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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 | |
5869c6ff | 14 | //! presence of aliasing. Both [`Cell<T>`] and [`RefCell<T>`] allow doing this in a single-threaded |
0531ce1d | 15 | //! way. However, neither `Cell<T>` nor `RefCell<T>` are thread safe (they do not implement |
5869c6ff XL |
16 | //! [`Sync`]). If you need to do aliasing and mutation between multiple threads it is possible to |
17 | //! use [`Mutex<T>`], [`RwLock<T>`] or [`atomic`] types. | |
0531ce1d | 18 | //! |
85aaf69f SL |
19 | //! Values of the `Cell<T>` and `RefCell<T>` types may be mutated through shared references (i.e. |
20 | //! the common `&T` type), whereas most Rust types can only be mutated through unique (`&mut T`) | |
21 | //! references. We say that `Cell<T>` and `RefCell<T>` provide 'interior mutability', in contrast | |
22 | //! with typical Rust types that exhibit 'inherited mutability'. | |
1a4d82fc | 23 | //! |
8bb4bdeb XL |
24 | //! Cell types come in two flavors: `Cell<T>` and `RefCell<T>`. `Cell<T>` implements interior |
25 | //! mutability by moving values in and out of the `Cell<T>`. To use references instead of values, | |
26 | //! one must use the `RefCell<T>` type, acquiring a write lock before mutating. `Cell<T>` provides | |
27 | //! methods to retrieve and change the current interior value: | |
28 | //! | |
5869c6ff XL |
29 | //! - For types that implement [`Copy`], the [`get`](Cell::get) method retrieves the current |
30 | //! interior value. | |
31 | //! - For types that implement [`Default`], the [`take`](Cell::take) method replaces the current | |
32 | //! interior value with [`Default::default()`] and returns the replaced value. | |
33 | //! - For all types, the [`replace`](Cell::replace) method replaces the current interior value and | |
34 | //! returns the replaced value and the [`into_inner`](Cell::into_inner) method consumes the | |
35 | //! `Cell<T>` and returns the interior value. Additionally, the [`set`](Cell::set) method | |
36 | //! replaces the interior value, dropping the replaced value. | |
1a4d82fc | 37 | //! |
85aaf69f SL |
38 | //! `RefCell<T>` uses Rust's lifetimes to implement 'dynamic borrowing', a process whereby one can |
39 | //! claim temporary, exclusive, mutable access to the inner value. Borrows for `RefCell<T>`s are | |
40 | //! tracked 'at runtime', unlike Rust's native reference types which are entirely tracked | |
41 | //! statically, at compile time. Because `RefCell<T>` borrows are dynamic it is possible to attempt | |
bd371182 AL |
42 | //! to borrow a value that is already mutably borrowed; when this happens it results in thread |
43 | //! panic. | |
1a4d82fc JJ |
44 | //! |
45 | //! # When to choose interior mutability | |
46 | //! | |
85aaf69f SL |
47 | //! The more common inherited mutability, where one must have unique access to mutate a value, is |
48 | //! one of the key language elements that enables Rust to reason strongly about pointer aliasing, | |
49 | //! statically preventing crash bugs. Because of that, inherited mutability is preferred, and | |
50 | //! interior mutability is something of a last resort. Since cell types enable mutation where it | |
51 | //! would otherwise be disallowed though, there are occasions when interior mutability might be | |
52 | //! appropriate, or even *must* be used, e.g. | |
1a4d82fc | 53 | //! |
c1a9b12d | 54 | //! * Introducing mutability 'inside' of something immutable |
1a4d82fc | 55 | //! * Implementation details of logically-immutable methods. |
5869c6ff | 56 | //! * Mutating implementations of [`Clone`]. |
1a4d82fc | 57 | //! |
c1a9b12d | 58 | //! ## Introducing mutability 'inside' of something immutable |
1a4d82fc | 59 | //! |
5869c6ff XL |
60 | //! Many shared smart pointer types, including [`Rc<T>`] and [`Arc<T>`], provide containers that can |
61 | //! be cloned and shared between multiple parties. Because the contained values may be | |
c1a9b12d SL |
62 | //! multiply-aliased, they can only be borrowed with `&`, not `&mut`. Without cells it would be |
63 | //! impossible to mutate data inside of these smart pointers at all. | |
1a4d82fc | 64 | //! |
85aaf69f SL |
65 | //! It's very common then to put a `RefCell<T>` inside shared pointer types to reintroduce |
66 | //! mutability: | |
1a4d82fc JJ |
67 | //! |
68 | //! ``` | |
dc9dc135 | 69 | //! use std::cell::{RefCell, RefMut}; |
1a4d82fc | 70 | //! use std::collections::HashMap; |
1a4d82fc JJ |
71 | //! use std::rc::Rc; |
72 | //! | |
73 | //! fn main() { | |
74 | //! let shared_map: Rc<RefCell<_>> = Rc::new(RefCell::new(HashMap::new())); | |
dc9dc135 XL |
75 | //! // Create a new block to limit the scope of the dynamic borrow |
76 | //! { | |
77 | //! let mut map: RefMut<_> = shared_map.borrow_mut(); | |
78 | //! map.insert("africa", 92388); | |
79 | //! map.insert("kyoto", 11837); | |
80 | //! map.insert("piccadilly", 11826); | |
81 | //! map.insert("marbles", 38); | |
82 | //! } | |
83 | //! | |
84 | //! // Note that if we had not let the previous borrow of the cache fall out | |
85 | //! // of scope then the subsequent borrow would cause a dynamic thread panic. | |
86 | //! // This is the major hazard of using `RefCell`. | |
87 | //! let total: i32 = shared_map.borrow().values().sum(); | |
88 | //! println!("{}", total); | |
1a4d82fc JJ |
89 | //! } |
90 | //! ``` | |
91 | //! | |
85aaf69f | 92 | //! Note that this example uses `Rc<T>` and not `Arc<T>`. `RefCell<T>`s are for single-threaded |
5869c6ff | 93 | //! scenarios. Consider using [`RwLock<T>`] or [`Mutex<T>`] if you need shared mutability in a |
c1a9b12d | 94 | //! multi-threaded situation. |
85aaf69f | 95 | //! |
1a4d82fc JJ |
96 | //! ## Implementation details of logically-immutable methods |
97 | //! | |
85aaf69f | 98 | //! Occasionally it may be desirable not to expose in an API that there is mutation happening |
0731742a | 99 | //! "under the hood". This may be because logically the operation is immutable, but e.g., caching |
85aaf69f SL |
100 | //! forces the implementation to perform mutation; or because you must employ mutation to implement |
101 | //! a trait method that was originally defined to take `&self`. | |
1a4d82fc JJ |
102 | //! |
103 | //! ``` | |
92a42be0 | 104 | //! # #![allow(dead_code)] |
1a4d82fc JJ |
105 | //! use std::cell::RefCell; |
106 | //! | |
107 | //! struct Graph { | |
85aaf69f SL |
108 | //! edges: Vec<(i32, i32)>, |
109 | //! span_tree_cache: RefCell<Option<Vec<(i32, i32)>>> | |
1a4d82fc JJ |
110 | //! } |
111 | //! | |
112 | //! impl Graph { | |
85aaf69f | 113 | //! fn minimum_spanning_tree(&self) -> Vec<(i32, i32)> { |
dc9dc135 XL |
114 | //! self.span_tree_cache.borrow_mut() |
115 | //! .get_or_insert_with(|| self.calc_span_tree()) | |
116 | //! .clone() | |
117 | //! } | |
1a4d82fc | 118 | //! |
dc9dc135 XL |
119 | //! fn calc_span_tree(&self) -> Vec<(i32, i32)> { |
120 | //! // Expensive computation goes here | |
121 | //! vec![] | |
1a4d82fc | 122 | //! } |
1a4d82fc | 123 | //! } |
1a4d82fc JJ |
124 | //! ``` |
125 | //! | |
62682a34 | 126 | //! ## Mutating implementations of `Clone` |
1a4d82fc | 127 | //! |
85aaf69f | 128 | //! This is simply a special - but common - case of the previous: hiding mutability for operations |
5869c6ff XL |
129 | //! that appear to be immutable. The [`clone`](Clone::clone) method is expected to not change the |
130 | //! source value, and is declared to take `&self`, not `&mut self`. Therefore, any mutation that | |
131 | //! happens in the `clone` method must use cell types. For example, [`Rc<T>`] maintains its | |
132 | //! reference counts within a `Cell<T>`. | |
1a4d82fc JJ |
133 | //! |
134 | //! ``` | |
135 | //! use std::cell::Cell; | |
0531ce1d | 136 | //! use std::ptr::NonNull; |
f9f354fc | 137 | //! use std::process::abort; |
60c5eb7d | 138 | //! use std::marker::PhantomData; |
1a4d82fc | 139 | //! |
9e0c209e | 140 | //! struct Rc<T: ?Sized> { |
60c5eb7d XL |
141 | //! ptr: NonNull<RcBox<T>>, |
142 | //! phantom: PhantomData<RcBox<T>>, | |
1a4d82fc JJ |
143 | //! } |
144 | //! | |
9e0c209e SL |
145 | //! struct RcBox<T: ?Sized> { |
146 | //! strong: Cell<usize>, | |
147 | //! refcount: Cell<usize>, | |
1a4d82fc | 148 | //! value: T, |
1a4d82fc JJ |
149 | //! } |
150 | //! | |
9e0c209e | 151 | //! impl<T: ?Sized> Clone for Rc<T> { |
1a4d82fc | 152 | //! fn clone(&self) -> Rc<T> { |
9e0c209e | 153 | //! self.inc_strong(); |
60c5eb7d XL |
154 | //! Rc { |
155 | //! ptr: self.ptr, | |
156 | //! phantom: PhantomData, | |
157 | //! } | |
9e0c209e SL |
158 | //! } |
159 | //! } | |
160 | //! | |
161 | //! trait RcBoxPtr<T: ?Sized> { | |
162 | //! | |
163 | //! fn inner(&self) -> &RcBox<T>; | |
164 | //! | |
165 | //! fn strong(&self) -> usize { | |
166 | //! self.inner().strong.get() | |
167 | //! } | |
168 | //! | |
169 | //! fn inc_strong(&self) { | |
170 | //! self.inner() | |
171 | //! .strong | |
172 | //! .set(self.strong() | |
173 | //! .checked_add(1) | |
f9f354fc | 174 | //! .unwrap_or_else(|| abort() )); |
1a4d82fc JJ |
175 | //! } |
176 | //! } | |
9e0c209e SL |
177 | //! |
178 | //! impl<T: ?Sized> RcBoxPtr<T> for Rc<T> { | |
179 | //! fn inner(&self) -> &RcBox<T> { | |
180 | //! unsafe { | |
7cac9316 | 181 | //! self.ptr.as_ref() |
9e0c209e SL |
182 | //! } |
183 | //! } | |
184 | //! } | |
1a4d82fc JJ |
185 | //! ``` |
186 | //! | |
5869c6ff XL |
187 | //! [`Arc<T>`]: ../../std/sync/struct.Arc.html |
188 | //! [`Rc<T>`]: ../../std/rc/struct.Rc.html | |
189 | //! [`RwLock<T>`]: ../../std/sync/struct.RwLock.html | |
190 | //! [`Mutex<T>`]: ../../std/sync/struct.Mutex.html | |
cdc7bbd5 | 191 | //! [`atomic`]: crate::sync::atomic |
1a4d82fc | 192 | |
85aaf69f | 193 | #![stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 194 | |
48663c56 XL |
195 | use crate::cmp::Ordering; |
196 | use crate::fmt::{self, Debug, Display}; | |
197 | use crate::marker::Unsize; | |
198 | use crate::mem; | |
dfeec247 | 199 | use crate::ops::{CoerceUnsized, Deref, DerefMut}; |
48663c56 | 200 | use crate::ptr; |
