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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 | //! ``` | |
9e0c209e | 136 | //! #![feature(core_intrinsics)] |
1a4d82fc | 137 | //! use std::cell::Cell; |
0531ce1d | 138 | //! use std::ptr::NonNull; |
9e0c209e | 139 | //! use std::intrinsics::abort; |
60c5eb7d | 140 | //! use std::marker::PhantomData; |
1a4d82fc | 141 | //! |
9e0c209e | 142 | //! struct Rc<T: ?Sized> { |
60c5eb7d XL |
143 | //! ptr: NonNull<RcBox<T>>, |
144 | //! phantom: PhantomData<RcBox<T>>, | |
1a4d82fc JJ |
145 | //! } |
146 | //! | |
9e0c209e SL |
147 | //! struct RcBox<T: ?Sized> { |
148 | //! strong: Cell<usize>, | |
149 | //! refcount: Cell<usize>, | |
1a4d82fc | 150 | //! value: T, |
1a4d82fc JJ |
151 | //! } |
152 | //! | |
9e0c209e | 153 | //! impl<T: ?Sized> Clone for Rc<T> { |
1a4d82fc | 154 | //! fn clone(&self) -> Rc<T> { |
9e0c209e | 155 | //! self.inc_strong(); |
60c5eb7d XL |
156 | //! Rc { |
157 | //! ptr: self.ptr, | |
158 | //! phantom: PhantomData, | |
159 | //! } | |
9e0c209e SL |
160 | //! } |
161 | //! } | |
162 | //! | |
163 | //! trait RcBoxPtr<T: ?Sized> { | |
164 | //! | |
165 | //! fn inner(&self) -> &RcBox<T>; | |
166 | //! | |
167 | //! fn strong(&self) -> usize { | |
168 | //! self.inner().strong.get() | |
169 | //! } | |
170 | //! | |
171 | //! fn inc_strong(&self) { | |
172 | //! self.inner() | |
173 | //! .strong | |
174 | //! .set(self.strong() | |
175 | //! .checked_add(1) | |
176 | //! .unwrap_or_else(|| unsafe { abort() })); | |
1a4d82fc JJ |
177 | //! } |
178 | //! } | |
9e0c209e SL |
179 | //! |
180 | //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> { | |
181 | //! fn inner(&self) -> &RcBox<T> { | |
182 | //! unsafe { | |
7cac9316 | 183 | //! self.ptr.as_ref() |
9e0c209e SL |
184 | //! } |
185 | //! } | |
186 | //! } | |
1a4d82fc JJ |
187 | //! ``` |
188 | //! | |
1a4d82fc | 189 | |
60c5eb7d XL |
190 | // ignore-tidy-undocumented-unsafe |
191 | ||
85aaf69f | 192 | #![stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 193 | |
48663c56 XL |
194 | use crate::cmp::Ordering; |
195 | use crate::fmt::{self, Debug, Display}; | |
196 | use crate::marker::Unsize; | |
197 | use crate::mem; | |
198 | use crate::ops::{Deref, DerefMut, CoerceUnsized}; | |
199 | use crate::ptr; | |
1a4d82fc | 200 | |
8bb4bdeb | 201 | /// A mutable memory location. |
85aaf69f | 202 | /// |
3b2f2976 XL |
203 | /// # Examples |
204 | /// | |
a1dfa0c6 XL |
205 | /// In this example, you can see that `Cell<T>` enables mutation inside an |
206 | /// immutable struct. In other words, it enables "interior mutability". | |
3b2f2976 XL |
207 | /// |
208 | /// ``` | |
209 | /// use std::cell::Cell; | |
210 | /// | |
211 | /// struct SomeStruct { | |
212 | /// regular_field: u8, | |
213 | /// special_field: Cell<u8>, | |
214 | /// } | |
215 | /// | |
216 | /// let my_struct = SomeStruct { | |
217 | /// regular_field: 0, | |
218 | /// special_field: Cell::new(1), | |
219 | /// }; | |
220 | /// | |
221 | /// let new_value = 100; | |
222 | /// | |
a1dfa0c6 | 223 | /// // ERROR: `my_struct` is immutable |
3b2f2976 XL |
224 | /// // my_struct.regular_field = new_value; |
225 | /// | |
a1dfa0c6 XL |
226 | /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, |
227 | /// // which can always be mutated | |
3b2f2976 XL |
228 | /// my_struct.special_field.set(new_value); |
229 | /// assert_eq!(my_struct.special_field.get(), new_value); | |
230 | /// ``` | |
231 | /// | |
85aaf69f SL |
232 | /// See the [module-level documentation](index.html) for more. |
233 | #[stable(feature = "rust1", since = "1.0.0")] | |
8faf50e0 XL |
234 | #[repr(transparent)] |
235 | pub struct Cell<T: ?Sized> { | |
1a4d82fc JJ |
236 | value: UnsafeCell<T>, |
237 | } | |
238 | ||
8bb4bdeb | 239 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 240 | unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} |
8bb4bdeb XL |
241 | |
242 | #[stable(feature = "rust1", since = "1.0.0")] | |
8faf50e0 | 243 | impl<T: ?Sized> !Sync for Cell<T> {} |
8bb4bdeb XL |
244 | |
245 | #[stable(feature = "rust1", since = "1.0.0")] | |
246 | impl<T:Copy> Clone for Cell<T> { | |
247 | #[inline] | |
248 | fn clone(&self) -> Cell<T> { | |
249 | Cell::new(self.get()) | |
250 | } | |
251 | } | |
252 | ||
253 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 254 | impl<T: Default> Default for Cell<T> { |
8bb4bdeb XL |
255 | /// Creates a `Cell<T>`, with the `Default` value for T. |
256 | #[inline] | |
257 | fn default() -> Cell<T> { | |
258 | Cell::new(Default::default()) | |
259 | } | |
260 | } | |
261 | ||
262 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 263 | impl<T: PartialEq + Copy> PartialEq for Cell<T> { |
8bb4bdeb XL |
264 | #[inline] |
265 | fn eq(&self, other: &Cell<T>) -> bool { | |
266 | self.get() == other.get() | |
267 | } | |
268 | } | |
269 | ||
270 | #[stable(feature = "cell_eq", since = "1.2.0")] | |
416331ca | 271 | impl<T: Eq + Copy> Eq for Cell<T> {} |
8bb4bdeb XL |
272 | |
273 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 274 | impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { |
8bb4bdeb XL |
275 | #[inline] |
276 | fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { | |
277 | self.get().partial_cmp(&other.get()) | |
278 | } | |
279 | ||
280 | #[inline] | |
281 | fn lt(&self, other: &Cell<T>) -> bool { | |
282 | self.get() < other.get() | |
283 | } | |
284 | ||
285 | #[inline] | |
286 | fn le(&self, other: &Cell<T>) -> bool { | |
287 | self.get() <= other.get() | |
288 | } | |
289 | ||
290 | #[inline] | |
291 | fn gt(&self, other: &Cell<T>) -> bool { | |
292 | self.get() > other.get() | |
293 | } | |
294 | ||
295 | #[inline] | |
296 | fn ge(&self, other: &Cell<T>) -> bool { | |
297 | self.get() >= other.get() | |
298 | } | |
299 | } | |
300 | ||
301 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 302 | impl<T: Ord + Copy> Ord for Cell<T> { |
8bb4bdeb XL |
303 | #[inline] |
304 | fn cmp(&self, other: &Cell<T>) -> Ordering { | |
305 | self.get().cmp(&other.get()) | |
306 | } | |
307 | } | |
308 | ||
309 | #[stable(feature = "cell_from", since = "1.12.0")] | |
310 | impl<T> From<T> for Cell<T> { | |
311 | fn from(t: T) -> Cell<T> { | |
312 | Cell::new(t) | |
313 | } | |
314 | } | |
315 | ||
316 | impl<T> Cell<T> { | |
317 | /// Creates a new `Cell` containing the given value. | |
85aaf69f SL |
318 | /// |
319 | /// # Examples | |
320 | /// | |
321 | /// ``` | |
322 | /// use std::cell::Cell; | |
323 | /// | |
324 | /// let c = Cell::new(5); | |
85aaf69f | 325 | /// ``` |
85aaf69f | 326 | #[stable(feature = "rust1", since = "1.0.0")] |
60c5eb7d | 327 | #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_cell_new", since = "1.32.0"))] |
8bb4bdeb XL |
328 | #[inline] |
329 | pub const fn new(value: T) -> Cell<T> { | |
330 | Cell { | |
331 | value: UnsafeCell::new(value), | |
1a4d82fc JJ |
332 | } |
333 | } | |
334 | ||
8bb4bdeb XL |
335 | /// Sets the contained value. |
336 | /// | |
337 | /// # Examples | |
