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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(); | |
5e7ed085 | 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}; | |
923072b8 | 197 | use crate::marker::{PhantomData, Unsize}; |
48663c56 | 198 | use crate::mem; |
dfeec247 | 199 | use crate::ops::{CoerceUnsized, Deref, DerefMut}; |
923072b8 FG |
200 | use crate::ptr::{self, NonNull}; |
201 | ||
202 | mod lazy; | |
203 | mod once; | |
204 | ||
205 | #[unstable(feature = "once_cell", issue = "74465")] | |
206 | pub use lazy::LazyCell; | |
207 | #[unstable(feature = "once_cell", issue = "74465")] | |
208 | pub use once::OnceCell; | |
1a4d82fc | 209 | |
8bb4bdeb | 210 | /// A mutable memory location. |
85aaf69f | 211 | /// |
3b2f2976 XL |
212 | /// # Examples |
213 | /// | |
a1dfa0c6 XL |
214 | /// In this example, you can see that `Cell<T>` enables mutation inside an |
215 | /// immutable struct. In other words, it enables "interior mutability". | |
3b2f2976 XL |
216 | /// |
217 | /// ``` | |
218 | /// use std::cell::Cell; | |
219 | /// | |
220 | /// struct SomeStruct { | |
221 | /// regular_field: u8, | |
222 | /// special_field: Cell<u8>, | |
223 | /// } | |
224 | /// | |
225 | /// let my_struct = SomeStruct { | |
226 | /// regular_field: 0, | |
227 | /// special_field: Cell::new(1), | |
228 | /// }; | |
229 | /// | |
230 | /// let new_value = 100; | |
231 | /// | |
a1dfa0c6 | 232 | /// // ERROR: `my_struct` is immutable |
3b2f2976 XL |
233 | /// // my_struct.regular_field = new_value; |
234 | /// | |
a1dfa0c6 XL |
235 | /// // WORKS: although `my_struct` is immutable, `special_field` is a `Cell`, |
236 | /// // which can always be mutated | |
3b2f2976 XL |
237 | /// my_struct.special_field.set(new_value); |
238 | /// assert_eq!(my_struct.special_field.get(), new_value); | |
239 | /// ``` | |
240 | /// | |
29967ef6 | 241 | /// See the [module-level documentation](self) for more. |
85aaf69f | 242 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 XL |
243 | #[repr(transparent)] |
244 | pub struct Cell<T: ?Sized> { | |
1a4d82fc JJ |
245 | value: UnsafeCell<T>, |
246 | } | |
247 | ||
8bb4bdeb | 248 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 249 | unsafe impl<T: ?Sized> Send for Cell<T> where T: Send {} |
8bb4bdeb | 250 | |
3c0e092e XL |
251 | // Note that this negative impl isn't strictly necessary for correctness, |
252 | // as `Cell` wraps `UnsafeCell`, which is itself `!Sync`. | |
253 | // However, given how important `Cell`'s `!Sync`-ness is, | |
254 | // having an explicit negative impl is nice for documentation purposes | |
255 | // and results in nicer error messages. | |
8bb4bdeb | 256 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 257 | impl<T: ?Sized> !Sync for Cell<T> {} |
8bb4bdeb XL |
258 | |
259 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 260 | impl<T: Copy> Clone for Cell<T> { |
8bb4bdeb XL |
261 | #[inline] |
262 | fn clone(&self) -> Cell<T> { | |
263 | Cell::new(self.get()) | |
264 | } | |
265 | } | |
266 | ||
267 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 268 | impl<T: Default> Default for Cell<T> { |
8bb4bdeb XL |
269 | /// Creates a `Cell<T>`, with the `Default` value for T. |
270 | #[inline] | |
271 | fn default() -> Cell<T> { | |
272 | Cell::new(Default::default()) | |
273 | } | |
274 | } | |
275 | ||
276 | #[stable(feature = "rust1", since = "1.0.0")] | |
416331ca | 277 | impl<T: PartialEq + Copy> PartialEq for Cell<T> { |
8bb4bdeb XL |
278 | #[inline] |
279 | fn eq(&self, other: &Cell<T>) -> bool { | |
280 | self.get() == other.get() | |
281 | } | |
282 | } | |
283 | ||
284 | #[stable(feature = "cell_eq", since = "1.2.0")] | |
416331ca | 285 | impl<T: Eq + Copy> Eq for Cell<T> {} |
8bb4bdeb XL |
286 | |
287 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 288 | impl<T: PartialOrd + Copy> PartialOrd for Cell<T> { |
8bb4bdeb XL |
289 | #[inline] |
290 | fn partial_cmp(&self, other: &Cell<T>) -> Option<Ordering> { | |
291 | self.get().partial_cmp(&other.get()) | |
292 | } | |
293 | ||
294 | #[inline] | |
295 | fn lt(&self, other: &Cell<T>) -> bool { | |
296 | self.get() < other.get() | |
297 | } | |
298 | ||
299 | #[inline] | |
300 | fn le(&self, other: &Cell<T>) -> bool { | |
301 | self.get() <= other.get() | |
302 | } | |
303 | ||
304 | #[inline] | |
305 | fn gt(&self, other: &Cell<T>) -> bool { | |
306 | self.get() > other.get() | |
307 | } | |
308 | ||
309 | #[inline] | |
310 | fn ge(&self, other: &Cell<T>) -> bool { | |
311 | self.get() >= other.get() | |
312 | } | |
313 | } | |
314 | ||
315 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
416331ca | 316 | impl<T: Ord + Copy> Ord for Cell<T> { |
8bb4bdeb XL |
317 | #[inline] |
318 | fn cmp(&self, other: &Cell<T>) -> Ordering { | |
319 | self.get().cmp(&other.get()) | |
320 | } | |
321 | } | |
322 | ||
323 | #[stable(feature = "cell_from", since = "1.12.0")] | |
3c0e092e XL |
324 | #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
325 | impl<T> const From<T> for Cell<T> { | |
5099ac24 | 326 | /// Creates a new `Cell<T>` containing the given value. |
8bb4bdeb XL |
327 | fn from(t: T) -> Cell<T> { |
328 | Cell::new(t) | |
329 | } | |
330 | } | |
331 | ||
332 | impl<T> Cell<T> { | |
333 | /// Creates a new `Cell` containing the given value. | |
85aaf69f SL |
334 | /// |
335 | /// # Examples | |
336 | /// | |
337 | /// ``` | |
338 | /// use std::cell::Cell; | |
339 | /// | |
340 | /// let c = Cell::new(5); | |
85aaf69f | 341 | /// ``` |
85aaf69f | 342 | #[stable(feature = "rust1", since = "1.0.0")] |
cdc7bbd5 | 343 | #[rustc_const_stable(feature = "const_cell_new", since = "1.24.0")] |
8bb4bdeb XL |
344 | #[inline] |
345 | pub const fn new(value: T) -> Cell<T> { | |
dfeec247 | 346 | Cell { value: UnsafeCell::new(value) } |
1a4d82fc JJ |
347 | } |
348 | ||
8bb4bdeb XL |
349 | /// Sets the contained value. |
350 | /// | |
351 | /// # Examples | |
352 | /// | |
353 | /// ``` | |
354 | /// use std::cell::Cell; | |
355 | /// | |
356 | /// let c = Cell::new(5); | |
357 | /// | |
358 | /// c.set(10); | |
359 | /// ``` | |
a7813a04 | 360 | #[inline] |
8bb4bdeb XL |
361 | #[stable(feature = "rust1", since = "1.0.0")] |
362 | pub fn set(&self, val: T) { | |
363 | let old = self.replace(val); | |
364 | drop(old); | |
a7813a04 XL |
365 | } |
366 | ||
c295e0f8 | 367 | /// Swaps the values of two `Cell`s. |
8bb4bdeb XL |
368 | /// Difference with `std::mem::swap` is that this function doesn't require `&mut` reference. |
369 | /// | |
370 | /// # Examples | |
371 | /// | |
372 | /// ``` | |
373 | /// use std::cell::Cell; | |
374 | /// | |
375 | /// let c1 = Cell::new(5i32); | |
376 | /// let c2 = Cell::new(10i32); | |
377 | /// c1.swap(&c2); | |
378 | /// assert_eq!(10, c1.get()); | |
379 | /// assert_eq!(5, c2.get()); | |
380 | /// ``` | |
a7813a04 | 381 | #[inline] |
8bb4bdeb XL |
382 | #[stable(feature = "move_cell", since = "1.17.0")] |
383 | pub fn swap(&self, other: &Self) { | |
384 | if ptr::eq(self, other) { | |
385 | return; | |
386 | } | |
dfeec247 XL |
387 | // SAFETY: This can be risky if called from separate threads, but `Cell` |
388 | // is `!Sync` so this won't happen. This also won't invalidate any | |
389 | // pointers since `Cell` makes sure nothing else will be pointing into | |
390 | // either of these `Cell`s. | |
8bb4bdeb XL |
391 | unsafe { |
392 | ptr::swap(self.value.get(), other.value.get()); | |
393 | } | |
a7813a04 XL |
394 | } |
395 | ||
6a06907d | 396 | /// Replaces the contained value with `val`, and returns the old contained value. |
8bb4bdeb XL |
397 | /// |
398 | /// # Examples | |
399 | /// | |
400 | /// ``` | |
401 | /// use std::cell::Cell; | |
402 | /// | |
041b39d2 XL |
403 | /// let cell = Cell::new(5); |
404 | /// assert_eq!(cell.get(), 5); | |
405 | /// assert_eq!(cell.replace(10), 5); | |
406 | /// assert_eq!(cell.get(), 10); | |
8bb4bdeb | 407 | /// ``` |
2b03887a | 408 | #[inline] |
8bb4bdeb XL |
409 | #[stable(feature = "move_cell", since = "1.17.0")] |
410 | pub fn replace(&self, val: T) -> T { | |
dfeec247 XL |
411 | // SAFETY: This can cause data races if called from a separate thread, |
412 | // but `Cell` is `!Sync` so this won't happen. | |
8bb4bdeb | 413 | mem::replace(unsafe { &mut *self.value.get() }, val) |
a7813a04 | 414 | } |
a7813a04 | 415 | |
8bb4bdeb XL |
416 | /// Unwraps the value. |
417 | /// | |
418 | /// # Examples | |
419 | /// | |
420 | /// ``` | |
421 | /// use std::cell::Cell; | |
422 | /// | |
423 | /// let c = Cell::new(5); | |
424 | /// let five = c.into_inner(); | |
425 | /// | |
426 | /// assert_eq!(five, 5); | |
427 | /// ``` | |
428 | #[stable(feature = "move_cell", since = "1.17.0")] | |
29967ef6 XL |
429 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
430 | pub const fn into_inner(self) -> T { | |
2c00a5a8 | 431 | self.value.into_inner() |
a7813a04 | 432 | } |
e74abb32 XL |
433 | } |
434 | ||
dfeec247 | 435 | impl<T: Copy> Cell<T> { |
e74abb32 XL |
436 | /// Returns a copy of the contained value. |
437 | /// | |
438 | /// # Examples | |
439 | /// | |
440 | /// ``` | |
441 | /// use std::cell::Cell; | |
442 | /// | |
443 | /// let c = Cell::new(5); | |
444 | /// | |
445 | /// let five = c.get(); | |
446 | /// ``` | |
447 | #[inline] | |
448 | #[stable(feature = "rust1", since = "1.0.0")] | |
449 | pub fn get(&self) -> T { | |
dfeec247 XL |
450 | // SAFETY: This can cause data races if called from a separate thread, |
451 | // but `Cell` is `!Sync` so this won't happen. | |
452 | unsafe { *self.value.get() } | |
e74abb32 XL |
453 | } |
454 | ||
455 | /// Updates the contained value using a function and returns the new value. | |
456 | /// | |
457 | /// # Examples | |
458 | /// | |
459 | /// ``` | |
460 | /// #![feature(cell_update)] | |
461 | /// | |
462 | /// use std::cell::Cell; | |
463 | /// | |
464 | /// let c = Cell::new(5); | |
465 | /// let new = c.update(|x| x + 1); | |
466 | /// | |
467 | /// assert_eq!(new, 6); | |
468 | /// assert_eq!(c.get(), 6); | |
469 | /// ``` | |
470 | #[inline] | |
471 | #[unstable(feature = "cell_update", issue = "50186")] | |
472 | pub fn update<F>(&self, f: F) -> T | |
473 | where | |
474 | F: FnOnce(T) -> T, | |
475 | { | |
476 | let old = self.get(); | |
477 | let new = f(old); | |
478 | self.set(new); | |
479 | new | |
480 | } | |
a7813a04 XL |
481 | } |
482 | ||
8faf50e0 XL |
483 | impl<T: ?Sized> Cell<T> { |
484 | /// Returns a raw pointer to the underlying data in this cell. | |
485 | /// | |
486 | /// # Examples | |
487 | /// | |
488 | /// ``` | |
489 | /// use std::cell::Cell; | |
490 | /// | |
491 | /// let c = Cell::new(5); | |
492 | /// | |
493 | /// let ptr = c.as_ptr(); | |
494 | /// ``` | |
495 | #[inline] | |
496 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
