core/mem/maybe_uninit.rs
1use crate::any::type_name;
2use crate::mem::ManuallyDrop;
3use crate::{fmt, intrinsics, ptr, slice};
4
5/// A wrapper type to construct uninitialized instances of `T`.
6///
7/// # Initialization invariant
8///
9/// The compiler, in general, assumes that a variable is properly initialized
10/// according to the requirements of the variable's type. For example, a variable of
11/// reference type must be aligned and non-null. This is an invariant that must
12/// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
13/// variable of reference type causes instantaneous [undefined behavior][ub],
14/// no matter whether that reference ever gets used to access memory:
15///
16/// ```rust,no_run
17/// # #![allow(invalid_value)]
18/// use std::mem::{self, MaybeUninit};
19///
20/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️
21/// // The equivalent code with `MaybeUninit<&i32>`:
22/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️
23/// ```
24///
25/// This is exploited by the compiler for various optimizations, such as eliding
26/// run-time checks and optimizing `enum` layout.
27///
28/// Similarly, entirely uninitialized memory may have any content, while a `bool` must
29/// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
30///
31/// ```rust,no_run
32/// # #![allow(invalid_value)]
33/// use std::mem::{self, MaybeUninit};
34///
35/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
36/// // The equivalent code with `MaybeUninit<bool>`:
37/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
38/// ```
39///
40/// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed"
41/// meaning "it won't change without being written to"). Reading the same uninitialized byte
42/// multiple times can give different results. This makes it undefined behavior to have
43/// uninitialized data in a variable even if that variable has an integer type, which otherwise can
44/// hold any *fixed* bit pattern:
45///
46/// ```rust,no_run
47/// # #![allow(invalid_value)]
48/// use std::mem::{self, MaybeUninit};
49///
50/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
51/// // The equivalent code with `MaybeUninit<i32>`:
52/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
53/// ```
54/// On top of that, remember that most types have additional invariants beyond merely
55/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
56/// is considered initialized (under the current implementation; this does not constitute
57/// a stable guarantee) because the only requirement the compiler knows about it
58/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
59/// *immediate* undefined behavior, but will cause undefined behavior with most
60/// safe operations (including dropping it).
61///
62/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
63///
64/// # Examples
65///
66/// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
67/// It is a signal to the compiler indicating that the data here might *not*
68/// be initialized:
69///
70/// ```rust
71/// use std::mem::MaybeUninit;
72///
73/// // Create an explicitly uninitialized reference. The compiler knows that data inside
74/// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
75/// let mut x = MaybeUninit::<&i32>::uninit();
76/// // Set it to a valid value.
77/// x.write(&0);
78/// // Extract the initialized data -- this is only allowed *after* properly
79/// // initializing `x`!
80/// let x = unsafe { x.assume_init() };
81/// ```
82///
83/// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
84///
85/// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
86/// any of the run-time tracking and without any of the safety checks.
87///
88/// ## out-pointers
89///
90/// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
91/// from a function, pass it a pointer to some (uninitialized) memory to put the
92/// result into. This can be useful when it is important for the caller to control
93/// how the memory the result is stored in gets allocated, and you want to avoid
94/// unnecessary moves.
95///
96/// ```
97/// use std::mem::MaybeUninit;
98///
99/// unsafe fn make_vec(out: *mut Vec<i32>) {
100/// // `write` does not drop the old contents, which is important.
101/// out.write(vec![1, 2, 3]);
102/// }
103///
104/// let mut v = MaybeUninit::uninit();
105/// unsafe { make_vec(v.as_mut_ptr()); }
106/// // Now we know `v` is initialized! This also makes sure the vector gets
107/// // properly dropped.
108/// let v = unsafe { v.assume_init() };
109/// assert_eq!(&v, &[1, 2, 3]);
110/// ```
111///
112/// ## Initializing an array element-by-element
113///
114/// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
115///
116/// ```
117/// use std::mem::{self, MaybeUninit};
118///
119/// let data = {
120/// // Create an uninitialized array of `MaybeUninit`.
121/// let mut data: [MaybeUninit<Vec<u32>>; 1000] = [const { MaybeUninit::uninit() }; 1000];
122///
123/// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop,
124/// // we have a memory leak, but there is no memory safety issue.
125/// for elem in &mut data[..] {
126/// elem.write(vec![42]);
127/// }
128///
129/// // Everything is initialized. Transmute the array to the
130/// // initialized type.
131/// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
132/// };
133///
134/// assert_eq!(&data[0], &[42]);
135/// ```
136///
137/// You can also work with partially initialized arrays, which could
138/// be found in low-level datastructures.
139///
140/// ```
141/// use std::mem::MaybeUninit;
142///
143/// // Create an uninitialized array of `MaybeUninit`.
144/// let mut data: [MaybeUninit<String>; 1000] = [const { MaybeUninit::uninit() }; 1000];
145/// // Count the number of elements we have assigned.
146/// let mut data_len: usize = 0;
147///
148/// for elem in &mut data[0..500] {
149/// elem.write(String::from("hello"));
150/// data_len += 1;
151/// }
152///
153/// // For each item in the array, drop if we allocated it.
154/// for elem in &mut data[0..data_len] {
155/// unsafe { elem.assume_init_drop(); }
156/// }
157/// ```
158///
159/// ## Initializing a struct field-by-field
160///
161/// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
162///
163/// ```rust
164/// use std::mem::MaybeUninit;
165/// use std::ptr::addr_of_mut;
166///
167/// #[derive(Debug, PartialEq)]
168/// pub struct Foo {
169/// name: String,
170/// list: Vec<u8>,
171/// }
172///
173/// let foo = {
174/// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
175/// let ptr = uninit.as_mut_ptr();
176///
177/// // Initializing the `name` field
178/// // Using `write` instead of assignment via `=` to not call `drop` on the
179/// // old, uninitialized value.
180/// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }
181///
182/// // Initializing the `list` field
183/// // If there is a panic here, then the `String` in the `name` field leaks.
184/// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); }
185///
186/// // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
187/// unsafe { uninit.assume_init() }
188/// };
189///
190/// assert_eq!(
191/// foo,
192/// Foo {
193/// name: "Bob".to_string(),
194/// list: vec![0, 1, 2]
195/// }
196/// );
197/// ```
198/// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut
199/// [ub]: ../../reference/behavior-considered-undefined.html
200///
201/// # Layout
202///
203/// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
204///
205/// ```rust
206/// use std::mem::{MaybeUninit, size_of, align_of};
207/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
208/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
209/// ```
210///
211/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
212/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
213/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
214/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
215/// optimizations, potentially resulting in a larger size:
216///
217/// ```rust
218/// # use std::mem::{MaybeUninit, size_of};
219/// assert_eq!(size_of::<Option<bool>>(), 1);
220/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
221/// ```
222///
223/// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
224///
225/// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
226/// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
227/// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
228/// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
229/// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
230/// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
231/// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
232/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
233/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
234/// guarantee may evolve.
