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/// 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 = 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 `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 `ManuallyDrop`, 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 // We do this via a raw ptr read instead of `ManuallyDrop::into_inner` so that there's
620 // no trace of `ManuallyDrop` in Miri's error messages here.
621 (&raw const self.value).cast::<T>().read()
622 }
623 }
624
625 /// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
626 /// to the usual drop handling.
627 ///
628 /// Whenever possible, it is preferable to use [`assume_init`] instead, which
629 /// prevents duplicating the content of the `MaybeUninit<T>`.
630 ///
631 /// # Safety
632 ///
633 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
634 /// state. Calling this when the content is not yet fully initialized causes undefined
635 /// behavior. The [type-level documentation][inv] contains more information about
636 /// this initialization invariant.
637 ///
638 /// Moreover, similar to the [`ptr::read`] function, this function creates a
639 /// bitwise copy of the contents, regardless whether the contained type
640 /// implements the [`Copy`] trait or not. When using multiple copies of the
641 /// data (by calling `assume_init_read` multiple times, or first calling
642 /// `assume_init_read` and then [`assume_init`]), it is your responsibility
643 /// to ensure that data may indeed be duplicated.
644 ///
645 /// [inv]: #initialization-invariant
646 /// [`assume_init`]: MaybeUninit::assume_init
647 ///
648 /// # Examples
649 ///
650 /// Correct usage of this method:
651 ///
652 /// ```rust
653 /// use std::mem::MaybeUninit;
654 ///
655 /// let mut x = MaybeUninit::<u32>::uninit();
656 /// x.write(13);
657 /// let x1 = unsafe { x.assume_init_read() };
658 /// // `u32` is `Copy`, so we may read multiple times.
659 /// let x2 = unsafe { x.assume_init_read() };
660 /// assert_eq!(x1, x2);
661 ///
662 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
663 /// x.write(None);
664 /// let x1 = unsafe { x.assume_init_read() };
665 /// // Duplicating a `None` value is okay, so we may read multiple times.
666 /// let x2 = unsafe { x.assume_init_read() };
667 /// assert_eq!(x1, x2);
668 /// ```
669 ///
670 /// *Incorrect* usage of this method:
671 ///
672 /// ```rust,no_run
673 /// use std::mem::MaybeUninit;
674 ///
675 /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
676 /// x.write(Some(vec![0, 1, 2]));
677 /// let x1 = unsafe { x.assume_init_read() };
678 /// let x2 = unsafe { x.assume_init_read() };
679 /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
680 /// // they both get dropped!
681 /// ```
682 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
683 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_read", since = "1.75.0")]
684 #[inline(always)]
685 #[track_caller]
686 pub const unsafe fn assume_init_read(&self) -> T {
687 // SAFETY: the caller must guarantee that `self` is initialized.
688 // Reading from `self.as_ptr()` is safe since `self` should be initialized.
689 unsafe {
690 intrinsics::assert_inhabited::<T>();
691 self.as_ptr().read()
692 }
693 }
694
695 /// Drops the contained value in place.
696 ///
697 /// If you have ownership of the `MaybeUninit`, you can also use
698 /// [`assume_init`] as an alternative.
699 ///
700 /// # Safety
701 ///
702 /// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
703 /// in an initialized state. Calling this when the content is not yet fully
704 /// initialized causes undefined behavior.
705 ///
706 /// On top of that, all additional invariants of the type `T` must be
707 /// satisfied, as the `Drop` implementation of `T` (or its members) may
708 /// rely on this. For example, setting a `Vec<T>` to an invalid but
709 /// non-null address makes it initialized (under the current implementation;
710 /// this does not constitute a stable guarantee), because the only
711 /// requirement the compiler knows about it is that the data pointer must be
712 /// non-null. Dropping such a `Vec<T>` however will cause undefined
713 /// behavior.
714 ///
715 /// [`assume_init`]: MaybeUninit::assume_init
716 #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
717 pub unsafe fn assume_init_drop(&mut self) {
718 // SAFETY: the caller must guarantee that `self` is initialized and
719 // satisfies all invariants of `T`.
720 // Dropping the value in place is safe if that is the case.
721 unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
722 }
723
724 /// Gets a shared reference to the contained value.
725 ///
726 /// This can be useful when we want to access a `MaybeUninit` that has been
727 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
728 /// of `.assume_init()`).
729 ///
730 /// # Safety
731 ///
732 /// Calling this when the content is not yet fully initialized causes undefined
733 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
734 /// is in an initialized state.
