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/// unsafe { 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;
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;
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: &'static str = type_name::<Self>();
280 let prefix_len: usize = 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 `MaybeUninit<T>` in an uninitialized state, with the memory being
335 /// filled with `0` bytes. It depends on `T` whether that already makes for
336 /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
337 /// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
338 /// be null.
339 ///
340 /// Note that if `T` has padding bytes, those bytes are *not* preserved when the
341 /// `MaybeUninit<T>` value is returned from this function, so those bytes will *not* be zeroed.
342 ///
343 /// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
344 /// It is your responsibility to make sure `T` gets dropped if it got initialized.
345 ///
346 /// # Example
347 ///
348 /// Correct usage of this function: initializing a struct with zero, where all
349 /// fields of the struct can hold the bit-pattern 0 as a valid value.
350 ///
351 /// ```rust
352 /// use std::mem::MaybeUninit;
353 ///
354 /// let x = MaybeUninit::<(u8, bool)>::zeroed();
355 /// let x = unsafe { x.assume_init() };
356 /// assert_eq!(x, (0, false));
357 /// ```
358 ///
359 /// This can be used in const contexts, such as to indicate the end of static arrays for
360 /// plugin registration.
361 ///
362 /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
363 /// when `0` is not a valid bit-pattern for the type:
364 ///
365 /// ```rust,no_run
366 /// use std::mem::MaybeUninit;
367 ///
368 /// enum NotZero { One = 1, Two = 2 }
369 ///
370 /// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
371 /// let x = unsafe { x.assume_init() };
372 /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
373 /// // This is undefined behavior. ⚠️
374 /// ```
375 #[inline]
376 #[must_use]
377 #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
378 #[stable(feature = "maybe_uninit", since = "1.36.0")]
379 #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "1.75.0")]
380 pub const fn zeroed() -> MaybeUninit<T> {
381 let mut u = MaybeUninit::<T>::uninit();
382 // SAFETY: `u.as_mut_ptr()` points to allocated memory.
383 unsafe { u.as_mut_ptr().write_bytes(0u8, 1) };
384 u
385 }
386
387 /// Sets the value of the `MaybeUninit<T>`.
388 ///
389 /// This overwrites any previous value without dropping it, so be careful
390 /// not to use this twice unless you want to skip running the destructor.
391 /// For your convenience, this also returns a mutable reference to the
392 /// (now safely initialized) contents of `self`.
393 ///
394 /// As the content is stored inside a `MaybeUninit`, the destructor is not
395 /// run for the inner data if the MaybeUninit leaves scope without a call to
396 /// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
397 /// the mutable reference returned by this function needs to keep this in
398 /// mind. The safety model of Rust regards leaks as safe, but they are
399 /// usually still undesirable. This being said, the mutable reference
400 /// behaves like any other mutable reference would, so assigning a new value
401 /// to it will drop the old content.
402 ///
403 /// [`assume_init`]: Self::assume_init
404 /// [`assume_init_drop`]: Self::assume_init_drop
405 ///
406 /// # Examples
407 ///
408 /// Correct usage of this method:
409 ///
410 /// ```rust
411 /// use std::mem::MaybeUninit;
412 ///
413 /// let mut x = MaybeUninit::<Vec<u8>>::uninit();
414 ///
415 /// {
416 /// let hello = x.write((&b"Hello, world!").to_vec());
417 /// // Setting hello does not leak prior allocations, but drops them
418 /// *hello = (&b"Hello").to_vec();
419 /// hello[0] = 'h' as u8;
420 /// }
421 /// // x is initialized now:
422 /// let s = unsafe { x.assume_init() };
423 /// assert_eq!(b"hello", s.as_slice());
424 /// ```
425 ///
426 /// This usage of the method causes a leak:
427 ///
428 /// ```rust
429 /// use std::mem::MaybeUninit;
430 ///
431 /// let mut x = MaybeUninit::<String>::uninit();
432 ///
433 /// x.write("Hello".to_string());
434 /// # // FIXME(https://github.com/rust-lang/miri/issues/3670):
435 /// # // use -Zmiri-disable-leak-check instead of unleaking in tests meant to leak.
436 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
437 /// // This leaks the contained string:
438 /// x.write("hello".to_string());
439 /// // x is initialized now:
440 /// let s = unsafe { x.assume_init() };
441 /// ```
442 ///
443 /// This method can be used to avoid unsafe in some cases. The example below
444 /// shows a part of an implementation of a fixed sized arena that lends out
445 /// pinned references.
446 /// With `write`, we can avoid the need to write through a raw pointer:
447 ///
448 /// ```rust
449 /// use core::pin::Pin;
450 /// use core::mem::MaybeUninit;
451 ///
452 /// struct PinArena<T> {
453 /// memory: Box<[MaybeUninit<T>]>,
454 /// len: usize,
455 /// }
456 ///
457 /// impl <T> PinArena<T> {
458 /// pub fn capacity(&self) -> usize {
459 /// self.memory.len()
460 /// }
461 /// pub fn push(&mut self, val: T) -> Pin<&mut T> {
462 /// if self.len >= self.capacity() {
463 /// panic!("Attempted to push to a full pin arena!");
464 /// }
465 /// let ref_ = self.memory[self.len].write(val);
466 /// self.len += 1;
467 /// unsafe { Pin::new_unchecked(ref_) }
468 /// }
469 /// }
470 /// ```
471 #[inline(always)]
472 #[stable(feature = "maybe_uninit_write", since = "1.55.0")]
473 #[rustc_const_stable(feature = "const_maybe_uninit_write", since = "1.85.0")]
474 pub const fn write(&mut self, val: T) -> &mut T {
475 *self = MaybeUninit::new(val);
476 // SAFETY: We just initialized this value.