1a4d82fc | 201 | |
8bb4bdeb | 202 | /// A mutable memory location. |
85aaf69f | 203 | /// |
3b2f2976 XL |
204 | /// # Examples |
205 | /// | |
a1dfa0c6 XL |
206 | /// In this example, you can see that `Cell<T>` enables mutation inside an |
207 | /// immutable struct. In other words, it enables "interior mutability". | |
3b2f2976 XL |
208 | /// |
209 | /// ``` | |
210 | /// use std::cell::Cell; | |
211 | /// | |
212 | /// struct SomeStruct { | |
213 | /// regular_field: u8, | |
214 | /// special_field: Cell<u8>, | |
215 | /// } | |
216 | /// | |
217 | /// let my_struct = SomeStruct { | |
218 | /// regular_field: 0, | |
219 | /// special_field: Cell::new(1), | |
220 | /// }; | |
221 | /// | |
222 | /// let new_value = 100; | |
223 | /// | |
a1dfa0c6 | 224 | /// // ERROR: `my_struct` is immutable |
3b2f2976 XL |
225 | /// // my_struct.regular_field = new_value; |
226 | /// | |
a1dfa0c6 XL |
227 | /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, |
228 | /// // which can always be mutated | |
3b2f2976 XL |
229 | /// my_struct.special_field.set(new_value); |
230 | /// assert_eq!(my_struct.special_field.get(), new_value); | |
231 | /// ``` | |
232 | /// | |
29967ef6 | 233 | /// See the [module-level documentation](self) for more. |
85aaf69f | 234 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 XL |
235 | #[repr(transparent)] |
236 | pub struct Cell<T: ?Sized> { | |
1a4d82fc JJ |
237 | value: UnsafeCell<T>, |
238 | } | |
239 | ||
8bb4bdeb | 240 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 241 | unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} |
8bb4bdeb XL |
242 | |
243 | #[stable(feature = "rust1", since = "1.0.0")] | |
8faf50e0 | 244 | impl<T: ?Sized> !Sync for Cell<T> {} |
8bb4bdeb XL |
245 | |
246 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 247 | impl<T: Copy> Clone for Cell<T> { |
8bb4bdeb XL |
248 | #[inline] |
249 | fn clone(&self) -> Cell<T> { | |
250 | Cell::new(self.get()) | |
251 | } | |
252 | } | |
253 | ||
254 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 255 | impl<T: Default> Default for Cell<T> { |
8bb4bdeb XL |
256 | /// Creates a `Cell<T>`, with the `Default` value for T. |
257 | #[inline] | |
258 | fn default() -> Cell<T> { | |
259 | Cell::new(Default::default()) | |
260 | } | |
261 | } | |
262 | ||
263 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 264 | impl<T: PartialEq + Copy> PartialEq for Cell<T> { |
8bb4bdeb XL |
265 | #[inline] |
266 | fn eq(&self, other: &Cell<T>) -> bool { | |
267 | self.get() == other.get() | |
268 | } | |
269 | } | |
270 | ||
271 | #[stable(feature = "cell_eq", since = "1.2.0")] | |
416331ca | 272 | impl<T: Eq + Copy> Eq for Cell<T> {} |
8bb4bdeb XL |
273 | |
274 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 275 | impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { |
8bb4bdeb XL |
276 | #[inline] |
277 | fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { | |
278 | self.get().partial_cmp(&other.get()) | |
279 | } | |
280 | ||
281 | #[inline] | |
282 | fn lt(&self, other: &Cell<T>) -> bool { | |
283 | self.get() < other.get() | |
284 | } | |
285 | ||
286 | #[inline] | |
287 | fn le(&self, other: &Cell<T>) -> bool { | |
288 | self.get() <= other.get() | |
289 | } | |
290 | ||
291 | #[inline] | |
292 | fn gt(&self, other: &Cell<T>) -> bool { | |
293 | self.get() > other.get() | |
294 | } | |
295 | ||
296 | #[inline] | |
297 | fn ge(&self, other: &Cell<T>) -> bool { | |
298 | self.get() >= other.get() | |
299 | } | |
300 | } | |
301 | ||
302 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 303 | impl<T: Ord + Copy> Ord for Cell<T> { |
8bb4bdeb XL |
304 | #[inline] |
305 | fn cmp(&self, other: &Cell<T>) -> Ordering { | |
306 | self.get().cmp(&other.get()) | |
307 | } | |
308 | } | |
309 | ||
310 | #[stable(feature = "cell_from", since = "1.12.0")] | |
311 | impl<T> From<T> for Cell<T> { | |
312 | fn from(t: T) -> Cell<T> { | |
313 | Cell::new(t) | |
314 | } | |
315 | } | |
316 | ||
317 | impl<T> Cell<T> { | |
318 | /// Creates a new `Cell` containing the given value. | |
85aaf69f SL |
319 | /// |
320 | /// # Examples | |
321 | /// | |
322 | /// ``` | |
323 | /// use std::cell::Cell; | |
324 | /// | |
325 | /// let c = Cell::new(5); | |
85aaf69f | 326 | /// ``` |
85aaf69f | 327 | #[stable(feature = "rust1", since = "1.0.0")] |
cdc7bbd5 | 328 | #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")] |
8bb4bdeb XL |
329 | #[inline] |
330 | pub const fn new(value: T) -> Cell<T> { | |
dfeec247 | 331 | Cell { value: UnsafeCell::new(value) } |
1a4d82fc JJ |
332 | } |
333 | ||
8bb4bdeb XL |
334 | /// Sets the contained value. |
335 | /// | |
336 | /// # Examples | |
337 | /// | |
338 | /// ``` | |
339 | /// use std::cell::Cell; | |
340 | /// | |
341 | /// let c = Cell::new(5); | |
342 | /// | |
343 | /// c.set(10); | |
344 | /// ``` | |
a7813a04 | 345 | #[inline] |
8bb4bdeb XL |
346 | #[stable(feature = "rust1", since = "1.0.0")] |
347 | pub fn set(&self, val: T) { | |
348 | let old = self.replace(val); | |
349 | drop(old); | |
a7813a04 XL |
350 | } |
351 | ||
8bb4bdeb XL |
352 | /// Swaps the values of two Cells. |
353 | /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. | |
354 | /// | |
355 | /// # Examples | |
356 | /// | |
357 | /// ``` | |
358 | /// use std::cell::Cell; | |
359 | /// | |
360 | /// let c1 = Cell::new(5i32); | |
361 | /// let c2 = Cell::new(10i32); | |
362 | /// c1.swap(&c2); | |
363 | /// assert_eq!(10, c1.get()); | |
364 | /// assert_eq!(5, c2.get()); | |
365 | /// ``` | |
a7813a04 | 366 | #[inline] |
8bb4bdeb XL |
367 | #[stable(feature = "move_cell", since = "1.17.0")] |
368 | pub fn swap(&self, other: &Self) { | |
369 | if ptr::eq(self, other) { | |
370 | return; | |
371 | } | |
dfeec247 XL |
372 | // SAFETY: This can be risky if called from separate threads, but `Cell` |
373 | // is `!Sync` so this won't happen. This also won't invalidate any | |
374 | // pointers since `Cell` makes sure nothing else will be pointing into | |
375 | // either of these `Cell`s. | |
8bb4bdeb XL |
376 | unsafe { |
377 | ptr::swap(self.value.get(), other.value.get()); | |
378 | } | |
a7813a04 XL |
379 | } |
380 | ||
6a06907d | 381 | /// Replaces the contained value with `val`, and returns the old contained value. |
8bb4bdeb XL |
382 | /// |
383 | /// # Examples | |
384 | /// | |
385 | /// ``` | |
386 | /// use std::cell::Cell; | |
387 | /// | |
041b39d2 XL |
388 | /// let cell = Cell::new(5); |
389 | /// assert_eq!(cell.get(), 5); | |
390 | /// assert_eq!(cell.replace(10), 5); | |
391 | /// assert_eq!(cell.get(), 10); | |
8bb4bdeb XL |
392 | /// ``` |
393 | #[stable(feature = "move_cell", since = "1.17.0")] | |
394 | pub fn replace(&self, val: T) -> T { | |
dfeec247 XL |
395 | // SAFETY: This can cause data races if called from a separate thread, |
396 | // but `Cell` is `!Sync` so this won't happen. | |
8bb4bdeb | 397 | mem::replace(unsafe { &mut *self.value.get() }, val) |
a7813a04 | 398 | } |
a7813a04 | 399 | |
8bb4bdeb XL |
400 | /// Unwraps the value. |
401 | /// | |
402 | /// # Examples | |
403 | /// | |
404 | /// ``` | |
405 | /// use std::cell::Cell; | |
406 | /// | |
407 | /// let c = Cell::new(5); | |
408 | /// let five = c.into_inner(); | |
409 | /// | |
410 | /// assert_eq!(five, 5); | |
411 | /// ``` | |
412 | #[stable(feature = "move_cell", since = "1.17.0")] | |
29967ef6 XL |
413 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
414 | pub const fn into_inner(self) -> T { | |
2c00a5a8 | 415 | self.value.into_inner() |
a7813a04 | 416 | } |
e74abb32 XL |
417 | } |
418 | ||
dfeec247 | 419 | impl<T: Copy> Cell<T> { |
e74abb32 XL |
420 | /// Returns a copy of the contained value. |
421 | /// | |
422 | /// # Examples | |
423 | /// | |
424 | /// ``` | |
425 | /// use std::cell::Cell; | |
426 | /// | |
427 | /// let c = Cell::new(5); | |
428 | /// | |
429 | /// let five = c.get(); | |
430 | /// ``` | |
431 | #[inline] | |
432 | #[stable(feature = "rust1", since = "1.0.0")] | |
433 | pub fn get(&self) -> T { | |
dfeec247 XL |
434 | // SAFETY: This can cause data races if called from a separate thread, |
435 | // but `Cell` is `!Sync` so this won't happen. | |
436 | unsafe { *self.value.get() } | |
e74abb32 XL |
437 | } |
438 | ||
439 | /// Updates the contained value using a function and returns the new value. | |
440 | /// | |
441 | /// # Examples | |
442 | /// | |
443 | /// ``` | |
444 | /// #![feature(cell_update)] | |
445 | /// | |
446 | /// use std::cell::Cell; | |
447 | /// | |
448 | /// let c = Cell::new(5); | |
449 | /// let new = c.update(|x| x + 1); | |
450 | /// | |
451 | /// assert_eq!(new, 6); | |
452 | /// assert_eq!(c.get(), 6); | |
453 | /// ``` | |
454 | #[inline] | |
455 | #[unstable(feature = "cell_update", issue = "50186")] | |
456 | pub fn update<F>(&self, f: F) -> T | |
457 | where | |
458 | F: FnOnce(T) -> T, | |
459 | { | |
460 | let old = self.get(); | |
461 | let new = f(old); | |
462 | self.set(new); | |
463 | new | |
464 | } | |
a7813a04 XL |
465 | } |
466 | ||
8faf50e0 XL |
467 | impl<T: ?Sized> Cell<T> { |
468 | /// Returns a raw pointer to the underlying data in this cell. | |
469 | /// | |
470 | /// # Examples | |
471 | /// | |
472 | /// ``` | |
473 | /// use std::cell::Cell; | |
474 | /// | |
475 | /// let c = Cell::new(5); | |
476 | /// | |
477 | /// let ptr = c.as_ptr(); | |
478 | /// ``` | |
479 | #[inline] | |
480 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
dfeec247 | 481 | #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")] |
a1dfa0c6 | 482 | pub const fn as_ptr(&self) -> *mut T { |
8faf50e0 XL |
483 | self.value.get() |
484 | } | |
485 | ||
486 | /// Returns a mutable reference to the underlying data. | |
487 | /// | |
488 | /// This call borrows `Cell` mutably (at compile-time) which guarantees | |
489 | /// that we possess the only reference. | |
490 | /// | |
491 | /// # Examples | |
492 | /// | |
493 | /// ``` | |
494 | /// use std::cell::Cell; | |
495 | /// | |
496 | /// let mut c = Cell::new(5); | |
497 | /// *c.get_mut() += 1; | |
498 | /// | |
