1use crate::any::type_name;
2use crate::fmt;
3use crate::intrinsics;
4use crate::mem::{self, ManuallyDrop};
5use crate::ptr;
6use crate::slice;
7
8/// A wrapper type to construct uninitialized instances of `T`.
9///
10/// # Initialization invariant
11///
12/// The compiler, in general, assumes that a variable is properly initialized
13/// according to the requirements of the variable's type. For example, a variable of
14/// reference type must be aligned and non-null. This is an invariant that must
15/// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
16/// variable of reference type causes instantaneous [undefined behavior][ub],
17/// no matter whether that reference ever gets used to access memory:
18///
19/// ```rust,no_run
20/// # #![allow(invalid_value)]
21/// use std::mem::{self, MaybeUninit};
22///
23/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️
24/// // The equivalent code with `MaybeUninit<&i32>`:
25/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️
26/// ```
27///
28/// This is exploited by the compiler for various optimizations, such as eliding
29/// run-time checks and optimizing `enum` layout.
30///
31/// Similarly, entirely uninitialized memory may have any content, while a `bool` must
32/// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
33///
34/// ```rust,no_run
35/// # #![allow(invalid_value)]
36/// use std::mem::{self, MaybeUninit};
37///
38/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
39/// // The equivalent code with `MaybeUninit<bool>`:
40/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
41/// ```
42///
43/// Moreover, uninitialized memory is special in that 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:
48///
49/// ```rust,no_run
50/// # #![allow(invalid_value)]
51/// use std::mem::{self, MaybeUninit};
52///
53/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
54/// // The equivalent code with `MaybeUninit<i32>`:
55/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
56/// ```
57/// On top of that, remember that most types have additional invariants beyond merely
58/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
59/// is considered initialized (under the current implementation; this does not constitute
60/// a stable guarantee) because the only requirement the compiler knows about it
61/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
62/// *immediate* undefined behavior, but will cause undefined behavior with most
63/// safe operations (including dropping it).
64///
65/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
66///
67/// # Examples
68///
69/// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
70/// It is a signal to the compiler indicating that the data here might *not*
71/// be initialized:
72///
73/// ```rust
74/// use std::mem::MaybeUninit;
75///
76/// // Create an explicitly uninitialized reference. The compiler knows that data inside
77/// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
78/// let mut x = MaybeUninit::<&i32>::uninit();
79/// // Set it to a valid value.
80/// x.write(&0);
81/// // Extract the initialized data -- this is only allowed *after* properly
82/// // initializing `x`!
83/// let x = unsafe { x.assume_init() };
84/// ```
85///
86/// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
87///
88/// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
89/// any of the run-time tracking and without any of the safety checks.
90///
91/// ## out-pointers
92///
93/// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
94/// from a function, pass it a pointer to some (uninitialized) memory to put the
95/// result into. This can be useful when it is important for the caller to control
96/// how the memory the result is stored in gets allocated, and you want to avoid
97/// unnecessary moves.
98///
99/// ```
100/// use std::mem::MaybeUninit;
101///
102/// unsafe fn make_vec(out: *mut Vec<i32>) {
103/// // `write` does not drop the old contents, which is important.
104/// out.write(vec![1, 2, 3]);
105/// }
106///
107/// let mut v = MaybeUninit::uninit();
108/// unsafe { make_vec(v.as_mut_ptr()); }
109/// // Now we know `v` is initialized! This also makes sure the vector gets
110/// // properly dropped.
111/// let v = unsafe { v.assume_init() };
112/// assert_eq!(&v, &[1, 2, 3]);
113/// ```
114///
115/// ## Initializing an array element-by-element
116///
117/// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
118///
119/// ```
120/// use std::mem::{self, MaybeUninit};
121///
122/// let data = {
123/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
124/// // safe because the type we are claiming to have initialized here is a
125/// // bunch of `MaybeUninit`s, which do not require initialization.
126/// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
127/// MaybeUninit::uninit().assume_init()
128/// };
129///
130/// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop,
131/// // we have a memory leak, but there is no memory safety issue.
132/// for elem in &mut data[..] {
133/// elem.write(vec![42]);
134/// }
135///
136/// // Everything is initialized. Transmute the array to the
137/// // initialized type.
138/// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
139/// };
140///
141/// assert_eq!(&data[0], &[42]);
142/// ```
143///
144/// You can also work with partially initialized arrays, which could
145/// be found in low-level datastructures.
146///
147/// ```
148/// use std::mem::MaybeUninit;
149///
150/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
151/// // safe because the type we are claiming to have initialized here is a
152/// // bunch of `MaybeUninit`s, which do not require initialization.
153/// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
154/// // Count the number of elements we have assigned.
155/// let mut data_len: usize = 0;
156///
157/// for elem in &mut data[0..500] {
158/// elem.write(String::from("hello"));
159/// data_len += 1;
160/// }
161///
162/// // For each item in the array, drop if we allocated it.
163/// for elem in &mut data[0..data_len] {
164/// unsafe { elem.assume_init_drop(); }
165/// }
166/// ```
167///
168/// ## Initializing a struct field-by-field
169///
170/// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
171///
172/// ```rust
173/// use std::mem::MaybeUninit;
174/// use std::ptr::addr_of_mut;
175///
176/// #[derive(Debug, PartialEq)]
177/// pub struct Foo {
178/// name: String,
179/// list: Vec<u8>,
180/// }
181///
182/// let foo = {
183/// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
184/// let ptr = uninit.as_mut_ptr();
185///
186/// // Initializing the `name` field
187/// // Using `write` instead of assignment via `=` to not call `drop` on the
188/// // old, uninitialized value.
189/// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }
190///
191/// // Initializing the `list` field
192/// // If there is a panic here, then the `String` in the `name` field leaks.
