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Commit | Line | Data |
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60c5eb7d XL |
1 | use crate::any::type_name; |
2 | use crate::fmt; | |
dc9dc135 | 3 | use crate::intrinsics; |
5099ac24 | 4 | use crate::mem::{self, ManuallyDrop}; |
1b1a35ee | 5 | use crate::ptr; |
5099ac24 | 6 | use crate::slice; |
60c5eb7d | 7 | |
dc9dc135 XL |
8 | /// A wrapper type to construct uninitialized instances of `T`. |
9 | /// | |
10 | /// # Initialization invariant | |
11 | /// | |
e74abb32 XL |
12 | /// The compiler, in general, assumes that a variable is properly initialized |
13 | /// according to the requirements of the variable's type. For example, a variable of | |
17df50a5 | 14 | /// reference type must be aligned and non-null. This is an invariant that must |
e74abb32 XL |
15 | /// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a |
16 | /// variable of reference type causes instantaneous [undefined behavior][ub], | |
17 | /// no matter whether that reference ever gets used to access memory: | |
dc9dc135 XL |
18 | /// |
19 | /// ```rust,no_run | |
416331ca | 20 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
21 | /// use std::mem::{self, MaybeUninit}; |
22 | /// | |
f9f354fc | 23 | /// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️ |
dc9dc135 | 24 | /// // The equivalent code with `MaybeUninit<&i32>`: |
f9f354fc | 25 | /// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
26 | /// ``` |
27 | /// | |
28 | /// This is exploited by the compiler for various optimizations, such as eliding | |
29 | /// run-time checks and optimizing `enum` layout. | |
30 | /// | |
31 | /// Similarly, entirely uninitialized memory may have any content, while a `bool` must | |
32 | /// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior: | |
33 | /// | |
34 | /// ```rust,no_run | |
416331ca | 35 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
36 | /// use std::mem::{self, MaybeUninit}; |
37 | /// | |
f9f354fc | 38 | /// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
dc9dc135 | 39 | /// // The equivalent code with `MaybeUninit<bool>`: |
f9f354fc | 40 | /// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
41 | /// ``` |
42 | /// | |
6a06907d XL |
43 | /// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed" |
44 | /// meaning "it won't change without being written to"). Reading the same uninitialized byte | |
45 | /// multiple times can give different results. This makes it undefined behavior to have | |
46 | /// uninitialized data in a variable even if that variable has an integer type, which otherwise can | |
47 | /// hold any *fixed* bit pattern: | |
dc9dc135 XL |
48 | /// |
49 | /// ```rust,no_run | |
416331ca | 50 | /// # #![allow(invalid_value)] |
dc9dc135 XL |
51 | /// use std::mem::{self, MaybeUninit}; |
52 | /// | |
f9f354fc | 53 | /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️ |
dc9dc135 | 54 | /// // The equivalent code with `MaybeUninit<i32>`: |
f9f354fc | 55 | /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️ |
dc9dc135 XL |
56 | /// ``` |
57 | /// (Notice that the rules around uninitialized integers are not finalized yet, but | |
58 | /// until they are, it is advisable to avoid them.) | |
59 | /// | |
60 | /// On top of that, remember that most types have additional invariants beyond merely | |
61 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] | |
416331ca XL |
62 | /// is considered initialized (under the current implementation; this does not constitute |
63 | /// a stable guarantee) because the only requirement the compiler knows about it | |
dc9dc135 XL |
64 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause |
65 | /// *immediate* undefined behavior, but will cause undefined behavior with most | |
66 | /// safe operations (including dropping it). | |
67 | /// | |
68 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html | |
69 | /// | |
70 | /// # Examples | |
71 | /// | |
72 | /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data. | |
73 | /// It is a signal to the compiler indicating that the data here might *not* | |
74 | /// be initialized: | |
75 | /// | |
76 | /// ```rust | |
77 | /// use std::mem::MaybeUninit; | |
78 | /// | |
79 | /// // Create an explicitly uninitialized reference. The compiler knows that data inside | |
80 | /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB: | |
81 | /// let mut x = MaybeUninit::<&i32>::uninit(); | |
82 | /// // Set it to a valid value. | |
136023e0 | 83 | /// x.write(&0); |
dc9dc135 XL |
84 | /// // Extract the initialized data -- this is only allowed *after* properly |
85 | /// // initializing `x`! | |
86 | /// let x = unsafe { x.assume_init() }; | |
87 | /// ``` | |
88 | /// | |
89 | /// The compiler then knows to not make any incorrect assumptions or optimizations on this code. | |
90 | /// | |
91 | /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without | |
92 | /// any of the run-time tracking and without any of the safety checks. | |
93 | /// | |
94 | /// ## out-pointers | |
95 | /// | |
96 | /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data | |
97 | /// from a function, pass it a pointer to some (uninitialized) memory to put the | |
98 | /// result into. This can be useful when it is important for the caller to control | |
99 | /// how the memory the result is stored in gets allocated, and you want to avoid | |
100 | /// unnecessary moves. | |
101 | /// | |
102 | /// ``` | |
103 | /// use std::mem::MaybeUninit; | |
104 | /// | |
105 | /// unsafe fn make_vec(out: *mut Vec<i32>) { | |
106 | /// // `write` does not drop the old contents, which is important. | |
107 | /// out.write(vec![1, 2, 3]); | |
108 | /// } | |
109 | /// | |
110 | /// let mut v = MaybeUninit::uninit(); | |
111 | /// unsafe { make_vec(v.as_mut_ptr()); } | |
112 | /// // Now we know `v` is initialized! This also makes sure the vector gets | |
113 | /// // properly dropped. | |
114 | /// let v = unsafe { v.assume_init() }; | |
115 | /// assert_eq!(&v, &[1, 2, 3]); | |
116 | /// ``` | |
117 | /// | |
118 | /// ## Initializing an array element-by-element | |
119 | /// | |
120 | /// `MaybeUninit<T>` can be used to initialize a large array element-by-element: | |
121 | /// | |
122 | /// ``` | |
123 | /// use std::mem::{self, MaybeUninit}; | |
dc9dc135 XL |
124 | /// |
125 | /// let data = { | |
126 | /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is | |
127 | /// // safe because the type we are claiming to have initialized here is a | |
128 | /// // bunch of `MaybeUninit`s, which do not require initialization. | |
129 | /// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe { | |
130 | /// MaybeUninit::uninit().assume_init() | |
131 | /// }; | |
132 | /// | |
416331ca XL |
133 | /// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer |
134 | /// // assignment instead of `ptr::write` does not cause the old | |
135 | /// // uninitialized value to be dropped. Also if there is a panic during | |
136 | /// // this loop, we have a memory leak, but there is no memory safety | |
137 | /// // issue. | |
dc9dc135 | 138 | /// for elem in &mut data[..] { |
136023e0 | 139 | /// elem.write(vec![42]); |
dc9dc135 XL |