499 | /// assert_eq!(c.get(), 6); | |
500 | /// ``` | |
501 | #[inline] | |
502 | #[stable(feature = "cell_get_mut", since = "1.11.0")] | |
503 | pub fn get_mut(&mut self) -> &mut T { | |
1b1a35ee | 504 | self.value.get_mut() |
8faf50e0 XL |
505 | } |
506 | ||
507 | /// Returns a `&Cell<T>` from a `&mut T` | |
508 | /// | |
509 | /// # Examples | |
510 | /// | |
511 | /// ``` | |
8faf50e0 XL |
512 | /// use std::cell::Cell; |
513 | /// | |
514 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
515 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
516 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
517 | /// | |
518 | /// assert_eq!(slice_cell.len(), 3); | |
519 | /// ``` | |
520 | #[inline] | |
dc9dc135 | 521 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 522 | pub fn from_mut(t: &mut T) -> &Cell<T> { |
dfeec247 XL |
523 | // SAFETY: `&mut` ensures unique access. |
524 | unsafe { &*(t as *mut T as *const Cell<T>) } | |
8faf50e0 XL |
525 | } |
526 | } | |
527 | ||
8bb4bdeb XL |
528 | impl<T: Default> Cell<T> { |
529 | /// Takes the value of the cell, leaving `Default::default()` in its place. | |
530 | /// | |
531 | /// # Examples | |
532 | /// | |
533 | /// ``` | |
534 | /// use std::cell::Cell; | |
535 | /// | |
536 | /// let c = Cell::new(5); | |
537 | /// let five = c.take(); | |
538 | /// | |
539 | /// assert_eq!(five, 5); | |
540 | /// assert_eq!(c.into_inner(), 0); | |
541 | /// ``` | |
542 | #[stable(feature = "move_cell", since = "1.17.0")] | |
543 | pub fn take(&self) -> T { | |
544 | self.replace(Default::default()) | |
5bcae85e SL |
545 | } |
546 | } | |
547 | ||
9e0c209e SL |
548 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
549 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {} | |
550 | ||
8faf50e0 XL |
551 | impl<T> Cell<[T]> { |
552 | /// Returns a `&[Cell<T>]` from a `&Cell<[T]>` | |
553 | /// | |
554 | /// # Examples | |
555 | /// | |
556 | /// ``` | |
8faf50e0 XL |
557 | /// use std::cell::Cell; |
558 | /// | |
559 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
560 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
561 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
562 | /// | |
563 | /// assert_eq!(slice_cell.len(), 3); | |
564 | /// ``` | |
dc9dc135 | 565 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 566 | pub fn as_slice_of_cells(&self) -> &[Cell<T>] { |
dfeec247 XL |
567 | // SAFETY: `Cell<T>` has the same memory layout as `T`. |
568 | unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) } | |
8faf50e0 XL |
569 | } |
570 | } | |
571 | ||
1a4d82fc | 572 | /// A mutable memory location with dynamically checked borrow rules |
85aaf69f | 573 | /// |
29967ef6 | 574 | /// See the [module-level documentation](self) for more. |
85aaf69f | 575 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 576 | pub struct RefCell<T: ?Sized> { |
1a4d82fc | 577 | borrow: Cell<BorrowFlag>, |
cdc7bbd5 XL |
578 | // Stores the location of the earliest currently active borrow. |
579 | // This gets updated whenver we go from having zero borrows | |
580 | // to having a single borrow. When a borrow occurs, this gets included | |
581 | // in the generated `BorroeError/`BorrowMutError` | |
582 | #[cfg(feature = "debug_refcell")] | |
583 | borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>, | |
d9579d0f | 584 | value: UnsafeCell<T>, |
1a4d82fc JJ |
585 | } |
586 | ||
29967ef6 | 587 | /// An error returned by [`RefCell::try_borrow`]. |
9e0c209e SL |
588 | #[stable(feature = "try_borrow", since = "1.13.0")] |
589 | pub struct BorrowError { | |
590 | _private: (), | |
cdc7bbd5 XL |
591 | #[cfg(feature = "debug_refcell")] |
592 | location: &'static crate::panic::Location<'static>, | |
5bcae85e SL |
593 | } |
594 | ||
9e0c209e SL |
595 | #[stable(feature = "try_borrow", since = "1.13.0")] |
596 | impl Debug for BorrowError { | |
48663c56 | 597 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
cdc7bbd5 XL |
598 | let mut builder = f.debug_struct("BorrowError"); |
599 | ||
600 | #[cfg(feature = "debug_refcell")] | |
601 | builder.field("location", self.location); | |
602 | ||
603 | builder.finish() | |
5bcae85e SL |
604 | } |
605 | } | |
606 | ||
9e0c209e SL |
607 | #[stable(feature = "try_borrow", since = "1.13.0")] |
608 | impl Display for BorrowError { | |
48663c56 | 609 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
610 | Display::fmt("already mutably borrowed", f) |
611 | } | |
612 | } | |
613 | ||
29967ef6 | 614 | /// An error returned by [`RefCell::try_borrow_mut`]. |
9e0c209e SL |
615 | #[stable(feature = "try_borrow", since = "1.13.0")] |
616 | pub struct BorrowMutError { | |
617 | _private: (), | |
cdc7bbd5 XL |
618 | #[cfg(feature = "debug_refcell")] |
619 | location: &'static crate::panic::Location<'static>, | |
5bcae85e SL |
620 | } |
621 | ||
9e0c209e SL |
622 | #[stable(feature = "try_borrow", since = "1.13.0")] |
623 | impl Debug for BorrowMutError { | |
48663c56 | 624 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
cdc7bbd5 XL |
625 | let mut builder = f.debug_struct("BorrowMutError"); |
626 | ||
627 | #[cfg(feature = "debug_refcell")] | |
628 | builder.field("location", self.location); | |
629 | ||
630 | builder.finish() | |
5bcae85e SL |
631 | } |
632 | } | |
633 | ||
9e0c209e SL |
634 | #[stable(feature = "try_borrow", since = "1.13.0")] |
635 | impl Display for BorrowMutError { | |
48663c56 | 636 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
637 | Display::fmt("already borrowed", f) |
638 | } | |
639 | } | |
640 | ||
8faf50e0 XL |
641 | // Positive values represent the number of `Ref` active. Negative values |
642 | // represent the number of `RefMut` active. Multiple `RefMut`s can only be | |
643 | // active at a time if they refer to distinct, nonoverlapping components of a | |
644 | // `RefCell` (e.g., different ranges of a slice). | |
94b46f34 XL |
645 | // |
646 | // `Ref` and `RefMut` are both two words in size, and so there will likely never | |
647 | // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` | |
8faf50e0 XL |
648 | // range. Thus, a `BorrowFlag` will probably never overflow or underflow. |
649 | // However, this is not a guarantee, as a pathological program could repeatedly | |
650 | // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must | |
651 | // explicitly check for overflow and underflow in order to avoid unsafety, or at | |
652 | // least behave correctly in the event that overflow or underflow happens (e.g., | |
653 | // see BorrowRef::new). | |
654 | type BorrowFlag = isize; | |
1a4d82fc | 655 | const UNUSED: BorrowFlag = 0; |
8faf50e0 XL |
656 | |
657 | #[inline(always)] | |
658 | fn is_writing(x: BorrowFlag) -> bool { | |
659 | x < UNUSED | |
660 | } | |
661 | ||
662 | #[inline(always)] | |
663 | fn is_reading(x: BorrowFlag) -> bool { | |
664 | x > UNUSED | |
665 | } | |
1a4d82fc JJ |
666 | |
667 | impl<T> RefCell<T> { | |
85aaf69f SL |
668 | /// Creates a new `RefCell` containing `value`. |
669 | /// | |
670 | /// # Examples | |
671 | /// | |
672 | /// ``` | |
673 | /// use std::cell::RefCell; | |
674 | /// | |
675 | /// let c = RefCell::new(5); | |
676 | /// ``` | |
677 | #[stable(feature = "rust1", since = "1.0.0")] | |
cdc7bbd5 | 678 | #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")] |
c34b1796 | 679 | #[inline] |
62682a34 | 680 | pub const fn new(value: T) -> RefCell<T> { |
cdc7bbd5 XL |
681 | RefCell { |
682 | value: UnsafeCell::new(value), | |
683 | borrow: Cell::new(UNUSED), | |
684 | #[cfg(feature = "debug_refcell")] | |
685 | borrowed_at: Cell::new(None), | |
686 | } | |
1a4d82fc JJ |
687 | } |
688 | ||
689 | /// Consumes the `RefCell`, returning the wrapped value. | |
85aaf69f SL |
690 | /// |
691 | /// # Examples | |
692 | /// | |
693 | /// ``` | |
694 | /// use std::cell::RefCell; | |
695 | /// | |
696 | /// let c = RefCell::new(5); | |
697 | /// | |
698 | /// let five = c.into_inner(); | |
699 | /// ``` | |
700 | #[stable(feature = "rust1", since = "1.0.0")] | |
29967ef6 | 701 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
c34b1796 | 702 | #[inline] |
29967ef6 | 703 | pub const fn into_inner(self) -> T { |
1a4d82fc JJ |
704 | // Since this function takes `self` (the `RefCell`) by value, the |
705 | // compiler statically verifies that it is not currently borrowed. | |
2c00a5a8 | 706 | self.value.into_inner() |
1a4d82fc | 707 | } |
3b2f2976 XL |
708 | |
709 | /// Replaces the wrapped value with a new one, returning the old value, | |
710 | /// without deinitializing either one. | |
711 | /// | |
712 | /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). | |
713 | /// | |
abe05a73 XL |
714 | /// # Panics |
715 | /// | |
716 | /// Panics if the value is currently borrowed. | |
717 | /// | |
3b2f2976 XL |
718 | /// # Examples |
719 | /// | |
720 | /// ``` | |
3b2f2976 | 721 | /// use std::cell::RefCell; |
abe05a73 XL |
722 | /// let cell = RefCell::new(5); |
723 | /// let old_value = cell.replace(6); | |
724 | /// assert_eq!(old_value, 5); | |
725 | /// assert_eq!(cell, RefCell::new(6)); | |
3b2f2976 | 726 | /// ``` |
abe05a73 | 727 | #[inline] |
dfeec247 | 728 | #[stable(feature = "refcell_replace", since = "1.24.0")] |
1b1a35ee | 729 | #[track_caller] |
abe05a73 XL |
730 | pub fn replace(&self, t: T) -> T { |
731 | mem::replace(&mut *self.borrow_mut(), t) | |
732 | } | |
733 | ||
734 | /// Replaces the wrapped value with a new one computed from `f`, returning | |
735 | /// the old value, without deinitializing either one. | |
736 | /// | |
3b2f2976 XL |
737 | /// # Panics |
738 | /// | |
abe05a73 XL |
739 | /// Panics if the value is currently borrowed. |
740 | /// | |
741 | /// # Examples | |
742 | /// | |
743 | /// ``` | |
abe05a73 XL |
744 | /// use std::cell::RefCell; |
745 | /// let cell = RefCell::new(5); | |
746 | /// let old_value = cell.replace_with(|&mut old| old + 1); | |
747 | /// assert_eq!(old_value, 5); | |
748 | /// assert_eq!(cell, RefCell::new(6)); | |
749 | /// ``` | |
3b2f2976 | 750 | #[inline] |
dfeec247 | 751 | #[stable(feature = "refcell_replace_swap", since = "1.35.0")] |
