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Commit | Line | Data |
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1a4d82fc JJ |
1 | //! Shareable mutable containers. |
2 | //! | |
0531ce1d XL |
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 | //! - Having several immutable references (`&T`) to the object (also known as **aliasing**). | |
7 | //! - Having one mutable reference (`&mut T`) to the object (also known as **mutability**). | |
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 | |
dc9dc135 | 14 | //! presence of aliasing. Both `Cell<T>` and `RefCell<T>` allow doing this in a single-threaded |
0531ce1d XL |
15 | //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement |
16 | //! `Sync`). If you need to do aliasing and mutation between multiple threads it is possible to | |
17 | //! use [`Mutex`](../../std/sync/struct.Mutex.html), | |
18 | //! [`RwLock`](../../std/sync/struct.RwLock.html) or | |
19 | //! [`atomic`](../../core/sync/atomic/index.html) types. | |
20 | //! | |
85aaf69f SL |
21 | //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e. |
22 | //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`) | |
23 | //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast | |
24 | //! with typical Rust types that exhibit 'inherited mutability'. | |
1a4d82fc | 25 | //! |
8bb4bdeb XL |
26 | //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior |
27 | //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values, | |
28 | //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides | |
29 | //! methods to retrieve and change the current interior value: | |
30 | //! | |
31 | //! - For types that implement `Copy`, the `get` method retrieves the current interior value. | |
32 | //! - For types that implement `Default`, the `take` method replaces the current interior value | |
33 | //! with `Default::default()` and returns the replaced value. | |
34 | //! - For all types, the `replace` method replaces the current interior value and returns the | |
35 | //! replaced value and the `into_inner` method consumes the `Cell<T>` and returns the interior | |
36 | //! value. Additionally, the `set` method replaces the interior value, dropping the replaced | |
37 | //! value. | |
1a4d82fc | 38 | //! |
85aaf69f SL |
39 | //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can |
40 | //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are | |
41 | //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked | |
42 | //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt | |
bd371182 AL |
43 | //! to borrow a value that is already mutably borrowed; when this happens it results in thread |
44 | //! panic. | |
1a4d82fc JJ |
45 | //! |
46 | //! # When to choose interior mutability | |
47 | //! | |
85aaf69f SL |
48 | //! The more common inherited mutability, where one must have unique access to mutate a value, is |
49 | //! one of the key language elements that enables Rust to reason strongly about pointer aliasing, | |
50 | //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and | |
51 | //! interior mutability is something of a last resort. Since cell types enable mutation where it | |
52 | //! would otherwise be disallowed though, there are occasions when interior mutability might be | |
53 | //! appropriate, or even *must* be used, e.g. | |
1a4d82fc | 54 | //! |
c1a9b12d | 55 | //! * Introducing mutability 'inside' of something immutable |
1a4d82fc | 56 | //! * Implementation details of logically-immutable methods. |
62682a34 | 57 | //! * Mutating implementations of `Clone`. |
1a4d82fc | 58 | //! |
c1a9b12d | 59 | //! ## Introducing mutability 'inside' of something immutable |
1a4d82fc | 60 | //! |
c1a9b12d | 61 | //! Many shared smart pointer types, including `Rc<T>` and `Arc<T>`, provide containers that can be |
85aaf69f | 62 | //! cloned and shared between multiple parties. Because the contained values may be |
c1a9b12d SL |
63 | //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be |
64 | //! impossible to mutate data inside of these smart pointers at all. | |
1a4d82fc | 65 | //! |
85aaf69f SL |
66 | //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce |
67 | //! mutability: | |
1a4d82fc JJ |
68 | //! |
69 | //! ``` | |
dc9dc135 | 70 | //! use std::cell::{RefCell, RefMut}; |
1a4d82fc | 71 | //! use std::collections::HashMap; |
1a4d82fc JJ |
72 | //! use std::rc::Rc; |
73 | //! | |
74 | //! fn main() { | |
75 | //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new())); | |
dc9dc135 XL |
76 | //! // Create a new block to limit the scope of the dynamic borrow |
77 | //! { | |
78 | //! let mut map: RefMut<_> = shared_map.borrow_mut(); | |
79 | //! map.insert("africa", 92388); | |
80 | //! map.insert("kyoto", 11837); | |
81 | //! map.insert("piccadilly", 11826); | |
82 | //! map.insert("marbles", 38); | |
83 | //! } | |
84 | //! | |
85 | //! // Note that if we had not let the previous borrow of the cache fall out | |
86 | //! // of scope then the subsequent borrow would cause a dynamic thread panic. | |
87 | //! // This is the major hazard of using `RefCell`. | |
88 | //! let total: i32 = shared_map.borrow().values().sum(); | |
89 | //! println!("{}", total); | |
1a4d82fc JJ |
90 | //! } |
91 | //! ``` | |
92 | //! | |
85aaf69f | 93 | //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded |
c1a9b12d SL |
94 | //! scenarios. Consider using `RwLock<T>` or `Mutex<T>` if you need shared mutability in a |
95 | //! multi-threaded situation. | |
85aaf69f | 96 | //! |
1a4d82fc JJ |
97 | //! ## Implementation details of logically-immutable methods |
98 | //! | |
85aaf69f | 99 | //! Occasionally it may be desirable not to expose in an API that there is mutation happening |
0731742a | 100 | //! "under the hood". This may be because logically the operation is immutable, but e.g., caching |
85aaf69f SL |
101 | //! forces the implementation to perform mutation; or because you must employ mutation to implement |
102 | //! a trait method that was originally defined to take `&self`. | |
1a4d82fc JJ |
103 | //! |
104 | //! ``` | |
92a42be0 | 105 | //! # #![allow(dead_code)] |
1a4d82fc JJ |
106 | //! use std::cell::RefCell; |
107 | //! | |
108 | //! struct Graph { | |
85aaf69f SL |
109 | //! edges: Vec<(i32, i32)>, |
110 | //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>> | |
1a4d82fc JJ |
111 | //! } |
112 | //! | |
113 | //! impl Graph { | |
85aaf69f | 114 | //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> { |
dc9dc135 XL |
115 | //! self.span_tree_cache.borrow_mut() |
116 | //! .get_or_insert_with(|| self.calc_span_tree()) | |
117 | //! .clone() | |
118 | //! } | |
1a4d82fc | 119 | //! |
dc9dc135 XL |
120 | //! fn calc_span_tree(&self) -> Vec<(i32, i32)> { |
121 | //! // Expensive computation goes here | |
122 | //! vec![] | |
1a4d82fc | 123 | //! } |
1a4d82fc | 124 | //! } |
1a4d82fc JJ |
125 | //! ``` |
126 | //! | |
62682a34 | 127 | //! ## Mutating implementations of `Clone` |
1a4d82fc | 128 | //! |
85aaf69f SL |
129 | //! This is simply a special - but common - case of the previous: hiding mutability for operations |
130 | //! that appear to be immutable. The `clone` method is expected to not change the source value, and | |
9fa01778 | 131 | //! is declared to take `&self`, not `&mut self`. Therefore, any mutation that happens in the |
85aaf69f SL |
132 | //! `clone` method must use cell types. For example, `Rc<T>` maintains its reference counts within a |
133 | //! `Cell<T>`. | |
1a4d82fc JJ |
134 | //! |
135 | //! ``` | |
136 | //! use std::cell::Cell; | |
0531ce1d | 137 | //! use std::ptr::NonNull; |
f9f354fc | 138 | //! use std::process::abort; |
60c5eb7d | 139 | //! use std::marker::PhantomData; |
1a4d82fc | 140 | //! |
9e0c209e | 141 | //! struct Rc<T: ?Sized> { |
60c5eb7d XL |
142 | //! ptr: NonNull<RcBox<T>>, |
143 | //! phantom: PhantomData<RcBox<T>>, | |
1a4d82fc JJ |
144 | //! } |
145 | //! | |
9e0c209e SL |
146 | //! struct RcBox<T: ?Sized> { |
147 | //! strong: Cell<usize>, | |
148 | //! refcount: Cell<usize>, | |
1a4d82fc | 149 | //! value: T, |
1a4d82fc JJ |
150 | //! } |
151 | //! | |
9e0c209e | 152 | //! impl<T: ?Sized> Clone for Rc<T> { |
1a4d82fc | 153 | //! fn clone(&self) -> Rc<T> { |
9e0c209e | 154 | //! self.inc_strong(); |
60c5eb7d XL |
155 | //! Rc { |
156 | //! ptr: self.ptr, | |
157 | //! phantom: PhantomData, | |
158 | //! } | |
9e0c209e SL |
159 | //! } |
160 | //! } | |
161 | //! | |
162 | //! trait RcBoxPtr<T: ?Sized> { | |
163 | //! | |
164 | //! fn inner(&self) -> &RcBox<T>; | |
165 | //! | |
166 | //! fn strong(&self) -> usize { | |
167 | //! self.inner().strong.get() | |
168 | //! } | |
169 | //! | |
170 | //! fn inc_strong(&self) { | |
171 | //! self.inner() | |
172 | //! .strong | |
173 | //! .set(self.strong() | |
174 | //! .checked_add(1) | |
f9f354fc | 175 | //! .unwrap_or_else(|| abort() )); |
1a4d82fc JJ |
176 | //! } |
177 | //! } | |
9e0c209e SL |
178 | //! |
179 | //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> { | |
180 | //! fn inner(&self) -> &RcBox<T> { | |
181 | //! unsafe { | |
7cac9316 | 182 | //! self.ptr.as_ref() |
9e0c209e SL |
183 | //! } |
184 | //! } | |
185 | //! } | |
1a4d82fc JJ |
186 | //! ``` |