235///
236/// Note that even though `T` and `MaybeUninit<T>` are ABI compatible it is still unsound to
237/// transmute `&mut T` to `&mut MaybeUninit<T>` and expose that to safe code because it would allow
238/// safe code to access uninitialized memory:
239///
240/// ```rust,no_run
241/// use core::mem::MaybeUninit;
242///
243/// fn unsound_transmute<T>(val: &mut T) -> &mut MaybeUninit<T> {
244/// unsafe { core::mem::transmute(val) }
245/// }
246///
247/// fn main() {
248/// let mut code = 0;
249/// let code = &mut code;
250/// let code2 = unsound_transmute(code);
251/// *code2 = MaybeUninit::uninit();
252/// std::process::exit(*code); // UB! Accessing uninitialized memory.
253/// }
254/// ```
255#[stable(feature = "maybe_uninit", since = "1.36.0")]
256// Lang item so we can wrap other types in it. This is useful for coroutines.
257#[lang = "maybe_uninit"]
258#[derive(Copy)]
259#[repr(transparent)]
260#[rustc_pub_transparent]
261pub union MaybeUninit<T> {
262 uninit: (),
263 value: ManuallyDrop<T>,
264}
265
266#[stable(feature = "maybe_uninit", since = "1.36.0")]
267impl<T: Copy> Clone for MaybeUninit<T> {
268 #[inline(always)]
269 fn clone(&self) -> Self {
270 // Not calling `T::clone()`, we cannot know if we are initialized enough for that.
271 *self
272 }
273}
274
275#[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
276impl<T> fmt::Debug for MaybeUninit<T> {
277 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
278 // NB: there is no `.pad_fmt` so we can't use a simpler `format_args!("MaybeUninit<{..}>").
279 let full_name = type_name::<Self>();
280 let prefix_len = full_name.find("MaybeUninit").unwrap();
281 f.pad(&full_name[prefix_len..])
282 }
283}
284
285impl<T> MaybeUninit<T> {
286 /// Creates a new `MaybeUninit<T>` initialized with the given value.
287 /// It is safe to call [`assume_init`] on the return value of this function.
288 ///
289 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
290 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
291 ///
292 /// # Example
293 ///
294 /// ```
295 /// use std::mem::MaybeUninit;
296 ///
297 /// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
298 /// # // Prevent leaks for Miri
299 /// # unsafe { let _ = MaybeUninit::assume_init(v); }
300 /// ```
301 ///
302 /// [`assume_init`]: MaybeUninit::assume_init
303 #[stable(feature = "maybe_uninit", since = "1.36.0")]
304 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
305 #[must_use = "use `forget` to avoid running Drop code"]
306 #[inline(always)]
307 pub const fn new(val: T) -> MaybeUninit<T> {
308 MaybeUninit { value: ManuallyDrop::new(val) }
309 }
310
311 /// Creates a new `MaybeUninit<T>` in an uninitialized state.
312 ///
313 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
314 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
315 ///
316 /// See the [type-level documentation][MaybeUninit] for some examples.
317 ///
318 /// # Example
319 ///
320 /// ```
321 /// use std::mem::MaybeUninit;
322 ///
323 /// let v: MaybeUninit<String> = MaybeUninit::uninit();
324 /// ```
325 #[stable(feature = "maybe_uninit", since = "1.36.0")]
326 #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
327 #[must_use]
328 #[inline(always)]
329 #[rustc_diagnostic_item = "maybe_uninit_uninit"]
330 pub const fn uninit() -> MaybeUninit<T> {
331 MaybeUninit { uninit: () }
332 }
333
334 /// Creates a new array of `MaybeUninit<T>` items, in an uninitialized state.
335 ///
336 /// Note: in a future Rust version this method may become unnecessary
337 /// when Rust allows
338 /// [inline const expressions](https://github.com/rust-lang/rust/issues/76001).
339 /// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
340 ///
341 /// # Examples
342 ///
343 /// ```no_run
344 /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
345 ///
346 /// use std::mem::MaybeUninit;
347 ///
348 /// unsafe extern "C" {
349 /// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
350 /// }
351 ///
352 /// /// Returns a (possibly smaller) slice of data that was actually read
353 /// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
354 /// unsafe {
355 /// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
356 /// buf[..len].assume_init_ref()
357 /// }
358 /// }
359 ///
360 /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
361 /// let data = read(&mut buf);
362 /// ```
363 #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
364 #[must_use]
365 #[inline(always)]
366 pub const fn uninit_array<const N: usize>() -> [Self; N] {
367 [const { MaybeUninit::uninit() }; N]
368 }
369
370 /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
371 /// filled with `0` bytes. It depends on `T` whether that already makes for
372 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
373 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
374 /// be null.
375 ///
376 /// Note that if `T` has padding bytes, those bytes are *not* preserved when the
377 /// `MaybeUninit<T>` value is returned from this function, so those bytes will *not* be zeroed.
378 ///
379 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
380 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
381 ///
382 /// # Example
383 ///
384 /// Correct usage of this function: initializing a struct with zero, where all
385 /// fields of the struct can hold the bit-pattern 0 as a valid value.
386 ///
387 /// ```rust
388 /// use std::mem::MaybeUninit;
389 ///
390 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
391 /// let x = unsafe { x.assume_init() };
392 /// assert_eq!(x, (0, false));
393 /// ```
394 ///
395 /// This can be used in const contexts, such as to indicate the end of static arrays for
396 /// plugin registration.
397 ///
398 /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
399 /// when `0` is not a valid bit-pattern for the type:
400 ///
401 /// ```rust,no_run
402 /// use std::mem::MaybeUninit;
403 ///
404 /// enum NotZero { One = 1, Two = 2 }
405 ///
406 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
407 /// let x = unsafe { x.assume_init() };
408 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
409 /// // This is undefined behavior. ⚠️
410 /// ```
411 #[inline]
412 #[must_use]
413 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
414 #[stable(feature = "maybe_uninit", since = "1.36.0")]
415 #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "1.75.0")]
416 pub const fn zeroed() -> MaybeUninit<T> {
417 let mut u = MaybeUninit::<T>::uninit();
418 // SAFETY: `u.as_mut_ptr()` points to allocated memory.
419 unsafe { u.as_mut_ptr().write_bytes(0u8, 1) };
420 u
421 }
422
423 /// Sets the value of the `MaybeUninit<T>`.