1b1a35ee | 752 | #[track_caller] |
abe05a73 XL |
753 | pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { |
754 | let mut_borrow = &mut *self.borrow_mut(); | |
755 | let replacement = f(mut_borrow); | |
756 | mem::replace(mut_borrow, replacement) | |
3b2f2976 XL |
757 | } |
758 | ||
759 | /// Swaps the wrapped value of `self` with the wrapped value of `other`, | |
760 | /// without deinitializing either one. | |
761 | /// | |
762 | /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). | |
763 | /// | |
abe05a73 XL |
764 | /// # Panics |
765 | /// | |
766 | /// Panics if the value in either `RefCell` is currently borrowed. | |
767 | /// | |
3b2f2976 XL |
768 | /// # Examples |
769 | /// | |
770 | /// ``` | |
3b2f2976 XL |
771 | /// use std::cell::RefCell; |
772 | /// let c = RefCell::new(5); | |
773 | /// let d = RefCell::new(6); | |
774 | /// c.swap(&d); | |
775 | /// assert_eq!(c, RefCell::new(6)); | |
776 | /// assert_eq!(d, RefCell::new(5)); | |
777 | /// ``` | |
3b2f2976 | 778 | #[inline] |
dfeec247 | 779 | #[stable(feature = "refcell_swap", since = "1.24.0")] |
3b2f2976 XL |
780 | pub fn swap(&self, other: &Self) { |
781 | mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) | |
782 | } | |
d9579d0f | 783 | } |
1a4d82fc | 784 | |
d9579d0f | 785 | impl<T: ?Sized> RefCell<T> { |
1a4d82fc JJ |
786 | /// Immutably borrows the wrapped value. |
787 | /// | |
788 | /// The borrow lasts until the returned `Ref` exits scope. Multiple | |
789 | /// immutable borrows can be taken out at the same time. | |
790 | /// | |
791 | /// # Panics | |
792 | /// | |
5bcae85e SL |
793 | /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use |
794 | /// [`try_borrow`](#method.try_borrow). | |
85aaf69f SL |
795 | /// |
796 | /// # Examples | |
797 | /// | |
798 | /// ``` | |
799 | /// use std::cell::RefCell; | |
800 | /// | |
801 | /// let c = RefCell::new(5); | |
802 | /// | |
803 | /// let borrowed_five = c.borrow(); | |
804 | /// let borrowed_five2 = c.borrow(); | |
805 | /// ``` | |
806 | /// | |
807 | /// An example of panic: | |
808 | /// | |
f035d41b | 809 | /// ```should_panic |
85aaf69f | 810 | /// use std::cell::RefCell; |
85aaf69f | 811 | /// |
f035d41b | 812 | /// let c = RefCell::new(5); |
85aaf69f | 813 | /// |
f035d41b XL |
814 | /// let m = c.borrow_mut(); |
815 | /// let b = c.borrow(); // this causes a panic | |
85aaf69f SL |
816 | /// ``` |
817 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 818 | #[inline] |
3dfed10e | 819 | #[track_caller] |
48663c56 | 820 | pub fn borrow(&self) -> Ref<'_, T> { |
5bcae85e SL |
821 | self.try_borrow().expect("already mutably borrowed") |
822 | } | |
823 | ||
824 | /// Immutably borrows the wrapped value, returning an error if the value is currently mutably | |
825 | /// borrowed. | |
826 | /// | |
827 | /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be | |
828 | /// taken out at the same time. | |
829 | /// | |
830 | /// This is the non-panicking variant of [`borrow`](#method.borrow). | |
831 | /// | |
832 | /// # Examples | |
833 | /// | |
834 | /// ``` | |
5bcae85e SL |
835 | /// use std::cell::RefCell; |
836 | /// | |
837 | /// let c = RefCell::new(5); | |
838 | /// | |
839 | /// { | |
840 | /// let m = c.borrow_mut(); | |
841 | /// assert!(c.try_borrow().is_err()); | |
842 | /// } | |
843 | /// | |
844 | /// { | |
845 | /// let m = c.borrow(); | |
846 | /// assert!(c.try_borrow().is_ok()); | |
847 | /// } | |
848 | /// ``` | |
9e0c209e | 849 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 850 | #[inline] |
cdc7bbd5 | 851 | #[cfg_attr(feature = "debug_refcell", track_caller)] |
48663c56 | 852 | pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { |
85aaf69f | 853 | match BorrowRef::new(&self.borrow) { |
cdc7bbd5 XL |
854 | Some(b) => { |
855 | #[cfg(feature = "debug_refcell")] | |
856 | { | |
857 | // `borrowed_at` is always the *first* active borrow | |
858 | if b.borrow.get() == 1 { | |
859 | self.borrowed_at.set(Some(crate::panic::Location::caller())); | |
860 | } | |
861 | } | |
862 | ||
863 | // SAFETY: `BorrowRef` ensures that there is only immutable access | |
864 | // to the value while borrowed. | |
865 | Ok(Ref { value: unsafe { &*self.value.get() }, borrow: b }) | |
866 | } | |
867 | None => Err(BorrowError { | |
868 | _private: (), | |
869 | // If a borrow occured, then we must already have an outstanding borrow, | |
870 | // so `borrowed_at` will be `Some` | |
871 | #[cfg(feature = "debug_refcell")] | |
872 | location: self.borrowed_at.get().unwrap(), | |
873 | }), | |
1a4d82fc JJ |
874 | } |
875 | } | |
876 | ||
1a4d82fc JJ |
877 | /// Mutably borrows the wrapped value. |
878 | /// | |
94b46f34 XL |
879 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
880 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
881 | /// active. | |
1a4d82fc JJ |
882 | /// |
883 | /// # Panics | |
884 | /// | |
5bcae85e SL |
885 | /// Panics if the value is currently borrowed. For a non-panicking variant, use |
886 | /// [`try_borrow_mut`](#method.try_borrow_mut). | |
85aaf69f SL |
887 | /// |
888 | /// # Examples | |
889 | /// | |
890 | /// ``` | |
891 | /// use std::cell::RefCell; | |
892 | /// | |
f9f354fc | 893 | /// let c = RefCell::new("hello".to_owned()); |
85aaf69f | 894 | /// |
f9f354fc | 895 | /// *c.borrow_mut() = "bonjour".to_owned(); |
7453a54e | 896 | /// |
f9f354fc | 897 | /// assert_eq!(&*c.borrow(), "bonjour"); |
85aaf69f SL |
898 | /// ``` |
899 | /// | |
900 | /// An example of panic: | |
901 | /// | |
f035d41b | 902 | /// ```should_panic |
85aaf69f | 903 | /// use std::cell::RefCell; |
85aaf69f | 904 | /// |
f035d41b XL |
905 | /// let c = RefCell::new(5); |
906 | /// let m = c.borrow(); | |
85aaf69f | 907 | /// |
f035d41b | 908 | /// let b = c.borrow_mut(); // this causes a panic |
85aaf69f SL |
909 | /// ``` |
910 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 911 | #[inline] |
3dfed10e | 912 | #[track_caller] |
48663c56 | 913 | pub fn borrow_mut(&self) -> RefMut<'_, T> { |
5bcae85e SL |
914 | self.try_borrow_mut().expect("already borrowed") |
915 | } | |
916 | ||
917 | /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. | |
918 | /// | |
94b46f34 XL |
919 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
920 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
921 | /// active. | |
5bcae85e SL |
922 | /// |
923 | /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). | |
924 | /// | |
925 | /// # Examples | |
926 | /// | |
927 | /// ``` | |
5bcae85e SL |
928 | /// use std::cell::RefCell; |
929 | /// | |
930 | /// let c = RefCell::new(5); | |
931 | /// | |
932 | /// { | |
933 | /// let m = c.borrow(); | |
934 | /// assert!(c.try_borrow_mut().is_err()); | |
935 | /// } | |
936 | /// | |
937 | /// assert!(c.try_borrow_mut().is_ok()); | |
938 | /// ``` | |
9e0c209e | 939 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 940 | #[inline] |
cdc7bbd5 | 941 | #[cfg_attr(feature = "debug_refcell", track_caller)] |
48663c56 | 942 | pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { |
85aaf69f | 943 | match BorrowRefMut::new(&self.borrow) { |
cdc7bbd5 XL |
944 | Some(b) => { |
945 | #[cfg(feature = "debug_refcell")] | |
946 | { | |
947 | self.borrowed_at.set(Some(crate::panic::Location::caller())); | |
948 | } | |
949 | ||
950 | // SAFETY: `BorrowRef` guarantees unique access. | |
951 | Ok(RefMut { value: unsafe { &mut *self.value.get() }, borrow: b }) | |
952 | } | |
953 | None => Err(BorrowMutError { | |
954 | _private: (), | |
955 | // If a borrow occured, then we must already have an outstanding borrow, | |
956 | // so `borrowed_at` will be `Some` | |
957 | #[cfg(feature = "debug_refcell")] | |
958 | location: self.borrowed_at.get().unwrap(), | |
959 | }), | |
1a4d82fc JJ |
960 | } |
961 | } | |
962 | ||
5bcae85e SL |
963 | /// Returns a raw pointer to the underlying data in this cell. |
964 | /// | |
965 | /// # Examples | |
966 | /// | |
967 | /// ``` | |
968 | /// use std::cell::RefCell; | |
969 | /// | |
970 | /// let c = RefCell::new(5); | |
971 | /// | |
972 | /// let ptr = c.as_ptr(); | |
973 | /// ``` | |
974 | #[inline] | |
975 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
976 | pub fn as_ptr(&self) -> *mut T { | |
977 | self.value.get() | |
978 | } | |
979 | ||
a7813a04 XL |
980 | /// Returns a mutable reference to the underlying data. |
981 | /// | |
982 | /// This call borrows `RefCell` mutably (at compile-time) so there is no | |
983 | /// need for dynamic checks. | |
5bcae85e | 984 | /// |
cc61c64b XL |
985 | /// However be cautious: this method expects `self` to be mutable, which is |
986 | /// generally not the case when using a `RefCell`. Take a look at the | |
987 | /// [`borrow_mut`] method instead if `self` isn't mutable. | |
988 | /// | |
989 | /// Also, please be aware that this method is only for special circumstances and is usually | |
3b2f2976 | 990 | /// not what you want. In case of doubt, use [`borrow_mut`] instead. |
cc61c64b | 991 | /// |
fc512014 | 992 | /// [`borrow_mut`]: RefCell::borrow_mut() |
cc61c64b | 993 | /// |
5bcae85e SL |
994 | /// # Examples |
995 | /// | |
996 | /// ``` | |
997 | /// use std::cell::RefCell; | |
998 | /// | |
999 | /// let mut c = RefCell::new(5); | |
1000 | /// *c.get_mut() += 1; | |
1001 | /// | |
1002 | /// assert_eq!(c, RefCell::new(6)); | |
1003 | /// ``` | |
a7813a04 | 1004 | #[inline] |
3157f602 | 1005 | #[stable(feature = "cell_get_mut", since = "1.11.0")] |
a7813a04 | 1006 | pub fn get_mut(&mut self) -> &mut T { |
1b1a35ee | 1007 | self.value.get_mut() |
a7813a04 | 1008 | } |
48663c56 | 1009 | |
ba9703b0 XL |
1010 | /// Undo the effect of leaked guards on the borrow state of the `RefCell`. |
1011 | /// | |
1012 | /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to | |
1013 | /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant | |
1014 | /// if some `Ref` or `RefMut` borrows have been leaked. | |
1015 | /// | |
fc512014 | 1016 | /// [`get_mut`]: RefCell::get_mut() |
ba9703b0 XL |
1017 | /// |
1018 | /// # Examples | |
