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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; | |
1b1a35ee | 5 | use crate::ptr; |
60c5eb7d | 6 | |
dc9dc135 XL |
7 | /// A wrapper type to construct uninitialized instances of `T`. |
8 | /// | |
9 | /// # Initialization invariant | |
10 | /// | |
e74abb32 XL |
11 | /// The compiler, in general, assumes that a variable is properly initialized |
12 | /// according to the requirements of the variable's type. For example, a variable of | |
13 | /// reference type must be aligned and non-NULL. This is an invariant that must | |
14 | /// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a | |
15 | /// variable of reference type causes instantaneous [undefined behavior][ub], | |
16 | /// no matter whether that reference ever gets used to access memory: | |
dc9dc135 XL |
17 | /// |
18 | /// ```rust,no_run | |
416331ca | 19 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
20 | /// use std::mem::{self, MaybeUninit}; |
21 | /// | |
f9f354fc | 22 | /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️ |
dc9dc135 | 23 | /// // The equivalent code with `MaybeUninit<&i32>`: |
f9f354fc | 24 | /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
25 | /// ``` |
26 | /// | |
27 | /// This is exploited by the compiler for various optimizations, such as eliding | |
28 | /// run-time checks and optimizing `enum` layout. | |
29 | /// | |
30 | /// Similarly, entirely uninitialized memory may have any content, while a `bool` must | |
31 | /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior: | |
32 | /// | |
33 | /// ```rust,no_run | |
416331ca | 34 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
35 | /// use std::mem::{self, MaybeUninit}; |
36 | /// | |
f9f354fc | 37 | /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
dc9dc135 | 38 | /// // The equivalent code with `MaybeUninit<bool>`: |
f9f354fc | 39 | /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
40 | /// ``` |
41 | /// | |
6a06907d XL |
42 | /// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed" |
43 | /// meaning "it won't change without being written to"). Reading the same uninitialized byte | |
44 | /// multiple times can give different results. This makes it undefined behavior to have | |
45 | /// uninitialized data in a variable even if that variable has an integer type, which otherwise can | |
46 | /// hold any *fixed* bit pattern: | |
dc9dc135 XL |
47 | /// |
48 | /// ```rust,no_run | |
416331ca | 49 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
50 | /// use std::mem::{self, MaybeUninit}; |
51 | /// | |
f9f354fc | 52 | /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
dc9dc135 | 53 | /// // The equivalent code with `MaybeUninit<i32>`: |
f9f354fc | 54 | /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
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 | /// | |
5869c6ff | 176 | /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field: |
dc9dc135 | 177 | /// |
5869c6ff XL |
178 | /// ```rust |
179 | /// use std::mem::MaybeUninit; | |
180 | /// use std::ptr::addr_of_mut; | |
181 | /// | |
182 | /// #[derive(Debug, PartialEq)] | |
183 | /// pub struct Foo { | |
184 | /// name: String, | |
185 | /// list: Vec<u8>, | |
186 | /// } | |
187 | /// | |
188 | /// let foo = { | |
189 | /// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit(); | |
190 | /// let ptr = uninit.as_mut_ptr(); | |
191 | /// | |
192 | /// // Initializing the `name` field | |
193 | /// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); } | |
194 | /// | |
195 | /// // Initializing the `list` field | |
196 | /// // If there is a panic here, then the `String` in the `name` field leaks. | |
197 | /// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); } | |
198 | /// | |
199 | /// // All the fields are initialized, so we call `assume_init` to get an initialized Foo. | |
200 | /// unsafe { uninit.assume_init() } | |
201 | /// }; | |
202 | /// | |
203 | /// assert_eq!( | |
204 | /// foo, | |
205 | /// Foo { | |
206 | /// name: "Bob".to_string(), | |
207 | /// list: vec![0, 1, 2] | |
208 | /// } | |
209 | /// ); | |
210 | /// ``` | |
211 | /// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut | |
dc9dc135 XL |
212 | /// [ub]: ../../reference/behavior-considered-undefined.html |
213 | /// | |
214 | /// # Layout | |
215 | /// | |
216 | /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: | |
217 | /// | |
218 | /// ```rust | |
219 | /// use std::mem::{MaybeUninit, size_of, align_of}; | |
220 | /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>()); | |
221 | /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>()); | |
222 | /// ``` | |
223 | /// | |
224 | /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same | |
225 | /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as | |
226 | /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit | |
227 | /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling | |
228 | /// optimizations, potentially resulting in a larger size: | |
229 | /// | |
230 | /// ```rust | |
231 | /// # use std::mem::{MaybeUninit, size_of}; | |
232 | /// assert_eq!(size_of::<Option<bool>>(), 1); | |
233 | /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); | |
234 | /// ``` | |
235 | /// | |
236 | /// If `T` is FFI-safe, then so is `MaybeUninit<T>`. | |
237 | /// | |