193/// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); }
194///
195/// // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
196/// unsafe { uninit.assume_init() }
197/// };
198///
199/// assert_eq!(
200/// foo,
201/// Foo {
202/// name: "Bob".to_string(),
203/// list: vec![0, 1, 2]
204/// }
205/// );
206/// ```
207/// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut
208/// [ub]: ../../reference/behavior-considered-undefined.html
209///
210/// # Layout
211///
212/// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
213///
214/// ```rust
215/// use std::mem::{MaybeUninit, size_of, align_of};
216/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
217/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
218/// ```
219///
220/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
221/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
222/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
223/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
224/// optimizations, potentially resulting in a larger size:
225///
226/// ```rust
227/// # use std::mem::{MaybeUninit, size_of};
228/// assert_eq!(size_of::<Option<bool>>(), 1);
229/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
230/// ```
231///
232/// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
233///
234/// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
235/// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
236/// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
237/// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
238/// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
239/// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
240/// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
241/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
242/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
243/// guarantee may evolve.
244#[stable(feature = "maybe_uninit", since = "1.36.0")]
245// Lang item so we can wrap other types in it. This is useful for coroutines.
246#[lang = "maybe_uninit"]
247#[derive(Copy)]
248#[repr(transparent)]
249pub union MaybeUninit<T> {
250 uninit: (),
251 value: ManuallyDrop<T>,
252}
253
254#[stable(feature = "maybe_uninit", since = "1.36.0")]
255impl<T: Copy> Clone for MaybeUninit<T> {
256 #[inline(always)]
257 fn clone(&self) -> Self {
258 // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
259 *self
260 }
261}
262
263#[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
264impl<T> fmt::Debug for MaybeUninit<T> {
265 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
266 f.pad(type_name::<Self>())
267 }
268}
269
270impl<T> MaybeUninit<T> {
271 /// Creates a new `MaybeUninit<T>` initialized with the given value.
272 /// It is safe to call [`assume_init`] on the return value of this function.
273 ///
274 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
275 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
276 ///
277 /// # Example
278 ///
279 /// ```
280 /// use std::mem::MaybeUninit;
281 ///
282 /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
283 /// ```
284 ///
285 /// [`assume_init`]: MaybeUninit::assume_init
286 #[stable(feature = "maybe_uninit", since = "1.36.0")]
287 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
288 #[must_use = "use `forget` to avoid running Drop code"]
289 #[inline(always)]
290 pub const fn new(val: T) -> MaybeUninit<T> {
291 MaybeUninit { value: ManuallyDrop::new(val) }
292 }
293
294 /// Creates a new `MaybeUninit<T>` in an uninitialized state.
295 ///
296 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
297 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
298 ///
299 /// See the [type-level documentation][MaybeUninit] for some examples.
300 ///
301 /// # Example
302 ///
303 /// ```
304 /// use std::mem::MaybeUninit;
305 ///
306 /// let v: MaybeUninit<String> = MaybeUninit::uninit();
307 /// ```
308 #[stable(feature = "maybe_uninit", since = "1.36.0")]
309 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
310 #[must_use]
311 #[inline(always)]
312 #[rustc_diagnostic_item = "maybe_uninit_uninit"]
313 pub const fn uninit() -> MaybeUninit<T> {
314 MaybeUninit { uninit: () }
315 }
316
317 /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
318 ///
319 /// Note: in a future Rust version this method may become unnecessary
320 /// when Rust allows
321 /// [inline const expressions](https://github.com/rust-lang/rust/issues/76001).
322 /// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
323 ///
324 /// # Examples
325 ///
326 /// ```no_run
327 /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
328 ///
329 /// use std::mem::MaybeUninit;
330 ///
331 /// extern "C" {
332 /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
333 /// }
334 ///
335 /// /// Returns a (possibly smaller) slice of data that was actually read
336 /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
337 /// unsafe {
338 /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
339 /// MaybeUninit::slice_assume_init_ref(&buf[..len])
340 /// }
341 /// }
342 ///
343 /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
344 /// let data = read(&mut buf);
345 /// ```
346 #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
347 #[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")]
348 #[must_use]
349 #[inline(always)]
350 pub const fn uninit_array<const N: usize>() -> [Self; N] {
351 // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
352 unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() }
353 }
354
355 /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
356 /// filled with `0` bytes. It depends on `T` whether that already makes for
357 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
358 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
359 /// be null.
360 ///
361 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
362 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
363 ///
364 /// # Example
365 ///
366 /// Correct usage of this function: initializing a struct with zero, where all
367 /// fields of the struct can hold the bit-pattern 0 as a valid value.
368 ///
369 /// ```rust
370 /// use std::mem::MaybeUninit;
371 ///
372 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
373 /// let x = unsafe { x.assume_init() };
374 /// assert_eq!(x, (0, false));
375 /// ```
376 ///
377 /// This can be used in const contexts, such as to indicate the end of static arrays for
378 /// plugin registration.
379 ///
380 /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
381 /// when `0` is not a valid bit-pattern for the type:
382 ///
383 /// ```rust,no_run
384 /// use std::mem::MaybeUninit;
385 ///
386 /// enum NotZero { One = 1, Two = 2 }
387 ///
388 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
389 /// let x = unsafe { x.assume_init() };
390 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
391 /// // This is undefined behavior. ⚠️
392 /// ```
393 #[inline]
394 #[must_use]
395 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
396 #[stable(feature = "maybe_uninit", since = "1.36.0")]
397 // These are OK to allow since we do not leak &mut to user-visible API
398 #[rustc_allow_const_fn_unstable(const_mut_refs)]
399 #[rustc_allow_const_fn_unstable(const_ptr_write)]
400 #[rustc_allow_const_fn_unstable(const_maybe_uninit_as_mut_ptr)]
401 #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "1.75.0")]
402 pub const fn zeroed() -> MaybeUninit<T> {
403 let mut u = MaybeUninit::<T>::uninit();
404 // SAFETY: `u.as_mut_ptr()` points to allocated memory.
405 unsafe { u.as_mut_ptr().write_bytes(0u8, 1) };
406 u
407 }
408
409 /// Sets the value of the `MaybeUninit<T>`.