1019 | /// | |
1020 | /// ``` | |
1021 | /// #![feature(cell_leak)] | |
1022 | /// use std::cell::RefCell; | |
1023 | /// | |
1024 | /// let mut c = RefCell::new(0); | |
1025 | /// std::mem::forget(c.borrow_mut()); | |
1026 | /// | |
1027 | /// assert!(c.try_borrow().is_err()); | |
1028 | /// c.undo_leak(); | |
1029 | /// assert!(c.try_borrow().is_ok()); | |
1030 | /// ``` | |
1031 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1032 | pub fn undo_leak(&mut self) -> &mut T { | |
1033 | *self.borrow.get_mut() = UNUSED; | |
1034 | self.get_mut() | |
1035 | } | |
1036 | ||
48663c56 XL |
1037 | /// Immutably borrows the wrapped value, returning an error if the value is |
1038 | /// currently mutably borrowed. | |
1039 | /// | |
1040 | /// # Safety | |
1041 | /// | |
1042 | /// Unlike `RefCell::borrow`, this method is unsafe because it does not | |
1043 | /// return a `Ref`, thus leaving the borrow flag untouched. Mutably | |
1044 | /// borrowing the `RefCell` while the reference returned by this method | |
1045 | /// is alive is undefined behaviour. | |
1046 | /// | |
1047 | /// # Examples | |
1048 | /// | |
1049 | /// ``` | |
48663c56 XL |
1050 | /// use std::cell::RefCell; |
1051 | /// | |
1052 | /// let c = RefCell::new(5); | |
1053 | /// | |
1054 | /// { | |
1055 | /// let m = c.borrow_mut(); | |
1056 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); | |
1057 | /// } | |
1058 | /// | |
1059 | /// { | |
1060 | /// let m = c.borrow(); | |
1061 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); | |
1062 | /// } | |
1063 | /// ``` | |
dc9dc135 | 1064 | #[stable(feature = "borrow_state", since = "1.37.0")] |
48663c56 XL |
1065 | #[inline] |
1066 | pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { | |
1067 | if !is_writing(self.borrow.get()) { | |
f035d41b XL |
1068 | // SAFETY: We check that nobody is actively writing now, but it is |
1069 | // the caller's responsibility to ensure that nobody writes until | |
1070 | // the returned reference is no longer in use. | |
1071 | // Also, `self.value.get()` refers to the value owned by `self` | |
1072 | // and is thus guaranteed to be valid for the lifetime of `self`. | |
1073 | Ok(unsafe { &*self.value.get() }) | |
48663c56 | 1074 | } else { |
cdc7bbd5 XL |
1075 | Err(BorrowError { |
1076 | _private: (), | |
1077 | // If a borrow occured, then we must already have an outstanding borrow, | |
1078 | // so `borrowed_at` will be `Some` | |
1079 | #[cfg(feature = "debug_refcell")] | |
1080 | location: self.borrowed_at.get().unwrap(), | |
1081 | }) | |
48663c56 XL |
1082 | } |
1083 | } | |
1a4d82fc JJ |
1084 | } |
1085 | ||
f9f354fc XL |
1086 | impl<T: Default> RefCell<T> { |
1087 | /// Takes the wrapped value, leaving `Default::default()` in its place. | |
1088 | /// | |
1089 | /// # Panics | |
1090 | /// | |
1091 | /// Panics if the value is currently borrowed. | |
1092 | /// | |
1093 | /// # Examples | |
1094 | /// | |
1095 | /// ``` | |
f9f354fc XL |
1096 | /// use std::cell::RefCell; |
1097 | /// | |
1098 | /// let c = RefCell::new(5); | |
1099 | /// let five = c.take(); | |
1100 | /// | |
1101 | /// assert_eq!(five, 5); | |
1102 | /// assert_eq!(c.into_inner(), 0); | |
1103 | /// ``` | |
fc512014 | 1104 | #[stable(feature = "refcell_take", since = "1.50.0")] |
f9f354fc XL |
1105 | pub fn take(&self) -> T { |
1106 | self.replace(Default::default()) | |
1107 | } | |
1108 | } | |
1109 | ||
85aaf69f | 1110 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1111 | unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {} |
1a4d82fc | 1112 | |
54a0048b SL |
1113 | #[stable(feature = "rust1", since = "1.0.0")] |
1114 | impl<T: ?Sized> !Sync for RefCell<T> {} | |
1115 | ||
85aaf69f | 1116 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 1117 | impl<T: Clone> Clone for RefCell<T> { |
0531ce1d XL |
1118 | /// # Panics |
1119 | /// | |
1120 | /// Panics if the value is currently mutably borrowed. | |
c34b1796 | 1121 | #[inline] |
1b1a35ee | 1122 | #[track_caller] |
1a4d82fc JJ |
1123 | fn clone(&self) -> RefCell<T> { |
1124 | RefCell::new(self.borrow().clone()) | |
1125 | } | |
1126 | } | |
1127 | ||
85aaf69f | 1128 | #[stable(feature = "rust1", since = "1.0.0")] |
416331ca | 1129 | impl<T: Default> Default for RefCell<T> { |
9e0c209e | 1130 | /// Creates a `RefCell<T>`, with the `Default` value for T. |
c34b1796 | 1131 | #[inline] |
1a4d82fc JJ |
1132 | fn default() -> RefCell<T> { |
1133 | RefCell::new(Default::default()) | |
1134 | } | |
1135 | } | |
1136 | ||
85aaf69f | 1137 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1138 | impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { |
0531ce1d XL |
1139 | /// # Panics |
1140 | /// | |
1141 | /// Panics if the value in either `RefCell` is currently borrowed. | |
c34b1796 | 1142 | #[inline] |
1a4d82fc JJ |
1143 | fn eq(&self, other: &RefCell<T>) -> bool { |
1144 | *self.borrow() == *other.borrow() | |
1145 | } | |
1146 | } | |
1147 | ||
62682a34 SL |
1148 | #[stable(feature = "cell_eq", since = "1.2.0")] |
1149 | impl<T: ?Sized + Eq> Eq for RefCell<T> {} | |
1150 | ||
a7813a04 XL |
1151 | #[stable(feature = "cell_ord", since = "1.10.0")] |
1152 | impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { | |
0531ce1d XL |
1153 | /// # Panics |
1154 | /// | |
1155 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1156 | #[inline] |
1157 | fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { | |
1158 | self.borrow().partial_cmp(&*other.borrow()) | |
1159 | } | |
1160 | ||
0531ce1d XL |
1161 | /// # Panics |
1162 | /// | |
1163 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1164 | #[inline] |
1165 | fn lt(&self, other: &RefCell<T>) -> bool { | |
1166 | *self.borrow() < *other.borrow() | |
1167 | } | |
1168 | ||
0531ce1d XL |
1169 | /// # Panics |
1170 | /// | |
1171 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1172 | #[inline] |
1173 | fn le(&self, other: &RefCell<T>) -> bool { | |
1174 | *self.borrow() <= *other.borrow() | |
1175 | } | |
1176 | ||
0531ce1d XL |
1177 | /// # Panics |
1178 | /// | |
1179 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1180 | #[inline] |
1181 | fn gt(&self, other: &RefCell<T>) -> bool { | |
1182 | *self.borrow() > *other.borrow() | |
1183 | } | |
1184 | ||
0531ce1d XL |
1185 | /// # Panics |
1186 | /// | |
1187 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1188 | #[inline] |
1189 | fn ge(&self, other: &RefCell<T>) -> bool { | |
1190 | *self.borrow() >= *other.borrow() | |
1191 | } | |
1192 | } | |
1193 | ||
1194 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
1195 | impl<T: ?Sized + Ord> Ord for RefCell<T> { | |
0531ce1d XL |
1196 | /// # Panics |
1197 | /// | |
1198 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1199 | #[inline] |
1200 | fn cmp(&self, other: &RefCell<T>) -> Ordering { | |
1201 | self.borrow().cmp(&*other.borrow()) | |
1202 | } | |
1203 | } | |
1204 | ||
5bcae85e SL |
1205 | #[stable(feature = "cell_from", since = "1.12.0")] |
1206 | impl<T> From<T> for RefCell<T> { | |
1207 | fn from(t: T) -> RefCell<T> { | |
1208 | RefCell::new(t) | |
1209 | } | |
1210 | } | |
1211 | ||
9e0c209e SL |
1212 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1213 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} | |
1214 | ||
1a4d82fc | 1215 | struct BorrowRef<'b> { |
54a0048b | 1216 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1217 | } |
1218 | ||
1219 | impl<'b> BorrowRef<'b> { | |
c34b1796 | 1220 | #[inline] |
1a4d82fc | 1221 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { |
416331ca XL |
1222 | let b = borrow.get().wrapping_add(1); |
1223 | if !is_reading(b) { | |
1224 | // Incrementing borrow can result in a non-reading value (<= 0) in these cases: | |
1225 | // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow | |
1226 | // due to Rust's reference aliasing rules | |
f035d41b XL |
1227 | // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed |
1228 | // into isize::MIN (the max amount of writing borrows) so we can't allow | |
416331ca XL |
1229 | // an additional read borrow because isize can't represent so many read borrows |
1230 | // (this can only happen if you mem::forget more than a small constant amount of | |
1231 | // `Ref`s, which is not good practice) | |
94b46f34 XL |
1232 | None |
1233 | } else { | |
416331ca XL |
1234 | // Incrementing borrow can result in a reading value (> 0) in these cases: |
1235 | // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow | |
f035d41b | 1236 | // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize |
416331ca XL |
1237 | // is large enough to represent having one more read borrow |
1238 | borrow.set(b); | |
94b46f34 | 1239 | Some(BorrowRef { borrow }) |
1a4d82fc JJ |
1240 | } |
1241 | } | |
1242 | } | |
1243 | ||
0bf4aa26 | 1244 | impl Drop for BorrowRef<'_> { |
c34b1796 | 1245 | #[inline] |
1a4d82fc | 1246 | fn drop(&mut self) { |
54a0048b | 1247 | let borrow = self.borrow.get(); |
8faf50e0 | 1248 | debug_assert!(is_reading(borrow)); |
54a0048b | 1249 | self.borrow.set(borrow - 1); |
1a4d82fc JJ |
1250 | } |
1251 | } | |
1252 | ||
0bf4aa26 | 1253 | impl Clone for BorrowRef<'_> { |
c34b1796 | 1254 | #[inline] |
0bf4aa26 | 1255 | fn clone(&self) -> Self { |
1a4d82fc | 1256 | // Since this Ref exists, we know the borrow flag |
8faf50e0 | 1257 | // is a reading borrow. |
54a0048b | 1258 | let borrow = self.borrow.get(); |
8faf50e0 | 1259 | debug_assert!(is_reading(borrow)); |
94b46f34 XL |
1260 | // Prevent the borrow counter from overflowing into |
1261 | // a writing borrow. | |
f035d41b | 1262 | assert!(borrow != isize::MAX); |
54a0048b SL |
1263 | self.borrow.set(borrow + 1); |
1264 | BorrowRef { borrow: self.borrow } | |
1a4d82fc JJ |
1265 | } |
1266 | } | |
1267 | ||
1268 | /// Wraps a borrowed reference to a value in a `RefCell` box. | |
85aaf69f SL |
1269 | /// A wrapper type for an immutably borrowed value from a `RefCell<T>`. |
1270 | /// | |
29967ef6 | 1271 | /// See the [module-level documentation](self) for more. |