424 ///
425 /// This overwrites any previous value without dropping it, so be careful
426 /// not to use this twice unless you want to skip running the destructor.
427 /// For your convenience, this also returns a mutable reference to the
428 /// (now safely initialized) contents of `self`.
429 ///
430 /// As the content is stored inside a `MaybeUninit`, the destructor is not
431 /// run for the inner data if the MaybeUninit leaves scope without a call to
432 /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
433 /// the mutable reference returned by this function needs to keep this in
434 /// mind. The safety model of Rust regards leaks as safe, but they are
435 /// usually still undesirable. This being said, the mutable reference
436 /// behaves like any other mutable reference would, so assigning a new value
437 /// to it will drop the old content.
438 ///
439 /// [`assume_init`]: Self::assume_init
440 /// [`assume_init_drop`]: Self::assume_init_drop
441 ///
442 /// # Examples
443 ///
444 /// Correct usage of this method:
445 ///
446 /// ```rust
447 /// use std::mem::MaybeUninit;
448 ///
449 /// let mut x = MaybeUninit::<Vec<u8>>::uninit();
450 ///
451 /// {
452 /// let hello = x.write((&b"Hello, world!").to_vec());
453 /// // Setting hello does not leak prior allocations, but drops them
454 /// *hello = (&b"Hello").to_vec();
455 /// hello[0] = 'h' as u8;
456 /// }
457 /// // x is initialized now:
458 /// let s = unsafe { x.assume_init() };
459 /// assert_eq!(b"hello", s.as_slice());
460 /// ```
461 ///
462 /// This usage of the method causes a leak:
463 ///
464 /// ```rust
465 /// use std::mem::MaybeUninit;
466 ///
467 /// let mut x = MaybeUninit::<String>::uninit();
468 ///
469 /// x.write("Hello".to_string());
470 /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
471 /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
472 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
473 /// // This leaks the contained string:
474 /// x.write("hello".to_string());
475 /// // x is initialized now:
476 /// let s = unsafe { x.assume_init() };
477 /// ```
478 ///
479 /// This method can be used to avoid unsafe in some cases. The example below
480 /// shows a part of an implementation of a fixed sized arena that lends out
481 /// pinned references.
482 /// With `write`, we can avoid the need to write through a raw pointer:
483 ///
484 /// ```rust
485 /// use core::pin::Pin;
486 /// use core::mem::MaybeUninit;
487 ///
488 /// struct PinArena<T> {
489 /// memory: Box<[MaybeUninit<T>]>,
490 /// len: usize,
491 /// }
492 ///
493 /// impl <T> PinArena<T> {
494 /// pub fn capacity(&self) -> usize {
495 /// self.memory.len()
496 /// }
497 /// pub fn push(&mut self, val: T) -> Pin<&mut T> {
498 /// if self.len >= self.capacity() {
499 /// panic!("Attempted to push to a full pin arena!");
500 /// }
501 /// let ref_ = self.memory[self.len].write(val);
502 /// self.len += 1;
503 /// unsafe { Pin::new_unchecked(ref_) }
504 /// }
505 /// }
506 /// ```
507 #[inline(always)]
508 #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
509 #[rustc_const_stable(feature = "const_maybe_uninit_write", since = "1.85.0")]
510 pub const fn write(&mut self, val: T) -> &mut T {
511 *self = MaybeUninit::new(val);
512 // SAFETY: We just initialized this value.
513 unsafe { self.assume_init_mut() }
514 }
515
516 /// Gets a pointer to the contained value. Reading from this pointer or turning it
517 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
518 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
519 /// (except inside an `UnsafeCell<T>`).
520 ///
521 /// # Examples
522 ///
523 /// Correct usage of this method:
524 ///
525 /// ```rust
526 /// use std::mem::MaybeUninit;
527 ///
528 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
529 /// x.write(vec![0, 1, 2]);
530 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
531 /// let x_vec = unsafe { &*x.as_ptr() };
532 /// assert_eq!(x_vec.len(), 3);
533 /// # // Prevent leaks for Miri
534 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
535 /// ```
536 ///
537 /// *Incorrect* usage of this method:
538 ///
539 /// ```rust,no_run
540 /// use std::mem::MaybeUninit;
541 ///
542 /// let x = MaybeUninit::<Vec<u32>>::uninit();
543 /// let x_vec = unsafe { &*x.as_ptr() };
544 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
545 /// ```
546 ///
547 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
548 /// until they are, it is advisable to avoid them.)
549 #[stable(feature = "maybe_uninit", since = "1.36.0")]
550 #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
551 #[rustc_as_ptr]
552 #[inline(always)]
553 pub const fn as_ptr(&self) -> *const T {
554 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
555 self as *const _ as *const T
556 }
557
558 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
559 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
560 ///
561 /// # Examples
562 ///
563 /// Correct usage of this method:
564 ///
565 /// ```rust
566 /// use std::mem::MaybeUninit;
567 ///
568 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
569 /// x.write(vec![0, 1, 2]);
570 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
571 /// // This is okay because we initialized it.
572 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
573 /// x_vec.push(3);
574 /// assert_eq!(x_vec.len(), 4);
575 /// # // Prevent leaks for Miri
576 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
577 /// ```
578 ///
579 /// *Incorrect* usage of this method:
580 ///
581 /// ```rust,no_run
582 /// use std::mem::MaybeUninit;
583 ///
584 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
585 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
586 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
587 /// ```
588 ///
589 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
590 /// until they are, it is advisable to avoid them.)
591 #[stable(feature = "maybe_uninit", since = "1.36.0")]
592 #[rustc_const_stable(feature = "const_maybe_uninit_as_mut_ptr", since = "1.83.0")]
593 #[rustc_as_ptr]
594 #[inline(always)]
595 pub const fn as_mut_ptr(&mut self) -> *mut T {
596 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
597 self as *mut _ as *mut T
598 }
599
600 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
601 /// to ensure that the data will get dropped, because the resulting `T` is
602 /// subject to the usual drop handling.
603 ///
604 /// # Safety
605 ///
606 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
607 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
608 /// behavior. The [type-level documentation][inv] contains more information about
609 /// this initialization invariant.
610 ///
611 /// [inv]: #initialization-invariant
612 ///
613 /// On top of that, remember that most types have additional invariants beyond merely
614 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
615 /// is considered initialized (under the current implementation; this does not constitute
616 /// a stable guarantee) because the only requirement the compiler knows about it
617 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
618 /// *immediate* undefined behavior, but will cause undefined behavior with most
619 /// safe operations (including dropping it).
