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;
151 /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
152 /// // safe because the type we are claiming to have initialized here is a
153 /// // bunch of `MaybeUninit`s, which do not require initialization.
154 /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
155 /// // Count the number of elements we have assigned.
156 /// let mut data_len: usize = 0;
158 /// for elem in &mut data[0..500] {
159 /// elem.write(String::from("hello"));
163 /// // For each item in the array, drop if we allocated it.
164 /// for elem in &mut data[0..data_len] {
165 /// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
169 /// ## Initializing a struct field-by-field
171 /// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
174 /// use std::mem::MaybeUninit;
175 /// use std::ptr::addr_of_mut;
177 /// #[derive(Debug, PartialEq)]
184 /// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
185 /// let ptr = uninit.as_mut_ptr();
187 /// // Initializing the `name` field
188 /// // Using `write` instead of assignment via `=` to not call `drop` on the
189 /// // old, uninitialized value.
190 /// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }
192 /// // Initializing the `list` field
193 /// // If there is a panic here, then the `String` in the `name` field leaks.
194 /// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); }
196 /// // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
197 /// unsafe { uninit.assume_init() }
203 /// name: "Bob".to_string(),
204 /// list: vec![0, 1, 2]
208 /// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut
209 /// [ub]: ../../reference/behavior-considered-undefined.html
213 /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
216 /// use std::mem::{MaybeUninit, size_of, align_of};
217 /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
218 /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
221 /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
222 /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
223 /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
224 /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
225 /// optimizations, potentially resulting in a larger size:
228 /// # use std::mem::{MaybeUninit, size_of};
229 /// assert_eq!(size_of::<Option<bool>>(), 1);
230 /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
233 /// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
235 /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
236 /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
237 /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
238 /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
239 /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
240 /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
241 /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
242 /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
243 /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
244 /// guarantee may evolve.
245 #[stable(feature = "maybe_uninit", since = "1.36.0")]
246 // Lang item so we can wrap other types in it. This is useful for generators.
247 #[lang = "maybe_uninit"]
250 pub union MaybeUninit<T> {
252 value: ManuallyDrop<T>,
255 #[stable(feature = "maybe_uninit", since = "1.36.0")]
256 impl<T: Copy> Clone for MaybeUninit<T> {
258 fn clone(&self) -> Self {
259 // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
264 #[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
265 impl<T> fmt::Debug for MaybeUninit<T> {
266 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
267 f.pad(type_name::<Self>())
271 impl<T> MaybeUninit<T> {
272 /// Creates a new `MaybeUninit<T>` initialized with the given value.
273 /// It is safe to call [`assume_init`] on the return value of this function.
275 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
276 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
281 /// use std::mem::MaybeUninit;
283 /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
286 /// [`assume_init`]: MaybeUninit::assume_init
287 #[stable(feature = "maybe_uninit", since = "1.36.0")]
288 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
289 #[must_use = "use `forget` to avoid running Drop code"]
291 pub const fn new(val: T) -> MaybeUninit<T> {
292 MaybeUninit { value: ManuallyDrop::new(val) }
295 /// Creates a new `MaybeUninit<T>` in an uninitialized state.
297 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
298 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
300 /// See the [type-level documentation][MaybeUninit] for some examples.
305 /// use std::mem::MaybeUninit;
307 /// let v: MaybeUninit<String> = MaybeUninit::uninit();
309 #[stable(feature = "maybe_uninit", since = "1.36.0")]
310 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
313 #[rustc_diagnostic_item = "maybe_uninit_uninit"]
314 pub const fn uninit() -> MaybeUninit<T> {
315 MaybeUninit { uninit: () }
318 /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
320 /// Note: in a future Rust version this method may become unnecessary
322 /// [inline const expressions](https://github.com/rust-lang/rust/issues/76001).
323 /// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
328 /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
330 /// use std::mem::MaybeUninit;
333 /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
336 /// /// Returns a (possibly smaller) slice of data that was actually read
337 /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
339 /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
340 /// MaybeUninit::slice_assume_init_ref(&buf[..len])
344 /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
345 /// let data = read(&mut buf);
347 #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
348 #[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")]
351 pub const fn uninit_array<const N: usize>() -> [Self; N] {
352 // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
353 unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() }
356 /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
357 /// filled with `0` bytes. It depends on `T` whether that already makes for
358 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
359 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
362 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
363 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
367 /// Correct usage of this function: initializing a struct with zero, where all
368 /// fields of the struct can hold the bit-pattern 0 as a valid value.
