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::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 = "CURRENT_RUSTC_VERSION")]
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 = "CURRENT_RUSTC_VERSION")]
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    /// #![feature(cell_update)]
553    ///
554    /// use std::cell::Cell;
555    ///
556    /// let c = Cell::new(5);
557    /// c.update(|x| x + 1);
558    /// assert_eq!(c.get(), 6);
559    /// ```
560    #[inline]
561    #[unstable(feature = "cell_update", issue = "50186")]
562    pub fn update(&self, f: impl FnOnce(T) -> T) {
563        let old = self.get();
564        self.set(f(old));
565    }
566}
567
568impl<T: ?Sized> Cell<T> {
569    /// Returns a raw pointer to the underlying data in this cell.
570    ///
571    /// # Examples
572    ///
573    /// ```
574    /// use std::cell::Cell;
575    ///
576    /// let c = Cell::new(5);
577    ///
578    /// let ptr = c.as_ptr();
579    /// ```
580    #[inline]
581    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
582    #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")]
583    #[rustc_as_ptr]
584    #[rustc_never_returns_null_ptr]
585    pub const fn as_ptr(&self) -> *mut T {
586        self.value.get()
587    }
588
589    /// Returns a mutable reference to the underlying data.
590    ///
591    /// This call borrows `Cell` mutably (at compile-time) which guarantees
592    /// that we possess the only reference.
593    ///
594    /// However be cautious: this method expects `self` to be mutable, which is
595    /// generally not the case when using a `Cell`. If you require interior
596    /// mutability by reference, consider using `RefCell` which provides
597    /// run-time checked mutable borrows through its [`borrow_mut`] method.
598    ///
599    /// [`borrow_mut`]: RefCell::borrow_mut()
600    ///
601    /// # Examples
602    ///
603    /// ```
604    /// use std::cell::Cell;
605    ///
606    /// let mut c = Cell::new(5);
607    /// *c.get_mut() += 1;
608    ///
609    /// assert_eq!(c.get(), 6);
610    /// ```
611    #[inline]
612    #[stable(feature = "cell_get_mut", since = "1.11.0")]
613    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
614    pub const fn get_mut(&mut self) -> &mut T {
615        self.value.get_mut()
616    }
617
618    /// Returns a `&Cell<T>` from a `&mut T`
619    ///
620    /// # Examples
621    ///
622    /// ```
623    /// use std::cell::Cell;
624    ///
625    /// let slice: &mut [i32] = &mut [1, 2, 3];
626    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
627    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
628    ///
629    /// assert_eq!(slice_cell.len(), 3);
630    /// ```
631    #[inline]
632    #[stable(feature = "as_cell", since = "1.37.0")]
633    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
634    pub const fn from_mut(t: &mut T) -> &Cell<T> {
635        // SAFETY: `&mut` ensures unique access.
636        unsafe { &*(t as *mut T as *const Cell<T>) }
637    }
638}
639
640impl<T: Default> Cell<T> {
641    /// Takes the value of the cell, leaving `Default::default()` in its place.
642    ///
643    /// # Examples
644    ///
645    /// ```
646    /// use std::cell::Cell;
647    ///
648    /// let c = Cell::new(5);
649    /// let five = c.take();
650    ///
651    /// assert_eq!(five, 5);
652    /// assert_eq!(c.into_inner(), 0);
653    /// ```
654    #[stable(feature = "move_cell", since = "1.17.0")]
655    pub fn take(&self) -> T {
656        self.replace(Default::default())
657    }
658}
659
660#[unstable(feature = "coerce_unsized", issue = "18598")]
661impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {}
662
663// Allow types that wrap `Cell` to also implement `DispatchFromDyn`
664// and become dyn-compatible method receivers.
665// Note that currently `Cell` itself cannot be a method receiver
666// because it does not implement Deref.
667// In other words:
668// `self: Cell<&Self>` won't work
669// `self: CellWrapper<Self>` becomes possible
670#[unstable(feature = "dispatch_from_dyn", issue = "none")]
671impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<Cell<U>> for Cell<T> {}
672
673#[unstable(feature = "pointer_like_trait", issue = "none")]
674impl<T: PointerLike> PointerLike for Cell<T> {}
675
676impl<T> Cell<[T]> {
677    /// Returns a `&[Cell<T>]` from a `&Cell<[T]>`
678    ///
679    /// # Examples
680    ///
681    /// ```
682    /// use std::cell::Cell;
683    ///
684    /// let slice: &mut [i32] = &mut [1, 2, 3];
685    /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice);
686    /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells();
687    ///
688    /// assert_eq!(slice_cell.len(), 3);
689    /// ```
690    #[stable(feature = "as_cell", since = "1.37.0")]
691    #[rustc_const_stable(feature = "const_cell", since = "CURRENT_RUSTC_VERSION")]
692    pub const fn as_slice_of_cells(&self) -> &[Cell<T>] {
693        // SAFETY: `Cell<T>` has the same memory layout as `T`.
694        unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) }
695    }
696}
697
698impl<T, const N: usize> Cell<[T; N]> {
699    /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>`
700    ///
701    /// # Examples
702    ///
703    /// ```
704    /// #![feature(as_array_of_cells)]
705    /// use std::cell::Cell;
706    ///
707    /// let mut array: [i32; 3] = [1, 2, 3];
708    /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array);
709    /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells();
710    /// ```
711    #[unstable(feature = "as_array_of_cells", issue = "88248")]
712    pub const fn as_array_of_cells(&self) -> &[Cell<T>; N] {
713        // SAFETY: `Cell<T>` has the same memory layout as `T`.
714        unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) }
715    }
716}
717
718/// A mutable memory location with dynamically checked borrow rules
719///
720/// See the [module-level documentation](self) for more.
721#[rustc_diagnostic_item = "RefCell"]
722#[stable(feature = "rust1", since = "1.0.0")]
723pub struct RefCell<T: ?Sized> {
724    borrow: Cell<BorrowFlag>,
725    // Stores the location of the earliest currently active borrow.
726    // This gets updated whenever we go from having zero borrows
727    // to having a single borrow. When a borrow occurs, this gets included
728    // in the generated `BorrowError`/`BorrowMutError`
729    #[cfg(feature = "debug_refcell")]
730    borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>,
731    value: UnsafeCell<T>,
732}
733
734/// An error returned by [`RefCell::try_borrow`].
735#[stable(feature = "try_borrow", since = "1.13.0")]
736#[non_exhaustive]
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 Debug for BorrowError {
744    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
745        let mut builder = f.debug_struct("BorrowError");
746
747        #[cfg(feature = "debug_refcell")]
748        builder.field("location", self.location);
749
750        builder.finish()
751    }
752}
753
754#[stable(feature = "try_borrow", since = "1.13.0")]
755impl Display for BorrowError {
756    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
757        Display::fmt("already mutably borrowed", f)
758    }
759}
760
761/// An error returned by [`RefCell::try_borrow_mut`].
762#[stable(feature = "try_borrow", since = "1.13.0")]
763#[non_exhaustive]
764pub struct BorrowMutError {
765    #[cfg(feature = "debug_refcell")]
766    location: &'static crate::panic::Location<'static>,
767}
768
769#[stable(feature = "try_borrow", since = "1.13.0")]
770impl Debug for BorrowMutError {
771    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
772        let mut builder = f.debug_struct("BorrowMutError");
773
774        #[cfg(feature = "debug_refcell")]
775        builder.field("location", self.location);
776
777        builder.finish()
778    }
779}
780
781#[stable(feature = "try_borrow", since = "1.13.0")]
782impl Display for BorrowMutError {
783    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
784        Display::fmt("already borrowed", f)
785    }
786}
787
788// This ensures the panicking code is outlined from `borrow_mut` for `RefCell`.