477 unsafe { self.assume_init_mut() }
478 }
479
480 /// Gets a pointer to the contained value. Reading from this pointer or turning it
481 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
482 /// Writing to memory that this pointer (non-transitively) points to is undefined behavior
483 /// (except inside an `UnsafeCell<T>`).
484 ///
485 /// # Examples
486 ///
487 /// Correct usage of this method:
488 ///
489 /// ```rust
490 /// use std::mem::MaybeUninit;
491 ///
492 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
493 /// x.write(vec![0, 1, 2]);
494 /// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
495 /// let x_vec = unsafe { &*x.as_ptr() };
496 /// assert_eq!(x_vec.len(), 3);
497 /// # // Prevent leaks for Miri
498 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
499 /// ```
500 ///
501 /// *Incorrect* usage of this method:
502 ///
503 /// ```rust,no_run
504 /// use std::mem::MaybeUninit;
505 ///
506 /// let x = MaybeUninit::<Vec<u32>>::uninit();
507 /// let x_vec = unsafe { &*x.as_ptr() };
508 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
509 /// ```
510 ///
511 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
512 /// until they are, it is advisable to avoid them.)
513 #[stable(feature = "maybe_uninit", since = "1.36.0")]
514 #[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
515 #[rustc_as_ptr]
516 #[inline(always)]
517 pub const fn as_ptr(&self) -> *const T {
518 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
519 self as *const _ as *const T
520 }
521
522 /// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
523 /// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
524 ///
525 /// # Examples
526 ///
527 /// Correct usage of this method:
528 ///
529 /// ```rust
530 /// use std::mem::MaybeUninit;
531 ///
532 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
533 /// x.write(vec![0, 1, 2]);
534 /// // Create a reference into the `MaybeUninit<Vec<u32>>`.
535 /// // This is okay because we initialized it.
536 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
537 /// x_vec.push(3);
538 /// assert_eq!(x_vec.len(), 4);
539 /// # // Prevent leaks for Miri
540 /// # unsafe { MaybeUninit::assume_init_drop(&mut x); }
541 /// ```
542 ///
543 /// *Incorrect* usage of this method:
544 ///
545 /// ```rust,no_run
546 /// use std::mem::MaybeUninit;
547 ///
548 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
549 /// let x_vec = unsafe { &mut *x.as_mut_ptr() };
550 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
551 /// ```
552 ///
553 /// (Notice that the rules around references to uninitialized data are not finalized yet, but
554 /// until they are, it is advisable to avoid them.)
555 #[stable(feature = "maybe_uninit", since = "1.36.0")]
556 #[rustc_const_stable(feature = "const_maybe_uninit_as_mut_ptr", since = "1.83.0")]
557 #[rustc_as_ptr]
558 #[inline(always)]
559 pub const fn as_mut_ptr(&mut self) -> *mut T {
560 // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
561 self as *mut _ as *mut T
562 }
563
564 /// Extracts the value from the `MaybeUninit<T>` container. This is a great way
565 /// to ensure that the data will get dropped, because the resulting `T` is
566 /// subject to the usual drop handling.
567 ///
568 /// # Safety
569 ///
570 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
571 /// state. Calling this when the content is not yet fully initialized causes immediate undefined
572 /// behavior. The [type-level documentation][inv] contains more information about
573 /// this initialization invariant.
574 ///
575 /// [inv]: #initialization-invariant
576 ///
577 /// On top of that, remember that most types have additional invariants beyond merely
578 /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
579 /// is considered initialized (under the current implementation; this does not constitute
580 /// a stable guarantee) because the only requirement the compiler knows about it
581 /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
582 /// *immediate* undefined behavior, but will cause undefined behavior with most
583 /// safe operations (including dropping it).
584 ///
585 /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
586 ///
587 /// # Examples
588 ///
589 /// Correct usage of this method:
590 ///
591 /// ```rust
592 /// use std::mem::MaybeUninit;
593 ///
594 /// let mut x = MaybeUninit::<bool>::uninit();
595 /// x.write(true);
596 /// let x_init = unsafe { x.assume_init() };
597 /// assert_eq!(x_init, true);
598 /// ```
599 ///
600 /// *Incorrect* usage of this method:
601 ///
602 /// ```rust,no_run
603 /// use std::mem::MaybeUninit;
604 ///
605 /// let x = MaybeUninit::<Vec<u32>>::uninit();
606 /// let x_init = unsafe { x.assume_init() };
607 /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
608 /// ```
609 #[stable(feature = "maybe_uninit", since = "1.36.0")]
610 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
611 #[inline(always)]
612 #[rustc_diagnostic_item = "assume_init"]
613 #[track_caller]
614 pub const unsafe fn assume_init(self) -> T {
615 // SAFETY: the caller must guarantee that `self` is initialized.
616 // This also means that `self` must be a `value` variant.
617 unsafe {
618 intrinsics::assert_inhabited::<T>();
619 ManuallyDrop::into_inner(self.value)
620 }
621 }
622
623 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
624 /// to the usual drop handling.
625 ///
626 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
627 /// prevents duplicating the content of the `MaybeUninit<T>`.
628 ///
629 /// # Safety
630 ///
631 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
632 /// state. Calling this when the content is not yet fully initialized causes undefined
633 /// behavior. The [type-level documentation][inv] contains more information about
634 /// this initialization invariant.