371 /// use std::mem::MaybeUninit;
373 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
374 /// let x = unsafe { x.assume_init() };
375 /// assert_eq!(x, (0, false));
378 /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
379 /// when `0` is not a valid bit-pattern for the type:
382 /// use std::mem::MaybeUninit;
384 /// enum NotZero { One = 1, Two = 2 }
386 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
387 /// let x = unsafe { x.assume_init() };
388 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
389 /// // This is undefined behavior. ⚠️
391 #[stable(feature = "maybe_uninit", since = "1.36.0")]
392 #[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")]
395 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
396 pub const fn zeroed() -> MaybeUninit<T> {
397 let mut u = MaybeUninit::<T>::uninit();
398 // SAFETY: `u.as_mut_ptr()` points to allocated memory.
400 u.as_mut_ptr().write_bytes(0u8, 1);
405 /// Sets the value of the `MaybeUninit<T>`.
407 /// This overwrites any previous value without dropping it, so be careful
408 /// not to use this twice unless you want to skip running the destructor.
409 /// For your convenience, this also returns a mutable reference to the
410 /// (now safely initialized) contents of `self`.
412 /// As the content is stored inside a `MaybeUninit`, the destructor is not
413 /// run for the inner data if the MaybeUninit leaves scope without a call to
414 /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
415 /// the mutable reference returned by this function needs to keep this in
416 /// mind. The safety model of Rust regards leaks as safe, but they are
417 /// usually still undesirable. This being said, the mutable reference
418 /// behaves like any other mutable reference would, so assigning a new value
419 /// to it will drop the old content.
421 /// [`assume_init`]: Self::assume_init
422 /// [`assume_init_drop`]: Self::assume_init_drop
426 /// Correct usage of this method:
429 /// use std::mem::MaybeUninit;
431 /// let mut x = MaybeUninit::<Vec<u8>>::uninit();
434 /// let hello = x.write((&b"Hello, world!").to_vec());
435 /// // Setting hello does not leak prior allocations, but drops them
436 /// *hello = (&b"Hello").to_vec();
437 /// hello[0] = 'h' as u8;
439 /// // x is initialized now:
440 /// let s = unsafe { x.assume_init() };
441 /// assert_eq!(b"hello", s.as_slice());
444 /// This usage of the method causes a leak:
447 /// use std::mem::MaybeUninit;
449 /// let mut x = MaybeUninit::<String>::uninit();
451 /// x.write("Hello".to_string());
452 /// // This leaks the contained string:
453 /// x.write("hello".to_string());
454 /// // x is initialized now:
455 /// let s = unsafe { x.assume_init() };
458 /// This method can be used to avoid unsafe in some cases. The example below
459 /// shows a part of an implementation of a fixed sized arena that lends out
460 /// pinned references.
461 /// With `write`, we can avoid the need to write through a raw pointer:
464 /// use core::pin::Pin;
465 /// use core::mem::MaybeUninit;
467 /// struct PinArena<T> {
468 /// memory: Box<[MaybeUninit<T>]>,
472 /// impl <T> PinArena<T> {
473 /// pub fn capacity(&self) -> usize {
474 /// self.memory.len()
476 /// pub fn push(&mut self, val: T) -> Pin<&mut T> {
477 /// if self.len >= self.capacity() {
478 /// panic!("Attempted to push to a full pin arena!");
480 /// let ref_ = self.memory[self.len].write(val);
482 /// unsafe { Pin::new_unchecked(ref_) }
486 #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
487 #[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")]
489 pub const fn write(&mut self, val: T) -> &mut T {
490 *self = MaybeUninit::new(val);
491 // SAFETY: We just initialized this value.
492 unsafe { self.assume_init_mut() }
495 /// Gets a pointer to the contained value. Reading from this pointer or turning it
496 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
497 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
498 /// (except inside an `UnsafeCell<T>`).
502 /// Correct usage of this method:
505 /// use std::mem::MaybeUninit;
507 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
508 /// x.write(vec![0, 1, 2]);
509 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
510 /// let x_vec = unsafe { &*x.as_ptr() };
511 /// assert_eq!(x_vec.len(), 3);
514 /// *Incorrect* usage of this method:
517 /// use std::mem::MaybeUninit;
519 /// let x = MaybeUninit::<Vec<u32>>::uninit();
520 /// let x_vec = unsafe { &*x.as_ptr() };
521 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
524 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
525 /// until they are, it is advisable to avoid them.)
