1 | use crate::any::type_name; |
2 | use crate::mem::ManuallyDrop; |
3 | use crate::{fmt, intrinsics, ptr, slice}; |
4 | |
5 | /// A wrapper type to construct uninitialized instances of `T`. |
6 | /// |
7 | /// # Initialization invariant |
8 | /// |
9 | /// The compiler, in general, assumes that a variable is properly initialized |
10 | /// according to the requirements of the variable's type. For example, a variable of |
11 | /// reference type must be aligned and non-null. This is an invariant that must |
12 | /// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a |
13 | /// variable of reference type causes instantaneous [undefined behavior][ub], |
14 | /// no matter whether that reference ever gets used to access memory: |
15 | /// |
16 | /// ```rust,no_run |
17 | /// # #![allow (invalid_value)] |
18 | /// use std::mem::{self, MaybeUninit}; |
19 | /// |
20 | /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️ |
21 | /// // The equivalent code with `MaybeUninit<&i32>`: |
22 | /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️ |
23 | /// ``` |
24 | /// |
25 | /// This is exploited by the compiler for various optimizations, such as eliding |
26 | /// run-time checks and optimizing `enum` layout. |
27 | /// |
28 | /// Similarly, entirely uninitialized memory may have any content, while a `bool` must |
29 | /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior: |
30 | /// |
31 | /// ```rust,no_run |
32 | /// # #![allow(invalid_value)] |
33 | /// use std::mem::{self, MaybeUninit}; |
34 | /// |
35 | /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
36 | /// // The equivalent code with `MaybeUninit<bool>`: |
37 | /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
38 | /// ``` |
39 | /// |
40 | /// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed" |
41 | /// meaning "it won't change without being written to"). Reading the same uninitialized byte |
42 | /// multiple times can give different results. This makes it undefined behavior to have |
43 | /// uninitialized data in a variable even if that variable has an integer type, which otherwise can |
44 | /// hold any *fixed* bit pattern: |
45 | /// |
46 | /// ```rust,no_run |
47 | /// # #![allow(invalid_value)] |
48 | /// use std::mem::{self, MaybeUninit}; |
49 | /// |
50 | /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
51 | /// // The equivalent code with `MaybeUninit<i32>`: |
52 | /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
53 | /// ``` |
54 | /// On top of that, remember that most types have additional invariants beyond merely |
55 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] |
56 | /// is considered initialized (under the current implementation; this does not constitute |
57 | /// a stable guarantee) because the only requirement the compiler knows about it |
58 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause |
59 | /// *immediate* undefined behavior, but will cause undefined behavior with most |
60 | /// safe operations (including dropping it). |
61 | /// |
62 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html |
63 | /// |
64 | /// # Examples |
65 | /// |
66 | /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data. |
67 | /// It is a signal to the compiler indicating that the data here might *not* |
68 | /// be initialized: |
69 | /// |
70 | /// ```rust |
71 | /// use std::mem::MaybeUninit; |
72 | /// |
73 | /// // Create an explicitly uninitialized reference. The compiler knows that data inside |
74 | /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB: |
75 | /// let mut x = MaybeUninit::<&i32>::uninit(); |
76 | /// // Set it to a valid value. |
77 | /// x.write(&0); |
78 | /// // Extract the initialized data -- this is only allowed *after* properly |
79 | /// // initializing `x`! |
80 | /// let x = unsafe { x.assume_init() }; |
81 | /// ``` |
82 | /// |
83 | /// The compiler then knows to not make any incorrect assumptions or optimizations on this code. |
84 | /// |
85 | /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without |
86 | /// any of the run-time tracking and without any of the safety checks. |
87 | /// |
88 | /// ## out-pointers |
89 | /// |
90 | /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data |
91 | /// from a function, pass it a pointer to some (uninitialized) memory to put the |
92 | /// result into. This can be useful when it is important for the caller to control |
93 | /// how the memory the result is stored in gets allocated, and you want to avoid |
94 | /// unnecessary moves. |
95 | /// |
96 | /// ``` |
97 | /// use std::mem::MaybeUninit; |
98 | /// |
99 | /// unsafe fn make_vec(out: *mut Vec<i32>) { |
100 | /// // `write` does not drop the old contents, which is important. |
101 | /// unsafe { out.write(vec![1, 2, 3]); } |
102 | /// } |
103 | /// |
104 | /// let mut v = MaybeUninit::uninit(); |
105 | /// unsafe { make_vec(v.as_mut_ptr()); } |
106 | /// // Now we know `v` is initialized! This also makes sure the vector gets |
107 | /// // properly dropped. |
108 | /// let v = unsafe { v.assume_init() }; |
109 | /// assert_eq!(&v, &[1, 2, 3]); |
110 | /// ``` |
111 | /// |
112 | /// ## Initializing an array element-by-element |
113 | /// |
114 | /// `MaybeUninit<T>` can be used to initialize a large array element-by-element: |
115 | /// |
116 | /// ``` |
117 | /// use std::mem::{self, MaybeUninit}; |
118 | /// |
119 | /// let data = { |
120 | /// // Create an uninitialized array of `MaybeUninit`. |
121 | /// let mut data: [MaybeUninit<Vec<u32>>; 1000] = [const { MaybeUninit::uninit() }; 1000]; |
122 | /// |
123 | /// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop, |
124 | /// // we have a memory leak, but there is no memory safety issue. |
125 | /// for elem in &mut data[..] { |
126 | /// elem.write(vec![42]); |
127 | /// } |
128 | /// |
129 | /// // Everything is initialized. Transmute the array to the |
130 | /// // initialized type. |
131 | /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) } |
132 | /// }; |
133 | /// |
134 | /// assert_eq!(&data[0], &[42]); |
135 | /// ``` |
136 | /// |
137 | /// You can also work with partially initialized arrays, which could |
138 | /// be found in low-level datastructures. |
139 | /// |
140 | /// ``` |
141 | /// use std::mem::MaybeUninit; |
142 | /// |
143 | /// // Create an uninitialized array of `MaybeUninit`. |
144 | /// let mut data: [MaybeUninit<String>; 1000] = [const { MaybeUninit::uninit() }; 1000]; |
145 | /// // Count the number of elements we have assigned. |
146 | /// let mut data_len: usize = 0; |
147 | /// |
148 | /// for elem in &mut data[0..500] { |
149 | /// elem.write(String::from("hello" )); |
150 | /// data_len += 1; |
151 | /// } |
152 | /// |
153 | /// // For each item in the array, drop if we allocated it. |
154 | /// for elem in &mut data[0..data_len] { |
155 | /// unsafe { elem.assume_init_drop(); } |
156 | /// } |
157 | /// ``` |
158 | /// |
159 | /// ## Initializing a struct field-by-field |
160 | /// |
161 | /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field: |
162 | /// |
163 | /// ```rust |
164 | /// use std::mem::MaybeUninit; |
165 | /// use std::ptr::addr_of_mut; |
166 | /// |
167 | /// #[derive(Debug, PartialEq)] |
168 | /// pub struct Foo { |
169 | /// name: String, |
170 | /// list: Vec<u8>, |
171 | /// } |
172 | /// |
173 | /// let foo = { |
174 | /// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit(); |
175 | /// let ptr = uninit.as_mut_ptr(); |
176 | /// |
177 | /// // Initializing the `name` field |
178 | /// // Using `write` instead of assignment via `=` to not call `drop` on the |
179 | /// // old, uninitialized value. |
180 | /// unsafe { addr_of_mut!((*ptr).name).write("Bob" .to_string()); } |
181 | /// |
182 | /// // Initializing the `list` field |
183 | /// // If there is a panic here, then the `String` in the `name` field leaks. |
184 | /// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); } |
185 | /// |
186 | /// // All the fields are initialized, so we call `assume_init` to get an initialized Foo. |
187 | /// unsafe { uninit.assume_init() } |
188 | /// }; |
189 | /// |
190 | /// assert_eq!( |