620 ///
621 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
622 ///
623 /// # Examples
624 ///
625 /// Correct usage of this method:
626 ///
627 /// ```rust
628 /// use std::mem::MaybeUninit;
629 ///
630 /// let mut x = MaybeUninit::<bool>::uninit();
631 /// x.write(true);
632 /// let x_init = unsafe { x.assume_init() };
633 /// assert_eq!(x_init, true);
634 /// ```
635 ///
636 /// *Incorrect* usage of this method:
637 ///
638 /// ```rust,no_run
639 /// use std::mem::MaybeUninit;
640 ///
641 /// let x = MaybeUninit::<Vec<u32>>::uninit();
642 /// let x_init = unsafe { x.assume_init() };
643 /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
644 /// ```
645 #[stable(feature = "maybe_uninit", since = "1.36.0")]
646 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
647 #[inline(always)]
648 #[rustc_diagnostic_item = "assume_init"]
649 #[track_caller]
650 pub const unsafe fn assume_init(self) -> T {
651 // SAFETY: the caller must guarantee that `self` is initialized.
652 // This also means that `self` must be a `value` variant.
653 unsafe {
654 intrinsics::assert_inhabited::<T>();
655 ManuallyDrop::into_inner(self.value)
656 }
657 }
658
659 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
660 /// to the usual drop handling.
661 ///
662 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
663 /// prevents duplicating the content of the `MaybeUninit<T>`.
664 ///
665 /// # Safety
666 ///
667 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
668 /// state. Calling this when the content is not yet fully initialized causes undefined
669 /// behavior. The [type-level documentation][inv] contains more information about
670 /// this initialization invariant.
671 ///
672 /// Moreover, similar to the [`ptr::read`] function, this function creates a
673 /// bitwise copy of the contents, regardless whether the contained type
674 /// implements the [`Copy`] trait or not. When using multiple copies of the
675 /// data (by calling `assume_init_read` multiple times, or first calling
676 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
677 /// to ensure that data may indeed be duplicated.
678 ///
679 /// [inv]: #initialization-invariant
680 /// [`assume_init`]: MaybeUninit::assume_init
681 ///
682 /// # Examples
683 ///
684 /// Correct usage of this method:
685 ///
686 /// ```rust
687 /// use std::mem::MaybeUninit;
688 ///
689 /// let mut x = MaybeUninit::<u32>::uninit();
690 /// x.write(13);
691 /// let x1 = unsafe { x.assume_init_read() };
692 /// // `u32` is `Copy`, so we may read multiple times.
693 /// let x2 = unsafe { x.assume_init_read() };
694 /// assert_eq!(x1, x2);
695 ///
696 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
697 /// x.write(None);
698 /// let x1 = unsafe { x.assume_init_read() };
699 /// // Duplicating a `None` value is okay, so we may read multiple times.
700 /// let x2 = unsafe { x.assume_init_read() };
701 /// assert_eq!(x1, x2);
702 /// ```
703 ///
704 /// *Incorrect* usage of this method:
705 ///
706 /// ```rust,no_run
707 /// use std::mem::MaybeUninit;
708 ///
709 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
710 /// x.write(Some(vec![0, 1, 2]));
711 /// let x1 = unsafe { x.assume_init_read() };
712 /// let x2 = unsafe { x.assume_init_read() };
713 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
714 /// // they both get dropped!
715 /// ```
716 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
717 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_read", since = "1.75.0")]
718 #[inline(always)]
719 #[track_caller]
720 pub const unsafe fn assume_init_read(&self) -> T {
721 // SAFETY: the caller must guarantee that `self` is initialized.
722 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
723 unsafe {
724 intrinsics::assert_inhabited::<T>();
725 self.as_ptr().read()
726 }
727 }
728
729 /// Drops the contained value in place.
730 ///
731 /// If you have ownership of the `MaybeUninit`, you can also use
732 /// [`assume_init`] as an alternative.
733 ///
734 /// # Safety
735 ///
736 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
737 /// in an initialized state. Calling this when the content is not yet fully
738 /// initialized causes undefined behavior.
739 ///
740 /// On top of that, all additional invariants of the type `T` must be
741 /// satisfied, as the `Drop` implementation of `T` (or its members) may
742 /// rely on this. For example, setting a `Vec<T>` to an invalid but
743 /// non-null address makes it initialized (under the current implementation;
744 /// this does not constitute a stable guarantee), because the only
745 /// requirement the compiler knows about it is that the data pointer must be
746 /// non-null. Dropping such a `Vec<T>` however will cause undefined
747 /// behavior.
748 ///
749 /// [`assume_init`]: MaybeUninit::assume_init
750 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
751 pub unsafe fn assume_init_drop(&mut self) {
752 // SAFETY: the caller must guarantee that `self` is initialized and
753 // satisfies all invariants of `T`.
754 // Dropping the value in place is safe if that is the case.
755 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
756 }
757
758 /// Gets a shared reference to the contained value.
759 ///
760 /// This can be useful when we want to access a `MaybeUninit` that has been
761 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
762 /// of `.assume_init()`).
763 ///
764 /// # Safety
765 ///
766 /// Calling this when the content is not yet fully initialized causes undefined
767 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
768 /// is in an initialized state.
769 ///
770 /// # Examples
771 ///
772 /// ### Correct usage of this method:
773 ///
774 /// ```rust
775 /// use std::mem::MaybeUninit;
776 ///
777 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
778 /// # let mut x_mu = x;
779 /// # let mut x = &mut x_mu;
780 /// // Initialize `x`:
781 /// x.write(vec![1, 2, 3]);
782 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
783 /// // create a shared reference to it:
784 /// let x: &Vec<u32> = unsafe {
785 /// // SAFETY: `x` has been initialized.
786 /// x.assume_init_ref()
787 /// };
788 /// assert_eq!(x, &vec![1, 2, 3]);
789 /// # // Prevent leaks for Miri
790 /// # unsafe { MaybeUninit::assume_init_drop(&mut x_mu); }
791 /// ```
792 ///
793 /// ### *Incorrect* usages of this method:
794 ///
795 /// ```rust,no_run
796 /// use std::mem::MaybeUninit;
797 ///
798 /// let x = MaybeUninit::<Vec<u32>>::uninit();
799 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
800 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
801 /// ```
802 ///
803 /// ```rust,no_run
804 /// use std::{cell::Cell, mem::MaybeUninit};
805 ///
806 /// let b = MaybeUninit::<Cell<bool>>::uninit();
807 /// // Initialize the `MaybeUninit` using `Cell::set`:
808 /// unsafe {
809 /// b.assume_init_ref().set(true);
810 /// // ^^^^^^^^^^^^^^^
811 /// // Reference to an uninitialized `Cell<bool>`: UB!