338 | /// | |
339 | /// ``` | |
340 | /// use std::cell::Cell; | |
341 | /// | |
342 | /// let c = Cell::new(5); | |
343 | /// | |
344 | /// c.set(10); | |
345 | /// ``` | |
a7813a04 | 346 | #[inline] |
8bb4bdeb XL |
347 | #[stable(feature = "rust1", since = "1.0.0")] |
348 | pub fn set(&self, val: T) { | |
349 | let old = self.replace(val); | |
350 | drop(old); | |
a7813a04 XL |
351 | } |
352 | ||
8bb4bdeb XL |
353 | /// Swaps the values of two Cells. |
354 | /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. | |
355 | /// | |
356 | /// # Examples | |
357 | /// | |
358 | /// ``` | |
359 | /// use std::cell::Cell; | |
360 | /// | |
361 | /// let c1 = Cell::new(5i32); | |
362 | /// let c2 = Cell::new(10i32); | |
363 | /// c1.swap(&c2); | |
364 | /// assert_eq!(10, c1.get()); | |
365 | /// assert_eq!(5, c2.get()); | |
366 | /// ``` | |
a7813a04 | 367 | #[inline] |
8bb4bdeb XL |
368 | #[stable(feature = "move_cell", since = "1.17.0")] |
369 | pub fn swap(&self, other: &Self) { | |
370 | if ptr::eq(self, other) { | |
371 | return; | |
372 | } | |
373 | unsafe { | |
374 | ptr::swap(self.value.get(), other.value.get()); | |
375 | } | |
a7813a04 XL |
376 | } |
377 | ||
041b39d2 | 378 | /// Replaces the contained value, and returns it. |
8bb4bdeb XL |
379 | /// |
380 | /// # Examples | |
381 | /// | |
382 | /// ``` | |
383 | /// use std::cell::Cell; | |
384 | /// | |
041b39d2 XL |
385 | /// let cell = Cell::new(5); |
386 | /// assert_eq!(cell.get(), 5); | |
387 | /// assert_eq!(cell.replace(10), 5); | |
388 | /// assert_eq!(cell.get(), 10); | |
8bb4bdeb XL |
389 | /// ``` |
390 | #[stable(feature = "move_cell", since = "1.17.0")] | |
391 | pub fn replace(&self, val: T) -> T { | |
392 | mem::replace(unsafe { &mut *self.value.get() }, val) | |
a7813a04 | 393 | } |
a7813a04 | 394 | |
8bb4bdeb XL |
395 | /// Unwraps the value. |
396 | /// | |
397 | /// # Examples | |
398 | /// | |
399 | /// ``` | |
400 | /// use std::cell::Cell; | |
401 | /// | |
402 | /// let c = Cell::new(5); | |
403 | /// let five = c.into_inner(); | |
404 | /// | |
405 | /// assert_eq!(five, 5); | |
406 | /// ``` | |
407 | #[stable(feature = "move_cell", since = "1.17.0")] | |
408 | pub fn into_inner(self) -> T { | |
2c00a5a8 | 409 | self.value.into_inner() |
a7813a04 | 410 | } |
e74abb32 XL |
411 | } |
412 | ||
413 | impl<T:Copy> Cell<T> { | |
414 | /// Returns a copy of the contained value. | |
415 | /// | |
416 | /// # Examples | |
417 | /// | |
418 | /// ``` | |
419 | /// use std::cell::Cell; | |
420 | /// | |
421 | /// let c = Cell::new(5); | |
422 | /// | |
423 | /// let five = c.get(); | |
424 | /// ``` | |
425 | #[inline] | |
426 | #[stable(feature = "rust1", since = "1.0.0")] | |
427 | pub fn get(&self) -> T { | |
428 | unsafe{ *self.value.get() } | |
429 | } | |
430 | ||
431 | /// Updates the contained value using a function and returns the new value. | |
432 | /// | |
433 | /// # Examples | |
434 | /// | |
435 | /// ``` | |
436 | /// #![feature(cell_update)] | |
437 | /// | |
438 | /// use std::cell::Cell; | |
439 | /// | |
440 | /// let c = Cell::new(5); | |
441 | /// let new = c.update(|x| x + 1); | |
442 | /// | |
443 | /// assert_eq!(new, 6); | |
444 | /// assert_eq!(c.get(), 6); | |
445 | /// ``` | |
446 | #[inline] | |
447 | #[unstable(feature = "cell_update", issue = "50186")] | |
448 | pub fn update<F>(&self, f: F) -> T | |
449 | where | |
450 | F: FnOnce(T) -> T, | |
451 | { | |
452 | let old = self.get(); | |
453 | let new = f(old); | |
454 | self.set(new); | |
455 | new | |
456 | } | |
a7813a04 XL |
457 | } |
458 | ||
8faf50e0 XL |
459 | impl<T: ?Sized> Cell<T> { |
460 | /// Returns a raw pointer to the underlying data in this cell. | |
461 | /// | |
462 | /// # Examples | |
463 | /// | |
464 | /// ``` | |
465 | /// use std::cell::Cell; | |
466 | /// | |
467 | /// let c = Cell::new(5); | |
468 | /// | |
469 | /// let ptr = c.as_ptr(); | |
470 | /// ``` | |
471 | #[inline] | |
472 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
60c5eb7d | 473 | #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0"))] |
a1dfa0c6 | 474 | pub const fn as_ptr(&self) -> *mut T { |
8faf50e0 XL |
475 | self.value.get() |
476 | } | |
477 | ||
478 | /// Returns a mutable reference to the underlying data. | |
479 | /// | |
480 | /// This call borrows `Cell` mutably (at compile-time) which guarantees | |
481 | /// that we possess the only reference. | |
482 | /// | |
483 | /// # Examples | |
484 | /// | |
485 | /// ``` | |
486 | /// use std::cell::Cell; | |
487 | /// | |
488 | /// let mut c = Cell::new(5); | |
489 | /// *c.get_mut() += 1; | |
490 | /// | |
491 | /// assert_eq!(c.get(), 6); | |
492 | /// ``` | |
493 | #[inline] | |
494 | #[stable(feature = "cell_get_mut", since = "1.11.0")] | |
495 | pub fn get_mut(&mut self) -> &mut T { | |
496 | unsafe { | |
497 | &mut *self.value.get() | |
498 | } | |
499 | } | |
500 | ||
501 | /// Returns a `&Cell<T>` from a `&mut T` | |
502 | /// | |
503 | /// # Examples | |
504 | /// | |
505 | /// ``` | |
8faf50e0 XL |
506 | /// use std::cell::Cell; |
507 | /// | |
508 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
509 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
510 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
511 | /// | |
512 | /// assert_eq!(slice_cell.len(), 3); | |
513 | /// ``` | |
514 | #[inline] | |
dc9dc135 | 515 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 XL |
516 | pub fn from_mut(t: &mut T) -> &Cell<T> { |
517 | unsafe { | |
518 | &*(t as *mut T as *const Cell<T>) | |
519 | } | |
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 XL |
561 | pub fn as_slice_of_cells(&self) -> &[Cell<T>] { |
562 | unsafe { | |
563 | &*(self as *const Cell<[T]> as *const [Cell<T>]) | |
564 | } | |
565 | } | |
566 | } | |
567 | ||
1a4d82fc | 568 | /// A mutable memory location with dynamically checked borrow rules |
85aaf69f SL |
569 | /// |
570 | /// See the [module-level documentation](index.html) for more. | |
571 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 572 | pub struct RefCell<T: ?Sized> { |
1a4d82fc | 573 | borrow: Cell<BorrowFlag>, |
d9579d0f | 574 | value: UnsafeCell<T>, |
1a4d82fc JJ |
575 | } |
576 | ||
5bcae85e | 577 | /// An error returned by [`RefCell::try_borrow`](struct.RefCell.html#method.try_borrow). |
9e0c209e SL |
578 | #[stable(feature = "try_borrow", since = "1.13.0")] |
579 | pub struct BorrowError { | |
580 | _private: (), | |
5bcae85e SL |
581 | } |
582 | ||
9e0c209e SL |
583 | #[stable(feature = "try_borrow", since = "1.13.0")] |
584 | impl Debug for BorrowError { | |
48663c56 | 585 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
586 | f.debug_struct("BorrowError").finish() |
587 | } | |
588 | } | |
589 | ||
9e0c209e SL |
590 | #[stable(feature = "try_borrow", since = "1.13.0")] |
591 | impl Display for BorrowError { | |
48663c56 | 592 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
593 | Display::fmt("already mutably borrowed", f) |
594 | } | |