187 | //! | |
1a4d82fc | 188 | |
85aaf69f | 189 | #![stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 190 | |
48663c56 XL |
191 | use crate::cmp::Ordering; |
192 | use crate::fmt::{self, Debug, Display}; | |
193 | use crate::marker::Unsize; | |
194 | use crate::mem; | |
dfeec247 | 195 | use crate::ops::{CoerceUnsized, Deref, DerefMut}; |
48663c56 | 196 | use crate::ptr; |
1a4d82fc | 197 | |
8bb4bdeb | 198 | /// A mutable memory location. |
85aaf69f | 199 | /// |
3b2f2976 XL |
200 | /// # Examples |
201 | /// | |
a1dfa0c6 XL |
202 | /// In this example, you can see that `Cell<T>` enables mutation inside an |
203 | /// immutable struct. In other words, it enables "interior mutability". | |
3b2f2976 XL |
204 | /// |
205 | /// ``` | |
206 | /// use std::cell::Cell; | |
207 | /// | |
208 | /// struct SomeStruct { | |
209 | /// regular_field: u8, | |
210 | /// special_field: Cell<u8>, | |
211 | /// } | |
212 | /// | |
213 | /// let my_struct = SomeStruct { | |
214 | /// regular_field: 0, | |
215 | /// special_field: Cell::new(1), | |
216 | /// }; | |
217 | /// | |
218 | /// let new_value = 100; | |
219 | /// | |
a1dfa0c6 | 220 | /// // ERROR: `my_struct` is immutable |
3b2f2976 XL |
221 | /// // my_struct.regular_field = new_value; |
222 | /// | |
a1dfa0c6 XL |
223 | /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, |
224 | /// // which can always be mutated | |
3b2f2976 XL |
225 | /// my_struct.special_field.set(new_value); |
226 | /// assert_eq!(my_struct.special_field.get(), new_value); | |
227 | /// ``` | |
228 | /// | |
85aaf69f SL |
229 | /// See the [module-level documentation](index.html) for more. |
230 | #[stable(feature = "rust1", since = "1.0.0")] | |
8faf50e0 XL |
231 | #[repr(transparent)] |
232 | pub struct Cell<T: ?Sized> { | |
1a4d82fc JJ |
233 | value: UnsafeCell<T>, |
234 | } | |
235 | ||
8bb4bdeb | 236 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 237 | unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} |
8bb4bdeb XL |
238 | |
239 | #[stable(feature = "rust1", since = "1.0.0")] | |
8faf50e0 | 240 | impl<T: ?Sized> !Sync for Cell<T> {} |
8bb4bdeb XL |
241 | |
242 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 243 | impl<T: Copy> Clone for Cell<T> { |
8bb4bdeb XL |
244 | #[inline] |
245 | fn clone(&self) -> Cell<T> { | |
246 | Cell::new(self.get()) | |
247 | } | |
248 | } | |
249 | ||
250 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 251 | impl<T: Default> Default for Cell<T> { |
8bb4bdeb XL |
252 | /// Creates a `Cell<T>`, with the `Default` value for T. |
253 | #[inline] | |
254 | fn default() -> Cell<T> { | |
255 | Cell::new(Default::default()) | |
256 | } | |
257 | } | |
258 | ||
259 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 260 | impl<T: PartialEq + Copy> PartialEq for Cell<T> { |
8bb4bdeb XL |
261 | #[inline] |
262 | fn eq(&self, other: &Cell<T>) -> bool { | |
263 | self.get() == other.get() | |
264 | } | |
265 | } | |
266 | ||
267 | #[stable(feature = "cell_eq", since = "1.2.0")] | |
416331ca | 268 | impl<T: Eq + Copy> Eq for Cell<T> {} |
8bb4bdeb XL |
269 | |
270 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 271 | impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { |
8bb4bdeb XL |
272 | #[inline] |
273 | fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { | |
274 | self.get().partial_cmp(&other.get()) | |
275 | } | |
276 | ||
277 | #[inline] | |
278 | fn lt(&self, other: &Cell<T>) -> bool { | |
279 | self.get() < other.get() | |
280 | } | |
281 | ||
282 | #[inline] | |
283 | fn le(&self, other: &Cell<T>) -> bool { | |
284 | self.get() <= other.get() | |
285 | } | |
286 | ||
287 | #[inline] | |
288 | fn gt(&self, other: &Cell<T>) -> bool { | |
289 | self.get() > other.get() | |
290 | } | |
291 | ||
292 | #[inline] | |
293 | fn ge(&self, other: &Cell<T>) -> bool { | |
294 | self.get() >= other.get() | |
295 | } | |
296 | } | |
297 | ||
298 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 299 | impl<T: Ord + Copy> Ord for Cell<T> { |
8bb4bdeb XL |
300 | #[inline] |
301 | fn cmp(&self, other: &Cell<T>) -> Ordering { | |
302 | self.get().cmp(&other.get()) | |
303 | } | |
304 | } | |
305 | ||
306 | #[stable(feature = "cell_from", since = "1.12.0")] | |
307 | impl<T> From<T> for Cell<T> { | |
308 | fn from(t: T) -> Cell<T> { | |
309 | Cell::new(t) | |
310 | } | |
311 | } | |
312 | ||
313 | impl<T> Cell<T> { | |
314 | /// Creates a new `Cell` containing the given value. | |
85aaf69f SL |
315 | /// |
316 | /// # Examples | |
317 | /// | |
318 | /// ``` | |
319 | /// use std::cell::Cell; | |
320 | /// | |
321 | /// let c = Cell::new(5); | |
85aaf69f | 322 | /// ``` |
85aaf69f | 323 | #[stable(feature = "rust1", since = "1.0.0")] |
dfeec247 | 324 | #[rustc_const_stable(feature = "const_cell_new", since = "1.32.0")] |
8bb4bdeb XL |
325 | #[inline] |
326 | pub const fn new(value: T) -> Cell<T> { | |
dfeec247 | 327 | Cell { value: UnsafeCell::new(value) } |
1a4d82fc JJ |
328 | } |
329 | ||
8bb4bdeb XL |
330 | /// Sets the contained value. |
331 | /// | |
332 | /// # Examples | |
333 | /// | |
334 | /// ``` | |
335 | /// use std::cell::Cell; | |
336 | /// | |
337 | /// let c = Cell::new(5); | |
338 | /// | |
339 | /// c.set(10); | |
340 | /// ``` | |
a7813a04 | 341 | #[inline] |
8bb4bdeb XL |
342 | #[stable(feature = "rust1", since = "1.0.0")] |
343 | pub fn set(&self, val: T) { | |
344 | let old = self.replace(val); | |
345 | drop(old); | |
a7813a04 XL |
346 | } |
347 | ||
8bb4bdeb XL |
348 | /// Swaps the values of two Cells. |
349 | /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. | |
350 | /// | |
351 | /// # Examples | |
352 | /// | |
353 | /// ``` | |
354 | /// use std::cell::Cell; | |
355 | /// | |
356 | /// let c1 = Cell::new(5i32); | |
357 | /// let c2 = Cell::new(10i32); | |
358 | /// c1.swap(&c2); | |
359 | /// assert_eq!(10, c1.get()); | |
360 | /// assert_eq!(5, c2.get()); | |
361 | /// ``` | |
a7813a04 | 362 | #[inline] |
8bb4bdeb XL |
363 | #[stable(feature = "move_cell", since = "1.17.0")] |
364 | pub fn swap(&self, other: &Self) { | |
365 | if ptr::eq(self, other) { | |
366 | return; | |
367 | } | |
dfeec247 XL |
368 | // SAFETY: This can be risky if called from separate threads, but `Cell` |
369 | // is `!Sync` so this won't happen. This also won't invalidate any | |
370 | // pointers since `Cell` makes sure nothing else will be pointing into | |
371 | // either of these `Cell`s. | |
8bb4bdeb XL |
372 | unsafe { |
373 | ptr::swap(self.value.get(), other.value.get()); | |
374 | } | |
a7813a04 XL |
375 | } |
376 | ||
041b39d2 | 377 | /// Replaces the contained value, and returns it. |
8bb4bdeb XL |
378 | /// |
379 | /// # Examples | |
380 | /// | |
381 | /// ``` | |
382 | /// use std::cell::Cell; | |
383 | /// | |
041b39d2 XL |
384 | /// let cell = Cell::new(5); |
385 | /// assert_eq!(cell.get(), 5); | |
386 | /// assert_eq!(cell.replace(10), 5); | |
387 | /// assert_eq!(cell.get(), 10); | |
8bb4bdeb XL |
388 | /// ``` |
389 | #[stable(feature = "move_cell", since = "1.17.0")] | |
390 | pub fn replace(&self, val: T) -> T { | |
dfeec247 XL |
391 | // SAFETY: This can cause data races if called from a separate thread, |
392 | // but `Cell` is `!Sync` so this won't happen. | |
8bb4bdeb | 393 | mem::replace(unsafe { &mut *self.value.get() }, val) |
a7813a04 | 394 | } |
a7813a04 | 395 | |
8bb4bdeb XL |
396 | /// Unwraps the value. |
397 | /// | |
398 | /// # Examples | |
399 | /// | |
400 | /// ``` | |
401 | /// use std::cell::Cell; | |
402 | /// | |
403 | /// let c = Cell::new(5); | |
404 | /// let five = c.into_inner(); | |
405 | /// | |
406 | /// assert_eq!(five, 5); | |
407 | /// ``` | |
408 | #[stable(feature = "move_cell", since = "1.17.0")] | |
409 | pub fn into_inner(self) -> T { | |
2c00a5a8 | 410 | self.value.into_inner() |
a7813a04 | 411 | } |
e74abb32 XL |
412 | } |
413 | ||
dfeec247 | 414 | impl<T: Copy> Cell<T> { |
e74abb32 XL |
415 | /// Returns a copy of the contained value. |
416 | /// | |
417 | /// # Examples | |
418 | /// | |
419 | /// ``` | |
420 | /// use std::cell::Cell; | |
421 | /// | |
422 | /// let c = Cell::new(5); | |
423 | /// | |
424 | /// let five = c.get(); | |
425 | /// ``` | |
426 | #[inline] | |
427 | #[stable(feature = "rust1", since = "1.0.0")] | |
428 | pub fn get(&self) -> T { | |
dfeec247 XL |
429 | // SAFETY: This can cause data races if called from a separate thread, |
430 | // but `Cell` is `!Sync` so this won't happen. | |
431 | unsafe { *self.value.get() } | |
e74abb32 XL |
432 | } |
433 | ||
434 | /// Updates the contained value using a function and returns the new value. | |
435 | /// | |
436 | /// # Examples | |
437 | /// | |
438 | /// ``` | |
439 | /// #![feature(cell_update)] | |
440 | /// | |
441 | /// use std::cell::Cell; | |
442 | /// | |
443 | /// let c = Cell::new(5); | |
444 | /// let new = c.update(|x| x + 1); | |
445 | /// | |
446 | /// assert_eq!(new, 6); | |
447 | /// assert_eq!(c.get(), 6); | |
448 | /// ``` | |
449 | #[inline] | |
450 | #[unstable(feature = "cell_update", issue = "50186")] | |
451 | pub fn update<F>(&self, f: F) -> T | |
452 | where | |
453 | F: FnOnce(T) -> T, | |
454 | { | |
455 | let old = self.get(); | |
456 | let new = f(old); | |