735 ///
736 /// # Examples
737 ///
738 /// ### Correct usage of this method:
739 ///
740 /// ```rust
741 /// use std::mem::MaybeUninit;
742 ///
743 /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
744 /// # let mut x_mu = x;
745 /// # let mut x = &mut x_mu;
746 /// // Initialize `x`:
747 /// x.write(vec![1, 2, 3]);
748 /// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
749 /// // create a shared reference to it:
750 /// let x: &Vec<u32> = unsafe {
751 /// // SAFETY: `x` has been initialized.
752 /// x.assume_init_ref()
753 /// };
754 /// assert_eq!(x, &vec![1, 2, 3]);
755 /// # // Prevent leaks for Miri
756 /// # unsafe { MaybeUninit::assume_init_drop(&mut x_mu); }
757 /// ```
758 ///
759 /// ### *Incorrect* usages of this method:
760 ///
761 /// ```rust,no_run
762 /// use std::mem::MaybeUninit;
763 ///
764 /// let x = MaybeUninit::<Vec<u32>>::uninit();
765 /// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
766 /// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
767 /// ```
768 ///
769 /// ```rust,no_run
770 /// use std::{cell::Cell, mem::MaybeUninit};
771 ///
772 /// let b = MaybeUninit::<Cell<bool>>::uninit();
773 /// // Initialize the `MaybeUninit` using `Cell::set`:
774 /// unsafe {
775 /// b.assume_init_ref().set(true);
776 /// // ^^^^^^^^^^^^^^^
777 /// // Reference to an uninitialized `Cell<bool>`: UB!
778 /// }
779 /// ```
780 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
781 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
782 #[inline(always)]
783 pub const unsafe fn assume_init_ref(&self) -> &T {
784 // SAFETY: the caller must guarantee that `self` is initialized.
785 // This also means that `self` must be a `value` variant.
786 unsafe {
787 intrinsics::assert_inhabited::<T>();
788 &*self.as_ptr()
789 }
790 }
791
792 /// Gets a mutable (unique) reference to the contained value.
793 ///
794 /// This can be useful when we want to access a `MaybeUninit` that has been
795 /// initialized but don't have ownership of the `MaybeUninit` (preventing the use
796 /// of `.assume_init()`).
797 ///
798 /// # Safety
799 ///
800 /// Calling this when the content is not yet fully initialized causes undefined
801 /// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
802 /// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
803 /// initialize a `MaybeUninit`.
804 ///
805 /// # Examples
806 ///
807 /// ### Correct usage of this method:
808 ///
809 /// ```rust
810 /// # #![allow(unexpected_cfgs)]
811 /// use std::mem::MaybeUninit;
812 ///
813 /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { unsafe { *buf = [0; 1024] } }
814 /// # #[cfg(FALSE)]
815 /// extern "C" {
816 /// /// Initializes *all* the bytes of the input buffer.
817 /// fn initialize_buffer(buf: *mut [u8; 1024]);
818 /// }
819 ///
820 /// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
821 ///
822 /// // Initialize `buf`:
823 /// unsafe { initialize_buffer(buf.as_mut_ptr()); }
824 /// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
825 /// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
826 /// // To assert our buffer has been initialized without copying it, we upgrade
827 /// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
828 /// let buf: &mut [u8; 1024] = unsafe {
829 /// // SAFETY: `buf` has been initialized.
830 /// buf.assume_init_mut()
831 /// };
832 ///
833 /// // Now we can use `buf` as a normal slice:
834 /// buf.sort_unstable();
835 /// assert!(
836 /// buf.windows(2).all(|pair| pair[0] <= pair[1]),
837 /// "buffer is sorted",
838 /// );
839 /// ```
840 ///
841 /// ### *Incorrect* usages of this method:
842 ///
843 /// You cannot use `.assume_init_mut()` to initialize a value:
844 ///
845 /// ```rust,no_run
846 /// use std::mem::MaybeUninit;
847 ///
848 /// let mut b = MaybeUninit::<bool>::uninit();
849 /// unsafe {
850 /// *b.assume_init_mut() = true;
851 /// // We have created a (mutable) reference to an uninitialized `bool`!
852 /// // This is undefined behavior. ⚠️
853 /// }
854 /// ```
855 ///
856 /// For instance, you cannot [`Read`] into an uninitialized buffer:
857 ///
858 /// [`Read`]: ../../std/io/trait.Read.html
859 ///
860 /// ```rust,no_run
861 /// use std::{io, mem::MaybeUninit};
862 ///
863 /// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
864 /// {
865 /// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
866 /// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
867 /// // ^^^^^^^^^^^^^^^^^^^^^^^^
868 /// // (mutable) reference to uninitialized memory!