dfeec247 | 497 | #[rustc_const_stable(feature = "const_cell_as_ptr", since = "1.32.0")] |
a1dfa0c6 | 498 | pub const fn as_ptr(&self) -> *mut T { |
8faf50e0 XL |
499 | self.value.get() |
500 | } | |
501 | ||
502 | /// Returns a mutable reference to the underlying data. | |
503 | /// | |
504 | /// This call borrows `Cell` mutably (at compile-time) which guarantees | |
505 | /// that we possess the only reference. | |
506 | /// | |
136023e0 XL |
507 | /// However be cautious: this method expects `self` to be mutable, which is |
508 | /// generally not the case when using a `Cell`. If you require interior | |
509 | /// mutability by reference, consider using `RefCell` which provides | |
510 | /// run-time checked mutable borrows through its [`borrow_mut`] method. | |
511 | /// | |
512 | /// [`borrow_mut`]: RefCell::borrow_mut() | |
513 | /// | |
8faf50e0 XL |
514 | /// # Examples |
515 | /// | |
516 | /// ``` | |
517 | /// use std::cell::Cell; | |
518 | /// | |
519 | /// let mut c = Cell::new(5); | |
520 | /// *c.get_mut() += 1; | |
521 | /// | |
522 | /// assert_eq!(c.get(), 6); | |
523 | /// ``` | |
524 | #[inline] | |
525 | #[stable(feature = "cell_get_mut", since = "1.11.0")] | |
526 | pub fn get_mut(&mut self) -> &mut T { | |
1b1a35ee | 527 | self.value.get_mut() |
8faf50e0 XL |
528 | } |
529 | ||
530 | /// Returns a `&Cell<T>` from a `&mut T` | |
531 | /// | |
532 | /// # Examples | |
533 | /// | |
534 | /// ``` | |
8faf50e0 XL |
535 | /// use std::cell::Cell; |
536 | /// | |
537 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
538 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
539 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
540 | /// | |
541 | /// assert_eq!(slice_cell.len(), 3); | |
542 | /// ``` | |
543 | #[inline] | |
dc9dc135 | 544 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 545 | pub fn from_mut(t: &mut T) -> &Cell<T> { |
dfeec247 XL |
546 | // SAFETY: `&mut` ensures unique access. |
547 | unsafe { &*(t as *mut T as *const Cell<T>) } | |
8faf50e0 XL |
548 | } |
549 | } | |
550 | ||
8bb4bdeb XL |
551 | impl<T: Default> Cell<T> { |
552 | /// Takes the value of the cell, leaving `Default::default()` in its place. | |
553 | /// | |
554 | /// # Examples | |
555 | /// | |
556 | /// ``` | |
557 | /// use std::cell::Cell; | |
558 | /// | |
559 | /// let c = Cell::new(5); | |
560 | /// let five = c.take(); | |
561 | /// | |
562 | /// assert_eq!(five, 5); | |
563 | /// assert_eq!(c.into_inner(), 0); | |
564 | /// ``` | |
565 | #[stable(feature = "move_cell", since = "1.17.0")] | |
566 | pub fn take(&self) -> T { | |
567 | self.replace(Default::default()) | |
5bcae85e SL |
568 | } |
569 | } | |
570 | ||
9e0c209e SL |
571 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
572 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<Cell<U>> for Cell<T> {} | |
573 | ||
8faf50e0 XL |
574 | impl<T> Cell<[T]> { |
575 | /// Returns a `&[Cell<T>]` from a `&Cell<[T]>` | |
576 | /// | |
577 | /// # Examples | |
578 | /// | |
579 | /// ``` | |
8faf50e0 XL |
580 | /// use std::cell::Cell; |
581 | /// | |
582 | /// let slice: &mut [i32] = &mut [1, 2, 3]; | |
583 | /// let cell_slice: &Cell<[i32]> = Cell::from_mut(slice); | |
584 | /// let slice_cell: &[Cell<i32>] = cell_slice.as_slice_of_cells(); | |
585 | /// | |
586 | /// assert_eq!(slice_cell.len(), 3); | |
587 | /// ``` | |
dc9dc135 | 588 | #[stable(feature = "as_cell", since = "1.37.0")] |
8faf50e0 | 589 | pub fn as_slice_of_cells(&self) -> &[Cell<T>] { |
dfeec247 XL |
590 | // SAFETY: `Cell<T>` has the same memory layout as `T`. |
591 | unsafe { &*(self as *const Cell<[T]> as *const [Cell<T>]) } | |
8faf50e0 XL |
592 | } |
593 | } | |
594 | ||
94222f64 XL |
595 | impl<T, const N: usize> Cell<[T; N]> { |
596 | /// Returns a `&[Cell<T>; N]` from a `&Cell<[T; N]>` | |
597 | /// | |
598 | /// # Examples | |
599 | /// | |
600 | /// ``` | |
601 | /// #![feature(as_array_of_cells)] | |
602 | /// use std::cell::Cell; | |
603 | /// | |
604 | /// let mut array: [i32; 3] = [1, 2, 3]; | |
605 | /// let cell_array: &Cell<[i32; 3]> = Cell::from_mut(&mut array); | |
606 | /// let array_cell: &[Cell<i32>; 3] = cell_array.as_array_of_cells(); | |
607 | /// ``` | |
608 | #[unstable(feature = "as_array_of_cells", issue = "88248")] | |
609 | pub fn as_array_of_cells(&self) -> &[Cell<T>; N] { | |
610 | // SAFETY: `Cell<T>` has the same memory layout as `T`. | |
611 | unsafe { &*(self as *const Cell<[T; N]> as *const [Cell<T>; N]) } | |
612 | } | |
613 | } | |
614 | ||
1a4d82fc | 615 | /// A mutable memory location with dynamically checked borrow rules |
85aaf69f | 616 | /// |
29967ef6 | 617 | /// See the [module-level documentation](self) for more. |
2b03887a | 618 | #[cfg_attr(not(test), rustc_diagnostic_item = "RefCell")] |
85aaf69f | 619 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 620 | pub struct RefCell<T: ?Sized> { |
1a4d82fc | 621 | borrow: Cell<BorrowFlag>, |
cdc7bbd5 | 622 | // Stores the location of the earliest currently active borrow. |
94222f64 | 623 | // This gets updated whenever we go from having zero borrows |
cdc7bbd5 | 624 | // to having a single borrow. When a borrow occurs, this gets included |
136023e0 | 625 | // in the generated `BorrowError/`BorrowMutError` |
cdc7bbd5 XL |
626 | #[cfg(feature = "debug_refcell")] |
627 | borrowed_at: Cell<Option<&'static crate::panic::Location<'static>>>, | |
d9579d0f | 628 | value: UnsafeCell<T>, |
1a4d82fc JJ |
629 | } |
630 | ||
29967ef6 | 631 | /// An error returned by [`RefCell::try_borrow`]. |
9e0c209e | 632 | #[stable(feature = "try_borrow", since = "1.13.0")] |
136023e0 | 633 | #[non_exhaustive] |
9e0c209e | 634 | pub struct BorrowError { |
cdc7bbd5 XL |
635 | #[cfg(feature = "debug_refcell")] |
636 | location: &'static crate::panic::Location<'static>, | |
5bcae85e SL |
637 | } |
638 | ||
9e0c209e SL |
639 | #[stable(feature = "try_borrow", since = "1.13.0")] |
640 | impl Debug for BorrowError { | |
48663c56 | 641 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
cdc7bbd5 XL |
642 | let mut builder = f.debug_struct("BorrowError"); |
643 | ||
644 | #[cfg(feature = "debug_refcell")] | |
645 | builder.field("location", self.location); | |
646 | ||
647 | builder.finish() | |
5bcae85e SL |
648 | } |
649 | } | |
650 | ||
9e0c209e SL |
651 | #[stable(feature = "try_borrow", since = "1.13.0")] |
652 | impl Display for BorrowError { | |
48663c56 | 653 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
654 | Display::fmt("already mutably borrowed", f) |
655 | } | |
656 | } | |
657 | ||
29967ef6 | 658 | /// An error returned by [`RefCell::try_borrow_mut`]. |
9e0c209e | 659 | #[stable(feature = "try_borrow", since = "1.13.0")] |
136023e0 | 660 | #[non_exhaustive] |
9e0c209e | 661 | pub struct BorrowMutError { |
cdc7bbd5 XL |
662 | #[cfg(feature = "debug_refcell")] |
663 | location: &'static crate::panic::Location<'static>, | |
5bcae85e SL |
664 | } |
665 | ||
9e0c209e SL |
666 | #[stable(feature = "try_borrow", since = "1.13.0")] |
667 | impl Debug for BorrowMutError { | |
48663c56 | 668 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
cdc7bbd5 XL |
669 | let mut builder = f.debug_struct("BorrowMutError"); |
670 | ||
671 | #[cfg(feature = "debug_refcell")] | |
672 | builder.field("location", self.location); | |
673 | ||
674 | builder.finish() | |
5bcae85e SL |
675 | } |
676 | } | |
677 | ||
9e0c209e SL |
678 | #[stable(feature = "try_borrow", since = "1.13.0")] |
679 | impl Display for BorrowMutError { | |
48663c56 | 680 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
5bcae85e SL |
681 | Display::fmt("already borrowed", f) |
682 | } | |
683 | } | |
684 | ||
8faf50e0 XL |
685 | // Positive values represent the number of `Ref` active. Negative values |
686 | // represent the number of `RefMut` active. Multiple `RefMut`s can only be | |
687 | // active at a time if they refer to distinct, nonoverlapping components of a | |
688 | // `RefCell` (e.g., different ranges of a slice). | |
94b46f34 XL |
689 | // |
690 | // `Ref` and `RefMut` are both two words in size, and so there will likely never | |
691 | // be enough `Ref`s or `RefMut`s in existence to overflow half of the `usize` | |
8faf50e0 XL |
692 | // range. Thus, a `BorrowFlag` will probably never overflow or underflow. |
693 | // However, this is not a guarantee, as a pathological program could repeatedly | |
694 | // create and then mem::forget `Ref`s or `RefMut`s. Thus, all code must | |
695 | // explicitly check for overflow and underflow in order to avoid unsafety, or at | |
696 | // least behave correctly in the event that overflow or underflow happens (e.g., | |
697 | // see BorrowRef::new). | |
698 | type BorrowFlag = isize; | |
1a4d82fc | 699 | const UNUSED: BorrowFlag = 0; |
8faf50e0 XL |
700 | |
701 | #[inline(always)] | |
702 | fn is_writing(x: BorrowFlag) -> bool { | |
703 | x < UNUSED | |
704 | } | |
705 | ||
706 | #[inline(always)] | |
707 | fn is_reading(x: BorrowFlag) -> bool { | |
708 | x > UNUSED | |
709 | } | |
1a4d82fc JJ |
710 | |
711 | impl<T> RefCell<T> { | |
85aaf69f SL |
712 | /// Creates a new `RefCell` containing `value`. |
713 | /// | |
714 | /// # Examples | |
715 | /// | |
716 | /// ``` | |
717 | /// use std::cell::RefCell; | |
718 | /// | |
719 | /// let c = RefCell::new(5); | |
720 | /// ``` | |
721 | #[stable(feature = "rust1", since = "1.0.0")] | |
cdc7bbd5 | 722 | #[rustc_const_stable(feature = "const_refcell_new", since = "1.24.0")] |
c34b1796 | 723 | #[inline] |
62682a34 | 724 | pub const fn new(value: T) -> RefCell<T> { |
cdc7bbd5 XL |
725 | RefCell { |
726 | value: UnsafeCell::new(value), | |
727 | borrow: Cell::new(UNUSED), | |
728 | #[cfg(feature = "debug_refcell")] | |
729 | borrowed_at: Cell::new(None), | |
730 | } | |
1a4d82fc JJ |
731 | } |
732 | ||
733 | /// Consumes the `RefCell`, returning the wrapped value. | |
85aaf69f SL |
734 | /// |
735 | /// # Examples | |
736 | /// | |
737 | /// ``` | |
738 | /// use std::cell::RefCell; | |
739 | /// | |
740 | /// let c = RefCell::new(5); | |
741 | /// | |
742 | /// let five = c.into_inner(); | |
743 | /// ``` | |
744 | #[stable(feature = "rust1", since = "1.0.0")] | |
29967ef6 | 745 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
c34b1796 | 746 | #[inline] |
29967ef6 | 747 | pub const fn into_inner(self) -> T { |
1a4d82fc JJ |
748 | // Since this function takes `self` (the `RefCell`) by value, the |
749 | // compiler statically verifies that it is not currently borrowed. | |
2c00a5a8 | 750 | self.value.into_inner() |
1a4d82fc | 751 | } |
3b2f2976 XL |
752 | |
753 | /// Replaces the wrapped value with a new one, returning the old value, | |
754 | /// without deinitializing either one. | |
755 | /// | |
756 | /// This function corresponds to [`std::mem::replace`](../mem/fn.replace.html). | |