457 | self.set(new); | |
458 | new | |
459 | } | |
a7813a04 XL |
460 | } |
461 | ||
8faf50e0 XL |
462 | impl<T: ?Sized> Cell<T> { |
463 | /// Returns a raw pointer to the underlying data in this cell. | |
464 | /// | |
465 | /// # Examples | |
466 | /// | |
467 | /// ``` | |
468 | /// use std::cell::Cell; | |
469 | /// | |
470 | /// let c = Cell::new(5); | |
471 | /// | |
472 | /// let ptr = c.as_ptr(); | |
473 | /// ``` | |
474 | #[inline] | |
475 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
dfeec247 | 476 | #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")] |
a1dfa0c6 | 477 | pub const fn as_ptr(&self) -> *mut T { |
8faf50e0 XL |
478 | self.value.get() |
479 | } | |
480 | ||
481 | /// Returns a mutable reference to the underlying data. | |
482 | /// | |
483 | /// This call borrows `Cell` mutably (at compile-time) which guarantees | |
484 | /// that we possess the only reference. | |
485 | /// | |
486 | /// # Examples | |
487 | /// | |
488 | /// ``` | |
489 | /// use std::cell::Cell; | |
490 | /// | |
491 | /// let mut c = Cell::new(5); | |
492 | /// *c.get_mut() += 1; | |
493 | /// | |
494 | /// assert_eq!(c.get(), 6); | |
495 | /// ``` | |
496 | #[inline] | |
497 | #[stable(feature = "cell_get_mut", since = "1.11.0")] | |
498 | pub fn get_mut(&mut self) -> &mut T { | |
1b1a35ee | 499 | self.value.get_mut() |
8faf50e0 XL |
500 | } |
501 | ||
502 | /// Returns a `&Cell<T>` from a `&mut T` | |
503 | /// | |
504 | /// # Examples | |
505 | /// | |
506 | /// ``` | |
8faf50e0 XL |
507 | /// use std::cell::Cell; |
508 | /// | |
509 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
510 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
511 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
512 | /// | |
513 | /// assert_eq!(slice_cell.len(), 3); | |
514 | /// ``` | |
515 | #[inline] | |
dc9dc135 | 516 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 517 | pub fn from_mut(t: &mut T) -> &Cell<T> { |
dfeec247 XL |
518 | // SAFETY: `&mut` ensures unique access. |
519 | unsafe { &*(t as *mut T as *const Cell<T>) } | |
8faf50e0 XL |
520 | } |
521 | } | |
522 | ||
8bb4bdeb XL |
523 | impl<T: Default> Cell<T> { |
524 | /// Takes the value of the cell, leaving `Default::default()` in its place. | |
525 | /// | |
526 | /// # Examples | |
527 | /// | |
528 | /// ``` | |
529 | /// use std::cell::Cell; | |
530 | /// | |
531 | /// let c = Cell::new(5); | |
532 | /// let five = c.take(); | |
533 | /// | |
534 | /// assert_eq!(five, 5); | |
535 | /// assert_eq!(c.into_inner(), 0); | |
536 | /// ``` | |
537 | #[stable(feature = "move_cell", since = "1.17.0")] | |
538 | pub fn take(&self) -> T { | |
539 | self.replace(Default::default()) | |
5bcae85e SL |
540 | } |
541 | } | |
542 | ||
9e0c209e SL |
543 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
544 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {} | |
545 | ||
8faf50e0 XL |
546 | impl<T> Cell<[T]> { |
547 | /// Returns a `&[Cell<T>]` from a `&Cell<[T]>` | |
548 | /// | |
549 | /// # Examples | |
550 | /// | |
551 | /// ``` | |
8faf50e0 XL |
552 | /// use std::cell::Cell; |
553 | /// | |
554 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
555 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
556 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
557 | /// | |
558 | /// assert_eq!(slice_cell.len(), 3); | |
559 | /// ``` | |
dc9dc135 | 560 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 561 | pub fn as_slice_of_cells(&self) -> &[Cell<T>] { |
dfeec247 XL |
562 | // SAFETY: `Cell<T>` has the same memory layout as `T`. |
563 | unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) } | |
8faf50e0 XL |
564 | } |
565 | } | |
566 | ||
1a4d82fc | 567 | /// A mutable memory location with dynamically checked borrow rules |
85aaf69f SL |
568 | /// |
569 | /// See the [module-level documentation](index.html) for more. | |
570 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 571 | pub struct RefCell<T: ?Sized> { |
1a4d82fc | 572 | borrow: Cell<BorrowFlag>, |
d9579d0f | 573 | value: UnsafeCell<T>, |
1a4d82fc JJ |
574 | } |
575 | ||
5bcae85e | 576 | /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow). |
9e0c209e SL |
577 | #[stable(feature = "try_borrow", since = "1.13.0")] |
578 | pub struct BorrowError { | |
579 | _private: (), | |
5bcae85e SL |
580 | } |
581 | ||
9e0c209e SL |
582 | #[stable(feature = "try_borrow", since = "1.13.0")] |
583 | impl Debug for BorrowError { | |
48663c56 | 584 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
585 | f.debug_struct("BorrowError").finish() |
586 | } | |
587 | } | |
588 | ||
9e0c209e SL |
589 | #[stable(feature = "try_borrow", since = "1.13.0")] |
590 | impl Display for BorrowError { | |
48663c56 | 591 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
592 | Display::fmt("already mutably borrowed", f) |
593 | } | |
594 | } | |
595 | ||
596 | /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut). | |
9e0c209e SL |
597 | #[stable(feature = "try_borrow", since = "1.13.0")] |
598 | pub struct BorrowMutError { | |
599 | _private: (), | |
5bcae85e SL |
600 | } |
601 | ||
9e0c209e SL |
602 | #[stable(feature = "try_borrow", since = "1.13.0")] |
603 | impl Debug for BorrowMutError { | |
48663c56 | 604 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
605 | f.debug_struct("BorrowMutError").finish() |
606 | } | |
607 | } | |
608 | ||
9e0c209e SL |
609 | #[stable(feature = "try_borrow", since = "1.13.0")] |
610 | impl Display for BorrowMutError { | |
48663c56 | 611 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
612 | Display::fmt("already borrowed", f) |
613 | } | |
614 | } | |
615 | ||
8faf50e0 XL |
616 | // Positive values represent the number of `Ref` active. Negative values |
617 | // represent the number of `RefMut` active. Multiple `RefMut`s can only be | |
618 | // active at a time if they refer to distinct, nonoverlapping components of a | |
619 | // `RefCell` (e.g., different ranges of a slice). | |
94b46f34 XL |
620 | // |
621 | // `Ref` and `RefMut` are both two words in size, and so there will likely never | |
622 | // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` | |
8faf50e0 XL |
623 | // range. Thus, a `BorrowFlag` will probably never overflow or underflow. |
624 | // However, this is not a guarantee, as a pathological program could repeatedly | |
625 | // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must | |
626 | // explicitly check for overflow and underflow in order to avoid unsafety, or at | |
627 | // least behave correctly in the event that overflow or underflow happens (e.g., | |
628 | // see BorrowRef::new). | |
629 | type BorrowFlag = isize; | |
1a4d82fc | 630 | const UNUSED: BorrowFlag = 0; |
8faf50e0 XL |
631 | |
632 | #[inline(always)] | |
633 | fn is_writing(x: BorrowFlag) -> bool { | |
634 | x < UNUSED | |
635 | } | |
636 | ||
637 | #[inline(always)] | |
638 | fn is_reading(x: BorrowFlag) -> bool { | |
639 | x > UNUSED | |
640 | } | |
1a4d82fc JJ |
641 | |
642 | impl<T> RefCell<T> { | |
85aaf69f SL |
643 | /// Creates a new `RefCell` containing `value`. |
644 | /// | |
645 | /// # Examples | |
646 | /// | |
647 | /// ``` | |
648 | /// use std::cell::RefCell; | |
649 | /// | |
650 | /// let c = RefCell::new(5); | |
651 | /// ``` | |
652 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 653 | #[rustc_const_stable(feature = "const_refcell_new", since = "1.32.0")] |
c34b1796 | 654 | #[inline] |
62682a34 | 655 | pub const fn new(value: T) -> RefCell<T> { |
dfeec247 | 656 | RefCell { value: UnsafeCell::new(value), borrow: Cell::new(UNUSED) } |
1a4d82fc JJ |
657 | } |
658 | ||
659 | /// Consumes the `RefCell`, returning the wrapped value. | |
85aaf69f SL |
660 | /// |
661 | /// # Examples | |
662 | /// | |
663 | /// ``` | |
664 | /// use std::cell::RefCell; | |
665 | /// | |
666 | /// let c = RefCell::new(5); | |
667 | /// | |
668 | /// let five = c.into_inner(); | |
669 | /// ``` | |
670 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 671 | #[inline] |
1a4d82fc JJ |
672 | pub fn into_inner(self) -> T { |
673 | // Since this function takes `self` (the `RefCell`) by value, the | |
674 | // compiler statically verifies that it is not currently borrowed. | |
675 | // Therefore the following assertion is just a `debug_assert!`. | |
676 | debug_assert!(self.borrow.get() == UNUSED); | |
2c00a5a8 | 677 | self.value.into_inner() |
1a4d82fc | 678 | } |
3b2f2976 XL |
679 | |
680 | /// Replaces the wrapped value with a new one, returning the old value, | |
681 | /// without deinitializing either one. | |
682 | /// | |
683 | /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). | |
684 | /// | |
abe05a73 XL |
685 | /// # Panics |
686 | /// | |
687 | /// Panics if the value is currently borrowed. | |
688 | /// | |
3b2f2976 XL |
689 | /// # Examples |
690 | /// | |
691 | /// ``` | |
3b2f2976 | 692 | /// use std::cell::RefCell; |
abe05a73 XL |
693 | /// let cell = RefCell::new(5); |
694 | /// let old_value = cell.replace(6); | |
695 | /// assert_eq!(old_value, 5); | |
696 | /// assert_eq!(cell, RefCell::new(6)); | |
3b2f2976 | 697 | /// ``` |
abe05a73 | 698 | #[inline] |
dfeec247 | 699 | #[stable(feature = "refcell_replace", since = "1.24.0")] |