869 /// // This is undefined behavior.
870 /// Ok(unsafe { buffer.assume_init() })
871 /// }
872 /// ```
873 ///
874 /// Nor can you use direct field access to do field-by-field gradual initialization:
875 ///
876 /// ```rust,no_run
877 /// use std::{mem::MaybeUninit, ptr};
878 ///
879 /// struct Foo {
880 /// a: u32,
881 /// b: u8,
882 /// }
883 ///
884 /// let foo: Foo = unsafe {
885 /// let mut foo = MaybeUninit::<Foo>::uninit();
886 /// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
887 /// // ^^^^^^^^^^^^^^^^^^^^^
888 /// // (mutable) reference to uninitialized memory!
889 /// // This is undefined behavior.
890 /// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
891 /// // ^^^^^^^^^^^^^^^^^^^^^
892 /// // (mutable) reference to uninitialized memory!
893 /// // This is undefined behavior.
894 /// foo.assume_init()
895 /// };
896 /// ```
897 #[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
898 #[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.84.0")]
899 #[inline(always)]
900 pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
901 // SAFETY: the caller must guarantee that `self` is initialized.
902 // This also means that `self` must be a `value` variant.
903 unsafe {
904 intrinsics::assert_inhabited::<T>();
905 &mut *self.as_mut_ptr()
906 }
907 }
908
909 /// Extracts the values from an array of `MaybeUninit` containers.
910 ///
911 /// # Safety
912 ///
913 /// It is up to the caller to guarantee that all elements of the array are
914 /// in an initialized state.
915 ///
916 /// # Examples
917 ///
918 /// ```
919 /// #![feature(maybe_uninit_array_assume_init)]
920 /// use std::mem::MaybeUninit;
921 ///
922 /// let mut array: [MaybeUninit<i32>; 3] = [MaybeUninit::uninit(); 3];
923 /// array[0].write(0);
924 /// array[1].write(1);
925 /// array[2].write(2);
926 ///
927 /// // SAFETY: Now safe as we initialised all elements
928 /// let array = unsafe {
929 /// MaybeUninit::array_assume_init(array)
930 /// };
931 ///
932 /// assert_eq!(array, [0, 1, 2]);
933 /// ```
934 #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
935 #[inline(always)]
936 #[track_caller]
937 pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
938 // SAFETY:
939 // * The caller guarantees that all elements of the array are initialized
940 // * `MaybeUninit<T>` and T are guaranteed to have the same layout
941 // * `MaybeUninit` does not drop, so there are no double-frees
942 // And thus the conversion is safe
943 unsafe {
944 intrinsics::assert_inhabited::<[T; N]>();
945 intrinsics::transmute_unchecked(array)
946 }
947 }
948
949 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
950 ///
951 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
952 /// contain padding bytes which are left uninitialized.
953 ///
954 /// # Examples
955 ///
956 /// ```
957 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
958 /// use std::mem::MaybeUninit;
959 ///
960 /// let val = 0x12345678_i32;
961 /// let uninit = MaybeUninit::new(val);
962 /// let uninit_bytes = uninit.as_bytes();
963 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
964 /// assert_eq!(bytes, val.to_ne_bytes());
965 /// ```
966 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
967 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
968 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
969 unsafe {
970 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of::<T>())
971 }
972 }
973
974 /// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
975 /// bytes.
976 ///
977 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
978 /// contain padding bytes which are left uninitialized.
979 ///
980 /// # Examples
981 ///
982 /// ```
983 /// #![feature(maybe_uninit_as_bytes)]
984 /// use std::mem::MaybeUninit;
985 ///
986 /// let val = 0x12345678_i32;
987 /// let mut uninit = MaybeUninit::new(val);
988 /// let uninit_bytes = uninit.as_bytes_mut();
989 /// if cfg!(target_endian = "little") {
990 /// uninit_bytes[0].write(0xcd);
991 /// } else {
992 /// uninit_bytes[3].write(0xcd);
993 /// }
994 /// let val2 = unsafe { uninit.assume_init() };
995 /// assert_eq!(val2, 0x123456cd);
996 /// ```
997 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
998 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
999 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1000 unsafe {
1001 slice::from_raw_parts_mut(
1002 self.as_mut_ptr().cast::<MaybeUninit<u8>>(),
1003 super::size_of::<T>(),
1004 )
1005 }
1006 }
1007
1008 /// Deprecated version of [`slice::assume_init_ref`].