812 /// }
813 /// ```
814 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
815 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
816 #[inline(always)]
817 pub const unsafe fn assume_init_ref(&self) -> &T {
818 // SAFETY: the caller must guarantee that `self` is initialized.
819 // This also means that `self` must be a `value` variant.
820 unsafe {
821 intrinsics::assert_inhabited::<T>();
822 &*self.as_ptr()
823 }
824 }
825
826 /// Gets a mutable (unique) reference to the contained value.
827 ///
828 /// This can be useful when we want to access a `MaybeUninit` that has been
829 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
830 /// of `.assume_init()`).
831 ///
832 /// # Safety
833 ///
834 /// Calling this when the content is not yet fully initialized causes undefined
835 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
836 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
837 /// initialize a `MaybeUninit`.
838 ///
839 /// # Examples
840 ///
841 /// ### Correct usage of this method:
842 ///
843 /// ```rust
844 /// # #![allow(unexpected_cfgs)]
845 /// use std::mem::MaybeUninit;
846 ///
847 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }
848 /// # #[cfg(FALSE)]
849 /// extern "C" {
850 /// /// Initializes *all* the bytes of the input buffer.
851 /// fn initialize_buffer(buf: *mut [u8; 1024]);
852 /// }
853 ///
854 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
855 ///
856 /// // Initialize `buf`:
857 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
858 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
859 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
860 /// // To assert our buffer has been initialized without copying it, we upgrade
861 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
862 /// let buf: &mut [u8; 1024] = unsafe {
863 /// // SAFETY: `buf` has been initialized.
864 /// buf.assume_init_mut()
865 /// };
866 ///
867 /// // Now we can use `buf` as a normal slice:
868 /// buf.sort_unstable();
869 /// assert!(
870 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
871 /// "buffer is sorted",
872 /// );
873 /// ```
874 ///
875 /// ### *Incorrect* usages of this method:
876 ///
877 /// You cannot use `.assume_init_mut()` to initialize a value:
878 ///
879 /// ```rust,no_run
880 /// use std::mem::MaybeUninit;
881 ///
882 /// let mut b = MaybeUninit::<bool>::uninit();
883 /// unsafe {
884 /// *b.assume_init_mut() = true;
885 /// // We have created a (mutable) reference to an uninitialized `bool`!
886 /// // This is undefined behavior. ⚠️
887 /// }
888 /// ```
889 ///
890 /// For instance, you cannot [`Read`] into an uninitialized buffer:
891 ///
892 /// [`Read`]: ../../std/io/trait.Read.html
893 ///
894 /// ```rust,no_run
895 /// use std::{io, mem::MaybeUninit};
896 ///
897 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
898 /// {
899 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
900 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
901 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
902 /// // (mutable) reference to uninitialized memory!
903 /// // This is undefined behavior.
904 /// Ok(unsafe { buffer.assume_init() })
905 /// }
906 /// ```
907 ///
908 /// Nor can you use direct field access to do field-by-field gradual initialization:
909 ///
910 /// ```rust,no_run
911 /// use std::{mem::MaybeUninit, ptr};
912 ///
913 /// struct Foo {
914 /// a: u32,
915 /// b: u8,
916 /// }
917 ///
918 /// let foo: Foo = unsafe {
919 /// let mut foo = MaybeUninit::<Foo>::uninit();
920 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
921 /// // ^^^^^^^^^^^^^^^^^^^^^
922 /// // (mutable) reference to uninitialized memory!
923 /// // This is undefined behavior.
924 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
925 /// // ^^^^^^^^^^^^^^^^^^^^^
926 /// // (mutable) reference to uninitialized memory!
927 /// // This is undefined behavior.
928 /// foo.assume_init()
929 /// };
930 /// ```
931 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
932 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.84.0")]
933 #[inline(always)]
934 pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
935 // SAFETY: the caller must guarantee that `self` is initialized.
936 // This also means that `self` must be a `value` variant.
937 unsafe {
938 intrinsics::assert_inhabited::<T>();
939 &mut *self.as_mut_ptr()
940 }
941 }
942
943 /// Extracts the values from an array of `MaybeUninit` containers.
944 ///
945 /// # Safety
946 ///
947 /// It is up to the caller to guarantee that all elements of the array are
948 /// in an initialized state.
949 ///
950 /// # Examples
951 ///
952 /// ```
953 /// #![feature(maybe_uninit_array_assume_init)]
954 /// use std::mem::MaybeUninit;
955 ///
956 /// let mut array: [MaybeUninit<i32>; 3] = [MaybeUninit::uninit(); 3];
957 /// array[0].write(0);
958 /// array[1].write(1);
959 /// array[2].write(2);
960 ///
961 /// // SAFETY: Now safe as we initialised all elements
962 /// let array = unsafe {
963 /// MaybeUninit::array_assume_init(array)
964 /// };
965 ///
966 /// assert_eq!(array, [0, 1, 2]);
967 /// ```
968 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
969 #[inline(always)]
970 #[track_caller]
971 pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
972 // SAFETY:
973 // * The caller guarantees that all elements of the array are initialized
974 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
975 // * `MaybeUninit` does not drop, so there are no double-frees
976 // And thus the conversion is safe
977 unsafe {
978 intrinsics::assert_inhabited::<[T; N]>();
979 intrinsics::transmute_unchecked(array)
980 }
981 }
982
983 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
984 ///
985 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
986 /// contain padding bytes which are left uninitialized.
987 ///
988 /// # Examples
989 ///
990 /// ```
991 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
992 /// use std::mem::MaybeUninit;
993 ///
994 /// let val = 0x12345678_i32;
995 /// let uninit = MaybeUninit::new(val);
996 /// let uninit_bytes = uninit.as_bytes();
997 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
998 /// assert_eq!(bytes, val.to_ne_bytes());
999 /// ```
1000 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1001 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1002 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1003 unsafe {
1004 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of::<T>())
1005 }
1006 }
1007
1008 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
1009 /// bytes.
1010 ///
1011 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1012 /// contain padding bytes which are left uninitialized.
1013 ///
1014 /// # Examples
1015 ///
1016 /// ```
1017 /// #![feature(maybe_uninit_as_bytes)]
1018 /// use std::mem::MaybeUninit;
1019 ///
1020 /// let val = 0x12345678_i32;
1021 /// let mut uninit = MaybeUninit::new(val);
1022 /// let uninit_bytes = uninit.as_bytes_mut();
1023 /// if cfg!(target_endian = "little") {
1024 /// uninit_bytes[0].write(0xcd);
1025 /// } else {
1026 /// uninit_bytes[3].write(0xcd);
1027 /// }
1028 /// let val2 = unsafe { uninit.assume_init() };
1029 /// assert_eq!(val2, 0x123456cd);
1030 /// ```
1031 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1032 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1033 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1034 unsafe {
1035 slice::from_raw_parts_mut(
1036 self.as_mut_ptr().cast::<MaybeUninit<u8>>(),
1037 super::size_of::<T>(),
1038 )
1039 }
1040 }
1041
1042 /// Deprecated version of [`slice::assume_init_ref`].