1b1a35ee | 700 | #[track_caller] |
abe05a73 XL |
701 | pub fn replace(&self, t: T) -> T { |
702 | mem::replace(&mut *self.borrow_mut(), t) | |
703 | } | |
704 | ||
705 | /// Replaces the wrapped value with a new one computed from `f`, returning | |
706 | /// the old value, without deinitializing either one. | |
707 | /// | |
3b2f2976 XL |
708 | /// # Panics |
709 | /// | |
abe05a73 XL |
710 | /// Panics if the value is currently borrowed. |
711 | /// | |
712 | /// # Examples | |
713 | /// | |
714 | /// ``` | |
abe05a73 XL |
715 | /// use std::cell::RefCell; |
716 | /// let cell = RefCell::new(5); | |
717 | /// let old_value = cell.replace_with(|&mut old| old + 1); | |
718 | /// assert_eq!(old_value, 5); | |
719 | /// assert_eq!(cell, RefCell::new(6)); | |
720 | /// ``` | |
3b2f2976 | 721 | #[inline] |
dfeec247 | 722 | #[stable(feature = "refcell_replace_swap", since = "1.35.0")] |
1b1a35ee | 723 | #[track_caller] |
abe05a73 XL |
724 | pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { |
725 | let mut_borrow = &mut *self.borrow_mut(); | |
726 | let replacement = f(mut_borrow); | |
727 | mem::replace(mut_borrow, replacement) | |
3b2f2976 XL |
728 | } |
729 | ||
730 | /// Swaps the wrapped value of `self` with the wrapped value of `other`, | |
731 | /// without deinitializing either one. | |
732 | /// | |
733 | /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). | |
734 | /// | |
abe05a73 XL |
735 | /// # Panics |
736 | /// | |
737 | /// Panics if the value in either `RefCell` is currently borrowed. | |
738 | /// | |
3b2f2976 XL |
739 | /// # Examples |
740 | /// | |
741 | /// ``` | |
3b2f2976 XL |
742 | /// use std::cell::RefCell; |
743 | /// let c = RefCell::new(5); | |
744 | /// let d = RefCell::new(6); | |
745 | /// c.swap(&d); | |
746 | /// assert_eq!(c, RefCell::new(6)); | |
747 | /// assert_eq!(d, RefCell::new(5)); | |
748 | /// ``` | |
3b2f2976 | 749 | #[inline] |
dfeec247 | 750 | #[stable(feature = "refcell_swap", since = "1.24.0")] |
3b2f2976 XL |
751 | pub fn swap(&self, other: &Self) { |
752 | mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) | |
753 | } | |
d9579d0f | 754 | } |
1a4d82fc | 755 | |
d9579d0f | 756 | impl<T: ?Sized> RefCell<T> { |
1a4d82fc JJ |
757 | /// Immutably borrows the wrapped value. |
758 | /// | |
759 | /// The borrow lasts until the returned `Ref` exits scope. Multiple | |
760 | /// immutable borrows can be taken out at the same time. | |
761 | /// | |
762 | /// # Panics | |
763 | /// | |
5bcae85e SL |
764 | /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use |
765 | /// [`try_borrow`](#method.try_borrow). | |
85aaf69f SL |
766 | /// |
767 | /// # Examples | |
768 | /// | |
769 | /// ``` | |
770 | /// use std::cell::RefCell; | |
771 | /// | |
772 | /// let c = RefCell::new(5); | |
773 | /// | |
774 | /// let borrowed_five = c.borrow(); | |
775 | /// let borrowed_five2 = c.borrow(); | |
776 | /// ``` | |
777 | /// | |
778 | /// An example of panic: | |
779 | /// | |
f035d41b | 780 | /// ```should_panic |
85aaf69f | 781 | /// use std::cell::RefCell; |
85aaf69f | 782 | /// |
f035d41b | 783 | /// let c = RefCell::new(5); |
85aaf69f | 784 | /// |
f035d41b XL |
785 | /// let m = c.borrow_mut(); |
786 | /// let b = c.borrow(); // this causes a panic | |
85aaf69f SL |
787 | /// ``` |
788 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 789 | #[inline] |
3dfed10e | 790 | #[track_caller] |
48663c56 | 791 | pub fn borrow(&self) -> Ref<'_, T> { |
5bcae85e SL |
792 | self.try_borrow().expect("already mutably borrowed") |
793 | } | |
794 | ||
795 | /// Immutably borrows the wrapped value, returning an error if the value is currently mutably | |
796 | /// borrowed. | |
797 | /// | |
798 | /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be | |
799 | /// taken out at the same time. | |
800 | /// | |
801 | /// This is the non-panicking variant of [`borrow`](#method.borrow). | |
802 | /// | |
803 | /// # Examples | |
804 | /// | |
805 | /// ``` | |
5bcae85e SL |
806 | /// use std::cell::RefCell; |
807 | /// | |
808 | /// let c = RefCell::new(5); | |
809 | /// | |
810 | /// { | |
811 | /// let m = c.borrow_mut(); | |
812 | /// assert!(c.try_borrow().is_err()); | |
813 | /// } | |
814 | /// | |
815 | /// { | |
816 | /// let m = c.borrow(); | |
817 | /// assert!(c.try_borrow().is_ok()); | |
818 | /// } | |
819 | /// ``` | |
9e0c209e | 820 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 821 | #[inline] |
48663c56 | 822 | pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { |
85aaf69f | 823 | match BorrowRef::new(&self.borrow) { |
dfeec247 XL |
824 | // SAFETY: `BorrowRef` ensures that there is only immutable access |
825 | // to the value while borrowed. | |
826 | Some(b) => Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }), | |
9e0c209e | 827 | None => Err(BorrowError { _private: () }), |
1a4d82fc JJ |
828 | } |
829 | } | |
830 | ||
1a4d82fc JJ |
831 | /// Mutably borrows the wrapped value. |
832 | /// | |
94b46f34 XL |
833 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
834 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
835 | /// active. | |
1a4d82fc JJ |
836 | /// |
837 | /// # Panics | |
838 | /// | |
5bcae85e SL |
839 | /// Panics if the value is currently borrowed. For a non-panicking variant, use |
840 | /// [`try_borrow_mut`](#method.try_borrow_mut). | |
85aaf69f SL |
841 | /// |
842 | /// # Examples | |
843 | /// | |
844 | /// ``` | |
845 | /// use std::cell::RefCell; | |
846 | /// | |
f9f354fc | 847 | /// let c = RefCell::new("hello".to_owned()); |
85aaf69f | 848 | /// |
f9f354fc | 849 | /// *c.borrow_mut() = "bonjour".to_owned(); |
7453a54e | 850 | /// |
f9f354fc | 851 | /// assert_eq!(&*c.borrow(), "bonjour"); |
85aaf69f SL |
852 | /// ``` |
853 | /// | |
854 | /// An example of panic: | |
855 | /// | |
f035d41b | 856 | /// ```should_panic |
85aaf69f | 857 | /// use std::cell::RefCell; |
85aaf69f | 858 | /// |
f035d41b XL |
859 | /// let c = RefCell::new(5); |
860 | /// let m = c.borrow(); | |
85aaf69f | 861 | /// |
f035d41b | 862 | /// let b = c.borrow_mut(); // this causes a panic |
85aaf69f SL |
863 | /// ``` |
864 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 865 | #[inline] |
3dfed10e | 866 | #[track_caller] |
48663c56 | 867 | pub fn borrow_mut(&self) -> RefMut<'_, T> { |
5bcae85e SL |
868 | self.try_borrow_mut().expect("already borrowed") |
869 | } | |
870 | ||
871 | /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. | |
872 | /// | |
94b46f34 XL |
873 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
874 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
875 | /// active. | |
5bcae85e SL |
876 | /// |
877 | /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). | |
878 | /// | |
879 | /// # Examples | |
880 | /// | |
881 | /// ``` | |
5bcae85e SL |
882 | /// use std::cell::RefCell; |
883 | /// | |
884 | /// let c = RefCell::new(5); | |
885 | /// | |
886 | /// { | |
887 | /// let m = c.borrow(); | |
888 | /// assert!(c.try_borrow_mut().is_err()); | |
889 | /// } | |
890 | /// | |
891 | /// assert!(c.try_borrow_mut().is_ok()); | |
892 | /// ``` | |
9e0c209e | 893 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 894 | #[inline] |
48663c56 | 895 | pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { |
85aaf69f | 896 | match BorrowRefMut::new(&self.borrow) { |
dfeec247 XL |
897 | // SAFETY: `BorrowRef` guarantees unique access. |
898 | Some(b) => Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }), | |
9e0c209e | 899 | None => Err(BorrowMutError { _private: () }), |
1a4d82fc JJ |
900 | } |
901 | } | |
902 | ||
5bcae85e SL |
903 | /// Returns a raw pointer to the underlying data in this cell. |
904 | /// | |
905 | /// # Examples | |
906 | /// | |
907 | /// ``` | |
908 | /// use std::cell::RefCell; | |
909 | /// | |
910 | /// let c = RefCell::new(5); | |
911 | /// | |
912 | /// let ptr = c.as_ptr(); | |
913 | /// ``` | |
914 | #[inline] | |
915 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
916 | pub fn as_ptr(&self) -> *mut T { | |
917 | self.value.get() | |
918 | } | |
919 | ||
a7813a04 XL |
920 | /// Returns a mutable reference to the underlying data. |
921 | /// | |
922 | /// This call borrows `RefCell` mutably (at compile-time) so there is no | |
923 | /// need for dynamic checks. | |
5bcae85e | 924 | /// |
cc61c64b XL |
925 | /// However be cautious: this method expects `self` to be mutable, which is |
926 | /// generally not the case when using a `RefCell`. Take a look at the | |
927 | /// [`borrow_mut`] method instead if `self` isn't mutable. | |
928 | /// | |
929 | /// Also, please be aware that this method is only for special circumstances and is usually | |
3b2f2976 | 930 | /// not what you want. In case of doubt, use [`borrow_mut`] instead. |
cc61c64b XL |
931 | /// |
932 | /// [`borrow_mut`]: #method.borrow_mut | |
933 | /// | |
5bcae85e SL |
934 | /// # Examples |
935 | /// | |
936 | /// ``` | |
937 | /// use std::cell::RefCell; | |
938 | /// | |
939 | /// let mut c = RefCell::new(5); | |
940 | /// *c.get_mut() += 1; | |
941 | /// | |
942 | /// assert_eq!(c, RefCell::new(6)); | |
943 | /// ``` | |
a7813a04 | 944 | #[inline] |
3157f602 | 945 | #[stable(feature = "cell_get_mut", since = "1.11.0")] |
a7813a04 | 946 | pub fn get_mut(&mut self) -> &mut T { |
1b1a35ee | 947 | self.value.get_mut() |
a7813a04 | 948 | } |
48663c56 | 949 | |
ba9703b0 XL |
950 | /// Undo the effect of leaked guards on the borrow state of the `RefCell`. |
951 | /// | |
952 | /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to | |
953 | /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant | |
954 | /// if some `Ref` or `RefMut` borrows have been leaked. | |
955 | /// | |
956 | /// [`get_mut`]: #method.get_mut | |
957 | /// | |
958 | /// # Examples | |
959 | /// | |
960 | /// ``` | |
961 | /// #![feature(cell_leak)] | |
962 | /// use std::cell::RefCell; | |
963 | /// | |
964 | /// let mut c = RefCell::new(0); | |
965 | /// std::mem::forget(c.borrow_mut()); | |
966 | /// | |
967 | /// assert!(c.try_borrow().is_err()); | |
968 | /// c.undo_leak(); | |
969 | /// assert!(c.try_borrow().is_ok()); | |
970 | /// ``` | |
971 | #[unstable(feature = "cell_leak", issue = "69099")] | |
972 | pub fn undo_leak(&mut self) -> &mut T { | |
973 | *self.borrow.get_mut() = UNUSED; | |
974 | self.get_mut() | |
975 | } | |
976 | ||
48663c56 XL |
977 | /// Immutably borrows the wrapped value, returning an error if the value is |
978 | /// currently mutably borrowed. | |
979 | /// | |
980 | /// # Safety | |
981 | /// | |
982 | /// Unlike `RefCell::borrow`, this method is unsafe because it does not | |
983 | /// return a `Ref`, thus leaving the borrow flag untouched. Mutably | |
984 | /// borrowing the `RefCell` while the reference returned by this method | |
985 | /// is alive is undefined behaviour. | |
986 | /// | |
987 | /// # Examples | |
988 | /// | |
989 | /// ``` | |
48663c56 XL |
990 | /// use std::cell::RefCell; |
991 | /// | |
992 | /// let c = RefCell::new(5); | |
993 | /// | |
994 | /// { | |
995 | /// let m = c.borrow_mut(); | |
996 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); | |
997 | /// } | |
998 | /// | |
999 | /// { | |
1000 | /// let m = c.borrow(); | |
1001 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); | |
1002 | /// } | |
1003 | /// ``` | |
dc9dc135 | 1004 | #[stable(feature = "borrow_state", since = "1.37.0")] |
48663c56 XL |
1005 | #[inline] |
1006 | pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { | |
1007 | if !is_writing(self.borrow.get()) { | |
f035d41b XL |
1008 | // SAFETY: We check that nobody is actively writing now, but it is |
1009 | // the caller's responsibility to ensure that nobody writes until | |
1010 | // the returned reference is no longer in use. | |
1011 | // Also, `self.value.get()` refers to the value owned by `self` | |
1012 | // and is thus guaranteed to be valid for the lifetime of `self`. | |
1013 | Ok(unsafe { &*self.value.get() }) | |
48663c56 XL |
1014 | } else { |
1015 | Err(BorrowError { _private: () }) | |
1016 | } | |
1017 | } | |
1a4d82fc JJ |
1018 | } |
1019 | ||
f9f354fc XL |
1020 | impl<T: Default> RefCell<T> { |
1021 | /// Takes the wrapped value, leaving `Default::default()` in its place. | |
1022 | /// | |
1023 | /// # Panics | |
1024 | /// | |
1025 | /// Panics if the value is currently borrowed. | |
1026 | /// | |
1027 | /// # Examples | |
1028 | /// | |
1029 | /// ``` | |
1030 | /// #![feature(refcell_take)] | |
1031 | /// use std::cell::RefCell; | |
1032 | /// | |
1033 | /// let c = RefCell::new(5); | |
1034 | /// let five = c.take(); | |
1035 | /// | |
1036 | /// assert_eq!(five, 5); | |
1037 | /// assert_eq!(c.into_inner(), 0); | |
1038 | /// ``` | |
1039 | #[unstable(feature = "refcell_take", issue = "71395")] | |
1040 | pub fn take(&self) -> T { | |
1041 | self.replace(Default::default()) | |
1042 | } | |
1043 | } | |
1044 | ||
85aaf69f | 1045 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1046 | unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {} |
1a4d82fc | 1047 | |
54a0048b SL |
1048 | #[stable(feature = "rust1", since = "1.0.0")] |
1049 | impl<T: ?Sized> !Sync for RefCell<T> {} | |
1050 | ||
85aaf69f | 1051 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 1052 | impl<T: Clone> Clone for RefCell<T> { |
0531ce1d XL |
1053 | /// # Panics |
1054 | /// | |
1055 | /// Panics if the value is currently mutably borrowed. | |
c34b1796 | 1056 | #[inline] |
1b1a35ee | 1057 | #[track_caller] |
1a4d82fc JJ |
1058 | fn clone(&self) -> RefCell<T> { |
1059 | RefCell::new(self.borrow().clone()) | |
1060 | } | |
1061 | } | |
1062 | ||
85aaf69f | 1063 | #[stable(feature = "rust1", since = "1.0.0")] |
416331ca | 1064 | impl<T: Default> Default for RefCell<T> { |
9e0c209e | 1065 | /// Creates a `RefCell<T>`, with the `Default` value for T. |
c34b1796 | 1066 | #[inline] |
1a4d82fc JJ |
1067 | fn default() -> RefCell<T> { |
1068 | RefCell::new(Default::default()) | |
1069 | } | |
1070 | } | |
1071 | ||
85aaf69f | 1072 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1073 | impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { |
0531ce1d XL |
1074 | /// # Panics |
1075 | /// | |
1076 | /// Panics if the value in either `RefCell` is currently borrowed. | |
c34b1796 | 1077 | #[inline] |
1a4d82fc JJ |
1078 | fn eq(&self, other: &RefCell<T>) -> bool { |
1079 | *self.borrow() == *other.borrow() | |
1080 | } | |
1081 | } | |
1082 | ||
62682a34 SL |
1083 | #[stable(feature = "cell_eq", since = "1.2.0")] |
1084 | impl<T: ?Sized + Eq> Eq for RefCell<T> {} | |
1085 | ||
a7813a04 XL |
1086 | #[stable(feature = "cell_ord", since = "1.10.0")] |
1087 | impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { | |
0531ce1d XL |
1088 | /// # Panics |
1089 | /// | |
1090 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1091 | #[inline] |
1092 | fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { | |
1093 | self.borrow().partial_cmp(&*other.borrow()) | |
1094 | } | |
1095 | ||
0531ce1d XL |
1096 | /// # Panics |
1097 | /// | |
1098 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1099 | #[inline] |
1100 | fn lt(&self, other: &RefCell<T>) -> bool { | |
1101 | *self.borrow() < *other.borrow() | |
1102 | } | |
1103 | ||
0531ce1d XL |
1104 | /// # Panics |
1105 | /// | |
1106 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1107 | #[inline] |
1108 | fn le(&self, other: &RefCell<T>) -> bool { | |
1109 | *self.borrow() <= *other.borrow() | |
1110 | } | |
1111 | ||
0531ce1d XL |
1112 | /// # Panics |
1113 | /// | |
1114 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1115 | #[inline] |
1116 | fn gt(&self, other: &RefCell<T>) -> bool { | |
1117 | *self.borrow() > *other.borrow() | |
1118 | } | |
1119 | ||
0531ce1d XL |
1120 | /// # Panics |
1121 | /// | |
1122 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1123 | #[inline] |
1124 | fn ge(&self, other: &RefCell<T>) -> bool { | |
1125 | *self.borrow() >= *other.borrow() | |
1126 | } | |
1127 | } | |
1128 | ||
1129 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
1130 | impl<T: ?Sized + Ord> Ord for RefCell<T> { | |
0531ce1d XL |
1131 | /// # Panics |
1132 | /// | |
1133 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1134 | #[inline] |
1135 | fn cmp(&self, other: &RefCell<T>) -> Ordering { | |
1136 | self.borrow().cmp(&*other.borrow()) | |
1137 | } | |
1138 | } | |
1139 | ||
5bcae85e SL |
1140 | #[stable(feature = "cell_from", since = "1.12.0")] |
1141 | impl<T> From<T> for RefCell<T> { | |
1142 | fn from(t: T) -> RefCell<T> { | |
1143 | RefCell::new(t) | |
1144 | } | |
1145 | } | |
1146 | ||
9e0c209e SL |
1147 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1148 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} | |
1149 | ||
1a4d82fc | 1150 | struct BorrowRef<'b> { |
54a0048b | 1151 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1152 | } |
1153 | ||
1154 | impl<'b> BorrowRef<'b> { | |
c34b1796 | 1155 | #[inline] |
1a4d82fc | 1156 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { |
416331ca XL |
1157 | let b = borrow.get().wrapping_add(1); |
1158 | if !is_reading(b) { | |
1159 | // Incrementing borrow can result in a non-reading value (<= 0) in these cases: | |
1160 | // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow | |
1161 | // due to Rust's reference aliasing rules | |
f035d41b XL |
1162 | // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed |
1163 | // into isize::MIN (the max amount of writing borrows) so we can't allow | |
416331ca XL |
1164 | // an additional read borrow because isize can't represent so many read borrows |
1165 | // (this can only happen if you mem::forget more than a small constant amount of | |
1166 | // `Ref`s, which is not good practice) | |
94b46f34 XL |
1167 | None |
1168 | } else { | |
416331ca XL |
1169 | // Incrementing borrow can result in a reading value (> 0) in these cases: |
1170 | // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow | |
f035d41b | 1171 | // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize |
416331ca XL |
1172 | // is large enough to represent having one more read borrow |
1173 | borrow.set(b); | |
94b46f34 | 1174 | Some(BorrowRef { borrow }) |
1a4d82fc JJ |
1175 | } |
1176 | } | |
1177 | } | |
1178 | ||
0bf4aa26 | 1179 | impl Drop for BorrowRef<'_> { |
c34b1796 | 1180 | #[inline] |
1a4d82fc | 1181 | fn drop(&mut self) { |
54a0048b | 1182 | let borrow = self.borrow.get(); |
8faf50e0 | 1183 | debug_assert!(is_reading(borrow)); |
54a0048b | 1184 | self.borrow.set(borrow - 1); |
1a4d82fc JJ |
1185 | } |
1186 | } | |
1187 | ||
0bf4aa26 | 1188 | impl Clone for BorrowRef<'_> { |
c34b1796 | 1189 | #[inline] |
0bf4aa26 | 1190 | fn clone(&self) -> Self { |
1a4d82fc | 1191 | // Since this Ref exists, we know the borrow flag |
8faf50e0 | 1192 | // is a reading borrow. |
54a0048b | 1193 | let borrow = self.borrow.get(); |
8faf50e0 | 1194 | debug_assert!(is_reading(borrow)); |
94b46f34 XL |
1195 | // Prevent the borrow counter from overflowing into |
1196 | // a writing borrow. | |
f035d41b | 1197 | assert!(borrow != isize::MAX); |
54a0048b SL |
1198 | self.borrow.set(borrow + 1); |
1199 | BorrowRef { borrow: self.borrow } | |
1a4d82fc JJ |
1200 | } |
1201 | } | |
1202 | ||
1203 | /// Wraps a borrowed reference to a value in a `RefCell` box. | |
85aaf69f SL |
1204 | /// A wrapper type for an immutably borrowed value from a `RefCell<T>`. |
1205 | /// | |
1206 | /// See the [module-level documentation](index.html) for more. | |
1207 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 1208 | pub struct Ref<'b, T: ?Sized + 'b> { |
54a0048b SL |
1209 | value: &'b T, |
1210 | borrow: BorrowRef<'b>, | |
1a4d82fc JJ |
1211 | } |
1212 | ||
85aaf69f | 1213 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1214 | impl<T: ?Sized> Deref for Ref<'_, T> { |
1a4d82fc JJ |
1215 | type Target = T; |
1216 | ||
1217 | #[inline] | |
e9174d1e | 1218 | fn deref(&self) -> &T { |
54a0048b | 1219 | self.value |
1a4d82fc JJ |
1220 | } |
1221 | } | |
1222 | ||
62682a34 SL |
1223 | impl<'b, T: ?Sized> Ref<'b, T> { |
1224 | /// Copies a `Ref`. | |
1225 | /// | |
1226 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1227 | /// | |
1228 | /// This is an associated function that needs to be used as | |
9fa01778 | 1229 | /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere |
62682a34 SL |
1230 | /// with the widespread use of `r.borrow().clone()` to clone the contents of |
1231 | /// a `RefCell`. | |
476ff2be | 1232 | #[stable(feature = "cell_extras", since = "1.15.0")] |
62682a34 SL |
1233 | #[inline] |
1234 | pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { | |
dfeec247 | 1235 | Ref { value: orig.value, borrow: orig.borrow.clone() } |
62682a34 SL |
1236 | } |
1237 | ||
9fa01778 | 1238 | /// Makes a new `Ref` for a component of the borrowed data. |
62682a34 SL |
1239 | /// |
1240 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1241 | /// | |
1242 | /// This is an associated function that needs to be used as `Ref::map(...)`. | |
1243 | /// A method would interfere with methods of the same name on the contents | |
1244 | /// of a `RefCell` used through `Deref`. | |
1245 | /// | |
3b2f2976 | 1246 | /// # Examples |
62682a34 SL |
1247 | /// |
1248 | /// ``` | |
62682a34 SL |
1249 | /// use std::cell::{RefCell, Ref}; |
1250 | /// | |
1251 | /// let c = RefCell::new((5, 'b')); | |
1252 | /// let b1: Ref<(u32, char)> = c.borrow(); | |
1253 | /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); | |
1254 | /// assert_eq!(*b2, 5) | |
1255 | /// ``` | |
7453a54e | 1256 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1257 | #[inline] |
1258 | pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> | |
dfeec247 XL |
1259 | where |
1260 | F: FnOnce(&T) -> &U, | |
62682a34 | 1261 | { |
dfeec247 | 1262 | Ref { value: f(orig.value), borrow: orig.borrow } |
62682a34 | 1263 | } |
94b46f34 | 1264 | |
9fa01778 | 1265 | /// Splits a `Ref` into multiple `Ref`s for different components of the |
94b46f34 XL |
1266 | /// borrowed data. |
1267 | /// | |
1268 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1269 | /// | |
1270 | /// This is an associated function that needs to be used as | |
1271 | /// `Ref::map_split(...)`. A method would interfere with methods of the same | |
1272 | /// name on the contents of a `RefCell` used through `Deref`. | |
1273 | /// | |
1274 | /// # Examples | |
1275 | /// | |
1276 | /// ``` | |
94b46f34 XL |
1277 | /// use std::cell::{Ref, RefCell}; |
1278 | /// | |
1279 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1280 | /// let borrow = cell.borrow(); | |
1281 | /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2)); | |
1282 | /// assert_eq!(*begin, [1, 2]); | |
1283 | /// assert_eq!(*end, [3, 4]); | |
1284 | /// ``` | |
532ac7d7 | 1285 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1286 | #[inline] |
1287 | pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>) | |
dfeec247 XL |
1288 | where |
1289 | F: FnOnce(&T) -> (&U, &V), | |
94b46f34 XL |
1290 | { |
1291 | let (a, b) = f(orig.value); | |
1292 | let borrow = orig.borrow.clone(); | |
1293 | (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow }) | |
1294 | } | |
74b04a01 XL |
1295 | |
1296 | /// Convert into a reference to the underlying data. | |
1297 | /// | |
1298 | /// The underlying `RefCell` can never be mutably borrowed from again and will always appear | |
1299 | /// already immutably borrowed. It is not a good idea to leak more than a constant number of | |
1300 | /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks | |
1301 | /// have occurred in total. | |
1302 | /// | |
1303 | /// This is an associated function that needs to be used as | |
1304 | /// `Ref::leak(...)`. A method would interfere with methods of the | |
1305 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1306 | /// | |
1307 | /// # Examples | |
1308 | /// | |
1309 | /// ``` | |
1310 | /// #![feature(cell_leak)] | |
1311 | /// use std::cell::{RefCell, Ref}; | |
1312 | /// let cell = RefCell::new(0); | |
1313 | /// | |
1314 | /// let value = Ref::leak(cell.borrow()); | |
1315 | /// assert_eq!(*value, 0); | |
1316 | /// | |
1317 | /// assert!(cell.try_borrow().is_ok()); | |
1318 | /// assert!(cell.try_borrow_mut().is_err()); | |
1319 | /// ``` | |
1320 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1321 | pub fn leak(orig: Ref<'b, T>) -> &'b T { | |
ba9703b0 XL |
1322 | // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to |
1323 | // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a | |
1324 | // unique reference to the borrowed RefCell. No further mutable references can be created | |
1325 | // from the original cell. | |
74b04a01 XL |
1326 | mem::forget(orig.borrow); |
1327 | orig.value | |
1328 | } | |
62682a34 SL |
1329 | } |
1330 | ||
54a0048b SL |
1331 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1332 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} | |
1333 | ||
041b39d2 | 1334 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1335 | impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { |
48663c56 | 1336 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1337 | self.value.fmt(f) |
1338 | } | |
1339 | } | |
1340 | ||
62682a34 | 1341 | impl<'b, T: ?Sized> RefMut<'b, T> { |
9fa01778 | 1342 | /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum |
62682a34 SL |
1343 | /// variant. |
1344 | /// | |
1345 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1346 | /// | |
1347 | /// This is an associated function that needs to be used as | |
9fa01778 | 1348 | /// `RefMut::map(...)`. A method would interfere with methods of the same |
62682a34 SL |
1349 | /// name on the contents of a `RefCell` used through `Deref`. |
1350 | /// | |
3b2f2976 | 1351 | /// # Examples |
62682a34 SL |
1352 | /// |
1353 | /// ``` | |
62682a34 SL |
1354 | /// use std::cell::{RefCell, RefMut}; |
1355 | /// | |
1356 | /// let c = RefCell::new((5, 'b')); | |
1357 | /// { | |
1358 | /// let b1: RefMut<(u32, char)> = c.borrow_mut(); | |
1359 | /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0); | |
1360 | /// assert_eq!(*b2, 5); | |
1361 | /// *b2 = 42; | |
1362 | /// } | |
1363 | /// assert_eq!(*c.borrow(), (42, 'b')); | |
1364 | /// ``` | |
7453a54e | 1365 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1366 | #[inline] |
1367 | pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U> | |
dfeec247 XL |
1368 | where |
1369 | F: FnOnce(&mut T) -> &mut U, | |
62682a34 | 1370 | { |
ff7c6d11 XL |
1371 | // FIXME(nll-rfc#40): fix borrow-check |
1372 | let RefMut { value, borrow } = orig; | |
dfeec247 | 1373 | RefMut { value: f(value), borrow } |
62682a34 | 1374 | } |
94b46f34 | 1375 | |
9fa01778 | 1376 | /// Splits a `RefMut` into multiple `RefMut`s for different components of the |
94b46f34 XL |
1377 | /// borrowed data. |
1378 | /// | |
1379 | /// The underlying `RefCell` will remain mutably borrowed until both | |
1380 | /// returned `RefMut`s go out of scope. | |
1381 | /// | |
1382 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1383 | /// | |
1384 | /// This is an associated function that needs to be used as | |
1385 | /// `RefMut::map_split(...)`. A method would interfere with methods of the | |
1386 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1387 | /// | |
1388 | /// # Examples | |
1389 | /// | |
1390 | /// ``` | |
94b46f34 XL |
1391 | /// use std::cell::{RefCell, RefMut}; |
1392 | /// | |
1393 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1394 | /// let borrow = cell.borrow_mut(); | |
1395 | /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2)); | |
1396 | /// assert_eq!(*begin, [1, 2]); | |