85aaf69f | 1272 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1273 | pub struct Ref<'b, T: ?Sized + 'b> { |
54a0048b SL |
1274 | value: &'b T, |
1275 | borrow: BorrowRef<'b>, | |
1a4d82fc JJ |
1276 | } |
1277 | ||
85aaf69f | 1278 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1279 | impl<T: ?Sized> Deref for Ref<'_, T> { |
1a4d82fc JJ |
1280 | type Target = T; |
1281 | ||
1282 | #[inline] | |
e9174d1e | 1283 | fn deref(&self) -> &T { |
54a0048b | 1284 | self.value |
1a4d82fc JJ |
1285 | } |
1286 | } | |
1287 | ||
62682a34 SL |
1288 | impl<'b, T: ?Sized> Ref<'b, T> { |
1289 | /// Copies a `Ref`. | |
1290 | /// | |
1291 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1292 | /// | |
1293 | /// This is an associated function that needs to be used as | |
9fa01778 | 1294 | /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere |
62682a34 SL |
1295 | /// with the widespread use of `r.borrow().clone()` to clone the contents of |
1296 | /// a `RefCell`. | |
476ff2be | 1297 | #[stable(feature = "cell_extras", since = "1.15.0")] |
62682a34 SL |
1298 | #[inline] |
1299 | pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { | |
dfeec247 | 1300 | Ref { value: orig.value, borrow: orig.borrow.clone() } |
62682a34 SL |
1301 | } |
1302 | ||
9fa01778 | 1303 | /// Makes a new `Ref` for a component of the borrowed data. |
62682a34 SL |
1304 | /// |
1305 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1306 | /// | |
1307 | /// This is an associated function that needs to be used as `Ref::map(...)`. | |
1308 | /// A method would interfere with methods of the same name on the contents | |
1309 | /// of a `RefCell` used through `Deref`. | |
1310 | /// | |
3b2f2976 | 1311 | /// # Examples |
62682a34 SL |
1312 | /// |
1313 | /// ``` | |
62682a34 SL |
1314 | /// use std::cell::{RefCell, Ref}; |
1315 | /// | |
1316 | /// let c = RefCell::new((5, 'b')); | |
1317 | /// let b1: Ref<(u32, char)> = c.borrow(); | |
1318 | /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); | |
1319 | /// assert_eq!(*b2, 5) | |
1320 | /// ``` | |
7453a54e | 1321 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1322 | #[inline] |
1323 | pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> | |
dfeec247 XL |
1324 | where |
1325 | F: FnOnce(&T) -> &U, | |
62682a34 | 1326 | { |
dfeec247 | 1327 | Ref { value: f(orig.value), borrow: orig.borrow } |
62682a34 | 1328 | } |
94b46f34 | 1329 | |
5869c6ff XL |
1330 | /// Makes a new `Ref` for an optional component of the borrowed data. The |
1331 | /// original guard is returned as an `Err(..)` if the closure returns | |
1332 | /// `None`. | |
1333 | /// | |
1334 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1335 | /// | |
1336 | /// This is an associated function that needs to be used as | |
1337 | /// `Ref::filter_map(...)`. A method would interfere with methods of the same | |
1338 | /// name on the contents of a `RefCell` used through `Deref`. | |
1339 | /// | |
1340 | /// # Examples | |
1341 | /// | |
1342 | /// ``` | |
1343 | /// #![feature(cell_filter_map)] | |
1344 | /// | |
1345 | /// use std::cell::{RefCell, Ref}; | |
1346 | /// | |
1347 | /// let c = RefCell::new(vec![1, 2, 3]); | |
1348 | /// let b1: Ref<Vec<u32>> = c.borrow(); | |
1349 | /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1)); | |
1350 | /// assert_eq!(*b2.unwrap(), 2); | |
1351 | /// ``` | |
1352 | #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")] | |
1353 | #[inline] | |
1354 | pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self> | |
1355 | where | |
1356 | F: FnOnce(&T) -> Option<&U>, | |
1357 | { | |
1358 | match f(orig.value) { | |
1359 | Some(value) => Ok(Ref { value, borrow: orig.borrow }), | |
1360 | None => Err(orig), | |
1361 | } | |
1362 | } | |
1363 | ||
9fa01778 | 1364 | /// Splits a `Ref` into multiple `Ref`s for different components of the |
94b46f34 XL |
1365 | /// borrowed data. |
1366 | /// | |
1367 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1368 | /// | |
1369 | /// This is an associated function that needs to be used as | |
1370 | /// `Ref::map_split(...)`. A method would interfere with methods of the same | |
1371 | /// name on the contents of a `RefCell` used through `Deref`. | |
1372 | /// | |
1373 | /// # Examples | |
1374 | /// | |
1375 | /// ``` | |
94b46f34 XL |
1376 | /// use std::cell::{Ref, RefCell}; |
1377 | /// | |
1378 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1379 | /// let borrow = cell.borrow(); | |
1380 | /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2)); | |
1381 | /// assert_eq!(*begin, [1, 2]); | |
1382 | /// assert_eq!(*end, [3, 4]); | |
1383 | /// ``` | |
532ac7d7 | 1384 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1385 | #[inline] |
1386 | pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>) | |
dfeec247 XL |
1387 | where |
1388 | F: FnOnce(&T) -> (&U, &V), | |
94b46f34 XL |
1389 | { |
1390 | let (a, b) = f(orig.value); | |
1391 | let borrow = orig.borrow.clone(); | |
1392 | (Ref { value: a, borrow }, Ref { value: b, borrow: orig.borrow }) | |
1393 | } | |
74b04a01 XL |
1394 | |
1395 | /// Convert into a reference to the underlying data. | |
1396 | /// | |
1397 | /// The underlying `RefCell` can never be mutably borrowed from again and will always appear | |
1398 | /// already immutably borrowed. It is not a good idea to leak more than a constant number of | |
1399 | /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks | |
1400 | /// have occurred in total. | |
1401 | /// | |
1402 | /// This is an associated function that needs to be used as | |
1403 | /// `Ref::leak(...)`. A method would interfere with methods of the | |
1404 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1405 | /// | |
1406 | /// # Examples | |
1407 | /// | |
1408 | /// ``` | |
1409 | /// #![feature(cell_leak)] | |
1410 | /// use std::cell::{RefCell, Ref}; | |
1411 | /// let cell = RefCell::new(0); | |
1412 | /// | |
1413 | /// let value = Ref::leak(cell.borrow()); | |
1414 | /// assert_eq!(*value, 0); | |
1415 | /// | |
1416 | /// assert!(cell.try_borrow().is_ok()); | |
1417 | /// assert!(cell.try_borrow_mut().is_err()); | |
1418 | /// ``` | |
1419 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1420 | pub fn leak(orig: Ref<'b, T>) -> &'b T { | |
ba9703b0 XL |
1421 | // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to |
1422 | // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a | |
1423 | // unique reference to the borrowed RefCell. No further mutable references can be created | |
1424 | // from the original cell. | |
74b04a01 XL |
1425 | mem::forget(orig.borrow); |
1426 | orig.value | |
1427 | } | |
62682a34 SL |
1428 | } |
1429 | ||
54a0048b SL |
1430 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1431 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} | |
1432 | ||
041b39d2 | 1433 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1434 | impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { |
48663c56 | 1435 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1436 | self.value.fmt(f) |
1437 | } | |
1438 | } | |
1439 | ||
62682a34 | 1440 | impl<'b, T: ?Sized> RefMut<'b, T> { |
9fa01778 | 1441 | /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum |
62682a34 SL |
1442 | /// variant. |
1443 | /// | |
1444 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1445 | /// | |
1446 | /// This is an associated function that needs to be used as | |
9fa01778 | 1447 | /// `RefMut::map(...)`. A method would interfere with methods of the same |
62682a34 SL |
1448 | /// name on the contents of a `RefCell` used through `Deref`. |
1449 | /// | |
3b2f2976 | 1450 | /// # Examples |
62682a34 SL |
1451 | /// |
1452 | /// ``` | |
62682a34 SL |
1453 | /// use std::cell::{RefCell, RefMut}; |
1454 | /// | |
1455 | /// let c = RefCell::new((5, 'b')); | |
1456 | /// { | |
1457 | /// let b1: RefMut<(u32, char)> = c.borrow_mut(); | |
1458 | /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0); | |
1459 | /// assert_eq!(*b2, 5); | |
1460 | /// *b2 = 42; | |
1461 | /// } | |
1462 | /// assert_eq!(*c.borrow(), (42, 'b')); | |
1463 | /// ``` | |
7453a54e | 1464 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1465 | #[inline] |
1466 | pub fn map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> RefMut<'b, U> | |
dfeec247 XL |
1467 | where |
1468 | F: FnOnce(&mut T) -> &mut U, | |
62682a34 | 1469 | { |
ff7c6d11 XL |
1470 | // FIXME(nll-rfc#40): fix borrow-check |
1471 | let RefMut { value, borrow } = orig; | |
dfeec247 | 1472 | RefMut { value: f(value), borrow } |
62682a34 | 1473 | } |
94b46f34 | 1474 | |
5869c6ff XL |
1475 | /// Makes a new `RefMut` for an optional component of the borrowed data. The |
1476 | /// original guard is returned as an `Err(..)` if the closure returns | |
1477 | /// `None`. | |
1478 | /// | |
1479 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1480 | /// | |
1481 | /// This is an associated function that needs to be used as | |
1482 | /// `RefMut::filter_map(...)`. A method would interfere with methods of the | |
1483 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1484 | /// | |
1485 | /// # Examples | |
1486 | /// | |
1487 | /// ``` | |
1488 | /// #![feature(cell_filter_map)] | |
1489 | /// | |
1490 | /// use std::cell::{RefCell, RefMut}; | |
1491 | /// | |
1492 | /// let c = RefCell::new(vec![1, 2, 3]); | |
1493 | /// | |
1494 | /// { | |
1495 | /// let b1: RefMut<Vec<u32>> = c.borrow_mut(); | |
1496 | /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1)); | |
1497 | /// | |
1498 | /// if let Ok(mut b2) = b2 { | |
1499 | /// *b2 += 2; | |
1500 | /// } | |
1501 | /// } | |
1502 | /// | |
1503 | /// assert_eq!(*c.borrow(), vec![1, 4, 3]); | |
1504 | /// ``` | |
1505 | #[unstable(feature = "cell_filter_map", reason = "recently added", issue = "81061")] | |
1506 | #[inline] | |
1507 | pub fn filter_map<U: ?Sized, F>(orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self> | |
1508 | where | |
1509 | F: FnOnce(&mut T) -> Option<&mut U>, | |
1510 | { | |
1511 | // FIXME(nll-rfc#40): fix borrow-check | |
1512 | let RefMut { value, borrow } = orig; | |