1043 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1044 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1045 #[deprecated(
1046 note = "replaced by inherent assume_init_ref method; will eventually be removed",
1047 since = "1.83.0"
1048 )]
1049 pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
1050 // SAFETY: Same for both methods.
1051 unsafe { slice.assume_init_ref() }
1052 }
1053
1054 /// Deprecated version of [`slice::assume_init_mut`].
1055 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1056 #[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
1057 #[deprecated(
1058 note = "replaced by inherent assume_init_mut method; will eventually be removed",
1059 since = "1.83.0"
1060 )]
1061 pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
1062 // SAFETY: Same for both methods.
1063 unsafe { slice.assume_init_mut() }
1064 }
1065
1066 /// Gets a pointer to the first element of the array.
1067 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1068 #[inline(always)]
1069 pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
1070 this.as_ptr() as *const T
1071 }
1072
1073 /// Gets a mutable pointer to the first element of the array.
1074 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1075 #[inline(always)]
1076 pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
1077 this.as_mut_ptr() as *mut T
1078 }
1079
1080 /// Deprecated version of [`slice::write_copy_of_slice`].
1081 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1082 #[deprecated(
1083 note = "replaced by inherent write_copy_of_slice method; will eventually be removed",
1084 since = "1.83.0"
1085 )]
1086 pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1087 where
1088 T: Copy,
1089 {
1090 this.write_copy_of_slice(src)
1091 }
1092
1093 /// Deprecated version of [`slice::write_clone_of_slice`].
1094 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1095 #[deprecated(
1096 note = "replaced by inherent write_clone_of_slice method; will eventually be removed",
1097 since = "1.83.0"
1098 )]
1099 pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1100 where
1101 T: Clone,
1102 {
1103 this.write_clone_of_slice(src)
1104 }
1105
1106 /// Fills a slice with elements by cloning `value`, returning a mutable reference to the now
1107 /// initialized contents of the slice.
1108 /// Any previously initialized elements will not be dropped.
1109 ///
1110 /// This is similar to [`slice::fill`].
1111 ///
1112 /// # Panics
1113 ///
1114 /// This function will panic if any call to `Clone` panics.
1115 ///
1116 /// If such a panic occurs, any elements previously initialized during this operation will be
1117 /// dropped.
1118 ///
1119 /// # Examples
1120 ///
1121 /// ```
1122 /// #![feature(maybe_uninit_fill)]
1123 /// use std::mem::MaybeUninit;
1124 ///
1125 /// let mut buf = [const { MaybeUninit::uninit() }; 10];
1126 /// let initialized = MaybeUninit::fill(&mut buf, 1);
1127 /// assert_eq!(initialized, &mut [1; 10]);
1128 /// ```
1129 #[doc(alias = "memset")]
1130 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1131 pub fn fill(this: &mut [MaybeUninit<T>], value: T) -> &mut [T]
1132 where
1133 T: Clone,
1134 {
1135 SpecFill::spec_fill(this, value);
1136 // SAFETY: Valid elements have just been filled into `this` so it is initialized
1137 unsafe { this.assume_init_mut() }
1138 }
1139
1140 /// Fills a slice with elements returned by calling a closure repeatedly.
1141 ///
1142 /// This method uses a closure to create new values. If you'd rather `Clone` a given value, use
1143 /// [`MaybeUninit::fill`]. If you want to use the `Default` trait to generate values, you can
1144 /// pass [`Default::default`] as the argument.
1145 ///
1146 /// # Panics
1147 ///
1148 /// This function will panic if any call to the provided closure panics.
1149 ///
1150 /// If such a panic occurs, any elements previously initialized during this operation will be
1151 /// dropped.
1152 ///
1153 /// # Examples
1154 ///
1155 /// ```
1156 /// #![feature(maybe_uninit_fill)]
1157 /// use std::mem::MaybeUninit;
1158 ///
1159 /// let mut buf = [const { MaybeUninit::<i32>::uninit() }; 10];
1160 /// let initialized = MaybeUninit::fill_with(&mut buf, Default::default);
1161 /// assert_eq!(initialized, &mut [0; 10]);
1162 /// ```
1163 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1164 pub fn fill_with<F>(this: &mut [MaybeUninit<T>], mut f: F) -> &mut [T]
1165 where
1166 F: FnMut() -> T,
1167 {
1168 let mut guard = Guard { slice: this, initialized: 0 };
1169
1170 for element in guard.slice.iter_mut() {
1171 element.write(f());
1172 guard.initialized += 1;
1173 }
1174
1175 super::forget(guard);
1176
1177 // SAFETY: Valid elements have just been written into `this` so it is initialized
1178 unsafe { this.assume_init_mut() }
1179 }
1180
1181 /// Fills a slice with elements yielded by an iterator until either all elements have been
1182 /// initialized or the iterator is empty.
1183 ///
1184 /// Returns two slices. The first slice contains the initialized portion of the original slice.
1185 /// The second slice is the still-uninitialized remainder of the original slice.
1186 ///
1187 /// # Panics
1188 ///
1189 /// This function panics if the iterator's `next` function panics.
1190 ///
1191 /// If such a panic occurs, any elements previously initialized during this operation will be
1192 /// dropped.