526 #[stable(feature = "maybe_uninit", since = "1.36.0")]
527 #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
529 pub const fn as_ptr(&self) -> *const T {
530 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
531 self as *const _ as *const T
534 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
535 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
539 /// Correct usage of this method:
542 /// use std::mem::MaybeUninit;
544 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
545 /// x.write(vec![0, 1, 2]);
546 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
547 /// // This is okay because we initialized it.
548 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
550 /// assert_eq!(x_vec.len(), 4);
553 /// *Incorrect* usage of this method:
556 /// use std::mem::MaybeUninit;
558 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
559 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
560 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
563 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
564 /// until they are, it is advisable to avoid them.)
565 #[stable(feature = "maybe_uninit", since = "1.36.0")]
566 #[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
568 pub const fn as_mut_ptr(&mut self) -> *mut T {
569 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
570 self as *mut _ as *mut T
573 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
574 /// to ensure that the data will get dropped, because the resulting `T` is
575 /// subject to the usual drop handling.
579 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
580 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
581 /// behavior. The [type-level documentation][inv] contains more information about
582 /// this initialization invariant.
584 /// [inv]: #initialization-invariant
586 /// On top of that, remember that most types have additional invariants beyond merely
587 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
588 /// is considered initialized (under the current implementation; this does not constitute
589 /// a stable guarantee) because the only requirement the compiler knows about it
590 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
591 /// *immediate* undefined behavior, but will cause undefined behavior with most
592 /// safe operations (including dropping it).
594 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
598 /// Correct usage of this method:
601 /// use std::mem::MaybeUninit;
603 /// let mut x = MaybeUninit::<bool>::uninit();
605 /// let x_init = unsafe { x.assume_init() };
606 /// assert_eq!(x_init, true);
609 /// *Incorrect* usage of this method:
612 /// use std::mem::MaybeUninit;
614 /// let x = MaybeUninit::<Vec<u32>>::uninit();
615 /// let x_init = unsafe { x.assume_init() };
616 /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
618 #[stable(feature = "maybe_uninit", since = "1.36.0")]
619 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
621 #[rustc_diagnostic_item = "assume_init"]
623 pub const unsafe fn assume_init(self) -> T {
624 // SAFETY: the caller must guarantee that `self` is initialized.
625 // This also means that `self` must be a `value` variant.
627 intrinsics::assert_inhabited::<T>();
628 ManuallyDrop::into_inner(self.value)
632 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
633 /// to the usual drop handling.
635 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
636 /// prevents duplicating the content of the `MaybeUninit<T>`.
640 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
641 /// state. Calling this when the content is not yet fully initialized causes undefined
642 /// behavior. The [type-level documentation][inv] contains more information about
643 /// this initialization invariant.
645 /// Moreover, similar to the [`ptr::read`] function, this function creates a
646 /// bitwise copy of the contents, regardless whether the contained type
647 /// implements the [`Copy`] trait or not. When using multiple copies of the
648 /// data (by calling `assume_init_read` multiple times, or first calling
649 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
650 /// to ensure that data may indeed be duplicated.
652 /// [inv]: #initialization-invariant
653 /// [`assume_init`]: MaybeUninit::assume_init
657 /// Correct usage of this method:
660 /// use std::mem::MaybeUninit;
662 /// let mut x = MaybeUninit::<u32>::uninit();
664 /// let x1 = unsafe { x.assume_init_read() };
665 /// // `u32` is `Copy`, so we may read multiple times.
666 /// let x2 = unsafe { x.assume_init_read() };
667 /// assert_eq!(x1, x2);
669 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
671 /// let x1 = unsafe { x.assume_init_read() };
672 /// // Duplicating a `None` value is okay, so we may read multiple times.
673 /// let x2 = unsafe { x.assume_init_read() };
674 /// assert_eq!(x1, x2);
677 /// *Incorrect* usage of this method:
680 /// use std::mem::MaybeUninit;
682 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
683 /// x.write(Some(vec![0, 1, 2]));
684 /// let x1 = unsafe { x.assume_init_read() };
685 /// let x2 = unsafe { x.assume_init_read() };
686 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
687 /// // they both get dropped!
689 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
690 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init_read", issue = "63567")]
693 pub const unsafe fn assume_init_read(&self) -> T {
694 // SAFETY: the caller must guarantee that `self` is initialized.
695 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
697 intrinsics::assert_inhabited::<T>();
702 /// Drops the contained value in place.