757 | /// | |
abe05a73 XL |
758 | /// # Panics |
759 | /// | |
760 | /// Panics if the value is currently borrowed. | |
761 | /// | |
3b2f2976 XL |
762 | /// # Examples |
763 | /// | |
764 | /// ``` | |
3b2f2976 | 765 | /// use std::cell::RefCell; |
abe05a73 XL |
766 | /// let cell = RefCell::new(5); |
767 | /// let old_value = cell.replace(6); | |
768 | /// assert_eq!(old_value, 5); | |
769 | /// assert_eq!(cell, RefCell::new(6)); | |
3b2f2976 | 770 | /// ``` |
abe05a73 | 771 | #[inline] |
dfeec247 | 772 | #[stable(feature = "refcell_replace", since = "1.24.0")] |
1b1a35ee | 773 | #[track_caller] |
abe05a73 XL |
774 | pub fn replace(&self, t: T) -> T { |
775 | mem::replace(&mut *self.borrow_mut(), t) | |
776 | } | |
777 | ||
778 | /// Replaces the wrapped value with a new one computed from `f`, returning | |
779 | /// the old value, without deinitializing either one. | |
780 | /// | |
3b2f2976 XL |
781 | /// # Panics |
782 | /// | |
abe05a73 XL |
783 | /// Panics if the value is currently borrowed. |
784 | /// | |
785 | /// # Examples | |
786 | /// | |
787 | /// ``` | |
abe05a73 XL |
788 | /// use std::cell::RefCell; |
789 | /// let cell = RefCell::new(5); | |
790 | /// let old_value = cell.replace_with(|&mut old| old + 1); | |
791 | /// assert_eq!(old_value, 5); | |
792 | /// assert_eq!(cell, RefCell::new(6)); | |
793 | /// ``` | |
3b2f2976 | 794 | #[inline] |
dfeec247 | 795 | #[stable(feature = "refcell_replace_swap", since = "1.35.0")] |
1b1a35ee | 796 | #[track_caller] |
abe05a73 XL |
797 | pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T { |
798 | let mut_borrow = &mut *self.borrow_mut(); | |
799 | let replacement = f(mut_borrow); | |
800 | mem::replace(mut_borrow, replacement) | |
3b2f2976 XL |
801 | } |
802 | ||
803 | /// Swaps the wrapped value of `self` with the wrapped value of `other`, | |
804 | /// without deinitializing either one. | |
805 | /// | |
806 | /// This function corresponds to [`std::mem::swap`](../mem/fn.swap.html). | |
807 | /// | |
abe05a73 XL |
808 | /// # Panics |
809 | /// | |
810 | /// Panics if the value in either `RefCell` is currently borrowed. | |
811 | /// | |
3b2f2976 XL |
812 | /// # Examples |
813 | /// | |
814 | /// ``` | |
3b2f2976 XL |
815 | /// use std::cell::RefCell; |
816 | /// let c = RefCell::new(5); | |
817 | /// let d = RefCell::new(6); | |
818 | /// c.swap(&d); | |
819 | /// assert_eq!(c, RefCell::new(6)); | |
820 | /// assert_eq!(d, RefCell::new(5)); | |
821 | /// ``` | |
3b2f2976 | 822 | #[inline] |
dfeec247 | 823 | #[stable(feature = "refcell_swap", since = "1.24.0")] |
3b2f2976 XL |
824 | pub fn swap(&self, other: &Self) { |
825 | mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut()) | |
826 | } | |
d9579d0f | 827 | } |
1a4d82fc | 828 | |
d9579d0f | 829 | impl<T: ?Sized> RefCell<T> { |
1a4d82fc JJ |
830 | /// Immutably borrows the wrapped value. |
831 | /// | |
832 | /// The borrow lasts until the returned `Ref` exits scope. Multiple | |
833 | /// immutable borrows can be taken out at the same time. | |
834 | /// | |
835 | /// # Panics | |
836 | /// | |
5bcae85e SL |
837 | /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use |
838 | /// [`try_borrow`](#method.try_borrow). | |
85aaf69f SL |
839 | /// |
840 | /// # Examples | |
841 | /// | |
842 | /// ``` | |
843 | /// use std::cell::RefCell; | |
844 | /// | |
845 | /// let c = RefCell::new(5); | |
846 | /// | |
847 | /// let borrowed_five = c.borrow(); | |
848 | /// let borrowed_five2 = c.borrow(); | |
849 | /// ``` | |
850 | /// | |
851 | /// An example of panic: | |
852 | /// | |
f035d41b | 853 | /// ```should_panic |
85aaf69f | 854 | /// use std::cell::RefCell; |
85aaf69f | 855 | /// |
f035d41b | 856 | /// let c = RefCell::new(5); |
85aaf69f | 857 | /// |
f035d41b XL |
858 | /// let m = c.borrow_mut(); |
859 | /// let b = c.borrow(); // this causes a panic | |
85aaf69f SL |
860 | /// ``` |
861 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 862 | #[inline] |
3dfed10e | 863 | #[track_caller] |
48663c56 | 864 | pub fn borrow(&self) -> Ref<'_, T> { |
5bcae85e SL |
865 | self.try_borrow().expect("already mutably borrowed") |
866 | } | |
867 | ||
868 | /// Immutably borrows the wrapped value, returning an error if the value is currently mutably | |
869 | /// borrowed. | |
870 | /// | |
871 | /// The borrow lasts until the returned `Ref` exits scope. Multiple immutable borrows can be | |
872 | /// taken out at the same time. | |
873 | /// | |
874 | /// This is the non-panicking variant of [`borrow`](#method.borrow). | |
875 | /// | |
876 | /// # Examples | |
877 | /// | |
878 | /// ``` | |
5bcae85e SL |
879 | /// use std::cell::RefCell; |
880 | /// | |
881 | /// let c = RefCell::new(5); | |
882 | /// | |
883 | /// { | |
884 | /// let m = c.borrow_mut(); | |
885 | /// assert!(c.try_borrow().is_err()); | |
886 | /// } | |
887 | /// | |
888 | /// { | |
889 | /// let m = c.borrow(); | |
890 | /// assert!(c.try_borrow().is_ok()); | |
891 | /// } | |
892 | /// ``` | |
9e0c209e | 893 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 894 | #[inline] |
cdc7bbd5 | 895 | #[cfg_attr(feature = "debug_refcell", track_caller)] |
48663c56 | 896 | pub fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> { |
85aaf69f | 897 | match BorrowRef::new(&self.borrow) { |
cdc7bbd5 XL |
898 | Some(b) => { |
899 | #[cfg(feature = "debug_refcell")] | |
900 | { | |
901 | // `borrowed_at` is always the *first* active borrow | |
902 | if b.borrow.get() == 1 { | |
903 | self.borrowed_at.set(Some(crate::panic::Location::caller())); | |
904 | } | |
905 | } | |
906 | ||
907 | // SAFETY: `BorrowRef` ensures that there is only immutable access | |
908 | // to the value while borrowed. | |
923072b8 FG |
909 | let value = unsafe { NonNull::new_unchecked(self.value.get()) }; |
910 | Ok(Ref { value, borrow: b }) | |
cdc7bbd5 XL |
911 | } |
912 | None => Err(BorrowError { | |
a2a8927a | 913 | // If a borrow occurred, then we must already have an outstanding borrow, |
cdc7bbd5 XL |
914 | // so `borrowed_at` will be `Some` |
915 | #[cfg(feature = "debug_refcell")] | |
916 | location: self.borrowed_at.get().unwrap(), | |
917 | }), | |
1a4d82fc JJ |
918 | } |
919 | } | |
920 | ||
1a4d82fc JJ |
921 | /// Mutably borrows the wrapped value. |
922 | /// | |
94b46f34 XL |
923 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
924 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
925 | /// active. | |
1a4d82fc JJ |
926 | /// |
927 | /// # Panics | |
928 | /// | |
5bcae85e SL |
929 | /// Panics if the value is currently borrowed. For a non-panicking variant, use |
930 | /// [`try_borrow_mut`](#method.try_borrow_mut). | |
85aaf69f SL |
931 | /// |
932 | /// # Examples | |
933 | /// | |
934 | /// ``` | |
935 | /// use std::cell::RefCell; | |
936 | /// | |
f9f354fc | 937 | /// let c = RefCell::new("hello".to_owned()); |
85aaf69f | 938 | /// |
f9f354fc | 939 | /// *c.borrow_mut() = "bonjour".to_owned(); |
7453a54e | 940 | /// |
f9f354fc | 941 | /// assert_eq!(&*c.borrow(), "bonjour"); |
85aaf69f SL |
942 | /// ``` |
943 | /// | |
944 | /// An example of panic: | |
945 | /// | |
f035d41b | 946 | /// ```should_panic |
85aaf69f | 947 | /// use std::cell::RefCell; |
85aaf69f | 948 | /// |
f035d41b XL |
949 | /// let c = RefCell::new(5); |
950 | /// let m = c.borrow(); | |
85aaf69f | 951 | /// |
f035d41b | 952 | /// let b = c.borrow_mut(); // this causes a panic |
85aaf69f SL |
953 | /// ``` |
954 | #[stable(feature = "rust1", since = "1.0.0")] | |
c34b1796 | 955 | #[inline] |
3dfed10e | 956 | #[track_caller] |
48663c56 | 957 | pub fn borrow_mut(&self) -> RefMut<'_, T> { |
5bcae85e SL |
958 | self.try_borrow_mut().expect("already borrowed") |
959 | } | |
960 | ||
961 | /// Mutably borrows the wrapped value, returning an error if the value is currently borrowed. | |
962 | /// | |
94b46f34 XL |
963 | /// The borrow lasts until the returned `RefMut` or all `RefMut`s derived |
964 | /// from it exit scope. The value cannot be borrowed while this borrow is | |
965 | /// active. | |
5bcae85e SL |
966 | /// |
967 | /// This is the non-panicking variant of [`borrow_mut`](#method.borrow_mut). | |
968 | /// | |
969 | /// # Examples | |
970 | /// | |
971 | /// ``` | |
5bcae85e SL |
972 | /// use std::cell::RefCell; |
973 | /// | |
974 | /// let c = RefCell::new(5); | |
975 | /// | |
976 | /// { | |
977 | /// let m = c.borrow(); | |
978 | /// assert!(c.try_borrow_mut().is_err()); | |
979 | /// } | |
980 | /// | |
981 | /// assert!(c.try_borrow_mut().is_ok()); | |
982 | /// ``` | |
9e0c209e | 983 | #[stable(feature = "try_borrow", since = "1.13.0")] |
5bcae85e | 984 | #[inline] |
cdc7bbd5 | 985 | #[cfg_attr(feature = "debug_refcell", track_caller)] |
48663c56 | 986 | pub fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> { |
85aaf69f | 987 | match BorrowRefMut::new(&self.borrow) { |
cdc7bbd5 XL |
988 | Some(b) => { |
989 | #[cfg(feature = "debug_refcell")] | |
990 | { | |
991 | self.borrowed_at.set(Some(crate::panic::Location::caller())); | |
992 | } | |
993 | ||
923072b8 FG |
994 | // SAFETY: `BorrowRefMut` guarantees unique access. |
995 | let value = unsafe { NonNull::new_unchecked(self.value.get()) }; | |
996 | Ok(RefMut { value, borrow: b, marker: PhantomData }) | |
cdc7bbd5 XL |
997 | } |
998 | None => Err(BorrowMutError { | |
a2a8927a | 999 | // If a borrow occurred, then we must already have an outstanding borrow, |
cdc7bbd5 XL |
1000 | // so `borrowed_at` will be `Some` |
1001 | #[cfg(feature = "debug_refcell")] | |
1002 | location: self.borrowed_at.get().unwrap(), | |
1003 | }), | |
1a4d82fc JJ |
1004 | } |
1005 | } | |
1006 | ||
5bcae85e SL |
1007 | /// Returns a raw pointer to the underlying data in this cell. |
1008 | /// | |
1009 | /// # Examples | |
1010 | /// | |
1011 | /// ``` | |
1012 | /// use std::cell::RefCell; | |
1013 | /// | |
1014 | /// let c = RefCell::new(5); | |
1015 | /// | |
1016 | /// let ptr = c.as_ptr(); | |
1017 | /// ``` | |
1018 | #[inline] | |
1019 | #[stable(feature = "cell_as_ptr", since = "1.12.0")] | |
1020 | pub fn as_ptr(&self) -> *mut T { | |
1021 | self.value.get() | |
1022 | } | |
1023 | ||
a7813a04 XL |
1024 | /// Returns a mutable reference to the underlying data. |
1025 | /// | |
2b03887a FG |
1026 | /// Since this method borrows `RefCell` mutably, it is statically guaranteed |
1027 | /// that no borrows to the underlying data exist. The dynamic checks inherent | |
487cf647 | 1028 | /// in [`borrow_mut`] and most other methods of `RefCell` are therefore |
2b03887a | 1029 | /// unnecessary. |
5bcae85e | 1030 | /// |
2b03887a FG |
1031 | /// This method can only be called if `RefCell` can be mutably borrowed, |
1032 | /// which in general is only the case directly after the `RefCell` has | |
1033 | /// been created. In these situations, skipping the aforementioned dynamic | |
1034 | /// borrowing checks may yield better ergonomics and runtime-performance. | |
cc61c64b | 1035 | /// |
2b03887a FG |
1036 | /// In most situations where `RefCell` is used, it can't be borrowed mutably. |
1037 | /// Use [`borrow_mut`] to get mutable access to the underlying data then. | |
cc61c64b | 1038 | /// |
fc512014 | 1039 | /// [`borrow_mut`]: RefCell::borrow_mut() |
cc61c64b | 1040 | /// |
5bcae85e SL |
1041 | /// # Examples |
1042 | /// | |
1043 | /// ``` | |
1044 | /// use std::cell::RefCell; | |
1045 | /// | |
1046 | /// let mut c = RefCell::new(5); | |
1047 | /// *c.get_mut() += 1; | |
1048 | /// | |
1049 | /// assert_eq!(c, RefCell::new(6)); | |
1050 | /// ``` | |
a7813a04 | 1051 | #[inline] |
3157f602 | 1052 | #[stable(feature = "cell_get_mut", since = "1.11.0")] |
a7813a04 | 1053 | pub fn get_mut(&mut self) -> &mut T { |
1b1a35ee | 1054 | self.value.get_mut() |
a7813a04 | 1055 | } |
48663c56 | 1056 | |
ba9703b0 XL |
1057 | /// Undo the effect of leaked guards on the borrow state of the `RefCell`. |
1058 | /// | |
1059 | /// This call is similar to [`get_mut`] but more specialized. It borrows `RefCell` mutably to | |
1060 | /// ensure no borrows exist and then resets the state tracking shared borrows. This is relevant | |
1061 | /// if some `Ref` or `RefMut` borrows have been leaked. | |
1062 | /// | |
fc512014 | 1063 | /// [`get_mut`]: RefCell::get_mut() |
ba9703b0 XL |
1064 | /// |
1065 | /// # Examples | |
1066 | /// | |
1067 | /// ``` | |
1068 | /// #![feature(cell_leak)] | |
1069 | /// use std::cell::RefCell; | |
1070 | /// | |
1071 | /// let mut c = RefCell::new(0); | |
1072 | /// std::mem::forget(c.borrow_mut()); | |
1073 | /// | |
1074 | /// assert!(c.try_borrow().is_err()); | |
1075 | /// c.undo_leak(); | |
1076 | /// assert!(c.try_borrow().is_ok()); | |
1077 | /// ``` | |
1078 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1079 | pub fn undo_leak(&mut self) -> &mut T { | |
1080 | *self.borrow.get_mut() = UNUSED; | |
1081 | self.get_mut() | |
1082 | } | |
1083 | ||
48663c56 XL |
1084 | /// Immutably borrows the wrapped value, returning an error if the value is |
1085 | /// currently mutably borrowed. | |
1086 | /// | |
1087 | /// # Safety | |
1088 | /// | |
1089 | /// Unlike `RefCell::borrow`, this method is unsafe because it does not | |
1090 | /// return a `Ref`, thus leaving the borrow flag untouched. Mutably | |
1091 | /// borrowing the `RefCell` while the reference returned by this method | |
1092 | /// is alive is undefined behaviour. | |
1093 | /// | |
1094 | /// # Examples | |
1095 | /// | |
1096 | /// ``` | |
48663c56 XL |
1097 | /// use std::cell::RefCell; |
1098 | /// | |
1099 | /// let c = RefCell::new(5); | |
1100 | /// | |
1101 | /// { | |
1102 | /// let m = c.borrow_mut(); | |
1103 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_err()); | |
1104 | /// } | |
1105 | /// | |
1106 | /// { | |
1107 | /// let m = c.borrow(); | |
1108 | /// assert!(unsafe { c.try_borrow_unguarded() }.is_ok()); | |
1109 | /// } | |
1110 | /// ``` | |
dc9dc135 | 1111 | #[stable(feature = "borrow_state", since = "1.37.0")] |
48663c56 XL |
1112 | #[inline] |
1113 | pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, BorrowError> { | |
1114 | if !is_writing(self.borrow.get()) { | |
f035d41b XL |
1115 | // SAFETY: We check that nobody is actively writing now, but it is |
1116 | // the caller's responsibility to ensure that nobody writes until | |
1117 | // the returned reference is no longer in use. | |
1118 | // Also, `self.value.get()` refers to the value owned by `self` | |
1119 | // and is thus guaranteed to be valid for the lifetime of `self`. | |
1120 | Ok(unsafe { &*self.value.get() }) | |
48663c56 | 1121 | } else { |
cdc7bbd5 | 1122 | Err(BorrowError { |
a2a8927a | 1123 | // If a borrow occurred, then we must already have an outstanding borrow, |
cdc7bbd5 XL |
1124 | // so `borrowed_at` will be `Some` |
1125 | #[cfg(feature = "debug_refcell")] | |
1126 | location: self.borrowed_at.get().unwrap(), | |
1127 | }) | |
48663c56 XL |
1128 | } |
1129 | } | |
1a4d82fc JJ |
1130 | } |
1131 | ||
f9f354fc XL |
1132 | impl<T: Default> RefCell<T> { |
1133 | /// Takes the wrapped value, leaving `Default::default()` in its place. | |
1134 | /// | |
1135 | /// # Panics | |
1136 | /// | |
1137 | /// Panics if the value is currently borrowed. | |
1138 | /// | |
1139 | /// # Examples | |
1140 | /// | |
1141 | /// ``` | |
f9f354fc XL |
1142 | /// use std::cell::RefCell; |
1143 | /// | |
1144 | /// let c = RefCell::new(5); | |
1145 | /// let five = c.take(); | |
1146 | /// | |
1147 | /// assert_eq!(five, 5); | |
1148 | /// assert_eq!(c.into_inner(), 0); | |
1149 | /// ``` | |
fc512014 | 1150 | #[stable(feature = "refcell_take", since = "1.50.0")] |
f9f354fc XL |
1151 | pub fn take(&self) -> T { |
1152 | self.replace(Default::default()) | |
1153 | } | |
1154 | } | |
1155 | ||
85aaf69f | 1156 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1157 | unsafe impl<T: ?Sized> Send for RefCell<T> where T: Send {} |
1a4d82fc | 1158 | |
54a0048b SL |
1159 | #[stable(feature = "rust1", since = "1.0.0")] |
1160 | impl<T: ?Sized> !Sync for RefCell<T> {} | |
1161 | ||
85aaf69f | 1162 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 1163 | impl<T: Clone> Clone for RefCell<T> { |
0531ce1d XL |
1164 | /// # Panics |
1165 | /// | |
1166 | /// Panics if the value is currently mutably borrowed. | |
c34b1796 | 1167 | #[inline] |
1b1a35ee | 1168 | #[track_caller] |
1a4d82fc JJ |
1169 | fn clone(&self) -> RefCell<T> { |
1170 | RefCell::new(self.borrow().clone()) | |
1171 | } | |
17df50a5 XL |
1172 | |
1173 | /// # Panics | |
1174 | /// | |
1175 | /// Panics if `other` is currently mutably borrowed. | |
1176 | #[inline] | |
1177 | #[track_caller] | |
1178 | fn clone_from(&mut self, other: &Self) { | |
1179 | self.get_mut().clone_from(&other.borrow()) | |
1180 | } | |
1a4d82fc JJ |
1181 | } |
1182 | ||
85aaf69f | 1183 | #[stable(feature = "rust1", since = "1.0.0")] |
416331ca | 1184 | impl<T: Default> Default for RefCell<T> { |
9e0c209e | 1185 | /// Creates a `RefCell<T>`, with the `Default` value for T. |
c34b1796 | 1186 | #[inline] |
1a4d82fc JJ |
1187 | fn default() -> RefCell<T> { |
1188 | RefCell::new(Default::default()) | |
1189 | } | |
1190 | } | |
1191 | ||
85aaf69f | 1192 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1193 | impl<T: ?Sized + PartialEq> PartialEq for RefCell<T> { |
0531ce1d XL |
1194 | /// # Panics |
1195 | /// | |
1196 | /// Panics if the value in either `RefCell` is currently borrowed. | |
c34b1796 | 1197 | #[inline] |
1a4d82fc JJ |
1198 | fn eq(&self, other: &RefCell<T>) -> bool { |
1199 | *self.borrow() == *other.borrow() | |
1200 | } | |
1201 | } | |
1202 | ||
62682a34 SL |
1203 | #[stable(feature = "cell_eq", since = "1.2.0")] |
1204 | impl<T: ?Sized + Eq> Eq for RefCell<T> {} | |
1205 | ||
a7813a04 XL |
1206 | #[stable(feature = "cell_ord", since = "1.10.0")] |
1207 | impl<T: ?Sized + PartialOrd> PartialOrd for RefCell<T> { | |
0531ce1d XL |
1208 | /// # Panics |
1209 | /// | |
1210 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1211 | #[inline] |
1212 | fn partial_cmp(&self, other: &RefCell<T>) -> Option<Ordering> { | |
1213 | self.borrow().partial_cmp(&*other.borrow()) | |
1214 | } | |
1215 | ||
0531ce1d XL |
1216 | /// # Panics |
1217 | /// | |
1218 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1219 | #[inline] |
1220 | fn lt(&self, other: &RefCell<T>) -> bool { | |
1221 | *self.borrow() < *other.borrow() | |
1222 | } | |
1223 | ||
0531ce1d XL |
1224 | /// # Panics |
1225 | /// | |
1226 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1227 | #[inline] |
1228 | fn le(&self, other: &RefCell<T>) -> bool { | |
1229 | *self.borrow() <= *other.borrow() | |
1230 | } | |
1231 | ||
0531ce1d XL |
1232 | /// # Panics |
1233 | /// | |
1234 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1235 | #[inline] |
1236 | fn gt(&self, other: &RefCell<T>) -> bool { | |
1237 | *self.borrow() > *other.borrow() | |
1238 | } | |
1239 | ||
0531ce1d XL |
1240 | /// # Panics |
1241 | /// | |
1242 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1243 | #[inline] |
1244 | fn ge(&self, other: &RefCell<T>) -> bool { | |
1245 | *self.borrow() >= *other.borrow() | |
1246 | } | |
1247 | } | |
1248 | ||
1249 | #[stable(feature = "cell_ord", since = "1.10.0")] | |
1250 | impl<T: ?Sized + Ord> Ord for RefCell<T> { | |
0531ce1d XL |
1251 | /// # Panics |
1252 | /// | |
1253 | /// Panics if the value in either `RefCell` is currently borrowed. | |
a7813a04 XL |
1254 | #[inline] |
1255 | fn cmp(&self, other: &RefCell<T>) -> Ordering { | |
1256 | self.borrow().cmp(&*other.borrow()) | |
1257 | } | |
1258 | } | |
1259 | ||
5bcae85e | 1260 | #[stable(feature = "cell_from", since = "1.12.0")] |
3c0e092e XL |
1261 | #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
1262 | impl<T> const From<T> for RefCell<T> { | |
5099ac24 | 1263 | /// Creates a new `RefCell<T>` containing the given value. |
5bcae85e SL |
1264 | fn from(t: T) -> RefCell<T> { |
1265 | RefCell::new(t) | |
1266 | } | |
1267 | } | |
1268 | ||
9e0c209e SL |
1269 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1270 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<RefCell<U>> for RefCell<T> {} | |
1271 | ||
1a4d82fc | 1272 | struct BorrowRef<'b> { |
54a0048b | 1273 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1274 | } |
1275 | ||
1276 | impl<'b> BorrowRef<'b> { | |
c34b1796 | 1277 | #[inline] |
1a4d82fc | 1278 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRef<'b>> { |
416331ca XL |
1279 | let b = borrow.get().wrapping_add(1); |
1280 | if !is_reading(b) { | |
1281 | // Incrementing borrow can result in a non-reading value (<= 0) in these cases: | |
1282 | // 1. It was < 0, i.e. there are writing borrows, so we can't allow a read borrow | |
1283 | // due to Rust's reference aliasing rules | |
f035d41b XL |
1284 | // 2. It was isize::MAX (the max amount of reading borrows) and it overflowed |
1285 | // into isize::MIN (the max amount of writing borrows) so we can't allow | |
416331ca XL |
1286 | // an additional read borrow because isize can't represent so many read borrows |
1287 | // (this can only happen if you mem::forget more than a small constant amount of | |
1288 | // `Ref`s, which is not good practice) | |
94b46f34 XL |
1289 | None |
1290 | } else { | |
416331ca XL |
1291 | // Incrementing borrow can result in a reading value (> 0) in these cases: |
1292 | // 1. It was = 0, i.e. it wasn't borrowed, and we are taking the first read borrow | |
f035d41b | 1293 | // 2. It was > 0 and < isize::MAX, i.e. there were read borrows, and isize |