191 | /// foo, |
192 | /// Foo { |
193 | /// name: "Bob" .to_string(), |
194 | /// list: vec![0, 1, 2] |
195 | /// } |
196 | /// ); |
197 | /// ``` |
198 | /// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut |
199 | /// [ub]: ../../reference/behavior-considered-undefined.html |
200 | /// |
201 | /// # Layout |
202 | /// |
203 | /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: |
204 | /// |
205 | /// ```rust |
206 | /// use std::mem::MaybeUninit; |
207 | /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>()); |
208 | /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>()); |
209 | /// ``` |
210 | /// |
211 | /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same |
212 | /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as |
213 | /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit |
214 | /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling |
215 | /// optimizations, potentially resulting in a larger size: |
216 | /// |
217 | /// ```rust |
218 | /// # use std::mem::MaybeUninit; |
219 | /// assert_eq!(size_of::<Option<bool>>(), 1); |
220 | /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); |
221 | /// ``` |
222 | /// |
223 | /// If `T` is FFI-safe, then so is `MaybeUninit<T>`. |
224 | /// |
225 | /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, |
226 | /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and |
227 | /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type |
228 | /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. |
229 | /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the |
230 | /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact |
231 | /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not |
232 | /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has |
233 | /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that |
234 | /// guarantee may evolve. |
235 | /// |
236 | /// Note that even though `T` and `MaybeUninit<T>` are ABI compatible it is still unsound to |
237 | /// transmute `&mut T` to `&mut MaybeUninit<T>` and expose that to safe code because it would allow |
238 | /// safe code to access uninitialized memory: |
239 | /// |
240 | /// ```rust,no_run |
241 | /// use core::mem::MaybeUninit; |
242 | /// |
243 | /// fn unsound_transmute<T>(val: &mut T) -> &mut MaybeUninit<T> { |
244 | /// unsafe { core::mem::transmute(val) } |
245 | /// } |
246 | /// |
247 | /// fn main() { |
248 | /// let mut code = 0; |
249 | /// let code = &mut code; |
250 | /// let code2 = unsound_transmute(code); |
251 | /// *code2 = MaybeUninit::uninit(); |
252 | /// std::process::exit(*code); // UB! Accessing uninitialized memory. |
253 | /// } |
254 | /// ``` |
255 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
256 | // Lang item so we can wrap other types in it. This is useful for coroutines. |
257 | #[lang = "maybe_uninit" ] |
258 | #[derive (Copy)] |
259 | #[repr (transparent)] |
260 | #[rustc_pub_transparent] |
261 | pub union MaybeUninit<T> { |
262 | uninit: (), |
263 | value: ManuallyDrop<T>, |
264 | } |
265 | |
266 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
267 | impl<T: Copy> Clone for MaybeUninit<T> { |
268 | #[inline (always)] |
269 | fn clone(&self) -> Self { |
270 | // Not calling `T::clone()`, we cannot know if we are initialized enough for that. |
271 | *self |
272 | } |
273 | } |
274 | |
275 | #[stable (feature = "maybe_uninit_debug" , since = "1.41.0" )] |
276 | impl<T> fmt::Debug for MaybeUninit<T> { |
277 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
278 | // NB: there is no `.pad_fmt` so we can't use a simpler `format_args!("MaybeUninit<{..}>"). |
279 | let full_name: &'static str = type_name::<Self>(); |
280 | let prefix_len: usize = full_name.find("MaybeUninit" ).unwrap(); |
281 | f.pad(&full_name[prefix_len..]) |
282 | } |
283 | } |
284 | |
285 | impl<T> MaybeUninit<T> { |
286 | /// Creates a new `MaybeUninit<T>` initialized with the given value. |
287 | /// It is safe to call [`assume_init`] on the return value of this function. |
288 | /// |
289 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. |
290 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
291 | /// |
292 | /// # Example |
293 | /// |
294 | /// ``` |
295 | /// use std::mem::MaybeUninit; |
296 | /// |
297 | /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]); |
298 | /// # // Prevent leaks for Miri |
299 | /// # unsafe { let _ = MaybeUninit::assume_init(v); } |
300 | /// ``` |
301 | /// |
302 | /// [`assume_init`]: MaybeUninit::assume_init |
303 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
304 | #[rustc_const_stable (feature = "const_maybe_uninit" , since = "1.36.0" )] |
305 | #[must_use = "use `forget` to avoid running Drop code" ] |
306 | #[inline (always)] |
307 | pub const fn new(val: T) -> MaybeUninit<T> { |
308 | MaybeUninit { value: ManuallyDrop::new(val) } |
309 | } |
310 | |
311 | /// Creates a new `MaybeUninit<T>` in an uninitialized state. |
312 | /// |
313 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. |
314 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
315 | /// |
316 | /// See the [type-level documentation][MaybeUninit] for some examples. |
317 | /// |
318 | /// # Example |
319 | /// |
320 | /// ``` |
321 | /// use std::mem::MaybeUninit; |
322 | /// |
323 | /// let v: MaybeUninit<String> = MaybeUninit::uninit(); |
324 | /// ``` |
325 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
326 | #[rustc_const_stable (feature = "const_maybe_uninit" , since = "1.36.0" )] |
327 | #[must_use ] |
328 | #[inline (always)] |
329 | #[rustc_diagnostic_item = "maybe_uninit_uninit" ] |
330 | pub const fn uninit() -> MaybeUninit<T> { |
331 | MaybeUninit { uninit: () } |
332 | } |
333 | |
334 | /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being |
335 | /// filled with `0` bytes. It depends on `T` whether that already makes for |
336 | /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, |
337 | /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not |
338 | /// be null. |
339 | /// |
340 | /// Note that if `T` has padding bytes, those bytes are *not* preserved when the |
341 | /// `MaybeUninit<T>` value is returned from this function, so those bytes will *not* be zeroed. |
342 | /// |
343 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. |
344 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. |
345 | /// |
346 | /// # Example |
347 | /// |
348 | /// Correct usage of this function: initializing a struct with zero, where all |
349 | /// fields of the struct can hold the bit-pattern 0 as a valid value. |
350 | /// |
351 | /// ```rust |
352 | /// use std::mem::MaybeUninit; |
353 | /// |
354 | /// let x = MaybeUninit::<(u8, bool)>::zeroed(); |
355 | /// let x = unsafe { x.assume_init() }; |
356 | /// assert_eq!(x, (0, false)); |
357 | /// ``` |
358 | /// |
359 | /// This can be used in const contexts, such as to indicate the end of static arrays for |
360 | /// plugin registration. |
361 | /// |
362 | /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()` |
363 | /// when `0` is not a valid bit-pattern for the type: |
364 | /// |
365 | /// ```rust,no_run |
366 | /// use std::mem::MaybeUninit; |
367 | /// |
368 | /// enum NotZero { One = 1, Two = 2 } |
369 | /// |
370 | /// let x = MaybeUninit::<(u8, NotZero)>::zeroed(); |
371 | /// let x = unsafe { x.assume_init() }; |
372 | /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. |
373 | /// // This is undefined behavior. ⚠️ |
374 | /// ``` |
375 | #[inline ] |
376 | #[must_use ] |
377 | #[rustc_diagnostic_item = "maybe_uninit_zeroed" ] |
378 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
379 | #[rustc_const_stable (feature = "const_maybe_uninit_zeroed" , since = "1.75.0" )] |
380 | pub const fn zeroed() -> MaybeUninit<T> { |
381 | let mut u = MaybeUninit::<T>::uninit(); |
382 | // SAFETY: `u.as_mut_ptr()` points to allocated memory. |
383 | unsafe { u.as_mut_ptr().write_bytes(0u8, 1) }; |
384 | u |
385 | } |
386 | |
387 | /// Sets the value of the `MaybeUninit<T>`. |
388 | /// |
389 | /// This overwrites any previous value without dropping it, so be careful |