635 ///
636 /// Moreover, similar to the [`ptr::read`] function, this function creates a
637 /// bitwise copy of the contents, regardless whether the contained type
638 /// implements the [`Copy`] trait or not. When using multiple copies of the
639 /// data (by calling `assume_init_read` multiple times, or first calling
640 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
641 /// to ensure that data may indeed be duplicated.
642 ///
643 /// [inv]: #initialization-invariant
644 /// [`assume_init`]: MaybeUninit::assume_init
645 ///
646 /// # Examples
647 ///
648 /// Correct usage of this method:
649 ///
650 /// ```rust
651 /// use std::mem::MaybeUninit;
652 ///
653 /// let mut x = MaybeUninit::<u32>::uninit();
654 /// x.write(13);
655 /// let x1 = unsafe { x.assume_init_read() };
656 /// // `u32` is `Copy`, so we may read multiple times.
657 /// let x2 = unsafe { x.assume_init_read() };
658 /// assert_eq!(x1, x2);
659 ///
660 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
661 /// x.write(None);
662 /// let x1 = unsafe { x.assume_init_read() };
663 /// // Duplicating a `None` value is okay, so we may read multiple times.
664 /// let x2 = unsafe { x.assume_init_read() };
665 /// assert_eq!(x1, x2);
666 /// ```
667 ///
668 /// *Incorrect* usage of this method:
669 ///
670 /// ```rust,no_run
671 /// use std::mem::MaybeUninit;
672 ///
673 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
674 /// x.write(Some(vec![0, 1, 2]));
675 /// let x1 = unsafe { x.assume_init_read() };
676 /// let x2 = unsafe { x.assume_init_read() };
677 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
678 /// // they both get dropped!
679 /// ```
680 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
681 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_read", since = "1.75.0")]
682 #[inline(always)]
683 #[track_caller]
684 pub const unsafe fn assume_init_read(&self) -> T {
685 // SAFETY: the caller must guarantee that `self` is initialized.
686 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
687 unsafe {
688 intrinsics::assert_inhabited::<T>();
689 self.as_ptr().read()
690 }
691 }
692
693 /// Drops the contained value in place.
694 ///
695 /// If you have ownership of the `MaybeUninit`, you can also use
696 /// [`assume_init`] as an alternative.
697 ///
698 /// # Safety
699 ///
700 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
701 /// in an initialized state. Calling this when the content is not yet fully
702 /// initialized causes undefined behavior.
703 ///
704 /// On top of that, all additional invariants of the type `T` must be
705 /// satisfied, as the `Drop` implementation of `T` (or its members) may
706 /// rely on this. For example, setting a `Vec<T>` to an invalid but
707 /// non-null address makes it initialized (under the current implementation;
708 /// this does not constitute a stable guarantee), because the only
709 /// requirement the compiler knows about it is that the data pointer must be
710 /// non-null. Dropping such a `Vec<T>` however will cause undefined
711 /// behavior.
712 ///
713 /// [`assume_init`]: MaybeUninit::assume_init
714 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
715 pub unsafe fn assume_init_drop(&mut self) {
716 // SAFETY: the caller must guarantee that `self` is initialized and
717 // satisfies all invariants of `T`.
718 // Dropping the value in place is safe if that is the case.
719 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
720 }
721
722 /// Gets a shared reference to the contained value.
723 ///
724 /// This can be useful when we want to access a `MaybeUninit` that has been
725 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
726 /// of `.assume_init()`).
727 ///
728 /// # Safety
729 ///
730 /// Calling this when the content is not yet fully initialized causes undefined
731 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
732 /// is in an initialized state.
733 ///
734 /// # Examples
735 ///
736 /// ### Correct usage of this method:
737 ///
738 /// ```rust
739 /// use std::mem::MaybeUninit;
740 ///
741 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
742 /// # let mut x_mu = x;
743 /// # let mut x = &mut x_mu;
744 /// // Initialize `x`:
745 /// x.write(vec![1, 2, 3]);
746 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
747 /// // create a shared reference to it:
748 /// let x: &Vec<u32> = unsafe {
749 /// // SAFETY: `x` has been initialized.
750 /// x.assume_init_ref()
751 /// };
752 /// assert_eq!(x, &vec![1, 2, 3]);
753 /// # // Prevent leaks for Miri
754 /// # unsafe { MaybeUninit::assume_init_drop(&mut x_mu); }
755 /// ```
756 ///
757 /// ### *Incorrect* usages of this method:
758 ///
759 /// ```rust,no_run
760 /// use std::mem::MaybeUninit;
761 ///
762 /// let x = MaybeUninit::<Vec<u32>>::uninit();
763 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
764 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
765 /// ```
766 ///
767 /// ```rust,no_run
768 /// use std::{cell::Cell, mem::MaybeUninit};
769 ///
770 /// let b = MaybeUninit::<Cell<bool>>::uninit();
771 /// // Initialize the `MaybeUninit` using `Cell::set`:
772 /// unsafe {
773 /// b.assume_init_ref().set(true);
774 /// // ^^^^^^^^^^^^^^^
775 /// // Reference to an uninitialized `Cell<bool>`: UB!
776 /// }
777 /// ```
778 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
779 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
780 #[inline(always)]
781 pub const unsafe fn assume_init_ref(&self) -> &T {
782 // SAFETY: the caller must guarantee that `self` is initialized.
783 // This also means that `self` must be a `value` variant.
784 unsafe {
785 intrinsics::assert_inhabited::<T>();
786 &*self.as_ptr()
787 }
788 }
789
790 /// Gets a mutable (unique) reference to the contained value.
791 ///
792 /// This can be useful when we want to access a `MaybeUninit` that has been
793 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
794 /// of `.assume_init()`).