416331ca XL |
1294 | // is large enough to represent having one more read borrow |
1295 | borrow.set(b); | |
94b46f34 | 1296 | Some(BorrowRef { borrow }) |
1a4d82fc JJ |
1297 | } |
1298 | } | |
1299 | } | |
1300 | ||
0bf4aa26 | 1301 | impl Drop for BorrowRef<'_> { |
c34b1796 | 1302 | #[inline] |
1a4d82fc | 1303 | fn drop(&mut self) { |
54a0048b | 1304 | let borrow = self.borrow.get(); |
8faf50e0 | 1305 | debug_assert!(is_reading(borrow)); |
54a0048b | 1306 | self.borrow.set(borrow - 1); |
1a4d82fc JJ |
1307 | } |
1308 | } | |
1309 | ||
0bf4aa26 | 1310 | impl Clone for BorrowRef<'_> { |
c34b1796 | 1311 | #[inline] |
0bf4aa26 | 1312 | fn clone(&self) -> Self { |
1a4d82fc | 1313 | // Since this Ref exists, we know the borrow flag |
8faf50e0 | 1314 | // is a reading borrow. |
54a0048b | 1315 | let borrow = self.borrow.get(); |
8faf50e0 | 1316 | debug_assert!(is_reading(borrow)); |
94b46f34 XL |
1317 | // Prevent the borrow counter from overflowing into |
1318 | // a writing borrow. | |
f035d41b | 1319 | assert!(borrow != isize::MAX); |
54a0048b SL |
1320 | self.borrow.set(borrow + 1); |
1321 | BorrowRef { borrow: self.borrow } | |
1a4d82fc JJ |
1322 | } |
1323 | } | |
1324 | ||
1325 | /// Wraps a borrowed reference to a value in a `RefCell` box. | |
85aaf69f SL |
1326 | /// A wrapper type for an immutably borrowed value from a `RefCell<T>`. |
1327 | /// | |
29967ef6 | 1328 | /// See the [module-level documentation](self) for more. |
85aaf69f | 1329 | #[stable(feature = "rust1", since = "1.0.0")] |
5099ac24 | 1330 | #[must_not_suspend = "holding a Ref across suspend points can cause BorrowErrors"] |
d9579d0f | 1331 | pub struct Ref<'b, T: ?Sized + 'b> { |
923072b8 FG |
1332 | // NB: we use a pointer instead of `&'b T` to avoid `noalias` violations, because a |
1333 | // `Ref` argument doesn't hold immutability for its whole scope, only until it drops. | |
1334 | // `NonNull` is also covariant over `T`, just like we would have with `&T`. | |
1335 | value: NonNull<T>, | |
54a0048b | 1336 | borrow: BorrowRef<'b>, |
1a4d82fc JJ |
1337 | } |
1338 | ||
85aaf69f | 1339 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1340 | impl<T: ?Sized> Deref for Ref<'_, T> { |
1a4d82fc JJ |
1341 | type Target = T; |
1342 | ||
1343 | #[inline] | |
e9174d1e | 1344 | fn deref(&self) -> &T { |
923072b8 FG |
1345 | // SAFETY: the value is accessible as long as we hold our borrow. |
1346 | unsafe { self.value.as_ref() } | |
1a4d82fc JJ |
1347 | } |
1348 | } | |
1349 | ||
62682a34 SL |
1350 | impl<'b, T: ?Sized> Ref<'b, T> { |
1351 | /// Copies a `Ref`. | |
1352 | /// | |
1353 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1354 | /// | |
1355 | /// This is an associated function that needs to be used as | |
9fa01778 | 1356 | /// `Ref::clone(...)`. A `Clone` implementation or a method would interfere |
62682a34 SL |
1357 | /// with the widespread use of `r.borrow().clone()` to clone the contents of |
1358 | /// a `RefCell`. | |
476ff2be | 1359 | #[stable(feature = "cell_extras", since = "1.15.0")] |
3c0e092e | 1360 | #[must_use] |
62682a34 SL |
1361 | #[inline] |
1362 | pub fn clone(orig: &Ref<'b, T>) -> Ref<'b, T> { | |
dfeec247 | 1363 | Ref { value: orig.value, borrow: orig.borrow.clone() } |
62682a34 SL |
1364 | } |
1365 | ||
9fa01778 | 1366 | /// Makes a new `Ref` for a component of the borrowed data. |
62682a34 SL |
1367 | /// |
1368 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1369 | /// | |
1370 | /// This is an associated function that needs to be used as `Ref::map(...)`. | |
1371 | /// A method would interfere with methods of the same name on the contents | |
1372 | /// of a `RefCell` used through `Deref`. | |
1373 | /// | |
3b2f2976 | 1374 | /// # Examples |
62682a34 SL |
1375 | /// |
1376 | /// ``` | |
62682a34 SL |
1377 | /// use std::cell::{RefCell, Ref}; |
1378 | /// | |
1379 | /// let c = RefCell::new((5, 'b')); | |
1380 | /// let b1: Ref<(u32, char)> = c.borrow(); | |
1381 | /// let b2: Ref<u32> = Ref::map(b1, |t| &t.0); | |
1382 | /// assert_eq!(*b2, 5) | |
1383 | /// ``` | |
7453a54e | 1384 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 SL |
1385 | #[inline] |
1386 | pub fn map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Ref<'b, U> | |
dfeec247 XL |
1387 | where |
1388 | F: FnOnce(&T) -> &U, | |
62682a34 | 1389 | { |
923072b8 | 1390 | Ref { value: NonNull::from(f(&*orig)), borrow: orig.borrow } |
62682a34 | 1391 | } |
94b46f34 | 1392 | |
5869c6ff XL |
1393 | /// Makes a new `Ref` for an optional component of the borrowed data. The |
1394 | /// original guard is returned as an `Err(..)` if the closure returns | |
1395 | /// `None`. | |
1396 | /// | |
1397 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1398 | /// | |
1399 | /// This is an associated function that needs to be used as | |
1400 | /// `Ref::filter_map(...)`. A method would interfere with methods of the same | |
1401 | /// name on the contents of a `RefCell` used through `Deref`. | |
1402 | /// | |
1403 | /// # Examples | |
1404 | /// | |
1405 | /// ``` | |
5869c6ff XL |
1406 | /// use std::cell::{RefCell, Ref}; |
1407 | /// | |
1408 | /// let c = RefCell::new(vec![1, 2, 3]); | |
1409 | /// let b1: Ref<Vec<u32>> = c.borrow(); | |
1410 | /// let b2: Result<Ref<u32>, _> = Ref::filter_map(b1, |v| v.get(1)); | |
1411 | /// assert_eq!(*b2.unwrap(), 2); | |
1412 | /// ``` | |
923072b8 | 1413 | #[stable(feature = "cell_filter_map", since = "1.63.0")] |
5869c6ff XL |
1414 | #[inline] |
1415 | pub fn filter_map<U: ?Sized, F>(orig: Ref<'b, T>, f: F) -> Result<Ref<'b, U>, Self> | |
1416 | where | |
1417 | F: FnOnce(&T) -> Option<&U>, | |
1418 | { | |
923072b8 FG |
1419 | match f(&*orig) { |
1420 | Some(value) => Ok(Ref { value: NonNull::from(value), borrow: orig.borrow }), | |
5869c6ff XL |
1421 | None => Err(orig), |
1422 | } | |
1423 | } | |
1424 | ||
9fa01778 | 1425 | /// Splits a `Ref` into multiple `Ref`s for different components of the |
94b46f34 XL |
1426 | /// borrowed data. |
1427 | /// | |
1428 | /// The `RefCell` is already immutably borrowed, so this cannot fail. | |
1429 | /// | |
1430 | /// This is an associated function that needs to be used as | |
1431 | /// `Ref::map_split(...)`. A method would interfere with methods of the same | |
1432 | /// name on the contents of a `RefCell` used through `Deref`. | |
1433 | /// | |
1434 | /// # Examples | |
1435 | /// | |
1436 | /// ``` | |
94b46f34 XL |
1437 | /// use std::cell::{Ref, RefCell}; |
1438 | /// | |
1439 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1440 | /// let borrow = cell.borrow(); | |
1441 | /// let (begin, end) = Ref::map_split(borrow, |slice| slice.split_at(2)); | |
1442 | /// assert_eq!(*begin, [1, 2]); | |
1443 | /// assert_eq!(*end, [3, 4]); | |
1444 | /// ``` | |
532ac7d7 | 1445 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1446 | #[inline] |
1447 | pub fn map_split<U: ?Sized, V: ?Sized, F>(orig: Ref<'b, T>, f: F) -> (Ref<'b, U>, Ref<'b, V>) | |
dfeec247 XL |
1448 | where |
1449 | F: FnOnce(&T) -> (&U, &V), | |
94b46f34 | 1450 | { |
923072b8 | 1451 | let (a, b) = f(&*orig); |
94b46f34 | 1452 | let borrow = orig.borrow.clone(); |
923072b8 FG |
1453 | ( |
1454 | Ref { value: NonNull::from(a), borrow }, | |
1455 | Ref { value: NonNull::from(b), borrow: orig.borrow }, | |
1456 | ) | |
94b46f34 | 1457 | } |
74b04a01 XL |
1458 | |
1459 | /// Convert into a reference to the underlying data. | |
1460 | /// | |
1461 | /// The underlying `RefCell` can never be mutably borrowed from again and will always appear | |
1462 | /// already immutably borrowed. It is not a good idea to leak more than a constant number of | |
1463 | /// references. The `RefCell` can be immutably borrowed again if only a smaller number of leaks | |
1464 | /// have occurred in total. | |
1465 | /// | |
1466 | /// This is an associated function that needs to be used as | |
1467 | /// `Ref::leak(...)`. A method would interfere with methods of the | |
1468 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1469 | /// | |
1470 | /// # Examples | |
1471 | /// | |
1472 | /// ``` | |
1473 | /// #![feature(cell_leak)] | |
1474 | /// use std::cell::{RefCell, Ref}; | |
1475 | /// let cell = RefCell::new(0); | |
1476 | /// | |
1477 | /// let value = Ref::leak(cell.borrow()); | |
1478 | /// assert_eq!(*value, 0); | |
1479 | /// | |
1480 | /// assert!(cell.try_borrow().is_ok()); | |
1481 | /// assert!(cell.try_borrow_mut().is_err()); | |
1482 | /// ``` | |
1483 | #[unstable(feature = "cell_leak", issue = "69099")] | |
1484 | pub fn leak(orig: Ref<'b, T>) -> &'b T { | |
ba9703b0 XL |
1485 | // By forgetting this Ref we ensure that the borrow counter in the RefCell can't go back to |
1486 | // UNUSED within the lifetime `'b`. Resetting the reference tracking state would require a | |
1487 | // unique reference to the borrowed RefCell. No further mutable references can be created | |
1488 | // from the original cell. | |
74b04a01 | 1489 | mem::forget(orig.borrow); |
923072b8 FG |
1490 | // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`. |
1491 | unsafe { orig.value.as_ref() } | |
74b04a01 | 1492 | } |
62682a34 SL |
1493 | } |
1494 | ||
54a0048b SL |
1495 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1496 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {} | |
1497 | ||
041b39d2 | 1498 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1499 | impl<T: ?Sized + fmt::Display> fmt::Display for Ref<'_, T> { |
48663c56 | 1500 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
923072b8 | 1501 | (**self).fmt(f) |
041b39d2 XL |
1502 | } |
1503 | } | |
1504 | ||
62682a34 | 1505 | impl<'b, T: ?Sized> RefMut<'b, T> { |
9fa01778 | 1506 | /// Makes a new `RefMut` for a component of the borrowed data, e.g., an enum |
62682a34 SL |
1507 | /// variant. |
1508 | /// | |
1509 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1510 | /// | |
1511 | /// This is an associated function that needs to be used as | |
9fa01778 | 1512 | /// `RefMut::map(...)`. A method would interfere with methods of the same |
62682a34 SL |
1513 | /// name on the contents of a `RefCell` used through `Deref`. |
1514 | /// | |
3b2f2976 | 1515 | /// # Examples |
62682a34 SL |
1516 | /// |
1517 | /// ``` | |
62682a34 SL |
1518 | /// use std::cell::{RefCell, RefMut}; |
1519 | /// | |
1520 | /// let c = RefCell::new((5, 'b')); | |
1521 | /// { | |
1522 | /// let b1: RefMut<(u32, char)> = c.borrow_mut(); | |
1523 | /// let mut b2: RefMut<u32> = RefMut::map(b1, |t| &mut t.0); | |
1524 | /// assert_eq!(*b2, 5); | |
1525 | /// *b2 = 42; | |
1526 | /// } | |
1527 | /// assert_eq!(*c.borrow(), (42, 'b')); | |
1528 | /// ``` | |
7453a54e | 1529 | #[stable(feature = "cell_map", since = "1.8.0")] |
62682a34 | 1530 | #[inline] |