140 | /// } |
141 | /// | |
142 | /// // Everything is initialized. Transmute the array to the | |
143 | /// // initialized type. | |
144 | /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) } | |
145 | /// }; | |
146 | /// | |
147 | /// assert_eq!(&data[0], &[42]); | |
148 | /// ``` | |
149 | /// | |
150 | /// You can also work with partially initialized arrays, which could | |
151 | /// be found in low-level datastructures. | |
152 | /// | |
153 | /// ``` | |
154 | /// use std::mem::MaybeUninit; | |
155 | /// use std::ptr; | |
156 | /// | |
157 | /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is | |
158 | /// // safe because the type we are claiming to have initialized here is a | |
159 | /// // bunch of `MaybeUninit`s, which do not require initialization. | |
160 | /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() }; | |
161 | /// // Count the number of elements we have assigned. | |
162 | /// let mut data_len: usize = 0; | |
163 | /// | |
164 | /// for elem in &mut data[0..500] { | |
136023e0 | 165 | /// elem.write(String::from("hello")); |
dc9dc135 XL |
166 | /// data_len += 1; |
167 | /// } | |
168 | /// | |
169 | /// // For each item in the array, drop if we allocated it. | |
170 | /// for elem in &mut data[0..data_len] { | |
171 | /// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); } | |
172 | /// } | |
173 | /// ``` | |
174 | /// | |
175 | /// ## Initializing a struct field-by-field | |
176 | /// | |
5869c6ff | 177 | /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field: |
dc9dc135 | 178 | /// |
5869c6ff XL |
179 | /// ```rust |
180 | /// use std::mem::MaybeUninit; | |
181 | /// use std::ptr::addr_of_mut; | |
182 | /// | |
183 | /// #[derive(Debug, PartialEq)] | |
184 | /// pub struct Foo { | |
185 | /// name: String, | |
186 | /// list: Vec<u8>, | |
187 | /// } | |
188 | /// | |
189 | /// let foo = { | |
190 | /// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit(); | |
191 | /// let ptr = uninit.as_mut_ptr(); | |
192 | /// | |
193 | /// // Initializing the `name` field | |
cdc7bbd5 XL |
194 | /// // Using `write` instead of assignment via `=` to not call `drop` on the |
195 | /// // old, uninitialized value. | |
5869c6ff XL |
196 | /// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); } |
197 | /// | |
198 | /// // Initializing the `list` field | |
199 | /// // If there is a panic here, then the `String` in the `name` field leaks. | |
200 | /// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); } | |
201 | /// | |
202 | /// // All the fields are initialized, so we call `assume_init` to get an initialized Foo. | |
203 | /// unsafe { uninit.assume_init() } | |
204 | /// }; | |
205 | /// | |
206 | /// assert_eq!( | |
207 | /// foo, | |
208 | /// Foo { | |
209 | /// name: "Bob".to_string(), | |
210 | /// list: vec![0, 1, 2] | |
211 | /// } | |
212 | /// ); | |
213 | /// ``` | |
214 | /// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut | |
dc9dc135 XL |
215 | /// [ub]: ../../reference/behavior-considered-undefined.html |
216 | /// | |
217 | /// # Layout | |
218 | /// | |
219 | /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`: | |
220 | /// | |
221 | /// ```rust | |
222 | /// use std::mem::{MaybeUninit, size_of, align_of}; | |
223 | /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>()); | |
224 | /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>()); | |
225 | /// ``` | |
226 | /// | |
227 | /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same | |
228 | /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as | |
229 | /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit | |
230 | /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling | |
231 | /// optimizations, potentially resulting in a larger size: | |
232 | /// | |
233 | /// ```rust | |
234 | /// # use std::mem::{MaybeUninit, size_of}; | |
235 | /// assert_eq!(size_of::<Option<bool>>(), 1); | |
236 | /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2); | |
237 | /// ``` | |
238 | /// | |
239 | /// If `T` is FFI-safe, then so is `MaybeUninit<T>`. | |
240 | /// | |
241 | /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size, | |
242 | /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and | |
243 | /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type | |
244 | /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`. | |
245 | /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the | |
246 | /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact | |
247 | /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not | |
248 | /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has | |
249 | /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that | |
250 | /// guarantee may evolve. | |
dc9dc135 | 251 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
416331ca | 252 | // Lang item so we can wrap other types in it. This is useful for generators. |
e1599b0c | 253 | #[lang = "maybe_uninit"] |
dc9dc135 | 254 | #[derive(Copy)] |
416331ca | 255 | #[repr(transparent)] |
dc9dc135 XL |
256 | pub union MaybeUninit<T> { |
257 | uninit: (), | |
258 | value: ManuallyDrop<T>, | |
259 | } | |
260 | ||
261 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
262 | impl<T: Copy> Clone for MaybeUninit<T> { | |
263 | #[inline(always)] | |
264 | fn clone(&self) -> Self { | |
265 | // Not calling `T::clone()`, we cannot know if we are initialized enough for that. | |
266 | *self | |
267 | } | |
268 | } | |
269 | ||
60c5eb7d XL |
270 | #[stable(feature = "maybe_uninit_debug", since = "1.41.0")] |
271 | impl<T> fmt::Debug for MaybeUninit<T> { | |
272 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { | |
273 | f.pad(type_name::<Self>()) | |
274 | } | |
275 | } | |
276 | ||
dc9dc135 XL |
277 | impl<T> MaybeUninit<T> { |
278 | /// Creates a new `MaybeUninit<T>` initialized with the given value. | |
279 | /// It is safe to call [`assume_init`] on the return value of this function. | |
280 | /// | |
281 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
282 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
283 | /// | |
29967ef6 XL |
284 | /// # Example |
285 | /// | |
286 | /// ``` | |
287 | /// use std::mem::MaybeUninit; | |
288 | /// | |
289 | /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]); | |
290 | /// ``` | |
291 | /// | |
3dfed10e | 292 | /// [`assume_init`]: MaybeUninit::assume_init |
dc9dc135 | 293 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
dfeec247 | 294 | #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")] |
c295e0f8 | 295 | #[must_use = "use `forget` to avoid running Drop code"] |
dc9dc135 XL |
296 | #[inline(always)] |
297 | pub const fn new(val: T) -> MaybeUninit<T> { | |
298 | MaybeUninit { value: ManuallyDrop::new(val) } | |
299 | } | |
300 | ||
301 | /// Creates a new `MaybeUninit<T>` in an uninitialized state. | |
302 | /// | |
303 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
304 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
305 | /// | |
29967ef6 | 306 | /// See the [type-level documentation][MaybeUninit] for some examples. |
dc9dc135 | 307 | /// |
29967ef6 XL |
308 | /// # Example |
309 | /// | |
310 | /// ``` | |
311 | /// use std::mem::MaybeUninit; | |
