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