923072b8 | 1531 | pub fn map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> RefMut<'b, U> |
dfeec247 XL |
1532 | where |
1533 | F: FnOnce(&mut T) -> &mut U, | |
62682a34 | 1534 | { |
923072b8 FG |
1535 | let value = NonNull::from(f(&mut *orig)); |
1536 | RefMut { value, borrow: orig.borrow, marker: PhantomData } | |
62682a34 | 1537 | } |
94b46f34 | 1538 | |
5869c6ff XL |
1539 | /// Makes a new `RefMut` for an optional component of the borrowed data. The |
1540 | /// original guard is returned as an `Err(..)` if the closure returns | |
1541 | /// `None`. | |
1542 | /// | |
1543 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1544 | /// | |
1545 | /// This is an associated function that needs to be used as | |
1546 | /// `RefMut::filter_map(...)`. A method would interfere with methods of the | |
1547 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1548 | /// | |
1549 | /// # Examples | |
1550 | /// | |
1551 | /// ``` | |
5869c6ff XL |
1552 | /// use std::cell::{RefCell, RefMut}; |
1553 | /// | |
1554 | /// let c = RefCell::new(vec![1, 2, 3]); | |
1555 | /// | |
1556 | /// { | |
1557 | /// let b1: RefMut<Vec<u32>> = c.borrow_mut(); | |
1558 | /// let mut b2: Result<RefMut<u32>, _> = RefMut::filter_map(b1, |v| v.get_mut(1)); | |
1559 | /// | |
1560 | /// if let Ok(mut b2) = b2 { | |
1561 | /// *b2 += 2; | |
1562 | /// } | |
1563 | /// } | |
1564 | /// | |
1565 | /// assert_eq!(*c.borrow(), vec![1, 4, 3]); | |
1566 | /// ``` | |
923072b8 | 1567 | #[stable(feature = "cell_filter_map", since = "1.63.0")] |
5869c6ff | 1568 | #[inline] |
923072b8 | 1569 | pub fn filter_map<U: ?Sized, F>(mut orig: RefMut<'b, T>, f: F) -> Result<RefMut<'b, U>, Self> |
5869c6ff XL |
1570 | where |
1571 | F: FnOnce(&mut T) -> Option<&mut U>, | |
1572 | { | |
5869c6ff XL |
1573 | // SAFETY: function holds onto an exclusive reference for the duration |
1574 | // of its call through `orig`, and the pointer is only de-referenced | |
1575 | // inside of the function call never allowing the exclusive reference to | |
1576 | // escape. | |
923072b8 FG |
1577 | match f(&mut *orig) { |
1578 | Some(value) => { | |
1579 | Ok(RefMut { value: NonNull::from(value), borrow: orig.borrow, marker: PhantomData }) | |
5869c6ff | 1580 | } |
923072b8 | 1581 | None => Err(orig), |
5869c6ff XL |
1582 | } |
1583 | } | |
1584 | ||
9fa01778 | 1585 | /// Splits a `RefMut` into multiple `RefMut`s for different components of the |
94b46f34 XL |
1586 | /// borrowed data. |
1587 | /// | |
1588 | /// The underlying `RefCell` will remain mutably borrowed until both | |
1589 | /// returned `RefMut`s go out of scope. | |
1590 | /// | |
1591 | /// The `RefCell` is already mutably borrowed, so this cannot fail. | |
1592 | /// | |
1593 | /// This is an associated function that needs to be used as | |
1594 | /// `RefMut::map_split(...)`. A method would interfere with methods of the | |
1595 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1596 | /// | |
1597 | /// # Examples | |
1598 | /// | |
1599 | /// ``` | |
94b46f34 XL |
1600 | /// use std::cell::{RefCell, RefMut}; |
1601 | /// | |
1602 | /// let cell = RefCell::new([1, 2, 3, 4]); | |
1603 | /// let borrow = cell.borrow_mut(); | |
1604 | /// let (mut begin, mut end) = RefMut::map_split(borrow, |slice| slice.split_at_mut(2)); | |
1605 | /// assert_eq!(*begin, [1, 2]); | |
1606 | /// assert_eq!(*end, [3, 4]); | |
1607 | /// begin.copy_from_slice(&[4, 3]); | |
1608 | /// end.copy_from_slice(&[2, 1]); | |
1609 | /// ``` | |
532ac7d7 | 1610 | #[stable(feature = "refcell_map_split", since = "1.35.0")] |
94b46f34 XL |
1611 | #[inline] |
1612 | pub fn map_split<U: ?Sized, V: ?Sized, F>( | |
923072b8 | 1613 | mut orig: RefMut<'b, T>, |
dfeec247 | 1614 | f: F, |
94b46f34 | 1615 | ) -> (RefMut<'b, U>, RefMut<'b, V>) |
dfeec247 XL |
1616 | where |
1617 | F: FnOnce(&mut T) -> (&mut U, &mut V), | |
94b46f34 | 1618 | { |
94b46f34 | 1619 | let borrow = orig.borrow.clone(); |
923072b8 FG |
1620 | let (a, b) = f(&mut *orig); |
1621 | ( | |
1622 | RefMut { value: NonNull::from(a), borrow, marker: PhantomData }, | |
1623 | RefMut { value: NonNull::from(b), borrow: orig.borrow, marker: PhantomData }, | |
1624 | ) | |
94b46f34 | 1625 | } |
74b04a01 XL |
1626 | |
1627 | /// Convert into a mutable reference to the underlying data. | |
1628 | /// | |
1629 | /// The underlying `RefCell` can not be borrowed from again and will always appear already | |
1630 | /// mutably borrowed, making the returned reference the only to the interior. | |
1631 | /// | |
1632 | /// This is an associated function that needs to be used as | |
1633 | /// `RefMut::leak(...)`. A method would interfere with methods of the | |
1634 | /// same name on the contents of a `RefCell` used through `Deref`. | |
1635 | /// | |
1636 | /// # Examples | |
1637 | /// | |
1638 | /// ``` | |
1639 | /// #![feature(cell_leak)] | |
1640 | /// use std::cell::{RefCell, RefMut}; | |
1641 | /// let cell = RefCell::new(0); | |
1642 | /// | |
1643 | /// let value = RefMut::leak(cell.borrow_mut()); | |
1644 | /// assert_eq!(*value, 0); | |
1645 | /// *value = 1; | |
1646 | /// | |
1647 | /// assert!(cell.try_borrow_mut().is_err()); | |
1648 | /// ``` | |
1649 | #[unstable(feature = "cell_leak", issue = "69099")] | |
923072b8 | 1650 | pub fn leak(mut orig: RefMut<'b, T>) -> &'b mut T { |
ba9703b0 XL |
1651 | // By forgetting this BorrowRefMut we ensure that the borrow counter in the RefCell can't |
1652 | // go back to UNUSED within the lifetime `'b`. Resetting the reference tracking state would | |
1653 | // require a unique reference to the borrowed RefCell. No further references can be created | |
1654 | // from the original cell within that lifetime, making the current borrow the only | |
1655 | // reference for the remaining lifetime. | |
74b04a01 | 1656 | mem::forget(orig.borrow); |
923072b8 FG |
1657 | // SAFETY: after forgetting, we can form a reference for the rest of lifetime `'b`. |
1658 | unsafe { orig.value.as_mut() } | |
74b04a01 | 1659 | } |
1a4d82fc JJ |
1660 | } |
1661 | ||
1662 | struct BorrowRefMut<'b> { | |
54a0048b | 1663 | borrow: &'b Cell<BorrowFlag>, |
1a4d82fc JJ |
1664 | } |
1665 | ||
0bf4aa26 | 1666 | impl Drop for BorrowRefMut<'_> { |
c34b1796 | 1667 | #[inline] |
1a4d82fc | 1668 | fn drop(&mut self) { |
54a0048b | 1669 | let borrow = self.borrow.get(); |
8faf50e0 XL |
1670 | debug_assert!(is_writing(borrow)); |
1671 | self.borrow.set(borrow + 1); | |
1a4d82fc JJ |
1672 | } |
1673 | } | |
1674 | ||
1675 | impl<'b> BorrowRefMut<'b> { | |
c34b1796 | 1676 | #[inline] |
1a4d82fc | 1677 | fn new(borrow: &'b Cell<BorrowFlag>) -> Option<BorrowRefMut<'b>> { |
94b46f34 XL |
1678 | // NOTE: Unlike BorrowRefMut::clone, new is called to create the initial |
1679 | // mutable reference, and so there must currently be no existing | |
1680 | // references. Thus, while clone increments the mutable refcount, here | |
8faf50e0 | 1681 | // we explicitly only allow going from UNUSED to UNUSED - 1. |
1a4d82fc JJ |
1682 | match borrow.get() { |
1683 | UNUSED => { | |
8faf50e0 | 1684 | borrow.set(UNUSED - 1); |
b7449926 | 1685 | Some(BorrowRefMut { borrow }) |
dfeec247 | 1686 | } |
1a4d82fc JJ |
1687 | _ => None, |
1688 | } | |
1689 | } | |
94b46f34 | 1690 | |
dc9dc135 | 1691 | // Clones a `BorrowRefMut`. |
94b46f34 XL |
1692 | // |
1693 | // This is only valid if each `BorrowRefMut` is used to track a mutable | |
1694 | // reference to a distinct, nonoverlapping range of the original object. | |
1695 | // This isn't in a Clone impl so that code doesn't call this implicitly. | |
1696 | #[inline] | |
1697 | fn clone(&self) -> BorrowRefMut<'b> { | |
1698 | let borrow = self.borrow.get(); | |
8faf50e0 XL |
1699 | debug_assert!(is_writing(borrow)); |
1700 | // Prevent the borrow counter from underflowing. | |
f035d41b | 1701 | assert!(borrow != isize::MIN); |
8faf50e0 | 1702 | self.borrow.set(borrow - 1); |
94b46f34 XL |
1703 | BorrowRefMut { borrow: self.borrow } |
1704 | } | |
1a4d82fc JJ |
1705 | } |
1706 | ||
85aaf69f SL |
1707 | /// A wrapper type for a mutably borrowed value from a `RefCell<T>`. |
1708 | /// | |
29967ef6 | 1709 | /// See the [module-level documentation](self) for more. |
85aaf69f | 1710 | #[stable(feature = "rust1", since = "1.0.0")] |
5099ac24 | 1711 | #[must_not_suspend = "holding a RefMut across suspend points can cause BorrowErrors"] |
d9579d0f | 1712 | pub struct RefMut<'b, T: ?Sized + 'b> { |
923072b8 FG |
1713 | // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a |
1714 | // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops. | |
1715 | value: NonNull<T>, | |
54a0048b | 1716 | borrow: BorrowRefMut<'b>, |
923072b8 FG |
1717 | // `NonNull` is covariant over `T`, so we need to reintroduce invariance. |
1718 | marker: PhantomData<&'b mut T>, | |
1a4d82fc JJ |
1719 | } |
1720 | ||
85aaf69f | 1721 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1722 | impl<T: ?Sized> Deref for RefMut<'_, T> { |
1a4d82fc JJ |
1723 | type Target = T; |
1724 | ||
1725 | #[inline] | |
e9174d1e | 1726 | fn deref(&self) -> &T { |
923072b8 FG |
1727 | // SAFETY: the value is accessible as long as we hold our borrow. |
1728 | unsafe { self.value.as_ref() } | |
1a4d82fc JJ |
1729 | } |
1730 | } | |
1731 | ||
85aaf69f | 1732 | #[stable(feature = "rust1", since = "1.0.0")] |
0bf4aa26 | 1733 | impl<T: ?Sized> DerefMut for RefMut<'_, T> { |
1a4d82fc | 1734 | #[inline] |
e9174d1e | 1735 | fn deref_mut(&mut self) -> &mut T { |
923072b8 FG |
1736 | // SAFETY: the value is accessible as long as we hold our borrow. |
1737 | unsafe { self.value.as_mut() } | |
1a4d82fc JJ |
1738 | } |
1739 | } | |
1740 | ||
54a0048b SL |
1741 | #[unstable(feature = "coerce_unsized", issue = "27732")] |
1742 | impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {} | |
1743 | ||
041b39d2 | 1744 | #[stable(feature = "std_guard_impls", since = "1.20.0")] |
0bf4aa26 | 1745 | impl<T: ?Sized + fmt::Display> fmt::Display for RefMut<'_, T> { |
48663c56 | 1746 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
923072b8 | 1747 | (**self).fmt(f) |
041b39d2 XL |
1748 | } |
1749 | } | |
1750 | ||
1a4d82fc JJ |
1751 | /// The core primitive for interior mutability in Rust. |
1752 | /// | |
6a06907d XL |
1753 | /// If you have a reference `&T`, then normally in Rust the compiler performs optimizations based on |
1754 | /// the knowledge that `&T` points to immutable data. Mutating that data, for example through an | |
1755 | /// alias or by transmuting an `&T` into an `&mut T`, is considered undefined behavior. | |
1756 | /// `UnsafeCell<T>` opts-out of the immutability guarantee for `&T`: a shared reference | |
1757 | /// `&UnsafeCell<T>` may point to data that is being mutated. This is called "interior mutability". | |
1a4d82fc | 1758 | /// |