789#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
790#[track_caller]
791#[cold]
792fn panic_already_borrowed(err: BorrowMutError) -> ! {
793    panic!("already borrowed: {:?}", err)
794}
795
796// This ensures the panicking code is outlined from `borrow` for `RefCell`.
797#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
798#[track_caller]
799#[cold]
800fn panic_already_mutably_borrowed(err: BorrowError) -> ! {
801    panic!("already mutably borrowed: {:?}", err)
802}
803
804// Positive values represent the number of `Ref` active. Negative values
805// represent the number of `RefMut` active. Multiple `RefMut`s can only be
806// active at a time if they refer to distinct, nonoverlapping components of a
807// `RefCell` (e.g., different ranges of a slice).
808//
809// `Ref` and `RefMut` are both two words in size, and so there will likely never
810// be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize`
811// range. Thus, a `BorrowFlag` will probably never overflow or underflow.
812// However, this is not a guarantee, as a pathological program could repeatedly
813// create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must
814// explicitly check for overflow and underflow in order to avoid unsafety, or at
815// least behave correctly in the event that overflow or underflow happens (e.g.,
816// see BorrowRef::new).
817type BorrowFlag = isize;
818const UNUSED: BorrowFlag = 0;
819
820#[inline(always)]
821fn is_writing(x: BorrowFlag) -> bool {
822    x < UNUSED
823}
824
825#[inline(always)]
826fn is_reading(x: BorrowFlag) -> bool {
827    x > UNUSED
828}
829
830impl<T> RefCell<T> {
831    /// Creates a new `RefCell` containing `value`.
832    ///
833    /// # Examples
834    ///
835    /// ```
836    /// use std::cell::RefCell;
837    ///
838    /// let c = RefCell::new(5);
839    /// ```
840    #[stable(feature = "rust1", since = "1.0.0")]
841    #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")]
842    #[inline]
843    pub const fn new(value: T) -> RefCell<T> {
844        RefCell {
845            value: UnsafeCell::new(value),
846            borrow: Cell::new(UNUSED),
847            #[cfg(feature = "debug_refcell")]
848            borrowed_at: Cell::new(None),
849        }
850    }
851
852    /// Consumes the `RefCell`, returning the wrapped value.
853    ///
854    /// # Examples
855    ///
856    /// ```
857    /// use std::cell::RefCell;
858    ///
859    /// let c = RefCell::new(5);
860    ///
861    /// let five = c.into_inner();
862    /// ```
863    #[stable(feature = "rust1", since = "1.0.0")]
864    #[rustc_const_stable(feature = "const_cell_into_inner", since = "1.83.0")]
865    #[rustc_allow_const_fn_unstable(const_precise_live_drops)]
866    #[inline]
867    pub const fn into_inner(self) -> T {
868        // Since this function takes `self` (the `RefCell`) by value, the
869        // compiler statically verifies that it is not currently borrowed.
870        self.value.into_inner()
871    }
872
873    /// Replaces the wrapped value with a new one, returning the old value,
874    /// without deinitializing either one.
875    ///
876    /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html).
877    ///
878    /// # Panics
879    ///
880    /// Panics if the value is currently borrowed.
881    ///
882    /// # Examples
883    ///
884    /// ```
885    /// use std::cell::RefCell;
886    /// let cell = RefCell::new(5);
887    /// let old_value = cell.replace(6);
888    /// assert_eq!(old_value, 5);
889    /// assert_eq!(cell, RefCell::new(6));
890    /// ```
891    #[inline]
892    #[stable(feature = "refcell_replace", since = "1.24.0")]
893    #[track_caller]
894    #[rustc_confusables("swap")]
895    pub fn replace(&self, t: T) -> T {
896        mem::replace(&mut *self.borrow_mut(), t)
897    }
898
899    /// Replaces the wrapped value with a new one computed from `f`, returning
900    /// the old value, without deinitializing either one.
901    ///
902    /// # Panics
903    ///
904    /// Panics if the value is currently borrowed.
905    ///
906    /// # Examples
907    ///
908    /// ```
909    /// use std::cell::RefCell;
910    /// let cell = RefCell::new(5);
911    /// let old_value = cell.replace_with(|&mut old| old + 1);
912    /// assert_eq!(old_value, 5);
913    /// assert_eq!(cell, RefCell::new(6));
914    /// ```
915    #[inline]
916    #[stable(feature = "refcell_replace_swap", since = "1.35.0")]
917    #[track_caller]
918    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
919        let mut_borrow = &mut *self.borrow_mut();
920        let replacement = f(mut_borrow);
921        mem::replace(mut_borrow, replacement)
922    }
923
924    /// Swaps the wrapped value of `self` with the wrapped value of `other`,
925    /// without deinitializing either one.
926    ///
927    /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html).
928    ///
929    /// # Panics
930    ///
931    /// Panics if the value in either `RefCell` is currently borrowed, or
932    /// if `self` and `other` point to the same `RefCell`.
933    ///
934    /// # Examples
935    ///
936    /// ```
937    /// use std::cell::RefCell;
938    /// let c = RefCell::new(5);
939    /// let d = RefCell::new(6);
940    /// c.swap(&d);
941    /// assert_eq!(c, RefCell::new(6));
942    /// assert_eq!(d, RefCell::new(5));
943    /// ```
944    #[inline]
945    #[stable(feature = "refcell_swap", since = "1.24.0")]
946    pub fn swap(&self, other: &Self) {
947        mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
948    }
949}
950
951impl<T: ?Sized> RefCell<T> {
952    /// Immutably borrows the wrapped value.
953    ///
954    /// The borrow lasts until the returned `Ref` exits scope. Multiple
955    /// immutable borrows can be taken out at the same time.
956    ///
957    /// # Panics
958    ///
959    /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
960    /// [`try_borrow`](#method.try_borrow).
961    ///
962    /// # Examples
963    ///
964    /// ```
965    /// use std::cell::RefCell;
966    ///
967    /// let c = RefCell::new(5);
968    ///
969    /// let borrowed_five = c.borrow();
970    /// let borrowed_five2 = c.borrow();
971    /// ```
972    ///
973    /// An example of panic:
974    ///
975    /// ```should_panic
976    /// use std::cell::RefCell;
977    ///
978    /// let c = RefCell::new(5);
979    ///
980    /// let m = c.borrow_mut();
981    /// let b = c.borrow(); // this causes a panic
982    /// ```
983    #[stable(feature = "rust1", since = "1.0.0")]
984    #[inline]
985    #[track_caller]
986    pub fn borrow(&self) -> Ref<'_, T> {
987        match self.try_borrow() {
988            Ok(b) => b,
989            Err(err) => panic_already_mutably_borrowed(err),
990        }
991    }
992
993    /// Immutably borrows the wrapped value, returning an error if the value is currently mutably
994    /// borrowed.
995    ///
996    /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be
997    /// taken out at the same time.
998    ///
999    /// This is the non-panicking variant of [`borrow`](#method.borrow).