595 | } | |
596 | ||
597 | /// An error returned by [`RefCell::try_borrow_mut`](struct.RefCell.html#method.try_borrow_mut). | |
9e0c209e SL |
598 | #[stable(feature = "try_borrow", since = "1.13.0")] |
599 | pub struct BorrowMutError { | |
600 | _private: (), | |
5bcae85e SL |
601 | } |
602 | ||
9e0c209e SL |
603 | #[stable(feature = "try_borrow", since = "1.13.0")] |
604 | impl Debug for BorrowMutError { | |
48663c56 | 605 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
606 | f.debug_struct("BorrowMutError").finish() |
607 | } | |
608 | } | |
609 | ||
9e0c209e SL |
610 | #[stable(feature = "try_borrow", since = "1.13.0")] |
611 | impl Display for BorrowMutError { | |
48663c56 | 612 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
613 | Display::fmt("already borrowed", f) |
614 | } | |
615 | } | |
616 | ||
8faf50e0 XL |
617 | // Positive values represent the number of `Ref` active. Negative values |
618 | // represent the number of `RefMut` active. Multiple `RefMut`s can only be | |
619 | // active at a time if they refer to distinct, nonoverlapping components of a | |
620 | // `RefCell` (e.g., different ranges of a slice). | |
94b46f34 XL |
621 | // |
622 | // `Ref` and `RefMut` are both two words in size, and so there will likely never | |
623 | // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` | |
8faf50e0 XL |
624 | // range. Thus, a `BorrowFlag` will probably never overflow or underflow. |
625 | // However, this is not a guarantee, as a pathological program could repeatedly | |
626 | // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must | |
627 | // explicitly check for overflow and underflow in order to avoid unsafety, or at | |
628 | // least behave correctly in the event that overflow or underflow happens (e.g., | |
629 | // see BorrowRef::new). | |
630 | type BorrowFlag = isize; | |
1a4d82fc | 631 | const UNUSED: BorrowFlag = 0; |
8faf50e0 XL |
632 | |
633 | #[inline(always)] | |
634 | fn is_writing(x: BorrowFlag) -> bool { | |
635 | x < UNUSED | |
636 | } | |
637 | ||
638 | #[inline(always)] | |
639 | fn is_reading(x: BorrowFlag) -> bool { | |
640 | x > UNUSED | |
641 | } | |
1a4d82fc JJ |
642 | |
643 | impl<T> RefCell<T> { | |
85aaf69f SL |
644 | /// Creates a new `RefCell` containing `value`. |
645 | /// | |
646 | /// # Examples | |
647 | /// | |
648 | /// ``` | |
649 | /// use std::cell::RefCell; | |
650 | /// | |
651 | /// let c = RefCell::new(5); | |
652 | /// ``` | |
653 | #[stable(feature = "rust1", since = "1.0.0")] | |
60c5eb7d | 654 | #[cfg_attr(not(bootstrap), rustc_const_stable(feature = "const_refcell_new", since = "1.32.0"))] |
c34b1796 | 655 | #[inline] |
62682a34 | 656 | pub const fn new(value: T) -> RefCell<T> { |
1a4d82fc JJ |
657 | RefCell { |
658 | value: UnsafeCell::new(value), | |
659 | borrow: Cell::new(UNUSED), | |
660 | } | |
661 | } | |
662 | ||
663 | /// Consumes the `RefCell`, returning the wrapped value. | |
85aaf69f SL |
664 | /// |
665 | /// # Examples | |
666 | /// | |
667 | /// ``` | |
668 | /// use std::cell::RefCell; | |
669 | /// | |
670 | /// let c = RefCell::new(5); | |
671 | /// | |
672 | /// let five = c.into_inner(); | |
673 | /// ``` | |
674 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 675 | #[inline] |
1a4d82fc JJ |
676 | pub fn into_inner(self) -> T { |
677 | // Since this function takes `self` (the `RefCell`) by value, the | |
678 | // compiler statically verifies that it is not currently borrowed. | |
679 | // Therefore the following assertion is just a `debug_assert!`. | |
680 | debug_assert!(self.borrow.get() == UNUSED); | |
2c00a5a8 | 681 | self.value.into_inner() |
1a4d82fc | 682 | } |
3b2f2976 XL |
683 | |
684 | /// Replaces the wrapped value with a new one, returning the old value, | |
685 | /// without deinitializing either one. | |
686 | /// | |
687 | /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). | |
688 | /// | |
abe05a73 XL |
689 | /// # Panics |
690 | /// | |
691 | /// Panics if the value is currently borrowed. | |
692 | /// | |
3b2f2976 XL |
693 | /// # Examples |
694 | /// | |
695 | /// ``` | |
3b2f2976 | 696 | /// use std::cell::RefCell; |
abe05a73 XL |
697 | /// let cell = RefCell::new(5); |
698 | /// let old_value = cell.replace(6); | |
699 | /// assert_eq!(old_value, 5); | |
700 | /// assert_eq!(cell, RefCell::new(6)); | |
3b2f2976 | 701 | /// ``` |
abe05a73 | 702 | #[inline] |
ff7c6d11 | 703 | #[stable(feature = "refcell_replace", since="1.24.0")] |
abe05a73 XL |
704 | pub fn replace(&self, t: T) -> T { |
705 | mem::replace(&mut *self.borrow_mut(), t) | |
706 | } | |
707 | ||
708 | /// Replaces the wrapped value with a new one computed from `f`, returning | |
709 | /// the old value, without deinitializing either one. | |
710 | /// | |
3b2f2976 XL |
711 | /// # Panics |
712 | /// | |
abe05a73 XL |
713 | /// Panics if the value is currently borrowed. |
714 | /// | |
715 | /// # Examples | |
716 | /// | |
717 | /// ``` | |
abe05a73 XL |
718 | /// use std::cell::RefCell; |
719 | /// let cell = RefCell::new(5); | |
720 | /// let old_value = cell.replace_with(|&mut old| old + 1); | |
721 | /// assert_eq!(old_value, 5); | |
722 | /// assert_eq!(cell, RefCell::new(6)); | |
723 | /// ``` | |
3b2f2976 | 724 | #[inline] |
532ac7d7 | 725 | #[stable(feature = "refcell_replace_swap", since="1.35.0")] |
abe05a73 XL |
726 | pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { |
727 | let mut_borrow = &mut *self.borrow_mut(); | |
728 | let replacement = f(mut_borrow); | |
729 | mem::replace(mut_borrow, replacement) | |
3b2f2976 XL |
730 | } |
731 | ||
732 | /// Swaps the wrapped value of `self` with the wrapped value of `other`, | |
733 | /// without deinitializing either one. | |
734 | /// | |
735 | /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). | |
736 | /// | |
abe05a73 XL |
737 | /// # Panics |
738 | /// | |
739 | /// Panics if the value in either `RefCell` is currently borrowed. | |
740 | /// | |
3b2f2976 XL |
741 | /// # Examples |
742 | /// | |
743 | /// ``` | |
3b2f2976 XL |
744 | /// use std::cell::RefCell; |
745 | /// let c = RefCell::new(5); | |
746 | /// let d = RefCell::new(6); | |
747 | /// c.swap(&d); | |
748 | /// assert_eq!(c, RefCell::new(6)); | |
749 | /// assert_eq!(d, RefCell::new(5)); | |
750 | /// ``` | |
3b2f2976 | 751 | #[inline] |
ff7c6d11 | 752 | #[stable(feature = "refcell_swap", since="1.24.0")] |
3b2f2976 XL |
753 | pub fn swap(&self, other: &Self) { |
754 | mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) | |
755 | } | |
d9579d0f | 756 | } |
1a4d82fc | 757 | |
d9579d0f | 758 | impl<T: ?Sized> RefCell<T> { |
1a4d82fc JJ |
759 | /// Immutably borrows the wrapped value. |
760 | /// | |
761 | /// The borrow lasts until the returned `Ref` exits scope. Multiple | |
762 | /// immutable borrows can be taken out at the same time. | |
763 | /// | |
764 | /// # Panics | |
765 | /// | |
5bcae85e SL |
766 | /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use |
767 | /// [`try_borrow`](#method.try_borrow). | |