1193 ///
1194 /// # Examples
1195 ///
1196 /// Completely filling the slice:
1197 ///
1198 /// ```
1199 /// #![feature(maybe_uninit_fill)]
1200 /// use std::mem::MaybeUninit;
1201 ///
1202 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1203 ///
1204 /// let iter = [1, 2, 3].into_iter().cycle();
1205 /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
1206 ///
1207 /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]);
1208 /// assert_eq!(remainder.len(), 0);
1209 /// ```
1210 ///
1211 /// Partially filling the slice:
1212 ///
1213 /// ```
1214 /// #![feature(maybe_uninit_fill)]
1215 /// use std::mem::MaybeUninit;
1216 ///
1217 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1218 /// let iter = [1, 2];
1219 /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter);
1220 ///
1221 /// assert_eq!(initialized, &mut [1, 2]);
1222 /// assert_eq!(remainder.len(), 3);
1223 /// ```
1224 ///
1225 /// Checking an iterator after filling a slice:
1226 ///
1227 /// ```
1228 /// #![feature(maybe_uninit_fill)]
1229 /// use std::mem::MaybeUninit;
1230 ///
1231 /// let mut buf = [const { MaybeUninit::uninit() }; 3];
1232 /// let mut iter = [1, 2, 3, 4, 5].into_iter();
1233 /// let (initialized, remainder) = MaybeUninit::fill_from(&mut buf, iter.by_ref());
1234 ///
1235 /// assert_eq!(initialized, &mut [1, 2, 3]);
1236 /// assert_eq!(remainder.len(), 0);
1237 /// assert_eq!(iter.as_slice(), &[4, 5]);
1238 /// ```
1239 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1240 pub fn fill_from<I>(this: &mut [MaybeUninit<T>], it: I) -> (&mut [T], &mut [MaybeUninit<T>])
1241 where
1242 I: IntoIterator<Item = T>,
1243 {
1244 let iter = it.into_iter();
1245 let mut guard = Guard { slice: this, initialized: 0 };
1246
1247 for (element, val) in guard.slice.iter_mut().zip(iter) {
1248 element.write(val);
1249 guard.initialized += 1;
1250 }
1251
1252 let initialized_len = guard.initialized;
1253 super::forget(guard);
1254
1255 // SAFETY: guard.initialized <= this.len()
1256 let (initted, remainder) = unsafe { this.split_at_mut_unchecked(initialized_len) };
1257
1258 // SAFETY: Valid elements have just been written into `init`, so that portion
1259 // of `this` is initialized.
1260 (unsafe { initted.assume_init_mut() }, remainder)
1261 }
1262
1263 /// Deprecated version of [`slice::as_bytes`].
1264 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1265 #[deprecated(
1266 note = "replaced by inherent as_bytes method; will eventually be removed",
1267 since = "1.83.0"
1268 )]
1269 pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
1270 this.as_bytes()
1271 }
1272
1273 /// Deprecated version of [`slice::as_bytes_mut`].
1274 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1275 #[deprecated(
1276 note = "replaced by inherent as_bytes_mut method; will eventually be removed",
1277 since = "1.83.0"
1278 )]
1279 pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
1280 this.as_bytes_mut()
1281 }
1282}
1283
1284impl<T> [MaybeUninit<T>] {
1285 /// Copies the elements from `src` to `self`,
1286 /// returning a mutable reference to the now initialized contents of `self`.
1287 ///
1288 /// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead.
1289 ///
1290 /// This is similar to [`slice::copy_from_slice`].
1291 ///
1292 /// # Panics
1293 ///
1294 /// This function will panic if the two slices have different lengths.
1295 ///
1296 /// # Examples
1297 ///
1298 /// ```
1299 /// #![feature(maybe_uninit_write_slice)]
1300 /// use std::mem::MaybeUninit;
1301 ///
1302 /// let mut dst = [MaybeUninit::uninit(); 32];
1303 /// let src = [0; 32];
1304 ///
1305 /// let init = dst.write_copy_of_slice(&src);
1306 ///
1307 /// assert_eq!(init, src);
1308 /// ```
1309 ///
1310 /// ```
1311 /// #![feature(maybe_uninit_write_slice)]
1312 ///
1313 /// let mut vec = Vec::with_capacity(32);
1314 /// let src = [0; 16];
1315 ///
1316 /// vec.spare_capacity_mut()[..src.len()].write_copy_of_slice(&src);
1317 ///
1318 /// // SAFETY: we have just copied all the elements of len into the spare capacity
1319 /// // the first src.len() elements of the vec are valid now.
1320 /// unsafe {
1321 /// vec.set_len(src.len());
1322 /// }
1323 ///
1324 /// assert_eq!(vec, src);
1325 /// ```
1326 ///
1327 /// [`write_clone_of_slice`]: slice::write_clone_of_slice
1328 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1329 #[rustc_const_unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1330 pub const fn write_copy_of_slice(&mut self, src: &[T]) -> &mut [T]
1331 where
1332 T: Copy,
1333 {
1334 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1335 let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
1336
1337 self.copy_from_slice(uninit_src);
1338
1339 // SAFETY: Valid elements have just been copied into `self` so it is initialized
1340 unsafe { self.assume_init_mut() }
1341 }
1342
1343 /// Clones the elements from `src` to `self`,
1344 /// returning a mutable reference to the now initialized contents of `self`.
1345 /// Any already initialized elements will not be dropped.
1346 ///
1347 /// If `T` implements `Copy`, use [`write_copy_of_slice`] instead.
1348 ///
1349 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1350 ///
1351 /// # Panics
1352 ///
1353 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1354 ///
1355 /// If there is a panic, the already cloned elements will be dropped.
1356 ///
1357 /// # Examples
1358 ///
1359 /// ```
1360 /// #![feature(maybe_uninit_write_slice)]
1361 /// use std::mem::MaybeUninit;
1362 ///
1363 /// let mut dst = [const { MaybeUninit::uninit() }; 5];
1364 /// let src = ["wibbly", "wobbly", "timey", "wimey", "stuff"].map(|s| s.to_string());
1365 ///
1366 /// let init = dst.write_clone_of_slice(&src);
1367 ///
1368 /// assert_eq!(init, src);
1369 ///
1370 /// # // Prevent leaks for Miri
1371 /// # unsafe { std::ptr::drop_in_place(init); }
1372 /// ```
1373 ///
1374 /// ```
1375 /// #![feature(maybe_uninit_write_slice)]
1376 ///
1377 /// let mut vec = Vec::with_capacity(32);
1378 /// let src = ["rust", "is", "a", "pretty", "cool", "language"].map(|s| s.to_string());
1379 ///
1380 /// vec.spare_capacity_mut()[..src.len()].write_clone_of_slice(&src);
1381 ///
1382 /// // SAFETY: we have just cloned all the elements of len into the spare capacity
1383 /// // the first src.len() elements of the vec are valid now.
1384 /// unsafe {
1385 /// vec.set_len(src.len());
1386 /// }
1387 ///
1388 /// assert_eq!(vec, src);
1389 /// ```
1390 ///
1391 /// [`write_copy_of_slice`]: slice::write_copy_of_slice
1392 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1393 pub fn write_clone_of_slice(&mut self, src: &[T]) -> &mut [T]
1394 where
1395 T: Clone,
1396 {
1397 // unlike copy_from_slice this does not call clone_from_slice on the slice
1398 // this is because `MaybeUninit<T: Clone>` does not implement Clone.
1399
1400 assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
1401
1402 // NOTE: We need to explicitly slice them to the same length
1403 // for bounds checking to be elided, and the optimizer will
1404 // generate memcpy for simple cases (for example T = u8).