704 /// If you have ownership of the `MaybeUninit`, you can also use
705 /// [`assume_init`] as an alternative.
709 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
710 /// in an initialized state. Calling this when the content is not yet fully
711 /// initialized causes undefined behavior.
713 /// On top of that, all additional invariants of the type `T` must be
714 /// satisfied, as the `Drop` implementation of `T` (or its members) may
715 /// rely on this. For example, setting a [`Vec<T>`] to an invalid but
716 /// non-null address makes it initialized (under the current implementation;
717 /// this does not constitute a stable guarantee), because the only
718 /// requirement the compiler knows about it is that the data pointer must be
719 /// non-null. Dropping such a `Vec<T>` however will cause undefined
722 /// [`assume_init`]: MaybeUninit::assume_init
723 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
724 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
725 pub unsafe fn assume_init_drop(&mut self) {
726 // SAFETY: the caller must guarantee that `self` is initialized and
727 // satisfies all invariants of `T`.
728 // Dropping the value in place is safe if that is the case.
729 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
732 /// Gets a shared reference to the contained value.
734 /// This can be useful when we want to access a `MaybeUninit` that has been
735 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
736 /// of `.assume_init()`).
740 /// Calling this when the content is not yet fully initialized causes undefined
741 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
742 /// is in an initialized state.
746 /// ### Correct usage of this method:
749 /// use std::mem::MaybeUninit;
751 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
752 /// // Initialize `x`:
753 /// x.write(vec![1, 2, 3]);
754 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
755 /// // create a shared reference to it:
756 /// let x: &Vec<u32> = unsafe {
757 /// // SAFETY: `x` has been initialized.
758 /// x.assume_init_ref()
760 /// assert_eq!(x, &vec![1, 2, 3]);
763 /// ### *Incorrect* usages of this method:
766 /// use std::mem::MaybeUninit;
768 /// let x = MaybeUninit::<Vec<u32>>::uninit();
769 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
770 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
774 /// use std::{cell::Cell, mem::MaybeUninit};
776 /// let b = MaybeUninit::<Cell<bool>>::uninit();
777 /// // Initialize the `MaybeUninit` using `Cell::set`:
779 /// b.assume_init_ref().set(true);
780 /// // ^^^^^^^^^^^^^^^
781 /// // Reference to an uninitialized `Cell<bool>`: UB!
784 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
785 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
787 pub const unsafe fn assume_init_ref(&self) -> &T {
788 // SAFETY: the caller must guarantee that `self` is initialized.
789 // This also means that `self` must be a `value` variant.
791 intrinsics::assert_inhabited::<T>();
796 /// Gets a mutable (unique) reference to the contained value.
798 /// This can be useful when we want to access a `MaybeUninit` that has been
799 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
800 /// of `.assume_init()`).
804 /// Calling this when the content is not yet fully initialized causes undefined
805 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
806 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
807 /// initialize a `MaybeUninit`.
811 /// ### Correct usage of this method:
814 /// # #![allow(unexpected_cfgs)]
815 /// use std::mem::MaybeUninit;
817 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }
820 /// /// Initializes *all* the bytes of the input buffer.
821 /// fn initialize_buffer(buf: *mut [u8; 1024]);
824 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
826 /// // Initialize `buf`:
827 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
828 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
829 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
830 /// // To assert our buffer has been initialized without copying it, we upgrade
831 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
832 /// let buf: &mut [u8; 1024] = unsafe {
833 /// // SAFETY: `buf` has been initialized.
834 /// buf.assume_init_mut()
837 /// // Now we can use `buf` as a normal slice:
838 /// buf.sort_unstable();
840 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
841 /// "buffer is sorted",
845 /// ### *Incorrect* usages of this method:
847 /// You cannot use `.assume_init_mut()` to initialize a value:
850 /// use std::mem::MaybeUninit;
852 /// let mut b = MaybeUninit::<bool>::uninit();
854 /// *b.assume_init_mut() = true;
855 /// // We have created a (mutable) reference to an uninitialized `bool`!
856 /// // This is undefined behavior. ⚠️
860 /// For instance, you cannot [`Read`] into an uninitialized buffer:
862 /// [`Read`]: ../../std/io/trait.Read.html
865 /// use std::{io, mem::MaybeUninit};
867 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
869 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
870 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
871 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
872 /// // (mutable) reference to uninitialized memory!
873 /// // This is undefined behavior.