1513 | let value = value as *mut T; | |
1514 | // SAFETY: function holds onto an exclusive reference for the duration | |
1515 | // of its call through `orig`, and the pointer is only de-referenced | |
1516 | // inside of the function call never allowing the exclusive reference to | |
1517 | // escape. | |
1518 | match f(unsafe { &mut *value }) { | |
1519 | Some(value) => Ok(RefMut { value, borrow }), | |
1520 | None => { | |
1521 | // SAFETY: same as above. | |
1522 | Err(RefMut { value: unsafe { &mut *value }, borrow }) | |
1523 | } | |
1524 | } | |
1525 | } | |
1526 | ||
9fa01778 | 1527 | /// Splits a `RefMut` into multiple `RefMut`s for different components of the |
94b46f34 XL |
1528 | /// borrowed data. |
1529 | /// | |
1530 | /// The underlying `RefCell` will remain mutably borrowed until both | |
1531 | /// returned `RefMut`s go out of scope. | |
1532 | /// | |
1533 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1534 | /// | |
1535 | /// This is an associated function that needs to be used as | |
1536 | /// `RefMut::map_split(...)`. A method would interfere with methods of the | |
1537 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1538 | /// | |
1539 | /// # Examples | |
1540 | /// | |
1541 | /// ``` | |
94b46f34 XL |
1542 | /// use std::cell::{RefCell, RefMut}; |
1543 | /// | |
1544 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1545 | /// let borrow = cell.borrow_mut(); | |
1546 | /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2)); | |
1547 | /// assert_eq!(*begin, [1, 2]); | |
1548 | /// assert_eq!(*end, [3, 4]); | |
1549 | /// begin.copy_from_slice(&[4, 3]); | |
1550 | /// end.copy_from_slice(&[2, 1]); | |
1551 | /// ``` | |
532ac7d7 | 1552 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1553 | #[inline] |
1554 | pub fn map_split<U: ?Sized, V: ?Sized, F>( | |
dfeec247 XL |
1555 | orig: RefMut<'b, T>, |
1556 | f: F, | |
94b46f34 | 1557 | ) -> (RefMut<'b, U>, RefMut<'b, V>) |
dfeec247 XL |
1558 | where |
1559 | F: FnOnce(&mut T) -> (&mut U, &mut V), | |
94b46f34 XL |
1560 | { |
1561 | let (a, b) = f(orig.value); | |
1562 | let borrow = orig.borrow.clone(); | |
1563 | (RefMut { value: a, borrow }, RefMut { value: b, borrow: orig.borrow }) | |
1564 | } | |
74b04a01 XL |
1565 | |
1566 | /// Convert into a mutable reference to the underlying data. | |
1567 | /// | |
1568 | /// The underlying `RefCell` can not be borrowed from again and will always appear already | |
1569 | /// mutably borrowed, making the returned reference the only to the interior. | |
1570 | /// | |
1571 | /// This is an associated function that needs to be used as | |
1572 | /// `RefMut::leak(...)`. A method would interfere with methods of the | |
1573 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1574 | /// | |
1575 | /// # Examples | |
1576 | /// | |
1577 | /// ``` | |
1578 | /// #![feature(cell_leak)] | |
1579 | /// use std::cell::{RefCell, RefMut}; | |
1580 | /// let cell = RefCell::new(0); | |
1581 | /// | |
1582 | /// let value = RefMut::leak(cell.borrow_mut()); | |
1583 | /// assert_eq!(*value, 0); | |
1584 | /// *value = 1; | |
1585 | /// | |
1586 | /// assert!(cell.try_borrow_mut().is_err()); | |
1587 | /// ``` | |
1588 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1589 | pub fn leak(orig: RefMut<'b, T>) -> &'b mut T { | |
ba9703b0 XL |
1590 | // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't |
1591 | // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would | |
1592 | // require a unique reference to the borrowed RefCell. No further references can be created | |
1593 | // from the original cell within that lifetime, making the current borrow the only | |
1594 | // reference for the remaining lifetime. | |
74b04a01 XL |
1595 | mem::forget(orig.borrow); |
1596 | orig.value | |
1597 | } | |
1a4d82fc JJ |
1598 | } |
1599 | ||
1600 | struct BorrowRefMut<'b> { | |
54a0048b | 1601 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1602 | } |
1603 | ||
0bf4aa26 | 1604 | impl Drop for BorrowRefMut<'_> { |
c34b1796 | 1605 | #[inline] |
1a4d82fc | 1606 | fn drop(&mut self) { |
54a0048b | 1607 | let borrow = self.borrow.get(); |
8faf50e0 XL |
1608 | debug_assert!(is_writing(borrow)); |
1609 | self.borrow.set(borrow + 1); | |
1a4d82fc JJ |
1610 | } |
1611 | } | |
1612 | ||
1613 | impl<'b> BorrowRefMut<'b> { | |
c34b1796 | 1614 | #[inline] |
1a4d82fc | 1615 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { |
94b46f34 XL |
1616 | // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial |
1617 | // mutable reference, and so there must currently be no existing | |
1618 | // references. Thus, while clone increments the mutable refcount, here | |
8faf50e0 | 1619 | // we explicitly only allow going from UNUSED to UNUSED - 1. |
1a4d82fc JJ |
1620 | match borrow.get() { |
1621 | UNUSED => { | |
8faf50e0 | 1622 | borrow.set(UNUSED - 1); |
b7449926 | 1623 | Some(BorrowRefMut { borrow }) |
dfeec247 | 1624 | } |
1a4d82fc JJ |
1625 | _ => None, |
1626 | } | |
1627 | } | |
94b46f34 | 1628 | |
dc9dc135 | 1629 | // Clones a `BorrowRefMut`. |
94b46f34 XL |
1630 | // |
1631 | // This is only valid if each `BorrowRefMut` is used to track a mutable | |
1632 | // reference to a distinct, nonoverlapping range of the original object. | |
1633 | // This isn't in a Clone impl so that code doesn't call this implicitly. | |
1634 | #[inline] | |
1635 | fn clone(&self) -> BorrowRefMut<'b> { | |
1636 | let borrow = self.borrow.get(); | |
8faf50e0 XL |
1637 | debug_assert!(is_writing(borrow)); |
1638 | // Prevent the borrow counter from underflowing. | |
f035d41b | 1639 | assert!(borrow != isize::MIN); |
8faf50e0 | 1640 | self.borrow.set(borrow - 1); |
94b46f34 XL |
1641 | BorrowRefMut { borrow: self.borrow } |
1642 | } | |
1a4d82fc JJ |
1643 | } |
1644 | ||
85aaf69f SL |
1645 | /// A wrapper type for a mutably borrowed value from a `RefCell<T>`. |
1646 | /// | |
29967ef6 | 1647 | /// See the [module-level documentation](self) for more. |
85aaf69f | 1648 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1649 | pub struct RefMut<'b, T: ?Sized + 'b> { |
54a0048b SL |
1650 | value: &'b mut T, |
1651 | borrow: BorrowRefMut<'b>, | |
1a4d82fc JJ |
1652 | } |
1653 | ||
85aaf69f | 1654 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1655 | impl<T: ?Sized> Deref for RefMut<'_, T> { |
1a4d82fc JJ |
1656 | type Target = T; |
1657 | ||
1658 | #[inline] | |
e9174d1e | 1659 | fn deref(&self) -> &T { |
54a0048b | 1660 | self.value |
1a4d82fc JJ |
1661 | } |
1662 | } | |
1663 | ||
85aaf69f | 1664 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1665 | impl<T: ?Sized> DerefMut for RefMut<'_, T> { |
1a4d82fc | 1666 | #[inline] |
e9174d1e | 1667 | fn deref_mut(&mut self) -> &mut T { |
54a0048b | 1668 | self.value |
1a4d82fc JJ |
1669 | } |
1670 | } | |
1671 | ||
54a0048b SL |
1672 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1673 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} | |
1674 | ||
041b39d2 | 1675 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1676 | impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { |
48663c56 | 1677 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
041b39d2 XL |
1678 | self.value.fmt(f) |
1679 | } | |
1680 | } | |
1681 | ||
1a4d82fc JJ |
1682 | /// The core primitive for interior mutability in Rust. |
1683 | /// | |
6a06907d XL |
1684 | /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on |
1685 | /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an | |
1686 | /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior. | |
1687 | /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference | |
1688 | /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability". | |
1a4d82fc | 1689 | /// |
6a06907d XL |
1690 | /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally |
1691 | /// use `UnsafeCell` to wrap their data. | |
1692 | /// | |
1693 | /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The | |
1694 | /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain | |
1695 | /// aliasing `&mut`, not even with `UnsafeCell<T>`. | |
5bcae85e | 1696 | /// |
1b1a35ee XL |
1697 | /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer |
1698 | /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer | |
1699 | /// correctly. | |
1700 | /// | |
1701 | /// [`.get()`]: `UnsafeCell::get` | |
0531ce1d XL |
1702 | /// |
1703 | /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: | |
1704 | /// | |
94b46f34 XL |
1705 | /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` |
1706 | /// reference) that is accessible by safe code (for example, because you returned it), | |
1707 | /// then you must not access the data in any way that contradicts that reference for the | |
1708 | /// remainder of `'a`. For example, this means that if you take the `*mut T` from an | |
1709 | /// `UnsafeCell<T>` and cast it to an `&T`, then the data in `T` must remain immutable | |
1710 | /// (modulo any `UnsafeCell` data found within `T`, of course) until that reference's | |
1711 | /// lifetime expires. Similarly, if you create a `&mut T` reference that is released to | |
1712 | /// safe code, then you must not access the data within the `UnsafeCell` until that | |
1713 | /// reference expires. | |
0531ce1d | 1714 | /// |
94b46f34 | 1715 | /// - At all times, you must avoid data races. If multiple threads have access to |
0531ce1d XL |
1716 | /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other |
1717 | /// accesses (or use atomics). | |
5bcae85e | 1718 | /// |
0531ce1d XL |
1719 | /// To assist with proper design, the following scenarios are explicitly declared legal |
1720 | /// for single-threaded code: | |
5bcae85e | 1721 | /// |
94b46f34 | 1722 | /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` |
0531ce1d XL |
1723 | /// references, but not with a `&mut T` |
1724 | /// | |
94b46f34 | 1725 | /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` |
0531ce1d XL |
1726 | /// co-exist with it. A `&mut T` must always be unique. |
1727 | /// | |
1b1a35ee XL |