795 ///
796 /// # Safety
797 ///
798 /// Calling this when the content is not yet fully initialized causes undefined
799 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
800 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
801 /// initialize a `MaybeUninit`.
802 ///
803 /// # Examples
804 ///
805 /// ### Correct usage of this method:
806 ///
807 /// ```rust
808 /// # #![allow(unexpected_cfgs)]
809 /// use std::mem::MaybeUninit;
810 ///
811 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { unsafe { *buf = [0; 1024] } }
812 /// # #[cfg(FALSE)]
813 /// extern "C" {
814 /// /// Initializes *all* the bytes of the input buffer.
815 /// fn initialize_buffer(buf: *mut [u8; 1024]);
816 /// }
817 ///
818 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
819 ///
820 /// // Initialize `buf`:
821 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
822 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
823 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
824 /// // To assert our buffer has been initialized without copying it, we upgrade
825 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
826 /// let buf: &mut [u8; 1024] = unsafe {
827 /// // SAFETY: `buf` has been initialized.
828 /// buf.assume_init_mut()
829 /// };
830 ///
831 /// // Now we can use `buf` as a normal slice:
832 /// buf.sort_unstable();
833 /// assert!(
834 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
835 /// "buffer is sorted",
836 /// );
837 /// ```
838 ///
839 /// ### *Incorrect* usages of this method:
840 ///
841 /// You cannot use `.assume_init_mut()` to initialize a value:
842 ///
843 /// ```rust,no_run
844 /// use std::mem::MaybeUninit;
845 ///
846 /// let mut b = MaybeUninit::<bool>::uninit();
847 /// unsafe {
848 /// *b.assume_init_mut() = true;
849 /// // We have created a (mutable) reference to an uninitialized `bool`!
850 /// // This is undefined behavior. ⚠️
851 /// }
852 /// ```
853 ///
854 /// For instance, you cannot [`Read`] into an uninitialized buffer:
855 ///
856 /// [`Read`]: ../../std/io/trait.Read.html
857 ///
858 /// ```rust,no_run
859 /// use std::{io, mem::MaybeUninit};
860 ///
861 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
862 /// {
863 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
864 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
865 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
866 /// // (mutable) reference to uninitialized memory!
867 /// // This is undefined behavior.
868 /// Ok(unsafe { buffer.assume_init() })
869 /// }
870 /// ```
871 ///
872 /// Nor can you use direct field access to do field-by-field gradual initialization:
873 ///
874 /// ```rust,no_run
875 /// use std::{mem::MaybeUninit, ptr};
876 ///
877 /// struct Foo {
878 /// a: u32,
879 /// b: u8,
880 /// }
881 ///
882 /// let foo: Foo = unsafe {
883 /// let mut foo = MaybeUninit::<Foo>::uninit();
884 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
885 /// // ^^^^^^^^^^^^^^^^^^^^^
886 /// // (mutable) reference to uninitialized memory!
887 /// // This is undefined behavior.
888 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
889 /// // ^^^^^^^^^^^^^^^^^^^^^
890 /// // (mutable) reference to uninitialized memory!
891 /// // This is undefined behavior.
892 /// foo.assume_init()
893 /// };
894 /// ```
895 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
896 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.84.0")]
897 #[inline(always)]
898 pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
899 // SAFETY: the caller must guarantee that `self` is initialized.
900 // This also means that `self` must be a `value` variant.
901 unsafe {
902 intrinsics::assert_inhabited::<T>();
903 &mut *self.as_mut_ptr()
904 }
905 }
906
907 /// Extracts the values from an array of `MaybeUninit` containers.
908 ///
909 /// # Safety
910 ///
911 /// It is up to the caller to guarantee that all elements of the array are
912 /// in an initialized state.
913 ///
914 /// # Examples
915 ///
916 /// ```
917 /// #![feature(maybe_uninit_array_assume_init)]
918 /// use std::mem::MaybeUninit;
919 ///
920 /// let mut array: [MaybeUninit<i32>; 3] = [MaybeUninit::uninit(); 3];
921 /// array[0].write(0);
922 /// array[1].write(1);
923 /// array[2].write(2);
924 ///
925 /// // SAFETY: Now safe as we initialised all elements
926 /// let array = unsafe {
927 /// MaybeUninit::array_assume_init(array)
928 /// };
929 ///
930 /// assert_eq!(array, [0, 1, 2]);
931 /// ```
932 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
933 #[inline(always)]
934 #[track_caller]
935 pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
936 // SAFETY:
937 // * The caller guarantees that all elements of the array are initialized
938 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
939 // * `MaybeUninit` does not drop, so there are no double-frees
940 // And thus the conversion is safe
941 unsafe {
942 intrinsics::assert_inhabited::<[T; N]>();
943 intrinsics::transmute_unchecked(array)
944 }
945 }
946
947 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
948 ///
949 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
950 /// contain padding bytes which are left uninitialized.
951 ///
952 /// # Examples
953 ///
954 /// ```
955 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
956 /// use std::mem::MaybeUninit;
957 ///
958 /// let val = 0x12345678_i32;
959 /// let uninit = MaybeUninit::new(val);
960 /// let uninit_bytes = uninit.as_bytes();
961 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
962 /// assert_eq!(bytes, val.to_ne_bytes());
963 /// ```
964 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
965 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
966 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
967 unsafe {
968 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of::<T>())
969 }
970 }
971
972 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
973 /// bytes.
974 ///
975 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
976 /// contain padding bytes which are left uninitialized.