390 | /// not to use this twice unless you want to skip running the destructor. |
391 | /// For your convenience, this also returns a mutable reference to the |
392 | /// (now safely initialized) contents of `self`. |
393 | /// |
394 | /// As the content is stored inside a `MaybeUninit`, the destructor is not |
395 | /// run for the inner data if the MaybeUninit leaves scope without a call to |
396 | /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives |
397 | /// the mutable reference returned by this function needs to keep this in |
398 | /// mind. The safety model of Rust regards leaks as safe, but they are |
399 | /// usually still undesirable. This being said, the mutable reference |
400 | /// behaves like any other mutable reference would, so assigning a new value |
401 | /// to it will drop the old content. |
402 | /// |
403 | /// [`assume_init`]: Self::assume_init |
404 | /// [`assume_init_drop`]: Self::assume_init_drop |
405 | /// |
406 | /// # Examples |
407 | /// |
408 | /// Correct usage of this method: |
409 | /// |
410 | /// ```rust |
411 | /// use std::mem::MaybeUninit; |
412 | /// |
413 | /// let mut x = MaybeUninit::<Vec<u8>>::uninit(); |
414 | /// |
415 | /// { |
416 | /// let hello = x.write((&b"Hello, world!" ).to_vec()); |
417 | /// // Setting hello does not leak prior allocations, but drops them |
418 | /// *hello = (&b"Hello" ).to_vec(); |
419 | /// hello[0] = 'h' as u8; |
420 | /// } |
421 | /// // x is initialized now: |
422 | /// let s = unsafe { x.assume_init() }; |
423 | /// assert_eq!(b"hello" , s.as_slice()); |
424 | /// ``` |
425 | /// |
426 | /// This usage of the method causes a leak: |
427 | /// |
428 | /// ```rust |
429 | /// use std::mem::MaybeUninit; |
430 | /// |
431 | /// let mut x = MaybeUninit::<String>::uninit(); |
432 | /// |
433 | /// x.write("Hello" .to_string()); |
434 | /// # // FIXME(https://github.com/rust-lang/miri/issues/3670): |
435 | /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak. |
436 | /// # unsafe { MaybeUninit::assume_init_drop(&mut x); } |
437 | /// // This leaks the contained string: |
438 | /// x.write("hello" .to_string()); |
439 | /// // x is initialized now: |
440 | /// let s = unsafe { x.assume_init() }; |
441 | /// ``` |
442 | /// |
443 | /// This method can be used to avoid unsafe in some cases. The example below |
444 | /// shows a part of an implementation of a fixed sized arena that lends out |
445 | /// pinned references. |
446 | /// With `write`, we can avoid the need to write through a raw pointer: |
447 | /// |
448 | /// ```rust |
449 | /// use core::pin::Pin; |
450 | /// use core::mem::MaybeUninit; |
451 | /// |
452 | /// struct PinArena<T> { |
453 | /// memory: Box<[MaybeUninit<T>]>, |
454 | /// len: usize, |
455 | /// } |
456 | /// |
457 | /// impl <T> PinArena<T> { |
458 | /// pub fn capacity(&self) -> usize { |
459 | /// self.memory.len() |
460 | /// } |
461 | /// pub fn push(&mut self, val: T) -> Pin<&mut T> { |
462 | /// if self.len >= self.capacity() { |
463 | /// panic!("Attempted to push to a full pin arena!" ); |
464 | /// } |
465 | /// let ref_ = self.memory[self.len].write(val); |
466 | /// self.len += 1; |
467 | /// unsafe { Pin::new_unchecked(ref_) } |
468 | /// } |
469 | /// } |
470 | /// ``` |
471 | #[inline (always)] |
472 | #[stable (feature = "maybe_uninit_write" , since = "1.55.0" )] |
473 | #[rustc_const_stable (feature = "const_maybe_uninit_write" , since = "1.85.0" )] |
474 | pub const fn write(&mut self, val: T) -> &mut T { |
475 | *self = MaybeUninit::new(val); |
476 | // SAFETY: We just initialized this value. |
477 | unsafe { self.assume_init_mut() } |
478 | } |
479 | |
480 | /// Gets a pointer to the contained value. Reading from this pointer or turning it |
481 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. |
482 | /// Writing to memory that this pointer (non-transitively) points to is undefined behavior |
483 | /// (except inside an `UnsafeCell<T>`). |
484 | /// |
485 | /// # Examples |
486 | /// |
487 | /// Correct usage of this method: |
488 | /// |
489 | /// ```rust |
490 | /// use std::mem::MaybeUninit; |
491 | /// |
492 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); |
493 | /// x.write(vec![0, 1, 2]); |
494 | /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it. |
495 | /// let x_vec = unsafe { &*x.as_ptr() }; |
496 | /// assert_eq!(x_vec.len(), 3); |
497 | /// # // Prevent leaks for Miri |
498 | /// # unsafe { MaybeUninit::assume_init_drop(&mut x); } |
499 | /// ``` |
500 | /// |
501 | /// *Incorrect* usage of this method: |
502 | /// |
503 | /// ```rust,no_run |
504 | /// use std::mem::MaybeUninit; |
505 | /// |
506 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); |
507 | /// let x_vec = unsafe { &*x.as_ptr() }; |
508 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
509 | /// ``` |
510 | /// |
511 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but |
512 | /// until they are, it is advisable to avoid them.) |
513 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
514 | #[rustc_const_stable (feature = "const_maybe_uninit_as_ptr" , since = "1.59.0" )] |
515 | #[rustc_as_ptr] |
516 | #[inline (always)] |
517 | pub const fn as_ptr(&self) -> *const T { |
518 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. |
519 | self as *const _ as *const T |
520 | } |
521 | |
522 | /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it |
523 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. |
524 | /// |
525 | /// # Examples |
526 | /// |
527 | /// Correct usage of this method: |
528 | /// |
529 | /// ```rust |
530 | /// use std::mem::MaybeUninit; |
531 | /// |
532 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); |
533 | /// x.write(vec![0, 1, 2]); |
534 | /// // Create a reference into the `MaybeUninit<Vec<u32>>`. |
535 | /// // This is okay because we initialized it. |
536 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; |
537 | /// x_vec.push(3); |
538 | /// assert_eq!(x_vec.len(), 4); |
539 | /// # // Prevent leaks for Miri |
540 | /// # unsafe { MaybeUninit::assume_init_drop(&mut x); } |
541 | /// ``` |
542 | /// |
543 | /// *Incorrect* usage of this method: |
544 | /// |
545 | /// ```rust,no_run |
546 | /// use std::mem::MaybeUninit; |
547 | /// |
548 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); |
549 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; |
550 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
551 | /// ``` |
552 | /// |
553 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but |
554 | /// until they are, it is advisable to avoid them.) |
555 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
556 | #[rustc_const_stable (feature = "const_maybe_uninit_as_mut_ptr" , since = "1.83.0" )] |
557 | #[rustc_as_ptr] |
558 | #[inline (always)] |
559 | pub const fn as_mut_ptr(&mut self) -> *mut T { |
560 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. |
561 | self as *mut _ as *mut T |
562 | } |
563 | |
564 | /// Extracts the value from the `MaybeUninit<T>` container. This is a great way |
565 | /// to ensure that the data will get dropped, because the resulting `T` is |
566 | /// subject to the usual drop handling. |
567 | /// |
568 | /// # Safety |
569 | /// |
570 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized |
571 | /// state. Calling this when the content is not yet fully initialized causes immediate undefined |
572 | /// behavior. The [type-level documentation][inv] contains more information about |
573 | /// this initialization invariant. |
574 | /// |
575 | /// [inv]: #initialization-invariant |
576 | /// |
577 | /// On top of that, remember that most types have additional invariants beyond merely |
578 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] |
579 | /// is considered initialized (under the current implementation; this does not constitute |
580 | /// a stable guarantee) because the only requirement the compiler knows about it |
581 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause |
582 | /// *immediate* undefined behavior, but will cause undefined behavior with most |
583 | /// safe operations (including dropping it). |
584 | /// |