85aaf69f SL |
768 | /// |
769 | /// # Examples | |
770 | /// | |
771 | /// ``` | |
772 | /// use std::cell::RefCell; | |
773 | /// | |
774 | /// let c = RefCell::new(5); | |
775 | /// | |
776 | /// let borrowed_five = c.borrow(); | |
777 | /// let borrowed_five2 = c.borrow(); | |
778 | /// ``` | |
779 | /// | |
780 | /// An example of panic: | |
781 | /// | |
782 | /// ``` | |
783 | /// use std::cell::RefCell; | |
784 | /// use std::thread; | |
785 | /// | |
786 | /// let result = thread::spawn(move || { | |
787 | /// let c = RefCell::new(5); | |
788 | /// let m = c.borrow_mut(); | |
789 | /// | |
790 | /// let b = c.borrow(); // this causes a panic | |
791 | /// }).join(); | |
792 | /// | |
793 | /// assert!(result.is_err()); | |
794 | /// ``` | |
795 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 796 | #[inline] |
48663c56 | 797 | pub fn borrow(&self) -> Ref<'_, T> { |
5bcae85e SL |
798 | self.try_borrow().expect("already mutably borrowed") |
799 | } | |
800 | ||
801 | /// Immutably borrows the wrapped value, returning an error if the value is currently mutably | |
802 | /// borrowed. | |
803 | /// | |
804 | /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be | |
805 | /// taken out at the same time. | |
806 | /// | |
807 | /// This is the non-panicking variant of [`borrow`](#method.borrow). | |
808 | /// | |
809 | /// # Examples | |
810 | /// | |
811 | /// ``` | |
5bcae85e SL |
812 | /// use std::cell::RefCell; |
813 | /// | |
814 | /// let c = RefCell::new(5); | |
815 | /// | |
816 | /// { | |
817 | /// let m = c.borrow_mut(); | |
818 | /// assert!(c.try_borrow().is_err()); | |
819 | /// } | |
820 | /// | |
821 | /// { | |
822 | /// let m = c.borrow(); | |
823 | /// assert!(c.try_borrow().is_ok()); | |
824 | /// } | |
825 | /// ``` | |
9e0c209e | 826 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 827 | #[inline] |
48663c56 | 828 | pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { |
85aaf69f | 829 | match BorrowRef::new(&self.borrow) { |
5bcae85e | 830 | Some(b) => Ok(Ref { |
54a0048b SL |
831 | value: unsafe { &*self.value.get() }, |
832 | borrow: b, | |
5bcae85e | 833 | }), |
9e0c209e | 834 | None => Err(BorrowError { _private: () }), |
1a4d82fc JJ |
835 | } |
836 | } | |
837 | ||
1a4d82fc JJ |
838 | /// Mutably borrows the wrapped value. |
839 | /// | |
94b46f34 XL |
840 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
841 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
842 | /// active. | |
1a4d82fc JJ |
843 | /// |
844 | /// # Panics | |
845 | /// | |
5bcae85e SL |
846 | /// Panics if the value is currently borrowed. For a non-panicking variant, use |
847 | /// [`try_borrow_mut`](#method.try_borrow_mut). | |
85aaf69f SL |
848 | /// |
849 | /// # Examples | |
850 | /// | |
851 | /// ``` | |
852 | /// use std::cell::RefCell; | |
853 | /// | |
854 | /// let c = RefCell::new(5); | |
855 | /// | |
7453a54e SL |
856 | /// *c.borrow_mut() = 7; |
857 | /// | |
858 | /// assert_eq!(*c.borrow(), 7); | |
85aaf69f SL |
859 | /// ``` |
860 | /// | |
861 | /// An example of panic: | |
862 | /// | |
863 | /// ``` | |
864 | /// use std::cell::RefCell; | |
865 | /// use std::thread; | |
866 | /// | |
867 | /// let result = thread::spawn(move || { | |
868 | /// let c = RefCell::new(5); | |
bd371182 | 869 | /// let m = c.borrow(); |
85aaf69f SL |
870 | /// |
871 | /// let b = c.borrow_mut(); // this causes a panic | |
872 | /// }).join(); | |
873 | /// | |
874 | /// assert!(result.is_err()); | |
875 | /// ``` | |
876 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 877 | #[inline] |
48663c56 | 878 | pub fn borrow_mut(&self) -> RefMut<'_, T> { |
5bcae85e SL |
879 | self.try_borrow_mut().expect("already borrowed") |
880 | } | |
881 | ||
882 | /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. | |
883 | /// | |
94b46f34 XL |
884 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
885 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
886 | /// active. | |
5bcae85e SL |
887 | /// |
888 | /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). | |
889 | /// | |
890 | /// # Examples | |
891 | /// | |
892 | /// ``` | |
5bcae85e SL |
893 | /// use std::cell::RefCell; |
894 | /// | |
895 | /// let c = RefCell::new(5); | |
896 | /// | |
897 | /// { | |
898 | /// let m = c.borrow(); | |
899 | /// assert!(c.try_borrow_mut().is_err()); | |
900 | /// } | |
901 | /// | |
902 | /// assert!(c.try_borrow_mut().is_ok()); | |
903 | /// ``` | |
9e0c209e | 904 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 905 | #[inline] |
48663c56 | 906 | pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { |
85aaf69f | 907 | match BorrowRefMut::new(&self.borrow) { |
5bcae85e | 908 | Some(b) => Ok(RefMut { |
54a0048b SL |
909 | value: unsafe { &mut *self.value.get() }, |
910 | borrow: b, | |
5bcae85e | 911 | }), |
9e0c209e | 912 | None => Err(BorrowMutError { _private: () }), |
1a4d82fc JJ |
913 | } |
914 | } | |
915 | ||
5bcae85e SL |
916 | /// Returns a raw pointer to the underlying data in this cell. |
917 | /// | |
918 | /// # Examples | |
919 | /// | |
920 | /// ``` | |
921 | /// use std::cell::RefCell; | |
922 | /// | |
923 | /// let c = RefCell::new(5); | |
924 | /// | |
925 | /// let ptr = c.as_ptr(); | |
926 | /// ``` | |
927 | #[inline] | |
928 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
929 | pub fn as_ptr(&self) -> *mut T { | |
930 | self.value.get() | |
931 | } | |
932 | ||
a7813a04 XL |
933 | /// Returns a mutable reference to the underlying data. |
934 | /// | |
935 | /// This call borrows `RefCell` mutably (at compile-time) so there is no | |
936 | /// need for dynamic checks. | |
5bcae85e | 937 | /// |
cc61c64b XL |
938 | /// However be cautious: this method expects `self` to be mutable, which is |
939 | /// generally not the case when using a `RefCell`. Take a look at the | |
940 | /// [`borrow_mut`] method instead if `self` isn't mutable. | |
941 | /// | |
942 | /// Also, please be aware that this method is only for special circumstances and is usually | |
3b2f2976 | 943 | /// not what you want. In case of doubt, use [`borrow_mut`] instead. |
cc61c64b XL |
944 | /// |
945 | /// [`borrow_mut`]: #method.borrow_mut | |
946 | /// | |
5bcae85e SL |
947 | /// # Examples |
948 | /// | |
949 | /// ``` | |
950 | /// use std::cell::RefCell; | |
951 | /// | |
952 | /// let mut c = RefCell::new(5); | |
953 | /// *c.get_mut() += 1; | |
954 | /// | |
955 | /// assert_eq!(c, RefCell::new(6)); | |
956 | /// ``` | |
a7813a04 | 957 | #[inline] |
3157f602 | 958 | #[stable(feature = "cell_get_mut", since = "1.11.0")] |
a7813a04 XL |
959 | pub fn get_mut(&mut self) -> &mut T { |
960 | unsafe { | |
961 | &mut *self.value.get() | |
962 | } | |
963 | } | |
48663c56 XL |
964 | |
965 | /// Immutably borrows the wrapped value, returning an error if the value is | |
966 | /// currently mutably borrowed. | |
967 | /// | |
968 | /// # Safety | |
969 | /// | |
970 | /// Unlike `RefCell::borrow`, this method is unsafe because it does not | |