1009 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1010 #[deprecated(
1011 note = "replaced by inherent assume_init_ref method; will eventually be removed",
1012 since = "1.83.0"
1013 )]
1014 pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
1015 // SAFETY: Same for both methods.
1016 unsafe { slice.assume_init_ref() }
1017 }
1018
1019 /// Deprecated version of [`slice::assume_init_mut`].
1020 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1021 #[deprecated(
1022 note = "replaced by inherent assume_init_mut method; will eventually be removed",
1023 since = "1.83.0"
1024 )]
1025 pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
1026 // SAFETY: Same for both methods.
1027 unsafe { slice.assume_init_mut() }
1028 }
1029
1030 /// Gets a pointer to the first element of the array.
1031 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1032 #[inline(always)]
1033 pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
1034 this.as_ptr() as *const T
1035 }
1036
1037 /// Gets a mutable pointer to the first element of the array.
1038 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1039 #[inline(always)]
1040 pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
1041 this.as_mut_ptr() as *mut T
1042 }
1043
1044 /// Deprecated version of [`slice::write_copy_of_slice`].
1045 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1046 #[deprecated(
1047 note = "replaced by inherent write_copy_of_slice method; will eventually be removed",
1048 since = "1.83.0"
1049 )]
1050 pub fn copy_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1051 where
1052 T: Copy,
1053 {
1054 this.write_copy_of_slice(src)
1055 }
1056
1057 /// Deprecated version of [`slice::write_clone_of_slice`].
1058 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1059 #[deprecated(
1060 note = "replaced by inherent write_clone_of_slice method; will eventually be removed",
1061 since = "1.83.0"
1062 )]
1063 pub fn clone_from_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
1064 where
1065 T: Clone,
1066 {
1067 this.write_clone_of_slice(src)
1068 }
1069
1070 /// Deprecated version of [`slice::write_filled`].
1071 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1072 #[deprecated(
1073 note = "replaced by inherent write_filled method; will eventually be removed",
1074 since = "1.83.0"
1075 )]
1076 pub fn fill<'a>(this: &'a mut [MaybeUninit<T>], value: T) -> &'a mut [T]
1077 where
1078 T: Clone,
1079 {
1080 this.write_filled(value)
1081 }
1082
1083 /// Deprecated version of [`slice::write_with`].
1084 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1085 #[deprecated(
1086 note = "replaced by inherent write_with method; will eventually be removed",
1087 since = "1.83.0"
1088 )]
1089 pub fn fill_with<'a, F>(this: &'a mut [MaybeUninit<T>], mut f: F) -> &'a mut [T]
1090 where
1091 F: FnMut() -> T,
1092 {
1093 this.write_with(|_| f())
1094 }
1095
1096 /// Deprecated version of [`slice::write_iter`].
1097 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1098 #[deprecated(
1099 note = "replaced by inherent write_iter method; will eventually be removed",
1100 since = "1.83.0"
1101 )]
1102 pub fn fill_from<'a, I>(
1103 this: &'a mut [MaybeUninit<T>],
1104 it: I,
1105 ) -> (&'a mut [T], &'a mut [MaybeUninit<T>])
1106 where
1107 I: IntoIterator<Item = T>,
1108 {
1109 this.write_iter(it)
1110 }
1111
1112 /// Deprecated version of [`slice::as_bytes`].
1113 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1114 #[deprecated(
1115 note = "replaced by inherent as_bytes method; will eventually be removed",
1116 since = "1.83.0"
1117 )]
1118 pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
1119 this.as_bytes()
1120 }
1121
1122 /// Deprecated version of [`slice::as_bytes_mut`].
1123 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1124 #[deprecated(
1125 note = "replaced by inherent as_bytes_mut method; will eventually be removed",
1126 since = "1.83.0"
1127 )]
1128 pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
1129 this.as_bytes_mut()
1130 }
1131}
1132
1133impl<T> [MaybeUninit<T>] {
1134 /// Copies the elements from `src` to `self`,
1135 /// returning a mutable reference to the now initialized contents of `self`.
1136 ///
1137 /// If `T` does not implement `Copy`, use [`write_clone_of_slice`] instead.
1138 ///
1139 /// This is similar to [`slice::copy_from_slice`].
1140 ///
1141 /// # Panics
1142 ///
1143 /// This function will panic if the two slices have different lengths.