1000    ///
1001    /// # Examples
1002    ///
1003    /// ```
1004    /// use std::cell::RefCell;
1005    ///
1006    /// let c = RefCell::new(5);
1007    ///
1008    /// {
1009    ///     let m = c.borrow_mut();
1010    ///     assert!(c.try_borrow().is_err());
1011    /// }
1012    ///
1013    /// {
1014    ///     let m = c.borrow();
1015    ///     assert!(c.try_borrow().is_ok());
1016    /// }
1017    /// ```
1018    #[stable(feature = "try_borrow", since = "1.13.0")]
1019    #[inline]
1020    #[cfg_attr(feature = "debug_refcell", track_caller)]
1021    pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
1022        match BorrowRef::new(&self.borrow) {
1023            Some(b) => {
1024                #[cfg(feature = "debug_refcell")]
1025                {
1026                    // `borrowed_at` is always the *first* active borrow
1027                    if b.borrow.get() == 1 {
1028                        self.borrowed_at.set(Some(crate::panic::Location::caller()));
1029                    }
1030                }
1031
1032                // SAFETY: `BorrowRef` ensures that there is only immutable access
1033                // to the value while borrowed.
1034                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1035                Ok(Ref { value, borrow: b })
1036            }
1037            None => Err(BorrowError {
1038                // If a borrow occurred, then we must already have an outstanding borrow,
1039                // so `borrowed_at` will be `Some`
1040                #[cfg(feature = "debug_refcell")]
1041                location: self.borrowed_at.get().unwrap(),
1042            }),
1043        }
1044    }
1045
1046    /// Mutably borrows the wrapped value.
1047    ///
1048    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1049    /// from it exit scope. The value cannot be borrowed while this borrow is
1050    /// active.
1051    ///
1052    /// # Panics
1053    ///
1054    /// Panics if the value is currently borrowed. For a non-panicking variant, use
1055    /// [`try_borrow_mut`](#method.try_borrow_mut).
1056    ///
1057    /// # Examples
1058    ///
1059    /// ```
1060    /// use std::cell::RefCell;
1061    ///
1062    /// let c = RefCell::new("hello".to_owned());
1063    ///
1064    /// *c.borrow_mut() = "bonjour".to_owned();
1065    ///
1066    /// assert_eq!(&*c.borrow(), "bonjour");
1067    /// ```
1068    ///
1069    /// An example of panic:
1070    ///
1071    /// ```should_panic
1072    /// use std::cell::RefCell;
1073    ///
1074    /// let c = RefCell::new(5);
1075    /// let m = c.borrow();
1076    ///
1077    /// let b = c.borrow_mut(); // this causes a panic
1078    /// ```
1079    #[stable(feature = "rust1", since = "1.0.0")]
1080    #[inline]
1081    #[track_caller]
1082    pub fn borrow_mut(&self) -> RefMut<'_, T> {
1083        match self.try_borrow_mut() {
1084            Ok(b) => b,
1085            Err(err) => panic_already_borrowed(err),
1086        }
1087    }
1088
1089    /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed.
1090    ///
1091    /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived
1092    /// from it exit scope. The value cannot be borrowed while this borrow is
1093    /// active.
1094    ///
1095    /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut).
1096    ///
1097    /// # Examples
1098    ///
1099    /// ```
1100    /// use std::cell::RefCell;
1101    ///
1102    /// let c = RefCell::new(5);
1103    ///
1104    /// {
1105    ///     let m = c.borrow();
1106    ///     assert!(c.try_borrow_mut().is_err());
1107    /// }
1108    ///
1109    /// assert!(c.try_borrow_mut().is_ok());
1110    /// ```
1111    #[stable(feature = "try_borrow", since = "1.13.0")]
1112    #[inline]
1113    #[cfg_attr(feature = "debug_refcell", track_caller)]
1114    pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
1115        match BorrowRefMut::new(&self.borrow) {
1116            Some(b) => {
1117                #[cfg(feature = "debug_refcell")]
1118                {
1119                    self.borrowed_at.set(Some(crate::panic::Location::caller()));
1120                }
1121
1122                // SAFETY: `BorrowRefMut` guarantees unique access.
1123                let value = unsafe { NonNull::new_unchecked(self.value.get()) };
1124                Ok(RefMut { value, borrow: b, marker: PhantomData })
1125            }
1126            None => Err(BorrowMutError {
1127                // If a borrow occurred, then we must already have an outstanding borrow,
1128                // so `borrowed_at` will be `Some`
1129                #[cfg(feature = "debug_refcell")]
1130                location: self.borrowed_at.get().unwrap(),
1131            }),
1132        }
1133    }
1134
1135    /// Returns a raw pointer to the underlying data in this cell.
1136    ///
1137    /// # Examples
1138    ///
1139    /// ```
1140    /// use std::cell::RefCell;
1141    ///
1142    /// let c = RefCell::new(5);
1143    ///
1144    /// let ptr = c.as_ptr();
1145    /// ```
1146    #[inline]
1147    #[stable(feature = "cell_as_ptr", since = "1.12.0")]
1148    #[rustc_as_ptr]
1149    #[rustc_never_returns_null_ptr]
1150    pub fn as_ptr(&self) -> *mut T {
1151        self.value.get()
1152    }
1153
1154    /// Returns a mutable reference to the underlying data.
1155    ///
1156    /// Since this method borrows `RefCell` mutably, it is statically guaranteed
1157    /// that no borrows to the underlying data exist. The dynamic checks inherent
1158    /// in [`borrow_mut`] and most other methods of `RefCell` are therefore
1159    /// unnecessary. Note that this method does not reset the borrowing state if borrows were previously leaked
1160    /// (e.g., via [`forget()`] on a [`Ref`] or [`RefMut`]). For that purpose,
1161    /// consider using the unstable [`undo_leak`] method.
1162    ///
1163    /// This method can only be called if `RefCell` can be mutably borrowed,
1164    /// which in general is only the case directly after the `RefCell` has
1165    /// been created. In these situations, skipping the aforementioned dynamic
1166    /// borrowing checks may yield better ergonomics and runtime-performance.
1167    ///
1168    /// In most situations where `RefCell` is used, it can't be borrowed mutably.
1169    /// Use [`borrow_mut`] to get mutable access to the underlying data then.
1170    ///
1171    /// [`borrow_mut`]: RefCell::borrow_mut()
1172    /// [`forget()`]: mem::forget
1173    /// [`undo_leak`]: RefCell::undo_leak()
1174    ///
1175    /// # Examples
1176    ///
1177    /// ```
1178    /// use std::cell::RefCell;
1179    ///
1180    /// let mut c = RefCell::new(5);
1181    /// *c.get_mut() += 1;
1182    ///
1183    /// assert_eq!(c, RefCell::new(6));
1184    /// ```
1185    #[inline]
1186    #[stable(feature = "cell_get_mut", since = "1.11.0")]
1187    pub fn get_mut(&mut self) -> &mut T {
1188        self.value.get_mut()
1189    }
1190
1191    /// Undo the effect of leaked guards on the borrow state of the `RefCell`.
1192    ///
1193    /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to
1194    /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant
1195    /// if some `Ref` or `RefMut` borrows have been leaked.
1196    ///
1197    /// [`get_mut`]: RefCell::get_mut()
1198    ///
1199    /// # Examples
1200    ///
1201    /// ```
1202    /// #![feature(cell_leak)]
1203    /// use std::cell::RefCell;
1204    ///
1205    /// let mut c = RefCell::new(0);
1206    /// std::mem::forget(c.borrow_mut());
1207    ///
1208    /// assert!(c.try_borrow().is_err());
1209    /// c.undo_leak();
1210    /// assert!(c.try_borrow().is_ok());
1211    /// ```
1212    #[unstable(feature = "cell_leak", issue = "69099")]
1213    pub fn undo_leak(&mut self) -> &mut T {
1214        *self.borrow.get_mut() = UNUSED;
1215        self.get_mut()
1216    }
1217
1218    /// Immutably borrows the wrapped value, returning an error if the value is
1219    /// currently mutably borrowed.