312 | /// | |
313 | /// let v: MaybeUninit<String> = MaybeUninit::uninit(); | |
314 | /// ``` | |
dc9dc135 | 315 | #[stable(feature = "maybe_uninit", since = "1.36.0")] |
dfeec247 | 316 | #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")] |
c295e0f8 | 317 | #[must_use] |
dc9dc135 | 318 | #[inline(always)] |
dfeec247 | 319 | #[rustc_diagnostic_item = "maybe_uninit_uninit"] |
dc9dc135 XL |
320 | pub const fn uninit() -> MaybeUninit<T> { |
321 | MaybeUninit { uninit: () } | |
322 | } | |
323 | ||
60c5eb7d XL |
324 | /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state. |
325 | /// | |
326 | /// Note: in a future Rust version this method may become unnecessary | |
cdc7bbd5 XL |
327 | /// when Rust allows |
328 | /// [inline const expressions](https://github.com/rust-lang/rust/issues/76001). | |
329 | /// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`. | |
60c5eb7d XL |
330 | /// |
331 | /// # Examples | |
332 | /// | |
333 | /// ```no_run | |
5099ac24 | 334 | /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)] |
60c5eb7d XL |
335 | /// |
336 | /// use std::mem::MaybeUninit; | |
337 | /// | |
338 | /// extern "C" { | |
339 | /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize; | |
340 | /// } | |
341 | /// | |
342 | /// /// Returns a (possibly smaller) slice of data that was actually read | |
343 | /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] { | |
344 | /// unsafe { | |
345 | /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len()); | |
1b1a35ee | 346 | /// MaybeUninit::slice_assume_init_ref(&buf[..len]) |
60c5eb7d XL |
347 | /// } |
348 | /// } | |
349 | /// | |
350 | /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array(); | |
351 | /// let data = read(&mut buf); | |
352 | /// ``` | |
04454e1e FG |
353 | #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")] |
354 | #[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")] | |
c295e0f8 | 355 | #[must_use] |
60c5eb7d | 356 | #[inline(always)] |
04454e1e | 357 | pub const fn uninit_array<const N: usize>() -> [Self; N] { |
1b1a35ee | 358 | // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid. |
04454e1e | 359 | unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() } |
60c5eb7d XL |
360 | } |
361 | ||
dc9dc135 XL |
362 | /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being |
363 | /// filled with `0` bytes. It depends on `T` whether that already makes for | |
364 | /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized, | |
365 | /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not | |
366 | /// be null. | |
367 | /// | |
368 | /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code. | |
369 | /// It is your responsibility to make sure `T` gets dropped if it got initialized. | |
370 | /// | |
371 | /// # Example | |
372 | /// | |
373 | /// Correct usage of this function: initializing a struct with zero, where all | |
374 | /// fields of the struct can hold the bit-pattern 0 as a valid value. | |
375 | /// | |
376 | /// ```rust | |
377 | /// use std::mem::MaybeUninit; | |
378 | /// | |
379 | /// let x = MaybeUninit::<(u8, bool)>::zeroed(); | |
380 | /// let x = unsafe { x.assume_init() }; | |
381 | /// assert_eq!(x, (0, false)); | |
382 | /// ``` | |
383 | /// | |
3dfed10e XL |
384 | /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()` |
385 | /// when `0` is not a valid bit-pattern for the type: | |
dc9dc135 XL |
386 | /// |
387 | /// ```rust,no_run | |
388 | /// use std::mem::MaybeUninit; | |
389 | /// | |
fc512014 | 390 | /// enum NotZero { One = 1, Two = 2 } |
dc9dc135 XL |
391 | /// |
392 | /// let x = MaybeUninit::<(u8, NotZero)>::zeroed(); | |
393 | /// let x = unsafe { x.assume_init() }; | |
394 | /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. | |
f9f354fc | 395 | /// // This is undefined behavior. ⚠️ |
dc9dc135 XL |
396 | /// ``` |
397 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
a2a8927a | 398 | #[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")] |
c295e0f8 | 399 | #[must_use] |
dc9dc135 | 400 | #[inline] |
dfeec247 | 401 | #[rustc_diagnostic_item = "maybe_uninit_zeroed"] |
a2a8927a | 402 | pub const fn zeroed() -> MaybeUninit<T> { |
dc9dc135 | 403 | let mut u = MaybeUninit::<T>::uninit(); |
1b1a35ee | 404 | // SAFETY: `u.as_mut_ptr()` points to allocated memory. |
dc9dc135 XL |
405 | unsafe { |
406 | u.as_mut_ptr().write_bytes(0u8, 1); | |
407 | } | |
408 | u | |
409 | } | |
410 | ||
136023e0 XL |
411 | /// Sets the value of the `MaybeUninit<T>`. |
412 | /// | |
413 | /// This overwrites any previous value without dropping it, so be careful | |
414 | /// not to use this twice unless you want to skip running the destructor. | |
415 | /// For your convenience, this also returns a mutable reference to the | |
416 | /// (now safely initialized) contents of `self`. | |
417 | /// | |
418 | /// As the content is stored inside a `MaybeUninit`, the destructor is not | |
419 | /// run for the inner data if the MaybeUninit leaves scope without a call to | |
420 | /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives | |
421 | /// the mutable reference returned by this function needs to keep this in | |
422 | /// mind. The safety model of Rust regards leaks as safe, but they are | |
423 | /// usually still undesirable. This being said, the mutable reference | |
424 | /// behaves like any other mutable reference would, so assigning a new value | |
425 | /// to it will drop the old content. | |
426 | /// | |
427 | /// [`assume_init`]: Self::assume_init | |
428 | /// [`assume_init_drop`]: Self::assume_init_drop | |
429 | /// | |
430 | /// # Examples | |
431 | /// | |
432 | /// Correct usage of this method: | |
433 | /// | |
434 | /// ```rust | |
435 | /// use std::mem::MaybeUninit; | |
436 | /// | |
437 | /// let mut x = MaybeUninit::<Vec<u8>>::uninit(); | |
438 | /// | |
439 | /// { | |
440 | /// let hello = x.write((&b"Hello, world!").to_vec()); | |
441 | /// // Setting hello does not leak prior allocations, but drops them | |
442 | /// *hello = (&b"Hello").to_vec(); | |
443 | /// hello[0] = 'h' as u8; | |
444 | /// } | |
445 | /// // x is initialized now: | |
446 | /// let s = unsafe { x.assume_init() }; | |
447 | /// assert_eq!(b"hello", s.as_slice()); | |
448 | /// ``` | |
449 | /// | |
450 | /// This usage of the method causes a leak: | |
451 | /// | |
452 | /// ```rust | |
453 | /// use std::mem::MaybeUninit; | |
454 | /// | |
455 | /// let mut x = MaybeUninit::<String>::uninit(); | |
456 | /// | |
457 | /// x.write("Hello".to_string()); | |
458 | /// // This leaks the contained string: | |
459 | /// x.write("hello".to_string()); | |
460 | /// // x is initialized now: | |
461 | /// let s = unsafe { x.assume_init() }; | |
462 | /// ``` | |
463 | /// | |
464 | /// This method can be used to avoid unsafe in some cases. The example below | |
465 | /// shows a part of an implementation of a fixed sized arena that lends out | |
466 | /// pinned references. | |
467 | /// With `write`, we can avoid the need to write through a raw pointer: | |
468 | /// | |
469 | /// ```rust | |
136023e0 XL |
470 | /// use core::pin::Pin; |
471 | /// use core::mem::MaybeUninit; | |
472 | /// | |
473 | /// struct PinArena<T> { | |