1397 | /// assert_eq!(*end, [3, 4]); | |
1398 | /// begin.copy_from_slice(&[4, 3]); | |
1399 | /// end.copy_from_slice(&[2, 1]); | |
1400 | /// ``` | |
532ac7d7 | 1401 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1402 | #[inline] |
1403 | pub fn map_split<U: ?Sized, V: ?Sized, F>( | |
dfeec247 XL |
1404 | orig: RefMut<'b, T>, |
1405 | f: F, | |
94b46f34 | 1406 | ) -> (RefMut<'b, U>, RefMut<'b, V>) |
dfeec247 XL |
1407 | where |
1408 | F: FnOnce(&mut T) -> (&mut U, &mut V), | |
94b46f34 XL |
1409 | { |
1410 | let (a, b) = f(orig.value); | |
1411 | let borrow = orig.borrow.clone(); | |
1412 | (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow }) | |
1413 | } | |
74b04a01 XL |
1414 | |
1415 | /// Convert into a mutable reference to the underlying data. | |
1416 | /// | |
1417 | /// The underlying `RefCell` can not be borrowed from again and will always appear already | |
1418 | /// mutably borrowed, making the returned reference the only to the interior. | |
1419 | /// | |
1420 | /// This is an associated function that needs to be used as | |
1421 | /// `RefMut::leak(...)`. A method would interfere with methods of the | |
1422 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1423 | /// | |
1424 | /// # Examples | |
1425 | /// | |
1426 | /// ``` | |
1427 | /// #![feature(cell_leak)] | |
1428 | /// use std::cell::{RefCell, RefMut}; | |
1429 | /// let cell = RefCell::new(0); | |
1430 | /// | |
1431 | /// let value = RefMut::leak(cell.borrow_mut()); | |
1432 | /// assert_eq!(*value, 0); | |
1433 | /// *value = 1; | |
1434 | /// | |
1435 | /// assert!(cell.try_borrow_mut().is_err()); | |
1436 | /// ``` | |
1437 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1438 | pub fn leak(orig: RefMut<'b, T>) -> &'b mut T { | |
ba9703b0 XL |
1439 | // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't |
1440 | // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would | |
1441 | // require a unique reference to the borrowed RefCell. No further references can be created | |
1442 | // from the original cell within that lifetime, making the current borrow the only | |
1443 | // reference for the remaining lifetime. | |
74b04a01 XL |
1444 | mem::forget(orig.borrow); |
1445 | orig.value | |
1446 | } | |
1a4d82fc JJ |
1447 | } |
1448 | ||
1449 | struct BorrowRefMut<'b> { | |
54a0048b | 1450 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1451 | } |
1452 | ||
0bf4aa26 | 1453 | impl Drop for BorrowRefMut<'_> { |
c34b1796 | 1454 | #[inline] |
1a4d82fc | 1455 | fn drop(&mut self) { |
54a0048b | 1456 | let borrow = self.borrow.get(); |
8faf50e0 XL |
1457 | debug_assert!(is_writing(borrow)); |
1458 | self.borrow.set(borrow + 1); | |
1a4d82fc JJ |
1459 | } |
1460 | } | |
1461 | ||
1462 | impl<'b> BorrowRefMut<'b> { | |
c34b1796 | 1463 | #[inline] |
1a4d82fc | 1464 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { |
94b46f34 XL |
1465 | // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial |
1466 | // mutable reference, and so there must currently be no existing | |
1467 | // references. Thus, while clone increments the mutable refcount, here | |
8faf50e0 | 1468 | // we explicitly only allow going from UNUSED to UNUSED - 1. |
1a4d82fc JJ |
1469 | match borrow.get() { |
1470 | UNUSED => { | |
8faf50e0 | 1471 | borrow.set(UNUSED - 1); |
b7449926 | 1472 | Some(BorrowRefMut { borrow }) |
dfeec247 | 1473 | } |
1a4d82fc JJ |
1474 | _ => None, |
1475 | } | |
1476 | } | |
94b46f34 | 1477 | |
dc9dc135 | 1478 | // Clones a `BorrowRefMut`. |
94b46f34 XL |
1479 | // |
1480 | // This is only valid if each `BorrowRefMut` is used to track a mutable | |
1481 | // reference to a distinct, nonoverlapping range of the original object. | |
1482 | // This isn't in a Clone impl so that code doesn't call this implicitly. | |
1483 | #[inline] | |
1484 | fn clone(&self) -> BorrowRefMut<'b> { | |
1485 | let borrow = self.borrow.get(); | |
8faf50e0 XL |
1486 | debug_assert!(is_writing(borrow)); |
1487 | // Prevent the borrow counter from underflowing. | |
f035d41b | 1488 | assert!(borrow != isize::MIN); |
8faf50e0 | 1489 | self.borrow.set(borrow - 1); |
94b46f34 XL |
1490 | BorrowRefMut { borrow: self.borrow } |
1491 | } | |
1a4d82fc JJ |
1492 | } |
1493 | ||
85aaf69f SL |
1494 | /// A wrapper type for a mutably borrowed value from a `RefCell<T>`. |
1495 | /// | |
1496 | /// See the [module-level documentation](index.html) for more. | |
1497 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 1498 | pub struct RefMut<'b, T: ?Sized + 'b> { |
54a0048b SL |
1499 | value: &'b mut T, |
1500 | borrow: BorrowRefMut<'b>, | |
1a4d82fc JJ |
1501 | } |
1502 | ||
85aaf69f | 1503 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1504 | impl<T: ?Sized> Deref for RefMut<'_, T> { |
1a4d82fc JJ |
1505 | type Target = T; |
1506 | ||
1507 | #[inline] | |
e9174d1e | 1508 | fn deref(&self) -> &T { |
54a0048b | 1509 | self.value |
1a4d82fc JJ |
1510 | } |
1511 | } | |
1512 | ||
85aaf69f | 1513 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1514 | impl<T: ?Sized> DerefMut for RefMut<'_, T> { |
1a4d82fc | 1515 | #[inline] |
e9174d1e | 1516 | fn deref_mut(&mut self) -> &mut T { |
54a0048b | 1517 | self.value |
1a4d82fc JJ |
1518 | } |
1519 | } | |
1520 | ||
54a0048b SL |
1521 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1522 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} | |
1523 | ||
041b39d2 | 1524 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1525 | impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { |
48663c56 | 1526 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1527 | self.value.fmt(f) |
1528 | } | |
1529 | } | |
1530 | ||
1a4d82fc JJ |
1531 | /// The core primitive for interior mutability in Rust. |
1532 | /// | |
85aaf69f SL |
1533 | /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the |
1534 | /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'. | |
1535 | /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered | |
1536 | /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior. | |
1a4d82fc | 1537 | /// |
0531ce1d XL |
1538 | /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are |
1539 | /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably | |
94b46f34 | 1540 | /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language |
416331ca XL |
1541 | /// feature to work around the restriction that `&T` may not be mutated. All other types that |
1542 | /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their | |
1543 | /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`. | |
5bcae85e | 1544 | /// |
1b1a35ee XL |
1545 | /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer |
1546 | /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer | |
1547 | /// correctly. | |
1548 | /// | |
1549 | /// [`.get()`]: `UnsafeCell::get` | |
0531ce1d XL |
1550 | /// |
1551 | /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: | |
1552 | /// | |
94b46f34 XL |
1553 | /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` |
1554 | /// reference) that is accessible by safe code (for example, because you returned it), | |
1555 | /// then you must not access the data in any way that contradicts that reference for the | |
1556 | /// remainder of `'a`. For example, this means that if you take the `*mut T` from an | |
1557 | /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable | |
1558 | /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's | |
1559 | /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to | |
1560 | /// safe code, then you must not access the data within the `UnsafeCell` until that | |
1561 | /// reference expires. | |
0531ce1d | 1562 | /// |
94b46f34 | 1563 | /// - At all times, you must avoid data races. If multiple threads have access to |
0531ce1d XL |
1564 | /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other |
1565 | /// accesses (or use atomics). | |
5bcae85e | 1566 | /// |
0531ce1d XL |
1567 | /// To assist with proper design, the following scenarios are explicitly declared legal |
1568 | /// for single-threaded code: | |
5bcae85e | 1569 | /// |
94b46f34 | 1570 | /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` |
0531ce1d XL |
1571 | /// references, but not with a `&mut T` |
1572 | /// | |
94b46f34 | 1573 | /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` |
0531ce1d XL |
1574 | /// co-exist with it. A `&mut T` must always be unique. |
1575 | /// | |
1b1a35ee XL |
1576 | /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other |
1577 | /// `&UnsafeCell<T>` references alias the cell) is | |
1578 | /// ok (provided you enforce the above invariants some other way), it is still undefined behavior | |
1579 | /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper | |
1580 | /// designed to have a special interaction with _shared_ accesses (_i.e._, through an | |
1581 | /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_ | |
1582 | /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value | |
1583 | /// may be aliased for the duration of that `&mut` borrow. | |
1584 | /// This is showcased by the [`.get_mut()`] accessor, which is a non-`unsafe` getter that yields | |
1585 | /// a `&mut T`. | |
1586 | /// | |
