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60c5eb7d XL |
1 | use crate::any::type_name; |
2 | use crate::fmt; | |
dc9dc135 XL |
3 | use crate::intrinsics; |
4 | use crate::mem::ManuallyDrop; | |
5 | ||
60c5eb7d XL |
6 | // ignore-tidy-undocumented-unsafe |
7 | ||
dc9dc135 XL |
8 | /// A wrapper type to construct uninitialized instances of `T`. |
9 | /// | |
10 | /// # Initialization invariant | |
11 | /// | |
e74abb32 XL |
12 | /// The compiler, in general, assumes that a variable is properly initialized |
13 | /// according to the requirements of the variable's type. For example, a variable of | |
14 | /// reference type must be aligned and non-NULL. This is an invariant that must | |
15 | /// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a | |
16 | /// variable of reference type causes instantaneous [undefined behavior][ub], | |
17 | /// no matter whether that reference ever gets used to access memory: | |
dc9dc135 XL |
18 | /// |
19 | /// ```rust,no_run | |
416331ca | 20 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
21 | /// use std::mem::{self, MaybeUninit}; |
22 | /// | |
23 | /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! | |
24 | /// // The equivalent code with `MaybeUninit<&i32>`: | |
25 | /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! | |
26 | /// ``` | |
27 | /// | |
28 | /// This is exploited by the compiler for various optimizations, such as eliding | |
29 | /// run-time checks and optimizing `enum` layout. | |
30 | /// | |
31 | /// Similarly, entirely uninitialized memory may have any content, while a `bool` must | |
32 | /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior: | |
33 | /// | |
34 | /// ```rust,no_run | |
416331ca | 35 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
36 | /// use std::mem::{self, MaybeUninit}; |
37 | /// | |
38 | /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! | |
39 | /// // The equivalent code with `MaybeUninit<bool>`: | |
40 | /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! | |
41 | /// ``` | |
42 | /// | |
43 | /// Moreover, uninitialized memory is special in that the compiler knows that | |
44 | /// it does not have a fixed value. This makes it undefined behavior to have | |
45 | /// uninitialized data in a variable even if that variable has an integer type, | |
46 | /// which otherwise can hold any *fixed* bit pattern: | |
47 | /// | |
48 | /// ```rust,no_run | |
416331ca | 49 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
50 | /// use std::mem::{self, MaybeUninit}; |
51 | /// | |
52 | /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! | |
53 | /// // The equivalent code with `MaybeUninit<i32>`: | |
54 | /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! | |
55 | /// ``` | |
56 | /// (Notice that the rules around uninitialized integers are not finalized yet, but | |
57 | /// until they are, it is advisable to avoid them.) | |
58 | /// | |
59 | /// On top of that, remember that most types have additional invariants beyond merely | |
60 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] | |
416331ca XL |
61 | /// is considered initialized (under the current implementation; this does not constitute |
62 | /// a stable guarantee) because the only requirement the compiler knows about it | |
dc9dc135 XL |
63 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause |
64 | /// *immediate* undefined behavior, but will cause undefined behavior with most | |
65 | /// safe operations (including dropping it). | |
66 | /// | |
67 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html | |
68 | /// | |
69 | /// # Examples | |
70 | /// | |
71 | /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data. | |
72 | /// It is a signal to the compiler indicating that the data here might *not* | |
73 | /// be initialized: | |
74 | /// | |
75 | /// ```rust | |
76 | /// use std::mem::MaybeUninit; | |
77 | /// | |
78 | /// // Create an explicitly uninitialized reference. The compiler knows that data inside | |
79 | /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB: | |
80 | /// let mut x = MaybeUninit::<&i32>::uninit(); | |
81 | /// // Set it to a valid value. | |
82 | /// unsafe { x.as_mut_ptr().write(&0); } | |
83 | /// // Extract the initialized data -- this is only allowed *after* properly | |
84 | /// // initializing `x`! | |
85 | /// let x = unsafe { x.assume_init() }; | |
86 | /// ``` | |
87 | /// | |
88 | /// The compiler then knows to not make any incorrect assumptions or optimizations on this code. | |
89 | /// | |
90 | /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without | |
91 | /// any of the run-time tracking and without any of the safety checks. | |
92 | /// | |
93 | /// ## out-pointers | |
94 | /// | |
95 | /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data | |
96 | /// from a function, pass it a pointer to some (uninitialized) memory to put the | |
97 | /// result into. This can be useful when it is important for the caller to control | |
98 | /// how the memory the result is stored in gets allocated, and you want to avoid | |
99 | /// unnecessary moves. | |
100 | /// | |
101 | /// ``` | |
102 | /// use std::mem::MaybeUninit; | |
103 | /// | |
104 | /// unsafe fn make_vec(out: *mut Vec<i32>) { | |
105 | /// // `write` does not drop the old contents, which is important. | |
106 | /// out.write(vec![1, 2, 3]); | |
107 | /// } | |
108 | /// | |
109 | /// let mut v = MaybeUninit::uninit(); | |
110 | /// unsafe { make_vec(v.as_mut_ptr()); } | |
111 | /// // Now we know `v` is initialized! This also makes sure the vector gets | |
112 | /// // properly dropped. | |
113 | /// let v = unsafe { v.assume_init() }; | |
114 | /// assert_eq!(&v, &[1, 2, 3]); | |
115 | /// ``` | |
116 | /// | |
117 | /// ## Initializing an array element-by-element | |
118 | /// | |
119 | /// `MaybeUninit<T>` can be used to initialize a large array element-by-element: | |
120 | /// | |
121 | /// ``` | |
122 | /// use std::mem::{self, MaybeUninit}; | |
dc9dc135 XL |
123 | /// |
124 | /// let data = { | |
125 | /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is | |
126 | /// // safe because the type we are claiming to have initialized here is a | |
127 | /// // bunch of `MaybeUninit`s, which do not require initialization. | |
128 | /// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe { | |
129 | /// MaybeUninit::uninit().assume_init() | |
130 | /// }; | |
131 | /// | |
416331ca XL |
132 | /// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer |
133 | /// // assignment instead of `ptr::write` does not cause the old | |
134 | /// // uninitialized value to be dropped. Also if there is a panic during | |
135 | /// // this loop, we have a memory leak, but there is no memory safety | |
136 | /// // issue. | |
dc9dc135 | 137 | /// for elem in &mut data[..] { |
416331ca | 138 | /// *elem = MaybeUninit::new(vec![42]); |
dc9dc135 XL |
139 | /// } |
140 | /// | |
141 | /// // Everything is initialized. Transmute the array to the | |
142 | /// // initialized type. | |
143 | /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) } | |
144 | /// }; | |
145 | /// | |
146 | /// assert_eq!(&data[0], &[42]); | |
147 | /// ``` | |
148 | /// | |
149 | /// You can also work with partially initialized arrays, which could | |
150 | /// be found in low-level datastructures. | |
151 | /// | |
152 | /// ``` | |
153 | /// use std::mem::MaybeUninit; | |
154 | /// use std::ptr; | |
155 | /// | |
156 | /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is | |
157 | /// // safe because the type we are claiming to have initialized here is a | |
158 | /// // bunch of `MaybeUninit`s, which do not require initialization. | |
159 | /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() }; | |
160 | /// // Count the number of elements we have assigned. | |
161 | /// let mut data_len: usize = 0; | |
162 | /// | |
163 | /// for elem in &mut data[0..500] { | |
416331ca | 164 | /// *elem = MaybeUninit::new(String::from("hello")); |
dc9dc135 XL |
165 | /// data_len += 1; |
166 | /// } | |
167 | /// | |
168 | /// // For each item in the array, drop if we allocated it. | |
169 | /// for elem in &mut data[0..data_len] { | |
170 | /// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); } | |
171 | /// } | |
172 | /// ``` | |
173 | /// | |
174 | /// ## Initializing a struct field-by-field | |
175 | /// | |
176 | /// There is currently no supported way to create a raw pointer or reference | |
177 | /// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible | |
178 | /// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing | |
179 | /// to its fields. | |
180 | /// | |
181 | /// [ub]: ../../reference/behavior-considered-undefined.html | |
182 | /// | |
183 | /// # Layout | |
184 | /// | |
185 | /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: | |
186 | /// | |
187 | /// ```rust | |
188 | /// use std::mem::{MaybeUninit, size_of, align_of}; | |
189 | /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>()); | |
190 | /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>()); | |
191 | /// ``` | |
192 | /// | |
193 | /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same | |
194 | /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as | |
195 | /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit | |
196 | /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling | |
197 | /// optimizations, potentially resulting in a larger size: | |
198 | /// | |
199 | /// ```rust | |
200 | /// # use std::mem::{MaybeUninit, size_of}; | |
201 | /// assert_eq!(size_of::<Option<bool>>(), 1); | |
202 | /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); | |
203 | /// ``` | |
204 | /// | |
205 | /// If `T` is FFI-safe, then so is `MaybeUninit<T>`. | |
206 | /// | |
207 | /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, | |
208 | /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and | |
209 | /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type | |
210 | /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. | |
211 | /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the | |
212 | /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact | |
213 | /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not | |
214 | /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has | |
215 | /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that | |
216 | /// guarantee may evolve. | |
217 | #[allow(missing_debug_implementations)] | |
218 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
416331ca | 219 | // Lang item so we can wrap other types in it. This is useful for generators. |
e1599b0c | 220 | #[lang = "maybe_uninit"] |
dc9dc135 | 221 | #[derive(Copy)] |
416331ca | 222 | #[repr(transparent)] |
dc9dc135 XL |
223 | pub union MaybeUninit<T> { |
224 | uninit: (), | |
225 | value: ManuallyDrop<T>, | |
226 | } | |
227 | ||
228 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
229 | impl<T: Copy> Clone for MaybeUninit<T> { | |
230 | #[inline(always)] | |
231 | fn clone(&self) -> Self { | |
232 | // Not calling `T::clone()`, we cannot know if we are initialized enough for that. | |
233 | *self | |
234 | } | |
235 | } | |
236 | ||
60c5eb7d XL |
237 | #[stable(feature = "maybe_uninit_debug", since = "1.41.0")] |
238 | impl<T> fmt::Debug for MaybeUninit<T> { | |
239 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
240 | f.pad(type_name::<Self>()) | |
241 | } | |
242 | } | |
243 | ||
dc9dc135 XL |
244 | impl<T> MaybeUninit<T> { |
245 | /// Creates a new `MaybeUninit<T>` initialized with the given value. | |
246 | /// It is safe to call [`assume_init`] on the return value of this function. | |
247 | /// | |
248 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
249 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
250 | /// | |
251 | /// [`assume_init`]: #method.assume_init | |
252 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
60c5eb7d XL |
253 | #[cfg_attr( |
254 | not(bootstrap), | |
255 | rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0"), | |
256 | )] | |
dc9dc135 XL |
257 | #[inline(always)] |
258 | pub const fn new(val: T) -> MaybeUninit<T> { | |
259 | MaybeUninit { value: ManuallyDrop::new(val) } | |
260 | } | |
261 | ||
262 | /// Creates a new `MaybeUninit<T>` in an uninitialized state. | |
263 | /// | |
264 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
265 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
266 | /// | |
267 | /// See the [type-level documentation][type] for some examples. | |
268 | /// | |
269 | /// [type]: union.MaybeUninit.html | |
270 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
60c5eb7d XL |
271 | #[cfg_attr( |
272 | not(bootstrap), | |
273 | rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0"), | |
274 | )] | |
dc9dc135 | 275 | #[inline(always)] |
60c5eb7d | 276 | #[cfg_attr(all(not(bootstrap)), rustc_diagnostic_item = "maybe_uninit_uninit")] |
dc9dc135 XL |
277 | pub const fn uninit() -> MaybeUninit<T> { |
278 | MaybeUninit { uninit: () } | |
279 | } | |
280 | ||
60c5eb7d XL |
281 | /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state. |
282 | /// | |
283 | /// Note: in a future Rust version this method may become unnecessary | |
284 | /// when array literal syntax allows | |
285 | /// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147). | |
286 | /// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`. | |
287 | /// | |
288 | /// # Examples | |
289 | /// | |
290 | /// ```no_run | |
291 | /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice_assume_init)] | |
292 | /// | |
293 | /// use std::mem::MaybeUninit; | |
294 | /// | |
295 | /// extern "C" { | |
296 | /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize; | |
297 | /// } | |
298 | /// | |
299 | /// /// Returns a (possibly smaller) slice of data that was actually read | |
300 | /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] { | |
301 | /// unsafe { | |
302 | /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len()); | |
303 | /// MaybeUninit::slice_get_ref(&buf[..len]) | |
304 | /// } | |
305 | /// } | |
306 | /// | |
307 | /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array(); | |
308 | /// let data = read(&mut buf); | |
309 | /// ``` | |
310 | #[unstable(feature = "maybe_uninit_uninit_array", issue = "0")] | |
311 | #[inline(always)] | |
312 | pub fn uninit_array<const LEN: usize>() -> [Self; LEN] { | |
313 | unsafe { | |
314 | MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() | |
315 | } | |
316 | } | |
317 | ||
416331ca XL |
318 | /// A promotable constant, equivalent to `uninit()`. |
319 | #[unstable(feature = "internal_uninit_const", issue = "0", | |
320 | reason = "hack to work around promotability")] | |
321 | pub const UNINIT: Self = Self::uninit(); | |
322 | ||
dc9dc135 XL |
323 | /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being |
324 | /// filled with `0` bytes. It depends on `T` whether that already makes for | |
325 | /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, | |
326 | /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not | |
327 | /// be null. | |
328 | /// | |
329 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
330 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
331 | /// | |
332 | /// # Example | |
333 | /// | |
334 | /// Correct usage of this function: initializing a struct with zero, where all | |
335 | /// fields of the struct can hold the bit-pattern 0 as a valid value. | |
336 | /// | |
337 | /// ```rust | |
338 | /// use std::mem::MaybeUninit; | |
339 | /// | |
340 | /// let x = MaybeUninit::<(u8, bool)>::zeroed(); | |
341 | /// let x = unsafe { x.assume_init() }; | |
342 | /// assert_eq!(x, (0, false)); | |
343 | /// ``` | |
344 | /// | |
345 | /// *Incorrect* usage of this function: initializing a struct with zero, where some fields | |
346 | /// cannot hold 0 as a valid value. | |
347 | /// | |
348 | /// ```rust,no_run | |
349 | /// use std::mem::MaybeUninit; | |
350 | /// | |
351 | /// enum NotZero { One = 1, Two = 2 }; | |
352 | /// | |
353 | /// let x = MaybeUninit::<(u8, NotZero)>::zeroed(); | |
354 | /// let x = unsafe { x.assume_init() }; | |
355 | /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. | |
356 | /// // This is undefined behavior. | |
357 | /// ``` | |
358 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
359 | #[inline] | |
60c5eb7d | 360 | #[cfg_attr(all(not(bootstrap)), rustc_diagnostic_item = "maybe_uninit_zeroed")] |
dc9dc135 XL |
361 | pub fn zeroed() -> MaybeUninit<T> { |
362 | let mut u = MaybeUninit::<T>::uninit(); | |
363 | unsafe { | |
364 | u.as_mut_ptr().write_bytes(0u8, 1); | |
365 | } | |
366 | u | |
367 | } | |
368 | ||
369 | /// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value | |
370 | /// without dropping it, so be careful not to use this twice unless you want to | |
371 | /// skip running the destructor. For your convenience, this also returns a mutable | |
372 | /// reference to the (now safely initialized) contents of `self`. | |
e1599b0c | 373 | #[unstable(feature = "maybe_uninit_extra", issue = "63567")] |
dc9dc135 XL |
374 | #[inline(always)] |
375 | pub fn write(&mut self, val: T) -> &mut T { | |
376 | unsafe { | |
377 | self.value = ManuallyDrop::new(val); | |
378 | self.get_mut() | |
379 | } | |
380 | } | |
381 | ||
382 | /// Gets a pointer to the contained value. Reading from this pointer or turning it | |
383 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
384 | /// Writing to memory that this pointer (non-transitively) points to is undefined behavior | |
385 | /// (except inside an `UnsafeCell<T>`). | |
386 | /// | |
387 | /// # Examples | |
388 | /// | |
389 | /// Correct usage of this method: | |
390 | /// | |
391 | /// ```rust | |
392 | /// use std::mem::MaybeUninit; | |
393 | /// | |
394 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
395 | /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); } | |
396 | /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it. | |
397 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
398 | /// assert_eq!(x_vec.len(), 3); | |
399 | /// ``` | |
400 | /// | |
401 | /// *Incorrect* usage of this method: | |
402 | /// | |
403 | /// ```rust,no_run | |
404 | /// use std::mem::MaybeUninit; | |
405 | /// | |
406 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
407 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
408 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. | |
409 | /// ``` | |
410 | /// | |
411 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
412 | /// until they are, it is advisable to avoid them.) | |
413 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
414 | #[inline(always)] | |
415 | pub fn as_ptr(&self) -> *const T { | |
416 | unsafe { &*self.value as *const T } | |
417 | } | |
418 | ||
419 | /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it | |
420 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
421 | /// | |
422 | /// # Examples | |
423 | /// | |
424 | /// Correct usage of this method: | |
425 | /// | |
426 | /// ```rust | |
427 | /// use std::mem::MaybeUninit; | |
428 | /// | |
429 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
430 | /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); } | |
431 | /// // Create a reference into the `MaybeUninit<Vec<u32>>`. | |
432 | /// // This is okay because we initialized it. | |
433 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
434 | /// x_vec.push(3); | |
435 | /// assert_eq!(x_vec.len(), 4); | |
436 | /// ``` | |
437 | /// | |
438 | /// *Incorrect* usage of this method: | |
439 | /// | |
440 | /// ```rust,no_run | |
441 | /// use std::mem::MaybeUninit; | |
442 | /// | |
443 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
444 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
445 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. | |
446 | /// ``` | |
447 | /// | |
448 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
449 | /// until they are, it is advisable to avoid them.) | |
450 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
451 | #[inline(always)] | |
452 | pub fn as_mut_ptr(&mut self) -> *mut T { | |
453 | unsafe { &mut *self.value as *mut T } | |
454 | } | |
455 | ||
456 | /// Extracts the value from the `MaybeUninit<T>` container. This is a great way | |
457 | /// to ensure that the data will get dropped, because the resulting `T` is | |
458 | /// subject to the usual drop handling. | |
459 | /// | |
460 | /// # Safety | |
461 | /// | |
462 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
463 | /// state. Calling this when the content is not yet fully initialized causes immediate undefined | |
464 | /// behavior. The [type-level documentation][inv] contains more information about | |
465 | /// this initialization invariant. | |
466 | /// | |
467 | /// [inv]: #initialization-invariant | |
468 | /// | |
416331ca XL |
469 | /// On top of that, remember that most types have additional invariants beyond merely |
470 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] | |
471 | /// is considered initialized (under the current implementation; this does not constitute | |
472 | /// a stable guarantee) because the only requirement the compiler knows about it | |
473 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause | |
474 | /// *immediate* undefined behavior, but will cause undefined behavior with most | |
475 | /// safe operations (including dropping it). | |
476 | /// | |
dc9dc135 XL |
477 | /// # Examples |
478 | /// | |
479 | /// Correct usage of this method: | |
480 | /// | |
481 | /// ```rust | |
482 | /// use std::mem::MaybeUninit; | |
483 | /// | |
484 | /// let mut x = MaybeUninit::<bool>::uninit(); | |
485 | /// unsafe { x.as_mut_ptr().write(true); } | |
486 | /// let x_init = unsafe { x.assume_init() }; | |
487 | /// assert_eq!(x_init, true); | |
488 | /// ``` | |
489 | /// | |
490 | /// *Incorrect* usage of this method: | |
491 | /// | |
492 | /// ```rust,no_run | |
493 | /// use std::mem::MaybeUninit; | |
494 | /// | |
495 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
496 | /// let x_init = unsafe { x.assume_init() }; | |
497 | /// // `x` had not been initialized yet, so this last line caused undefined behavior. | |
498 | /// ``` | |
499 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
500 | #[inline(always)] | |
60c5eb7d | 501 | #[cfg_attr(all(not(bootstrap)), rustc_diagnostic_item = "assume_init")] |
dc9dc135 XL |
502 | pub unsafe fn assume_init(self) -> T { |
503 | intrinsics::panic_if_uninhabited::<T>(); | |
504 | ManuallyDrop::into_inner(self.value) | |
505 | } | |
506 | ||
507 | /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject | |
508 | /// to the usual drop handling. | |
509 | /// | |
416331ca | 510 | /// Whenever possible, it is preferable to use [`assume_init`] instead, which |
dc9dc135 XL |
511 | /// prevents duplicating the content of the `MaybeUninit<T>`. |
512 | /// | |
513 | /// # Safety | |
514 | /// | |
515 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
516 | /// state. Calling this when the content is not yet fully initialized causes undefined | |
517 | /// behavior. The [type-level documentation][inv] contains more information about | |
518 | /// this initialization invariant. | |
519 | /// | |
520 | /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using | |
521 | /// multiple copies of the data (by calling `read` multiple times, or first | |
522 | /// calling `read` and then [`assume_init`]), it is your responsibility | |
523 | /// to ensure that that data may indeed be duplicated. | |
524 | /// | |
525 | /// [inv]: #initialization-invariant | |
526 | /// [`assume_init`]: #method.assume_init | |
527 | /// | |
528 | /// # Examples | |
529 | /// | |
530 | /// Correct usage of this method: | |
531 | /// | |
532 | /// ```rust | |
533 | /// #![feature(maybe_uninit_extra)] | |
534 | /// use std::mem::MaybeUninit; | |
535 | /// | |
536 | /// let mut x = MaybeUninit::<u32>::uninit(); | |
537 | /// x.write(13); | |
538 | /// let x1 = unsafe { x.read() }; | |
539 | /// // `u32` is `Copy`, so we may read multiple times. | |
540 | /// let x2 = unsafe { x.read() }; | |
541 | /// assert_eq!(x1, x2); | |
542 | /// | |
543 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
544 | /// x.write(None); | |
545 | /// let x1 = unsafe { x.read() }; | |
546 | /// // Duplicating a `None` value is okay, so we may read multiple times. | |
547 | /// let x2 = unsafe { x.read() }; | |
548 | /// assert_eq!(x1, x2); | |
549 | /// ``` | |
550 | /// | |
551 | /// *Incorrect* usage of this method: | |
552 | /// | |
553 | /// ```rust,no_run | |
554 | /// #![feature(maybe_uninit_extra)] | |
555 | /// use std::mem::MaybeUninit; | |
556 | /// | |
557 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
558 | /// x.write(Some(vec![0,1,2])); | |
559 | /// let x1 = unsafe { x.read() }; | |
560 | /// let x2 = unsafe { x.read() }; | |
561 | /// // We now created two copies of the same vector, leading to a double-free when | |
562 | /// // they both get dropped! | |
563 | /// ``` | |
e1599b0c | 564 | #[unstable(feature = "maybe_uninit_extra", issue = "63567")] |
dc9dc135 XL |
565 | #[inline(always)] |
566 | pub unsafe fn read(&self) -> T { | |
567 | intrinsics::panic_if_uninhabited::<T>(); | |
568 | self.as_ptr().read() | |
569 | } | |
570 | ||
60c5eb7d XL |
571 | /// Gets a shared reference to the contained value. |
572 | /// | |
573 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
574 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
575 | /// of `.assume_init()`). | |
dc9dc135 XL |
576 | /// |
577 | /// # Safety | |
578 | /// | |
60c5eb7d XL |
579 | /// Calling this when the content is not yet fully initialized causes undefined |
580 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
581 | /// is in an initialized state. | |
582 | /// | |
583 | /// # Examples | |
584 | /// | |
585 | /// ### Correct usage of this method: | |
586 | /// | |
587 | /// ```rust | |
588 | /// #![feature(maybe_uninit_ref)] | |
589 | /// use std::mem::MaybeUninit; | |
590 | /// | |
591 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
592 | /// // Initialize `x`: | |
593 | /// unsafe { x.as_mut_ptr().write(vec![1, 2, 3]); } | |
594 | /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to | |
595 | /// // create a shared reference to it: | |
596 | /// let x: &Vec<u32> = unsafe { | |
597 | /// // Safety: `x` has been initialized. | |
598 | /// x.get_ref() | |
599 | /// }; | |
600 | /// assert_eq!(x, &vec![1, 2, 3]); | |
601 | /// ``` | |
602 | /// | |
603 | /// ### *Incorrect* usages of this method: | |
604 | /// | |
605 | /// ```rust,no_run | |
606 | /// #![feature(maybe_uninit_ref)] | |
607 | /// use std::mem::MaybeUninit; | |
608 | /// | |
609 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
610 | /// let x_vec: &Vec<u32> = unsafe { x.get_ref() }; | |
611 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. | |
612 | /// ``` | |
613 | /// | |
614 | /// ```rust,no_run | |
615 | /// #![feature(maybe_uninit_ref)] | |
616 | /// use std::{cell::Cell, mem::MaybeUninit}; | |
617 | /// | |
618 | /// let b = MaybeUninit::<Cell<bool>>::uninit(); | |
619 | /// // Initialize the `MaybeUninit` using `Cell::set`: | |
620 | /// unsafe { | |
621 | /// b.get_ref().set(true); | |
622 | /// // ^^^^^^^^^^^ | |
623 | /// // Reference to an uninitialized `Cell<bool>`: UB! | |
624 | /// } | |
625 | /// ``` | |
e1599b0c | 626 | #[unstable(feature = "maybe_uninit_ref", issue = "63568")] |
dc9dc135 XL |
627 | #[inline(always)] |
628 | pub unsafe fn get_ref(&self) -> &T { | |
60c5eb7d | 629 | intrinsics::panic_if_uninhabited::<T>(); |
dc9dc135 XL |
630 | &*self.value |
631 | } | |
632 | ||
60c5eb7d XL |
633 | /// Gets a mutable (unique) reference to the contained value. |
634 | /// | |
635 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
636 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
637 | /// of `.assume_init()`). | |
dc9dc135 XL |
638 | /// |
639 | /// # Safety | |
640 | /// | |
60c5eb7d XL |
641 | /// Calling this when the content is not yet fully initialized causes undefined |
642 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
643 | /// is in an initialized state. For instance, `.get_mut()` cannot be used to | |
644 | /// initialize a `MaybeUninit`. | |
645 | /// | |
646 | /// # Examples | |
647 | /// | |
648 | /// ### Correct usage of this method: | |
649 | /// | |
650 | /// ```rust | |
651 | /// #![feature(maybe_uninit_ref)] | |
652 | /// use std::mem::MaybeUninit; | |
653 | /// | |
654 | /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 2048]) { *buf = [0; 2048] } | |
655 | /// # #[cfg(FALSE)] | |
656 | /// extern "C" { | |
657 | /// /// Initializes *all* the bytes of the input buffer. | |
658 | /// fn initialize_buffer(buf: *mut [u8; 2048]); | |
659 | /// } | |
660 | /// | |
661 | /// let mut buf = MaybeUninit::<[u8; 2048]>::uninit(); | |
662 | /// | |
663 | /// // Initialize `buf`: | |
664 | /// unsafe { initialize_buffer(buf.as_mut_ptr()); } | |
665 | /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it. | |
666 | /// // However, using `.assume_init()` may trigger a `memcpy` of the 2048 bytes. | |
667 | /// // To assert our buffer has been initialized without copying it, we upgrade | |
668 | /// // the `&mut MaybeUninit<[u8; 2048]>` to a `&mut [u8; 2048]`: | |
669 | /// let buf: &mut [u8; 2048] = unsafe { | |
670 | /// // Safety: `buf` has been initialized. | |
671 | /// buf.get_mut() | |
672 | /// }; | |
673 | /// | |
674 | /// // Now we can use `buf` as a normal slice: | |
675 | /// buf.sort_unstable(); | |
676 | /// assert!( | |
677 | /// buf.chunks(2).all(|chunk| chunk[0] <= chunk[1]), | |
678 | /// "buffer is sorted", | |
679 | /// ); | |
680 | /// ``` | |
681 | /// | |
682 | /// ### *Incorrect* usages of this method: | |
683 | /// | |
684 | /// You cannot use `.get_mut()` to initialize a value: | |
685 | /// | |
686 | /// ```rust,no_run | |
687 | /// #![feature(maybe_uninit_ref)] | |
688 | /// use std::mem::MaybeUninit; | |
689 | /// | |
690 | /// let mut b = MaybeUninit::<bool>::uninit(); | |
691 | /// unsafe { | |
692 | /// *b.get_mut() = true; | |
693 | /// // We have created a (mutable) reference to an uninitialized `bool`! | |
694 | /// // This is undefined behavior. | |
695 | /// } | |
696 | /// ``` | |
697 | /// | |
698 | /// For instance, you cannot [`Read`] into an uninitialized buffer: | |
699 | /// | |
700 | /// [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html | |
701 | /// | |
702 | /// ```rust,no_run | |
703 | /// #![feature(maybe_uninit_ref)] | |
704 | /// use std::{io, mem::MaybeUninit}; | |
705 | /// | |
706 | /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]> | |
707 | /// { | |
708 | /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit(); | |
709 | /// reader.read_exact(unsafe { buffer.get_mut() })?; | |
710 | /// // ^^^^^^^^^^^^^^^^ | |
711 | /// // (mutable) reference to uninitialized memory! | |
712 | /// // This is undefined behavior. | |
713 | /// Ok(unsafe { buffer.assume_init() }) | |
714 | /// } | |
715 | /// ``` | |
716 | /// | |
717 | /// Nor can you use direct field access to do field-by-field gradual initialization: | |
718 | /// | |
719 | /// ```rust,no_run | |
720 | /// #![feature(maybe_uninit_ref)] | |
721 | /// use std::{mem::MaybeUninit, ptr}; | |
722 | /// | |
723 | /// struct Foo { | |
724 | /// a: u32, | |
725 | /// b: u8, | |
726 | /// } | |
727 | /// | |
728 | /// let foo: Foo = unsafe { | |
729 | /// let mut foo = MaybeUninit::<Foo>::uninit(); | |
730 | /// ptr::write(&mut foo.get_mut().a as *mut u32, 1337); | |
731 | /// // ^^^^^^^^^^^^^ | |
732 | /// // (mutable) reference to uninitialized memory! | |
733 | /// // This is undefined behavior. | |
734 | /// ptr::write(&mut foo.get_mut().b as *mut u8, 42); | |
735 | /// // ^^^^^^^^^^^^^ | |
736 | /// // (mutable) reference to uninitialized memory! | |
737 | /// // This is undefined behavior. | |
738 | /// foo.assume_init() | |
739 | /// }; | |
740 | /// ``` | |
dc9dc135 XL |
741 | // FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references |
742 | // to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make | |
743 | // a final decision about the rules before stabilization. | |
e1599b0c | 744 | #[unstable(feature = "maybe_uninit_ref", issue = "63568")] |
dc9dc135 XL |
745 | #[inline(always)] |
746 | pub unsafe fn get_mut(&mut self) -> &mut T { | |
60c5eb7d | 747 | intrinsics::panic_if_uninhabited::<T>(); |
dc9dc135 XL |
748 | &mut *self.value |
749 | } | |
750 | ||
60c5eb7d XL |
751 | /// Assuming all the elements are initialized, get a slice to them. |
752 | /// | |
753 | /// # Safety | |
754 | /// | |
755 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
756 | /// really are in an initialized state. | |
757 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
758 | #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "0")] | |
759 | #[inline(always)] | |
760 | pub unsafe fn slice_get_ref(slice: &[Self]) -> &[T] { | |
761 | &*(slice as *const [Self] as *const [T]) | |
762 | } | |
763 | ||
764 | /// Assuming all the elements are initialized, get a mutable slice to them. | |
765 | /// | |
766 | /// # Safety | |
767 | /// | |
768 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
769 | /// really are in an initialized state. | |
770 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
771 | #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "0")] | |
772 | #[inline(always)] | |
773 | pub unsafe fn slice_get_mut(slice: &mut [Self]) -> &mut [T] { | |
774 | &mut *(slice as *mut [Self] as *mut [T]) | |
775 | } | |
776 | ||
dc9dc135 | 777 | /// Gets a pointer to the first element of the array. |
e1599b0c | 778 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 XL |
779 | #[inline(always)] |
780 | pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T { | |
781 | this as *const [MaybeUninit<T>] as *const T | |
782 | } | |
783 | ||
784 | /// Gets a mutable pointer to the first element of the array. | |
e1599b0c | 785 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 XL |
786 | #[inline(always)] |
787 | pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T { | |
788 | this as *mut [MaybeUninit<T>] as *mut T | |
789 | } | |
790 | } |