1405 let len = self.len();
1406 let src = &src[..len];
1407
1408 // guard is needed b/c panic might happen during a clone
1409 let mut guard = Guard { slice: self, initialized: 0 };
1410
1411 for i in 0..len {
1412 guard.slice[i].write(src[i].clone());
1413 guard.initialized += 1;
1414 }
1415
1416 super::forget(guard);
1417
1418 // SAFETY: Valid elements have just been written into `self` so it is initialized
1419 unsafe { self.assume_init_mut() }
1420 }
1421
1422 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1423 ///
1424 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1425 /// contain padding bytes which are left uninitialized.
1426 ///
1427 /// # Examples
1428 ///
1429 /// ```
1430 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1431 /// use std::mem::MaybeUninit;
1432 ///
1433 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1434 /// let uninit_bytes = uninit.as_bytes();
1435 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
1436 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1437 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1438 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1439 /// ```
1440 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1441 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1442 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1443 unsafe {
1444 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of_val(self))
1445 }
1446 }
1447
1448 /// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially
1449 /// uninitialized bytes.
1450 ///
1451 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1452 /// contain padding bytes which are left uninitialized.
1453 ///
1454 /// # Examples
1455 ///
1456 /// ```
1457 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1458 /// use std::mem::MaybeUninit;
1459 ///
1460 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1461 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1462 /// uninit_bytes.write_copy_of_slice(&[0x12, 0x34, 0x56, 0x78]);
1463 /// let vals = unsafe { uninit.assume_init_ref() };
1464 /// if cfg!(target_endian = "little") {
1465 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1466 /// } else {
1467 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1468 /// }
1469 /// ```
1470 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1471 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1472 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1473 unsafe {
1474 slice::from_raw_parts_mut(
1475 self.as_mut_ptr() as *mut MaybeUninit<u8>,
1476 super::size_of_val(self),
1477 )
1478 }
1479 }
1480
1481 /// Drops the contained values in place.
1482 ///
1483 /// # Safety
1484 ///
1485 /// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice
1486 /// really is in an initialized state. Calling this when the content is not yet
1487 /// fully initialized causes undefined behavior.
1488 ///
1489 /// On top of that, all additional invariants of the type `T` must be
1490 /// satisfied, as the `Drop` implementation of `T` (or its members) may
1491 /// rely on this. For example, setting a `Vec<T>` to an invalid but
1492 /// non-null address makes it initialized (under the current implementation;
1493 /// this does not constitute a stable guarantee), because the only
1494 /// requirement the compiler knows about it is that the data pointer must be
1495 /// non-null. Dropping such a `Vec<T>` however will cause undefined
1496 /// behaviour.
1497 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1498 #[inline(always)]
1499 pub unsafe fn assume_init_drop(&mut self) {
1500 if !self.is_empty() {
1501 // SAFETY: the caller must guarantee that every element of `self`
1502 // is initialized and satisfies all invariants of `T`.
1503 // Dropping the value in place is safe if that is the case.
1504 unsafe { ptr::drop_in_place(self as *mut [MaybeUninit<T>] as *mut [T]) }
1505 }
1506 }
1507
1508 /// Gets a shared reference to the contained value.
1509 ///
1510 /// # Safety
1511 ///
1512 /// Calling this when the content is not yet fully initialized causes undefined
1513 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in
1514 /// the slice really is in an initialized state.
1515 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1516 #[inline(always)]
1517 pub const unsafe fn assume_init_ref(&self) -> &[T] {
1518 // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
1519 // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
1520 // The pointer obtained is valid since it refers to memory owned by `slice` which is a
1521 // reference and thus guaranteed to be valid for reads.
1522 unsafe { &*(self as *const Self as *const [T]) }
1523 }
1524
1525 /// Gets a mutable (unique) reference to the contained value.
1526 ///
1527 /// # Safety
1528 ///
1529 /// Calling this when the content is not yet fully initialized causes undefined
1530 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
1531 /// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
1532 /// be used to initialize a `MaybeUninit` slice.
1533 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1534 #[inline(always)]
1535 pub const unsafe fn assume_init_mut(&mut self) -> &mut [T] {
1536 // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
1537 // mutable reference which is also guaranteed to be valid for writes.
1538 unsafe { &mut *(self as *mut Self as *mut [T]) }
1539 }
1540}
1541
1542impl<T, const N: usize> MaybeUninit<[T; N]> {
1543 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1544 ///
1545 /// # Examples
1546 ///
1547 /// ```
1548 /// #![feature(maybe_uninit_uninit_array_transpose)]
1549 /// # use std::mem::MaybeUninit;
1550 ///
1551 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1552 /// ```
1553 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1554 #[inline]
1555 pub const fn transpose(self) -> [MaybeUninit<T>; N] {
1556 // SAFETY: T and MaybeUninit<T> have the same layout
1557 unsafe { intrinsics::transmute_unchecked(self) }
1558 }
1559}
1560
1561impl<T, const N: usize> [MaybeUninit<T>; N] {
1562 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1563 ///
1564 /// # Examples
1565 ///
1566 /// ```
1567 /// #![feature(maybe_uninit_uninit_array_transpose)]
1568 /// # use std::mem::MaybeUninit;
1569 ///
1570 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1571 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1572 /// ```
1573 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1574 #[inline]
1575 pub const fn transpose(self) -> MaybeUninit<[T; N]> {
1576 // SAFETY: T and MaybeUninit<T> have the same layout
1577 unsafe { intrinsics::transmute_unchecked(self) }
1578 }
1579}
1580
1581struct Guard<'a, T> {
1582 slice: &'a mut [MaybeUninit<T>],
1583 initialized: usize,
1584}
1585
1586impl<'a, T> Drop for Guard<'a, T> {
1587 fn drop(&mut self) {
1588 let initialized_part = &mut self.slice[..self.initialized];
1589 // SAFETY: this raw sub-slice will contain only initialized objects.
1590 unsafe {
1591 initialized_part.assume_init_drop();
1592 }
1593 }
1594}
1595
1596trait SpecFill<T> {
1597 fn spec_fill(&mut self, value: T);
1598}
1599
1600impl<T: Clone> SpecFill<T> for [MaybeUninit<T>] {
1601 default fn spec_fill(&mut self, value: T) {
1602 let mut guard = Guard { slice: self, initialized: 0 };
1603
1604 if let Some((last, elems)) = guard.slice.split_last_mut() {
1605 for el in elems {
1606 el.write(value.clone());
1607 guard.initialized += 1;
1608 }
1609
1610 last.write(value);
1611 }
1612 super::forget(guard);
1613 }
1614}
1615
1616impl<T: Copy> SpecFill<T> for [MaybeUninit<T>] {
1617 fn spec_fill(&mut self, value: T) {
1618 self.fill(MaybeUninit::new(value));
1619 }
1620}