1220    ///
1221    /// # Safety
1222    ///
1223    /// Unlike `RefCell::borrow`, this method is unsafe because it does not
1224    /// return a `Ref`, thus leaving the borrow flag untouched. Mutably
1225    /// borrowing the `RefCell` while the reference returned by this method
1226    /// is alive is undefined behavior.
1227    ///
1228    /// # Examples
1229    ///
1230    /// ```
1231    /// use std::cell::RefCell;
1232    ///
1233    /// let c = RefCell::new(5);
1234    ///
1235    /// {
1236    ///     let m = c.borrow_mut();
1237    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_err());
1238    /// }
1239    ///
1240    /// {
1241    ///     let m = c.borrow();
1242    ///     assert!(unsafe { c.try_borrow_unguarded() }.is_ok());
1243    /// }
1244    /// ```
1245    #[stable(feature = "borrow_state", since = "1.37.0")]
1246    #[inline]
1247    pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> {
1248        if !is_writing(self.borrow.get()) {
1249            // SAFETY: We check that nobody is actively writing now, but it is
1250            // the caller's responsibility to ensure that nobody writes until
1251            // the returned reference is no longer in use.
1252            // Also, `self.value.get()` refers to the value owned by `self`
1253            // and is thus guaranteed to be valid for the lifetime of `self`.
1254            Ok(unsafe { &*self.value.get() })
1255        } else {
1256            Err(BorrowError {
1257                // If a borrow occurred, then we must already have an outstanding borrow,
1258                // so `borrowed_at` will be `Some`
1259                #[cfg(feature = "debug_refcell")]
1260                location: self.borrowed_at.get().unwrap(),
1261            })
1262        }
1263    }
1264}
1265
1266impl<T: Default> RefCell<T> {
1267    /// Takes the wrapped value, leaving `Default::default()` in its place.
1268    ///
1269    /// # Panics
1270    ///
1271    /// Panics if the value is currently borrowed.
1272    ///
1273    /// # Examples
1274    ///
1275    /// ```
1276    /// use std::cell::RefCell;
1277    ///
1278    /// let c = RefCell::new(5);
1279    /// let five = c.take();
1280    ///
1281    /// assert_eq!(five, 5);
1282    /// assert_eq!(c.into_inner(), 0);
1283    /// ```
1284    #[stable(feature = "refcell_take", since = "1.50.0")]
1285    pub fn take(&self) -> T {
1286        self.replace(Default::default())
1287    }
1288}
1289
1290#[stable(feature = "rust1", since = "1.0.0")]
1291unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {}
1292
1293#[stable(feature = "rust1", since = "1.0.0")]
1294impl<T: ?Sized> !Sync for RefCell<T> {}
1295
1296#[stable(feature = "rust1", since = "1.0.0")]
1297impl<T: Clone> Clone for RefCell<T> {
1298    /// # Panics
1299    ///
1300    /// Panics if the value is currently mutably borrowed.
1301    #[inline]
1302    #[track_caller]
1303    fn clone(&self) -> RefCell<T> {
1304        RefCell::new(self.borrow().clone())
1305    }
1306
1307    /// # Panics
1308    ///
1309    /// Panics if `source` is currently mutably borrowed.
1310    #[inline]
1311    #[track_caller]
1312    fn clone_from(&mut self, source: &Self) {
1313        self.get_mut().clone_from(&source.borrow())
1314    }
1315}
1316
1317#[stable(feature = "rust1", since = "1.0.0")]
1318impl<T: Default> Default for RefCell<T> {
1319    /// Creates a `RefCell<T>`, with the `Default` value for T.
1320    #[inline]
1321    fn default() -> RefCell<T> {
1322        RefCell::new(Default::default())
1323    }
1324}
1325
1326#[stable(feature = "rust1", since = "1.0.0")]
1327impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> {
1328    /// # Panics
1329    ///
1330    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1331    #[inline]
1332    fn eq(&self, other: &RefCell<T>) -> bool {
1333        *self.borrow() == *other.borrow()
1334    }
1335}
1336
1337#[stable(feature = "cell_eq", since = "1.2.0")]
1338impl<T: ?Sized + Eq> Eq for RefCell<T> {}
1339
1340#[stable(feature = "cell_ord", since = "1.10.0")]
1341impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> {
1342    /// # Panics
1343    ///
1344    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1345    #[inline]
1346    fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> {
1347        self.borrow().partial_cmp(&*other.borrow())
1348    }
1349
1350    /// # Panics
1351    ///
1352    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1353    #[inline]
1354    fn lt(&self, other: &RefCell<T>) -> bool {
1355        *self.borrow() < *other.borrow()
1356    }
1357
1358    /// # Panics
1359    ///
1360    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1361    #[inline]
1362    fn le(&self, other: &RefCell<T>) -> bool {
1363        *self.borrow() <= *other.borrow()
1364    }
1365
1366    /// # Panics
1367    ///
1368    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1369    #[inline]
1370    fn gt(&self, other: &RefCell<T>) -> bool {
1371        *self.borrow() > *other.borrow()
1372    }
1373
1374    /// # Panics
1375    ///
1376    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1377    #[inline]
1378    fn ge(&self, other: &RefCell<T>) -> bool {
1379        *self.borrow() >= *other.borrow()
1380    }
1381}
1382
1383#[stable(feature = "cell_ord", since = "1.10.0")]
1384impl<T: ?Sized + Ord> Ord for RefCell<T> {
1385    /// # Panics
1386    ///
1387    /// Panics if the value in either `RefCell` is currently mutably borrowed.
1388    #[inline]
1389    fn cmp(&self, other: &RefCell<T>) -> Ordering {
1390        self.borrow().cmp(&*other.borrow())
1391    }
1392}
1393
1394#[stable(feature = "cell_from", since = "1.12.0")]
1395impl<T> From<T> for RefCell<T> {
1396    /// Creates a new `RefCell<T>` containing the given value.
1397    fn from(t: T) -> RefCell<T> {
1398        RefCell::new(t)
1399    }
1400}
1401
1402#[unstable(feature = "coerce_unsized", issue = "18598")]
1403impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {}
1404
1405struct BorrowRef<'b> {
1406    borrow: &'b Cell<BorrowFlag>,
1407}
1408
1409impl<'b> BorrowRef<'b> {
1410    #[inline]
1411    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> {
1412        let b = borrow.get().wrapping_add(1);
1413        if !is_reading(b) {
1414            // Incrementing borrow can result in a non-reading value (<= 0) in these cases:
1415            // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow
1416            //    due to Rust's reference aliasing rules
1417            // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed
1418            //    into isize::MIN (the max amount of writing borrows) so we can't allow
1419            //    an additional read borrow because isize can't represent so many read borrows
1420            //    (this can only happen if you mem::forget more than a small constant amount of
1421            //    `Ref`s, which is not good practice)
1422            None
1423        } else {
1424            // Incrementing borrow can result in a reading value (> 0) in these cases:
1425            // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow
1426            // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize
1427            //    is large enough to represent having one more read borrow
1428            borrow.set(b);
1429            Some(BorrowRef { borrow })
1430        }
1431    }
1432}
1433
1434impl Drop for BorrowRef<'_> {
1435    #[inline]
1436    fn drop(&mut self) {
1437        let borrow = self.borrow.get();
1438        debug_assert!(is_reading(borrow));
1439        self.borrow.set(borrow - 1);
1440    }
1441}
1442
1443impl Clone for BorrowRef<'_> {
1444    #[inline]
1445    fn clone(&self) -> Self {
1446        // Since this Ref exists, we know the borrow flag
1447        // is a reading borrow.
1448        let borrow = self.borrow.get();
1449        debug_assert!(is_reading(borrow));
1450        // Prevent the borrow counter from overflowing into
1451        // a writing borrow.
1452        assert!(borrow != BorrowFlag::MAX);
1453        self.borrow.set(borrow + 1);
1454        BorrowRef { borrow: self.borrow }
1455    }
1456}
1457
1458/// Wraps a borrowed reference to a value in a `RefCell` box.
1459/// A wrapper type for an immutably borrowed value from a `RefCell<T>`.
1460///
1461/// See the [module-level documentation](self) for more.
1462#[stable(feature = "rust1", since = "1.0.0")]
1463#[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"]
1464#[rustc_diagnostic_item = "RefCellRef"]
1465pub struct Ref<'b, T: ?Sized + 'b> {
1466    // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a
1467    // `Ref` argument doesn't hold immutability for its whole scope, only until it drops.
1468    // `NonNull` is also covariant over `T`, just like we would have with `&T`.
1469    value: NonNull<T>,
1470    borrow: BorrowRef<'b>,
1471}
1472
1473#[stable(feature = "rust1", since = "1.0.0")]
1474impl<T: ?Sized> Deref for Ref<'_, T> {
1475    type Target = T;
1476
1477    #[inline]
1478    fn deref(&self) -> &T {
1479        // SAFETY: the value is accessible as long as we hold our borrow.
1480        unsafe { self.value.as_ref() }
1481    }
1482}
1483
1484#[unstable(feature = "deref_pure_trait", issue = "87121")]
1485unsafe impl<T: ?Sized> DerefPure for Ref<'_, T> {}
1486
1487impl<'b, T: ?Sized> Ref<'b, T> {
1488    /// Copies a `Ref`.
1489    ///
1490    /// The `RefCell` is already immutably borrowed, so this cannot fail.
1491    ///
1492    /// This is an associated function that needs to be used as
1493    /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere
1494    /// with the widespread use of `r.borrow().clone()` to clone the contents of
1495    /// a `RefCell`.
1496    #[stable(feature = "cell_extras", since = "1.15.0")]
1497    #[must_use]
1498    #[inline]
1499    pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> {
1500        Ref { value: orig.value, borrow: orig.borrow.clone() }
1501    }
1502
1503    /// Makes a new `Ref` for a component of the borrowed data.
1504    ///
1505    /// The `RefCell` is already immutably borrowed, so this cannot fail.
1506    ///
1507    /// This is an associated function that needs to be used as `Ref::map(...)`.
1508    /// A method would interfere with methods of the same name on the contents
1509    /// of a `RefCell` used through `Deref`.
1510    ///
1511    /// # Examples
1512    ///
1513    /// ```
1514    /// use std::cell::{RefCell, Ref};
1515    ///
1516    /// let c = RefCell::new((5, 'b'));
1517    /// let b1: Ref<'_, (u32, char)> = c.borrow();
1518    /// let b2: Ref<'_, u32> = Ref::map(b1, |t| &t.0);
1519    /// assert_eq!(*b2, 5)
1520    /// ```
1521    #[stable(feature = "cell_map", since = "1.8.0")]
1522    #[inline]
1523    pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U>
1524    where
1525        F: FnOnce(&T) -> &U,
1526    {
1527        Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow }
1528    }
1529
1530    /// Makes a new `Ref` for an optional component of the borrowed data. The
1531    /// original guard is returned as an `Err(..)` if the closure returns
1532    /// `None`.
1533    ///
1534    /// The `RefCell` is already immutably borrowed, so this cannot fail.
1535    ///
1536    /// This is an associated function that needs to be used as
1537    /// `Ref::filter_map(...)`. A method would interfere with methods of the same
1538    /// name on the contents of a `RefCell` used through `Deref`.
1539    ///
1540    /// # Examples
1541    ///
1542    /// ```
1543    /// use std::cell::{RefCell, Ref};
1544    ///
1545    /// let c = RefCell::new(vec![1, 2, 3]);
1546    /// let b1: Ref<'_, Vec<u32>> = c.borrow();
1547    /// let b2: Result<Ref<'_, u32>, _> = Ref::filter_map(b1, |v| v.get(1));
1548    /// assert_eq!(*b2.unwrap(), 2);
1549    /// ```
1550    #[stable(feature = "cell_filter_map", since = "1.63.0")]
1551    #[inline]
1552    pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self>
1553    where
1554        F: FnOnce(&T) -> Option<&U>,
1555    {
1556        match f(&*orig) {
1557            Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }),
1558            None => Err(orig),
1559        }
1560    }
1561
1562    /// Splits a `Ref` into multiple `Ref`s for different components of the
1563    /// borrowed data.
1564    ///
1565    /// The `RefCell` is already immutably borrowed, so this cannot fail.
1566    ///
1567    /// This is an associated function that needs to be used as
1568    /// `Ref::map_split(...)`. A method would interfere with methods of the same
1569    /// name on the contents of a `RefCell` used through `Deref`.
1570    ///
1571    /// # Examples
1572    ///
1573    /// ```
1574    /// use std::cell::{Ref, RefCell};
1575    ///
1576    /// let cell = RefCell::new([1, 2, 3, 4]);
1577    /// let borrow = cell.borrow();
1578    /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2));
1579    /// assert_eq!(*begin, [1, 2]);
1580    /// assert_eq!(*end, [3, 4]);
1581    /// ```
1582    #[stable(feature = "refcell_map_split", since = "1.35.0")]
1583    #[inline]
1584    pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>)
1585    where
1586        F: FnOnce(&T) -> (&U, &V),
1587    {
1588        let (a, b) = f(&*orig);
1589        let borrow = orig.borrow.clone();
1590        (
1591            Ref { value: NonNull::from(a), borrow },
1592            Ref { value: NonNull::from(b), borrow: orig.borrow },
1593        )
1594    }
1595
1596    /// Converts into a reference to the underlying data.
1597    ///
1598    /// The underlying `RefCell` can never be mutably borrowed from again and will always appear
1599    /// already immutably borrowed. It is not a good idea to leak more than a constant number of
1600    /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks
1601    /// have occurred in total.
1602    ///
1603    /// This is an associated function that needs to be used as
1604    /// `Ref::leak(...)`. A method would interfere with methods of the
1605    /// same name on the contents of a `RefCell` used through `Deref`.
1606    ///
1607    /// # Examples
1608    ///
1609    /// ```
1610    /// #![feature(cell_leak)]
1611    /// use std::cell::{RefCell, Ref};
1612    /// let cell = RefCell::new(0);
1613    ///
1614    /// let value = Ref::leak(cell.borrow());
1615    /// assert_eq!(*value, 0);
1616    ///
1617    /// assert!(cell.try_borrow().is_ok());
1618    /// assert!(cell.try_borrow_mut().is_err());
1619    /// ```
1620    #[unstable(feature = "cell_leak", issue = "69099")]
1621    pub fn leak(orig: Ref<'b, T>) -> &'b T {
1622        // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to
1623        // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a
1624        // unique reference to the borrowed RefCell. No further mutable references can be created
1625        // from the original cell.
1626        mem::forget(orig.borrow);
1627        // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1628        unsafe { orig.value.as_ref() }
1629    }
1630}
1631
1632#[unstable(feature = "coerce_unsized", issue = "18598")]
1633impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
1634
1635#[stable(feature = "std_guard_impls", since = "1.20.0")]
1636impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> {
1637    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1638        (**self).fmt(f)
1639    }
1640}
1641
1642impl<'b, T: ?Sized> RefMut<'b, T> {
1643    /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum
1644    /// variant.
1645    ///
1646    /// The `RefCell` is already mutably borrowed, so this cannot fail.
1647    ///
1648    /// This is an associated function that needs to be used as
1649    /// `RefMut::map(...)`. A method would interfere with methods of the same
1650    /// name on the contents of a `RefCell` used through `Deref`.
1651    ///
1652    /// # Examples
1653    ///
1654    /// ```
1655    /// use std::cell::{RefCell, RefMut};
1656    ///
1657    /// let c = RefCell::new((5, 'b'));
1658    /// {
1659    ///     let b1: RefMut<'_, (u32, char)> = c.borrow_mut();
1660    ///     let mut b2: RefMut<'_, u32> = RefMut::map(b1, |t| &mut t.0);
1661    ///     assert_eq!(*b2, 5);
1662    ///     *b2 = 42;
1663    /// }
1664    /// assert_eq!(*c.borrow(), (42, 'b'));
1665    /// ```
1666    #[stable(feature = "cell_map", since = "1.8.0")]
1667    #[inline]
1668    pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U>
1669    where
1670        F: FnOnce(&mut T) -> &mut U,
1671    {
1672        let value = NonNull::from(f(&mut *orig));
1673        RefMut { value, borrow: orig.borrow, marker: PhantomData }
1674    }
1675
1676    /// Makes a new `RefMut` for an optional component of the borrowed data. The
1677    /// original guard is returned as an `Err(..)` if the closure returns
1678    /// `None`.
1679    ///
1680    /// The `RefCell` is already mutably borrowed, so this cannot fail.
1681    ///
1682    /// This is an associated function that needs to be used as
1683    /// `RefMut::filter_map(...)`. A method would interfere with methods of the
1684    /// same name on the contents of a `RefCell` used through `Deref`.
1685    ///
1686    /// # Examples
1687    ///
1688    /// ```
1689    /// use std::cell::{RefCell, RefMut};
1690    ///
1691    /// let c = RefCell::new(vec![1, 2, 3]);
1692    ///
1693    /// {
1694    ///     let b1: RefMut<'_, Vec<u32>> = c.borrow_mut();
1695    ///     let mut b2: Result<RefMut<'_, u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1));
1696    ///
1697    ///     if let Ok(mut b2) = b2 {
1698    ///         *b2 += 2;
1699    ///     }
1700    /// }
1701    ///
1702    /// assert_eq!(*c.borrow(), vec![1, 4, 3]);
1703    /// ```
1704    #[stable(feature = "cell_filter_map", since = "1.63.0")]
1705    #[inline]
1706    pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self>
1707    where
1708        F: FnOnce(&mut T) -> Option<&mut U>,
1709    {
1710        // SAFETY: function holds onto an exclusive reference for the duration
1711        // of its call through `orig`, and the pointer is only de-referenced
1712        // inside of the function call never allowing the exclusive reference to
1713        // escape.
1714        match f(&mut *orig) {
1715            Some(value) => {
1716                Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData })
1717            }
1718            None => Err(orig),
1719        }
1720    }
1721
1722    /// Splits a `RefMut` into multiple `RefMut`s for different components of the
1723    /// borrowed data.
1724    ///
1725    /// The underlying `RefCell` will remain mutably borrowed until both
1726    /// returned `RefMut`s go out of scope.
1727    ///
1728    /// The `RefCell` is already mutably borrowed, so this cannot fail.
1729    ///
1730    /// This is an associated function that needs to be used as
1731    /// `RefMut::map_split(...)`. A method would interfere with methods of the
1732    /// same name on the contents of a `RefCell` used through `Deref`.
1733    ///
1734    /// # Examples
1735    ///
1736    /// ```
1737    /// use std::cell::{RefCell, RefMut};
1738    ///
1739    /// let cell = RefCell::new([1, 2, 3, 4]);
1740    /// let borrow = cell.borrow_mut();
1741    /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2));
1742    /// assert_eq!(*begin, [1, 2]);
1743    /// assert_eq!(*end, [3, 4]);
1744    /// begin.copy_from_slice(&[4, 3]);
1745    /// end.copy_from_slice(&[2, 1]);
1746    /// ```
1747    #[stable(feature = "refcell_map_split", since = "1.35.0")]
1748    #[inline]
1749    pub fn map_split<U: ?Sized, V: ?Sized, F>(
1750        mut orig: RefMut<'b, T>,
1751        f: F,
1752    ) -> (RefMut<'b, U>, RefMut<'b, V>)
1753    where
1754        F: FnOnce(&mut T) -> (&mut U, &mut V),
1755    {
1756        let borrow = orig.borrow.clone();
1757        let (a, b) = f(&mut *orig);
1758        (
1759            RefMut { value: NonNull::from(a), borrow, marker: PhantomData },
1760            RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData },
1761        )
1762    }
1763
1764    /// Converts into a mutable reference to the underlying data.
1765    ///
1766    /// The underlying `RefCell` can not be borrowed from again and will always appear already
1767    /// mutably borrowed, making the returned reference the only to the interior.
1768    ///
1769    /// This is an associated function that needs to be used as
1770    /// `RefMut::leak(...)`. A method would interfere with methods of the
1771    /// same name on the contents of a `RefCell` used through `Deref`.
1772    ///
1773    /// # Examples
1774    ///
1775    /// ```
1776    /// #![feature(cell_leak)]
1777    /// use std::cell::{RefCell, RefMut};
1778    /// let cell = RefCell::new(0);
1779    ///
1780    /// let value = RefMut::leak(cell.borrow_mut());
1781    /// assert_eq!(*value, 0);
1782    /// *value = 1;
1783    ///
1784    /// assert!(cell.try_borrow_mut().is_err());
1785    /// ```
1786    #[unstable(feature = "cell_leak", issue = "69099")]
1787    pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T {
1788        // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't
1789        // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would
1790        // require a unique reference to the borrowed RefCell. No further references can be created
1791        // from the original cell within that lifetime, making the current borrow the only
1792        // reference for the remaining lifetime.
1793        mem::forget(orig.borrow);
1794        // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`.
1795        unsafe { orig.value.as_mut() }
1796    }
1797}
1798
1799struct BorrowRefMut<'b> {
1800    borrow: &'b Cell<BorrowFlag>,
1801}
1802
1803impl Drop for BorrowRefMut<'_> {
1804    #[inline]
1805    fn drop(&mut self) {
1806        let borrow = self.borrow.get();
1807        debug_assert!(is_writing(borrow));
1808        self.borrow.set(borrow + 1);
1809    }
1810}
1811
1812impl<'b> BorrowRefMut<'b> {
1813    #[inline]
1814    fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> {
1815        // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial
1816        // mutable reference, and so there must currently be no existing
1817        // references. Thus, while clone increments the mutable refcount, here
1818        // we explicitly only allow going from UNUSED to UNUSED - 1.
1819        match borrow.get() {
1820            UNUSED => {
1821                borrow.set(UNUSED - 1);
1822                Some(BorrowRefMut { borrow })
1823            }
1824            _ => None,
1825        }
1826    }
1827
1828    // Clones a `BorrowRefMut`.
1829    //
1830    // This is only valid if each `BorrowRefMut` is used to track a mutable
1831    // reference to a distinct, nonoverlapping range of the original object.
1832    // This isn't in a Clone impl so that code doesn't call this implicitly.
1833    #[inline]
1834    fn clone(&self) -> BorrowRefMut<'b> {
1835        let borrow = self.borrow.get();
1836        debug_assert!(is_writing(borrow));
1837        // Prevent the borrow counter from underflowing.
1838        assert!(borrow != BorrowFlag::MIN);
1839        self.borrow.set(borrow - 1);
1840        BorrowRefMut { borrow: self.borrow }
1841    }
1842}
1843
1844/// A wrapper type for a mutably borrowed value from a `RefCell<T>`.
1845///
1846/// See the [module-level documentation](self) for more.
1847#[stable(feature = "rust1", since = "1.0.0")]
1848#[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"]
1849#[rustc_diagnostic_item = "RefCellRefMut"]
1850pub struct RefMut<'b, T: ?Sized + 'b> {
1851    // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
1852    // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
1853    value: NonNull<T>,
1854    borrow: BorrowRefMut<'b>,
1855    // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
1856    marker: PhantomData<&'b mut T>,
1857}
1858
1859#[stable(feature = "rust1", since = "1.0.0")]
1860impl<T: ?Sized> Deref for RefMut<'_, T> {
1861    type Target = T;
1862
1863    #[inline]
1864    fn deref(&self) -> &T {
1865        // SAFETY: the value is accessible as long as we hold our borrow.
1866        unsafe { self.value.as_ref() }
1867    }
1868}
1869
1870#[stable(feature = "rust1", since = "1.0.0")]
1871impl<T: ?Sized> DerefMut for RefMut<'_, T> {
1872    #[inline]
1873    fn deref_mut(&mut self) -> &mut T {
1874        // SAFETY: the value is accessible as long as we hold our borrow.
1875        unsafe { self.value.as_mut() }
1876    }
1877}
1878
1879#[unstable(feature = "deref_pure_trait", issue = "87121")]
1880unsafe impl<T: ?Sized> DerefPure for RefMut<'_, T> {}
1881
1882#[unstable(feature = "coerce_unsized", issue = "18598")]
1883impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
1884
1885#[stable(feature = "std_guard_impls", since = "1.20.0")]
1886impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> {
1887    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1888        (**self).fmt(f)
1889    }
1890}
1891
1892/// The core primitive for interior mutability in Rust.
1893///
1894/// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on
1895/// the knowledge that `&T` points to immutable data. Mutating that data, for example through an
1896/// alias or by transmuting a `&T` into a `&mut T`, is considered undefined behavior.
1897/// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference
1898/// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability".
1899///
1900/// All other types that allow internal mutability, such as [`Cell<T>`] and [`RefCell<T>`], internally
1901/// use `UnsafeCell` to wrap their data.
1902///
1903/// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The
1904/// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain
1905/// aliasing `&mut`, not even with `UnsafeCell<T>`.
1906///
1907/// `UnsafeCell` does nothing to avoid data races; they are still undefined behavior. If multiple
1908/// threads have access to the same `UnsafeCell`, they must follow the usual rules of the
1909/// [concurrent memory model]: conflicting non-synchronized accesses must be done via the APIs in
1910/// [`core::sync::atomic`].
1911///
1912/// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer
1913/// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer
1914/// correctly.
1915///
1916/// [`.get()`]: `UnsafeCell::get`
1917/// [concurrent memory model]: ../sync/atomic/index.html#memory-model-for-atomic-accesses
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.
2173    /// Ensure that the access is unique (no active references, mutable or not)
2174    /// when casting to `&mut T`, and ensure that there are no mutations
2175    /// or mutable aliases going on when casting to `&T`
2176    ///
2177    /// # Examples
2178    ///
2179    /// ```
2180    /// use std::cell::UnsafeCell;
2181    ///
2182    /// let uc = UnsafeCell::new(5);
2183    ///
2184    /// let five = uc.get();
2185    /// ```
2186    #[inline(always)]
2187    #[stable(feature = "rust1", since = "1.0.0")]
2188    #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")]
2189    #[rustc_as_ptr]
2190    #[rustc_never_returns_null_ptr]
2191    pub const fn get(&self) -> *mut T {
2192        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2193        // #[repr(transparent)]. This exploits std's special status, there is
2194        // no guarantee for user code that this will work in future versions of the compiler!
2195        self as *const UnsafeCell<T> as *const T as *mut T
2196    }
2197
2198    /// Returns a mutable reference to the underlying data.
2199    ///
2200    /// This call borrows the `UnsafeCell` mutably (at compile-time) which
2201    /// guarantees that we possess the only reference.
2202    ///
2203    /// # Examples
2204    ///
2205    /// ```
2206    /// use std::cell::UnsafeCell;
2207    ///
2208    /// let mut c = UnsafeCell::new(5);
2209    /// *c.get_mut() += 1;
2210    ///
2211    /// assert_eq!(*c.get_mut(), 6);
2212    /// ```
2213    #[inline(always)]
2214    #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")]
2215    #[rustc_const_stable(feature = "const_unsafecell_get_mut", since = "1.83.0")]
2216    pub const fn get_mut(&mut self) -> &mut T {
2217        &mut self.value
2218    }
2219
2220    /// Gets a mutable pointer to the wrapped value.
2221    /// The difference from [`get`] is that this function accepts a raw pointer,
2222    /// which is useful to avoid the creation of temporary references.
2223    ///
2224    /// The result can be cast to a pointer of any kind.
2225    /// Ensure that the access is unique (no active references, mutable or not)
2226    /// when casting to `&mut T`, and ensure that there are no mutations
2227    /// or mutable aliases going on when casting to `&T`.
2228    ///
2229    /// [`get`]: UnsafeCell::get()
2230    ///
2231    /// # Examples
2232    ///
2233    /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as
2234    /// calling `get` would require creating a reference to uninitialized data:
2235    ///
2236    /// ```
2237    /// use std::cell::UnsafeCell;
2238    /// use std::mem::MaybeUninit;
2239    ///
2240    /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit();
2241    /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); }
2242    /// // avoid below which references to uninitialized data
2243    /// // unsafe { UnsafeCell::get(&*m.as_ptr()).write(5); }
2244    /// let uc = unsafe { m.assume_init() };
2245    ///
2246    /// assert_eq!(uc.into_inner(), 5);
2247    /// ```
2248    #[inline(always)]
2249    #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2250    #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
2251    #[rustc_diagnostic_item = "unsafe_cell_raw_get"]
2252    pub const fn raw_get(this: *const Self) -> *mut T {
2253        // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
2254        // #[repr(transparent)]. This exploits std's special status, there is
2255        // no guarantee for user code that this will work in future versions of the compiler!
2256        this as *const T as *mut T
2257    }
2258
2259    /// Get a shared reference to the value within the `UnsafeCell`.
2260    ///
2261    /// # Safety
2262    ///
2263    /// - It is Undefined Behavior to call this while any mutable
2264    ///   reference to the wrapped value is alive.
2265    /// - Mutating the wrapped value while the returned
2266    ///   reference is alive is Undefined Behavior.
2267    ///
2268    /// # Examples
2269    ///
2270    /// ```
2271    /// #![feature(unsafe_cell_access)]
2272    /// use std::cell::UnsafeCell;
2273    ///
2274    /// let uc = UnsafeCell::new(5);
2275    ///
2276    /// let val = unsafe { uc.as_ref_unchecked() };
2277    /// assert_eq!(val, &5);
2278    /// ```
2279    #[inline]
2280    #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2281    pub const unsafe fn as_ref_unchecked(&self) -> &T {
2282        // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2283        unsafe { self.get().as_ref_unchecked() }
2284    }
2285
2286    /// Get an exclusive reference to the value within the `UnsafeCell`.
2287    ///
2288    /// # Safety
2289    ///
2290    /// - It is Undefined Behavior to call this while any other
2291    ///   reference(s) to the wrapped value are alive.
2292    /// - Mutating the wrapped value through other means while the
2293    ///   returned reference is alive is Undefined Behavior.
2294    ///
2295    /// # Examples
2296    ///
2297    /// ```
2298    /// #![feature(unsafe_cell_access)]
2299    /// use std::cell::UnsafeCell;
2300    ///
2301    /// let uc = UnsafeCell::new(5);
2302    ///
2303    /// unsafe { *uc.as_mut_unchecked() += 1; }
2304    /// assert_eq!(uc.into_inner(), 6);
2305    /// ```
2306    #[inline]
2307    #[unstable(feature = "unsafe_cell_access", issue = "136327")]
2308    #[allow(clippy::mut_from_ref)]
2309    pub const unsafe fn as_mut_unchecked(&self) -> &mut T {
2310        // SAFETY: pointer comes from `&self` so naturally satisfies ptr-to-ref invariants.
2311        unsafe { self.get().as_mut_unchecked() }
2312    }
2313}
2314
2315#[stable(feature = "unsafe_cell_default", since = "1.10.0")]
2316impl<T: Default> Default for UnsafeCell<T> {
2317    /// Creates an `UnsafeCell`, with the `Default` value for T.
2318    fn default() -> UnsafeCell<T> {
2319        UnsafeCell::new(Default::default())
2320    }
2321}
2322
2323#[stable(feature = "cell_from", since = "1.12.0")]
2324impl<T> From<T> for UnsafeCell<T> {
2325    /// Creates a new `UnsafeCell<T>` containing the given value.
2326    fn from(t: T) -> UnsafeCell<T> {
2327        UnsafeCell::new(t)
2328    }
2329}
2330
2331#[unstable(feature = "coerce_unsized", issue = "18598")]
2332impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {}
2333
2334// Allow types that wrap `UnsafeCell` to also implement `DispatchFromDyn`
2335// and become dyn-compatible method receivers.
2336// Note that currently `UnsafeCell` itself cannot be a method receiver
2337// because it does not implement Deref.
2338// In other words:
2339// `self: UnsafeCell<&Self>` won't work
2340// `self: UnsafeCellWrapper<Self>` becomes possible
2341#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2342impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<UnsafeCell<U>> for UnsafeCell<T> {}
2343
2344#[unstable(feature = "pointer_like_trait", issue = "none")]
2345impl<T: PointerLike> PointerLike for UnsafeCell<T> {}
2346
2347/// [`UnsafeCell`], but [`Sync`].
2348///
2349/// This is just an `UnsafeCell`, except it implements `Sync`
2350/// if `T` implements `Sync`.
2351///
2352/// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use.
2353/// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be
2354/// shared between threads, if that's intentional.
2355/// Providing proper synchronization is still the task of the user,
2356/// making this type just as unsafe to use.
2357///
2358/// See [`UnsafeCell`] for details.
2359#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2360#[repr(transparent)]
2361#[rustc_diagnostic_item = "SyncUnsafeCell"]
2362#[rustc_pub_transparent]
2363pub struct SyncUnsafeCell<T: ?Sized> {
2364    value: UnsafeCell<T>,
2365}
2366
2367#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2368unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {}
2369
2370#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2371impl<T> SyncUnsafeCell<T> {
2372    /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value.
2373    #[inline]
2374    pub const fn new(value: T) -> Self {
2375        Self { value: UnsafeCell { value } }
2376    }
2377
2378    /// Unwraps the value, consuming the cell.
2379    #[inline]
2380    #[rustc_const_unstable(feature = "sync_unsafe_cell", issue = "95439")]
2381    pub const fn into_inner(self) -> T {
2382        self.value.into_inner()
2383    }
2384}
2385
2386#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2387impl<T: ?Sized> SyncUnsafeCell<T> {
2388    /// Gets a mutable pointer to the wrapped value.
2389    ///
2390    /// This can be cast to a pointer of any kind.
2391    /// Ensure that the access is unique (no active references, mutable or not)
2392    /// when casting to `&mut T`, and ensure that there are no mutations
2393    /// or mutable aliases going on when casting to `&T`
2394    #[inline]
2395    #[rustc_as_ptr]
2396    #[rustc_never_returns_null_ptr]
2397    pub const fn get(&self) -> *mut T {
2398        self.value.get()
2399    }
2400
2401    /// Returns a mutable reference to the underlying data.
2402    ///
2403    /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which
2404    /// guarantees that we possess the only reference.
2405    #[inline]
2406    pub const fn get_mut(&mut self) -> &mut T {
2407        self.value.get_mut()
2408    }
2409
2410    /// Gets a mutable pointer to the wrapped value.
2411    ///
2412    /// See [`UnsafeCell::get`] for details.
2413    #[inline]
2414    pub const fn raw_get(this: *const Self) -> *mut T {
2415        // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because
2416        // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell.
2417        // See UnsafeCell::raw_get.
2418        this as *const T as *mut T
2419    }
2420}
2421
2422#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2423impl<T: Default> Default for SyncUnsafeCell<T> {
2424    /// Creates an `SyncUnsafeCell`, with the `Default` value for T.
2425    fn default() -> SyncUnsafeCell<T> {
2426        SyncUnsafeCell::new(Default::default())
2427    }
2428}
2429
2430#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2431impl<T> From<T> for SyncUnsafeCell<T> {
2432    /// Creates a new `SyncUnsafeCell<T>` containing the given value.
2433    fn from(t: T) -> SyncUnsafeCell<T> {
2434        SyncUnsafeCell::new(t)
2435    }
2436}
2437
2438#[unstable(feature = "coerce_unsized", issue = "18598")]
2439//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2440impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2441
2442// Allow types that wrap `SyncUnsafeCell` to also implement `DispatchFromDyn`
2443// and become dyn-compatible method receivers.
2444// Note that currently `SyncUnsafeCell` itself cannot be a method receiver
2445// because it does not implement Deref.
2446// In other words:
2447// `self: SyncUnsafeCell<&Self>` won't work
2448// `self: SyncUnsafeCellWrapper<Self>` becomes possible
2449#[unstable(feature = "dispatch_from_dyn", issue = "none")]
2450//#[unstable(feature = "sync_unsafe_cell", issue = "95439")]
2451impl<T: DispatchFromDyn<U>, U> DispatchFromDyn<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {}
2452
2453#[unstable(feature = "pointer_like_trait", issue = "none")]
2454impl<T: PointerLike> PointerLike for SyncUnsafeCell<T> {}
2455
2456#[allow(unused)]
2457fn assert_coerce_unsized(
2458    a: UnsafeCell<&i32>,
2459    b: SyncUnsafeCell<&i32>,
2460    c: Cell<&i32>,
2461    d: RefCell<&i32>,
2462) {
2463    let _: UnsafeCell<&dyn Send> = a;
2464    let _: SyncUnsafeCell<&dyn Send> = b;
2465    let _: Cell<&dyn Send> = c;
2466    let _: RefCell<&dyn Send> = d;
2467}
2468
2469#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2470unsafe impl<T: ?Sized> PinCoerceUnsized for UnsafeCell<T> {}
2471
2472#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2473unsafe impl<T: ?Sized> PinCoerceUnsized for SyncUnsafeCell<T> {}
2474
2475#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2476unsafe impl<T: ?Sized> PinCoerceUnsized for Cell<T> {}
2477
2478#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2479unsafe impl<T: ?Sized> PinCoerceUnsized for RefCell<T> {}
2480
2481#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2482unsafe impl<'b, T: ?Sized> PinCoerceUnsized for Ref<'b, T> {}
2483
2484#[unstable(feature = "pin_coerce_unsized_trait", issue = "123430")]
2485unsafe impl<'b, T: ?Sized> PinCoerceUnsized for RefMut<'b, T> {}