1587 | /// [`.get_mut()`]: `UnsafeCell::get_mut` | |
1a4d82fc | 1588 | /// |
85aaf69f | 1589 | /// # Examples |
1a4d82fc | 1590 | /// |
1b1a35ee XL |
1591 | /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite |
1592 | /// there being multiple references aliasing the cell: | |
1593 | /// | |
85aaf69f | 1594 | /// ``` |
1a4d82fc | 1595 | /// use std::cell::UnsafeCell; |
1a4d82fc | 1596 | /// |
1b1a35ee XL |
1597 | /// let x: UnsafeCell<i32> = 42.into(); |
1598 | /// // Get multiple / concurrent / shared references to the same `x`. | |
1599 | /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x); | |
1600 | /// | |
1601 | /// unsafe { | |
1602 | /// // SAFETY: within this scope there are no other references to `x`'s contents, | |
1603 | /// // so ours is effectively unique. | |
1604 | /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+ | |
1605 | /// *p1_exclusive += 27; // | | |
1606 | /// } // <---------- cannot go beyond this point -------------------+ | |
1607 | /// | |
1608 | /// unsafe { | |
1609 | /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents, | |
1610 | /// // so we can have multiple shared accesses concurrently. | |
1611 | /// let p2_shared: &i32 = &*p2.get(); | |
1612 | /// assert_eq!(*p2_shared, 42 + 27); | |
1613 | /// let p1_shared: &i32 = &*p1.get(); | |
1614 | /// assert_eq!(*p1_shared, *p2_shared); | |
1a4d82fc | 1615 | /// } |
1b1a35ee XL |
1616 | /// ``` |
1617 | /// | |
1618 | /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>` | |
1619 | /// implies exclusive access to its `T`: | |
1620 | /// | |
1621 | /// ```rust | |
1622 | /// #![feature(unsafe_cell_get_mut)] | |
1623 | /// #![forbid(unsafe_code)] // with exclusive accesses, | |
1624 | /// // `UnsafeCell` is a transparent no-op wrapper, | |
1625 | /// // so no need for `unsafe` here. | |
1626 | /// use std::cell::UnsafeCell; | |
1627 | /// | |
1628 | /// let mut x: UnsafeCell<i32> = 42.into(); | |
1629 | /// | |
1630 | /// // Get a compile-time-checked unique reference to `x`. | |
1631 | /// let p_unique: &mut UnsafeCell<i32> = &mut x; | |
1632 | /// // With an exclusive reference, we can mutate the contents for free. | |
1633 | /// *p_unique.get_mut() = 0; | |
1634 | /// // Or, equivalently: | |
1635 | /// x = UnsafeCell::new(0); | |
85aaf69f | 1636 | /// |
1b1a35ee XL |
1637 | /// // When we own the value, we can extract the contents for free. |
1638 | /// let contents: i32 = x.into_inner(); | |
1639 | /// assert_eq!(contents, 0); | |
1a4d82fc | 1640 | /// ``` |
d9579d0f | 1641 | #[lang = "unsafe_cell"] |
85aaf69f | 1642 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 1643 | #[repr(transparent)] |
ba9703b0 | 1644 | #[repr(no_niche)] // rust-lang/rust#68303. |
d9579d0f | 1645 | pub struct UnsafeCell<T: ?Sized> { |
e9174d1e | 1646 | value: T, |
1a4d82fc JJ |
1647 | } |
1648 | ||
92a42be0 | 1649 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1650 | impl<T: ?Sized> !Sync for UnsafeCell<T> {} |
c34b1796 | 1651 | |
1a4d82fc | 1652 | impl<T> UnsafeCell<T> { |
9346a6ac | 1653 | /// Constructs a new instance of `UnsafeCell` which will wrap the specified |
1a4d82fc JJ |
1654 | /// value. |
1655 | /// | |
b039eaaf | 1656 | /// All access to the inner value through methods is `unsafe`. |
85aaf69f SL |
1657 | /// |
1658 | /// # Examples | |
1659 | /// | |
1660 | /// ``` | |
1661 | /// use std::cell::UnsafeCell; | |
1662 | /// | |
1663 | /// let uc = UnsafeCell::new(5); | |
1664 | /// ``` | |
1665 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 1666 | #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")] |
c34b1796 | 1667 | #[inline] |
62682a34 | 1668 | pub const fn new(value: T) -> UnsafeCell<T> { |
b7449926 | 1669 | UnsafeCell { value } |
1a4d82fc JJ |
1670 | } |
1671 | ||
d9579d0f AL |
1672 | /// Unwraps the value. |
1673 | /// | |
85aaf69f SL |
1674 | /// # Examples |
1675 | /// | |
1676 | /// ``` | |
1677 | /// use std::cell::UnsafeCell; | |
1678 | /// | |
1679 | /// let uc = UnsafeCell::new(5); | |
1680 | /// | |
2c00a5a8 | 1681 | /// let five = uc.into_inner(); |
85aaf69f | 1682 | /// ``` |
1a4d82fc | 1683 | #[inline] |
85aaf69f | 1684 | #[stable(feature = "rust1", since = "1.0.0")] |
2c00a5a8 | 1685 | pub fn into_inner(self) -> T { |
62682a34 SL |
1686 | self.value |
1687 | } | |
d9579d0f | 1688 | } |
1a4d82fc | 1689 | |
d9579d0f AL |
1690 | impl<T: ?Sized> UnsafeCell<T> { |
1691 | /// Gets a mutable pointer to the wrapped value. | |
85aaf69f | 1692 | /// |
5bcae85e | 1693 | /// This can be cast to a pointer of any kind. |
0531ce1d XL |
1694 | /// Ensure that the access is unique (no active references, mutable or not) |
1695 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1696 | /// or mutable aliases going on when casting to `&T` | |
5bcae85e | 1697 | /// |
85aaf69f SL |
1698 | /// # Examples |
1699 | /// | |
1700 | /// ``` | |
1701 | /// use std::cell::UnsafeCell; | |
1702 | /// | |
1703 | /// let uc = UnsafeCell::new(5); | |
1704 | /// | |
d9579d0f | 1705 | /// let five = uc.get(); |
85aaf69f | 1706 | /// ``` |
1a4d82fc | 1707 | #[inline] |
85aaf69f | 1708 | #[stable(feature = "rust1", since = "1.0.0")] |
dfeec247 | 1709 | #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")] |
a1dfa0c6 XL |
1710 | pub const fn get(&self) -> *mut T { |
1711 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
60c5eb7d XL |
1712 | // #[repr(transparent)]. This exploits libstd's special status, there is |
1713 | // no guarantee for user code that this will work in future versions of the compiler! | |
a1dfa0c6 | 1714 | self as *const UnsafeCell<T> as *const T as *mut T |
d9579d0f | 1715 | } |
60c5eb7d | 1716 | |
1b1a35ee XL |
1717 | /// Returns a mutable reference to the underlying data. |
1718 | /// | |
1719 | /// This call borrows the `UnsafeCell` mutably (at compile-time) which | |
1720 | /// guarantees that we possess the only reference. | |
1721 | /// | |
1722 | /// # Examples | |
1723 | /// | |
1724 | /// ``` | |
1725 | /// #![feature(unsafe_cell_get_mut)] | |
1726 | /// use std::cell::UnsafeCell; | |
1727 | /// | |
1728 | /// let mut c = UnsafeCell::new(5); | |
1729 | /// *c.get_mut() += 1; | |
1730 | /// | |
1731 | /// assert_eq!(*c.get_mut(), 6); | |
1732 | /// ``` | |
1733 | #[inline] | |
1734 | #[unstable(feature = "unsafe_cell_get_mut", issue = "76943")] | |
1735 | pub fn get_mut(&mut self) -> &mut T { | |
1736 | // SAFETY: (outer) `&mut` guarantees unique access. | |
1737 | unsafe { &mut *self.get() } | |
1738 | } | |
1739 | ||
60c5eb7d XL |
1740 | /// Gets a mutable pointer to the wrapped value. |
1741 | /// The difference to [`get`] is that this function accepts a raw pointer, | |
1742 | /// which is useful to avoid the creation of temporary references. | |
1743 | /// | |
1744 | /// The result can be cast to a pointer of any kind. | |
1745 | /// Ensure that the access is unique (no active references, mutable or not) | |
1746 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1747 | /// or mutable aliases going on when casting to `&T`. | |
1748 | /// | |
1749 | /// [`get`]: #method.get | |
1750 | /// | |
1751 | /// # Examples | |
1752 | /// | |
1753 | /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as | |
1754 | /// calling `get` would require creating a reference to uninitialized data: | |
1755 | /// | |
1756 | /// ``` | |
1757 | /// #![feature(unsafe_cell_raw_get)] | |
1758 | /// use std::cell::UnsafeCell; | |
1759 | /// use std::mem::MaybeUninit; | |
1760 | /// | |
1761 | /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit(); | |
1762 | /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); } | |
1763 | /// let uc = unsafe { m.assume_init() }; | |
1764 | /// | |
1765 | /// assert_eq!(uc.into_inner(), 5); | |
1766 | /// ``` | |
1767 | #[inline] | |
1768 | #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")] | |
1769 | pub const fn raw_get(this: *const Self) -> *mut T { | |
1770 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
1771 | // #[repr(transparent)]. This exploits libstd's special status, there is | |
1772 | // no guarantee for user code that this will work in future versions of the compiler! | |
1773 | this as *const T as *mut T | |
1774 | } | |
1a4d82fc | 1775 | } |
a7813a04 | 1776 | |
7cac9316 | 1777 | #[stable(feature = "unsafe_cell_default", since = "1.10.0")] |
a7813a04 | 1778 | impl<T: Default> Default for UnsafeCell<T> { |
9e0c209e | 1779 | /// Creates an `UnsafeCell`, with the `Default` value for T. |
a7813a04 XL |
1780 | fn default() -> UnsafeCell<T> { |
1781 | UnsafeCell::new(Default::default()) | |
1782 | } | |
1783 | } | |
5bcae85e SL |
1784 | |
1785 | #[stable(feature = "cell_from", since = "1.12.0")] | |
1786 | impl<T> From<T> for UnsafeCell<T> { | |
1787 | fn from(t: T) -> UnsafeCell<T> { | |
1788 | UnsafeCell::new(t) | |
1789 | } | |
1790 | } | |
9e0c209e SL |
1791 | |
1792 | #[unstable(feature = "coerce_unsized", issue = "27732")] | |
1793 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {} | |
1794 | ||
1795 | #[allow(unused)] | |
1796 | fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) { | |
8faf50e0 XL |
1797 | let _: UnsafeCell<&dyn Send> = a; |
1798 | let _: Cell<&dyn Send> = b; | |
1799 | let _: RefCell<&dyn Send> = c; | |
9e0c209e | 1800 | } |