585 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html |
586 | /// |
587 | /// # Examples |
588 | /// |
589 | /// Correct usage of this method: |
590 | /// |
591 | /// ```rust |
592 | /// use std::mem::MaybeUninit; |
593 | /// |
594 | /// let mut x = MaybeUninit::<bool>::uninit(); |
595 | /// x.write(true); |
596 | /// let x_init = unsafe { x.assume_init() }; |
597 | /// assert_eq!(x_init, true); |
598 | /// ``` |
599 | /// |
600 | /// *Incorrect* usage of this method: |
601 | /// |
602 | /// ```rust,no_run |
603 | /// use std::mem::MaybeUninit; |
604 | /// |
605 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); |
606 | /// let x_init = unsafe { x.assume_init() }; |
607 | /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️ |
608 | /// ``` |
609 | #[stable (feature = "maybe_uninit" , since = "1.36.0" )] |
610 | #[rustc_const_stable (feature = "const_maybe_uninit_assume_init_by_value" , since = "1.59.0" )] |
611 | #[inline (always)] |
612 | #[rustc_diagnostic_item = "assume_init" ] |
613 | #[track_caller ] |
614 | pub const unsafe fn assume_init(self) -> T { |
615 | // SAFETY: the caller must guarantee that `self` is initialized. |
616 | // This also means that `self` must be a `value` variant. |
617 | unsafe { |
618 | intrinsics::assert_inhabited::<T>(); |
619 | ManuallyDrop::into_inner(self.value) |
620 | } |
621 | } |
622 | |
623 | /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject |
624 | /// to the usual drop handling. |
625 | /// |
626 | /// Whenever possible, it is preferable to use [`assume_init`] instead, which |
627 | /// prevents duplicating the content of the `MaybeUninit<T>`. |
628 | /// |
629 | /// # Safety |
630 | /// |
631 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized |
632 | /// state. Calling this when the content is not yet fully initialized causes undefined |
633 | /// behavior. The [type-level documentation][inv] contains more information about |
634 | /// this initialization invariant. |
635 | /// |
636 | /// Moreover, similar to the [`ptr::read`] function, this function creates a |
637 | /// bitwise copy of the contents, regardless whether the contained type |
638 | /// implements the [`Copy`] trait or not. When using multiple copies of the |
639 | /// data (by calling `assume_init_read` multiple times, or first calling |
640 | /// `assume_init_read` and then [`assume_init`]), it is your responsibility |
641 | /// to ensure that data may indeed be duplicated. |
642 | /// |
643 | /// [inv]: #initialization-invariant |
644 | /// [`assume_init`]: MaybeUninit::assume_init |
645 | /// |
646 | /// # Examples |
647 | /// |
648 | /// Correct usage of this method: |
649 | /// |
650 | /// ```rust |
651 | /// use std::mem::MaybeUninit; |
652 | /// |
653 | /// let mut x = MaybeUninit::<u32>::uninit(); |
654 | /// x.write(13); |
655 | /// let x1 = unsafe { x.assume_init_read() }; |
656 | /// // `u32` is `Copy`, so we may read multiple times. |
657 | /// let x2 = unsafe { x.assume_init_read() }; |
658 | /// assert_eq!(x1, x2); |
659 | /// |
660 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); |
661 | /// x.write(None); |
662 | /// let x1 = unsafe { x.assume_init_read() }; |
663 | /// // Duplicating a `None` value is okay, so we may read multiple times. |
664 | /// let x2 = unsafe { x.assume_init_read() }; |
665 | /// assert_eq!(x1, x2); |
666 | /// ``` |
667 | /// |
668 | /// *Incorrect* usage of this method: |
669 | /// |
670 | /// ```rust,no_run |
671 | /// use std::mem::MaybeUninit; |
672 | /// |
673 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); |
674 | /// x.write(Some(vec![0, 1, 2])); |
675 | /// let x1 = unsafe { x.assume_init_read() }; |
676 | /// let x2 = unsafe { x.assume_init_read() }; |
677 | /// // We now created two copies of the same vector, leading to a double-free ⚠️ when |
678 | /// // they both get dropped! |
679 | /// ``` |
680 | #[stable (feature = "maybe_uninit_extra" , since = "1.60.0" )] |
681 | #[rustc_const_stable (feature = "const_maybe_uninit_assume_init_read" , since = "1.75.0" )] |
682 | #[inline (always)] |
683 | #[track_caller ] |
684 | pub const unsafe fn assume_init_read(&self) -> T { |
685 | // SAFETY: the caller must guarantee that `self` is initialized. |
686 | // Reading from `self.as_ptr()` is safe since `self` should be initialized. |
687 | unsafe { |
688 | intrinsics::assert_inhabited::<T>(); |
689 | self.as_ptr().read() |
690 | } |
691 | } |
692 | |
693 | /// Drops the contained value in place. |
694 | /// |
695 | /// If you have ownership of the `MaybeUninit`, you can also use |
696 | /// [`assume_init`] as an alternative. |
697 | /// |
698 | /// # Safety |
699 | /// |
700 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is |
701 | /// in an initialized state. Calling this when the content is not yet fully |
702 | /// initialized causes undefined behavior. |
703 | /// |
704 | /// On top of that, all additional invariants of the type `T` must be |
705 | /// satisfied, as the `Drop` implementation of `T` (or its members) may |
706 | /// rely on this. For example, setting a `Vec<T>` to an invalid but |
707 | /// non-null address makes it initialized (under the current implementation; |
708 | /// this does not constitute a stable guarantee), because the only |
709 | /// requirement the compiler knows about it is that the data pointer must be |
710 | /// non-null. Dropping such a `Vec<T>` however will cause undefined |
711 | /// behavior. |
712 | /// |
713 | /// [`assume_init`]: MaybeUninit::assume_init |
714 | #[stable (feature = "maybe_uninit_extra" , since = "1.60.0" )] |
715 | pub unsafe fn assume_init_drop(&mut self) { |
716 | // SAFETY: the caller must guarantee that `self` is initialized and |
717 | // satisfies all invariants of `T`. |
718 | // Dropping the value in place is safe if that is the case. |
719 | unsafe { ptr::drop_in_place(self.as_mut_ptr()) } |
720 | } |
721 | |
722 | /// Gets a shared reference to the contained value. |
723 | /// |
724 | /// This can be useful when we want to access a `MaybeUninit` that has been |
725 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use |
726 | /// of `.assume_init()`). |
727 | /// |
728 | /// # Safety |
729 | /// |
730 | /// Calling this when the content is not yet fully initialized causes undefined |
731 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really |
732 | /// is in an initialized state. |
733 | /// |
734 | /// # Examples |
735 | /// |
736 | /// ### Correct usage of this method: |
737 | /// |
738 | /// ```rust |
739 | /// use std::mem::MaybeUninit; |
740 | /// |
741 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); |
742 | /// # let mut x_mu = x; |
743 | /// # let mut x = &mut x_mu; |
744 | /// // Initialize `x`: |
745 | /// x.write(vec![1, 2, 3]); |
746 | /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to |
747 | /// // create a shared reference to it: |
748 | /// let x: &Vec<u32> = unsafe { |
749 | /// // SAFETY: `x` has been initialized. |
750 | /// x.assume_init_ref() |
751 | /// }; |
752 | /// assert_eq!(x, &vec![1, 2, 3]); |
753 | /// # // Prevent leaks for Miri |
754 | /// # unsafe { MaybeUninit::assume_init_drop(&mut x_mu); } |
755 | /// ``` |
756 | /// |
757 | /// ### *Incorrect* usages of this method: |
758 | /// |
759 | /// ```rust,no_run |
760 | /// use std::mem::MaybeUninit; |
761 | /// |
762 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); |
763 | /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() }; |
764 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
765 | /// ``` |
766 | /// |
767 | /// ```rust,no_run |
768 | /// use std::{cell::Cell, mem::MaybeUninit}; |
769 | /// |
770 | /// let b = MaybeUninit::<Cell<bool>>::uninit(); |
771 | /// // Initialize the `MaybeUninit` using `Cell::set`: |
772 | /// unsafe { |
773 | /// b.assume_init_ref().set(true); |
774 | /// // ^^^^^^^^^^^^^^^ |
775 | /// // Reference to an uninitialized `Cell<bool>`: UB! |
776 | /// } |
777 | /// ``` |
778 | #[stable (feature = "maybe_uninit_ref" , since = "1.55.0" )] |
779 | #[rustc_const_stable (feature = "const_maybe_uninit_assume_init_ref" , since = "1.59.0" )] |
780 | #[inline (always)] |
