1 use crate::any::type_name;
4 use crate::mem::ManuallyDrop;
6 // ignore-tidy-undocumented-unsafe
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 the compiler knows that
44 /// it does not have a fixed value. This makes it undefined behavior to have
45 /// uninitialized data in a variable even if that variable has an integer type,
46 /// which otherwise can hold any *fixed* bit pattern:
49 /// # #![allow(invalid_value)]
50 /// use std::mem::{self, MaybeUninit};
52 /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
53 /// // The equivalent code with `MaybeUninit<i32>`:
54 /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
56 /// (Notice that the rules around uninitialized integers are not finalized yet, but
57 /// until they are, it is advisable to avoid them.)
59 /// On top of that, remember that most types have additional invariants beyond merely
60 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
61 /// is considered initialized (under the current implementation; this does not constitute
62 /// a stable guarantee) because the only requirement the compiler knows about it
63 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
64 /// *immediate* undefined behavior, but will cause undefined behavior with most
65 /// safe operations (including dropping it).
67 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
71 /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
72 /// It is a signal to the compiler indicating that the data here might *not*
76 /// use std::mem::MaybeUninit;
78 /// // Create an explicitly uninitialized reference. The compiler knows that data inside
79 /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
80 /// let mut x = MaybeUninit::<&i32>::uninit();
81 /// // Set it to a valid value.
82 /// unsafe { x.as_mut_ptr().write(&0); }
83 /// // Extract the initialized data -- this is only allowed *after* properly
84 /// // initializing `x`!
85 /// let x = unsafe { x.assume_init() };
88 /// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
90 /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
91 /// any of the run-time tracking and without any of the safety checks.
95 /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
96 /// from a function, pass it a pointer to some (uninitialized) memory to put the
97 /// result into. This can be useful when it is important for the caller to control
98 /// how the memory the result is stored in gets allocated, and you want to avoid
99 /// unnecessary moves.
102 /// use std::mem::MaybeUninit;
104 /// unsafe fn make_vec(out: *mut Vec<i32>) {
105 /// // `write` does not drop the old contents, which is important.
106 /// out.write(vec![1, 2, 3]);
109 /// let mut v = MaybeUninit::uninit();
110 /// unsafe { make_vec(v.as_mut_ptr()); }
111 /// // Now we know `v` is initialized! This also makes sure the vector gets
112 /// // properly dropped.
113 /// let v = unsafe { v.assume_init() };
114 /// assert_eq!(&v, &[1, 2, 3]);
117 /// ## Initializing an array element-by-element
119 /// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
122 /// use std::mem::{self, MaybeUninit};
125 /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
126 /// // safe because the type we are claiming to have initialized here is a
127 /// // bunch of `MaybeUninit`s, which do not require initialization.
128 /// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
129 /// MaybeUninit::uninit().assume_init()
132 /// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
133 /// // assignment instead of `ptr::write` does not cause the old
134 /// // uninitialized value to be dropped. Also if there is a panic during
135 /// // this loop, we have a memory leak, but there is no memory safety
137 /// for elem in &mut data[..] {
138 /// *elem = MaybeUninit::new(vec![42]);
141 /// // Everything is initialized. Transmute the array to the
142 /// // initialized type.
143 /// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
146 /// assert_eq!(&data[0], &[42]);
149 /// You can also work with partially initialized arrays, which could
150 /// be found in low-level datastructures.
153 /// use std::mem::MaybeUninit;
156 /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
157 /// // safe because the type we are claiming to have initialized here is a
158 /// // bunch of `MaybeUninit`s, which do not require initialization.
159 /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
160 /// // Count the number of elements we have assigned.
161 /// let mut data_len: usize = 0;
163 /// for elem in &mut data[0..500] {
164 /// *elem = MaybeUninit::new(String::from("hello"));
168 /// // For each item in the array, drop if we allocated it.