977 ///
978 /// # Examples
979 ///
980 /// ```
981 /// #![feature(maybe_uninit_as_bytes)]
982 /// use std::mem::MaybeUninit;
983 ///
984 /// let val = 0x12345678_i32;
985 /// let mut uninit = MaybeUninit::new(val);
986 /// let uninit_bytes = uninit.as_bytes_mut();
987 /// if cfg!(target_endian = "little") {
988 /// uninit_bytes[0].write(0xcd);
989 /// } else {
990 /// uninit_bytes[3].write(0xcd);
991 /// }
992 /// let val2 = unsafe { uninit.assume_init() };
993 /// assert_eq!(val2, 0x123456cd);
994 /// ```
995 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
996 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
997 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
998 unsafe {
999 slice::from_raw_parts_mut(
1000 self.as_mut_ptr().cast::<MaybeUninit<u8>>(),
1001 super::size_of::<T>(),
1002 )
1003 }
1004 }
1005
1006 /// Deprecated version of [`slice::assume_init_ref`].
1007 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1008 #[deprecated(
1009 note = "replaced by inherent assume_init_ref method; will eventually be removed",
1010 since = "1.83.0"
1011 )]
1012 pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
1013 // SAFETY: Same for both methods.
1014 unsafe { slice.assume_init_ref() }
1015 }
1016
1017 /// Deprecated version of [`slice::assume_init_mut`].
1018 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1019 #[deprecated(
1020 note = "replaced by inherent assume_init_mut method; will eventually be removed",
1021 since = "1.83.0"
1022 )]
1023 pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
1024 // SAFETY: Same for both methods.
1025 unsafe { slice.assume_init_mut() }
1026 }
1027
1028 /// Gets a pointer to the first element of the array.
1029 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1030 #[inline(always)]
1031 pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
1032 this.as_ptr() as *const T
1033 }
1034
1035 /// Gets a mutable pointer to the first element of the array.
1036 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1037 #[inline(always)]
1038 pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
1039 this.as_mut_ptr() as *mut T
1040 }
1041
1042 /// Deprecated version of [`slice::write_copy_of_slice`].
1043 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1044 #[deprecated(
1045 note = "replaced by inherent write_copy_of_slice method; will eventually be removed",
1046 since = "1.83.0"
1047 )]
1048 pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1049 where
1050 T: Copy,
1051 {
1052 this.write_copy_of_slice(src)
1053 }
1054
1055 /// Deprecated version of [`slice::write_clone_of_slice`].
1056 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1057 #[deprecated(
1058 note = "replaced by inherent write_clone_of_slice method; will eventually be removed",
1059 since = "1.83.0"
1060 )]
1061 pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1062 where
1063 T: Clone,
1064 {
1065 this.write_clone_of_slice(src)
1066 }
1067
1068 /// Deprecated version of [`slice::write_filled`].
1069 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1070 #[deprecated(
1071 note = "replaced by inherent write_filled method; will eventually be removed",
1072 since = "1.83.0"
1073 )]
1074 pub fn fill<'a>(this: &'a mut [MaybeUninit<T>], value: T) -> &'a mut [T]
1075 where
1076 T: Clone,
1077 {
1078 this.write_filled(value)
1079 }
1080
1081 /// Deprecated version of [`slice::write_with`].
1082 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1083 #[deprecated(
1084 note = "replaced by inherent write_with method; will eventually be removed",
1085 since = "1.83.0"
1086 )]
1087 pub fn fill_with<'a, F>(this: &'a mut [MaybeUninit<T>], mut f: F) -> &'a mut [T]
1088 where
1089 F: FnMut() -> T,
1090 {
1091 this.write_with(|_| f())
1092 }
1093
1094 /// Deprecated version of [`slice::write_iter`].
1095 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1096 #[deprecated(
1097 note = "replaced by inherent write_iter method; will eventually be removed",
1098 since = "1.83.0"
1099 )]
1100 pub fn fill_from<'a, I>(
1101 this: &'a mut [MaybeUninit<T>],
1102 it: I,
1103 ) -> (&'a mut [T], &'a mut [MaybeUninit<T>])
1104 where
1105 I: IntoIterator<Item = T>,
1106 {
1107 this.write_iter(it)
1108 }
1109
1110 /// Deprecated version of [`slice::as_bytes`].
1111 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1112 #[deprecated(
1113 note = "replaced by inherent as_bytes method; will eventually be removed",
1114 since = "1.83.0"
1115 )]
1116 pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
1117 this.as_bytes()
1118 }
1119
1120 /// Deprecated version of [`slice::as_bytes_mut`].
1121 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1122 #[deprecated(
1123 note = "replaced by inherent as_bytes_mut method; will eventually be removed",
1124 since = "1.83.0"
1125 )]
1126 pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
1127 this.as_bytes_mut()
1128 }
1129}
1130
1131impl<T> [MaybeUninit<T>] {
1132 /// Copies the elements from `src` to `self`,
1133 /// returning a mutable reference to the now initialized contents of `self`.
1134 ///
1135 /// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead.
1136 ///
1137 /// This is similar to [`slice::copy_from_slice`].
1138 ///
1139 /// # Panics
1140 ///
1141 /// This function will panic if the two slices have different lengths.