474 | /// memory: Box<[MaybeUninit<T>]>, | |
475 | /// len: usize, | |
476 | /// } | |
477 | /// | |
478 | /// impl <T> PinArena<T> { | |
479 | /// pub fn capacity(&self) -> usize { | |
480 | /// self.memory.len() | |
481 | /// } | |
482 | /// pub fn push(&mut self, val: T) -> Pin<&mut T> { | |
483 | /// if self.len >= self.capacity() { | |
484 | /// panic!("Attempted to push to a full pin arena!"); | |
485 | /// } | |
486 | /// let ref_ = self.memory[self.len].write(val); | |
487 | /// self.len += 1; | |
488 | /// unsafe { Pin::new_unchecked(ref_) } | |
489 | /// } | |
490 | /// } | |
491 | /// ``` | |
492 | #[stable(feature = "maybe_uninit_write", since = "1.55.0")] | |
493 | #[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")] | |
dc9dc135 | 494 | #[inline(always)] |
fc512014 | 495 | pub const fn write(&mut self, val: T) -> &mut T { |
1b1a35ee XL |
496 | *self = MaybeUninit::new(val); |
497 | // SAFETY: We just initialized this value. | |
498 | unsafe { self.assume_init_mut() } | |
dc9dc135 XL |
499 | } |
500 | ||
501 | /// Gets a pointer to the contained value. Reading from this pointer or turning it | |
502 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
503 | /// Writing to memory that this pointer (non-transitively) points to is undefined behavior | |
504 | /// (except inside an `UnsafeCell<T>`). | |
505 | /// | |
506 | /// # Examples | |
507 | /// | |
508 | /// Correct usage of this method: | |
509 | /// | |
510 | /// ```rust | |
511 | /// use std::mem::MaybeUninit; | |
512 | /// | |
513 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
136023e0 | 514 | /// x.write(vec![0, 1, 2]); |
dc9dc135 XL |
515 | /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it. |
516 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
517 | /// assert_eq!(x_vec.len(), 3); | |
518 | /// ``` | |
519 | /// | |
520 | /// *Incorrect* usage of this method: | |
521 | /// | |
522 | /// ```rust,no_run | |
523 | /// use std::mem::MaybeUninit; | |
524 | /// | |
525 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
526 | /// let x_vec = unsafe { &*x.as_ptr() }; | |
f9f354fc | 527 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
dc9dc135 XL |
528 | /// ``` |
529 | /// | |
530 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
531 | /// until they are, it is advisable to avoid them.) | |
532 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
a2a8927a | 533 | #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")] |
dc9dc135 | 534 | #[inline(always)] |
3dfed10e XL |
535 | pub const fn as_ptr(&self) -> *const T { |
536 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. | |
537 | self as *const _ as *const T | |
dc9dc135 XL |
538 | } |
539 | ||
540 | /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it | |
541 | /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized. | |
542 | /// | |
543 | /// # Examples | |
544 | /// | |
545 | /// Correct usage of this method: | |
546 | /// | |
547 | /// ```rust | |
548 | /// use std::mem::MaybeUninit; | |
549 | /// | |
550 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
136023e0 | 551 | /// x.write(vec![0, 1, 2]); |
dc9dc135 XL |
552 | /// // Create a reference into the `MaybeUninit<Vec<u32>>`. |
553 | /// // This is okay because we initialized it. | |
554 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
555 | /// x_vec.push(3); | |
556 | /// assert_eq!(x_vec.len(), 4); | |
557 | /// ``` | |
558 | /// | |
559 | /// *Incorrect* usage of this method: | |
560 | /// | |
561 | /// ```rust,no_run | |
562 | /// use std::mem::MaybeUninit; | |
563 | /// | |
564 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
565 | /// let x_vec = unsafe { &mut *x.as_mut_ptr() }; | |
f9f354fc | 566 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
dc9dc135 XL |
567 | /// ``` |
568 | /// | |
569 | /// (Notice that the rules around references to uninitialized data are not finalized yet, but | |
570 | /// until they are, it is advisable to avoid them.) | |
571 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
a2a8927a | 572 | #[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")] |
dc9dc135 | 573 | #[inline(always)] |
3dfed10e XL |
574 | pub const fn as_mut_ptr(&mut self) -> *mut T { |
575 | // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer. | |
576 | self as *mut _ as *mut T | |
dc9dc135 XL |
577 | } |
578 | ||
579 | /// Extracts the value from the `MaybeUninit<T>` container. This is a great way | |
580 | /// to ensure that the data will get dropped, because the resulting `T` is | |
581 | /// subject to the usual drop handling. | |
582 | /// | |
583 | /// # Safety | |
584 | /// | |
585 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
586 | /// state. Calling this when the content is not yet fully initialized causes immediate undefined | |
587 | /// behavior. The [type-level documentation][inv] contains more information about | |
588 | /// this initialization invariant. | |
589 | /// | |
590 | /// [inv]: #initialization-invariant | |
591 | /// | |
416331ca XL |
592 | /// On top of that, remember that most types have additional invariants beyond merely |
593 | /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`] | |
594 | /// is considered initialized (under the current implementation; this does not constitute | |
595 | /// a stable guarantee) because the only requirement the compiler knows about it | |
596 | /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause | |
597 | /// *immediate* undefined behavior, but will cause undefined behavior with most | |
598 | /// safe operations (including dropping it). | |
599 | /// | |
1b1a35ee XL |
600 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html |
601 | /// | |
dc9dc135 XL |
602 | /// # Examples |
603 | /// | |
604 | /// Correct usage of this method: | |
605 | /// | |
606 | /// ```rust | |
607 | /// use std::mem::MaybeUninit; | |
608 | /// | |
609 | /// let mut x = MaybeUninit::<bool>::uninit(); | |
136023e0 | 610 | /// x.write(true); |
dc9dc135 XL |
611 | /// let x_init = unsafe { x.assume_init() }; |
612 | /// assert_eq!(x_init, true); | |
613 | /// ``` | |
614 | /// | |
615 | /// *Incorrect* usage of this method: | |
616 | /// | |
617 | /// ```rust,no_run | |
618 | /// use std::mem::MaybeUninit; | |
619 | /// | |
620 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
621 | /// let x_init = unsafe { x.assume_init() }; | |
f9f354fc | 622 | /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️ |
dc9dc135 XL |
623 | /// ``` |
624 | #[stable(feature = "maybe_uninit", since = "1.36.0")] | |
5e7ed085 | 625 | #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")] |
dc9dc135 | 626 | #[inline(always)] |
dfeec247 | 627 | #[rustc_diagnostic_item = "assume_init"] |
94222f64 | 628 | #[track_caller] |
fc512014 | 629 | pub const unsafe fn assume_init(self) -> T { |
f035d41b XL |
630 | // SAFETY: the caller must guarantee that `self` is initialized. |
631 | // This also means that `self` must be a `value` variant. | |
632 | unsafe { | |
633 | intrinsics::assert_inhabited::<T>(); | |
634 | ManuallyDrop::into_inner(self.value) | |
635 | } | |
dc9dc135 XL |
636 | } |
637 | ||
638 | /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject | |
639 | /// to the usual drop handling. | |
640 | /// | |
416331ca | 641 | /// Whenever possible, it is preferable to use [`assume_init`] instead, which |
dc9dc135 XL |
642 | /// prevents duplicating the content of the `MaybeUninit<T>`. |
643 | /// | |
644 | /// # Safety | |
645 | /// | |
646 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized | |
647 | /// state. Calling this when the content is not yet fully initialized causes undefined | |
648 | /// behavior. The [type-level documentation][inv] contains more information about | |
649 | /// this initialization invariant. | |
650 | /// | |
136023e0 XL |
651 | /// Moreover, similar to the [`ptr::read`] function, this function creates a |
652 | /// bitwise copy of the contents, regardless whether the contained type | |
653 | /// implements the [`Copy`] trait or not. When using multiple copies of the | |
654 | /// data (by calling `assume_init_read` multiple times, or first calling | |
655 | /// `assume_init_read` and then [`assume_init`]), it is your responsibility | |
dc9dc135 XL |
656 | /// to ensure that that data may indeed be duplicated. |
657 | /// | |
658 | /// [inv]: #initialization-invariant | |
3dfed10e | 659 | /// [`assume_init`]: MaybeUninit::assume_init |
dc9dc135 XL |
660 | /// |
661 | /// # Examples | |
662 | /// | |
663 | /// Correct usage of this method: | |
664 | /// | |
665 | /// ```rust | |
dc9dc135 XL |
666 | /// use std::mem::MaybeUninit; |
667 | /// | |
668 | /// let mut x = MaybeUninit::<u32>::uninit(); | |
669 | /// x.write(13); | |
1b1a35ee | 670 | /// let x1 = unsafe { x.assume_init_read() }; |
dc9dc135 | 671 | /// // `u32` is `Copy`, so we may read multiple times. |
1b1a35ee | 672 | /// let x2 = unsafe { x.assume_init_read() }; |
dc9dc135 XL |
673 | /// assert_eq!(x1, x2); |
674 | /// | |
675 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
676 | /// x.write(None); | |
1b1a35ee | 677 | /// let x1 = unsafe { x.assume_init_read() }; |
dc9dc135 | 678 | /// // Duplicating a `None` value is okay, so we may read multiple times. |
1b1a35ee | 679 | /// let x2 = unsafe { x.assume_init_read() }; |
dc9dc135 XL |
680 | /// assert_eq!(x1, x2); |
681 | /// ``` | |
682 | /// | |
683 | /// *Incorrect* usage of this method: | |
684 | /// | |
685 | /// ```rust,no_run | |
dc9dc135 XL |
686 | /// use std::mem::MaybeUninit; |
687 | /// | |
688 | /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit(); | |
fc512014 | 689 | /// x.write(Some(vec![0, 1, 2])); |
1b1a35ee XL |
690 | /// let x1 = unsafe { x.assume_init_read() }; |
691 | /// let x2 = unsafe { x.assume_init_read() }; | |
f9f354fc | 692 | /// // We now created two copies of the same vector, leading to a double-free ⚠️ when |
dc9dc135 XL |
693 | /// // they both get dropped! |
694 | /// ``` | |
5099ac24 FG |
695 | #[stable(feature = "maybe_uninit_extra", since = "1.60.0")] |
696 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init_read", issue = "63567")] | |
dc9dc135 | 697 | #[inline(always)] |
94222f64 | 698 | #[track_caller] |
5869c6ff | 699 | pub const unsafe fn assume_init_read(&self) -> T { |
f035d41b XL |
700 | // SAFETY: the caller must guarantee that `self` is initialized. |
701 | // Reading from `self.as_ptr()` is safe since `self` should be initialized. | |
702 | unsafe { | |
703 | intrinsics::assert_inhabited::<T>(); | |
704 | self.as_ptr().read() | |
705 | } | |
dc9dc135 XL |
706 | } |
707 | ||
1b1a35ee XL |
708 | /// Drops the contained value in place. |
709 | /// | |
136023e0 XL |
710 | /// If you have ownership of the `MaybeUninit`, you can also use |
711 | /// [`assume_init`] as an alternative. | |
1b1a35ee XL |
712 | /// |
713 | /// # Safety | |
714 | /// | |
715 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is | |
716 | /// in an initialized state. Calling this when the content is not yet fully | |
717 | /// initialized causes undefined behavior. | |
718 | /// | |
719 | /// On top of that, all additional invariants of the type `T` must be | |
720 | /// satisfied, as the `Drop` implementation of `T` (or its members) may | |
136023e0 XL |
721 | /// rely on this. For example, setting a [`Vec<T>`] to an invalid but |
722 | /// non-null address makes it initialized (under the current implementation; | |
723 | /// this does not constitute a stable guarantee), because the only | |
724 | /// requirement the compiler knows about it is that the data pointer must be | |
725 | /// non-null. Dropping such a `Vec<T>` however will cause undefined | |
726 | /// behaviour. | |
1b1a35ee XL |
727 | /// |
728 | /// [`assume_init`]: MaybeUninit::assume_init | |
729 | /// [`Vec<T>`]: ../../std/vec/struct.Vec.html | |
5099ac24 | 730 | #[stable(feature = "maybe_uninit_extra", since = "1.60.0")] |
1b1a35ee XL |
731 | pub unsafe fn assume_init_drop(&mut self) { |
732 | // SAFETY: the caller must guarantee that `self` is initialized and | |
733 | // satisfies all invariants of `T`. | |
734 | // Dropping the value in place is safe if that is the case. | |
735 | unsafe { ptr::drop_in_place(self.as_mut_ptr()) } | |
736 | } | |
737 | ||
60c5eb7d XL |
738 | /// Gets a shared reference to the contained value. |
739 | /// | |
740 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
741 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
742 | /// of `.assume_init()`). | |
dc9dc135 XL |
743 | /// |
744 | /// # Safety | |
745 | /// | |
60c5eb7d XL |
746 | /// Calling this when the content is not yet fully initialized causes undefined |
747 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
748 | /// is in an initialized state. | |
749 | /// | |
750 | /// # Examples | |
751 | /// | |
752 | /// ### Correct usage of this method: | |
753 | /// | |
754 | /// ```rust | |
60c5eb7d XL |
755 | /// use std::mem::MaybeUninit; |
756 | /// | |
757 | /// let mut x = MaybeUninit::<Vec<u32>>::uninit(); | |
758 | /// // Initialize `x`: | |
136023e0 | 759 | /// x.write(vec![1, 2, 3]); |
60c5eb7d XL |
760 | /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to |
761 | /// // create a shared reference to it: | |
762 | /// let x: &Vec<u32> = unsafe { | |
1b1a35ee XL |
763 | /// // SAFETY: `x` has been initialized. |
764 | /// x.assume_init_ref() | |
60c5eb7d XL |
765 | /// }; |
766 | /// assert_eq!(x, &vec![1, 2, 3]); | |
767 | /// ``` | |
768 | /// | |
769 | /// ### *Incorrect* usages of this method: | |
770 | /// | |
771 | /// ```rust,no_run | |
60c5eb7d XL |
772 | /// use std::mem::MaybeUninit; |
773 | /// | |
774 | /// let x = MaybeUninit::<Vec<u32>>::uninit(); | |
1b1a35ee | 775 | /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() }; |
f9f354fc | 776 | /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️ |
60c5eb7d XL |
777 | /// ``` |
778 | /// | |
779 | /// ```rust,no_run | |
60c5eb7d XL |
780 | /// use std::{cell::Cell, mem::MaybeUninit}; |
781 | /// | |
782 | /// let b = MaybeUninit::<Cell<bool>>::uninit(); | |
783 | /// // Initialize the `MaybeUninit` using `Cell::set`: | |
784 | /// unsafe { | |
1b1a35ee XL |
785 | /// b.assume_init_ref().set(true); |
786 | /// // ^^^^^^^^^^^^^^^ | |
787 | /// // Reference to an uninitialized `Cell<bool>`: UB! | |
60c5eb7d XL |
788 | /// } |
789 | /// ``` | |
136023e0 | 790 | #[stable(feature = "maybe_uninit_ref", since = "1.55.0")] |
5e7ed085 | 791 | #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")] |
dc9dc135 | 792 | #[inline(always)] |
fc512014 | 793 | pub const unsafe fn assume_init_ref(&self) -> &T { |
f035d41b XL |
794 | // SAFETY: the caller must guarantee that `self` is initialized. |
795 | // This also means that `self` must be a `value` variant. | |
796 | unsafe { | |
797 | intrinsics::assert_inhabited::<T>(); | |
fc512014 | 798 | &*self.as_ptr() |
f035d41b | 799 | } |
dc9dc135 XL |
800 | } |
801 | ||
60c5eb7d XL |
802 | /// Gets a mutable (unique) reference to the contained value. |
803 | /// | |
804 | /// This can be useful when we want to access a `MaybeUninit` that has been | |
805 | /// initialized but don't have ownership of the `MaybeUninit` (preventing the use | |
806 | /// of `.assume_init()`). | |
dc9dc135 XL |
807 | /// |
808 | /// # Safety | |
809 | /// | |
60c5eb7d XL |
810 | /// Calling this when the content is not yet fully initialized causes undefined |
811 | /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really | |
1b1a35ee | 812 | /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to |
60c5eb7d XL |
813 | /// initialize a `MaybeUninit`. |
814 | /// | |
815 | /// # Examples | |
816 | /// | |
817 | /// ### Correct usage of this method: | |
818 | /// | |
819 | /// ```rust | |
5e7ed085 | 820 | /// # #![allow(unexpected_cfgs)] |
60c5eb7d XL |
821 | /// use std::mem::MaybeUninit; |
822 | /// | |
cdc7bbd5 | 823 | /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] } |
60c5eb7d XL |
824 | /// # #[cfg(FALSE)] |
825 | /// extern "C" { | |
826 | /// /// Initializes *all* the bytes of the input buffer. | |
cdc7bbd5 | 827 | /// fn initialize_buffer(buf: *mut [u8; 1024]); |
60c5eb7d XL |
828 | /// } |
829 | /// | |
cdc7bbd5 | 830 | /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit(); |
60c5eb7d XL |
831 | /// |
832 | /// // Initialize `buf`: | |
833 | /// unsafe { initialize_buffer(buf.as_mut_ptr()); } | |
834 | /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it. | |
cdc7bbd5 | 835 | /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes. |
60c5eb7d | 836 | /// // To assert our buffer has been initialized without copying it, we upgrade |
cdc7bbd5 XL |
837 | /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`: |
838 | /// let buf: &mut [u8; 1024] = unsafe { | |
1b1a35ee XL |
839 | /// // SAFETY: `buf` has been initialized. |
840 | /// buf.assume_init_mut() | |
60c5eb7d XL |
841 | /// }; |
842 | /// | |
843 | /// // Now we can use `buf` as a normal slice: | |
844 | /// buf.sort_unstable(); | |
845 | /// assert!( | |
74b04a01 | 846 | /// buf.windows(2).all(|pair| pair[0] <= pair[1]), |
60c5eb7d XL |
847 | /// "buffer is sorted", |
848 | /// ); | |
849 | /// ``` | |
850 | /// | |
851 | /// ### *Incorrect* usages of this method: | |
852 | /// | |
1b1a35ee | 853 | /// You cannot use `.assume_init_mut()` to initialize a value: |
60c5eb7d XL |
854 | /// |
855 | /// ```rust,no_run | |
60c5eb7d XL |
856 | /// use std::mem::MaybeUninit; |
857 | /// | |
858 | /// let mut b = MaybeUninit::<bool>::uninit(); | |
859 | /// unsafe { | |
1b1a35ee | 860 | /// *b.assume_init_mut() = true; |
60c5eb7d | 861 | /// // We have created a (mutable) reference to an uninitialized `bool`! |
f9f354fc | 862 | /// // This is undefined behavior. ⚠️ |
60c5eb7d XL |
863 | /// } |
864 | /// ``` | |
865 | /// | |
866 | /// For instance, you cannot [`Read`] into an uninitialized buffer: | |
867 | /// | |
923072b8 | 868 | /// [`Read`]: ../../std/io/trait.Read.html |
60c5eb7d XL |
869 | /// |
870 | /// ```rust,no_run | |
60c5eb7d XL |
871 | /// use std::{io, mem::MaybeUninit}; |
872 | /// | |
873 | /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]> | |
874 | /// { | |
875 | /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit(); | |
1b1a35ee XL |
876 | /// reader.read_exact(unsafe { buffer.assume_init_mut() })?; |
877 | /// // ^^^^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
878 | /// // (mutable) reference to uninitialized memory! |
879 | /// // This is undefined behavior. | |
880 | /// Ok(unsafe { buffer.assume_init() }) | |
881 | /// } | |
882 | /// ``` | |
883 | /// | |
884 | /// Nor can you use direct field access to do field-by-field gradual initialization: | |
885 | /// | |
886 | /// ```rust,no_run | |
60c5eb7d XL |
887 | /// use std::{mem::MaybeUninit, ptr}; |
888 | /// | |
889 | /// struct Foo { | |
890 | /// a: u32, | |
891 | /// b: u8, | |
892 | /// } | |
893 | /// | |
894 | /// let foo: Foo = unsafe { | |
895 | /// let mut foo = MaybeUninit::<Foo>::uninit(); | |
1b1a35ee XL |
896 | /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337); |
897 | /// // ^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
898 | /// // (mutable) reference to uninitialized memory! |
899 | /// // This is undefined behavior. | |
1b1a35ee XL |
900 | /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42); |
901 | /// // ^^^^^^^^^^^^^^^^^^^^^ | |
60c5eb7d XL |
902 | /// // (mutable) reference to uninitialized memory! |
903 | /// // This is undefined behavior. | |
904 | /// foo.assume_init() | |
905 | /// }; | |
906 | /// ``` | |
136023e0 | 907 | #[stable(feature = "maybe_uninit_ref", since = "1.55.0")] |
fc512014 | 908 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
dc9dc135 | 909 | #[inline(always)] |
fc512014 | 910 | pub const unsafe fn assume_init_mut(&mut self) -> &mut T { |
f035d41b XL |
911 | // SAFETY: the caller must guarantee that `self` is initialized. |
912 | // This also means that `self` must be a `value` variant. | |
913 | unsafe { | |
914 | intrinsics::assert_inhabited::<T>(); | |
fc512014 | 915 | &mut *self.as_mut_ptr() |
f035d41b | 916 | } |
dc9dc135 XL |
917 | } |
918 | ||
5869c6ff XL |
919 | /// Extracts the values from an array of `MaybeUninit` containers. |
920 | /// | |
921 | /// # Safety | |
922 | /// | |
923 | /// It is up to the caller to guarantee that all elements of the array are | |
924 | /// in an initialized state. | |
925 | /// | |
926 | /// # Examples | |
927 | /// | |
928 | /// ``` | |
929 | /// #![feature(maybe_uninit_uninit_array)] | |
930 | /// #![feature(maybe_uninit_array_assume_init)] | |
931 | /// use std::mem::MaybeUninit; | |
932 | /// | |
933 | /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array(); | |
136023e0 XL |
934 | /// array[0].write(0); |
935 | /// array[1].write(1); | |
936 | /// array[2].write(2); | |
5869c6ff XL |
937 | /// |
938 | /// // SAFETY: Now safe as we initialised all elements | |
939 | /// let array = unsafe { | |
940 | /// MaybeUninit::array_assume_init(array) | |
941 | /// }; | |
942 | /// | |
943 | /// assert_eq!(array, [0, 1, 2]); | |
944 | /// ``` | |
04454e1e FG |
945 | #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")] |
946 | #[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")] | |
5869c6ff | 947 | #[inline(always)] |
94222f64 | 948 | #[track_caller] |
04454e1e | 949 | pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] { |
5869c6ff XL |
950 | // SAFETY: |
951 | // * The caller guarantees that all elements of the array are initialized | |
952 | // * `MaybeUninit<T>` and T are guaranteed to have the same layout | |