238 | /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, | |
239 | /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and | |
240 | /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type | |
241 | /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. | |
242 | /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the | |
243 | /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact | |
244 | /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not | |
245 | /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has | |
246 | /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that | |
247 | /// guarantee may evolve. | |
dc9dc135 | 248 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
416331ca | 249 | // Lang item so we can wrap other types in it. This is useful for generators. |
e1599b0c | 250 | #[lang = "maybe_uninit"] |
dc9dc135 | 251 | #[derive(Copy)] |
416331ca | 252 | #[repr(transparent)] |
dc9dc135 XL |
253 | pub union MaybeUninit<T> { |
254 | uninit: (), | |
255 | value: ManuallyDrop<T>, | |
256 | } | |
257 | ||
258 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
259 | impl<T: Copy> Clone for MaybeUninit<T> { | |
260 | #[inline(always)] | |
261 | fn clone(&self) -> Self { | |
262 | // Not calling `T::clone()`, we cannot know if we are initialized enough for that. | |
263 | *self | |
264 | } | |
265 | } | |
266 | ||
60c5eb7d XL |
267 | #[stable(feature = "maybe_uninit_debug", since = "1.41.0")] |
268 | impl<T> fmt::Debug for MaybeUninit<T> { | |
269 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
270 | f.pad(type_name::<Self>()) | |
271 | } | |
272 | } | |
273 | ||
dc9dc135 XL |
274 | impl<T> MaybeUninit<T> { |
275 | /// Creates a new `MaybeUninit<T>` initialized with the given value. | |
276 | /// It is safe to call [`assume_init`] on the return value of this function. | |
277 | /// | |
278 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
279 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
280 | /// | |
29967ef6 XL |
281 | /// # Example |
282 | /// | |
283 | /// ``` | |
284 | /// use std::mem::MaybeUninit; | |
285 | /// | |
286 | /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]); | |
287 | /// ``` | |
288 | /// | |
3dfed10e | 289 | /// [`assume_init`]: MaybeUninit::assume_init |
dc9dc135 | 290 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
dfeec247 | 291 | #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")] |
dc9dc135 XL |
292 | #[inline(always)] |
293 | pub const fn new(val: T) -> MaybeUninit<T> { | |
294 | MaybeUninit { value: ManuallyDrop::new(val) } | |
295 | } | |
296 | ||
297 | /// Creates a new `MaybeUninit<T>` in an uninitialized state. | |
298 | /// | |
299 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
300 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
301 | /// | |
29967ef6 | 302 | /// See the [type-level documentation][MaybeUninit] for some examples. |
dc9dc135 | 303 | /// |
29967ef6 XL |
304 | /// # Example |
305 | /// | |
306 | /// ``` | |
307 | /// use std::mem::MaybeUninit; | |
308 | /// | |
309 | /// let v: MaybeUninit<String> = MaybeUninit::uninit(); | |
310 | /// ``` | |
dc9dc135 | 311 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
dfeec247 | 312 | #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")] |
dc9dc135 | 313 | #[inline(always)] |
dfeec247 | 314 | #[rustc_diagnostic_item = "maybe_uninit_uninit"] |
dc9dc135 XL |
315 | pub const fn uninit() -> MaybeUninit<T> { |
316 | MaybeUninit { uninit: () } | |
317 | } | |
318 | ||
60c5eb7d XL |
319 | /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state. |
320 | /// | |
321 | /// Note: in a future Rust version this method may become unnecessary | |
322 | /// when array literal syntax allows | |
323 | /// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147). | |
324 | /// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`. | |
325 | /// | |
326 | /// # Examples | |
327 | /// | |
328 | /// ```no_run | |
1b1a35ee | 329 | /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice)] |
60c5eb7d XL |
330 | /// |
331 | /// use std::mem::MaybeUninit; | |
332 | /// | |
333 | /// extern "C" { | |
334 | /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize; | |
335 | /// } | |
336 | /// | |
337 | /// /// Returns a (possibly smaller) slice of data that was actually read | |
338 | /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] { | |
339 | /// unsafe { | |
340 | /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len()); | |
1b1a35ee | 341 | /// MaybeUninit::slice_assume_init_ref(&buf[..len]) |
60c5eb7d XL |
342 | /// } |
343 | /// } | |
344 | /// | |
345 | /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array(); | |
346 | /// let data = read(&mut buf); | |
347 | /// ``` | |
dfeec247 | 348 | #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")] |
fc512014 | 349 | #[rustc_const_unstable(feature = "maybe_uninit_uninit_array", issue = "none")] |
60c5eb7d | 350 | #[inline(always)] |
fc512014 | 351 | pub const fn uninit_array<const LEN: usize>() -> [Self; LEN] { |
1b1a35ee | 352 | // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid. |
dfeec247 | 353 | unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() } |
60c5eb7d XL |
354 | } |
355 | ||
dc9dc135 XL |
356 | /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being |
357 | /// filled with `0` bytes. It depends on `T` whether that already makes for | |
358 | /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, | |
359 | /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not | |
360 | /// be null. | |
361 | /// | |
362 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
363 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
364 | /// | |
365 | /// # Example | |
366 | /// | |
367 | /// Correct usage of this function: initializing a struct with zero, where all | |
368 | /// fields of the struct can hold the bit-pattern 0 as a valid value. | |
369 | /// | |
370 | /// ```rust | |
371 | /// use std::mem::MaybeUninit; | |
372 | /// | |
373 | /// let x = MaybeUninit::<(u8, bool)>::zeroed(); | |
374 | /// let x = unsafe { x.assume_init() }; | |
375 | /// assert_eq!(x, (0, false)); | |
376 | /// ``` | |
377 | /// | |
3dfed10e XL |
378 | /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()` |
379 | /// when `0` is not a valid bit-pattern for the type: | |
dc9dc135 XL |
380 | /// |
381 | /// ```rust,no_run | |
382 | /// use std::mem::MaybeUninit; | |
383 | /// | |
fc512014 | 384 | /// enum NotZero { One = 1, Two = 2 } |
dc9dc135 XL |
385 | /// |
386 | /// let x = MaybeUninit::<(u8, NotZero)>::zeroed(); | |
387 | /// let x = unsafe { x.assume_init() }; | |
388 | /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. | |
f9f354fc | 389 | /// // This is undefined behavior. ⚠️ |
dc9dc135 XL |
390 | /// ``` |
391 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
392 | #[inline] | |
dfeec247 | 393 | #[rustc_diagnostic_item = "maybe_uninit_zeroed"] |
dc9dc135 XL |
394 | pub fn zeroed() -> MaybeUninit<T> { |
395 | let mut u = MaybeUninit::<T>::uninit(); | |
1b1a35ee | 396 | // SAFETY: `u.as_mut_ptr()` points to allocated memory. |
dc9dc135 XL |
397 | unsafe { |
398 | u.as_mut_ptr().write_bytes(0u8, 1); | |
399 | } | |
400 | u | |
401 | } | |
402 | ||
403 | /// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value | |
404 | /// without dropping it, so be careful not to use this twice unless you want to | |
405 | /// skip running the destructor. For your convenience, this also returns a mutable | |
406 | /// reference to the (now safely initialized) contents of `self`. | |
e1599b0c | 407 | #[unstable(feature = "maybe_uninit_extra", issue = "63567")] |
fc512014 | 408 | #[rustc_const_unstable(feature = "maybe_uninit_extra", issue = "63567")] |
dc9dc135 | 409 | #[inline(always)] |
fc512014 | 410 | pub const fn write(&mut self, val: T) -> &mut T { |
1b1a35ee XL |
411 | *self = MaybeUninit::new(val); |
412 | // SAFETY: We just initialized this value. | |
413 | unsafe { self.assume_init_mut() } | |
dc9dc135 XL |
414 | } |
415 | ||
416 | /// Gets a pointer to the contained value. Reading from this pointer or turning it | |
417 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
418 | /// Writing to memory that this pointer (non-transitively) points to is undefined behavior | |
419 | /// (except inside an `UnsafeCell<T>`). | |
420 | /// | |
421 | /// # Examples | |
422 | /// | |
423 | /// Correct usage of this method: | |
424 | /// | |
425 | /// ```rust | |
426 | /// use std::mem::MaybeUninit; | |
427 | /// | |
428 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
fc512014 | 429 | /// unsafe { x.as_mut_ptr().write(vec![0, 1, 2]); } |
dc9dc135 XL |
430 | /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it. |
431 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
432 | /// assert_eq!(x_vec.len(), 3); | |
433 | /// ``` | |
434 | /// | |
435 | /// *Incorrect* usage of this method: | |
436 | /// | |
437 | /// ```rust,no_run | |
438 | /// use std::mem::MaybeUninit; | |
439 | /// | |
440 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
441 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
f9f354fc | 442 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
dc9dc135 XL |
443 | /// ``` |
444 | /// | |
445 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
446 | /// until they are, it is advisable to avoid them.) | |
447 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
3dfed10e | 448 | #[rustc_const_unstable(feature = "const_maybe_uninit_as_ptr", issue = "75251")] |
dc9dc135 | 449 | #[inline(always)] |
3dfed10e XL |
450 | pub const fn as_ptr(&self) -> *const T { |
451 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. | |
452 | self as *const _ as *const T | |
dc9dc135 XL |
453 | } |
454 | ||
455 | /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it | |
456 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
457 | /// | |
458 | /// # Examples | |
459 | /// | |
460 | /// Correct usage of this method: | |
461 | /// | |
462 | /// ```rust | |
463 | /// use std::mem::MaybeUninit; | |
464 | /// | |
465 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
fc512014 | 466 | /// unsafe { x.as_mut_ptr().write(vec![0, 1, 2]); } |
dc9dc135 XL |
467 | /// // Create a reference into the `MaybeUninit<Vec<u32>>`. |
468 | /// // This is okay because we initialized it. | |
469 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
470 | /// x_vec.push(3); | |
471 | /// assert_eq!(x_vec.len(), 4); | |
472 | /// ``` | |
473 | /// | |
474 | /// *Incorrect* usage of this method: | |
475 | /// | |
476 | /// ```rust,no_run | |
477 | /// use std::mem::MaybeUninit; | |
478 | /// | |
479 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
480 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
f9f354fc | 481 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
dc9dc135 XL |
482 | /// ``` |
483 | /// | |
484 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
485 | /// until they are, it is advisable to avoid them.) | |
486 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
3dfed10e | 487 | #[rustc_const_unstable(feature = "const_maybe_uninit_as_ptr", issue = "75251")] |
dc9dc135 | 488 | #[inline(always)] |
3dfed10e XL |
489 | pub const fn as_mut_ptr(&mut self) -> *mut T { |
490 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. | |
491 | self as *mut _ as *mut T | |
dc9dc135 XL |
492 | } |
493 | ||
494 | /// Extracts the value from the `MaybeUninit<T>` container. This is a great way | |
495 | /// to ensure that the data will get dropped, because the resulting `T` is | |
496 | /// subject to the usual drop handling. | |
497 | /// | |
498 | /// # Safety | |
499 | /// | |
500 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
501 | /// state. Calling this when the content is not yet fully initialized causes immediate undefined | |
502 | /// behavior. The [type-level documentation][inv] contains more information about | |
503 | /// this initialization invariant. | |
504 | /// | |
505 | /// [inv]: #initialization-invariant | |
506 | /// | |
416331ca XL |
507 | /// On top of that, remember that most types have additional invariants beyond merely |
508 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] | |
509 | /// is considered initialized (under the current implementation; this does not constitute | |
510 | /// a stable guarantee) because the only requirement the compiler knows about it | |
511 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause | |
512 | /// *immediate* undefined behavior, but will cause undefined behavior with most | |
513 | /// safe operations (including dropping it). | |
514 | /// | |
1b1a35ee XL |
515 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html |
516 | /// | |
dc9dc135 XL |
517 | /// # Examples |
518 | /// | |
519 | /// Correct usage of this method: | |
520 | /// | |
521 | /// ```rust | |
522 | /// use std::mem::MaybeUninit; | |
523 | /// | |
524 | /// let mut x = MaybeUninit::<bool>::uninit(); | |
525 | /// unsafe { x.as_mut_ptr().write(true); } | |
526 | /// let x_init = unsafe { x.assume_init() }; | |
527 | /// assert_eq!(x_init, true); | |
528 | /// ``` | |
529 | /// | |
530 | /// *Incorrect* usage of this method: | |
531 | /// | |
532 | /// ```rust,no_run | |
533 | /// use std::mem::MaybeUninit; | |
534 | /// | |
535 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
536 | /// let x_init = unsafe { x.assume_init() }; | |
f9f354fc | 537 | /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️ |
dc9dc135 XL |
538 | /// ``` |
539 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
fc512014 | 540 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
dc9dc135 | 541 | #[inline(always)] |
dfeec247 | 542 | #[rustc_diagnostic_item = "assume_init"] |
fc512014 | 543 | pub const unsafe fn assume_init(self) -> T { |
f035d41b XL |
544 | // SAFETY: the caller must guarantee that `self` is initialized. |
545 | // This also means that `self` must be a `value` variant. | |
546 | unsafe { | |
547 | intrinsics::assert_inhabited::<T>(); | |
548 | ManuallyDrop::into_inner(self.value) | |
549 | } | |
dc9dc135 XL |
550 | } |
551 | ||
552 | /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject | |
553 | /// to the usual drop handling. | |
554 | /// | |
416331ca | 555 | /// Whenever possible, it is preferable to use [`assume_init`] instead, which |
dc9dc135 XL |
556 | /// prevents duplicating the content of the `MaybeUninit<T>`. |
557 | /// | |
558 | /// # Safety | |
559 | /// | |
560 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
561 | /// state. Calling this when the content is not yet fully initialized causes undefined | |
562 | /// behavior. The [type-level documentation][inv] contains more information about | |
563 | /// this initialization invariant. | |
564 | /// | |
565 | /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using | |
1b1a35ee XL |
566 | /// multiple copies of the data (by calling `assume_init_read` multiple times, or first |
567 | /// calling `assume_init_read` and then [`assume_init`]), it is your responsibility | |
dc9dc135 XL |
568 | /// to ensure that that data may indeed be duplicated. |
569 | /// | |
570 | /// [inv]: #initialization-invariant | |
3dfed10e | 571 | /// [`assume_init`]: MaybeUninit::assume_init |
dc9dc135 XL |
572 | /// |
573 | /// # Examples | |
574 | /// | |
575 | /// Correct usage of this method: | |
576 | /// | |
577 | /// ```rust | |
578 | /// #![feature(maybe_uninit_extra)] | |
579 | /// use std::mem::MaybeUninit; | |
580 | /// | |
581 | /// let mut x = MaybeUninit::<u32>::uninit(); | |
582 | /// x.write(13); | |
1b1a35ee | 583 | /// let x1 = unsafe { x.assume_init_read() }; |
dc9dc135 | 584 | /// // `u32` is `Copy`, so we may read multiple times. |
1b1a35ee | 585 | /// let x2 = unsafe { x.assume_init_read() }; |
dc9dc135 XL |
586 | /// assert_eq!(x1, x2); |
587 | /// | |
588 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
589 | /// x.write(None); | |
1b1a35ee | 590 | /// let x1 = unsafe { x.assume_init_read() }; |
dc9dc135 | 591 | /// // Duplicating a `None` value is okay, so we may read multiple times. |
1b1a35ee | 592 | /// let x2 = unsafe { x.assume_init_read() }; |
dc9dc135 XL |
593 | /// assert_eq!(x1, x2); |
594 | /// ``` | |
595 | /// | |
596 | /// *Incorrect* usage of this method: | |
597 | /// | |
598 | /// ```rust,no_run | |
599 | /// #![feature(maybe_uninit_extra)] | |
600 | /// use std::mem::MaybeUninit; | |
601 | /// | |
602 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
fc512014 | 603 | /// x.write(Some(vec![0, 1, 2])); |
1b1a35ee XL |
604 | /// let x1 = unsafe { x.assume_init_read() }; |
605 | /// let x2 = unsafe { x.assume_init_read() }; | |
f9f354fc | 606 | /// // We now created two copies of the same vector, leading to a double-free ⚠️ when |
dc9dc135 XL |
607 | /// // they both get dropped! |
608 | /// ``` | |
e1599b0c | 609 | #[unstable(feature = "maybe_uninit_extra", issue = "63567")] |
5869c6ff | 610 | #[rustc_const_unstable(feature = "maybe_uninit_extra", issue = "63567")] |
dc9dc135 | 611 | #[inline(always)] |
5869c6ff | 612 | pub const unsafe fn assume_init_read(&self) -> T { |
f035d41b XL |
613 | // SAFETY: the caller must guarantee that `self` is initialized. |
614 | // Reading from `self.as_ptr()` is safe since `self` should be initialized. | |
615 | unsafe { | |
616 | intrinsics::assert_inhabited::<T>(); | |
617 | self.as_ptr().read() | |
618 | } | |
dc9dc135 XL |
619 | } |
620 | ||
1b1a35ee XL |
621 | /// Drops the contained value in place. |
622 | /// | |
623 | /// If you have ownership of the `MaybeUninit`, you can use [`assume_init`] instead. | |
624 | /// | |
625 | /// # Safety | |
626 | /// | |
627 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is | |
628 | /// in an initialized state. Calling this when the content is not yet fully | |
629 | /// initialized causes undefined behavior. | |
630 | /// | |
631 | /// On top of that, all additional invariants of the type `T` must be | |
632 | /// satisfied, as the `Drop` implementation of `T` (or its members) may | |
633 | /// rely on this. For example, a `1`-initialized [`Vec<T>`] is considered | |
634 | /// initialized (under the current implementation; this does not constitute | |
635 | /// a stable guarantee) because the only requirement the compiler knows | |
636 | /// about it is that the data pointer must be non-null. Dropping such a | |
637 | /// `Vec<T>` however will cause undefined behaviour. | |
638 | /// | |
639 | /// [`assume_init`]: MaybeUninit::assume_init | |
640 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html | |
641 | #[unstable(feature = "maybe_uninit_extra", issue = "63567")] | |
642 | pub unsafe fn assume_init_drop(&mut self) { | |
643 | // SAFETY: the caller must guarantee that `self` is initialized and | |
644 | // satisfies all invariants of `T`. | |
645 | // Dropping the value in place is safe if that is the case. | |
646 | unsafe { ptr::drop_in_place(self.as_mut_ptr()) } | |
647 | } | |
648 | ||
60c5eb7d XL |
649 | /// Gets a shared reference to the contained value. |
650 | /// | |
651 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
652 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
653 | /// of `.assume_init()`). | |
dc9dc135 XL |
654 | /// |
655 | /// # Safety | |
656 | /// | |
60c5eb7d XL |
657 | /// Calling this when the content is not yet fully initialized causes undefined |
658 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
659 | /// is in an initialized state. | |
660 | /// | |
661 | /// # Examples | |
662 | /// | |
663 | /// ### Correct usage of this method: | |
664 | /// | |
665 | /// ```rust | |
666 | /// #![feature(maybe_uninit_ref)] | |
667 | /// use std::mem::MaybeUninit; | |
668 | /// | |
669 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
670 | /// // Initialize `x`: | |
671 | /// unsafe { x.as_mut_ptr().write(vec![1, 2, 3]); } | |
672 | /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to | |
673 | /// // create a shared reference to it: | |
674 | /// let x: &Vec<u32> = unsafe { | |
1b1a35ee XL |
675 | /// // SAFETY: `x` has been initialized. |
676 | /// x.assume_init_ref() | |
60c5eb7d XL |
677 | /// }; |
678 | /// assert_eq!(x, &vec![1, 2, 3]); | |
679 | /// ``` | |
680 | /// | |
681 | /// ### *Incorrect* usages of this method: | |
682 | /// | |
683 | /// ```rust,no_run | |
684 | /// #![feature(maybe_uninit_ref)] | |
685 | /// use std::mem::MaybeUninit; | |
686 | /// | |
687 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
1b1a35ee | 688 | /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() }; |
f9f354fc | 689 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
60c5eb7d XL |
690 | /// ``` |
691 | /// | |
692 | /// ```rust,no_run | |
693 | /// #![feature(maybe_uninit_ref)] | |
694 | /// use std::{cell::Cell, mem::MaybeUninit}; | |
695 | /// | |
696 | /// let b = MaybeUninit::<Cell<bool>>::uninit(); | |
697 | /// // Initialize the `MaybeUninit` using `Cell::set`: | |
698 | /// unsafe { | |
1b1a35ee XL |
699 | /// b.assume_init_ref().set(true); |
700 | /// // ^^^^^^^^^^^^^^^ | |
701 | /// // Reference to an uninitialized `Cell<bool>`: UB! | |
60c5eb7d XL |
702 | /// } |
703 | /// ``` | |
e1599b0c | 704 | #[unstable(feature = "maybe_uninit_ref", issue = "63568")] |
fc512014 | 705 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
dc9dc135 | 706 | #[inline(always)] |
fc512014 | 707 | pub const unsafe fn assume_init_ref(&self) -> &T { |
f035d41b XL |
708 | // SAFETY: the caller must guarantee that `self` is initialized. |