6a06907d XL |
1759 | /// All other types that allow internal mutability, such as `Cell<T>` and `RefCell<T>`, internally |
1760 | /// use `UnsafeCell` to wrap their data. | |
1761 | /// | |
1762 | /// Note that only the immutability guarantee for shared references is affected by `UnsafeCell`. The | |
1763 | /// uniqueness guarantee for mutable references is unaffected. There is *no* legal way to obtain | |
1764 | /// aliasing `&mut`, not even with `UnsafeCell<T>`. | |
5bcae85e | 1765 | /// |
1b1a35ee XL |
1766 | /// The `UnsafeCell` API itself is technically very simple: [`.get()`] gives you a raw pointer |
1767 | /// `*mut T` to its contents. It is up to _you_ as the abstraction designer to use that raw pointer | |
1768 | /// correctly. | |
1769 | /// | |
1770 | /// [`.get()`]: `UnsafeCell::get` | |
0531ce1d XL |
1771 | /// |
1772 | /// The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious: | |
1773 | /// | |
064997fb FG |
1774 | /// - If you create a safe reference with lifetime `'a` (either a `&T` or `&mut T` reference), then |
1775 | /// you must not access the data in any way that contradicts that reference for the remainder of | |
1776 | /// `'a`. For example, this means that if you take the `*mut T` from an `UnsafeCell<T>` and cast it | |
1777 | /// to an `&T`, then the data in `T` must remain immutable (modulo any `UnsafeCell` data found | |
1778 | /// within `T`, of course) until that reference's lifetime expires. Similarly, if you create a `&mut | |
1779 | /// T` reference that is released to safe code, then you must not access the data within the | |
1780 | /// `UnsafeCell` until that reference expires. | |
1781 | /// | |
1782 | /// - For both `&T` without `UnsafeCell<_>` and `&mut T`, you must also not deallocate the data | |
1783 | /// until the reference expires. As a special exception, given an `&T`, any part of it that is | |
1784 | /// inside an `UnsafeCell<_>` may be deallocated during the lifetime of the reference, after the | |
1785 | /// last time the reference is used (dereferenced or reborrowed). Since you cannot deallocate a part | |
1786 | /// of what a reference points to, this means the memory an `&T` points to can be deallocted only if | |
1787 | /// *every part of it* (including padding) is inside an `UnsafeCell`. | |
1788 | /// | |
1789 | /// However, whenever a `&UnsafeCell<T>` is constructed or dereferenced, it must still point to | |
1790 | /// live memory and the compiler is allowed to insert spurious reads if it can prove that this | |
1791 | /// memory has not yet been deallocated. | |
0531ce1d | 1792 | /// |
94b46f34 | 1793 | /// - At all times, you must avoid data races. If multiple threads have access to |
0531ce1d XL |
1794 | /// the same `UnsafeCell`, then any writes must have a proper happens-before relation to all other |
1795 | /// accesses (or use atomics). | |
5bcae85e | 1796 | /// |
0531ce1d XL |
1797 | /// To assist with proper design, the following scenarios are explicitly declared legal |
1798 | /// for single-threaded code: | |
5bcae85e | 1799 | /// |
94b46f34 | 1800 | /// 1. A `&T` reference can be released to safe code and there it can co-exist with other `&T` |
0531ce1d XL |
1801 | /// references, but not with a `&mut T` |
1802 | /// | |
94b46f34 | 1803 | /// 2. A `&mut T` reference may be released to safe code provided neither other `&mut T` nor `&T` |
0531ce1d XL |
1804 | /// co-exist with it. A `&mut T` must always be unique. |
1805 | /// | |
1b1a35ee XL |
1806 | /// Note that whilst mutating the contents of an `&UnsafeCell<T>` (even while other |
1807 | /// `&UnsafeCell<T>` references alias the cell) is | |
1808 | /// ok (provided you enforce the above invariants some other way), it is still undefined behavior | |
1809 | /// to have multiple `&mut UnsafeCell<T>` aliases. That is, `UnsafeCell` is a wrapper | |
1810 | /// designed to have a special interaction with _shared_ accesses (_i.e._, through an | |
1811 | /// `&UnsafeCell<_>` reference); there is no magic whatsoever when dealing with _exclusive_ | |
1812 | /// accesses (_e.g._, through an `&mut UnsafeCell<_>`): neither the cell nor the wrapped value | |
1813 | /// may be aliased for the duration of that `&mut` borrow. | |
fc512014 | 1814 | /// This is showcased by the [`.get_mut()`] accessor, which is a _safe_ getter that yields |
1b1a35ee XL |
1815 | /// a `&mut T`. |
1816 | /// | |
1817 | /// [`.get_mut()`]: `UnsafeCell::get_mut` | |
1a4d82fc | 1818 | /// |
2b03887a FG |
1819 | /// # Memory layout |
1820 | /// | |
1821 | /// `UnsafeCell<T>` has the same in-memory representation as its inner type `T`. A consequence | |
1822 | /// of this guarantee is that it is possible to convert between `T` and `UnsafeCell<T>`. | |
1823 | /// Special care has to be taken when converting a nested `T` inside of an `Outer<T>` type | |
1824 | /// to an `Outer<UnsafeCell<T>>` type: this is not sound when the `Outer<T>` type enables [niche] | |
1825 | /// optimizations. For example, the type `Option<NonNull<u8>>` is typically 8 bytes large on | |
1826 | /// 64-bit platforms, but the type `Option<UnsafeCell<NonNull<u8>>>` takes up 16 bytes of space. | |
1827 | /// Therefore this is not a valid conversion, despite `NonNull<u8>` and `UnsafeCell<NonNull<u8>>>` | |
1828 | /// having the same memory layout. This is because `UnsafeCell` disables niche optimizations in | |
1829 | /// order to avoid its interior mutability property from spreading from `T` into the `Outer` type, | |
1830 | /// thus this can cause distortions in the type size in these cases. | |
1831 | /// | |
1832 | /// Note that the only valid way to obtain a `*mut T` pointer to the contents of a | |
1833 | /// _shared_ `UnsafeCell<T>` is through [`.get()`] or [`.raw_get()`]. A `&mut T` reference | |
1834 | /// can be obtained by either dereferencing this pointer or by calling [`.get_mut()`] | |
1835 | /// on an _exclusive_ `UnsafeCell<T>`. Even though `T` and `UnsafeCell<T>` have the | |
1836 | /// same memory layout, the following is not allowed and undefined behavior: | |
1837 | /// | |
1838 | /// ```rust,no_run | |
1839 | /// # use std::cell::UnsafeCell; | |
1840 | /// unsafe fn not_allowed<T>(ptr: &UnsafeCell<T>) -> &mut T { | |
1841 | /// let t = ptr as *const UnsafeCell<T> as *mut T; | |
1842 | /// // This is undefined behavior, because the `*mut T` pointer | |
1843 | /// // was not obtained through `.get()` nor `.raw_get()`: | |
1844 | /// unsafe { &mut *t } | |
1845 | /// } | |
1846 | /// ``` | |
1847 | /// | |
1848 | /// Instead, do this: | |
1849 | /// | |
1850 | /// ```rust | |
1851 | /// # use std::cell::UnsafeCell; | |
1852 | /// // Safety: the caller must ensure that there are no references that | |
1853 | /// // point to the *contents* of the `UnsafeCell`. | |
1854 | /// unsafe fn get_mut<T>(ptr: &UnsafeCell<T>) -> &mut T { | |
1855 | /// unsafe { &mut *ptr.get() } | |
1856 | /// } | |
1857 | /// ``` | |
1858 | /// | |
487cf647 | 1859 | /// Converting in the other direction from a `&mut T` |
2b03887a FG |
1860 | /// to an `&UnsafeCell<T>` is allowed: |
1861 | /// | |
1862 | /// ```rust | |
1863 | /// # use std::cell::UnsafeCell; | |
1864 | /// fn get_shared<T>(ptr: &mut T) -> &UnsafeCell<T> { | |
1865 | /// let t = ptr as *mut T as *const UnsafeCell<T>; | |
1866 | /// // SAFETY: `T` and `UnsafeCell<T>` have the same memory layout | |
1867 | /// unsafe { &*t } | |
1868 | /// } | |
1869 | /// ``` | |
1870 | /// | |
1871 | /// [niche]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#niche | |
1872 | /// [`.raw_get()`]: `UnsafeCell::raw_get` | |
1873 | /// | |
85aaf69f | 1874 | /// # Examples |
1a4d82fc | 1875 | /// |
1b1a35ee XL |
1876 | /// Here is an example showcasing how to soundly mutate the contents of an `UnsafeCell<_>` despite |
1877 | /// there being multiple references aliasing the cell: | |
1878 | /// | |
85aaf69f | 1879 | /// ``` |
1a4d82fc | 1880 | /// use std::cell::UnsafeCell; |
1a4d82fc | 1881 | /// |
1b1a35ee XL |
1882 | /// let x: UnsafeCell<i32> = 42.into(); |
1883 | /// // Get multiple / concurrent / shared references to the same `x`. | |
1884 | /// let (p1, p2): (&UnsafeCell<i32>, &UnsafeCell<i32>) = (&x, &x); | |
1885 | /// | |
1886 | /// unsafe { | |
1887 | /// // SAFETY: within this scope there are no other references to `x`'s contents, | |
1888 | /// // so ours is effectively unique. | |
1889 | /// let p1_exclusive: &mut i32 = &mut *p1.get(); // -- borrow --+ | |
1890 | /// *p1_exclusive += 27; // | | |
1891 | /// } // <---------- cannot go beyond this point -------------------+ | |
1892 | /// | |
1893 | /// unsafe { | |
1894 | /// // SAFETY: within this scope nobody expects to have exclusive access to `x`'s contents, | |
1895 | /// // so we can have multiple shared accesses concurrently. | |
1896 | /// let p2_shared: &i32 = &*p2.get(); | |
1897 | /// assert_eq!(*p2_shared, 42 + 27); | |
1898 | /// let p1_shared: &i32 = &*p1.get(); | |
1899 | /// assert_eq!(*p1_shared, *p2_shared); | |
1a4d82fc | 1900 | /// } |
1b1a35ee XL |
1901 | /// ``` |
1902 | /// | |
1903 | /// The following example showcases the fact that exclusive access to an `UnsafeCell<T>` | |
1904 | /// implies exclusive access to its `T`: | |
1905 | /// | |
1906 | /// ```rust | |
1b1a35ee XL |
1907 | /// #![forbid(unsafe_code)] // with exclusive accesses, |
1908 | /// // `UnsafeCell` is a transparent no-op wrapper, | |
1909 | /// // so no need for `unsafe` here. | |
1910 | /// use std::cell::UnsafeCell; | |
1911 | /// | |
1912 | /// let mut x: UnsafeCell<i32> = 42.into(); | |
1913 | /// | |
1914 | /// // Get a compile-time-checked unique reference to `x`. | |
1915 | /// let p_unique: &mut UnsafeCell<i32> = &mut x; | |
1916 | /// // With an exclusive reference, we can mutate the contents for free. | |
1917 | /// *p_unique.get_mut() = 0; | |
1918 | /// // Or, equivalently: | |
1919 | /// x = UnsafeCell::new(0); | |
85aaf69f | 1920 | /// |
1b1a35ee XL |
1921 | /// // When we own the value, we can extract the contents for free. |
1922 | /// let contents: i32 = x.into_inner(); | |
1923 | /// assert_eq!(contents, 0); | |
1a4d82fc | 1924 | /// ``` |
d9579d0f | 1925 | #[lang = "unsafe_cell"] |
85aaf69f | 1926 | #[stable(feature = "rust1", since = "1.0.0")] |
8faf50e0 | 1927 | #[repr(transparent)] |
d9579d0f | 1928 | pub struct UnsafeCell<T: ?Sized> { |
e9174d1e | 1929 | value: T, |
1a4d82fc JJ |
1930 | } |
1931 | ||
92a42be0 | 1932 | #[stable(feature = "rust1", since = "1.0.0")] |
d9579d0f | 1933 | impl<T: ?Sized> !Sync for UnsafeCell<T> {} |
c34b1796 | 1934 | |
1a4d82fc | 1935 | impl<T> UnsafeCell<T> { |
9346a6ac | 1936 | /// Constructs a new instance of `UnsafeCell` which will wrap the specified |
1a4d82fc JJ |
1937 | /// value. |
1938 | /// | |