781 | pub const unsafe fn assume_init_ref(&self) -> &T { |
782 | // SAFETY: the caller must guarantee that `self` is initialized. |
783 | // This also means that `self` must be a `value` variant. |
784 | unsafe { |
785 | intrinsics::assert_inhabited::<T>(); |
786 | &*self.as_ptr() |
787 | } |
788 | } |
789 | |
790 | /// Gets a mutable (unique) reference to the contained value. |
791 | /// |
792 | /// This can be useful when we want to access a `MaybeUninit` that has been |
793 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use |
794 | /// of `.assume_init()`). |
795 | /// |
796 | /// # Safety |
797 | /// |
798 | /// Calling this when the content is not yet fully initialized causes undefined |
799 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really |
800 | /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to |
801 | /// initialize a `MaybeUninit`. |
802 | /// |
803 | /// # Examples |
804 | /// |
805 | /// ### Correct usage of this method: |
806 | /// |
807 | /// ```rust |
808 | /// # #![allow (unexpected_cfgs)] |
809 | /// use std::mem::MaybeUninit; |
810 | /// |
811 | /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { unsafe { *buf = [0; 1024] } } |
812 | /// # #[cfg (FALSE)] |
813 | /// extern "C" { |
814 | /// /// Initializes *all* the bytes of the input buffer. |
815 | /// fn initialize_buffer(buf: *mut [u8; 1024]); |
816 | /// } |
817 | /// |
818 | /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit(); |
819 | /// |
820 | /// // Initialize `buf`: |
821 | /// unsafe { initialize_buffer(buf.as_mut_ptr()); } |
822 | /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it. |
823 | /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes. |
824 | /// // To assert our buffer has been initialized without copying it, we upgrade |
825 | /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`: |
826 | /// let buf: &mut [u8; 1024] = unsafe { |
827 | /// // SAFETY: `buf` has been initialized. |
828 | /// buf.assume_init_mut() |
829 | /// }; |
830 | /// |
831 | /// // Now we can use `buf` as a normal slice: |
832 | /// buf.sort_unstable(); |
833 | /// assert!( |
834 | /// buf.windows(2).all(|pair| pair[0] <= pair[1]), |
835 | /// "buffer is sorted" , |
836 | /// ); |
837 | /// ``` |
838 | /// |
839 | /// ### *Incorrect* usages of this method: |
840 | /// |
841 | /// You cannot use `.assume_init_mut()` to initialize a value: |
842 | /// |
843 | /// ```rust,no_run |
844 | /// use std::mem::MaybeUninit; |
845 | /// |
846 | /// let mut b = MaybeUninit::<bool>::uninit(); |
847 | /// unsafe { |
848 | /// *b.assume_init_mut() = true; |
849 | /// // We have created a (mutable) reference to an uninitialized `bool`! |
850 | /// // This is undefined behavior. ⚠️ |
851 | /// } |
852 | /// ``` |
853 | /// |
854 | /// For instance, you cannot [`Read`] into an uninitialized buffer: |
855 | /// |
856 | /// [`Read`]: ../../std/io/trait.Read.html |
857 | /// |
858 | /// ```rust,no_run |
859 | /// use std::{io, mem::MaybeUninit}; |
860 | /// |
861 | /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]> |
862 | /// { |
863 | /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit(); |
864 | /// reader.read_exact(unsafe { buffer.assume_init_mut() })?; |
865 | /// // ^^^^^^^^^^^^^^^^^^^^^^^^ |
866 | /// // (mutable) reference to uninitialized memory! |
867 | /// // This is undefined behavior. |
868 | /// Ok(unsafe { buffer.assume_init() }) |
869 | /// } |
870 | /// ``` |
871 | /// |
872 | /// Nor can you use direct field access to do field-by-field gradual initialization: |
873 | /// |
874 | /// ```rust,no_run |
875 | /// use std::{mem::MaybeUninit, ptr}; |
876 | /// |
877 | /// struct Foo { |
878 | /// a: u32, |
879 | /// b: u8, |
880 | /// } |
881 | /// |
882 | /// let foo: Foo = unsafe { |
883 | /// let mut foo = MaybeUninit::<Foo>::uninit(); |
884 | /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337); |
885 | /// // ^^^^^^^^^^^^^^^^^^^^^ |
886 | /// // (mutable) reference to uninitialized memory! |
887 | /// // This is undefined behavior. |
888 | /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42); |
889 | /// // ^^^^^^^^^^^^^^^^^^^^^ |
890 | /// // (mutable) reference to uninitialized memory! |
891 | /// // This is undefined behavior. |
892 | /// foo.assume_init() |
893 | /// }; |
894 | /// ``` |
895 | #[stable (feature = "maybe_uninit_ref" , since = "1.55.0" )] |
896 | #[rustc_const_stable (feature = "const_maybe_uninit_assume_init" , since = "1.84.0" )] |
897 | #[inline (always)] |
898 | pub const unsafe fn assume_init_mut(&mut self) -> &mut T { |
899 | // SAFETY: the caller must guarantee that `self` is initialized. |
900 | // This also means that `self` must be a `value` variant. |
901 | unsafe { |
902 | intrinsics::assert_inhabited::<T>(); |
903 | &mut *self.as_mut_ptr() |
904 | } |
905 | } |
906 | |
907 | /// Extracts the values from an array of `MaybeUninit` containers. |
908 | /// |
909 | /// # Safety |
910 | /// |
911 | /// It is up to the caller to guarantee that all elements of the array are |
912 | /// in an initialized state. |
913 | /// |
914 | /// # Examples |
915 | /// |
916 | /// ``` |
917 | /// #![feature(maybe_uninit_array_assume_init)] |
918 | /// use std::mem::MaybeUninit; |
919 | /// |
920 | /// let mut array: [MaybeUninit<i32>; 3] = [MaybeUninit::uninit(); 3]; |
921 | /// array[0].write(0); |
922 | /// array[1].write(1); |
923 | /// array[2].write(2); |
924 | /// |
925 | /// // SAFETY: Now safe as we initialised all elements |
926 | /// let array = unsafe { |
927 | /// MaybeUninit::array_assume_init(array) |
928 | /// }; |
929 | /// |
930 | /// assert_eq!(array, [0, 1, 2]); |
931 | /// ``` |
932 | #[unstable (feature = "maybe_uninit_array_assume_init" , issue = "96097" )] |
933 | #[inline (always)] |
934 | #[track_caller ] |
935 | pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] { |
936 | // SAFETY: |
937 | // * The caller guarantees that all elements of the array are initialized |
938 | // * `MaybeUninit<T>` and T are guaranteed to have the same layout |
939 | // * `MaybeUninit` does not drop, so there are no double-frees |
940 | // And thus the conversion is safe |
941 | unsafe { |
942 | intrinsics::assert_inhabited::<[T; N]>(); |
943 | intrinsics::transmute_unchecked(array) |
944 | } |
945 | } |
946 | |
947 | /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes. |
948 | /// |
949 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still |
950 | /// contain padding bytes which are left uninitialized. |
951 | /// |
952 | /// # Examples |
953 | /// |
954 | /// ``` |
955 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)] |
956 | /// use std::mem::MaybeUninit; |
957 | /// |
958 | /// let val = 0x12345678_i32; |
959 | /// let uninit = MaybeUninit::new(val); |
960 | /// let uninit_bytes = uninit.as_bytes(); |
961 | /// let bytes = unsafe { uninit_bytes.assume_init_ref() }; |
962 | /// assert_eq!(bytes, val.to_ne_bytes()); |
963 | /// ``` |
964 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
965 | pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] { |
966 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes |
967 | unsafe { |
968 | slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of::<T>()) |
969 | } |
970 | } |
971 | |
972 | /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized |
973 | /// bytes. |
974 | /// |
975 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still |
976 | /// contain padding bytes which are left uninitialized. |
977 | /// |
978 | /// # Examples |
979 | /// |
980 | /// ``` |
981 | /// #![feature(maybe_uninit_as_bytes)] |
982 | /// use std::mem::MaybeUninit; |
983 | /// |
984 | /// let val = 0x12345678_i32; |
985 | /// let mut uninit = MaybeUninit::new(val); |
986 | /// let uninit_bytes = uninit.as_bytes_mut(); |
987 | /// if cfg!(target_endian = "little" ) { |
988 | /// uninit_bytes[0].write(0xcd); |
989 | /// } else { |
990 | /// uninit_bytes[3].write(0xcd); |
991 | /// } |
992 | /// let val2 = unsafe { uninit.assume_init() }; |
993 | /// assert_eq!(val2, 0x123456cd); |
994 | /// ``` |
995 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
996 | pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] { |