1142 ///
1143 /// # Examples
1144 ///
1145 /// ```
1146 /// #![feature(maybe_uninit_write_slice)]
1147 /// use std::mem::MaybeUninit;
1148 ///
1149 /// let mut dst = [MaybeUninit::uninit(); 32];
1150 /// let src = [0; 32];
1151 ///
1152 /// let init = dst.write_copy_of_slice(&src);
1153 ///
1154 /// assert_eq!(init, src);
1155 /// ```
1156 ///
1157 /// ```
1158 /// #![feature(maybe_uninit_write_slice)]
1159 ///
1160 /// let mut vec = Vec::with_capacity(32);
1161 /// let src = [0; 16];
1162 ///
1163 /// vec.spare_capacity_mut()[..src.len()].write_copy_of_slice(&src);
1164 ///
1165 /// // SAFETY: we have just copied all the elements of len into the spare capacity
1166 /// // the first src.len() elements of the vec are valid now.
1167 /// unsafe {
1168 /// vec.set_len(src.len());
1169 /// }
1170 ///
1171 /// assert_eq!(vec, src);
1172 /// ```
1173 ///
1174 /// [`write_clone_of_slice`]: slice::write_clone_of_slice
1175 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1176 pub const fn write_copy_of_slice(&mut self, src: &[T]) -> &mut [T]
1177 where
1178 T: Copy,
1179 {
1180 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1181 let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
1182
1183 self.copy_from_slice(uninit_src);
1184
1185 // SAFETY: Valid elements have just been copied into `self` so it is initialized
1186 unsafe { self.assume_init_mut() }
1187 }
1188
1189 /// Clones the elements from `src` to `self`,
1190 /// returning a mutable reference to the now initialized contents of `self`.
1191 /// Any already initialized elements will not be dropped.
1192 ///
1193 /// If `T` implements `Copy`, use [`write_copy_of_slice`] instead.
1194 ///
1195 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1196 ///
1197 /// # Panics
1198 ///
1199 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1200 ///
1201 /// If there is a panic, the already cloned elements will be dropped.
1202 ///
1203 /// # Examples
1204 ///
1205 /// ```
1206 /// #![feature(maybe_uninit_write_slice)]
1207 /// use std::mem::MaybeUninit;
1208 ///
1209 /// let mut dst = [const { MaybeUninit::uninit() }; 5];
1210 /// let src = ["wibbly", "wobbly", "timey", "wimey", "stuff"].map(|s| s.to_string());
1211 ///
1212 /// let init = dst.write_clone_of_slice(&src);
1213 ///
1214 /// assert_eq!(init, src);
1215 ///
1216 /// # // Prevent leaks for Miri
1217 /// # unsafe { std::ptr::drop_in_place(init); }
1218 /// ```
1219 ///
1220 /// ```
1221 /// #![feature(maybe_uninit_write_slice)]
1222 ///
1223 /// let mut vec = Vec::with_capacity(32);
1224 /// let src = ["rust", "is", "a", "pretty", "cool", "language"].map(|s| s.to_string());
1225 ///
1226 /// vec.spare_capacity_mut()[..src.len()].write_clone_of_slice(&src);
1227 ///
1228 /// // SAFETY: we have just cloned all the elements of len into the spare capacity
1229 /// // the first src.len() elements of the vec are valid now.
1230 /// unsafe {
1231 /// vec.set_len(src.len());
1232 /// }
1233 ///
1234 /// assert_eq!(vec, src);
1235 /// ```
1236 ///
1237 /// [`write_copy_of_slice`]: slice::write_copy_of_slice
1238 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1239 pub fn write_clone_of_slice(&mut self, src: &[T]) -> &mut [T]
1240 where
1241 T: Clone,
1242 {
1243 // unlike copy_from_slice this does not call clone_from_slice on the slice
1244 // this is because `MaybeUninit<T: Clone>` does not implement Clone.
1245
1246 assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
1247
1248 // NOTE: We need to explicitly slice them to the same length
1249 // for bounds checking to be elided, and the optimizer will
1250 // generate memcpy for simple cases (for example T = u8).
1251 let len = self.len();
1252 let src = &src[..len];
1253
1254 // guard is needed b/c panic might happen during a clone
1255 let mut guard = Guard { slice: self, initialized: 0 };
1256
1257 for i in 0..len {
1258 guard.slice[i].write(src[i].clone());
1259 guard.initialized += 1;
1260 }
1261
1262 super::forget(guard);
1263
1264 // SAFETY: Valid elements have just been written into `self` so it is initialized
1265 unsafe { self.assume_init_mut() }
1266 }
1267
1268 /// Fills a slice with elements by cloning `value`, returning a mutable reference to the now
1269 /// initialized contents of the slice.
1270 /// Any previously initialized elements will not be dropped.
1271 ///
1272 /// This is similar to [`slice::fill`].
1273 ///
1274 /// # Panics
1275 ///
1276 /// This function will panic if any call to `Clone` panics.
1277 ///
1278 /// If such a panic occurs, any elements previously initialized during this operation will be
1279 /// dropped.
1280 ///
1281 /// # Examples
1282 ///
1283 /// ```
1284 /// #![feature(maybe_uninit_fill)]
1285 /// use std::mem::MaybeUninit;
1286 ///
1287 /// let mut buf = [const { MaybeUninit::uninit() }; 10];
1288 /// let initialized = buf.write_filled(1);
1289 /// assert_eq!(initialized, &mut [1; 10]);
1290 /// ```
1291 #[doc(alias = "memset")]
1292 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1293 pub fn write_filled(&mut self, value: T) -> &mut [T]
1294 where
1295 T: Clone,
1296 {
1297 SpecFill::spec_fill(self, value);
1298 // SAFETY: Valid elements have just been filled into `self` so it is initialized
1299 unsafe { self.assume_init_mut() }
1300 }
1301
1302 /// Fills a slice with elements returned by calling a closure for each index.