487cf647 | 1939 | /// All access to the inner value through `&UnsafeCell<T>` requires `unsafe` code. |
85aaf69f SL |
1940 | /// |
1941 | /// # Examples | |
1942 | /// | |
1943 | /// ``` | |
1944 | /// use std::cell::UnsafeCell; | |
1945 | /// | |
1946 | /// let uc = UnsafeCell::new(5); | |
1947 | /// ``` | |
1948 | #[stable(feature = "rust1", since = "1.0.0")] | |
dfeec247 | 1949 | #[rustc_const_stable(feature = "const_unsafe_cell_new", since = "1.32.0")] |
cdc7bbd5 | 1950 | #[inline(always)] |
62682a34 | 1951 | pub const fn new(value: T) -> UnsafeCell<T> { |
b7449926 | 1952 | UnsafeCell { value } |
1a4d82fc JJ |
1953 | } |
1954 | ||
d9579d0f AL |
1955 | /// Unwraps the value. |
1956 | /// | |
85aaf69f SL |
1957 | /// # Examples |
1958 | /// | |
1959 | /// ``` | |
1960 | /// use std::cell::UnsafeCell; | |
1961 | /// | |
1962 | /// let uc = UnsafeCell::new(5); | |
1963 | /// | |
2c00a5a8 | 1964 | /// let five = uc.into_inner(); |
85aaf69f | 1965 | /// ``` |
cdc7bbd5 | 1966 | #[inline(always)] |
85aaf69f | 1967 | #[stable(feature = "rust1", since = "1.0.0")] |
29967ef6 XL |
1968 | #[rustc_const_unstable(feature = "const_cell_into_inner", issue = "78729")] |
1969 | pub const fn into_inner(self) -> T { | |
62682a34 SL |
1970 | self.value |
1971 | } | |
d9579d0f | 1972 | } |
1a4d82fc | 1973 | |
d9579d0f AL |
1974 | impl<T: ?Sized> UnsafeCell<T> { |
1975 | /// Gets a mutable pointer to the wrapped value. | |
85aaf69f | 1976 | /// |
5bcae85e | 1977 | /// This can be cast to a pointer of any kind. |
0531ce1d XL |
1978 | /// Ensure that the access is unique (no active references, mutable or not) |
1979 | /// when casting to `&mut T`, and ensure that there are no mutations | |
1980 | /// or mutable aliases going on when casting to `&T` | |
5bcae85e | 1981 | /// |
85aaf69f SL |
1982 | /// # Examples |
1983 | /// | |
1984 | /// ``` | |
1985 | /// use std::cell::UnsafeCell; | |
1986 | /// | |
1987 | /// let uc = UnsafeCell::new(5); | |
1988 | /// | |
d9579d0f | 1989 | /// let five = uc.get(); |
85aaf69f | 1990 | /// ``` |
cdc7bbd5 | 1991 | #[inline(always)] |
85aaf69f | 1992 | #[stable(feature = "rust1", since = "1.0.0")] |
dfeec247 | 1993 | #[rustc_const_stable(feature = "const_unsafecell_get", since = "1.32.0")] |
a1dfa0c6 XL |
1994 | pub const fn get(&self) -> *mut T { |
1995 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
60c5eb7d XL |
1996 | // #[repr(transparent)]. This exploits libstd's special status, there is |
1997 | // no guarantee for user code that this will work in future versions of the compiler! | |
a1dfa0c6 | 1998 | self as *const UnsafeCell<T> as *const T as *mut T |
d9579d0f | 1999 | } |
60c5eb7d | 2000 | |
1b1a35ee XL |
2001 | /// Returns a mutable reference to the underlying data. |
2002 | /// | |
2003 | /// This call borrows the `UnsafeCell` mutably (at compile-time) which | |
2004 | /// guarantees that we possess the only reference. | |
2005 | /// | |
2006 | /// # Examples | |
2007 | /// | |
2008 | /// ``` | |
1b1a35ee XL |
2009 | /// use std::cell::UnsafeCell; |
2010 | /// | |
2011 | /// let mut c = UnsafeCell::new(5); | |
2012 | /// *c.get_mut() += 1; | |
2013 | /// | |
2014 | /// assert_eq!(*c.get_mut(), 6); | |
2015 | /// ``` | |
cdc7bbd5 | 2016 | #[inline(always)] |
fc512014 | 2017 | #[stable(feature = "unsafe_cell_get_mut", since = "1.50.0")] |
c295e0f8 XL |
2018 | #[rustc_const_unstable(feature = "const_unsafecell_get_mut", issue = "88836")] |
2019 | pub const fn get_mut(&mut self) -> &mut T { | |
29967ef6 | 2020 | &mut self.value |
1b1a35ee XL |
2021 | } |
2022 | ||
60c5eb7d | 2023 | /// Gets a mutable pointer to the wrapped value. |
94222f64 | 2024 | /// The difference from [`get`] is that this function accepts a raw pointer, |
60c5eb7d XL |
2025 | /// which is useful to avoid the creation of temporary references. |
2026 | /// | |
2027 | /// The result can be cast to a pointer of any kind. | |
2028 | /// Ensure that the access is unique (no active references, mutable or not) | |
2029 | /// when casting to `&mut T`, and ensure that there are no mutations | |
2030 | /// or mutable aliases going on when casting to `&T`. | |
2031 | /// | |
fc512014 | 2032 | /// [`get`]: UnsafeCell::get() |
60c5eb7d XL |
2033 | /// |
2034 | /// # Examples | |
2035 | /// | |
2036 | /// Gradual initialization of an `UnsafeCell` requires `raw_get`, as | |
2037 | /// calling `get` would require creating a reference to uninitialized data: | |
2038 | /// | |
2039 | /// ``` | |
60c5eb7d XL |
2040 | /// use std::cell::UnsafeCell; |
2041 | /// use std::mem::MaybeUninit; | |
2042 | /// | |
2043 | /// let m = MaybeUninit::<UnsafeCell<i32>>::uninit(); | |
2044 | /// unsafe { UnsafeCell::raw_get(m.as_ptr()).write(5); } | |
2045 | /// let uc = unsafe { m.assume_init() }; | |
2046 | /// | |
2047 | /// assert_eq!(uc.into_inner(), 5); | |
2048 | /// ``` | |
cdc7bbd5 | 2049 | #[inline(always)] |
94222f64 | 2050 | #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")] |
5099ac24 | 2051 | #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")] |
60c5eb7d XL |
2052 | pub const fn raw_get(this: *const Self) -> *mut T { |
2053 | // We can just cast the pointer from `UnsafeCell<T>` to `T` because of | |
2054 | // #[repr(transparent)]. This exploits libstd's special status, there is | |
2055 | // no guarantee for user code that this will work in future versions of the compiler! | |
2056 | this as *const T as *mut T | |
2057 | } | |
1a4d82fc | 2058 | } |
a7813a04 | 2059 | |
7cac9316 | 2060 | #[stable(feature = "unsafe_cell_default", since = "1.10.0")] |
a7813a04 | 2061 | impl<T: Default> Default for UnsafeCell<T> { |
9e0c209e | 2062 | /// Creates an `UnsafeCell`, with the `Default` value for T. |
a7813a04 XL |
2063 | fn default() -> UnsafeCell<T> { |
2064 | UnsafeCell::new(Default::default()) | |
2065 | } | |
2066 | } | |
5bcae85e SL |
2067 | |
2068 | #[stable(feature = "cell_from", since = "1.12.0")] | |
3c0e092e XL |
2069 | #[rustc_const_unstable(feature = "const_convert", issue = "88674")] |
2070 | impl<T> const From<T> for UnsafeCell<T> { | |
5099ac24 | 2071 | /// Creates a new `UnsafeCell<T>` containing the given value. |
5bcae85e SL |
2072 | fn from(t: T) -> UnsafeCell<T> { |
2073 | UnsafeCell::new(t) | |
2074 | } | |
2075 | } | |
9e0c209e SL |
2076 | |
2077 | #[unstable(feature = "coerce_unsized", issue = "27732")] | |
2078 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<UnsafeCell<U>> for UnsafeCell<T> {} | |
2079 | ||
04454e1e FG |
2080 | /// [`UnsafeCell`], but [`Sync`]. |
2081 | /// | |
2082 | /// This is just an `UnsafeCell`, except it implements `Sync` | |
2083 | /// if `T` implements `Sync`. | |
2084 | /// | |
2085 | /// `UnsafeCell` doesn't implement `Sync`, to prevent accidental mis-use. | |
2086 | /// You can use `SyncUnsafeCell` instead of `UnsafeCell` to allow it to be | |
2087 | /// shared between threads, if that's intentional. | |
2088 | /// Providing proper synchronization is still the task of the user, | |
2089 | /// making this type just as unsafe to use. | |
2090 | /// | |
2091 | /// See [`UnsafeCell`] for details. | |
2092 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2093 | #[repr(transparent)] | |
2094 | pub struct SyncUnsafeCell<T: ?Sized> { | |
2095 | value: UnsafeCell<T>, | |
2096 | } | |
2097 | ||
2098 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2099 | unsafe impl<T: ?Sized + Sync> Sync for SyncUnsafeCell<T> {} | |
2100 | ||
2101 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2102 | impl<T> SyncUnsafeCell<T> { | |
2103 | /// Constructs a new instance of `SyncUnsafeCell` which will wrap the specified value. | |
2104 | #[inline] | |
2105 | pub const fn new(value: T) -> Self { | |
2106 | Self { value: UnsafeCell { value } } | |
2107 | } | |
2108 | ||
2109 | /// Unwraps the value. | |
2110 | #[inline] | |
2111 | pub const fn into_inner(self) -> T { | |
2112 | self.value.into_inner() | |
2113 | } | |
2114 | } | |
2115 | ||
2116 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2117 | impl<T: ?Sized> SyncUnsafeCell<T> { | |
2118 | /// Gets a mutable pointer to the wrapped value. | |
2119 | /// | |
2120 | /// This can be cast to a pointer of any kind. | |
2121 | /// Ensure that the access is unique (no active references, mutable or not) | |
2122 | /// when casting to `&mut T`, and ensure that there are no mutations | |
2123 | /// or mutable aliases going on when casting to `&T` | |
2124 | #[inline] | |
2125 | pub const fn get(&self) -> *mut T { | |
2126 | self.value.get() | |
2127 | } | |
2128 | ||
2129 | /// Returns a mutable reference to the underlying data. | |
2130 | /// | |
2131 | /// This call borrows the `SyncUnsafeCell` mutably (at compile-time) which | |
2132 | /// guarantees that we possess the only reference. | |
2133 | #[inline] | |
2134 | pub const fn get_mut(&mut self) -> &mut T { | |
2135 | self.value.get_mut() | |
2136 | } | |
2137 | ||
2138 | /// Gets a mutable pointer to the wrapped value. | |
2139 | /// | |
2140 | /// See [`UnsafeCell::get`] for details. | |
2141 | #[inline] | |
2142 | pub const fn raw_get(this: *const Self) -> *mut T { | |
2143 | // We can just cast the pointer from `SyncUnsafeCell<T>` to `T` because | |
2144 | // of #[repr(transparent)] on both SyncUnsafeCell and UnsafeCell. | |
2145 | // See UnsafeCell::raw_get. | |
2146 | this as *const T as *mut T | |
2147 | } | |
2148 | } | |
2149 | ||
2150 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2151 | impl<T: Default> Default for SyncUnsafeCell<T> { | |
2152 | /// Creates an `SyncUnsafeCell`, with the `Default` value for T. | |
2153 | fn default() -> SyncUnsafeCell<T> { | |
2154 | SyncUnsafeCell::new(Default::default()) | |
2155 | } | |
2156 | } | |
2157 | ||
2158 | #[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2159 | #[rustc_const_unstable(feature = "const_convert", issue = "88674")] | |
2160 | impl<T> const From<T> for SyncUnsafeCell<T> { | |
2161 | /// Creates a new `SyncUnsafeCell<T>` containing the given value. | |
2162 | fn from(t: T) -> SyncUnsafeCell<T> { | |
2163 | SyncUnsafeCell::new(t) | |
2164 | } | |
2165 | } | |
2166 | ||
2167 | #[unstable(feature = "coerce_unsized", issue = "27732")] | |
2168 | //#[unstable(feature = "sync_unsafe_cell", issue = "95439")] | |
2169 | impl<T: CoerceUnsized<U>, U> CoerceUnsized<SyncUnsafeCell<U>> for SyncUnsafeCell<T> {} | |
2170 | ||
9e0c209e | 2171 | #[allow(unused)] |
04454e1e FG |
2172 | fn assert_coerce_unsized( |
2173 | a: UnsafeCell<&i32>, | |
2174 | b: SyncUnsafeCell<&i32>, | |
2175 | c: Cell<&i32>, | |
2176 | d: RefCell<&i32>, | |
2177 | ) { | |
8faf50e0 | 2178 | let _: UnsafeCell<&dyn Send> = a; |
04454e1e FG |
2179 | let _: SyncUnsafeCell<&dyn Send> = b; |
2180 | let _: Cell<&dyn Send> = c; | |
2181 | let _: RefCell<&dyn Send> = d; | |
9e0c209e | 2182 | } |