709 | // This also means that `self` must be a `value` variant. | |
710 | unsafe { | |
711 | intrinsics::assert_inhabited::<T>(); | |
fc512014 | 712 | &*self.as_ptr() |
f035d41b | 713 | } |
dc9dc135 XL |
714 | } |
715 | ||
60c5eb7d XL |
716 | /// Gets a mutable (unique) reference to the contained value. |
717 | /// | |
718 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
719 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
720 | /// of `.assume_init()`). | |
dc9dc135 XL |
721 | /// |
722 | /// # Safety | |
723 | /// | |
60c5eb7d XL |
724 | /// Calling this when the content is not yet fully initialized causes undefined |
725 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
1b1a35ee | 726 | /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to |
60c5eb7d XL |
727 | /// initialize a `MaybeUninit`. |
728 | /// | |
729 | /// # Examples | |
730 | /// | |
731 | /// ### Correct usage of this method: | |
732 | /// | |
733 | /// ```rust | |
734 | /// #![feature(maybe_uninit_ref)] | |
735 | /// use std::mem::MaybeUninit; | |
736 | /// | |
737 | /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 2048]) { *buf = [0; 2048] } | |
738 | /// # #[cfg(FALSE)] | |
739 | /// extern "C" { | |
740 | /// /// Initializes *all* the bytes of the input buffer. | |
741 | /// fn initialize_buffer(buf: *mut [u8; 2048]); | |
742 | /// } | |
743 | /// | |
744 | /// let mut buf = MaybeUninit::<[u8; 2048]>::uninit(); | |
745 | /// | |
746 | /// // Initialize `buf`: | |
747 | /// unsafe { initialize_buffer(buf.as_mut_ptr()); } | |
748 | /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it. | |
749 | /// // However, using `.assume_init()` may trigger a `memcpy` of the 2048 bytes. | |
750 | /// // To assert our buffer has been initialized without copying it, we upgrade | |
751 | /// // the `&mut MaybeUninit<[u8; 2048]>` to a `&mut [u8; 2048]`: | |
752 | /// let buf: &mut [u8; 2048] = unsafe { | |
1b1a35ee XL |
753 | /// // SAFETY: `buf` has been initialized. |
754 | /// buf.assume_init_mut() | |
60c5eb7d XL |
755 | /// }; |
756 | /// | |
757 | /// // Now we can use `buf` as a normal slice: | |
758 | /// buf.sort_unstable(); | |
759 | /// assert!( | |
74b04a01 | 760 | /// buf.windows(2).all(|pair| pair[0] <= pair[1]), |
60c5eb7d XL |
761 | /// "buffer is sorted", |
762 | /// ); | |
763 | /// ``` | |
764 | /// | |
765 | /// ### *Incorrect* usages of this method: | |
766 | /// | |
1b1a35ee | 767 | /// You cannot use `.assume_init_mut()` to initialize a value: |
60c5eb7d XL |
768 | /// |
769 | /// ```rust,no_run | |
770 | /// #![feature(maybe_uninit_ref)] | |
771 | /// use std::mem::MaybeUninit; | |
772 | /// | |
773 | /// let mut b = MaybeUninit::<bool>::uninit(); | |
774 | /// unsafe { | |
1b1a35ee | 775 | /// *b.assume_init_mut() = true; |
60c5eb7d | 776 | /// // We have created a (mutable) reference to an uninitialized `bool`! |
f9f354fc | 777 | /// // This is undefined behavior. ⚠️ |
60c5eb7d XL |
778 | /// } |
779 | /// ``` | |
780 | /// | |
781 | /// For instance, you cannot [`Read`] into an uninitialized buffer: | |
782 | /// | |
783 | /// [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html | |
784 | /// | |
785 | /// ```rust,no_run | |
786 | /// #![feature(maybe_uninit_ref)] | |
787 | /// use std::{io, mem::MaybeUninit}; | |
788 | /// | |
789 | /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]> | |
790 | /// { | |
791 | /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit(); | |
1b1a35ee XL |
792 | /// reader.read_exact(unsafe { buffer.assume_init_mut() })?; |
793 | /// // ^^^^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
794 | /// // (mutable) reference to uninitialized memory! |
795 | /// // This is undefined behavior. | |
796 | /// Ok(unsafe { buffer.assume_init() }) | |
797 | /// } | |
798 | /// ``` | |
799 | /// | |
800 | /// Nor can you use direct field access to do field-by-field gradual initialization: | |
801 | /// | |
802 | /// ```rust,no_run | |
803 | /// #![feature(maybe_uninit_ref)] | |
804 | /// use std::{mem::MaybeUninit, ptr}; | |
805 | /// | |
806 | /// struct Foo { | |
807 | /// a: u32, | |
808 | /// b: u8, | |
809 | /// } | |
810 | /// | |
811 | /// let foo: Foo = unsafe { | |
812 | /// let mut foo = MaybeUninit::<Foo>::uninit(); | |
1b1a35ee XL |
813 | /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337); |
814 | /// // ^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
815 | /// // (mutable) reference to uninitialized memory! |
816 | /// // This is undefined behavior. | |
1b1a35ee XL |
817 | /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42); |
818 | /// // ^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
819 | /// // (mutable) reference to uninitialized memory! |
820 | /// // This is undefined behavior. | |
821 | /// foo.assume_init() | |
822 | /// }; | |
823 | /// ``` | |
1b1a35ee | 824 | // FIXME(#76092): We currently rely on the above being incorrect, i.e., we have references |
dc9dc135 XL |
825 | // to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make |
826 | // a final decision about the rules before stabilization. | |
e1599b0c | 827 | #[unstable(feature = "maybe_uninit_ref", issue = "63568")] |
fc512014 | 828 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
dc9dc135 | 829 | #[inline(always)] |
fc512014 | 830 | pub const unsafe fn assume_init_mut(&mut self) -> &mut T { |
f035d41b XL |