1303 ///
1304 /// This method uses a closure to create new values. If you'd rather `Clone` a given value, use
1305 /// [`MaybeUninit::fill`]. If you want to use the `Default` trait to generate values, you can
1306 /// pass [`|_| Default::default()`][Default::default] as the argument.
1307 ///
1308 /// # Panics
1309 ///
1310 /// This function will panic if any call to the provided closure panics.
1311 ///
1312 /// If such a panic occurs, any elements previously initialized during this operation will be
1313 /// dropped.
1314 ///
1315 /// # Examples
1316 ///
1317 /// ```
1318 /// #![feature(maybe_uninit_fill)]
1319 /// use std::mem::MaybeUninit;
1320 ///
1321 /// let mut buf = [const { MaybeUninit::<usize>::uninit() }; 5];
1322 /// let initialized = buf.write_with(|idx| idx + 1);
1323 /// assert_eq!(initialized, &mut [1, 2, 3, 4, 5]);
1324 /// ```
1325 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1326 pub fn write_with<F>(&mut self, mut f: F) -> &mut [T]
1327 where
1328 F: FnMut(usize) -> T,
1329 {
1330 let mut guard = Guard { slice: self, initialized: 0 };
1331
1332 for (idx, element) in guard.slice.iter_mut().enumerate() {
1333 element.write(f(idx));
1334 guard.initialized += 1;
1335 }
1336
1337 super::forget(guard);
1338
1339 // SAFETY: Valid elements have just been written into `this` so it is initialized
1340 unsafe { self.assume_init_mut() }
1341 }
1342
1343 /// Fills a slice with elements yielded by an iterator until either all elements have been
1344 /// initialized or the iterator is empty.
1345 ///
1346 /// Returns two slices. The first slice contains the initialized portion of the original slice.
1347 /// The second slice is the still-uninitialized remainder of the original slice.
1348 ///
1349 /// # Panics
1350 ///
1351 /// This function panics if the iterator's `next` function panics.
1352 ///
1353 /// If such a panic occurs, any elements previously initialized during this operation will be
1354 /// dropped.
1355 ///
1356 /// # Examples
1357 ///
1358 /// Completely filling the slice:
1359 ///
1360 /// ```
1361 /// #![feature(maybe_uninit_fill)]
1362 /// use std::mem::MaybeUninit;
1363 ///
1364 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1365 ///
1366 /// let iter = [1, 2, 3].into_iter().cycle();
1367 /// let (initialized, remainder) = buf.write_iter(iter);
1368 ///
1369 /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]);
1370 /// assert_eq!(remainder.len(), 0);
1371 /// ```
1372 ///
1373 /// Partially filling the slice:
1374 ///
1375 /// ```
1376 /// #![feature(maybe_uninit_fill)]
1377 /// use std::mem::MaybeUninit;
1378 ///
1379 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1380 /// let iter = [1, 2];
1381 /// let (initialized, remainder) = buf.write_iter(iter);
1382 ///
1383 /// assert_eq!(initialized, &mut [1, 2]);
1384 /// assert_eq!(remainder.len(), 3);
1385 /// ```
1386 ///
1387 /// Checking an iterator after filling a slice:
1388 ///
1389 /// ```
1390 /// #![feature(maybe_uninit_fill)]
1391 /// use std::mem::MaybeUninit;
1392 ///
1393 /// let mut buf = [const { MaybeUninit::uninit() }; 3];
1394 /// let mut iter = [1, 2, 3, 4, 5].into_iter();
1395 /// let (initialized, remainder) = buf.write_iter(iter.by_ref());
1396 ///
1397 /// assert_eq!(initialized, &mut [1, 2, 3]);
1398 /// assert_eq!(remainder.len(), 0);
1399 /// assert_eq!(iter.as_slice(), &[4, 5]);
1400 /// ```
1401 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1402 pub fn write_iter<I>(&mut self, it: I) -> (&mut [T], &mut [MaybeUninit<T>])
1403 where
1404 I: IntoIterator<Item = T>,
1405 {
1406 let iter = it.into_iter();
1407 let mut guard = Guard { slice: self, initialized: 0 };
1408
1409 for (element, val) in guard.slice.iter_mut().zip(iter) {
1410 element.write(val);
1411 guard.initialized += 1;
1412 }
1413
1414 let initialized_len = guard.initialized;
1415 super::forget(guard);
1416
1417 // SAFETY: guard.initialized <= self.len()
1418 let (initted, remainder) = unsafe { self.split_at_mut_unchecked(initialized_len) };
1419
1420 // SAFETY: Valid elements have just been written into `init`, so that portion
1421 // of `this` is initialized.
1422 (unsafe { initted.assume_init_mut() }, remainder)
1423 }
1424
1425 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1426 ///
1427 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1428 /// contain padding bytes which are left uninitialized.
1429 ///
1430 /// # Examples
1431 ///
1432 /// ```
1433 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1434 /// use std::mem::MaybeUninit;
1435 ///
1436 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1437 /// let uninit_bytes = uninit.as_bytes();
1438 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
1439 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1440 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1441 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1442 /// ```
1443 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1444 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1445 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1446 unsafe {
1447 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of_val(self))
1448 }
1449 }
1450
1451 /// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially
1452 /// uninitialized bytes.
1453 ///
1454 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1455 /// contain padding bytes which are left uninitialized.