997 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes |
998 | unsafe { |
999 | slice::from_raw_parts_mut( |
1000 | self.as_mut_ptr().cast::<MaybeUninit<u8>>(), |
1001 | super::size_of::<T>(), |
1002 | ) |
1003 | } |
1004 | } |
1005 | |
1006 | /// Deprecated version of [`slice::assume_init_ref`]. |
1007 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1008 | #[deprecated ( |
1009 | note = "replaced by inherent assume_init_ref method; will eventually be removed" , |
1010 | since = "1.83.0" |
1011 | )] |
1012 | pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] { |
1013 | // SAFETY: Same for both methods. |
1014 | unsafe { slice.assume_init_ref() } |
1015 | } |
1016 | |
1017 | /// Deprecated version of [`slice::assume_init_mut`]. |
1018 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1019 | #[deprecated ( |
1020 | note = "replaced by inherent assume_init_mut method; will eventually be removed" , |
1021 | since = "1.83.0" |
1022 | )] |
1023 | pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] { |
1024 | // SAFETY: Same for both methods. |
1025 | unsafe { slice.assume_init_mut() } |
1026 | } |
1027 | |
1028 | /// Gets a pointer to the first element of the array. |
1029 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1030 | #[inline (always)] |
1031 | pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T { |
1032 | this.as_ptr() as *const T |
1033 | } |
1034 | |
1035 | /// Gets a mutable pointer to the first element of the array. |
1036 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1037 | #[inline (always)] |
1038 | pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T { |
1039 | this.as_mut_ptr() as *mut T |
1040 | } |
1041 | |
1042 | /// Deprecated version of [`slice::write_copy_of_slice`]. |
1043 | #[unstable (feature = "maybe_uninit_write_slice" , issue = "79995" )] |
1044 | #[deprecated ( |
1045 | note = "replaced by inherent write_copy_of_slice method; will eventually be removed" , |
1046 | since = "1.83.0" |
1047 | )] |
1048 | pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] |
1049 | where |
1050 | T: Copy, |
1051 | { |
1052 | this.write_copy_of_slice(src) |
1053 | } |
1054 | |
1055 | /// Deprecated version of [`slice::write_clone_of_slice`]. |
1056 | #[unstable (feature = "maybe_uninit_write_slice" , issue = "79995" )] |
1057 | #[deprecated ( |
1058 | note = "replaced by inherent write_clone_of_slice method; will eventually be removed" , |
1059 | since = "1.83.0" |
1060 | )] |
1061 | pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] |
1062 | where |
1063 | T: Clone, |
1064 | { |
1065 | this.write_clone_of_slice(src) |
1066 | } |
1067 | |
1068 | /// Deprecated version of [`slice::write_filled`]. |
1069 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1070 | #[deprecated ( |
1071 | note = "replaced by inherent write_filled method; will eventually be removed" , |
1072 | since = "1.83.0" |
1073 | )] |
1074 | pub fn fill<'a>(this: &'a mut [MaybeUninit<T>], value: T) -> &'a mut [T] |
1075 | where |
1076 | T: Clone, |
1077 | { |
1078 | this.write_filled(value) |
1079 | } |
1080 | |
1081 | /// Deprecated version of [`slice::write_with`]. |
1082 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1083 | #[deprecated ( |
1084 | note = "replaced by inherent write_with method; will eventually be removed" , |
1085 | since = "1.83.0" |
1086 | )] |
1087 | pub fn fill_with<'a, F>(this: &'a mut [MaybeUninit<T>], mut f: F) -> &'a mut [T] |
1088 | where |
1089 | F: FnMut() -> T, |
1090 | { |
1091 | this.write_with(|_| f()) |
1092 | } |
1093 | |
1094 | /// Deprecated version of [`slice::write_iter`]. |
1095 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1096 | #[deprecated ( |
1097 | note = "replaced by inherent write_iter method; will eventually be removed" , |
1098 | since = "1.83.0" |
1099 | )] |
1100 | pub fn fill_from<'a, I>( |
1101 | this: &'a mut [MaybeUninit<T>], |
1102 | it: I, |
1103 | ) -> (&'a mut [T], &'a mut [MaybeUninit<T>]) |
1104 | where |
1105 | I: IntoIterator<Item = T>, |
1106 | { |
1107 | this.write_iter(it) |
1108 | } |
1109 | |
1110 | /// Deprecated version of [`slice::as_bytes`]. |
1111 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
1112 | #[deprecated ( |
1113 | note = "replaced by inherent as_bytes method; will eventually be removed" , |
1114 | since = "1.83.0" |
1115 | )] |
1116 | pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] { |
1117 | this.as_bytes() |
1118 | } |
1119 | |
1120 | /// Deprecated version of [`slice::as_bytes_mut`]. |
1121 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
1122 | #[deprecated ( |
1123 | note = "replaced by inherent as_bytes_mut method; will eventually be removed" , |
1124 | since = "1.83.0" |
1125 | )] |
1126 | pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] { |
1127 | this.as_bytes_mut() |
1128 | } |
1129 | } |
1130 | |
1131 | impl<T> [MaybeUninit<T>] { |
1132 | /// Copies the elements from `src` to `self`, |
1133 | /// returning a mutable reference to the now initialized contents of `self`. |
1134 | /// |
1135 | /// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead. |
1136 | /// |
1137 | /// This is similar to [`slice::copy_from_slice`]. |
1138 | /// |
1139 | /// # Panics |
1140 | /// |
1141 | /// This function will panic if the two slices have different lengths. |
1142 | /// |
1143 | /// # Examples |
1144 | /// |
1145 | /// ``` |
1146 | /// #![feature(maybe_uninit_write_slice)] |
1147 | /// use std::mem::MaybeUninit; |
1148 | /// |
1149 | /// let mut dst = [MaybeUninit::uninit(); 32]; |
1150 | /// let src = [0; 32]; |
1151 | /// |
1152 | /// let init = dst.write_copy_of_slice(&src); |
1153 | /// |
1154 | /// assert_eq!(init, src); |
1155 | /// ``` |
1156 | /// |
1157 | /// ``` |
1158 | /// #![feature(maybe_uninit_write_slice)] |
1159 | /// |
1160 | /// let mut vec = Vec::with_capacity(32); |
1161 | /// let src = [0; 16]; |
1162 | /// |
1163 | /// vec.spare_capacity_mut()[..src.len()].write_copy_of_slice(&src); |
1164 | /// |
1165 | /// // SAFETY: we have just copied all the elements of len into the spare capacity |
1166 | /// // the first src.len() elements of the vec are valid now. |
1167 | /// unsafe { |
1168 | /// vec.set_len(src.len()); |
1169 | /// } |
1170 | /// |
1171 | /// assert_eq!(vec, src); |
1172 | /// ``` |
1173 | /// |
1174 | /// [`write_clone_of_slice`]: slice::write_clone_of_slice |
1175 | #[unstable (feature = "maybe_uninit_write_slice" , issue = "79995" )] |
1176 | pub const fn write_copy_of_slice(&mut self, src: &[T]) -> &mut [T] |
1177 | where |
1178 | T: Copy, |
1179 | { |
1180 | // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout |
1181 | let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) }; |
1182 | |
1183 | self.copy_from_slice(uninit_src); |
1184 | |
1185 | // SAFETY: Valid elements have just been copied into `self` so it is initialized |
1186 | unsafe { self.assume_init_mut() } |
1187 | } |
1188 | |
1189 | /// Clones the elements from `src` to `self`, |
1190 | /// returning a mutable reference to the now initialized contents of `self`. |
1191 | /// Any already initialized elements will not be dropped. |
1192 | /// |
1193 | /// If `T` implements `Copy`, use [`write_copy_of_slice`] instead. |
1194 | /// |
1195 | /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements. |
1196 | /// |
1197 | /// # Panics |
1198 | /// |
1199 | /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics. |
1200 | /// |
1201 | /// If there is a panic, the already cloned elements will be dropped. |
1202 | /// |
1203 | /// # Examples |
1204 | /// |
1205 | /// ``` |
1206 | /// #![feature(maybe_uninit_write_slice)] |
1207 | /// use std::mem::MaybeUninit; |
1208 | /// |
1209 | /// let mut dst = [const { MaybeUninit::uninit() }; 5]; |
1210 | /// let src = ["wibbly" , "wobbly" , "timey" , "wimey" , "stuff" ].map(|s| s.to_string()); |
1211 | /// |
1212 | /// let init = dst.write_clone_of_slice(&src); |
1213 | /// |
1214 | /// assert_eq!(init, src); |
1215 | /// |
1216 | /// # // Prevent leaks for Miri |
1217 | /// # unsafe { std::ptr::drop_in_place(init); } |
1218 | /// ``` |
1219 | /// |
1220 | /// ``` |