874 /// Ok(unsafe { buffer.assume_init() })
878 /// Nor can you use direct field access to do field-by-field gradual initialization:
881 /// use std::{mem::MaybeUninit, ptr};
888 /// let foo: Foo = unsafe {
889 /// let mut foo = MaybeUninit::<Foo>::uninit();
890 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
891 /// // ^^^^^^^^^^^^^^^^^^^^^
892 /// // (mutable) reference to uninitialized memory!
893 /// // This is undefined behavior.
894 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
895 /// // ^^^^^^^^^^^^^^^^^^^^^
896 /// // (mutable) reference to uninitialized memory!
897 /// // This is undefined behavior.
898 /// foo.assume_init()
901 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
902 #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
904 pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
905 // SAFETY: the caller must guarantee that `self` is initialized.
906 // This also means that `self` must be a `value` variant.
908 intrinsics::assert_inhabited::<T>();
909 &mut *self.as_mut_ptr()
913 /// Extracts the values from an array of `MaybeUninit` containers.
917 /// It is up to the caller to guarantee that all elements of the array are
918 /// in an initialized state.
923 /// #![feature(maybe_uninit_uninit_array)]
924 /// #![feature(maybe_uninit_array_assume_init)]
925 /// use std::mem::MaybeUninit;
927 /// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
928 /// array[0].write(0);
929 /// array[1].write(1);
930 /// array[2].write(2);
932 /// // SAFETY: Now safe as we initialised all elements
933 /// let array = unsafe {
934 /// MaybeUninit::array_assume_init(array)
937 /// assert_eq!(array, [0, 1, 2]);
939 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
940 #[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")]
943 pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
945 // * The caller guarantees that all elements of the array are initialized
946 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
947 // * `MaybeUninit` does not drop, so there are no double-frees
948 // And thus the conversion is safe
950 intrinsics::assert_inhabited::<[T; N]>();
951 (&array as *const _ as *const [T; N]).read()
954 // FIXME: required to avoid `~const Destruct` bound
955 super::forget(array);
959 /// Assuming all the elements are initialized, get a slice to them.
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.
967 /// See [`assume_init_ref`] for more details and examples.
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")]
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]) }
981 /// Assuming all the elements are initialized, get a mutable slice to them.
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.
989 /// See [`assume_init_mut`] for more details and examples.
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")]
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]) }
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")]
1005 pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
1006 this.as_ptr() as *const T
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")]
1013 pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
1014 this.as_mut_ptr() as *mut T
1017 /// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
1019 /// If `T` does not implement `Copy`, use [`write_slice_cloned`]
1021 /// This is similar to [`slice::copy_from_slice`].
1025 /// This function will panic if the two slices have different lengths.
1030 /// #![feature(maybe_uninit_write_slice)]
1031 /// use std::mem::MaybeUninit;
1033 /// let mut dst = [MaybeUninit::uninit(); 32];
1034 /// let src = [0; 32];
1036 /// let init = MaybeUninit::write_slice(&mut dst, &src);
1038 /// assert_eq!(init, src);
1042 /// #![feature(maybe_uninit_write_slice)]
1043 /// use std::mem::MaybeUninit;
1045 /// let mut vec = Vec::with_capacity(32);
1046 /// let src = [0; 16];
1048 /// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
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.
1053 /// vec.set_len(src.len());
1056 /// assert_eq!(vec, src);
1059 /// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned
1060 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1061 pub fn write_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1065 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1066 let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
1068 this.copy_from_slice(uninit_src);
1070 // SAFETY: Valid elements have just been copied into `this` so it is initialized
1071 unsafe { MaybeUninit::slice_assume_init_mut(this) }
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.
1077 /// If `T` implements `Copy`, use [`write_slice`]
1079 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1083 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1085 /// If there is a panic, the already cloned elements will be dropped.
1090 /// #![feature(maybe_uninit_write_slice)]
1091 /// use std::mem::MaybeUninit;
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()];
1096 /// let init = MaybeUninit::write_slice_cloned(&mut dst, &src);
1098 /// assert_eq!(init, src);
1102 /// #![feature(maybe_uninit_write_slice)]
1103 /// use std::mem::MaybeUninit;
1105 /// let mut vec = Vec::with_capacity(32);
1106 /// let src = ["rust", "is", "a", "pretty", "cool", "language"];
1108 /// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src);
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.