169 /// for elem in &mut data[0..data_len] {
170 /// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
174 /// ## Initializing a struct field-by-field
176 /// There is currently no supported way to create a raw pointer or reference
177 /// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible
178 /// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
181 /// [ub]: ../../reference/behavior-considered-undefined.html
185 /// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
188 /// use std::mem::{MaybeUninit, size_of, align_of};
189 /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
190 /// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
193 /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
194 /// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
195 /// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
196 /// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
197 /// optimizations, potentially resulting in a larger size:
200 /// # use std::mem::{MaybeUninit, size_of};
201 /// assert_eq!(size_of::<Option<bool>>(), 1);
202 /// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
205 /// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
207 /// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
208 /// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
209 /// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
210 /// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
211 /// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
212 /// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
213 /// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
214 /// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
215 /// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
216 /// guarantee may evolve.
217 #[allow(missing_debug_implementations)]
218 #[stable(feature = "maybe_uninit", since = "1.36.0")]
219 // Lang item so we can wrap other types in it. This is useful for generators.
220 #[lang = "maybe_uninit"]
223 pub union MaybeUninit<T> {
225 value: ManuallyDrop<T>,
228 #[stable(feature = "maybe_uninit", since = "1.36.0")]
229 impl<T: Copy> Clone for MaybeUninit<T> {
231 fn clone(&self) -> Self {
232 // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
237 #[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
238 impl<T> fmt::Debug for MaybeUninit<T> {
239 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
240 f.pad(type_name::<Self>())
244 impl<T> MaybeUninit<T> {
245 /// Creates a new `MaybeUninit<T>` initialized with the given value.
246 /// It is safe to call [`assume_init`] on the return value of this function.
248 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
249 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
251 /// [`assume_init`]: #method.assume_init
252 #[stable(feature = "maybe_uninit", since = "1.36.0")]
253 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
255 pub const fn new(val: T) -> MaybeUninit<T> {
256 MaybeUninit { value: ManuallyDrop::new(val) }
259 /// Creates a new `MaybeUninit<T>` in an uninitialized state.
261 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
262 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
264 /// See the [type-level documentation][type] for some examples.
266 /// [type]: union.MaybeUninit.html
267 #[stable(feature = "maybe_uninit", since = "1.36.0")]
268 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
270 #[rustc_diagnostic_item = "maybe_uninit_uninit"]
271 pub const fn uninit() -> MaybeUninit<T> {
272 MaybeUninit { uninit: () }
275 /// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
277 /// Note: in a future Rust version this method may become unnecessary
278 /// when array literal syntax allows
279 /// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147).
280 /// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`.
285 /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice_assume_init)]
287 /// use std::mem::MaybeUninit;
290 /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
293 /// /// Returns a (possibly smaller) slice of data that was actually read
294 /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
296 /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
297 /// MaybeUninit::slice_get_ref(&buf[..len])
301 /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
302 /// let data = read(&mut buf);
304 #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
306 pub fn uninit_array<const LEN: usize>() -> [Self; LEN] {
307 unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
310 /// A promotable constant, equivalent to `uninit()`.
312 feature = "internal_uninit_const",
314 reason = "hack to work around promotability"
316 pub const UNINIT: Self = Self::uninit();
318 /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
319 /// filled with `0` bytes. It depends on `T` whether that already makes for
320 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
321 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
324 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
325 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
329 /// Correct usage of this function: initializing a struct with zero, where all
330 /// fields of the struct can hold the bit-pattern 0 as a valid value.
333 /// use std::mem::MaybeUninit;
335 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
336 /// let x = unsafe { x.assume_init() };
337 /// assert_eq!(x, (0, false));
340 /// *Incorrect* usage of this function: initializing a struct with zero, where some fields
341 /// cannot hold 0 as a valid value.
344 /// use std::mem::MaybeUninit;
346 /// enum NotZero { One = 1, Two = 2 };
348 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
349 /// let x = unsafe { x.assume_init() };
350 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
351 /// // This is undefined behavior.