1221 | /// #![feature(maybe_uninit_write_slice)] |
1222 | /// |
1223 | /// let mut vec = Vec::with_capacity(32); |
1224 | /// let src = ["rust" , "is" , "a" , "pretty" , "cool" , "language" ].map(|s| s.to_string()); |
1225 | /// |
1226 | /// vec.spare_capacity_mut()[..src.len()].write_clone_of_slice(&src); |
1227 | /// |
1228 | /// // SAFETY: we have just cloned all the elements of len into the spare capacity |
1229 | /// // the first src.len() elements of the vec are valid now. |
1230 | /// unsafe { |
1231 | /// vec.set_len(src.len()); |
1232 | /// } |
1233 | /// |
1234 | /// assert_eq!(vec, src); |
1235 | /// ``` |
1236 | /// |
1237 | /// [`write_copy_of_slice`]: slice::write_copy_of_slice |
1238 | #[unstable (feature = "maybe_uninit_write_slice" , issue = "79995" )] |
1239 | pub fn write_clone_of_slice(&mut self, src: &[T]) -> &mut [T] |
1240 | where |
1241 | T: Clone, |
1242 | { |
1243 | // unlike copy_from_slice this does not call clone_from_slice on the slice |
1244 | // this is because `MaybeUninit<T: Clone>` does not implement Clone. |
1245 | |
1246 | assert_eq!(self.len(), src.len(), "destination and source slices have different lengths" ); |
1247 | |
1248 | // NOTE: We need to explicitly slice them to the same length |
1249 | // for bounds checking to be elided, and the optimizer will |
1250 | // generate memcpy for simple cases (for example T = u8). |
1251 | let len = self.len(); |
1252 | let src = &src[..len]; |
1253 | |
1254 | // guard is needed b/c panic might happen during a clone |
1255 | let mut guard = Guard { slice: self, initialized: 0 }; |
1256 | |
1257 | for i in 0..len { |
1258 | guard.slice[i].write(src[i].clone()); |
1259 | guard.initialized += 1; |
1260 | } |
1261 | |
1262 | super::forget(guard); |
1263 | |
1264 | // SAFETY: Valid elements have just been written into `self` so it is initialized |
1265 | unsafe { self.assume_init_mut() } |
1266 | } |
1267 | |
1268 | /// Fills a slice with elements by cloning `value`, returning a mutable reference to the now |
1269 | /// initialized contents of the slice. |
1270 | /// Any previously initialized elements will not be dropped. |
1271 | /// |
1272 | /// This is similar to [`slice::fill`]. |
1273 | /// |
1274 | /// # Panics |
1275 | /// |
1276 | /// This function will panic if any call to `Clone` panics. |
1277 | /// |
1278 | /// If such a panic occurs, any elements previously initialized during this operation will be |
1279 | /// dropped. |
1280 | /// |
1281 | /// # Examples |
1282 | /// |
1283 | /// ``` |
1284 | /// #![feature(maybe_uninit_fill)] |
1285 | /// use std::mem::MaybeUninit; |
1286 | /// |
1287 | /// let mut buf = [const { MaybeUninit::uninit() }; 10]; |
1288 | /// let initialized = buf.write_filled(1); |
1289 | /// assert_eq!(initialized, &mut [1; 10]); |
1290 | /// ``` |
1291 | #[doc (alias = "memset" )] |
1292 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1293 | pub fn write_filled(&mut self, value: T) -> &mut [T] |
1294 | where |
1295 | T: Clone, |
1296 | { |
1297 | SpecFill::spec_fill(self, value); |
1298 | // SAFETY: Valid elements have just been filled into `self` so it is initialized |
1299 | unsafe { self.assume_init_mut() } |
1300 | } |
1301 | |
1302 | /// Fills a slice with elements returned by calling a closure for each index. |
1303 | /// |
1304 | /// This method uses a closure to create new values. If you'd rather `Clone` a given value, use |
1305 | /// [`MaybeUninit::fill`]. If you want to use the `Default` trait to generate values, you can |
1306 | /// pass [`|_| Default::default()`][Default::default] as the argument. |
1307 | /// |
1308 | /// # Panics |
1309 | /// |
1310 | /// This function will panic if any call to the provided closure panics. |
1311 | /// |
1312 | /// If such a panic occurs, any elements previously initialized during this operation will be |
1313 | /// dropped. |
1314 | /// |
1315 | /// # Examples |
1316 | /// |
1317 | /// ``` |
1318 | /// #![feature(maybe_uninit_fill)] |
1319 | /// use std::mem::MaybeUninit; |
1320 | /// |
1321 | /// let mut buf = [const { MaybeUninit::<usize>::uninit() }; 5]; |
1322 | /// let initialized = buf.write_with(|idx| idx + 1); |
1323 | /// assert_eq!(initialized, &mut [1, 2, 3, 4, 5]); |
1324 | /// ``` |
1325 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1326 | pub fn write_with<F>(&mut self, mut f: F) -> &mut [T] |
1327 | where |
1328 | F: FnMut(usize) -> T, |
1329 | { |
1330 | let mut guard = Guard { slice: self, initialized: 0 }; |
1331 | |
1332 | for (idx, element) in guard.slice.iter_mut().enumerate() { |
1333 | element.write(f(idx)); |
1334 | guard.initialized += 1; |
1335 | } |
1336 | |
1337 | super::forget(guard); |
1338 | |
1339 | // SAFETY: Valid elements have just been written into `this` so it is initialized |
1340 | unsafe { self.assume_init_mut() } |
1341 | } |
1342 | |
1343 | /// Fills a slice with elements yielded by an iterator until either all elements have been |
1344 | /// initialized or the iterator is empty. |
1345 | /// |
1346 | /// Returns two slices. The first slice contains the initialized portion of the original slice. |
1347 | /// The second slice is the still-uninitialized remainder of the original slice. |
1348 | /// |
1349 | /// # Panics |
1350 | /// |
1351 | /// This function panics if the iterator's `next` function panics. |
1352 | /// |
1353 | /// If such a panic occurs, any elements previously initialized during this operation will be |
1354 | /// dropped. |
1355 | /// |
1356 | /// # Examples |
1357 | /// |
1358 | /// Completely filling the slice: |
1359 | /// |
1360 | /// ``` |
1361 | /// #![feature(maybe_uninit_fill)] |
1362 | /// use std::mem::MaybeUninit; |
1363 | /// |
1364 | /// let mut buf = [const { MaybeUninit::uninit() }; 5]; |
1365 | /// |
1366 | /// let iter = [1, 2, 3].into_iter().cycle(); |
1367 | /// let (initialized, remainder) = buf.write_iter(iter); |
1368 | /// |
1369 | /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]); |
1370 | /// assert_eq!(remainder.len(), 0); |
1371 | /// ``` |
1372 | /// |
1373 | /// Partially filling the slice: |
1374 | /// |
1375 | /// ``` |
1376 | /// #![feature(maybe_uninit_fill)] |
1377 | /// use std::mem::MaybeUninit; |
1378 | /// |
1379 | /// let mut buf = [const { MaybeUninit::uninit() }; 5]; |
1380 | /// let iter = [1, 2]; |
1381 | /// let (initialized, remainder) = buf.write_iter(iter); |
1382 | /// |
1383 | /// assert_eq!(initialized, &mut [1, 2]); |
1384 | /// assert_eq!(remainder.len(), 3); |
1385 | /// ``` |
1386 | /// |
1387 | /// Checking an iterator after filling a slice: |
1388 | /// |
1389 | /// ``` |
1390 | /// #![feature(maybe_uninit_fill)] |
1391 | /// use std::mem::MaybeUninit; |
1392 | /// |
1393 | /// let mut buf = [const { MaybeUninit::uninit() }; 3]; |
1394 | /// let mut iter = [1, 2, 3, 4, 5].into_iter(); |
1395 | /// let (initialized, remainder) = buf.write_iter(iter.by_ref()); |
1396 | /// |
1397 | /// assert_eq!(initialized, &mut [1, 2, 3]); |
1398 | /// assert_eq!(remainder.len(), 0); |
1399 | /// assert_eq!(iter.as_slice(), &[4, 5]); |
1400 | /// ``` |
1401 | #[unstable (feature = "maybe_uninit_fill" , issue = "117428" )] |
1402 | pub fn write_iter<I>(&mut self, it: I) -> (&mut [T], &mut [MaybeUninit<T>]) |
1403 | where |
1404 | I: IntoIterator<Item = T>, |
1405 | { |
1406 | let iter = it.into_iter(); |
1407 | let mut guard = Guard { slice: self, initialized: 0 }; |
1408 | |
1409 | for (element, val) in guard.slice.iter_mut().zip(iter) { |
1410 | element.write(val); |
1411 | guard.initialized += 1; |
1412 | } |
1413 | |
1414 | let initialized_len = guard.initialized; |
1415 | super::forget(guard); |
1416 | |
1417 | // SAFETY: guard.initialized <= self.len() |
1418 | let (initted, remainder) = unsafe { self.split_at_mut_unchecked(initialized_len) }; |
1419 | |
1420 | // SAFETY: Valid elements have just been written into `init`, so that portion |
1421 | // of `this` is initialized. |
1422 | (unsafe { initted.assume_init_mut() }, remainder) |
1423 | } |
1424 | |
1425 | /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes. |
1426 | /// |
1427 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still |