1456 ///
1457 /// # Examples
1458 ///
1459 /// ```
1460 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1461 /// use std::mem::MaybeUninit;
1462 ///
1463 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1464 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1465 /// uninit_bytes.write_copy_of_slice(&[0x12, 0x34, 0x56, 0x78]);
1466 /// let vals = unsafe { uninit.assume_init_ref() };
1467 /// if cfg!(target_endian = "little") {
1468 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1469 /// } else {
1470 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1471 /// }
1472 /// ```
1473 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1474 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1475 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1476 unsafe {
1477 slice::from_raw_parts_mut(
1478 self.as_mut_ptr() as *mut MaybeUninit<u8>,
1479 super::size_of_val(self),
1480 )
1481 }
1482 }
1483
1484 /// Drops the contained values in place.
1485 ///
1486 /// # Safety
1487 ///
1488 /// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice
1489 /// really is in an initialized state. Calling this when the content is not yet
1490 /// fully initialized causes undefined behavior.
1491 ///
1492 /// On top of that, all additional invariants of the type `T` must be
1493 /// satisfied, as the `Drop` implementation of `T` (or its members) may
1494 /// rely on this. For example, setting a `Vec<T>` to an invalid but
1495 /// non-null address makes it initialized (under the current implementation;
1496 /// this does not constitute a stable guarantee), because the only
1497 /// requirement the compiler knows about it is that the data pointer must be
1498 /// non-null. Dropping such a `Vec<T>` however will cause undefined
1499 /// behaviour.
1500 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1501 #[inline(always)]
1502 pub unsafe fn assume_init_drop(&mut self) {
1503 if !self.is_empty() {
1504 // SAFETY: the caller must guarantee that every element of `self`
1505 // is initialized and satisfies all invariants of `T`.
1506 // Dropping the value in place is safe if that is the case.
1507 unsafe { ptr::drop_in_place(self as *mut [MaybeUninit<T>] as *mut [T]) }
1508 }
1509 }
1510
1511 /// Gets a shared reference to the contained value.
1512 ///
1513 /// # Safety
1514 ///
1515 /// Calling this when the content is not yet fully initialized causes undefined
1516 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in
1517 /// the slice really is in an initialized state.
1518 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1519 #[inline(always)]
1520 pub const unsafe fn assume_init_ref(&self) -> &[T] {
1521 // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
1522 // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
1523 // The pointer obtained is valid since it refers to memory owned by `slice` which is a
1524 // reference and thus guaranteed to be valid for reads.
1525 unsafe { &*(self as *const Self as *const [T]) }
1526 }
1527
1528 /// Gets a mutable (unique) reference to the contained value.
1529 ///
1530 /// # Safety
1531 ///
1532 /// Calling this when the content is not yet fully initialized causes undefined
1533 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
1534 /// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
1535 /// be used to initialize a `MaybeUninit` slice.
1536 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1537 #[inline(always)]
1538 pub const unsafe fn assume_init_mut(&mut self) -> &mut [T] {
1539 // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
1540 // mutable reference which is also guaranteed to be valid for writes.
1541 unsafe { &mut *(self as *mut Self as *mut [T]) }
1542 }
1543}
1544
1545impl<T, const N: usize> MaybeUninit<[T; N]> {
1546 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1547 ///
1548 /// # Examples
1549 ///
1550 /// ```
1551 /// #![feature(maybe_uninit_uninit_array_transpose)]
1552 /// # use std::mem::MaybeUninit;
1553 ///
1554 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1555 /// ```
1556 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1557 #[inline]
1558 pub const fn transpose(self) -> [MaybeUninit<T>; N] {
1559 // SAFETY: T and MaybeUninit<T> have the same layout
1560 unsafe { intrinsics::transmute_unchecked(self) }
1561 }
1562}
1563
1564impl<T, const N: usize> [MaybeUninit<T>; N] {
1565 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1566 ///
1567 /// # Examples
1568 ///
1569 /// ```
1570 /// #![feature(maybe_uninit_uninit_array_transpose)]
1571 /// # use std::mem::MaybeUninit;
1572 ///
1573 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1574 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1575 /// ```
1576 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1577 #[inline]
1578 pub const fn transpose(self) -> MaybeUninit<[T; N]> {
1579 // SAFETY: T and MaybeUninit<T> have the same layout
1580 unsafe { intrinsics::transmute_unchecked(self) }
1581 }
1582}
1583
1584struct Guard<'a, T> {
1585 slice: &'a mut [MaybeUninit<T>],
1586 initialized: usize,
1587}
1588
1589impl<'a, T> Drop for Guard<'a, T> {
1590 fn drop(&mut self) {
1591 let initialized_part: &mut [MaybeUninit] = &mut self.slice[..self.initialized];
1592 // SAFETY: this raw sub-slice will contain only initialized objects.
1593 unsafe {
1594 initialized_part.assume_init_drop();
1595 }
1596 }
1597}
1598
1599trait SpecFill<T> {
1600 fn spec_fill(&mut self, value: T);
1601}
1602
1603impl<T: Clone> SpecFill<T> for [MaybeUninit<T>] {
1604 default fn spec_fill(&mut self, value: T) {
1605 let mut guard: Guard<'_, T> = Guard { slice: self, initialized: 0 };
1606
1607 if let Some((last: &mut MaybeUninit, elems: &mut [MaybeUninit])) = guard.slice.split_last_mut() {
1608 for el: &mut MaybeUninit in elems {
1609 el.write(val:value.clone());
1610 guard.initialized += 1;
1611 }
1612
1613 last.write(val:value);
1614 }
1615 super::forget(guard);
1616 }
1617}
1618
1619impl<T: Copy> SpecFill<T> for [MaybeUninit<T>] {
1620 fn spec_fill(&mut self, value: T) {
1621 self.fill(MaybeUninit::new(val:value));
1622 }
1623}
1624

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