17df50a5 | 953 | // * `MaybeUninit` does not drop, so there are no double-frees |
5869c6ff | 954 | // And thus the conversion is safe |
04454e1e | 955 | let ret = unsafe { |
5869c6ff XL |
956 | intrinsics::assert_inhabited::<[T; N]>(); |
957 | (&array as *const _ as *const [T; N]).read() | |
04454e1e FG |
958 | }; |
959 | ||
960 | // FIXME: required to avoid `~const Destruct` bound | |
961 | super::forget(array); | |
962 | ret | |
5869c6ff XL |
963 | } |
964 | ||
60c5eb7d XL |
965 | /// Assuming all the elements are initialized, get a slice to them. |
966 | /// | |
967 | /// # Safety | |
968 | /// | |
969 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
970 | /// really are in an initialized state. | |
971 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
1b1a35ee XL |
972 | /// |
973 | /// See [`assume_init_ref`] for more details and examples. | |
974 | /// | |
975 | /// [`assume_init_ref`]: MaybeUninit::assume_init_ref | |
976 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] | |
a2a8927a | 977 | #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")] |
60c5eb7d | 978 | #[inline(always)] |
fc512014 | 979 | pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] { |
a2a8927a XL |
980 | // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that |
981 | // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`. | |
f035d41b XL |
982 | // The pointer obtained is valid since it refers to memory owned by `slice` which is a |
983 | // reference and thus guaranteed to be valid for reads. | |
984 | unsafe { &*(slice as *const [Self] as *const [T]) } | |
60c5eb7d XL |
985 | } |
986 | ||
987 | /// Assuming all the elements are initialized, get a mutable slice to them. | |
988 | /// | |
989 | /// # Safety | |
990 | /// | |
991 | /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements | |
992 | /// really are in an initialized state. | |
993 | /// Calling this when the content is not yet fully initialized causes undefined behavior. | |
1b1a35ee XL |
994 | /// |
995 | /// See [`assume_init_mut`] for more details and examples. | |
996 | /// | |
997 | /// [`assume_init_mut`]: MaybeUninit::assume_init_mut | |
998 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] | |
fc512014 | 999 | #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")] |
60c5eb7d | 1000 | #[inline(always)] |
fc512014 | 1001 | pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] { |
f035d41b XL |
1002 | // SAFETY: similar to safety notes for `slice_get_ref`, but we have a |
1003 | // mutable reference which is also guaranteed to be valid for writes. | |
1004 | unsafe { &mut *(slice as *mut [Self] as *mut [T]) } | |
60c5eb7d XL |
1005 | } |
1006 | ||
dc9dc135 | 1007 | /// Gets a pointer to the first element of the array. |
e1599b0c | 1008 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
fc512014 | 1009 | #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 | 1010 | #[inline(always)] |
fc512014 | 1011 | pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T { |
29967ef6 | 1012 | this.as_ptr() as *const T |
dc9dc135 XL |
1013 | } |
1014 | ||
1015 | /// Gets a mutable pointer to the first element of the array. | |
e1599b0c | 1016 | #[unstable(feature = "maybe_uninit_slice", issue = "63569")] |
fc512014 | 1017 | #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")] |
dc9dc135 | 1018 | #[inline(always)] |
fc512014 | 1019 | pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T { |
29967ef6 | 1020 | this.as_mut_ptr() as *mut T |
dc9dc135 | 1021 | } |
fc512014 | 1022 | |
136023e0 | 1023 | /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`. |
fc512014 XL |
1024 | /// |
1025 | /// If `T` does not implement `Copy`, use [`write_slice_cloned`] | |
1026 | /// | |
1027 | /// This is similar to [`slice::copy_from_slice`]. | |
1028 | /// | |
1029 | /// # Panics | |
1030 | /// | |
1031 | /// This function will panic if the two slices have different lengths. | |
1032 | /// | |
1033 | /// # Examples | |
1034 | /// | |
1035 | /// ``` | |
1036 | /// #![feature(maybe_uninit_write_slice)] | |
1037 | /// use std::mem::MaybeUninit; | |
1038 | /// | |
1039 | /// let mut dst = [MaybeUninit::uninit(); 32]; | |
1040 | /// let src = [0; 32]; | |
1041 | /// | |
1042 | /// let init = MaybeUninit::write_slice(&mut dst, &src); | |
1043 | /// | |
1044 | /// assert_eq!(init, src); | |
1045 | /// ``` | |
1046 | /// | |
1047 | /// ``` | |
5099ac24 | 1048 | /// #![feature(maybe_uninit_write_slice)] |
fc512014 XL |
1049 | /// use std::mem::MaybeUninit; |
1050 | /// | |
1051 | /// let mut vec = Vec::with_capacity(32); | |
1052 | /// let src = [0; 16]; | |
1053 | /// | |
1054 | /// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src); | |
1055 | /// | |
1056 | /// // SAFETY: we have just copied all the elements of len into the spare capacity | |
1057 | /// // the first src.len() elements of the vec are valid now. | |
1058 | /// unsafe { | |
1059 | /// vec.set_len(src.len()); | |
1060 | /// } | |
1061 | /// | |
1062 | /// assert_eq!(vec, src); | |
1063 | /// ``` | |
1064 | /// | |
1065 | /// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned | |
fc512014 XL |
1066 | #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")] |
1067 | pub fn write_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] | |
1068 | where | |
1069 | T: Copy, | |
1070 | { | |
1071 | // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout | |
1072 | let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) }; | |
1073 | ||
1074 | this.copy_from_slice(uninit_src); | |
1075 | ||
136023e0 | 1076 | // SAFETY: Valid elements have just been copied into `this` so it is initialized |
fc512014 XL |
1077 | unsafe { MaybeUninit::slice_assume_init_mut(this) } |
1078 | } | |
1079 | ||
136023e0 XL |
1080 | /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`. |
1081 | /// Any already initialized elements will not be dropped. | |
fc512014 XL |
1082 | /// |
1083 | /// If `T` implements `Copy`, use [`write_slice`] | |
1084 | /// | |
1085 | /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements. | |
1086 | /// | |
1087 | /// # Panics | |
1088 | /// | |
1089 | /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics. | |
1090 | /// | |
1091 | /// If there is a panic, the already cloned elements will be dropped. | |
1092 | /// | |
1093 | /// # Examples | |
1094 | /// | |
1095 | /// ``` | |
1096 | /// #![feature(maybe_uninit_write_slice)] | |
1097 | /// use std::mem::MaybeUninit; | |
1098 | /// | |
1099 | /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()]; | |
1100 | /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()]; | |
1101 | /// | |
1102 | /// let init = MaybeUninit::write_slice_cloned(&mut dst, &src); | |
1103 | /// | |
1104 | /// assert_eq!(init, src); | |
1105 | /// ``` | |
1106 | /// | |
1107 | /// ``` | |
5099ac24 | 1108 | /// #![feature(maybe_uninit_write_slice)] |
fc512014 XL |
1109 | /// use std::mem::MaybeUninit; |
1110 | /// | |
1111 | /// let mut vec = Vec::with_capacity(32); | |
1112 | /// let src = ["rust", "is", "a", "pretty", "cool", "language"]; | |
1113 | /// | |
1114 | /// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src); | |
1115 | /// | |
1116 | /// // SAFETY: we have just cloned all the elements of len into the spare capacity | |