1144 ///
1145 /// # Examples
1146 ///
1147 /// ```
1148 /// #![feature(maybe_uninit_write_slice)]
1149 /// use std::mem::MaybeUninit;
1150 ///
1151 /// let mut dst = [MaybeUninit::uninit(); 32];
1152 /// let src = [0; 32];
1153 ///
1154 /// let init = dst.write_copy_of_slice(&src);
1155 ///
1156 /// assert_eq!(init, src);
1157 /// ```
1158 ///
1159 /// ```
1160 /// #![feature(maybe_uninit_write_slice)]
1161 ///
1162 /// let mut vec = Vec::with_capacity(32);
1163 /// let src = [0; 16];
1164 ///
1165 /// vec.spare_capacity_mut()[..src.len()].write_copy_of_slice(&src);
1166 ///
1167 /// // SAFETY: we have just copied all the elements of len into the spare capacity
1168 /// // the first src.len() elements of the vec are valid now.
1169 /// unsafe {
1170 /// vec.set_len(src.len());
1171 /// }
1172 ///
1173 /// assert_eq!(vec, src);
1174 /// ```
1175 ///
1176 /// [`write_clone_of_slice`]: slice::write_clone_of_slice
1177 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1178 pub const fn write_copy_of_slice(&mut self, src: &[T]) -> &mut [T]
1179 where
1180 T: Copy,
1181 {
1182 // SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
1183 let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
1184
1185 self.copy_from_slice(uninit_src);
1186
1187 // SAFETY: Valid elements have just been copied into `self` so it is initialized
1188 unsafe { self.assume_init_mut() }
1189 }
1190
1191 /// Clones the elements from `src` to `self`,
1192 /// returning a mutable reference to the now initialized contents of `self`.
1193 /// Any already initialized elements will not be dropped.
1194 ///
1195 /// If `T` implements `Copy`, use [`write_copy_of_slice`] instead.
1196 ///
1197 /// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
1198 ///
1199 /// # Panics
1200 ///
1201 /// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
1202 ///
1203 /// If there is a panic, the already cloned elements will be dropped.
1204 ///
1205 /// # Examples
1206 ///
1207 /// ```
1208 /// #![feature(maybe_uninit_write_slice)]
1209 /// use std::mem::MaybeUninit;
1210 ///
1211 /// let mut dst = [const { MaybeUninit::uninit() }; 5];
1212 /// let src = ["wibbly", "wobbly", "timey", "wimey", "stuff"].map(|s| s.to_string());
1213 ///
1214 /// let init = dst.write_clone_of_slice(&src);
1215 ///
1216 /// assert_eq!(init, src);
1217 ///
1218 /// # // Prevent leaks for Miri
1219 /// # unsafe { std::ptr::drop_in_place(init); }
1220 /// ```
1221 ///
1222 /// ```
1223 /// #![feature(maybe_uninit_write_slice)]
1224 ///
1225 /// let mut vec = Vec::with_capacity(32);
1226 /// let src = ["rust", "is", "a", "pretty", "cool", "language"].map(|s| s.to_string());
1227 ///
1228 /// vec.spare_capacity_mut()[..src.len()].write_clone_of_slice(&src);
1229 ///
1230 /// // SAFETY: we have just cloned all the elements of len into the spare capacity
1231 /// // the first src.len() elements of the vec are valid now.
1232 /// unsafe {
1233 /// vec.set_len(src.len());
1234 /// }
1235 ///
1236 /// assert_eq!(vec, src);
1237 /// ```
1238 ///
1239 /// [`write_copy_of_slice`]: slice::write_copy_of_slice
1240 #[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
1241 pub fn write_clone_of_slice(&mut self, src: &[T]) -> &mut [T]
1242 where
1243 T: Clone,
1244 {
1245 // unlike copy_from_slice this does not call clone_from_slice on the slice
1246 // this is because `MaybeUninit<T: Clone>` does not implement Clone.
1247
1248 assert_eq!(self.len(), src.len(), "destination and source slices have different lengths");
1249
1250 // NOTE: We need to explicitly slice them to the same length
1251 // for bounds checking to be elided, and the optimizer will
1252 // generate memcpy for simple cases (for example T = u8).
1253 let len = self.len();
1254 let src = &src[..len];
1255
1256 // guard is needed b/c panic might happen during a clone
1257 let mut guard = Guard { slice: self, initialized: 0 };
1258
1259 for i in 0..len {
1260 guard.slice[i].write(src[i].clone());
1261 guard.initialized += 1;
1262 }
1263
1264 super::forget(guard);
1265
1266 // SAFETY: Valid elements have just been written into `self` so it is initialized
1267 unsafe { self.assume_init_mut() }
1268 }
1269
1270 /// Fills a slice with elements by cloning `value`, returning a mutable reference to the now
1271 /// initialized contents of the slice.
1272 /// Any previously initialized elements will not be dropped.
1273 ///
1274 /// This is similar to [`slice::fill`].
1275 ///
1276 /// # Panics
1277 ///
1278 /// This function will panic if any call to `Clone` panics.
1279 ///
1280 /// If such a panic occurs, any elements previously initialized during this operation will be
1281 /// dropped.
1282 ///
1283 /// # Examples
1284 ///
1285 /// ```
1286 /// #![feature(maybe_uninit_fill)]
1287 /// use std::mem::MaybeUninit;
1288 ///
1289 /// let mut buf = [const { MaybeUninit::uninit() }; 10];
1290 /// let initialized = buf.write_filled(1);
1291 /// assert_eq!(initialized, &mut [1; 10]);
1292 /// ```
1293 #[doc(alias = "memset")]
1294 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1295 pub fn write_filled(&mut self, value: T) -> &mut [T]
1296 where
1297 T: Clone,
1298 {
1299 SpecFill::spec_fill(self, value);
1300 // SAFETY: Valid elements have just been filled into `self` so it is initialized
1301 unsafe { self.assume_init_mut() }
1302 }
1303
1304 /// Fills a slice with elements returned by calling a closure for each index.
1305 ///
1306 /// This method uses a closure to create new values. If you'd rather `Clone` a given value, use
1307 /// [`MaybeUninit::fill`]. If you want to use the `Default` trait to generate values, you can
1308 /// pass [`|_| Default::default()`][Default::default] as the argument.
1309 ///
1310 /// # Panics
1311 ///
1312 /// This function will panic if any call to the provided closure panics.
1313 ///
1314 /// If such a panic occurs, any elements previously initialized during this operation will be
1315 /// dropped.
1316 ///
1317 /// # Examples
1318 ///
1319 /// ```
1320 /// #![feature(maybe_uninit_fill)]
1321 /// use std::mem::MaybeUninit;
1322 ///
1323 /// let mut buf = [const { MaybeUninit::<usize>::uninit() }; 5];
1324 /// let initialized = buf.write_with(|idx| idx + 1);
1325 /// assert_eq!(initialized, &mut [1, 2, 3, 4, 5]);
1326 /// ```
1327 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1328 pub fn write_with<F>(&mut self, mut f: F) -> &mut [T]
1329 where
1330 F: FnMut(usize) -> T,
1331 {
1332 let mut guard = Guard { slice: self, initialized: 0 };
1333
1334 for (idx, element) in guard.slice.iter_mut().enumerate() {
1335 element.write(f(idx));
1336 guard.initialized += 1;
1337 }
1338
1339 super::forget(guard);
1340
1341 // SAFETY: Valid elements have just been written into `this` so it is initialized
1342 unsafe { self.assume_init_mut() }
1343 }
1344
1345 /// Fills a slice with elements yielded by an iterator until either all elements have been
1346 /// initialized or the iterator is empty.
1347 ///
1348 /// Returns two slices. The first slice contains the initialized portion of the original slice.
1349 /// The second slice is the still-uninitialized remainder of the original slice.
1350 ///
1351 /// # Panics
1352 ///
1353 /// This function panics if the iterator's `next` function panics.
1354 ///
1355 /// If such a panic occurs, any elements previously initialized during this operation will be
1356 /// dropped.
1357 ///
1358 /// # Examples
1359 ///
1360 /// Completely filling the slice:
1361 ///
1362 /// ```
1363 /// #![feature(maybe_uninit_fill)]
1364 /// use std::mem::MaybeUninit;
1365 ///
1366 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1367 ///
1368 /// let iter = [1, 2, 3].into_iter().cycle();
1369 /// let (initialized, remainder) = buf.write_iter(iter);
1370 ///
1371 /// assert_eq!(initialized, &mut [1, 2, 3, 1, 2]);
1372 /// assert_eq!(remainder.len(), 0);
1373 /// ```
1374 ///
1375 /// Partially filling the slice:
1376 ///
1377 /// ```
1378 /// #![feature(maybe_uninit_fill)]
1379 /// use std::mem::MaybeUninit;
1380 ///
1381 /// let mut buf = [const { MaybeUninit::uninit() }; 5];
1382 /// let iter = [1, 2];
1383 /// let (initialized, remainder) = buf.write_iter(iter);
1384 ///
1385 /// assert_eq!(initialized, &mut [1, 2]);
1386 /// assert_eq!(remainder.len(), 3);
1387 /// ```
1388 ///
1389 /// Checking an iterator after filling a slice:
1390 ///
1391 /// ```
1392 /// #![feature(maybe_uninit_fill)]
1393 /// use std::mem::MaybeUninit;
1394 ///
1395 /// let mut buf = [const { MaybeUninit::uninit() }; 3];
1396 /// let mut iter = [1, 2, 3, 4, 5].into_iter();
1397 /// let (initialized, remainder) = buf.write_iter(iter.by_ref());
1398 ///
1399 /// assert_eq!(initialized, &mut [1, 2, 3]);
1400 /// assert_eq!(remainder.len(), 0);
1401 /// assert_eq!(iter.as_slice(), &[4, 5]);
1402 /// ```
1403 #[unstable(feature = "maybe_uninit_fill", issue = "117428")]
1404 pub fn write_iter<I>(&mut self, it: I) -> (&mut [T], &mut [MaybeUninit<T>])
1405 where
1406 I: IntoIterator<Item = T>,
1407 {
1408 let iter = it.into_iter();
1409 let mut guard = Guard { slice: self, initialized: 0 };
1410
1411 for (element, val) in guard.slice.iter_mut().zip(iter) {
1412 element.write(val);
1413 guard.initialized += 1;
1414 }
1415
1416 let initialized_len = guard.initialized;
1417 super::forget(guard);
1418
1419 // SAFETY: guard.initialized <= self.len()
1420 let (initted, remainder) = unsafe { self.split_at_mut_unchecked(initialized_len) };
1421
1422 // SAFETY: Valid elements have just been written into `init`, so that portion
1423 // of `this` is initialized.
1424 (unsafe { initted.assume_init_mut() }, remainder)
1425 }
1426
1427 /// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
1428 ///
1429 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1430 /// contain padding bytes which are left uninitialized.
1431 ///
1432 /// # Examples
1433 ///
1434 /// ```
1435 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1436 /// use std::mem::MaybeUninit;
1437 ///
1438 /// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
1439 /// let uninit_bytes = uninit.as_bytes();
1440 /// let bytes = unsafe { uninit_bytes.assume_init_ref() };
1441 /// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
1442 /// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
1443 /// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
1444 /// ```
1445 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1446 pub const fn as_bytes(&self) -> &[MaybeUninit<u8>] {
1447 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1448 unsafe {
1449 slice::from_raw_parts(self.as_ptr().cast::<MaybeUninit<u8>>(), super::size_of_val(self))
1450 }
1451 }
1452
1453 /// Returns the contents of this `MaybeUninit` slice as a mutable slice of potentially
1454 /// uninitialized bytes.
1455 ///
1456 /// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
1457 /// contain padding bytes which are left uninitialized.
1458 ///
1459 /// # Examples
1460 ///
1461 /// ```
1462 /// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
1463 /// use std::mem::MaybeUninit;
1464 ///
1465 /// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
1466 /// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
1467 /// uninit_bytes.write_copy_of_slice(&[0x12, 0x34, 0x56, 0x78]);
1468 /// let vals = unsafe { uninit.assume_init_ref() };
1469 /// if cfg!(target_endian = "little") {
1470 /// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
1471 /// } else {
1472 /// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
1473 /// }
1474 /// ```
1475 #[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
1476 pub const fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
1477 // SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
1478 unsafe {
1479 slice::from_raw_parts_mut(
1480 self.as_mut_ptr() as *mut MaybeUninit<u8>,
1481 super::size_of_val(self),
1482 )
1483 }
1484 }
1485
1486 /// Drops the contained values in place.
1487 ///
1488 /// # Safety
1489 ///
1490 /// It is up to the caller to guarantee that every `MaybeUninit<T>` in the slice
1491 /// really is in an initialized state. Calling this when the content is not yet
1492 /// fully initialized causes undefined behavior.
1493 ///
1494 /// On top of that, all additional invariants of the type `T` must be
1495 /// satisfied, as the `Drop` implementation of `T` (or its members) may
1496 /// rely on this. For example, setting a `Vec<T>` to an invalid but
1497 /// non-null address makes it initialized (under the current implementation;
1498 /// this does not constitute a stable guarantee), because the only
1499 /// requirement the compiler knows about it is that the data pointer must be
1500 /// non-null. Dropping such a `Vec<T>` however will cause undefined
1501 /// behaviour.