971 | /// return a `Ref`, thus leaving the borrow flag untouched. Mutably | |
972 | /// borrowing the `RefCell` while the reference returned by this method | |
973 | /// is alive is undefined behaviour. | |
974 | /// | |
975 | /// # Examples | |
976 | /// | |
977 | /// ``` | |
48663c56 XL |
978 | /// use std::cell::RefCell; |
979 | /// | |
980 | /// let c = RefCell::new(5); | |
981 | /// | |
982 | /// { | |
983 | /// let m = c.borrow_mut(); | |
984 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); | |
985 | /// } | |
986 | /// | |
987 | /// { | |
988 | /// let m = c.borrow(); | |
989 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); | |
990 | /// } | |
991 | /// ``` | |
dc9dc135 | 992 | #[stable(feature = "borrow_state", since = "1.37.0")] |
48663c56 XL |
993 | #[inline] |
994 | pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { | |
995 | if !is_writing(self.borrow.get()) { | |
996 | Ok(&*self.value.get()) | |
997 | } else { | |
998 | Err(BorrowError { _private: () }) | |
999 | } | |
1000 | } | |
1a4d82fc JJ |
1001 | } |
1002 | ||
85aaf69f | 1003 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1004 | unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {} |
1a4d82fc | 1005 | |
54a0048b SL |
1006 | #[stable(feature = "rust1", since = "1.0.0")] |
1007 | impl<T: ?Sized> !Sync for RefCell<T> {} | |
1008 | ||
85aaf69f | 1009 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 1010 | impl<T: Clone> Clone for RefCell<T> { |
0531ce1d XL |
1011 | /// # Panics |
1012 | /// | |
1013 | /// Panics if the value is currently mutably borrowed. | |
c34b1796 | 1014 | #[inline] |
1a4d82fc JJ |
1015 | fn clone(&self) -> RefCell<T> { |
1016 | RefCell::new(self.borrow().clone()) | |
1017 | } | |
1018 | } | |
1019 | ||
85aaf69f | 1020 | #[stable(feature = "rust1", since = "1.0.0")] |
416331ca | 1021 | impl<T: Default> Default for RefCell<T> { |
9e0c209e | 1022 | /// Creates a `RefCell<T>`, with the `Default` value for T. |
c34b1796 | 1023 | #[inline] |
1a4d82fc JJ |
1024 | fn default() -> RefCell<T> { |
1025 | RefCell::new(Default::default()) | |
1026 | } | |
1027 | } | |
1028 | ||
85aaf69f | 1029 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1030 | impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { |
0531ce1d XL |
1031 | /// # Panics |
1032 | /// | |
1033 | /// Panics if the value in either `RefCell` is currently borrowed. | |
c34b1796 | 1034 | #[inline] |
1a4d82fc JJ |
1035 | fn eq(&self, other: &RefCell<T>) -> bool { |
1036 | *self.borrow() == *other.borrow() | |
1037 | } | |
1038 | } | |
1039 | ||
62682a34 SL |
1040 | #[stable(feature = "cell_eq", since = "1.2.0")] |
1041 | impl<T: ?Sized + Eq> Eq for RefCell<T> {} | |
1042 | ||
a7813a04 XL |
1043 | #[stable(feature = "cell_ord", since = "1.10.0")] |
1044 | impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { | |
0531ce1d XL |
1045 | /// # Panics |
1046 | /// | |
1047 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1048 | #[inline] |
1049 | fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { | |
1050 | self.borrow().partial_cmp(&*other.borrow()) | |
1051 | } | |
1052 | ||
0531ce1d XL |
1053 | /// # Panics |
1054 | /// | |
1055 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1056 | #[inline] |
1057 | fn lt(&self, other: &RefCell<T>) -> bool { | |
1058 | *self.borrow() < *other.borrow() | |
1059 | } | |
1060 | ||
0531ce1d XL |
1061 | /// # Panics |
1062 | /// | |
1063 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1064 | #[inline] |
1065 | fn le(&self, other: &RefCell<T>) -> bool { | |
1066 | *self.borrow() <= *other.borrow() | |
1067 | } | |
1068 | ||
0531ce1d XL |
1069 | /// # Panics |
1070 | /// | |
1071 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1072 | #[inline] |
1073 | fn gt(&self, other: &RefCell<T>) -> bool { | |
1074 | *self.borrow() > *other.borrow() | |
1075 | } | |
1076 | ||
0531ce1d XL |
1077 | /// # Panics |
1078 | /// | |
1079 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1080 | #[inline] |
1081 | fn ge(&self, other: &RefCell<T>) -> bool { | |
1082 | *self.borrow() >= *other.borrow() | |
1083 | } | |
1084 | } | |
1085 | ||
1086 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
1087 | impl<T: ?Sized + Ord> Ord 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 cmp(&self, other: &RefCell<T>) -> Ordering { | |
1093 | self.borrow().cmp(&*other.borrow()) | |
1094 | } | |
1095 | } | |
1096 | ||
5bcae85e SL |
1097 | #[stable(feature = "cell_from", since = "1.12.0")] |
1098 | impl<T> From<T> for RefCell<T> { | |
1099 | fn from(t: T) -> RefCell<T> { | |
1100 | RefCell::new(t) | |
1101 | } | |
1102 | } | |
1103 | ||
9e0c209e SL |
1104 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1105 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} | |
1106 | ||
1a4d82fc | 1107 | struct BorrowRef<'b> { |
54a0048b | 1108 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1109 | } |
1110 | ||
1111 | impl<'b> BorrowRef<'b> { | |
c34b1796 | 1112 | #[inline] |
1a4d82fc | 1113 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { |
416331ca XL |
1114 | let b = borrow.get().wrapping_add(1); |
1115 | if !is_reading(b) { | |
1116 | // Incrementing borrow can result in a non-reading value (<= 0) in these cases: | |
1117 | // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow | |
1118 | // due to Rust's reference aliasing rules | |
1119 | // 2. It was isize::max_value() (the max amount of reading borrows) and it overflowed | |
1120 | // into isize::min_value() (the max amount of writing borrows) so we can't allow | |
1121 | // an additional read borrow because isize can't represent so many read borrows | |
1122 | // (this can only happen if you mem::forget more than a small constant amount of | |
1123 | // `Ref`s, which is not good practice) | |
94b46f34 XL |
1124 | None |
1125 | } else { | |
416331ca XL |
1126 | // Incrementing borrow can result in a reading value (> 0) in these cases: |
1127 | // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow | |
1128 | // 2. It was > 0 and < isize::max_value(), i.e. there were read borrows, and isize | |
1129 | // is large enough to represent having one more read borrow | |
1130 | borrow.set(b); | |
94b46f34 | 1131 | Some(BorrowRef { borrow }) |
1a4d82fc JJ |
1132 | } |
1133 | } | |
1134 | } | |
1135 | ||
0bf4aa26 | 1136 | impl Drop for BorrowRef<'_> { |
c34b1796 | 1137 | #[inline] |
1a4d82fc | 1138 | fn drop(&mut self) { |
54a0048b | 1139 | let borrow = self.borrow.get(); |
8faf50e0 | 1140 | debug_assert!(is_reading(borrow)); |
54a0048b | 1141 | self.borrow.set(borrow - 1); |
1a4d82fc JJ |
1142 | } |
1143 | } | |
1144 | ||
0bf4aa26 | 1145 | impl Clone for BorrowRef<'_> { |
c34b1796 | 1146 | #[inline] |
0bf4aa26 | 1147 | fn clone(&self) -> Self { |
1a4d82fc | 1148 | // Since this Ref exists, we know the borrow flag |
8faf50e0 | 1149 | // is a reading borrow. |
54a0048b | 1150 | let borrow = self.borrow.get(); |
8faf50e0 | 1151 | debug_assert!(is_reading(borrow)); |