353 #[stable(feature = "maybe_uninit", since = "1.36.0")]
355 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
356 pub fn zeroed() -> MaybeUninit<T> {
357 let mut u = MaybeUninit::<T>::uninit();
359 u.as_mut_ptr().write_bytes(0u8, 1);
364 /// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value
365 /// without dropping it, so be careful not to use this twice unless you want to
366 /// skip running the destructor. For your convenience, this also returns a mutable
367 /// reference to the (now safely initialized) contents of `self`.
368 #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
370 pub fn write(&mut self, val: T) -> &mut T {
372 self.value = ManuallyDrop::new(val);
377 /// Gets a pointer to the contained value. Reading from this pointer or turning it
378 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
379 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
380 /// (except inside an `UnsafeCell<T>`).
384 /// Correct usage of this method:
387 /// use std::mem::MaybeUninit;
389 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
390 /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
391 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
392 /// let x_vec = unsafe { &*x.as_ptr() };
393 /// assert_eq!(x_vec.len(), 3);
396 /// *Incorrect* usage of this method:
399 /// use std::mem::MaybeUninit;
401 /// let x = MaybeUninit::<Vec<u32>>::uninit();
402 /// let x_vec = unsafe { &*x.as_ptr() };
403 /// // We have created a reference to an uninitialized vector! This is undefined behavior.
406 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
407 /// until they are, it is advisable to avoid them.)
408 #[stable(feature = "maybe_uninit", since = "1.36.0")]
410 pub fn as_ptr(&self) -> *const T {
411 unsafe { &*self.value as *const T }
414 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
415 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
419 /// Correct usage of this method:
422 /// use std::mem::MaybeUninit;
424 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
425 /// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
426 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
427 /// // This is okay because we initialized it.
428 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
430 /// assert_eq!(x_vec.len(), 4);
433 /// *Incorrect* usage of this method:
436 /// use std::mem::MaybeUninit;
438 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
439 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
440 /// // We have created a reference to an uninitialized vector! This is undefined behavior.
443 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
444 /// until they are, it is advisable to avoid them.)
445 #[stable(feature = "maybe_uninit", since = "1.36.0")]
447 pub fn as_mut_ptr(&mut self) -> *mut T {
448 unsafe { &mut *self.value as *mut T }
451 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
452 /// to ensure that the data will get dropped, because the resulting `T` is
453 /// subject to the usual drop handling.
457 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
458 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
459 /// behavior. The [type-level documentation][inv] contains more information about
460 /// this initialization invariant.
462 /// [inv]: #initialization-invariant
464 /// On top of that, remember that most types have additional invariants beyond merely
465 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
466 /// is considered initialized (under the current implementation; this does not constitute
467 /// a stable guarantee) because the only requirement the compiler knows about it
468 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
469 /// *immediate* undefined behavior, but will cause undefined behavior with most
470 /// safe operations (including dropping it).
474 /// Correct usage of this method:
477 /// use std::mem::MaybeUninit;
479 /// let mut x = MaybeUninit::<bool>::uninit();
480 /// unsafe { x.as_mut_ptr().write(true); }
481 /// let x_init = unsafe { x.assume_init() };
482 /// assert_eq!(x_init, true);
485 /// *Incorrect* usage of this method:
488 /// use std::mem::MaybeUninit;
490 /// let x = MaybeUninit::<Vec<u32>>::uninit();
491 /// let x_init = unsafe { x.assume_init() };
492 /// // `x` had not been initialized yet, so this last line caused undefined behavior.
494 #[stable(feature = "maybe_uninit", since = "1.36.0")]
496 #[rustc_diagnostic_item = "assume_init"]
497 pub unsafe fn assume_init(self) -> T {
498 intrinsics::panic_if_uninhabited::<T>();
499 ManuallyDrop::into_inner(self.value)
502 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
503 /// to the usual drop handling.
505 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
506 /// prevents duplicating the content of the `MaybeUninit<T>`.