410 ///
411 /// This overwrites any previous value without dropping it, so be careful
412 /// not to use this twice unless you want to skip running the destructor.
413 /// For your convenience, this also returns a mutable reference to the
414 /// (now safely initialized) contents of `self`.
415 ///
416 /// As the content is stored inside a `MaybeUninit`, the destructor is not
417 /// run for the inner data if the MaybeUninit leaves scope without a call to
418 /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
419 /// the mutable reference returned by this function needs to keep this in
420 /// mind. The safety model of Rust regards leaks as safe, but they are
421 /// usually still undesirable. This being said, the mutable reference
422 /// behaves like any other mutable reference would, so assigning a new value
423 /// to it will drop the old content.
424 ///
425 /// [`assume_init`]: Self::assume_init
426 /// [`assume_init_drop`]: Self::assume_init_drop
427 ///
428 /// # Examples
429 ///
430 /// Correct usage of this method:
431 ///
432 /// ```rust
433 /// use std::mem::MaybeUninit;
434 ///
435 /// let mut x = MaybeUninit::<Vec<u8>>::uninit();
436 ///
437 /// {
438 /// let hello = x.write((&b"Hello, world!").to_vec());
439 /// // Setting hello does not leak prior allocations, but drops them
440 /// *hello = (&b"Hello").to_vec();
441 /// hello[0] = 'h' as u8;
442 /// }
443 /// // x is initialized now:
444 /// let s = unsafe { x.assume_init() };
445 /// assert_eq!(b"hello", s.as_slice());
446 /// ```
447 ///
448 /// This usage of the method causes a leak:
449 ///
450 /// ```rust
451 /// use std::mem::MaybeUninit;
452 ///
453 /// let mut x = MaybeUninit::<String>::uninit();
454 ///
455 /// x.write("Hello".to_string());
456 /// // This leaks the contained string:
457 /// x.write("hello".to_string());
458 /// // x is initialized now:
459 /// let s = unsafe { x.assume_init() };
460 /// ```
461 ///
462 /// This method can be used to avoid unsafe in some cases. The example below
463 /// shows a part of an implementation of a fixed sized arena that lends out
464 /// pinned references.
465 /// With `write`, we can avoid the need to write through a raw pointer:
466 ///
467 /// ```rust
468 /// use core::pin::Pin;
469 /// use core::mem::MaybeUninit;
470 ///
471 /// struct PinArena<T> {
472 /// memory: Box<[MaybeUninit<T>]>,
473 /// len: usize,
474 /// }
475 ///
476 /// impl <T> PinArena<T> {
477 /// pub fn capacity(&self) -> usize {
478 /// self.memory.len()
479 /// }
480 /// pub fn push(&mut self, val: T) -> Pin<&mut T> {
481 /// if self.len >= self.capacity() {
482 /// panic!("Attempted to push to a full pin arena!");
483 /// }
484 /// let ref_ = self.memory[self.len].write(val);
485 /// self.len += 1;
486 /// unsafe { Pin::new_unchecked(ref_) }
487 /// }
488 /// }
489 /// ```
490 #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
491 #[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")]
492 #[inline(always)]
493 pub const fn write(&mut self, val: T) -> &mut T {
494 *self = MaybeUninit::new(val);
495 // SAFETY: We just initialized this value.
496 unsafe { self.assume_init_mut() }
497 }
498
499 /// Gets a pointer to the contained value. Reading from this pointer or turning it
500 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
501 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
502 /// (except inside an `UnsafeCell<T>`).
503 ///
504 /// # Examples
505 ///
506 /// Correct usage of this method:
507 ///
508 /// ```rust
509 /// use std::mem::MaybeUninit;
510 ///
511 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
512 /// x.write(vec![0, 1, 2]);
513 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
514 /// let x_vec = unsafe { &*x.as_ptr() };
515 /// assert_eq!(x_vec.len(), 3);
516 /// ```
517 ///
518 /// *Incorrect* usage of this method:
519 ///
520 /// ```rust,no_run
521 /// use std::mem::MaybeUninit;
522 ///
523 /// let x = MaybeUninit::<Vec<u32>>::uninit();
524 /// let x_vec = unsafe { &*x.as_ptr() };
525 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
526 /// ```
527 ///
528 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
529 /// until they are, it is advisable to avoid them.)
530 #[stable(feature = "maybe_uninit", since = "1.36.0")]
531 #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
532 #[inline(always)]
533 pub const fn as_ptr(&self) -> *const T {
534 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
535 self as *const _ as *const T
536 }
537
538 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
539 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
540 ///
541 /// # Examples
542 ///
543 /// Correct usage of this method:
544 ///
545 /// ```rust
546 /// use std::mem::MaybeUninit;
547 ///
548 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
549 /// x.write(vec![0, 1, 2]);
550 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
551 /// // This is okay because we initialized it.
552 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
553 /// x_vec.push(3);
554 /// assert_eq!(x_vec.len(), 4);
555 /// ```
556 ///
557 /// *Incorrect* usage of this method:
558 ///
559 /// ```rust,no_run
560 /// use std::mem::MaybeUninit;
561 ///
562 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
563 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
564 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
565 /// ```
566 ///
567 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
568 /// until they are, it is advisable to avoid them.)
569 #[stable(feature = "maybe_uninit", since = "1.36.0")]
570 #[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
571 #[inline(always)]
572 pub const fn as_mut_ptr(&mut self) -> *mut T {
573 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
574 self as *mut _ as *mut T
575 }
576
577 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
578 /// to ensure that the data will get dropped, because the resulting `T` is
579 /// subject to the usual drop handling.
580 ///
581 /// # Safety
582 ///
583 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
584 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
585 /// behavior. The [type-level documentation][inv] contains more information about
586 /// this initialization invariant.
587 ///
588 /// [inv]: #initialization-invariant
589 ///
590 /// On top of that, remember that most types have additional invariants beyond merely
591 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
592 /// is considered initialized (under the current implementation; this does not constitute
593 /// a stable guarantee) because the only requirement the compiler knows about it
594 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
595 /// *immediate* undefined behavior, but will cause undefined behavior with most
596 /// safe operations (including dropping it).
597 ///
598 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
599 ///
600 /// # Examples
601 ///
602 /// Correct usage of this method:
603 ///
604 /// ```rust
605 /// use std::mem::MaybeUninit;
606 ///
607 /// let mut x = MaybeUninit::<bool>::uninit();
608 /// x.write(true);
609 /// let x_init = unsafe { x.assume_init() };
610 /// assert_eq!(x_init, true);
611 /// ```
612 ///
613 /// *Incorrect* usage of this method:
614 ///
615 /// ```rust,no_run
616 /// use std::mem::MaybeUninit;
617 ///
618 /// let x = MaybeUninit::<Vec<u32>>::uninit();
619 /// let x_init = unsafe { x.assume_init() };
620 /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
621 /// ```
622 #[stable(feature = "maybe_uninit", since = "1.36.0")]
623 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
624 #[inline(always)]
625 #[rustc_diagnostic_item = "assume_init"]
626 #[track_caller]
627 pub const unsafe fn assume_init(self) -> T {
628 // SAFETY: the caller must guarantee that `self` is initialized.
629 // This also means that `self` must be a `value` variant.
630 unsafe {
631 intrinsics::assert_inhabited::<T>();
632 ManuallyDrop::into_inner(self.value)
633 }
634 }
635
636 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
637 /// to the usual drop handling.
638 ///
639 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
640 /// prevents duplicating the content of the `MaybeUninit<T>`.
641 ///
642 /// # Safety
643 ///
644 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
645 /// state. Calling this when the content is not yet fully initialized causes undefined
646 /// behavior. The [type-level documentation][inv] contains more information about
647 /// this initialization invariant.
648 ///
649 /// Moreover, similar to the [`ptr::read`] function, this function creates a
650 /// bitwise copy of the contents, regardless whether the contained type
651 /// implements the [`Copy`] trait or not. When using multiple copies of the
652 /// data (by calling `assume_init_read` multiple times, or first calling
653 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
654 /// to ensure that data may indeed be duplicated.
655 ///
656 /// [inv]: #initialization-invariant
657 /// [`assume_init`]: MaybeUninit::assume_init
658 ///
659 /// # Examples
660 ///
661 /// Correct usage of this method:
662 ///
663 /// ```rust
664 /// use std::mem::MaybeUninit;
665 ///
666 /// let mut x = MaybeUninit::<u32>::uninit();
667 /// x.write(13);
668 /// let x1 = unsafe { x.assume_init_read() };
669 /// // `u32` is `Copy`, so we may read multiple times.
670 /// let x2 = unsafe { x.assume_init_read() };
671 /// assert_eq!(x1, x2);
672 ///
673 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
674 /// x.write(None);
675 /// let x1 = unsafe { x.assume_init_read() };
676 /// // Duplicating a `None` value is okay, so we may read multiple times.
677 /// let x2 = unsafe { x.assume_init_read() };
678 /// assert_eq!(x1, x2);
679 /// ```
680 ///
681 /// *Incorrect* usage of this method:
682 ///
683 /// ```rust,no_run
684 /// use std::mem::MaybeUninit;
685 ///
686 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
687 /// x.write(Some(vec![0, 1, 2]));
688 /// let x1 = unsafe { x.assume_init_read() };
689 /// let x2 = unsafe { x.assume_init_read() };
690 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
691 /// // they both get dropped!
692 /// ```
693 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
694 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_read", since = "1.75.0")]
695 #[inline(always)]
696 #[track_caller]
697 pub const unsafe fn assume_init_read(&self) -> T {
698 // SAFETY: the caller must guarantee that `self` is initialized.
699 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
700 unsafe {
701 intrinsics::assert_inhabited::<T>();
702 self.as_ptr().read()
703 }
704 }
705
706 /// Drops the contained value in place.
707 ///
708 /// If you have ownership of the `MaybeUninit`, you can also use
709 /// [`assume_init`] as an alternative.
710 ///
711 /// # Safety
712 ///
713 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
714 /// in an initialized state. Calling this when the content is not yet fully
715 /// initialized causes undefined behavior.
716 ///
717 /// On top of that, all additional invariants of the type `T` must be
718 /// satisfied, as the `Drop` implementation of `T` (or its members) may
719 /// rely on this. For example, setting a [`Vec<T>`] to an invalid but
720 /// non-null address makes it initialized (under the current implementation;
721 /// this does not constitute a stable guarantee), because the only
722 /// requirement the compiler knows about it is that the data pointer must be
723 /// non-null. Dropping such a `Vec<T>` however will cause undefined
724 /// behaviour.
725 ///
726 /// [`assume_init`]: MaybeUninit::assume_init
727 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
728 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
729 pub unsafe fn assume_init_drop(&mut self) {
730 // SAFETY: the caller must guarantee that `self` is initialized and
731 // satisfies all invariants of `T`.
732 // Dropping the value in place is safe if that is the case.
733 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
734 }
735
736 /// Gets a shared reference to the contained value.
737 ///
738 /// This can be useful when we want to access a `MaybeUninit` that has been
739 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
740 /// of `.assume_init()`).
741 ///
742 /// # Safety
743 ///
744 /// Calling this when the content is not yet fully initialized causes undefined
745 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
746 /// is in an initialized state.
747 ///
748 /// # Examples
749 ///
750 /// ### Correct usage of this method:
751 ///
752 /// ```rust
753 /// use std::mem::MaybeUninit;
754 ///
755 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
756 /// // Initialize `x`:
757 /// x.write(vec![1, 2, 3]);
758 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
759 /// // create a shared reference to it:
760 /// let x: &Vec<u32> = unsafe {
761 /// // SAFETY: `x` has been initialized.
762 /// x.assume_init_ref()
763 /// };
764 /// assert_eq!(x, &vec![1, 2, 3]);
765 /// ```
766 ///
767 /// ### *Incorrect* usages of this method:
768 ///
769 /// ```rust,no_run
770 /// use std::mem::MaybeUninit;
771 ///
772 /// let x = MaybeUninit::<Vec<u32>>::uninit();
773 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
774 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
775 /// ```
776 ///
777 /// ```rust,no_run
778 /// use std::{cell::Cell, mem::MaybeUninit};
779 ///
780 /// let b = MaybeUninit::<Cell<bool>>::uninit();
781 /// // Initialize the `MaybeUninit` using `Cell::set`:
782 /// unsafe {
783 /// b.assume_init_ref().set(true);
784 /// // ^^^^^^^^^^^^^^^
785 /// // Reference to an uninitialized `Cell<bool>`: UB!
786 /// }
787 /// ```
788 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
789 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
790 #[inline(always)]
791 pub const unsafe fn assume_init_ref(&self) -> &T {
792 // SAFETY: the caller must guarantee that `self` is initialized.
793 // This also means that `self` must be a `value` variant.
794 unsafe {
795 intrinsics::assert_inhabited::<T>();
796 &*self.as_ptr()
797 }
798 }
799
800 /// Gets a mutable (unique) reference to the contained value.
801 ///
802 /// This can be useful when we want to access a `MaybeUninit` that has been
803 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
804 /// of `.assume_init()`).
805 ///
806 /// # Safety
807 ///
808 /// Calling this when the content is not yet fully initialized causes undefined
809 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
810 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
811 /// initialize a `MaybeUninit`.
812 ///
813 /// # Examples
814 ///
815 /// ### Correct usage of this method:
816 ///
817 /// ```rust
818 /// # #![allow(unexpected_cfgs)]
819 /// use std::mem::MaybeUninit;
820 ///
821 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }
822 /// # #[cfg(FALSE)]
823 /// extern "C" {
824 /// /// Initializes *all* the bytes of the input buffer.
825 /// fn initialize_buffer(buf: *mut [u8; 1024]);
826 /// }
827 ///
828 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
829 ///
830 /// // Initialize `buf`:
831 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
832 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
833 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
834 /// // To assert our buffer has been initialized without copying it, we upgrade
835 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
836 /// let buf: &mut [u8; 1024] = unsafe {
837 /// // SAFETY: `buf` has been initialized.
838 /// buf.assume_init_mut()
839 /// };
840 ///
841 /// // Now we can use `buf` as a normal slice:
842 /// buf.sort_unstable();
843 /// assert!(
844 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
845 /// "buffer is sorted",
846 /// );
847 /// ```
848 ///
849 /// ### *Incorrect* usages of this method:
850 ///
851 /// You cannot use `.assume_init_mut()` to initialize a value:
852 ///
853 /// ```rust,no_run
854 /// use std::mem::MaybeUninit;
855 ///
856 /// let mut b = MaybeUninit::<bool>::uninit();
857 /// unsafe {
858 /// *b.assume_init_mut() = true;
859 /// // We have created a (mutable) reference to an uninitialized `bool`!
860 /// // This is undefined behavior. ⚠️
861 /// }
862 /// ```
863 ///
864 /// For instance, you cannot [`Read`] into an uninitialized buffer:
865 ///
866 /// [`Read`]: ../../std/io/trait.Read.html
867 ///
868 /// ```rust,no_run
869 /// use std::{io, mem::MaybeUninit};
870 ///
871 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
872 /// {
873 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
874 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
875 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
876 /// // (mutable) reference to uninitialized memory!
877 /// // This is undefined behavior.
878 /// Ok(unsafe { buffer.assume_init() })
879 /// }
880 /// ```
881 ///
882 /// Nor can you use direct field access to do field-by-field gradual initialization:
883 ///
884 /// ```rust,no_run
885 /// use std::{mem::MaybeUninit, ptr};
886 ///
887 /// struct Foo {
888 /// a: u32,
889 /// b: u8,
890 /// }
891 ///
892 /// let foo: Foo = unsafe {
893 /// let mut foo = MaybeUninit::<Foo>::uninit();
894 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
895 /// // ^^^^^^^^^^^^^^^^^^^^^
896 /// // (mutable) reference to uninitialized memory!
897 /// // This is undefined behavior.
898 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
899 /// // ^^^^^^^^^^^^^^^^^^^^^
900 /// // (mutable) reference to uninitialized memory!
901 /// // This is undefined behavior.
902 /// foo.assume_init()
903 /// };
904 /// ```
905 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
906 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
907 #[inline(always)]
908 pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
909 // SAFETY: the caller must guarantee that `self` is initialized.
910 // This also means that `self` must be a `value` variant.
911 unsafe {
912 intrinsics::assert_inhabited::<T>();
913 &mut *self.as_mut_ptr()
914 }
915 }
916
917 /// Extracts the values from an array of `MaybeUninit` containers.
918 ///
919 /// # Safety
920 ///
921 /// It is up to the caller to guarantee that all elements of the array are
922 /// in an initialized state.
923 ///
924 /// # Examples
925 ///
926 /// ```
927 /// #![feature(maybe_uninit_uninit_array)]
928 /// #![feature(maybe_uninit_array_assume_init)]
929 /// use std::mem::MaybeUninit;
930 ///
931 /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
932 /// array[0].write(0);
933 /// array[1].write(1);
934 /// array[2].write(2);
935 ///
936 /// // SAFETY: Now safe as we initialised all elements
937 /// let array = unsafe {
938 /// MaybeUninit::array_assume_init(array)
939 /// };
940 ///
941 /// assert_eq!(array, [0, 1, 2]);
942 /// ```
943 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
944 #[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")]
945 #[inline(always)]
946 #[track_caller]
947 pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
948 // SAFETY:
949 // * The caller guarantees that all elements of the array are initialized
950 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
951 // * `MaybeUninit` does not drop, so there are no double-frees
952 // And thus the conversion is safe
953 unsafe {
954 intrinsics::assert_inhabited::<[T; N]>();
955 intrinsics::transmute_unchecked(array)
956 }
957 }
958
959 /// Assuming all the elements are initialized, get a slice to them.
960 ///
961 /// # Safety
962 ///
963 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
964 /// really are in an initialized state.
965 /// Calling this when the content is not yet fully initialized causes undefined behavior.
966 ///
967 /// See [`assume_init_ref`] for more details and examples.
968 ///
969 /// [`assume_init_ref`]: MaybeUninit::assume_init_ref
970 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
971 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
972 #[inline(always)]
973 pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
974 // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
975 // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
976 // The pointer obtained is valid since it refers to memory owned by `slice` which is a
977 // reference and thus guaranteed to be valid for reads.
978 unsafe { &*(slice as *const [Self] as *const [T]) }
979 }
980
981 /// Assuming all the elements are initialized, get a mutable slice to them.
982 ///
983 /// # Safety
984 ///
985 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
986 /// really are in an initialized state.
987 /// Calling this when the content is not yet fully initialized causes undefined behavior.
988 ///
989 /// See [`assume_init_mut`] for more details and examples.
990 ///
991 /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
992 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
993 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
994 #[inline(always)]
995 pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
996 // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
997 // mutable reference which is also guaranteed to be valid for writes.
998 unsafe { &mut *(slice as *mut [Self] as *mut [T]) }
999 }
1000
1001 /// Gets a pointer to the first element of the array.
1002 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1003 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1004 #[inline(always)]
1005 pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
1006 this.as_ptr() as *const T
1007 }
1008
1009 /// Gets a mutable pointer to the first element of the array.
1010 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1011 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1012 #[inline(always)]
1013 pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
1014 this.as_mut_ptr() as *mut T
1015 }
1016
1017 /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
1018 ///
1019 /// If `T` does not implement `Copy`, use [`clone_from_slice`]
1020 ///
1021 /// This is similar to [`slice::copy_from_slice`].
1022 ///
1023 /// # Panics
1024 ///
1025 /// This function will panic if the two slices have different lengths.
1026 ///
1027 /// # Examples
1028 ///
1029 /// ```
1030 /// #![feature(maybe_uninit_write_slice)]
1031 /// use std::mem::MaybeUninit;
1032 ///
1033 /// let mut dst = [MaybeUninit::uninit(); 32];
1034 /// let src = [0; 32];
1035 ///
1036 /// let init = MaybeUninit::copy_from_slice(&mut dst, &src);
1037 ///
1038 /// assert_eq!(init, src);
1039 /// ```
1040 ///
1041 /// ```
1042 /// #![feature(maybe_uninit_write_slice)]
1043 /// use std::mem::MaybeUninit;
1044 ///
1045 /// let mut vec = Vec::with_capacity(32);
1046 /// let src = [0; 16];
1047 ///
1048 /// MaybeUninit::copy_from_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
1049 ///
1050 /// // SAFETY: we have just copied all the elements of len into the spare capacity
1051 /// // the first src.len() elements of the vec are valid now.
1052 /// unsafe {
1053 /// vec.set_len(src.len());
1054 /// }
1055 ///
1056 /// assert_eq!(vec, src);
1057 /// ```
1058 ///
1059 /// [`clone_from_slice`]: MaybeUninit::clone_from_slice
1060 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1061 pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1062 where
1063 T: Copy,
1064 {
1065 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1066 let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
1067
1068 this.copy_from_slice(uninit_src);
1069
1070 // SAFETY: Valid elements have just been copied into `this` so it is initialized
1071 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1072 }
1073
1074 /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
1075 /// Any already initialized elements will not be dropped.
1076 ///
1077 /// If `T` implements `Copy`, use [`copy_from_slice`]
1078 ///
1079 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1080 ///
1081 /// # Panics
1082 ///
1083 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1084 ///
1085 /// If there is a panic, the already cloned elements will be dropped.
1086 ///
1087 /// # Examples
1088 ///
1089 /// ```
1090 /// #![feature(maybe_uninit_write_slice)]
1091 /// use std::mem::MaybeUninit;
1092 ///
1093 /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
1094 /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
1095 ///
1096 /// let init = MaybeUninit::clone_from_slice(&mut dst, &src);
1097 ///
1098 /// assert_eq!(init, src);
1099 /// ```
1100 ///
1101 /// ```
1102 /// #![feature(maybe_uninit_write_slice)]
1103 /// use std::mem::MaybeUninit;
1104 ///
1105 /// let mut vec = Vec::with_capacity(32);
1106 /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
1107 ///
1108 /// MaybeUninit::clone_from_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
1109 ///
1110 /// // SAFETY: we have just cloned all the elements of len into the spare capacity
1111 /// // the first src.len() elements of the vec are valid now.
1112 /// unsafe {
1113 /// vec.set_len(src.len());
1114 /// }
1115 ///
1116 /// assert_eq!(vec, src);
1117 /// ```
1118 ///
1119 /// [`copy_from_slice`]: MaybeUninit::copy_from_slice
1120 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1121 pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1122 where
1123 T: Clone,
1124 {
1125 // unlike copy_from_slice this does not call clone_from_slice on the slice
1126 // this is because `MaybeUninit<T: Clone>` does not implement Clone.
1127
1128 assert_eq!(this.len(), src.len(), "destination and source slices have different lengths");
1129 // NOTE: We need to explicitly slice them to the same length
1130 // for bounds checking to be elided, and the optimizer will
1131 // generate memcpy for simple cases (for example T = u8).
1132 let len = this.len();
1133 let src = &src[..len];
1134
1135 // guard is needed b/c panic might happen during a clone
1136 let mut guard = Guard { slice: this, initialized: 0 };
1137
1138 for i in 0..len {
1139 guard.slice[i].write(src[i].clone());
1140 guard.initialized += 1;
1141 }
1142
1143 super::forget(guard);
1144
1145 // SAFETY: Valid elements have just been written into `this` so it is initialized
1146 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1147 }
1148
1149 /// Fills `this` with elements by cloning `value`, returning a mutable reference to the now
1150 /// initialized contents of `this`.
1151 /// Any previously initialized elements will not be dropped.
1152 ///
1153 /// This is similar to [`slice::fill`].
1154 ///
1155 /// # Panics
1156 ///
1157 /// This function will panic if any call to `Clone` panics.
1158 ///
1159 /// If such a panic occurs, any elements previously initialized during this operation will be
1160 /// dropped.
1161 ///
1162 /// # Examples
1163 ///
1164 /// Fill an uninit vec with 1.
1165 /// ```
1166 /// #![feature(maybe_uninit_fill)]
1167 /// use std::mem::MaybeUninit;
1168 ///
1169 /// let mut buf = vec![MaybeUninit::uninit(); 10];
1170 /// let initialized = MaybeUninit::fill(buf.as_mut_slice(), 1);
1171 /// assert_eq!(initialized, &mut [1; 10]);
1172 /// ```
1173 #[doc(alias = "memset")]
1174 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1175 pub fn fill<'a>(this: &'a mut [MaybeUninit<T>], value: T) -> &'a mut [T]
1176 where
1177 T: Clone,
1178 {
1179 SpecFill::spec_fill(this, value);
1180 // SAFETY: Valid elements have just been filled into `this` so it is initialized
1181 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1182 }
1183
1184 /// Fills `this` with elements returned by calling a closure repeatedly.
1185 ///
1186 /// This method uses a closure to create new values. If you'd rather `Clone` a given value, use
1187 /// [`MaybeUninit::fill`]. If you want to use the `Default` trait to generate values, you can
1188 /// pass [`Default::default`] as the argument.
1189 ///
1190 /// # Panics
1191 ///
1192 /// This function will panic if any call to the provided closure panics.
1193 ///
1194 /// If such a panic occurs, any elements previously initialized during this operation will be
1195 /// dropped.
1196 ///
1197 /// # Examples
1198 ///
1199 /// Fill an uninit vec with the default value.
1200 /// ```
1201 /// #![feature(maybe_uninit_fill)]
1202 /// use std::mem::MaybeUninit;
1203 ///
1204 /// let mut buf = vec![MaybeUninit::<i32>::uninit(); 10];
1205 /// let initialized = MaybeUninit::fill_with(buf.as_mut_slice(), Default::default);
1206 /// assert_eq!(initialized, &mut [0; 10]);
1207 /// ```
1208 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1209 pub fn fill_with<'a, F>(this: &'a mut [MaybeUninit<T>], mut f: F) -> &'a mut [T]
1210 where
1211 F: FnMut() -> T,
1212 {
1213 let mut guard = Guard { slice: this, initialized: 0 };
1214
1215 for element in guard.slice.iter_mut() {
1216 element.write(f());
1217 guard.initialized += 1;
1218 }
1219
1220 super::forget(guard);
1221
1222 // SAFETY: Valid elements have just been written into `this` so it is initialized
1223 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1224 }
1225
1226 /// Fills `this` with elements yielded by an iterator until either all elements have been
1227 /// initialized or the iterator is empty.
1228 ///
1229 /// Returns two slices. The first slice contains the initialized portion of the original slice.
1230 /// The second slice is the still-uninitialized remainder of the original slice.
1231 ///
1232 /// # Panics
1233 ///
1234 /// This function panics if the iterator's `next` function panics.
1235 ///
1236 /// If such a panic occurs, any elements previously initialized during this operation will be
1237 /// dropped.
1238 ///
1239 /// # Examples
1240 ///
1241 /// Fill an uninit vec with a cycling iterator.
1242 /// ```
1243 /// #![feature(maybe_uninit_fill)]
1244 /// use std::mem::MaybeUninit;
1245 ///
1246 /// let mut buf = vec![MaybeUninit::uninit(); 5];
1247 ///
1248 /// let iter = [1, 2, 3].into_iter().cycle();
1249 /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
1250 ///
1251 /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]);
1252 /// assert_eq!(0, remainder.len());
1253 /// ```
1254 ///
1255 /// Fill an uninit vec, but not completely.
1256 /// ```
1257 /// #![feature(maybe_uninit_fill)]
1258 /// use std::mem::MaybeUninit;
1259 ///
1260 /// let mut buf = vec![MaybeUninit::uninit(); 5];
1261 /// let iter = [1, 2];
1262 /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
1263 ///
1264 /// assert_eq!(initialized, &mut [1, 2]);
1265 /// assert_eq!(remainder.len(), 3);
1266 /// ```
1267 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1268 pub fn fill_from<'a, I>(
1269 this: &'a mut [MaybeUninit<T>],
1270 it: I,
1271 ) -> (&'a mut [T], &'a mut [MaybeUninit<T>])
1272 where
1273 I: IntoIterator<Item = T>,
1274 {
1275 let iter = it.into_iter();
1276 let mut guard = Guard { slice: this, initialized: 0 };
1277
1278 for (element, val) in guard.slice.iter_mut().zip(iter) {
1279 element.write(val);
1280 guard.initialized += 1;
1281 }
1282
1283 let initialized_len = guard.initialized;
1284 super::forget(guard);
1285
1286 // SAFETY: guard.initialized <= this.len()
1287 let (initted, remainder) = unsafe { this.split_at_mut_unchecked(initialized_len) };
1288
1289 // SAFETY: Valid elements have just been written into `init`, so that portion
1290 // of `this` is initialized.
1291 (unsafe { MaybeUninit::slice_assume_init_mut(initted) }, remainder)
1292 }
1293
1294 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1295 ///
1296 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1297 /// contain padding bytes which are left uninitialized.
1298 ///
1299 /// # Examples
1300 ///
1301 /// ```
1302 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
1303 /// use std::mem::MaybeUninit;
1304 ///
1305 /// let val = 0x12345678_i32;
1306 /// let uninit = MaybeUninit::new(val);
1307 /// let uninit_bytes = uninit.as_bytes();
1308 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
1309 /// assert_eq!(bytes, val.to_ne_bytes());
1310 /// ```
1311 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1312 pub fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1313 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1314 unsafe {
1315 slice::from_raw_parts(self.as_ptr() as *const MaybeUninit<u8>, mem::size_of::<T>())
1316 }
1317 }
1318
1319 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
1320 /// bytes.
1321 ///
1322 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1323 /// contain padding bytes which are left uninitialized.
1324 ///
1325 /// # Examples
1326 ///
1327 /// ```
1328 /// #![feature(maybe_uninit_as_bytes)]
1329 /// use std::mem::MaybeUninit;
1330 ///
1331 /// let val = 0x12345678_i32;
1332 /// let mut uninit = MaybeUninit::new(val);
1333 /// let uninit_bytes = uninit.as_bytes_mut();
1334 /// if cfg!(target_endian = "little") {
1335 /// uninit_bytes[0].write(0xcd);
1336 /// } else {
1337 /// uninit_bytes[3].write(0xcd);
1338 /// }
1339 /// let val2 = unsafe { uninit.assume_init() };
1340 /// assert_eq!(val2, 0x123456cd);
1341 /// ```
1342 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1343 pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1344 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1345 unsafe {
1346 slice::from_raw_parts_mut(
1347 self.as_mut_ptr() as *mut MaybeUninit<u8>,
1348 mem::size_of::<T>(),
1349 )
1350 }
1351 }
1352
1353 /// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized
1354 /// bytes.
1355 ///
1356 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1357 /// contain padding bytes which are left uninitialized.
1358 ///
1359 /// # Examples
1360 ///
1361 /// ```
1362 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1363 /// use std::mem::MaybeUninit;
1364 ///
1365 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1366 /// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit);
1367 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) };
1368 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1369 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1370 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1371 /// ```
1372 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1373 pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
1374 let bytes = mem::size_of_val(this);
1375 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1376 unsafe { slice::from_raw_parts(this.as_ptr() as *const MaybeUninit<u8>, bytes) }
1377 }
1378
1379 /// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of
1380 /// potentially uninitialized bytes.
1381 ///
1382 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1383 /// contain padding bytes which are left uninitialized.
1384 ///
1385 /// # Examples
1386 ///
1387 /// ```
1388 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1389 /// use std::mem::MaybeUninit;
1390 ///
1391 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1392 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1393 /// MaybeUninit::copy_from_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
1394 /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
1395 /// if cfg!(target_endian = "little") {
1396 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1397 /// } else {
1398 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1399 /// }
1400 /// ```
1401 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1402 pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
1403 let bytes = mem::size_of_val(this);
1404 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1405 unsafe { slice::from_raw_parts_mut(this.as_mut_ptr() as *mut MaybeUninit<u8>, bytes) }
1406 }
1407}
1408
1409impl<T, const N: usize> MaybeUninit<[T; N]> {
1410 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1411 ///
1412 /// # Examples
1413 ///
1414 /// ```
1415 /// #![feature(maybe_uninit_uninit_array_transpose)]
1416 /// # use std::mem::MaybeUninit;
1417 ///
1418 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1419 /// ```
1420 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1421 #[inline]
1422 pub const fn transpose(self) -> [MaybeUninit<T>; N] {
1423 // SAFETY: T and MaybeUninit<T> have the same layout
1424 unsafe { intrinsics::transmute_unchecked(self) }
1425 }
1426}
1427
1428impl<T, const N: usize> [MaybeUninit<T>; N] {
1429 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1430 ///
1431 /// # Examples
1432 ///
1433 /// ```
1434 /// #![feature(maybe_uninit_uninit_array_transpose)]
1435 /// # use std::mem::MaybeUninit;
1436 ///
1437 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1438 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1439 /// ```
1440 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1441 #[inline]
1442 pub const fn transpose(self) -> MaybeUninit<[T; N]> {
1443 // SAFETY: T and MaybeUninit<T> have the same layout
1444 unsafe { intrinsics::transmute_unchecked(self) }
1445 }
1446}
1447
1448struct Guard<'a, T> {
1449 slice: &'a mut [MaybeUninit<T>],
1450 initialized: usize,
1451}
1452
1453impl<'a, T> Drop for Guard<'a, T> {
1454 fn drop(&mut self) {
1455 let initialized_part: &mut [MaybeUninit] = &mut self.slice[..self.initialized];
1456 // SAFETY: this raw sub-slice will contain only initialized objects.
1457 unsafe {
1458 crate::ptr::drop_in_place(to_drop:MaybeUninit::slice_assume_init_mut(slice:initialized_part));
1459 }
1460 }
1461}
1462
1463trait SpecFill<T> {
1464 fn spec_fill(&mut self, value: T);
1465}
1466
1467impl<T: Clone> SpecFill<T> for [MaybeUninit<T>] {
1468 default fn spec_fill(&mut self, value: T) {
1469 let mut guard: Guard<'_, T> = Guard { slice: self, initialized: 0 };
1470
1471 if let Some((last: &mut MaybeUninit, elems: &mut [MaybeUninit])) = guard.slice.split_last_mut() {
1472 for el: &mut MaybeUninit in elems {
1473 el.write(val:value.clone());
1474 guard.initialized += 1;
1475 }
1476
1477 last.write(val:value);
1478 }
1479 super::forget(guard);
1480 }
1481}
1482
1483impl<T: Copy> SpecFill<T> for [MaybeUninit<T>] {
1484 fn spec_fill(&mut self, value: T) {
1485 self.fill(MaybeUninit::new(val:value));
1486 }
1487}
1488