.0")]
1866#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1867#[rustc_diagnostic_item = "RefCellRefMut"]
1868pub struct RefMut<'b, T: ?Sized + 'b> {
1869 // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
1870 // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
1871 value: NonNull<T>,
1872 borrow: BorrowRefMut<'b>,
1873 // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
1874 marker: PhantomData<&'b mut T>,
1875}
1876
1877#[stable(feature = "rust1", since = "1.0.0")]
1878#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1879impl<T: ?Sized> const Deref for RefMut<'_, T> {
1880 type Target = T;
1881
1882 #[inline]
1883 fn deref(&self) -> &T {
1884 // SAFETY: the value is accessible as long as we hold our borrow.
1885 unsafe { self.value.as_ref() }
1886 }
1887}
1888
1889#[stable(feature = "rust1", since = "1.0.0")]
1890#[rustc_const_unstable(feature = "const_deref", issue = "88955")]
1891impl<T: ?Sized> const DerefMut for RefMut<'_, T> {
1892 #[inline]
1893 fn deref_mut(&mut self) -> &mut T {
1894 // SAFETY: the value is accessible as long as we hold our borrow.
1895 unsafe { self.value.as_mut() }
1896 }
1897}
1898
1899#[unstable(feature = "deref_pure_trait", issue = "87121")]
1900unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}
1901
1902#[unstable(feature = "coerce_unsized", issue = "18598")]
1903impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1904
1905#[stable(feature = "std_guard_impls", since = "1.20.0")]
1906impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1907 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1908 (**self).fmt(f)
1909 }
1910}
1911
1912/// The core primitive for interior mutability in Rust.
1913///
1914/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1915/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1916/// alias or by transmuting a `&T` into a `&mut T`, is considered undefined behavior.
1917/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1918/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1919///
1920/// All other types that allow internal mutability, such as [`Cell<T>`] and [`RefCell<T>`], internally
1921/// use `UnsafeCell` to wrap their data.
1922///
1923/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1924/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1925/// aliasing `&mut`, not even with `UnsafeCell<T>`.
1926///
1927/// `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
1928/// threads have access to the same `UnsafeCell`, they must follow the usual rules of the
1929/// [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
1930/// [`core::sync::atomic`].
1931///
1932/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1933/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1934/// correctly.
1935///
1936/// [`.get()`]: `UnsafeCell::get`
1937/// [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
1938///
1939/// # Aliasing rules
1940///
1941/// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:
1942///
1943/// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then
1944/// you must not access the data in any way that contradicts that reference for the remainder of
1945/// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it
1946/// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found
1947/// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut
1948/// T` reference that is released to safe code, then you must not access the data within the
1949/// `UnsafeCell` until that reference expires.
1950///
1951/// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data
1952/// until the reference expires. As a special exception, given an `&T`, any part of it that is
1953/// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the
1954/// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part
1955/// of what a reference points to, this means the memory an `&T` points to can be deallocated only if
1956/// *every part of it* (including padding) is inside an `UnsafeCell`.
1957///
1958/// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to
1959/// live memory and the compiler is allowed to insert spurious reads if it can prove that this
1960/// memory has not yet been deallocated.
1961///
1962/// To assist with proper design, the following scenarios are explicitly declared legal
1963/// for single-threaded code:
1964///
1965/// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T`
1966/// references, but not with a `&mut T`
1967///
1968/// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T`
1969/// co-exist with it. A `&mut T` must always be unique.
1970///
1971/// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other
1972/// `&UnsafeCell<T>` references alias the cell) is
1973/// ok (provided you enforce the above invariants some other way), it is still undefined behavior
1974/// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper
1975/// designed to have a special interaction with _shared_ accesses (_i.e._, through an
1976/// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_
1977/// accesses (_e.g._, through a `&mut UnsafeCell<_>`): neither the cell nor the wrapped value
1978/// may be aliased for the duration of that `&mut` borrow.
1979/// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields
1980/// a `&mut T`.
1981///
1982/// [`.get_mut()`]: `UnsafeCell::get_mut`
1983///
1984/// # Memory layout
1985///
1986/// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence
1987/// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`.
1988/// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type
1989/// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche]
1990/// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on
1991/// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space.
1992/// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>`
1993/// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in
1994/// order to avoid its interior mutability property from spreading from `T` into the `Outer` type,
1995/// thus this can cause distortions in the type size in these cases.
1996///
1997/// Note that the only valid way to obtain a `*mut T` pointer to the contents of a
1998/// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference
1999/// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`]
2000/// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the
2001/// same memory layout, the following is not allowed and undefined behavior:
2002///
2003/// ```rust,compile_fail
2004/// # use std::cell::UnsafeCell;
2005/// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T {
2006/// let t = ptr as *const UnsafeCell<T> as *mut T;
2007/// // This is undefined behavior, because the `*mut T` pointer
2008/// // was not obtained through `.get()` nor `.raw_get()`:
2009/// unsafe { &mut *t }
2010/// }
2011/// ```
2012///
2013/// Instead, do this:
2014///
2015/// ```rust
2016/// # use std::cell::UnsafeCell;
2017/// // Safety: the caller must ensure that there are no references that
2018/// // point to the *contents* of the `UnsafeCell`.
2019/// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T {
2020/// unsafe { &mut *ptr.get() }
2021/// }
2022/// ```
2023///
2024/// Converting in the other direction from a `&mut T`
2025/// to an `&UnsafeCell<T>` is allowed:
2026///
2027/// ```rust
2028/// # use std::cell::UnsafeCell;
2029/// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> {
2030/// let t = ptr as *mut T as *const UnsafeCell<T>;
2031/// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout
2032/// unsafe { &*t }
2033/// }
2034/// ```
2035///
2036/// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche
2037/// [`.raw_get()`]: `UnsafeCell::raw_get`
2038///
2039/// # Examples
2040///
2041/// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite
2042/// there being multiple references aliasing the cell:
2043///
2044/// ```
2045/// use std::cell::UnsafeCell;
2046///
2047/// let x: UnsafeCell<i32> = 42.into();
2048/// // Get multiple / concurrent / shared references to the same `x`.
2049/// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x);
2050///
2051/// unsafe {
2052/// // SAFETY: within this scope there are no other references to `x`'s contents,
2053/// // so ours is effectively unique.
2054/// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+
2055/// *p1_exclusive += 27; // |
2056/// } // <---------- cannot go beyond this point -------------------+
2057///
2058/// unsafe {
2059/// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents,
2060/// // so we can have multiple shared accesses concurrently.
2061/// let p2_shared: &i32 = &*p2.get();
2062/// assert_eq!(*p2_shared, 42 + 27);
2063/// let p1_shared: &i32 = &*p1.get();
2064/// assert_eq!(*p1_shared, *p2_shared);
2065/// }
2066/// ```
2067///
2068/// The following example showcases the fact that exclusive access to an `UnsafeCell<T>`
2069/// implies exclusive access to its `T`:
2070///
2071/// ```rust
2072/// #![forbid(unsafe_code)] // with exclusive accesses,
2073/// // `UnsafeCell` is a transparent no-op wrapper,
2074/// // so no need for `unsafe` here.
2075/// use std::cell::UnsafeCell;
2076///
2077/// let mut x: UnsafeCell<i32> = 42.into();
2078///
2079/// // Get a compile-time-checked unique reference to `x`.
2080/// let p_unique: &mut UnsafeCell<i32> = &mut x;
2081/// // With an exclusive reference, we can mutate the contents for free.
2082/// *p_unique.get_mut() = 0;
2083/// // Or, equivalently:
2084/// x = UnsafeCell::new(0);
2085///
2086/// // When we own the value, we can extract the contents for free.
2087/// let contents: i32 = x.into_inner();
2088/// assert_eq!(contents, 0);
2089/// ```
2090#[lang = "unsafe_cell"]
2091#[stable(feature = "rust1", since = "1.0.0")]
2092#[repr(transparent)]
2093#[rustc_pub_transparent]
2094pub struct UnsafeCell<T: ?Sized> {
2095 value: T,
2096}
2097
2098#[stable(feature = "rust1", since = "1.0.0")]
2099impl<T: ?Sized> !Sync for UnsafeCell<T> {}
2100
2101impl<T> UnsafeCell<T> {
2102 /// Constructs a new instance of `UnsafeCell` which will wrap the specified
2103 /// value.
2104 ///
2105 /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code.
2106 ///
2107 /// # Examples
2108 ///
2109 /// ```
2110 /// use std::cell::UnsafeCell;
2111 ///
2112 /// let uc = UnsafeCell::new(5);
2113 /// ```
2114 #[stable(feature = "rust1", since = "1.0.0")]
2115 #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")]
2116 #[inline(always)]
2117 pub const fn new(value: T) -> UnsafeCell<T> {
2118 UnsafeCell { value }
2119 }
2120
2121 /// Unwraps the value, consuming the cell.
2122 ///
2123 /// # Examples
2124 ///
2125 /// ```
2126 /// use std::cell::UnsafeCell;
2127 ///
2128 /// let uc = UnsafeCell::new(5);
2129 ///
2130 /// let five = uc.into_inner();
2131 /// ```
2132 #[inline(always)]
2133 #[stable(feature = "rust1", since = "1.0.0")]
2134 #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
2135 #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
2136 pub const fn into_inner(self) -> T {
2137 self.value
2138 }
2139
2140 /// Replace the value in this `UnsafeCell` and return the old value.
2141 ///
2142 /// # Safety
2143 ///
2144 /// The caller must take care to avoid aliasing and data races.
2145 ///
2146 /// - It is Undefined Behavior to allow calls to race with
2147 /// any other access to the wrapped value.
2148 /// - It is Undefined Behavior to call this while any other
2149 /// reference(s) to the wrapped value are alive.
2150 ///
2151 /// # Examples
2152 ///
2153 /// ```
2154 /// #![feature(unsafe_cell_access)]
2155 /// use std::cell::UnsafeCell;
2156 ///
2157 /// let uc = UnsafeCell::new(5);
2158 ///
2159 /// let old = unsafe { uc.replace(10) };
2160 /// assert_eq!(old, 5);
2161 /// ```
2162 #[inline]
2163 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2164 pub const unsafe fn replace(&self, value: T) -> T {
2165 // SAFETY: pointer comes from `&self` so naturally satisfies invariants.
2166 unsafe { ptr::replace(self.get(), value) }
2167 }
2168}
2169
2170impl<T: ?Sized> UnsafeCell<T> {
2171 /// Converts from `&mut T` to `&mut UnsafeCell<T>`.
2172 ///
2173 /// # Examples
2174 ///
2175 /// ```
2176 /// use std::cell::UnsafeCell;
2177 ///
2178 /// let mut val = 42;
2179 /// let uc = UnsafeCell::from_mut(&mut val);
2180 ///
2181 /// *uc.get_mut() -= 1;
2182 /// assert_eq!(*uc.get_mut(), 41);
2183 /// ```
2184 #[inline(always)]
2185 #[stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
2186 #[rustc_const_stable(feature = "unsafe_cell_from_mut", since = "1.84.0")]
2187 pub const fn from_mut(value: &mut T) -> &mut UnsafeCell<T> {
2188 // SAFETY: `UnsafeCell<T>` has the same memory layout as `T` due to #[repr(transparent)].
2189 unsafe { &mut *(value as *mut T as *mut UnsafeCell<T>) }
2190 }
2191
2192 /// Gets a mutable pointer to the wrapped value.
2193 ///
2194 /// This can be cast to a pointer of any kind. When creating references, you must uphold the
2195 /// aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
2196 /// caveats.
2197 ///
2198 /// # Examples
2199 ///
2200 /// ```
2201 /// use std::cell::UnsafeCell;
2202 ///
2203 /// let uc = UnsafeCell::new(5);
2204 ///
2205 /// let five = uc.get();
2206 /// ```
2207 #[inline(always)]
2208 #[stable(feature = "rust1", since = "1.0.0")]
2209 #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
2210 #[rustc_as_ptr]
2211 #[rustc_never_returns_null_ptr]
2212 pub const fn get(&self) -> *mut T {
2213 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2214 // #[repr(transparent)]. This exploits std's special status, there is
2215 // no guarantee for user code that this will work in future versions of the compiler!
2216 self as *const UnsafeCell<T> as *const T as *mut T
2217 }
2218
2219 /// Returns a mutable reference to the underlying data.
2220 ///
2221 /// This call borrows the `UnsafeCell` mutably (at compile-time) which
2222 /// guarantees that we possess the only reference.
2223 ///
2224 /// # Examples
2225 ///
2226 /// ```
2227 /// use std::cell::UnsafeCell;
2228 ///
2229 /// let mut c = UnsafeCell::new(5);
2230 /// *c.get_mut() += 1;
2231 ///
2232 /// assert_eq!(*c.get_mut(), 6);
2233 /// ```
2234 #[inline(always)]
2235 #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
2236 #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "1.83.0")]
2237 pub const fn get_mut(&mut self) -> &mut T {
2238 &mut self.value
2239 }
2240
2241 /// Gets a mutable pointer to the wrapped value.
2242 /// The difference from [`get`] is that this function accepts a raw pointer,
2243 /// which is useful to avoid the creation of temporary references.
2244 ///
2245 /// This can be cast to a pointer of any kind. When creating references, you must uphold the
2246 /// aliasing rules; see [the type-level docs][UnsafeCell#aliasing-rules] for more discussion and
2247 /// caveats.
2248 ///
2249 /// [`get`]: UnsafeCell::get()
2250 ///
2251 /// # Examples
2252 ///
2253 /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
2254 /// calling `get` would require creating a reference to uninitialized data:
2255 ///
2256 /// ```
2257 /// use std::cell::UnsafeCell;
2258 /// use std::mem::MaybeUninit;
2259 ///
2260 /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
2261 /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
2262 /// // avoid below which references to uninitialized data
2263 /// // unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
2264 /// let uc = unsafe { m.assume_init() };
2265 ///
2266 /// assert_eq!(uc.into_inner(), 5);
2267 /// ```
2268 #[inline(always)]
2269 #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2270 #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2271 #[rustc_diagnostic_item = "unsafe_cell_raw_get"]
2272 pub const fn raw_get(this: *const Self) -> *mut T {
2273 // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2274 // #[repr(transparent)]. This exploits std's special status, there is
2275 // no guarantee for user code that this will work in future versions of the compiler!
2276 this as *const T as *mut T
2277 }
2278
2279 /// Get a shared reference to the value within the `UnsafeCell`.
2280 ///
2281 /// # Safety
2282 ///
2283 /// - It is Undefined Behavior to call this while any mutable
2284 /// reference to the wrapped value is alive.
2285 /// - Mutating the wrapped value while the returned
2286 /// reference is alive is Undefined Behavior.
2287 ///
2288 /// # Examples
2289 ///
2290 /// ```
2291 /// #![feature(unsafe_cell_access)]
2292 /// use std::cell::UnsafeCell;
2293 ///
2294 /// let uc = UnsafeCell::new(5);
2295 ///
2296 /// let val = unsafe { uc.as_ref_unchecked() };
2297 /// assert_eq!(val, &5);
2298 /// ```
2299 #[inline]
2300 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2301 pub const unsafe fn as_ref_unchecked(&self) -> &T {
2302 // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2303 unsafe { self.get().as_ref_unchecked() }
2304 }
2305
2306 /// Get an exclusive reference to the value within the `UnsafeCell`.
2307 ///
2308 /// # Safety
2309 ///
2310 /// - It is Undefined Behavior to call this while any other
2311 /// reference(s) to the wrapped value are alive.
2312 /// - Mutating the wrapped value through other means while the
2313 /// returned reference is alive is Undefined Behavior.
2314 ///
2315 /// # Examples
2316 ///
2317 /// ```
2318 /// #![feature(unsafe_cell_access)]
2319 /// use std::cell::UnsafeCell;
2320 ///
2321 /// let uc = UnsafeCell::new(5);
2322 ///
2323 /// unsafe { *uc.as_mut_unchecked() += 1; }
2324 /// assert_eq!(uc.into_inner(), 6);
2325 /// ```
2326 #[inline]
2327 #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2328 #[allow(clippy::mut_from_ref)]
2329 pub const unsafe fn as_mut_unchecked(&self) -> &mut T {
2330 // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2331 unsafe { self.get().as_mut_unchecked() }
2332 }
2333}
2334
2335#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
2336impl<T: Default> Default for UnsafeCell<T> {
2337 /// Creates an `UnsafeCell`, with the `Default` value for T.
2338 fn default() -> UnsafeCell<T> {
2339 UnsafeCell::new(Default::default())
2340 }
2341}
2342
2343#[stable(feature = "cell_from", since = "1.12.0")]
2344impl<T> From<T> for UnsafeCell<T> {
2345 /// Creates a new `UnsafeCell<T>` containing the given value.
2346 fn from(t: T) -> UnsafeCell<T> {
2347 UnsafeCell::new(t)
2348 }
2349}
2350
2351#[unstable(feature = "coerce_unsized", issue = "18598")]
2352impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
2353
2354// Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
2355// and become dyn-compatible method receivers.
2356// Note that currently `UnsafeCell` itself cannot be a method receiver
2357// because it does not implement Deref.
2358// In other words:
2359// `self: UnsafeCell<&Self>` won't work
2360// `self: UnsafeCellWrapper<Self>` becomes possible
2361#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2362impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
2363
2364#[unstable(feature = "pointer_like_trait", issue = "none")]
2365impl<T: PointerLike> PointerLike for UnsafeCell<T> {}
2366
2367/// [`UnsafeCell`], but [`Sync`].
2368///
2369/// This is just an `UnsafeCell`, except it implements `Sync`
2370/// if `T` implements `Sync`.
2371///
2372/// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
2373/// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
2374/// shared between threads, if that's intentional.
2375/// Providing proper synchronization is still the task of the user,
2376/// making this type just as unsafe to use.
2377///
2378/// See [`UnsafeCell`] for details.
2379#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2380#[repr(transparent)]
2381#[rustc_diagnostic_item = "SyncUnsafeCell"]
2382#[rustc_pub_transparent]
2383pub struct SyncUnsafeCell<T: ?Sized> {
2384 value: UnsafeCell<T>,
2385}
2386
2387#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2388unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
2389
2390#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2391impl<T> SyncUnsafeCell<T> {
2392 /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
2393 #[inline]
2394 pub const fn new(value: T) -> Self {
2395 Self { value: UnsafeCell { value } }
2396 }
2397
2398 /// Unwraps the value, consuming the cell.
2399 #[inline]
2400 #[rustc_const_unstable(feature = "sync_unsafe_cell", issue = "95439")]
2401 pub const fn into_inner(self) -> T {
2402 self.value.into_inner()
2403 }
2404}
2405
2406#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2407impl<T: ?Sized> SyncUnsafeCell<T> {
2408 /// Gets a mutable pointer to the wrapped value.
2409 ///
2410 /// This can be cast to a pointer of any kind.
2411 /// Ensure that the access is unique (no active references, mutable or not)
2412 /// when casting to `&mut T`, and ensure that there are no mutations
2413 /// or mutable aliases going on when casting to `&T`
2414 #[inline]
2415 #[rustc_as_ptr]
2416 #[rustc_never_returns_null_ptr]
2417 pub const fn get(&self) -> *mut T {
2418 self.value.get()
2419 }
2420
2421 /// Returns a mutable reference to the underlying data.
2422 ///
2423 /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
2424 /// guarantees that we possess the only reference.
2425 #[inline]
2426 pub const fn get_mut(&mut self) -> &mut T {
2427 self.value.get_mut()
2428 }
2429
2430 /// Gets a mutable pointer to the wrapped value.
2431 ///
2432 /// See [`UnsafeCell::get`] for details.
2433 #[inline]
2434 pub const fn raw_get(this: *const Self) -> *mut T {
2435 // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
2436 // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
2437 // See UnsafeCell::raw_get.
2438 this as *const T as *mut T
2439 }
2440}
2441
2442#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2443impl<T: Default> Default for SyncUnsafeCell<T> {
2444 /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
2445 fn default() -> SyncUnsafeCell<T> {
2446 SyncUnsafeCell::new(Default::default())
2447 }
2448}
2449
2450#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2451impl<T> From<T> for SyncUnsafeCell<T> {
2452 /// Creates a new `SyncUnsafeCell<T>` containing the given value.
2453 fn from(t: T) -> SyncUnsafeCell<T> {
2454 SyncUnsafeCell::new(t)
2455 }
2456}
2457
2458#[unstable(feature = "coerce_unsized", issue = "18598")]
2459//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2460impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2461
2462// Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
2463// and become dyn-compatible method receivers.
2464// Note that currently `SyncUnsafeCell` itself cannot be a method receiver
2465// because it does not implement Deref.
2466// In other words:
2467// `self: SyncUnsafeCell<&Self>` won't work
2468// `self: SyncUnsafeCellWrapper<Self>` becomes possible
2469#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2470//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2471impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2472
2473#[unstable(feature = "pointer_like_trait", issue = "none")]
2474impl<T: PointerLike> PointerLike for SyncUnsafeCell<T> {}
2475
2476#[allow(unused)]
2477fn assert_coerce_unsized(
2478 a: UnsafeCell<&i32>,
2479 b: SyncUnsafeCell<&i32>,
2480 c: Cell<&i32>,
2481 d: RefCell<&i32>,
2482) {
2483 let _: UnsafeCell<&dyn Send> = a;
2484 let _: SyncUnsafeCell<&dyn Send> = b;
2485 let _: Cell<&dyn Send> = c;
2486 let _: RefCell<&dyn Send> = d;
2487}
2488
2489#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2490unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
2491
2492#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2493unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}
2494
2495#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2496unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
2497
2498#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2499unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}
2500
2501#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2502unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}
2503
2504#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2505unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}