1728 | /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other |
1729 | /// `&UnsafeCell<T>` references alias the cell) is | |
1730 | /// ok (provided you enforce the above invariants some other way), it is still undefined behavior | |
1731 | /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper | |
1732 | /// designed to have a special interaction with _shared_ accesses (_i.e._, through an | |
1733 | /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_ | |
1734 | /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value | |
1735 | /// may be aliased for the duration of that `&mut` borrow. | |
fc512014 | 1736 | /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields |
1b1a35ee XL |
1737 | /// a `&mut T`. |
1738 | /// | |
1739 | /// [`.get_mut()`]: `UnsafeCell::get_mut` | |
1a4d82fc | 1740 | /// |
85aaf69f | 1741 | /// # Examples |
1a4d82fc | 1742 | /// |
1b1a35ee XL |
1743 | /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite |
1744 | /// there being multiple references aliasing the cell: | |
1745 | /// | |
85aaf69f | 1746 | /// ``` |
1a4d82fc | 1747 | /// use std::cell::UnsafeCell; |
1a4d82fc | 1748 | /// |
1b1a35ee XL |
1749 | /// let x: UnsafeCell<i32> = 42.into(); |
1750 | /// // Get multiple / concurrent / shared references to the same `x`. | |
1751 | /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x); | |
1752 | /// | |
1753 | /// unsafe { | |
1754 | /// // SAFETY: within this scope there are no other references to `x`'s contents, | |
1755 | /// // so ours is effectively unique. | |
1756 | /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+ | |
1757 | /// *p1_exclusive += 27; // | | |
1758 | /// } // <---------- cannot go beyond this point -------------------+ | |
1759 | /// | |
1760 | /// unsafe { | |
1761 | /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents, | |
1762 | /// // so we can have multiple shared accesses concurrently. | |
1763 | /// let p2_shared: &i32 = &*p2.get(); | |
1764 | /// assert_eq!(*p2_shared, 42 + 27); | |
1765 | /// let p1_shared: &i32 = &*p1.get(); | |
1766 | /// assert_eq!(*p1_shared, *p2_shared); | |
1a4d82fc | 1767 | /// } |
1b1a35ee XL |
1768 | /// ``` |
1769 | /// | |
1770 | /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>` | |
1771 | /// implies exclusive access to its `T`: | |
1772 | /// | |
1773 | /// ```rust | |
1b1a35ee XL |
1774 | /// #![forbid(unsafe_code)] // with exclusive accesses, |
1775 | /// // `UnsafeCell` is a transparent no-op wrapper, | |
1776 | /// // so no need for `unsafe` here. | |
1777 | /// use std::cell::UnsafeCell; | |
1778 | /// | |
1779 | /// let mut x: UnsafeCell<i32> = 42.into(); | |
1780 | /// | |
1781 | /// // Get a compile-time-checked unique reference to `x`. | |
1782 | /// let p_unique: &mut UnsafeCell<i32> = &mut x; | |
1783 | /// // With an exclusive reference, we can mutate the contents for free. | |
1784 | /// *p_unique.get_mut() = 0; | |
1785 | /// // Or, equivalently: | |
1786 | /// x = UnsafeCell::new(0); | |
85aaf69f | 1787 | /// |
1b1a35ee XL |
1788 | /// // When we own the value, we can extract the contents for free. |
1789 | /// let contents: i32 = x.into_inner(); | |
1790 | /// assert_eq!(contents, 0); | |
1a4d82fc | 1791 | /// ``` |
d9579d0f | 1792 | #[lang = "unsafe_cell"] |
85aaf69f | 1793 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 1794 | #[repr(transparent)] |
ba9703b0 | 1795 | #[repr(no_niche)] // rust-lang/rust#68303. |
d9579d0f | 1796 | pub struct UnsafeCell<T: ?Sized> { |
e9174d1e | 1797 | value: T, |
1a4d82fc JJ |
1798 | } |
1799 | ||
92a42be0 | 1800 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1801 | impl<T: ?Sized> !Sync for UnsafeCell<T> {} |
c34b1796 | 1802 | |
1a4d82fc | 1803 | impl<T> UnsafeCell<T> { |
9346a6ac | 1804 | /// Constructs a new instance of `UnsafeCell` which will wrap the specified |
1a4d82fc JJ |
1805 | /// value. |
1806 | /// | |
b039eaaf | 1807 | /// All access to the inner value through methods is `unsafe`. |
85aaf69f SL |
1808 | /// |
1809 | /// # Examples | |
1810 | /// | |
1811 | /// ``` | |
1812 | /// use std::cell::UnsafeCell; | |
1813 | /// | |
1814 | /// let uc = UnsafeCell::new(5); | |
1815 | /// ``` | |
1816 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 1817 | #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")] |
cdc7bbd5 | 1818 | #[inline(always)] |
62682a34 | 1819 | pub const fn new(value: T) -> UnsafeCell<T> { |
b7449926 | 1820 | UnsafeCell { value } |
1a4d82fc JJ |
1821 | } |
1822 | ||
d9579d0f AL |
1823 | /// Unwraps the value. |
1824 | /// | |
85aaf69f SL |
1825 | /// # Examples |
1826 | /// | |
1827 | /// ``` | |
1828 | /// use std::cell::UnsafeCell; | |
1829 | /// | |
1830 | /// let uc = UnsafeCell::new(5); | |
1831 | /// | |
2c00a5a8 | 1832 | /// let five = uc.into_inner(); |
85aaf69f | 1833 | /// ``` |
cdc7bbd5 | 1834 | #[inline(always)] |
85aaf69f | 1835 | #[stable(feature = "rust1", since = "1.0.0")] |
29967ef6 XL |
1836 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
1837 | pub const fn into_inner(self) -> T { | |
62682a34 SL |
1838 | self.value |
1839 | } | |
d9579d0f | 1840 | } |
1a4d82fc | 1841 | |
d9579d0f AL |
1842 | impl<T: ?Sized> UnsafeCell<T> { |
1843 | /// Gets a mutable pointer to the wrapped value. | |
85aaf69f | 1844 | /// |
5bcae85e | 1845 | /// This can be cast to a pointer of any kind. |
0531ce1d XL |
1846 | /// Ensure that the access is unique (no active references, mutable or not) |
1847 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1848 | /// or mutable aliases going on when casting to `&T` | |
5bcae85e | 1849 | /// |
85aaf69f SL |
1850 | /// # Examples |
1851 | /// | |
1852 | /// ``` | |
1853 | /// use std::cell::UnsafeCell; | |
1854 | /// | |
1855 | /// let uc = UnsafeCell::new(5); | |
1856 | /// | |
d9579d0f | 1857 | /// let five = uc.get(); |
85aaf69f | 1858 | /// ``` |
cdc7bbd5 | 1859 | #[inline(always)] |
85aaf69f | 1860 | #[stable(feature = "rust1", since = "1.0.0")] |
dfeec247 | 1861 | #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")] |
a1dfa0c6 XL |
1862 | pub const fn get(&self) -> *mut T { |
1863 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
60c5eb7d XL |
1864 | // #[repr(transparent)]. This exploits libstd's special status, there is |
1865 | // no guarantee for user code that this will work in future versions of the compiler! | |
a1dfa0c6 | 1866 | self as *const UnsafeCell<T> as *const T as *mut T |
d9579d0f | 1867 | } |
60c5eb7d | 1868 | |
1b1a35ee XL |
1869 | /// Returns a mutable reference to the underlying data. |
1870 | /// | |
1871 | /// This call borrows the `UnsafeCell` mutably (at compile-time) which | |
1872 | /// guarantees that we possess the only reference. | |
1873 | /// | |
1874 | /// # Examples | |
1875 | /// | |
1876 | /// ``` | |
1b1a35ee XL |
1877 | /// use std::cell::UnsafeCell; |
1878 | /// | |
1879 | /// let mut c = UnsafeCell::new(5); | |
1880 | /// *c.get_mut() += 1; | |
1881 | /// | |
1882 | /// assert_eq!(*c.get_mut(), 6); | |
1883 | /// ``` | |
cdc7bbd5 | 1884 | #[inline(always)] |
fc512014 | 1885 | #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")] |
1b1a35ee | 1886 | pub fn get_mut(&mut self) -> &mut T { |
29967ef6 | 1887 | &mut self.value |
1b1a35ee XL |
1888 | } |
1889 | ||
60c5eb7d XL |
1890 | /// Gets a mutable pointer to the wrapped value. |
1891 | /// The difference to [`get`] is that this function accepts a raw pointer, | |
1892 | /// which is useful to avoid the creation of temporary references. | |
1893 | /// | |
1894 | /// The result can be cast to a pointer of any kind. | |
1895 | /// Ensure that the access is unique (no active references, mutable or not) | |
1896 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1897 | /// or mutable aliases going on when casting to `&T`. | |
1898 | /// | |
fc512014 | 1899 | /// [`get`]: UnsafeCell::get() |
60c5eb7d XL |
1900 | /// |
1901 | /// # Examples | |
1902 | /// | |
1903 | /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as | |
1904 | /// calling `get` would require creating a reference to uninitialized data: | |
1905 | /// | |
1906 | /// ``` | |
1907 | /// #![feature(unsafe_cell_raw_get)] | |
1908 | /// use std::cell::UnsafeCell; | |
1909 | /// use std::mem::MaybeUninit; | |
1910 | /// | |
1911 | /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit(); | |
1912 | /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); } | |
1913 | /// let uc = unsafe { m.assume_init() }; | |
1914 | /// | |
1915 | /// assert_eq!(uc.into_inner(), 5); | |
1916 | /// ``` | |
cdc7bbd5 | 1917 | #[inline(always)] |
60c5eb7d XL |
1918 | #[unstable(feature = "unsafe_cell_raw_get", issue = "66358")] |
1919 | pub const fn raw_get(this: *const Self) -> *mut T { | |
1920 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
1921 | // #[repr(transparent)]. This exploits libstd's special status, there is | |
1922 | // no guarantee for user code that this will work in future versions of the compiler! | |
1923 | this as *const T as *mut T | |
1924 | } | |
1a4d82fc | 1925 | } |
a7813a04 | 1926 | |
7cac9316 | 1927 | #[stable(feature = "unsafe_cell_default", since = "1.10.0")] |
a7813a04 | 1928 | impl<T: Default> Default for UnsafeCell<T> { |
9e0c209e | 1929 | /// Creates an `UnsafeCell`, with the `Default` value for T. |
a7813a04 XL |
1930 | fn default() -> UnsafeCell<T> { |
1931 | UnsafeCell::new(Default::default()) | |
1932 | } | |
1933 | } | |
5bcae85e SL |
1934 | |
1935 | #[stable(feature = "cell_from", since = "1.12.0")] | |
1936 | impl<T> From<T> for UnsafeCell<T> { | |
1937 | fn from(t: T) -> UnsafeCell<T> { | |
1938 | UnsafeCell::new(t) | |
1939 | } | |
1940 | } | |
9e0c209e SL |
1941 | |
1942 | #[unstable(feature = "coerce_unsized", issue = "27732")] | |
1943 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {} | |
1944 | ||
1945 | #[allow(unused)] | |
1946 | fn assert_coerce_unsized(a: UnsafeCell<&i32>, b: Cell<&i32>, c: RefCell<&i32>) { | |
8faf50e0 XL |
1947 | let _: UnsafeCell<&dyn Send> = a; |
1948 | let _: Cell<&dyn Send> = b; | |
1949 | let _: RefCell<&dyn Send> = c; | |
9e0c209e | 1950 | } |