94b46f34 XL |
1152 | // Prevent the borrow counter from overflowing into |
1153 | // a writing borrow. | |
8faf50e0 | 1154 | assert!(borrow != isize::max_value()); |
54a0048b SL |
1155 | self.borrow.set(borrow + 1); |
1156 | BorrowRef { borrow: self.borrow } | |
1a4d82fc JJ |
1157 | } |
1158 | } | |
1159 | ||
1160 | /// Wraps a borrowed reference to a value in a `RefCell` box. | |
85aaf69f SL |
1161 | /// A wrapper type for an immutably borrowed value from a `RefCell<T>`. |
1162 | /// | |
1163 | /// See the [module-level documentation](index.html) for more. | |
1164 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 1165 | pub struct Ref<'b, T: ?Sized + 'b> { |
54a0048b SL |
1166 | value: &'b T, |
1167 | borrow: BorrowRef<'b>, | |
1a4d82fc JJ |
1168 | } |
1169 | ||
85aaf69f | 1170 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1171 | impl<T: ?Sized> Deref for Ref<'_, T> { |
1a4d82fc JJ |
1172 | type Target = T; |
1173 | ||
1174 | #[inline] | |
e9174d1e | 1175 | fn deref(&self) -> &T { |
54a0048b | 1176 | self.value |
1a4d82fc JJ |
1177 | } |
1178 | } | |
1179 | ||
62682a34 SL |
1180 | impl<'b, T: ?Sized> Ref<'b, T> { |
1181 | /// Copies a `Ref`. | |
1182 | /// | |
1183 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1184 | /// | |
1185 | /// This is an associated function that needs to be used as | |
9fa01778 | 1186 | /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere |
62682a34 SL |
1187 | /// with the widespread use of `r.borrow().clone()` to clone the contents of |
1188 | /// a `RefCell`. | |
476ff2be | 1189 | #[stable(feature = "cell_extras", since = "1.15.0")] |
62682a34 SL |
1190 | #[inline] |
1191 | pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { | |
1192 | Ref { | |
54a0048b SL |
1193 | value: orig.value, |
1194 | borrow: orig.borrow.clone(), | |
62682a34 SL |
1195 | } |
1196 | } | |
1197 | ||
9fa01778 | 1198 | /// Makes a new `Ref` for a component of the borrowed data. |
62682a34 SL |
1199 | /// |
1200 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1201 | /// | |
1202 | /// This is an associated function that needs to be used as `Ref::map(...)`. | |
1203 | /// A method would interfere with methods of the same name on the contents | |
1204 | /// of a `RefCell` used through `Deref`. | |
1205 | /// | |
3b2f2976 | 1206 | /// # Examples |
62682a34 SL |
1207 | /// |
1208 | /// ``` | |
62682a34 SL |
1209 | /// use std::cell::{RefCell, Ref}; |
1210 | /// | |
1211 | /// let c = RefCell::new((5, 'b')); | |
1212 | /// let b1: Ref<(u32, char)> = c.borrow(); | |
1213 | /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); | |
1214 | /// assert_eq!(*b2, 5) | |
1215 | /// ``` | |
7453a54e | 1216 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1217 | #[inline] |
1218 | pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> | |
1219 | where F: FnOnce(&T) -> &U | |
1220 | { | |
1221 | Ref { | |
54a0048b SL |
1222 | value: f(orig.value), |
1223 | borrow: orig.borrow, | |
62682a34 SL |
1224 | } |
1225 | } | |
94b46f34 | 1226 | |
9fa01778 | 1227 | /// Splits a `Ref` into multiple `Ref`s for different components of the |
94b46f34 XL |
1228 | /// borrowed data. |
1229 | /// | |
1230 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1231 | /// | |
1232 | /// This is an associated function that needs to be used as | |
1233 | /// `Ref::map_split(...)`. A method would interfere with methods of the same | |
1234 | /// name on the contents of a `RefCell` used through `Deref`. | |
1235 | /// | |
1236 | /// # Examples | |
1237 | /// | |
1238 | /// ``` | |
94b46f34 XL |
1239 | /// use std::cell::{Ref, RefCell}; |
1240 | /// | |
1241 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1242 | /// let borrow = cell.borrow(); | |
1243 | /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2)); | |
1244 | /// assert_eq!(*begin, [1, 2]); | |
1245 | /// assert_eq!(*end, [3, 4]); | |
1246 | /// ``` | |
532ac7d7 | 1247 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1248 | #[inline] |
1249 | pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>) | |
1250 | where F: FnOnce(&T) -> (&U, &V) | |
1251 | { | |
1252 | let (a, b) = f(orig.value); | |
1253 | let borrow = orig.borrow.clone(); | |
1254 | (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow }) | |
1255 | } | |
62682a34 SL |
1256 | } |
1257 | ||
54a0048b SL |
1258 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1259 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} | |
1260 | ||
041b39d2 | 1261 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1262 | impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { |
48663c56 | 1263 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1264 | self.value.fmt(f) |
1265 | } | |
1266 | } | |
1267 | ||
62682a34 | 1268 | impl<'b, T: ?Sized> RefMut<'b, T> { |
9fa01778 | 1269 | /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum |
62682a34 SL |
1270 | /// variant. |
1271 | /// | |
1272 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1273 | /// | |
1274 | /// This is an associated function that needs to be used as | |
9fa01778 | 1275 | /// `RefMut::map(...)`. A method would interfere with methods of the same |
62682a34 SL |
1276 | /// name on the contents of a `RefCell` used through `Deref`. |
1277 | /// | |
3b2f2976 | 1278 | /// # Examples |
62682a34 SL |
1279 | /// |
1280 | /// ``` | |
62682a34 SL |
1281 | /// use std::cell::{RefCell, RefMut}; |
1282 | /// | |
1283 | /// let c = RefCell::new((5, 'b')); | |
1284 | /// { | |
1285 | /// let b1: RefMut<(u32, char)> = c.borrow_mut(); | |
1286 | /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0); | |
1287 | /// assert_eq!(*b2, 5); | |
1288 | /// *b2 = 42; | |
1289 | /// } | |
1290 | /// assert_eq!(*c.borrow(), (42, 'b')); | |
1291 | /// ``` | |
7453a54e | 1292 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1293 | #[inline] |
1294 | pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U> | |
1295 | where F: FnOnce(&mut T) -> &mut U | |
1296 | { | |
ff7c6d11 XL |
1297 | // FIXME(nll-rfc#40): fix borrow-check |
1298 | let RefMut { value, borrow } = orig; | |
62682a34 | 1299 | RefMut { |
ff7c6d11 | 1300 | value: f(value), |
b7449926 | 1301 | borrow, |
62682a34 SL |
1302 | } |
1303 | } | |
94b46f34 | 1304 | |
9fa01778 | 1305 | /// Splits a `RefMut` into multiple `RefMut`s for different components of the |
94b46f34 XL |
1306 | /// borrowed data. |
1307 | /// | |
1308 | /// The underlying `RefCell` will remain mutably borrowed until both | |
1309 | /// returned `RefMut`s go out of scope. | |
1310 | /// | |
1311 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1312 | /// | |
1313 | /// This is an associated function that needs to be used as | |
1314 | /// `RefMut::map_split(...)`. A method would interfere with methods of the | |
1315 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1316 | /// | |
1317 | /// # Examples | |
1318 | /// | |
1319 | /// ``` | |
94b46f34 XL |
1320 | /// use std::cell::{RefCell, RefMut}; |
1321 | /// | |