510 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
511 /// state. Calling this when the content is not yet fully initialized causes undefined
512 /// behavior. The [type-level documentation][inv] contains more information about
513 /// this initialization invariant.
515 /// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using
516 /// multiple copies of the data (by calling `read` multiple times, or first
517 /// calling `read` and then [`assume_init`]), it is your responsibility
518 /// to ensure that that data may indeed be duplicated.
520 /// [inv]: #initialization-invariant
521 /// [`assume_init`]: #method.assume_init
525 /// Correct usage of this method:
528 /// #![feature(maybe_uninit_extra)]
529 /// use std::mem::MaybeUninit;
531 /// let mut x = MaybeUninit::<u32>::uninit();
533 /// let x1 = unsafe { x.read() };
534 /// // `u32` is `Copy`, so we may read multiple times.
535 /// let x2 = unsafe { x.read() };
536 /// assert_eq!(x1, x2);
538 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
540 /// let x1 = unsafe { x.read() };
541 /// // Duplicating a `None` value is okay, so we may read multiple times.
542 /// let x2 = unsafe { x.read() };
543 /// assert_eq!(x1, x2);
546 /// *Incorrect* usage of this method:
549 /// #![feature(maybe_uninit_extra)]
550 /// use std::mem::MaybeUninit;
552 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
553 /// x.write(Some(vec![0,1,2]));
554 /// let x1 = unsafe { x.read() };
555 /// let x2 = unsafe { x.read() };
556 /// // We now created two copies of the same vector, leading to a double-free when
557 /// // they both get dropped!
559 #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
561 pub unsafe fn read(&self) -> T {
562 intrinsics::panic_if_uninhabited::<T>();
566 /// Gets a shared reference to the contained value.
568 /// This can be useful when we want to access a `MaybeUninit` that has been
569 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
570 /// of `.assume_init()`).
574 /// Calling this when the content is not yet fully initialized causes undefined
575 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
576 /// is in an initialized state.
580 /// ### Correct usage of this method:
583 /// #![feature(maybe_uninit_ref)]
584 /// use std::mem::MaybeUninit;
586 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
587 /// // Initialize `x`:
588 /// unsafe { x.as_mut_ptr().write(vec![1, 2, 3]); }
589 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
590 /// // create a shared reference to it:
591 /// let x: &Vec<u32> = unsafe {
592 /// // Safety: `x` has been initialized.
595 /// assert_eq!(x, &vec![1, 2, 3]);
598 /// ### *Incorrect* usages of this method:
601 /// #![feature(maybe_uninit_ref)]
602 /// use std::mem::MaybeUninit;
604 /// let x = MaybeUninit::<Vec<u32>>::uninit();
605 /// let x_vec: &Vec<u32> = unsafe { x.get_ref() };
606 /// // We have created a reference to an uninitialized vector! This is undefined behavior.
610 /// #![feature(maybe_uninit_ref)]
611 /// use std::{cell::Cell, mem::MaybeUninit};
613 /// let b = MaybeUninit::<Cell<bool>>::uninit();
614 /// // Initialize the `MaybeUninit` using `Cell::set`:
616 /// b.get_ref().set(true);
618 /// // Reference to an uninitialized `Cell<bool>`: UB!
621 #[unstable(feature = "maybe_uninit_ref", issue = "63568")]
623 pub unsafe fn get_ref(&self) -> &T {
624 intrinsics::panic_if_uninhabited::<T>();
628 /// Gets a mutable (unique) reference to the contained value.
630 /// This can be useful when we want to access a `MaybeUninit` that has been
631 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
632 /// of `.assume_init()`).
636 /// Calling this when the content is not yet fully initialized causes undefined
637 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
638 /// is in an initialized state. For instance, `.get_mut()` cannot be used to
639 /// initialize a `MaybeUninit`.
643 /// ### Correct usage of this method:
646 /// #![feature(maybe_uninit_ref)]
647 /// use std::mem::MaybeUninit;
649 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 2048]) { *buf = [0; 2048] }
652 /// /// Initializes *all* the bytes of the input buffer.
653 /// fn initialize_buffer(buf: *mut [u8; 2048]);
656 /// let mut buf = MaybeUninit::<[u8; 2048]>::uninit();
658 /// // Initialize `buf`:
659 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
660 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
661 /// // However, using `.assume_init()` may trigger a `memcpy` of the 2048 bytes.
662 /// // To assert our buffer has been initialized without copying it, we upgrade
663 /// // the `&mut MaybeUninit<[u8; 2048]>` to a `&mut [u8; 2048]`:
664 /// let buf: &mut [u8; 2048] = unsafe {
665 /// // Safety: `buf` has been initialized.
669 /// // Now we can use `buf` as a normal slice:
670 /// buf.sort_unstable();
672 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
673 /// "buffer is sorted",
677 /// ### *Incorrect* usages of this method:
679 /// You cannot use `.get_mut()` to initialize a value:
682 /// #![feature(maybe_uninit_ref)]
683 /// use std::mem::MaybeUninit;
685 /// let mut b = MaybeUninit::<bool>::uninit();
687 /// *b.get_mut() = true;
688 /// // We have created a (mutable) reference to an uninitialized `bool`!
689 /// // This is undefined behavior.
693 /// For instance, you cannot [`Read`] into an uninitialized buffer:
695 /// [`Read`]: https://doc.rust-lang.org/std/io/trait.Read.html
698 /// #![feature(maybe_uninit_ref)]
699 /// use std::{io, mem::MaybeUninit};
701 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
703 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
704 /// reader.read_exact(unsafe { buffer.get_mut() })?;
705 /// // ^^^^^^^^^^^^^^^^
706 /// // (mutable) reference to uninitialized memory!
707 /// // This is undefined behavior.
708 /// Ok(unsafe { buffer.assume_init() })
712 /// Nor can you use direct field access to do field-by-field gradual initialization:
715 /// #![feature(maybe_uninit_ref)]
716 /// use std::{mem::MaybeUninit, ptr};
723 /// let foo: Foo = unsafe {
724 /// let mut foo = MaybeUninit::<Foo>::uninit();
725 /// ptr::write(&mut foo.get_mut().a as *mut u32, 1337);
727 /// // (mutable) reference to uninitialized memory!
728 /// // This is undefined behavior.
729 /// ptr::write(&mut foo.get_mut().b as *mut u8, 42);
731 /// // (mutable) reference to uninitialized memory!
732 /// // This is undefined behavior.
733 /// foo.assume_init()
736 // FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references
737 // to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make
738 // a final decision about the rules before stabilization.
739 #[unstable(feature = "maybe_uninit_ref", issue = "63568")]
741 pub unsafe fn get_mut(&mut self) -> &mut T {
742 intrinsics::panic_if_uninhabited::<T>();
746 /// Assuming all the elements are initialized, get a slice to them.
750 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
751 /// really are in an initialized state.
752 /// Calling this when the content is not yet fully initialized causes undefined behavior.
753 #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
755 pub unsafe fn slice_get_ref(slice: &[Self]) -> &[T] {
756 &*(slice as *const [Self] as *const [T])
759 /// Assuming all the elements are initialized, get a mutable slice to them.
763 /// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
764 /// really are in an initialized state.
765 /// Calling this when the content is not yet fully initialized causes undefined behavior.
766 #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
768 pub unsafe fn slice_get_mut(slice: &mut [Self]) -> &mut [T] {
769 &mut *(slice as *mut [Self] as *mut [T])
772 /// Gets a pointer to the first element of the array.
773 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
775 pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T {
776 this as *const [MaybeUninit<T>] as *const T
779 /// Gets a mutable pointer to the first element of the array.
780 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
782 pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T {
783 this as *mut [MaybeUninit<T>] as *mut T