1428 | /// contain padding bytes which are left uninitialized. |
1429 | /// |
1430 | /// # Examples |
1431 | /// |
1432 | /// ``` |
1433 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)] |
1434 | /// use std::mem::MaybeUninit; |
1435 | /// |
1436 | /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)]; |
1437 | /// let uninit_bytes = uninit.as_bytes(); |
1438 | /// let bytes = unsafe { uninit_bytes.assume_init_ref() }; |
1439 | /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap()); |
1440 | /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap()); |
1441 | /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]); |
1442 | /// ``` |
1443 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
1444 | pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] { |
1445 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes |
1446 | unsafe { |
1447 | slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of_val(self)) |
1448 | } |
1449 | } |
1450 | |
1451 | /// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially |
1452 | /// uninitialized bytes. |
1453 | /// |
1454 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still |
1455 | /// contain padding bytes which are left uninitialized. |
1456 | /// |
1457 | /// # Examples |
1458 | /// |
1459 | /// ``` |
1460 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)] |
1461 | /// use std::mem::MaybeUninit; |
1462 | /// |
1463 | /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()]; |
1464 | /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit); |
1465 | /// uninit_bytes.write_copy_of_slice(&[0x12, 0x34, 0x56, 0x78]); |
1466 | /// let vals = unsafe { uninit.assume_init_ref() }; |
1467 | /// if cfg!(target_endian = "little" ) { |
1468 | /// assert_eq!(vals, &[0x3412u16, 0x7856u16]); |
1469 | /// } else { |
1470 | /// assert_eq!(vals, &[0x1234u16, 0x5678u16]); |
1471 | /// } |
1472 | /// ``` |
1473 | #[unstable (feature = "maybe_uninit_as_bytes" , issue = "93092" )] |
1474 | pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] { |
1475 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes |
1476 | unsafe { |
1477 | slice::from_raw_parts_mut( |
1478 | self.as_mut_ptr() as *mut MaybeUninit<u8>, |
1479 | super::size_of_val(self), |
1480 | ) |
1481 | } |
1482 | } |
1483 | |
1484 | /// Drops the contained values in place. |
1485 | /// |
1486 | /// # Safety |
1487 | /// |
1488 | /// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice |
1489 | /// really is in an initialized state. Calling this when the content is not yet |
1490 | /// fully initialized causes undefined behavior. |
1491 | /// |
1492 | /// On top of that, all additional invariants of the type `T` must be |
1493 | /// satisfied, as the `Drop` implementation of `T` (or its members) may |
1494 | /// rely on this. For example, setting a `Vec<T>` to an invalid but |
1495 | /// non-null address makes it initialized (under the current implementation; |
1496 | /// this does not constitute a stable guarantee), because the only |
1497 | /// requirement the compiler knows about it is that the data pointer must be |
1498 | /// non-null. Dropping such a `Vec<T>` however will cause undefined |
1499 | /// behaviour. |
1500 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1501 | #[inline (always)] |
1502 | pub unsafe fn assume_init_drop(&mut self) { |
1503 | if !self.is_empty() { |
1504 | // SAFETY: the caller must guarantee that every element of `self` |
1505 | // is initialized and satisfies all invariants of `T`. |
1506 | // Dropping the value in place is safe if that is the case. |
1507 | unsafe { ptr::drop_in_place(self as *mut [MaybeUninit<T>] as *mut [T]) } |
1508 | } |
1509 | } |
1510 | |
1511 | /// Gets a shared reference to the contained value. |
1512 | /// |
1513 | /// # Safety |
1514 | /// |
1515 | /// Calling this when the content is not yet fully initialized causes undefined |
1516 | /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in |
1517 | /// the slice really is in an initialized state. |
1518 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1519 | #[inline (always)] |
1520 | pub const unsafe fn assume_init_ref(&self) -> &[T] { |
1521 | // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that |
1522 | // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`. |
1523 | // The pointer obtained is valid since it refers to memory owned by `slice` which is a |
1524 | // reference and thus guaranteed to be valid for reads. |
1525 | unsafe { &*(self as *const Self as *const [T]) } |
1526 | } |
1527 | |
1528 | /// Gets a mutable (unique) reference to the contained value. |
1529 | /// |
1530 | /// # Safety |
1531 | /// |
1532 | /// Calling this when the content is not yet fully initialized causes undefined |
1533 | /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the |
1534 | /// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot |
1535 | /// be used to initialize a `MaybeUninit` slice. |
1536 | #[unstable (feature = "maybe_uninit_slice" , issue = "63569" )] |
1537 | #[inline (always)] |
1538 | pub const unsafe fn assume_init_mut(&mut self) -> &mut [T] { |
1539 | // SAFETY: similar to safety notes for `slice_get_ref`, but we have a |
1540 | // mutable reference which is also guaranteed to be valid for writes. |
1541 | unsafe { &mut *(self as *mut Self as *mut [T]) } |
1542 | } |
1543 | } |
1544 | |
1545 | impl<T, const N: usize> MaybeUninit<[T; N]> { |
1546 | /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`. |
1547 | /// |
1548 | /// # Examples |
1549 | /// |
1550 | /// ``` |
1551 | /// #![feature(maybe_uninit_uninit_array_transpose)] |
1552 | /// # use std::mem::MaybeUninit; |
1553 | /// |
1554 | /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose(); |
1555 | /// ``` |
1556 | #[unstable (feature = "maybe_uninit_uninit_array_transpose" , issue = "96097" )] |
1557 | #[inline ] |
1558 | pub const fn transpose(self) -> [MaybeUninit<T>; N] { |
1559 | // SAFETY: T and MaybeUninit<T> have the same layout |
1560 | unsafe { intrinsics::transmute_unchecked(self) } |
1561 | } |
1562 | } |
1563 | |
1564 | impl<T, const N: usize> [MaybeUninit<T>; N] { |
1565 | /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`. |
1566 | /// |
1567 | /// # Examples |
1568 | /// |
1569 | /// ``` |
1570 | /// #![feature(maybe_uninit_uninit_array_transpose)] |
1571 | /// # use std::mem::MaybeUninit; |
1572 | /// |
1573 | /// let data = [MaybeUninit::<u8>::uninit(); 1000]; |
1574 | /// let data: MaybeUninit<[u8; 1000]> = data.transpose(); |
1575 | /// ``` |
1576 | #[unstable (feature = "maybe_uninit_uninit_array_transpose" , issue = "96097" )] |
1577 | #[inline ] |
1578 | pub const fn transpose(self) -> MaybeUninit<[T; N]> { |
1579 | // SAFETY: T and MaybeUninit<T> have the same layout |
1580 | unsafe { intrinsics::transmute_unchecked(self) } |
1581 | } |
1582 | } |
1583 | |
1584 | struct Guard<'a, T> { |
1585 | slice: &'a mut [MaybeUninit<T>], |
1586 | initialized: usize, |
1587 | } |
1588 | |
1589 | impl<'a, T> Drop for Guard<'a, T> { |
1590 | fn drop(&mut self) { |
1591 | let initialized_part: &mut [MaybeUninit] = &mut self.slice[..self.initialized]; |
1592 | // SAFETY: this raw sub-slice will contain only initialized objects. |
1593 | unsafe { |
1594 | initialized_part.assume_init_drop(); |
1595 | } |
1596 | } |
1597 | } |
1598 | |
1599 | trait SpecFill<T> { |
1600 | fn spec_fill(&mut self, value: T); |
1601 | } |
1602 | |
1603 | impl<T: Clone> SpecFill<T> for [MaybeUninit<T>] { |
1604 | default fn spec_fill(&mut self, value: T) { |
1605 | let mut guard: Guard<'_, T> = Guard { slice: self, initialized: 0 }; |
1606 | |
1607 | if let Some((last: &mut MaybeUninit, elems: &mut [MaybeUninit])) = guard.slice.split_last_mut() { |
1608 | for el: &mut MaybeUninit in elems { |
1609 | el.write(val:value.clone()); |
1610 | guard.initialized += 1; |
1611 | } |
1612 | |
1613 | last.write(val:value); |
1614 | } |
1615 | super::forget(guard); |
1616 | } |
1617 | } |
1618 | |
1619 | impl<T: Copy> SpecFill<T> for [MaybeUninit<T>] { |
1620 | fn spec_fill(&mut self, value: T) { |
1621 | self.fill(MaybeUninit::new(val:value)); |
1622 | } |
1623 | } |
1624 | |