1117 | /// // the first src.len() elements of the vec are valid now. | |
1118 | /// unsafe { | |
1119 | /// vec.set_len(src.len()); | |
1120 | /// } | |
1121 | /// | |
1122 | /// assert_eq!(vec, src); | |
1123 | /// ``` | |
1124 | /// | |
1125 | /// [`write_slice`]: MaybeUninit::write_slice | |
fc512014 XL |
1126 | #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")] |
1127 | pub fn write_slice_cloned<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T] | |
1128 | where | |
1129 | T: Clone, | |
1130 | { | |
1131 | // unlike copy_from_slice this does not call clone_from_slice on the slice | |
1132 | // this is because `MaybeUninit<T: Clone>` does not implement Clone. | |
1133 | ||
1134 | struct Guard<'a, T> { | |
1135 | slice: &'a mut [MaybeUninit<T>], | |
1136 | initialized: usize, | |
1137 | } | |
1138 | ||
1139 | impl<'a, T> Drop for Guard<'a, T> { | |
1140 | fn drop(&mut self) { | |
1141 | let initialized_part = &mut self.slice[..self.initialized]; | |
1142 | // SAFETY: this raw slice will contain only initialized objects | |
1143 | // that's why, it is allowed to drop it. | |
1144 | unsafe { | |
1145 | crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part)); | |
1146 | } | |
1147 | } | |
1148 | } | |
1149 | ||
1150 | assert_eq!(this.len(), src.len(), "destination and source slices have different lengths"); | |
1151 | // NOTE: We need to explicitly slice them to the same length | |
1152 | // for bounds checking to be elided, and the optimizer will | |
1153 | // generate memcpy for simple cases (for example T = u8). | |
1154 | let len = this.len(); | |
1155 | let src = &src[..len]; | |
1156 | ||
1157 | // guard is needed b/c panic might happen during a clone | |
1158 | let mut guard = Guard { slice: this, initialized: 0 }; | |
1159 | ||
1160 | for i in 0..len { | |
1161 | guard.slice[i].write(src[i].clone()); | |
1162 | guard.initialized += 1; | |
1163 | } | |
1164 | ||
1165 | super::forget(guard); | |
1166 | ||
136023e0 | 1167 | // SAFETY: Valid elements have just been written into `this` so it is initialized |
fc512014 XL |
1168 | unsafe { MaybeUninit::slice_assume_init_mut(this) } |
1169 | } | |
5099ac24 FG |
1170 | |
1171 | /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes. | |
1172 | /// | |
1173 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still | |
1174 | /// contain padding bytes which are left uninitialized. | |
1175 | /// | |
1176 | /// # Examples | |
1177 | /// | |
1178 | /// ``` | |
1179 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)] | |
1180 | /// use std::mem::MaybeUninit; | |
1181 | /// | |
1182 | /// let val = 0x12345678i32; | |
1183 | /// let uninit = MaybeUninit::new(val); | |
1184 | /// let uninit_bytes = uninit.as_bytes(); | |
1185 | /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) }; | |
1186 | /// assert_eq!(bytes, val.to_ne_bytes()); | |
1187 | /// ``` | |
1188 | #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")] | |
1189 | pub fn as_bytes(&self) -> &[MaybeUninit<u8>] { | |
1190 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes | |
1191 | unsafe { | |
1192 | slice::from_raw_parts(self.as_ptr() as *const MaybeUninit<u8>, mem::size_of::<T>()) | |
1193 | } | |
1194 | } | |
1195 | ||
1196 | /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized | |
1197 | /// bytes. | |
1198 | /// | |
1199 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still | |
1200 | /// contain padding bytes which are left uninitialized. | |
1201 | /// | |
1202 | /// # Examples | |
1203 | /// | |
1204 | /// ``` | |
1205 | /// #![feature(maybe_uninit_as_bytes)] | |
1206 | /// use std::mem::MaybeUninit; | |
1207 | /// | |
1208 | /// let val = 0x12345678i32; | |
1209 | /// let mut uninit = MaybeUninit::new(val); | |
1210 | /// let uninit_bytes = uninit.as_bytes_mut(); | |
1211 | /// if cfg!(target_endian = "little") { | |
1212 | /// uninit_bytes[0].write(0xcd); | |
1213 | /// } else { | |
1214 | /// uninit_bytes[3].write(0xcd); | |
1215 | /// } | |
1216 | /// let val2 = unsafe { uninit.assume_init() }; | |
1217 | /// assert_eq!(val2, 0x123456cd); | |
1218 | /// ``` | |
1219 | #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")] | |
1220 | pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] { | |
1221 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes | |
1222 | unsafe { | |
1223 | slice::from_raw_parts_mut( | |
1224 | self.as_mut_ptr() as *mut MaybeUninit<u8>, | |
1225 | mem::size_of::<T>(), | |
1226 | ) | |
1227 | } | |
1228 | } | |
1229 | ||
1230 | /// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized | |
1231 | /// bytes. | |
1232 | /// | |
1233 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still | |
1234 | /// contain padding bytes which are left uninitialized. | |
1235 | /// | |
1236 | /// # Examples | |
1237 | /// | |
1238 | /// ``` | |
1239 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)] | |
1240 | /// use std::mem::MaybeUninit; | |
1241 | /// | |
1242 | /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)]; | |
1243 | /// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit); | |
1244 | /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) }; | |
1245 | /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap()); | |
1246 | /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap()); | |
1247 | /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]); | |
1248 | /// ``` | |
1249 | #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")] | |
1250 | pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] { | |
1251 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes | |
1252 | unsafe { | |
1253 | slice::from_raw_parts( | |
1254 | this.as_ptr() as *const MaybeUninit<u8>, | |
1255 | this.len() * mem::size_of::<T>(), | |
1256 | ) | |
1257 | } | |
1258 | } | |
1259 | ||
1260 | /// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of | |
1261 | /// potentially uninitialized bytes. | |
1262 | /// | |
1263 | /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still | |
1264 | /// contain padding bytes which are left uninitialized. | |
1265 | /// | |
1266 | /// # Examples | |
1267 | /// | |
1268 | /// ``` | |
1269 | /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)] | |
1270 | /// use std::mem::MaybeUninit; | |
1271 | /// | |
1272 | /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()]; | |
1273 | /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit); | |
1274 | /// MaybeUninit::write_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]); | |
1275 | /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) }; | |
1276 | /// if cfg!(target_endian = "little") { | |
1277 | /// assert_eq!(vals, &[0x3412u16, 0x7856u16]); | |
1278 | /// } else { | |
1279 | /// assert_eq!(vals, &[0x1234u16, 0x5678u16]); | |
1280 | /// } | |
1281 | /// ``` | |
1282 | #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")] | |
1283 | pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] { | |
1284 | // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes | |
1285 | unsafe { | |
1286 | slice::from_raw_parts_mut( | |
1287 | this.as_mut_ptr() as *mut MaybeUninit<u8>, | |
1288 | this.len() * mem::size_of::<T>(), | |
1289 | ) | |
1290 | } | |
1291 | } | |
dc9dc135 | 1292 | } |