1322 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1323 | /// let borrow = cell.borrow_mut(); | |
1324 | /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2)); | |
1325 | /// assert_eq!(*begin, [1, 2]); | |
1326 | /// assert_eq!(*end, [3, 4]); | |
1327 | /// begin.copy_from_slice(&[4, 3]); | |
1328 | /// end.copy_from_slice(&[2, 1]); | |
1329 | /// ``` | |
532ac7d7 | 1330 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1331 | #[inline] |
1332 | pub fn map_split<U: ?Sized, V: ?Sized, F>( | |
1333 | orig: RefMut<'b, T>, f: F | |
1334 | ) -> (RefMut<'b, U>, RefMut<'b, V>) | |
1335 | where F: FnOnce(&mut T) -> (&mut U, &mut V) | |
1336 | { | |
1337 | let (a, b) = f(orig.value); | |
1338 | let borrow = orig.borrow.clone(); | |
1339 | (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow }) | |
1340 | } | |
1a4d82fc JJ |
1341 | } |
1342 | ||
1343 | struct BorrowRefMut<'b> { | |
54a0048b | 1344 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1345 | } |
1346 | ||
0bf4aa26 | 1347 | impl Drop for BorrowRefMut<'_> { |
c34b1796 | 1348 | #[inline] |
1a4d82fc | 1349 | fn drop(&mut self) { |
54a0048b | 1350 | let borrow = self.borrow.get(); |
8faf50e0 XL |
1351 | debug_assert!(is_writing(borrow)); |
1352 | self.borrow.set(borrow + 1); | |
1a4d82fc JJ |
1353 | } |
1354 | } | |
1355 | ||
1356 | impl<'b> BorrowRefMut<'b> { | |
c34b1796 | 1357 | #[inline] |
1a4d82fc | 1358 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { |
94b46f34 XL |
1359 | // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial |
1360 | // mutable reference, and so there must currently be no existing | |
1361 | // references. Thus, while clone increments the mutable refcount, here | |
8faf50e0 | 1362 | // we explicitly only allow going from UNUSED to UNUSED - 1. |
1a4d82fc JJ |
1363 | match borrow.get() { |
1364 | UNUSED => { | |
8faf50e0 | 1365 | borrow.set(UNUSED - 1); |
b7449926 | 1366 | Some(BorrowRefMut { borrow }) |
1a4d82fc JJ |
1367 | }, |
1368 | _ => None, | |
1369 | } | |
1370 | } | |
94b46f34 | 1371 | |
dc9dc135 | 1372 | // Clones a `BorrowRefMut`. |
94b46f34 XL |
1373 | // |
1374 | // This is only valid if each `BorrowRefMut` is used to track a mutable | |
1375 | // reference to a distinct, nonoverlapping range of the original object. | |
1376 | // This isn't in a Clone impl so that code doesn't call this implicitly. | |
1377 | #[inline] | |
1378 | fn clone(&self) -> BorrowRefMut<'b> { | |
1379 | let borrow = self.borrow.get(); | |
8faf50e0 XL |
1380 | debug_assert!(is_writing(borrow)); |
1381 | // Prevent the borrow counter from underflowing. | |
1382 | assert!(borrow != isize::min_value()); | |
1383 | self.borrow.set(borrow - 1); | |
94b46f34 XL |
1384 | BorrowRefMut { borrow: self.borrow } |
1385 | } | |
1a4d82fc JJ |
1386 | } |
1387 | ||
85aaf69f SL |
1388 | /// A wrapper type for a mutably borrowed value from a `RefCell<T>`. |
1389 | /// | |
1390 | /// See the [module-level documentation](index.html) for more. | |
1391 | #[stable(feature = "rust1", since = "1.0.0")] | |
d9579d0f | 1392 | pub struct RefMut<'b, T: ?Sized + 'b> { |
54a0048b SL |
1393 | value: &'b mut T, |
1394 | borrow: BorrowRefMut<'b>, | |
1a4d82fc JJ |
1395 | } |
1396 | ||
85aaf69f | 1397 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1398 | impl<T: ?Sized> Deref for RefMut<'_, T> { |
1a4d82fc JJ |
1399 | type Target = T; |
1400 | ||
1401 | #[inline] | |
e9174d1e | 1402 | fn deref(&self) -> &T { |
54a0048b | 1403 | self.value |
1a4d82fc JJ |
1404 | } |
1405 | } | |
1406 | ||
85aaf69f | 1407 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1408 | impl<T: ?Sized> DerefMut for RefMut<'_, T> { |
1a4d82fc | 1409 | #[inline] |
e9174d1e | 1410 | fn deref_mut(&mut self) -> &mut T { |
54a0048b | 1411 | self.value |
1a4d82fc JJ |
1412 | } |
1413 | } | |
1414 | ||
54a0048b SL |
1415 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1416 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} | |
1417 | ||
041b39d2 | 1418 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1419 | impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { |
48663c56 | 1420 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1421 | self.value.fmt(f) |
1422 | } | |
1423 | } | |
1424 | ||
1a4d82fc JJ |
1425 | /// The core primitive for interior mutability in Rust. |
1426 | /// | |
85aaf69f SL |
1427 | /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the |
1428 | /// wrapped type. Types with an `UnsafeCell<T>` field are considered to have an 'unsafe interior'. | |
1429 | /// The `UnsafeCell<T>` type is the only legal way to obtain aliasable data that is considered | |
1430 | /// mutable. In general, transmuting an `&T` type into an `&mut T` is considered undefined behavior. | |
1a4d82fc | 1431 | /// |
0531ce1d XL |
1432 | /// If you have a reference `&SomeStruct`, then normally in Rust all fields of `SomeStruct` are |
1433 | /// immutable. The compiler makes optimizations based on the knowledge that `&T` is not mutably | |
94b46f34 | 1434 | /// aliased or mutated, and that `&mut T` is unique. `UnsafeCell<T>` is the only core language |
416331ca XL |
1435 | /// feature to work around the restriction that `&T` may not be mutated. All other types that |
1436 | /// allow internal mutability, such as `Cell<T>` and `RefCell<T>`, use `UnsafeCell` to wrap their | |
1437 | /// internal data. There is *no* legal way to obtain aliasing `&mut`, not even with `UnsafeCell<T>`. | |
5bcae85e | 1438 | /// |
0531ce1d XL |
1439 | /// The `UnsafeCell` API itself is technically very simple: it gives you a raw pointer `*mut T` to |
1440 | /// its contents. It is up to _you_ as the abstraction designer to use that raw pointer correctly. | |
1441 | /// | |
1442 | /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: | |
1443 | /// | |
94b46f34 XL |
1444 | /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` |
1445 | /// reference) that is accessible by safe code (for example, because you returned it), | |
1446 | /// then you must not access the data in any way that contradicts that reference for the | |
1447 | /// remainder of `'a`. For example, this means that if you take the `*mut T` from an | |
1448 | /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable | |
1449 | /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's | |
1450 | /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to | |
1451 | /// safe code, then you must not access the data within the `UnsafeCell` until that | |
1452 | /// reference expires. | |
0531ce1d | 1453 | /// |
94b46f34 | 1454 | /// - At all times, you must avoid data races. If multiple threads have access to |
0531ce1d XL |
1455 | /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other |
1456 | /// accesses (or use atomics). | |
5bcae85e | 1457 | /// |
0531ce1d XL |
1458 | /// To assist with proper design, the following scenarios are explicitly declared legal |
1459 | /// for single-threaded code: | |
5bcae85e | 1460 | /// |