1502 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1503 #[inline(always)]
1504 pub unsafe fn assume_init_drop(&mut self) {
1505 if !self.is_empty() {
1506 // SAFETY: the caller must guarantee that every element of `self`
1507 // is initialized and satisfies all invariants of `T`.
1508 // Dropping the value in place is safe if that is the case.
1509 unsafe { ptr::drop_in_place(self as *mut [MaybeUninit<T>] as *mut [T]) }
1510 }
1511 }
1512
1513 /// Gets a shared reference to the contained value.
1514 ///
1515 /// # Safety
1516 ///
1517 /// Calling this when the content is not yet fully initialized causes undefined
1518 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in
1519 /// the slice really is in an initialized state.
1520 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1521 #[inline(always)]
1522 pub const unsafe fn assume_init_ref(&self) -> &[T] {
1523 // SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
1524 // `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
1525 // The pointer obtained is valid since it refers to memory owned by `slice` which is a
1526 // reference and thus guaranteed to be valid for reads.
1527 unsafe { &*(self as *const Self as *const [T]) }
1528 }
1529
1530 /// Gets a mutable (unique) reference to the contained value.
1531 ///
1532 /// # Safety
1533 ///
1534 /// Calling this when the content is not yet fully initialized causes undefined
1535 /// behavior: it is up to the caller to guarantee that every `MaybeUninit<T>` in the
1536 /// slice really is in an initialized state. For instance, `.assume_init_mut()` cannot
1537 /// be used to initialize a `MaybeUninit` slice.
1538 #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
1539 #[inline(always)]
1540 pub const unsafe fn assume_init_mut(&mut self) -> &mut [T] {
1541 // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
1542 // mutable reference which is also guaranteed to be valid for writes.
1543 unsafe { &mut *(self as *mut Self as *mut [T]) }
1544 }
1545}
1546
1547impl<T, const N: usize> MaybeUninit<[T; N]> {
1548 /// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
1549 ///
1550 /// # Examples
1551 ///
1552 /// ```
1553 /// #![feature(maybe_uninit_uninit_array_transpose)]
1554 /// # use std::mem::MaybeUninit;
1555 ///
1556 /// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
1557 /// ```
1558 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1559 #[inline]
1560 pub const fn transpose(self) -> [MaybeUninit<T>; N] {
1561 // SAFETY: T and MaybeUninit<T> have the same layout
1562 unsafe { intrinsics::transmute_unchecked(self) }
1563 }
1564}
1565
1566impl<T, const N: usize> [MaybeUninit<T>; N] {
1567 /// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
1568 ///
1569 /// # Examples
1570 ///
1571 /// ```
1572 /// #![feature(maybe_uninit_uninit_array_transpose)]
1573 /// # use std::mem::MaybeUninit;
1574 ///
1575 /// let data = [MaybeUninit::<u8>::uninit(); 1000];
1576 /// let data: MaybeUninit<[u8; 1000]> = data.transpose();
1577 /// ```
1578 #[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
1579 #[inline]
1580 pub const fn transpose(self) -> MaybeUninit<[T; N]> {
1581 // SAFETY: T and MaybeUninit<T> have the same layout
1582 unsafe { intrinsics::transmute_unchecked(self) }
1583 }
1584}
1585
1586struct Guard<'a, T> {
1587 slice: &'a mut [MaybeUninit<T>],
1588 initialized: usize,
1589}
1590
1591impl<'a, T> Drop for Guard<'a, T> {
1592 fn drop(&mut self) {
1593 let initialized_part = &mut self.slice[..self.initialized];
1594 // SAFETY: this raw sub-slice will contain only initialized objects.
1595 unsafe {
1596 initialized_part.assume_init_drop();
1597 }
1598 }
1599}
1600
1601trait SpecFill<T> {
1602 fn spec_fill(&mut self, value: T);
1603}
1604
1605impl<T: Clone> SpecFill<T> for [MaybeUninit<T>] {
1606 default fn spec_fill(&mut self, value: T) {
1607 let mut guard = Guard { slice: self, initialized: 0 };
1608
1609 if let Some((last, elems)) = guard.slice.split_last_mut() {
1610 for el in elems {
1611 el.write(value.clone());
1612 guard.initialized += 1;
1613 }
1614
1615 last.write(value);
1616 }
1617 super::forget(guard);
1618 }
1619}
1620
1621impl<T: Copy> SpecFill<T> for [MaybeUninit<T>] {
1622 fn spec_fill(&mut self, value: T) {
1623 self.fill(MaybeUninit::new(value));
1624 }
1625}