831 | // SAFETY: the caller must guarantee that `self` is initialized. |
832 | // This also means that `self` must be a `value` variant. | |
833 | unsafe { | |
834 | intrinsics::assert_inhabited::<T>(); | |
fc512014 | 835 | &mut *self.as_mut_ptr() |
f035d41b | 836 | } |
dc9dc135 XL |
837 | } |
838 | ||
5869c6ff XL |
839 | /// Extracts the values from an array of `MaybeUninit` containers. |
840 | /// | |
841 | /// # Safety | |
842 | /// | |
843 | /// It is up to the caller to guarantee that all elements of the array are | |
844 | /// in an initialized state. | |
845 | /// | |
846 | /// # Examples | |
847 | /// | |
848 | /// ``` | |
849 | /// #![feature(maybe_uninit_uninit_array)] | |
850 | /// #![feature(maybe_uninit_array_assume_init)] | |
851 | /// use std::mem::MaybeUninit; | |
852 | /// | |
853 | /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array(); | |
854 | /// array[0] = MaybeUninit::new(0); | |
855 | /// array[1] = MaybeUninit::new(1); | |
856 | /// array[2] = MaybeUninit::new(2); | |
857 | /// | |
858 | /// // SAFETY: Now safe as we initialised all elements | |
859 | /// let array = unsafe { | |
860 | /// MaybeUninit::array_assume_init(array) | |
861 | /// }; | |
862 | /// | |
863 | /// assert_eq!(array, [0, 1, 2]); | |
864 | /// ``` | |
865 | #[unstable(feature = "maybe_uninit_array_assume_init", issue = "80908")] | |
866 | #[inline(always)] | |
867 | pub unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] { | |
868 | // SAFETY: | |
869 | // * The caller guarantees that all elements of the array are initialized | |
870 | // * `MaybeUninit<T>` and T are guaranteed to have the same layout | |
871 | // * MaybeUnint does not drop, so there are no double-frees | |
872 | // And thus the conversion is safe | |
873 | unsafe { | |
874 | intrinsics::assert_inhabited::<[T; N]>(); | |
875 | (&array as *const _ as *const [T; N]).read() | |
876 | } | |
877 | } | |
878 | ||
60c5eb7d XL |
879 | /// Assuming all the elements are initialized, get a slice to them. |
880 | /// | |
881 | /// # Safety | |
882 | /// | |
883 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
884 | /// really are in an initialized state. | |
885 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
1b1a35ee XL |
886 | /// |
887 | /// See [`assume_init_ref`] for more details and examples. | |
888 | /// | |
889 | /// [`assume_init_ref`]: MaybeUninit::assume_init_ref | |
890 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] | |
fc512014 | 891 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
60c5eb7d | 892 | #[inline(always)] |
fc512014 | 893 | pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] { |
f035d41b XL |
894 | // SAFETY: casting slice to a `*const [T]` is safe since the caller guarantees that |
895 | // `slice` is initialized, and`MaybeUninit` is guaranteed to have the same layout as `T`. | |
896 | // The pointer obtained is valid since it refers to memory owned by `slice` which is a | |
897 | // reference and thus guaranteed to be valid for reads. | |
898 | unsafe { &*(slice as *const [Self] as *const [T]) } | |
60c5eb7d XL |
899 | } |
900 | ||
901 | /// Assuming all the elements are initialized, get a mutable slice to them. | |
902 | /// | |
903 | /// # Safety | |
904 | /// | |
905 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
906 | /// really are in an initialized state. | |
907 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
1b1a35ee XL |
908 | /// |
909 | /// See [`assume_init_mut`] for more details and examples. | |
910 | /// | |
911 | /// [`assume_init_mut`]: MaybeUninit::assume_init_mut | |
912 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] | |
fc512014 | 913 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
60c5eb7d | 914 | #[inline(always)] |
fc512014 | 915 | pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] { |
f035d41b XL |
916 | // SAFETY: similar to safety notes for `slice_get_ref`, but we have a |
917 | // mutable reference which is also guaranteed to be valid for writes. | |
918 | unsafe { &mut *(slice as *mut [Self] as *mut [T]) } | |
60c5eb7d XL |
919 | } |
920 | ||
dc9dc135 | 921 | /// Gets a pointer to the first element of the array. |
e1599b0c | 922 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
fc512014 | 923 | #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 | 924 | #[inline(always)] |
fc512014 | 925 | pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T { |
29967ef6 | 926 | this.as_ptr() as *const T |
dc9dc135 XL |
927 | } |
928 | ||
929 | /// Gets a mutable pointer to the first element of the array. | |
e1599b0c | 930 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
fc512014 | 931 | #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 | 932 | #[inline(always)] |
fc512014 | 933 | pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T { |
29967ef6 | 934 | this.as_mut_ptr() as *mut T |
dc9dc135 | 935 | } |
fc512014 XL |
936 | |
937 | /// Copies the elements from `src` to `this`, returning a mutable reference to the now initalized contents of `this`. | |
938 | /// | |
939 | /// If `T` does not implement `Copy`, use [`write_slice_cloned`] | |
940 | /// | |
941 | /// This is similar to [`slice::copy_from_slice`]. | |
942 | /// | |
943 | /// # Panics | |
944 | /// | |
945 | /// This function will panic if the two slices have different lengths. | |
946 | /// | |
947 | /// # Examples | |
948 | /// | |
949 | /// ``` | |
950 | /// #![feature(maybe_uninit_write_slice)] | |
951 | /// use std::mem::MaybeUninit; | |
952 | /// | |
953 | /// let mut dst = [MaybeUninit::uninit(); 32]; | |
954 | /// let src = [0; 32]; | |
955 | /// | |
956 | /// let init = MaybeUninit::write_slice(&mut dst, &src); | |
957 | /// | |
958 | /// assert_eq!(init, src); | |
959 | /// ``` | |
960 | /// | |
961 | /// ``` | |
962 | /// #![feature(maybe_uninit_write_slice, vec_spare_capacity)] | |
963 | /// use std::mem::MaybeUninit; | |
964 | /// | |
965 | /// let mut vec = Vec::with_capacity(32); | |
966 | /// let src = [0; 16]; | |
967 | /// | |
968 | /// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src); | |
969 | /// | |
970 | /// // SAFETY: we have just copied all the elements of len into the spare capacity | |
971 | /// // the first src.len() elements of the vec are valid now. | |
972 | /// unsafe { | |
973 | /// vec.set_len(src.len()); | |
974 | /// } | |
975 | /// | |
976 | /// assert_eq!(vec, src); | |
977 | /// ``` | |
978 | /// | |
979 | /// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned | |
fc512014 XL |
980 | #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")] |
981 | pub fn write_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] | |
982 | where | |
983 | T: Copy, | |
984 | { | |
985 | // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout | |
986 | let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) }; | |
987 | ||
988 | this.copy_from_slice(uninit_src); | |
989 | ||
990 | // SAFETY: Valid elements have just been copied into `this` so it is initalized | |
991 | unsafe { MaybeUninit::slice_assume_init_mut(this) } | |
992 | } | |
993 | ||
994 | /// Clones the elements from `src` to `this`, returning a mutable reference to the now initalized contents of `this`. | |
995 | /// Any already initalized elements will not be dropped. | |
996 | /// | |
997 | /// If `T` implements `Copy`, use [`write_slice`] | |
998 | /// | |
999 | /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements. | |
1000 | /// | |
1001 | /// # Panics | |
1002 | /// | |
1003 | /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics. | |
1004 | /// | |
1005 | /// If there is a panic, the already cloned elements will be dropped. | |
1006 | /// | |
1007 | /// # Examples | |
1008 | /// | |
1009 | /// ``` | |
1010 | /// #![feature(maybe_uninit_write_slice)] | |
1011 | /// use std::mem::MaybeUninit; | |
1012 | /// | |
1013 | /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()]; | |
1014 | /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()]; | |
1015 | /// | |
1016 | /// let init = MaybeUninit::write_slice_cloned(&mut dst, &src); | |
1017 | /// | |
1018 | /// assert_eq!(init, src); | |
1019 | /// ``` | |
1020 | /// | |
1021 | /// ``` | |
1022 | /// #![feature(maybe_uninit_write_slice, vec_spare_capacity)] | |
1023 | /// use std::mem::MaybeUninit; | |
1024 | /// | |
1025 | /// let mut vec = Vec::with_capacity(32); | |
1026 | /// let src = ["rust", "is", "a", "pretty", "cool", "language"]; | |
1027 | /// | |
1028 | /// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src); | |
1029 | /// | |
1030 | /// // SAFETY: we have just cloned all the elements of len into the spare capacity | |
1031 | /// // the first src.len() elements of the vec are valid now. | |
1032 | /// unsafe { | |
1033 | /// vec.set_len(src.len()); | |
1034 | /// } | |
1035 | /// | |
1036 | /// assert_eq!(vec, src); | |
1037 | /// ``` | |
1038 | /// | |
1039 | /// [`write_slice`]: MaybeUninit::write_slice | |
fc512014 XL |
1040 | #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")] |
1041 | pub fn write_slice_cloned<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] | |
1042 | where | |
1043 | T: Clone, | |
1044 | { | |
1045 | // unlike copy_from_slice this does not call clone_from_slice on the slice | |
1046 | // this is because `MaybeUninit<T: Clone>` does not implement Clone. | |
1047 | ||
1048 | struct Guard<'a, T> { | |
1049 | slice: &'a mut [MaybeUninit<T>], | |
1050 | initialized: usize, | |
1051 | } | |
1052 | ||
1053 | impl<'a, T> Drop for Guard<'a, T> { | |
1054 | fn drop(&mut self) { | |
1055 | let initialized_part = &mut self.slice[..self.initialized]; | |
1056 | // SAFETY: this raw slice will contain only initialized objects | |
1057 | // that's why, it is allowed to drop it. | |
1058 | unsafe { | |
1059 | crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part)); | |
1060 | } | |
1061 | } | |
1062 | } | |
1063 | ||
1064 | assert_eq!(this.len(), src.len(), "destination and source slices have different lengths"); | |
1065 | // NOTE: We need to explicitly slice them to the same length | |
1066 | // for bounds checking to be elided, and the optimizer will | |
1067 | // generate memcpy for simple cases (for example T = u8). | |
1068 | let len = this.len(); | |
1069 | let src = &src[..len]; | |
1070 | ||
1071 | // guard is needed b/c panic might happen during a clone | |
1072 | let mut guard = Guard { slice: this, initialized: 0 }; | |
1073 | ||
1074 | for i in 0..len { | |
1075 | guard.slice[i].write(src[i].clone()); | |
1076 | guard.initialized += 1; | |
1077 | } | |
1078 | ||
1079 | super::forget(guard); | |
1080 | ||
1081 | // SAFETY: Valid elements have just been written into `this` so it is initalized | |
1082 | unsafe { MaybeUninit::slice_assume_init_mut(this) } | |
1083 | } | |
dc9dc135 | 1084 | } |