94b46f34 | 1461 | /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` |
0531ce1d XL |
1462 | /// references, but not with a `&mut T` |
1463 | /// | |
94b46f34 | 1464 | /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` |
0531ce1d XL |
1465 | /// co-exist with it. A `&mut T` must always be unique. |
1466 | /// | |
94b46f34 | 1467 | /// Note that while mutating or mutably aliasing the contents of an `&UnsafeCell<T>` is |
9fa01778 | 1468 | /// ok (provided you enforce the invariants some other way), it is still undefined behavior |
0531ce1d | 1469 | /// to have multiple `&mut UnsafeCell<T>` aliases. |
1a4d82fc | 1470 | /// |
85aaf69f | 1471 | /// # Examples |
1a4d82fc | 1472 | /// |
85aaf69f | 1473 | /// ``` |
1a4d82fc | 1474 | /// use std::cell::UnsafeCell; |
1a4d82fc | 1475 | /// |
92a42be0 | 1476 | /// # #[allow(dead_code)] |
1a4d82fc JJ |
1477 | /// struct NotThreadSafe<T> { |
1478 | /// value: UnsafeCell<T>, | |
1a4d82fc | 1479 | /// } |
85aaf69f SL |
1480 | /// |
1481 | /// unsafe impl<T> Sync for NotThreadSafe<T> {} | |
1a4d82fc | 1482 | /// ``` |
d9579d0f | 1483 | #[lang = "unsafe_cell"] |
85aaf69f | 1484 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 1485 | #[repr(transparent)] |
d9579d0f | 1486 | pub struct UnsafeCell<T: ?Sized> { |
e9174d1e | 1487 | value: T, |
1a4d82fc JJ |
1488 | } |
1489 | ||
92a42be0 | 1490 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1491 | impl<T: ?Sized> !Sync for UnsafeCell<T> {} |
c34b1796 | 1492 | |
1a4d82fc | 1493 | impl<T> UnsafeCell<T> { |
9346a6ac | 1494 | /// Constructs a new instance of `UnsafeCell` which will wrap the specified |
1a4d82fc JJ |
1495 | /// value. |
1496 | /// | |
b039eaaf | 1497 | /// All access to the inner value through methods is `unsafe`. |
85aaf69f SL |
1498 | /// |
1499 | /// # Examples | |
1500 | /// | |
1501 | /// ``` | |
1502 | /// use std::cell::UnsafeCell; | |
1503 | /// | |
1504 | /// let uc = UnsafeCell::new(5); | |
1505 | /// ``` | |
1506 | #[stable(feature = "rust1", since = "1.0.0")] | |
60c5eb7d XL |
1507 | #[cfg_attr( |
1508 | not(bootstrap), | |
1509 | rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0"), | |
1510 | )] | |
c34b1796 | 1511 | #[inline] |
62682a34 | 1512 | pub const fn new(value: T) -> UnsafeCell<T> { |
b7449926 | 1513 | UnsafeCell { value } |
1a4d82fc JJ |
1514 | } |
1515 | ||
d9579d0f AL |
1516 | /// Unwraps the value. |
1517 | /// | |
85aaf69f SL |
1518 | /// # Examples |
1519 | /// | |
1520 | /// ``` | |
1521 | /// use std::cell::UnsafeCell; | |
1522 | /// | |
1523 | /// let uc = UnsafeCell::new(5); | |
1524 | /// | |
2c00a5a8 | 1525 | /// let five = uc.into_inner(); |
85aaf69f | 1526 | /// ``` |
1a4d82fc | 1527 | #[inline] |
85aaf69f | 1528 | #[stable(feature = "rust1", since = "1.0.0")] |
2c00a5a8 | 1529 | pub fn into_inner(self) -> T { |
62682a34 SL |
1530 | self.value |
1531 | } | |
d9579d0f | 1532 | } |
1a4d82fc | 1533 | |
d9579d0f AL |
1534 | impl<T: ?Sized> UnsafeCell<T> { |
1535 | /// Gets a mutable pointer to the wrapped value. | |
85aaf69f | 1536 | /// |
5bcae85e | 1537 | /// This can be cast to a pointer of any kind. |
0531ce1d XL |
1538 | /// Ensure that the access is unique (no active references, mutable or not) |
1539 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1540 | /// or mutable aliases going on when casting to `&T` | |
5bcae85e | 1541 | /// |
85aaf69f SL |
1542 | /// # Examples |
1543 | /// | |
1544 | /// ``` | |
1545 | /// use std::cell::UnsafeCell; | |
1546 | /// | |
1547 | /// let uc = UnsafeCell::new(5); | |
1548 | /// | |
d9579d0f | 1549 | /// let five = uc.get(); |
85aaf69f | 1550 | /// ``` |
1a4d82fc | 1551 | #[inline] |
85aaf69f | 1552 | #[stable(feature = "rust1", since = "1.0.0")] |
60c5eb7d XL |
1553 | #[cfg_attr( |
1554 | not(bootstrap), | |
1555 | rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0"), | |
1556 | )] | |
a1dfa0c6 XL |
1557 | pub const fn get(&self) -> *mut T { |
1558 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
60c5eb7d XL |
1559 | // #[repr(transparent)]. This exploits libstd's special status, there is |
1560 | // no guarantee for user code that this will work in future versions of the compiler! | |
a1dfa0c6 | 1561 | self as *const UnsafeCell<T> as *const T as *mut T |
d9579d0f | 1562 | } |
60c5eb7d XL |
1563 | |
1564 | /// Gets a mutable pointer to the wrapped value. | |
1565 | /// The difference to [`get`] is that this function accepts a raw pointer, | |
1566 | /// which is useful to avoid the creation of temporary references. | |
1567 | /// | |
1568 | /// The result can be cast to a pointer of any kind. | |
1569 | /// Ensure that the access is unique (no active references, mutable or not) | |
1570 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1571 | /// or mutable aliases going on when casting to `&T`. | |
1572 | /// | |
1573 | /// [`get`]: #method.get | |
1574 | /// | |
1575 | /// # Examples | |
1576 | /// | |
1577 | /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as | |
1578 | /// calling `get` would require creating a reference to uninitialized data: | |
1579 | /// | |
1580 | /// ``` | |
1581 | /// #![feature(unsafe_cell_raw_get)] | |
1582 | /// use std::cell::UnsafeCell; | |
1583 | /// use std::mem::MaybeUninit; | |
1584 | /// | |
1585 | /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit(); | |
1586 | /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); } | |
1587 | /// let uc = unsafe { m.assume_init() }; | |
1588 | /// | |
1589 | /// assert_eq!(uc.into_inner(), 5); | |
1590 | /// ``` | |
1591 | #[inline] | |
1592 | #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")] | |
1593 | pub const fn raw_get(this: *const Self) -> *mut T { | |
1594 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
1595 | // #[repr(transparent)]. This exploits libstd's special status, there is | |
1596 | // no guarantee for user code that this will work in future versions of the compiler! | |
1597 | this as *const T as *mut T | |
1598 | } | |
1a4d82fc | 1599 | } |
a7813a04 | 1600 | |
7cac9316 | 1601 | #[stable(feature = "unsafe_cell_default", since = "1.10.0")] |
a7813a04 | 1602 | impl<T: Default> Default for UnsafeCell<T> { |
9e0c209e | 1603 | /// Creates an `UnsafeCell`, with the `Default` value for T. |
a7813a04 XL |
1604 | fn default() -> UnsafeCell<T> { |
1605 | UnsafeCell::new(Default::default()) | |
1606 | } | |
1607 | } | |
5bcae85e SL |
1608 | |
1609 | #[stable(feature = "cell_from", since = "1.12.0")] | |
1610 | impl<T> From<T> for UnsafeCell<T> { | |
1611 | fn from(t: T) -> UnsafeCell<T> { | |
1612 | UnsafeCell::new(t) | |
1613 | } | |
1614 | } | |
9e0c209e SL |
1615 | |
1616 | #[unstable(feature = "coerce_unsized", issue = "27732")] | |
1617 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {} | |
1618 | ||
1619 | #[allow(unused)] | |
1620 | fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) { | |
8faf50e0 XL |
1621 | let _: UnsafeCell<&dyn Send> = a; |
1622 | let _: Cell<&dyn Send> = b; | |
1623 | let _: RefCell<&dyn Send> = c; | |
9e0c209e | 1624 | } |