1113 /// vec.set_len(src.len());
1116 /// assert_eq!(vec, src);
1119 /// [`write_slice`]: MaybeUninit::write_slice
1120 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1121 pub fn write_slice_cloned<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
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.
1128 struct Guard<'a, T> {
1129 slice: &'a mut [MaybeUninit<T>],
1133 impl<'a, T> Drop for Guard<'a, T> {
1134 fn drop(&mut self) {
1135 let initialized_part = &mut self.slice[..self.initialized];
1136 // SAFETY: this raw slice will contain only initialized objects
1137 // that's why, it is allowed to drop it.
1139 crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part));
1144 assert_eq!(this.len(), src.len(), "destination and source slices have different lengths");
1145 // NOTE: We need to explicitly slice them to the same length
1146 // for bounds checking to be elided, and the optimizer will
1147 // generate memcpy for simple cases (for example T = u8).
1148 let len = this.len();
1149 let src = &src[..len];
1151 // guard is needed b/c panic might happen during a clone
1152 let mut guard = Guard { slice: this, initialized: 0 };
1155 guard.slice[i].write(src[i].clone());
1156 guard.initialized += 1;
1159 super::forget(guard);
1161 // SAFETY: Valid elements have just been written into `this` so it is initialized
1162 unsafe { MaybeUninit::slice_assume_init_mut(this) }
1165 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1167 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1168 /// contain padding bytes which are left uninitialized.
1173 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
1174 /// use std::mem::MaybeUninit;
1176 /// let val = 0x12345678i32;
1177 /// let uninit = MaybeUninit::new(val);
1178 /// let uninit_bytes = uninit.as_bytes();
1179 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
1180 /// assert_eq!(bytes, val.to_ne_bytes());
1182 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1183 pub fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1184 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1186 slice::from_raw_parts(self.as_ptr() as *const MaybeUninit<u8>, mem::size_of::<T>())
1190 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
1193 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1194 /// contain padding bytes which are left uninitialized.
1199 /// #![feature(maybe_uninit_as_bytes)]
1200 /// use std::mem::MaybeUninit;
1202 /// let val = 0x12345678i32;
1203 /// let mut uninit = MaybeUninit::new(val);
1204 /// let uninit_bytes = uninit.as_bytes_mut();
1205 /// if cfg!(target_endian = "little") {
1206 /// uninit_bytes[0].write(0xcd);
1208 /// uninit_bytes[3].write(0xcd);
1210 /// let val2 = unsafe { uninit.assume_init() };
1211 /// assert_eq!(val2, 0x123456cd);
1213 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1214 pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1215 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1217 slice::from_raw_parts_mut(
1218 self.as_mut_ptr() as *mut MaybeUninit<u8>,
1219 mem::size_of::<T>(),
1224 /// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized
1227 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1228 /// contain padding bytes which are left uninitialized.
1233 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1234 /// use std::mem::MaybeUninit;
1236 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1237 /// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit);
1238 /// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) };
1239 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1240 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1241 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1243 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1244 pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
1245 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1247 slice::from_raw_parts(
1248 this.as_ptr() as *const MaybeUninit<u8>,
1249 this.len() * mem::size_of::<T>(),
1254 /// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of
1255 /// potentially uninitialized bytes.
1257 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1258 /// contain padding bytes which are left uninitialized.
1263 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1264 /// use std::mem::MaybeUninit;
1266 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1267 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1268 /// MaybeUninit::write_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
1269 /// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
1270 /// if cfg!(target_endian = "little") {
1271 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1273 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1276 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1277 pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
1278 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1280 slice::from_raw_parts_mut(
1281 this.as_mut_ptr() as *mut MaybeUninit<u8>,
1282 this.len() * mem::size_of::<T>(),
1288 impl<T, const N: usize> MaybeUninit<[T; N]> {
1289 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1294 /// #![feature(maybe_uninit_uninit_array_transpose)]
1295 /// # use std::mem::MaybeUninit;
1297 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1299 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1301 pub const fn transpose(self) -> [MaybeUninit<T>; N] {
1302 // SAFETY: T and MaybeUninit<T> have the same layout
1303 unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
1307 impl<T, const N: usize> [MaybeUninit<T>; N] {
1308 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1313 /// #![feature(maybe_uninit_uninit_array_transpose)]
1314 /// # use std::mem::MaybeUninit;
1316 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1317 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1319 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1321 pub const fn transpose(self) -> MaybeUninit<[T; N]> {
1322 // SAFETY: T and MaybeUninit<T> have the same layout
1323 unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }