1 use crate::any
::type_name
;
4 use crate::mem
::{self, ManuallyDrop}
;
8 /// A wrapper type to construct uninitialized instances of `T`.
10 /// # Initialization invariant
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:
20 /// # #![allow(invalid_value)]
21 /// use std::mem::{self, MaybeUninit};
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! ⚠️
28 /// This is exploited by the compiler for various optimizations, such as eliding
29 /// run-time checks and optimizing `enum` layout.
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:
35 /// # #![allow(invalid_value)]
36 /// use std::mem::{self, MaybeUninit};
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! ⚠️
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:
50 /// # #![allow(invalid_value)]
51 /// use std::mem::{self, MaybeUninit};
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! ⚠️
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).
65 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
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*
74 /// use std::mem::MaybeUninit;
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.
81 /// // Extract the initialized data -- this is only allowed *after* properly
82 /// // initializing `x`!
83 /// let x = unsafe { x.assume_init() };
86 /// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
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.
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.
100 /// use std::mem::MaybeUninit;
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]);
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]);
115 /// ## Initializing an array element-by-element
117 /// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
120 /// use std::mem::{self, MaybeUninit};
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()
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]);
136 /// // Everything is initialized. Transmute the array to the
137 /// // initialized type.
138 /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
141 /// assert_eq!(&data[0], &[42]);
144 /// You can also work with partially initialized arrays, which could
145 /// be found in low-level datastructures.
148 /// use std::mem::MaybeUninit;
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;
157 /// for elem in &mut data[0..500] {
158 /// elem.write(String::from("hello"));
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(); }
168 /// ## Initializing a struct field-by-field
170 /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
173 /// use std::mem::MaybeUninit;
174 /// use std::ptr::addr_of_mut;
176 /// #[derive(Debug, PartialEq)]
183 /// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
184 /// let ptr = uninit.as_mut_ptr();
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()); }
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]); }
195 /// // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
196 /// unsafe { uninit.assume_init() }
202 /// name: "Bob".to_string(),
203 /// list: vec![0, 1, 2]
207 /// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut
208 /// [ub]: ../../reference/behavior-considered-undefined.html
212 /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
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>());
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:
227 /// # use std::mem::{MaybeUninit, size_of};
228 /// assert_eq!(size_of::<Option<bool>>(), 1);
229 /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
232 /// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
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 generators.
246 #[lang = "maybe_uninit"]
249 pub union MaybeUninit
<T
> {
251 value
: ManuallyDrop
<T
>,
254 #[stable(feature = "maybe_uninit", since = "1.36.0")]
255 impl<T
: Copy
> Clone
for MaybeUninit
<T
> {
257 fn clone(&self) -> Self {
258 // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
263 #[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
264 impl<T
> fmt
::Debug
for MaybeUninit
<T
> {
265 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
266 f
.pad(type_name
::<Self>())
270 impl<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.
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.
280 /// use std::mem::MaybeUninit;
282 /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
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"]
290 pub const fn new(val
: T
) -> MaybeUninit
<T
> {
291 MaybeUninit { value: ManuallyDrop::new(val) }
294 /// Creates a new `MaybeUninit<T>` in an uninitialized state.
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.
299 /// See the [type-level documentation][MaybeUninit] for some examples.
304 /// use std::mem::MaybeUninit;
306 /// let v: MaybeUninit<String> = MaybeUninit::uninit();
308 #[stable(feature = "maybe_uninit", since = "1.36.0")]
309 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
312 #[rustc_diagnostic_item = "maybe_uninit_uninit"]
313 pub const fn uninit() -> MaybeUninit
<T
> {
314 MaybeUninit { uninit: () }
317 /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
319 /// Note: in a future Rust version this method may become unnecessary
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];`.
327 /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
329 /// use std::mem::MaybeUninit;
332 /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
335 /// /// Returns a (possibly smaller) slice of data that was actually read
336 /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
338 /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
339 /// MaybeUninit::slice_assume_init_ref(&buf[..len])
343 /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
344 /// let data = read(&mut buf);
346 #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
347 #[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")]
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() }
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
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.
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.
370 /// use std::mem::MaybeUninit;
372 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
373 /// let x = unsafe { x.assume_init() };
374 /// assert_eq!(x, (0, false));
377 /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
378 /// when `0` is not a valid bit-pattern for the type:
381 /// use std::mem::MaybeUninit;
383 /// enum NotZero { One = 1, Two = 2 }
385 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
386 /// let x = unsafe { x.assume_init() };
387 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
388 /// // This is undefined behavior. ⚠️
390 #[stable(feature = "maybe_uninit", since = "1.36.0")]
391 #[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")]
394 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
395 pub const fn zeroed() -> MaybeUninit
<T
> {
396 let mut u
= MaybeUninit
::<T
>::uninit();
397 // SAFETY: `u.as_mut_ptr()` points to allocated memory.
399 u
.as_mut_ptr().write_bytes(0u8, 1);
404 /// Sets the value of the `MaybeUninit<T>`.
406 /// This overwrites any previous value without dropping it, so be careful
407 /// not to use this twice unless you want to skip running the destructor.
408 /// For your convenience, this also returns a mutable reference to the
409 /// (now safely initialized) contents of `self`.
411 /// As the content is stored inside a `MaybeUninit`, the destructor is not
412 /// run for the inner data if the MaybeUninit leaves scope without a call to
413 /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
414 /// the mutable reference returned by this function needs to keep this in
415 /// mind. The safety model of Rust regards leaks as safe, but they are
416 /// usually still undesirable. This being said, the mutable reference
417 /// behaves like any other mutable reference would, so assigning a new value
418 /// to it will drop the old content.
420 /// [`assume_init`]: Self::assume_init
421 /// [`assume_init_drop`]: Self::assume_init_drop
425 /// Correct usage of this method:
428 /// use std::mem::MaybeUninit;
430 /// let mut x = MaybeUninit::<Vec<u8>>::uninit();
433 /// let hello = x.write((&b"Hello, world!").to_vec());
434 /// // Setting hello does not leak prior allocations, but drops them
435 /// *hello = (&b"Hello").to_vec();
436 /// hello[0] = 'h' as u8;
438 /// // x is initialized now:
439 /// let s = unsafe { x.assume_init() };
440 /// assert_eq!(b"hello", s.as_slice());
443 /// This usage of the method causes a leak:
446 /// use std::mem::MaybeUninit;
448 /// let mut x = MaybeUninit::<String>::uninit();
450 /// x.write("Hello".to_string());
451 /// // This leaks the contained string:
452 /// x.write("hello".to_string());
453 /// // x is initialized now:
454 /// let s = unsafe { x.assume_init() };
457 /// This method can be used to avoid unsafe in some cases. The example below
458 /// shows a part of an implementation of a fixed sized arena that lends out
459 /// pinned references.
460 /// With `write`, we can avoid the need to write through a raw pointer:
463 /// use core::pin::Pin;
464 /// use core::mem::MaybeUninit;
466 /// struct PinArena<T> {
467 /// memory: Box<[MaybeUninit<T>]>,
471 /// impl <T> PinArena<T> {
472 /// pub fn capacity(&self) -> usize {
473 /// self.memory.len()
475 /// pub fn push(&mut self, val: T) -> Pin<&mut T> {
476 /// if self.len >= self.capacity() {
477 /// panic!("Attempted to push to a full pin arena!");
479 /// let ref_ = self.memory[self.len].write(val);
481 /// unsafe { Pin::new_unchecked(ref_) }
485 #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
486 #[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")]
488 pub const fn write(&mut self, val
: T
) -> &mut T
{
489 *self = MaybeUninit
::new(val
);
490 // SAFETY: We just initialized this value.
491 unsafe { self.assume_init_mut() }
494 /// Gets a pointer to the contained value. Reading from this pointer or turning it
495 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
496 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
497 /// (except inside an `UnsafeCell<T>`).
501 /// Correct usage of this method:
504 /// use std::mem::MaybeUninit;
506 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
507 /// x.write(vec![0, 1, 2]);
508 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
509 /// let x_vec = unsafe { &*x.as_ptr() };
510 /// assert_eq!(x_vec.len(), 3);
513 /// *Incorrect* usage of this method:
516 /// use std::mem::MaybeUninit;
518 /// let x = MaybeUninit::<Vec<u32>>::uninit();
519 /// let x_vec = unsafe { &*x.as_ptr() };
520 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
523 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
524 /// until they are, it is advisable to avoid them.)
525 #[stable(feature = "maybe_uninit", since = "1.36.0")]
526 #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
528 pub const fn as_ptr(&self) -> *const T
{
529 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
530 self as *const _
as *const T
533 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
534 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
538 /// Correct usage of this method:
541 /// use std::mem::MaybeUninit;
543 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
544 /// x.write(vec![0, 1, 2]);
545 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
546 /// // This is okay because we initialized it.
547 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
549 /// assert_eq!(x_vec.len(), 4);
552 /// *Incorrect* usage of this method:
555 /// use std::mem::MaybeUninit;
557 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
558 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
559 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
562 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
563 /// until they are, it is advisable to avoid them.)
564 #[stable(feature = "maybe_uninit", since = "1.36.0")]
565 #[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
567 pub const fn as_mut_ptr(&mut self) -> *mut T
{
568 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
569 self as *mut _
as *mut T
572 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
573 /// to ensure that the data will get dropped, because the resulting `T` is
574 /// subject to the usual drop handling.
578 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
579 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
580 /// behavior. The [type-level documentation][inv] contains more information about
581 /// this initialization invariant.
583 /// [inv]: #initialization-invariant
585 /// On top of that, remember that most types have additional invariants beyond merely
586 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
587 /// is considered initialized (under the current implementation; this does not constitute
588 /// a stable guarantee) because the only requirement the compiler knows about it
589 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
590 /// *immediate* undefined behavior, but will cause undefined behavior with most
591 /// safe operations (including dropping it).
593 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
597 /// Correct usage of this method:
600 /// use std::mem::MaybeUninit;
602 /// let mut x = MaybeUninit::<bool>::uninit();
604 /// let x_init = unsafe { x.assume_init() };
605 /// assert_eq!(x_init, true);
608 /// *Incorrect* usage of this method:
611 /// use std::mem::MaybeUninit;
613 /// let x = MaybeUninit::<Vec<u32>>::uninit();
614 /// let x_init = unsafe { x.assume_init() };
615 /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
617 #[stable(feature = "maybe_uninit", since = "1.36.0")]
618 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
620 #[rustc_diagnostic_item = "assume_init"]
622 pub const unsafe fn assume_init(self) -> T
{
623 // SAFETY: the caller must guarantee that `self` is initialized.
624 // This also means that `self` must be a `value` variant.
626 intrinsics
::assert_inhabited
::<T
>();
627 ManuallyDrop
::into_inner(self.value
)
631 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
632 /// to the usual drop handling.
634 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
635 /// prevents duplicating the content of the `MaybeUninit<T>`.
639 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
640 /// state. Calling this when the content is not yet fully initialized causes undefined
641 /// behavior. The [type-level documentation][inv] contains more information about
642 /// this initialization invariant.
644 /// Moreover, similar to the [`ptr::read`] function, this function creates a
645 /// bitwise copy of the contents, regardless whether the contained type
646 /// implements the [`Copy`] trait or not. When using multiple copies of the
647 /// data (by calling `assume_init_read` multiple times, or first calling
648 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
649 /// to ensure that data may indeed be duplicated.
651 /// [inv]: #initialization-invariant
652 /// [`assume_init`]: MaybeUninit::assume_init
656 /// Correct usage of this method:
659 /// use std::mem::MaybeUninit;
661 /// let mut x = MaybeUninit::<u32>::uninit();
663 /// let x1 = unsafe { x.assume_init_read() };
664 /// // `u32` is `Copy`, so we may read multiple times.
665 /// let x2 = unsafe { x.assume_init_read() };
666 /// assert_eq!(x1, x2);
668 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
670 /// let x1 = unsafe { x.assume_init_read() };
671 /// // Duplicating a `None` value is okay, so we may read multiple times.
672 /// let x2 = unsafe { x.assume_init_read() };
673 /// assert_eq!(x1, x2);
676 /// *Incorrect* usage of this method:
679 /// use std::mem::MaybeUninit;
681 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
682 /// x.write(Some(vec![0, 1, 2]));
683 /// let x1 = unsafe { x.assume_init_read() };
684 /// let x2 = unsafe { x.assume_init_read() };
685 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
686 /// // they both get dropped!
688 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
689 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init_read", issue = "63567")]
692 pub const unsafe fn assume_init_read(&self) -> T
{
693 // SAFETY: the caller must guarantee that `self` is initialized.
694 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
696 intrinsics
::assert_inhabited
::<T
>();
701 /// Drops the contained value in place.
703 /// If you have ownership of the `MaybeUninit`, you can also use
704 /// [`assume_init`] as an alternative.
708 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
709 /// in an initialized state. Calling this when the content is not yet fully
710 /// initialized causes undefined behavior.
712 /// On top of that, all additional invariants of the type `T` must be
713 /// satisfied, as the `Drop` implementation of `T` (or its members) may
714 /// rely on this. For example, setting a [`Vec<T>`] to an invalid but
715 /// non-null address makes it initialized (under the current implementation;
716 /// this does not constitute a stable guarantee), because the only
717 /// requirement the compiler knows about it is that the data pointer must be
718 /// non-null. Dropping such a `Vec<T>` however will cause undefined
721 /// [`assume_init`]: MaybeUninit::assume_init
722 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
723 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
724 pub unsafe fn assume_init_drop(&mut self) {
725 // SAFETY: the caller must guarantee that `self` is initialized and
726 // satisfies all invariants of `T`.
727 // Dropping the value in place is safe if that is the case.
728 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
731 /// Gets a shared reference to the contained value.
733 /// This can be useful when we want to access a `MaybeUninit` that has been
734 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
735 /// of `.assume_init()`).
739 /// Calling this when the content is not yet fully initialized causes undefined
740 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
741 /// is in an initialized state.
745 /// ### Correct usage of this method:
748 /// use std::mem::MaybeUninit;
750 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
751 /// // Initialize `x`:
752 /// x.write(vec![1, 2, 3]);
753 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
754 /// // create a shared reference to it:
755 /// let x: &Vec<u32> = unsafe {
756 /// // SAFETY: `x` has been initialized.
757 /// x.assume_init_ref()
759 /// assert_eq!(x, &vec![1, 2, 3]);
762 /// ### *Incorrect* usages of this method:
765 /// use std::mem::MaybeUninit;
767 /// let x = MaybeUninit::<Vec<u32>>::uninit();
768 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
769 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
773 /// use std::{cell::Cell, mem::MaybeUninit};
775 /// let b = MaybeUninit::<Cell<bool>>::uninit();
776 /// // Initialize the `MaybeUninit` using `Cell::set`:
778 /// b.assume_init_ref().set(true);
779 /// // ^^^^^^^^^^^^^^^
780 /// // Reference to an uninitialized `Cell<bool>`: UB!
783 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
784 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
786 pub const unsafe fn assume_init_ref(&self) -> &T
{
787 // SAFETY: the caller must guarantee that `self` is initialized.
788 // This also means that `self` must be a `value` variant.
790 intrinsics
::assert_inhabited
::<T
>();
795 /// Gets a mutable (unique) reference to the contained value.
797 /// This can be useful when we want to access a `MaybeUninit` that has been
798 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
799 /// of `.assume_init()`).
803 /// Calling this when the content is not yet fully initialized causes undefined
804 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
805 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
806 /// initialize a `MaybeUninit`.
810 /// ### Correct usage of this method:
813 /// # #![allow(unexpected_cfgs)]
814 /// use std::mem::MaybeUninit;
816 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }
819 /// /// Initializes *all* the bytes of the input buffer.
820 /// fn initialize_buffer(buf: *mut [u8; 1024]);
823 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
825 /// // Initialize `buf`:
826 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
827 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
828 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
829 /// // To assert our buffer has been initialized without copying it, we upgrade
830 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
831 /// let buf: &mut [u8; 1024] = unsafe {
832 /// // SAFETY: `buf` has been initialized.
833 /// buf.assume_init_mut()
836 /// // Now we can use `buf` as a normal slice:
837 /// buf.sort_unstable();
839 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
840 /// "buffer is sorted",
844 /// ### *Incorrect* usages of this method:
846 /// You cannot use `.assume_init_mut()` to initialize a value:
849 /// use std::mem::MaybeUninit;
851 /// let mut b = MaybeUninit::<bool>::uninit();
853 /// *b.assume_init_mut() = true;
854 /// // We have created a (mutable) reference to an uninitialized `bool`!
855 /// // This is undefined behavior. ⚠️
859 /// For instance, you cannot [`Read`] into an uninitialized buffer:
861 /// [`Read`]: ../../std/io/trait.Read.html
864 /// use std::{io, mem::MaybeUninit};
866 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
868 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
869 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
870 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
871 /// // (mutable) reference to uninitialized memory!
872 /// // This is undefined behavior.
873 /// Ok(unsafe { buffer.assume_init() })
877 /// Nor can you use direct field access to do field-by-field gradual initialization:
880 /// use std::{mem::MaybeUninit, ptr};
887 /// let foo: Foo = unsafe {
888 /// let mut foo = MaybeUninit::<Foo>::uninit();
889 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
890 /// // ^^^^^^^^^^^^^^^^^^^^^
891 /// // (mutable) reference to uninitialized memory!
892 /// // This is undefined behavior.
893 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
894 /// // ^^^^^^^^^^^^^^^^^^^^^
895 /// // (mutable) reference to uninitialized memory!
896 /// // This is undefined behavior.
897 /// foo.assume_init()
900 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
901 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
903 pub const unsafe fn assume_init_mut(&mut self) -> &mut T
{
904 // SAFETY: the caller must guarantee that `self` is initialized.
905 // This also means that `self` must be a `value` variant.
907 intrinsics
::assert_inhabited
::<T
>();
908 &mut *self.as_mut_ptr()
912 /// Extracts the values from an array of `MaybeUninit` containers.
916 /// It is up to the caller to guarantee that all elements of the array are
917 /// in an initialized state.
922 /// #![feature(maybe_uninit_uninit_array)]
923 /// #![feature(maybe_uninit_array_assume_init)]
924 /// use std::mem::MaybeUninit;
926 /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
927 /// array[0].write(0);
928 /// array[1].write(1);
929 /// array[2].write(2);
931 /// // SAFETY: Now safe as we initialised all elements
932 /// let array = unsafe {
933 /// MaybeUninit::array_assume_init(array)
936 /// assert_eq!(array, [0, 1, 2]);
938 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
939 #[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")]
942 pub const unsafe fn array_assume_init
<const N
: usize>(array
: [Self; N
]) -> [T
; N
] {
944 // * The caller guarantees that all elements of the array are initialized
945 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
946 // * `MaybeUninit` does not drop, so there are no double-frees
947 // And thus the conversion is safe
949 intrinsics
::assert_inhabited
::<[T
; N
]>();
950 (&array
as *const _
as *const [T
; N
]).read()
953 // FIXME: required to avoid `~const Destruct` bound
954 super::forget(array
);
958 /// Assuming all the elements are initialized, get a slice to them.
962 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
963 /// really are in an initialized state.
964 /// Calling this when the content is not yet fully initialized causes undefined behavior.
966 /// See [`assume_init_ref`] for more details and examples.
968 /// [`assume_init_ref`]: MaybeUninit::assume_init_ref
969 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
970 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
972 pub const unsafe fn slice_assume_init_ref(slice
: &[Self]) -> &[T
] {
973 // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
974 // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
975 // The pointer obtained is valid since it refers to memory owned by `slice` which is a
976 // reference and thus guaranteed to be valid for reads.
977 unsafe { &*(slice as *const [Self] as *const [T]) }
980 /// Assuming all the elements are initialized, get a mutable slice to them.
984 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
985 /// really are in an initialized state.
986 /// Calling this when the content is not yet fully initialized causes undefined behavior.
988 /// See [`assume_init_mut`] for more details and examples.
990 /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
991 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
992 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
994 pub const unsafe fn slice_assume_init_mut(slice
: &mut [Self]) -> &mut [T
] {
995 // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
996 // mutable reference which is also guaranteed to be valid for writes.
997 unsafe { &mut *(slice as *mut [Self] as *mut [T]) }
1000 /// Gets a pointer to the first element of the array.
1001 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1002 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1004 pub const fn slice_as_ptr(this
: &[MaybeUninit
<T
>]) -> *const T
{
1005 this
.as_ptr() as *const T
1008 /// Gets a mutable pointer to the first element of the array.
1009 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1010 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1012 pub const fn slice_as_mut_ptr(this
: &mut [MaybeUninit
<T
>]) -> *mut T
{
1013 this
.as_mut_ptr() as *mut T
1016 /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
1018 /// If `T` does not implement `Copy`, use [`write_slice_cloned`]
1020 /// This is similar to [`slice::copy_from_slice`].
1024 /// This function will panic if the two slices have different lengths.
1029 /// #![feature(maybe_uninit_write_slice)]
1030 /// use std::mem::MaybeUninit;
1032 /// let mut dst = [MaybeUninit::uninit(); 32];
1033 /// let src = [0; 32];
1035 /// let init = MaybeUninit::write_slice(&mut dst, &src);
1037 /// assert_eq!(init, src);
1041 /// #![feature(maybe_uninit_write_slice)]
1042 /// use std::mem::MaybeUninit;
1044 /// let mut vec = Vec::with_capacity(32);
1045 /// let src = [0; 16];
1047 /// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
1049 /// // SAFETY: we have just copied all the elements of len into the spare capacity
1050 /// // the first src.len() elements of the vec are valid now.
1052 /// vec.set_len(src.len());
1055 /// assert_eq!(vec, src);
1058 /// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned
1059 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1060 pub fn write_slice
<'a
>(this
: &'a
mut [MaybeUninit
<T
>], src
: &[T
]) -> &'a
mut [T
]
1064 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1065 let uninit_src
: &[MaybeUninit
<T
>] = unsafe { super::transmute(src) }
;
1067 this
.copy_from_slice(uninit_src
);
1069 // SAFETY: Valid elements have just been copied into `this` so it is initialized
1070 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1073 /// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
1074 /// Any already initialized elements will not be dropped.
1076 /// If `T` implements `Copy`, use [`write_slice`]
1078 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1082 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1084 /// If there is a panic, the already cloned elements will be dropped.
1089 /// #![feature(maybe_uninit_write_slice)]
1090 /// use std::mem::MaybeUninit;
1092 /// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
1093 /// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
1095 /// let init = MaybeUninit::write_slice_cloned(&mut dst, &src);
1097 /// assert_eq!(init, src);
1101 /// #![feature(maybe_uninit_write_slice)]
1102 /// use std::mem::MaybeUninit;
1104 /// let mut vec = Vec::with_capacity(32);
1105 /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
1107 /// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src);
1109 /// // SAFETY: we have just cloned all the elements of len into the spare capacity
1110 /// // the first src.len() elements of the vec are valid now.
1112 /// vec.set_len(src.len());
1115 /// assert_eq!(vec, src);
1118 /// [`write_slice`]: MaybeUninit::write_slice
1119 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1120 pub fn write_slice_cloned
<'a
>(this
: &'a
mut [MaybeUninit
<T
>], src
: &[T
]) -> &'a
mut [T
]
1124 // unlike copy_from_slice this does not call clone_from_slice on the slice
1125 // this is because `MaybeUninit<T: Clone>` does not implement Clone.
1127 struct Guard
<'a
, T
> {
1128 slice
: &'a
mut [MaybeUninit
<T
>],
1132 impl<'a
, T
> Drop
for Guard
<'a
, T
> {
1133 fn drop(&mut self) {
1134 let initialized_part
= &mut self.slice
[..self.initialized
];
1135 // SAFETY: this raw slice will contain only initialized objects
1136 // that's why, it is allowed to drop it.
1138 crate::ptr
::drop_in_place(MaybeUninit
::slice_assume_init_mut(initialized_part
));
1143 assert_eq
!(this
.len(), src
.len(), "destination and source slices have different lengths");
1144 // NOTE: We need to explicitly slice them to the same length
1145 // for bounds checking to be elided, and the optimizer will
1146 // generate memcpy for simple cases (for example T = u8).
1147 let len
= this
.len();
1148 let src
= &src
[..len
];
1150 // guard is needed b/c panic might happen during a clone
1151 let mut guard
= Guard { slice: this, initialized: 0 }
;
1154 guard
.slice
[i
].write(src
[i
].clone());
1155 guard
.initialized
+= 1;
1158 super::forget(guard
);
1160 // SAFETY: Valid elements have just been written into `this` so it is initialized
1161 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1164 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1166 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1167 /// contain padding bytes which are left uninitialized.
1172 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
1173 /// use std::mem::MaybeUninit;
1175 /// let val = 0x12345678_i32;
1176 /// let uninit = MaybeUninit::new(val);
1177 /// let uninit_bytes = uninit.as_bytes();
1178 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
1179 /// assert_eq!(bytes, val.to_ne_bytes());
1181 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1182 pub fn as_bytes(&self) -> &[MaybeUninit
<u8>] {
1183 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1185 slice
::from_raw_parts(self.as_ptr() as *const MaybeUninit
<u8>, mem
::size_of
::<T
>())
1189 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
1192 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1193 /// contain padding bytes which are left uninitialized.
1198 /// #![feature(maybe_uninit_as_bytes)]
1199 /// use std::mem::MaybeUninit;
1201 /// let val = 0x12345678_i32;
1202 /// let mut uninit = MaybeUninit::new(val);
1203 /// let uninit_bytes = uninit.as_bytes_mut();
1204 /// if cfg!(target_endian = "little") {
1205 /// uninit_bytes[0].write(0xcd);
1207 /// uninit_bytes[3].write(0xcd);
1209 /// let val2 = unsafe { uninit.assume_init() };
1210 /// assert_eq!(val2, 0x123456cd);
1212 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1213 pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit
<u8>] {
1214 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1216 slice
::from_raw_parts_mut(
1217 self.as_mut_ptr() as *mut MaybeUninit
<u8>,
1218 mem
::size_of
::<T
>(),
1223 /// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized
1226 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1227 /// contain padding bytes which are left uninitialized.
1232 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1233 /// use std::mem::MaybeUninit;
1235 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1236 /// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit);
1237 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) };
1238 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1239 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1240 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1242 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1243 pub fn slice_as_bytes(this
: &[MaybeUninit
<T
>]) -> &[MaybeUninit
<u8>] {
1244 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1246 slice
::from_raw_parts(
1247 this
.as_ptr() as *const MaybeUninit
<u8>,
1248 this
.len() * mem
::size_of
::<T
>(),
1253 /// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of
1254 /// potentially uninitialized bytes.
1256 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1257 /// contain padding bytes which are left uninitialized.
1262 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1263 /// use std::mem::MaybeUninit;
1265 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1266 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1267 /// MaybeUninit::write_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
1268 /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
1269 /// if cfg!(target_endian = "little") {
1270 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1272 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1275 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1276 pub fn slice_as_bytes_mut(this
: &mut [MaybeUninit
<T
>]) -> &mut [MaybeUninit
<u8>] {
1277 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1279 slice
::from_raw_parts_mut(
1280 this
.as_mut_ptr() as *mut MaybeUninit
<u8>,
1281 this
.len() * mem
::size_of
::<T
>(),
1287 impl<T
, const N
: usize> MaybeUninit
<[T
; N
]> {
1288 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1293 /// #![feature(maybe_uninit_uninit_array_transpose)]
1294 /// # use std::mem::MaybeUninit;
1296 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1298 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1300 pub const fn transpose(self) -> [MaybeUninit
<T
>; N
] {
1301 // SAFETY: T and MaybeUninit<T> have the same layout
1302 unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
1306 impl<T
, const N
: usize> [MaybeUninit
<T
>; N
] {
1307 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1312 /// #![feature(maybe_uninit_uninit_array_transpose)]
1313 /// # use std::mem::MaybeUninit;
1315 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1316 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1318 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1320 pub const fn transpose(self) -> MaybeUninit
<[T
; N
]> {
1321 // SAFETY: T and MaybeUninit<T> have the same layout
1322 unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }