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use crate::any::type_name;
use crate::fmt;
use crate::intrinsics;
use crate::mem::{self, ManuallyDrop};
use crate::ptr;
use crate::slice;
/// A wrapper type to construct uninitialized instances of `T`.
///
/// # Initialization invariant
///
/// The compiler, in general, assumes that a variable is properly initialized
/// according to the requirements of the variable's type. For example, a variable of
/// reference type must be aligned and non-null. This is an invariant that must
/// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
/// variable of reference type causes instantaneous [undefined behavior][ub],
/// no matter whether that reference ever gets used to access memory:
///
/// ```rust,no_run
/// # #![allow(invalid_value)]
/// use std::mem::{self, MaybeUninit};
///
/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior! ⚠️
/// // The equivalent code with `MaybeUninit<&i32>`:
/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior! ⚠️
/// ```
///
/// This is exploited by the compiler for various optimizations, such as eliding
/// run-time checks and optimizing `enum` layout.
///
/// Similarly, entirely uninitialized memory may have any content, while a `bool` must
/// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
///
/// ```rust,no_run
/// # #![allow(invalid_value)]
/// use std::mem::{self, MaybeUninit};
///
/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
/// // The equivalent code with `MaybeUninit<bool>`:
/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
/// ```
///
/// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed"
/// meaning "it won't change without being written to"). Reading the same uninitialized byte
/// multiple times can give different results. This makes it undefined behavior to have
/// uninitialized data in a variable even if that variable has an integer type, which otherwise can
/// hold any *fixed* bit pattern:
///
/// ```rust,no_run
/// # #![allow(invalid_value)]
/// use std::mem::{self, MaybeUninit};
///
/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
/// // The equivalent code with `MaybeUninit<i32>`:
/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
/// ```
/// On top of that, remember that most types have additional invariants beyond merely
/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
/// is considered initialized (under the current implementation; this does not constitute
/// a stable guarantee) because the only requirement the compiler knows about it
/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
/// *immediate* undefined behavior, but will cause undefined behavior with most
/// safe operations (including dropping it).
///
/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
///
/// # Examples
///
/// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
/// It is a signal to the compiler indicating that the data here might *not*
/// be initialized:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// // Create an explicitly uninitialized reference. The compiler knows that data inside
/// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
/// let mut x = MaybeUninit::<&i32>::uninit();
/// // Set it to a valid value.
/// x.write(&0);
/// // Extract the initialized data -- this is only allowed *after* properly
/// // initializing `x`!
/// let x = unsafe { x.assume_init() };
/// ```
///
/// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
///
/// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
/// any of the run-time tracking and without any of the safety checks.
///
/// ## out-pointers
///
/// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
/// from a function, pass it a pointer to some (uninitialized) memory to put the
/// result into. This can be useful when it is important for the caller to control
/// how the memory the result is stored in gets allocated, and you want to avoid
/// unnecessary moves.
///
/// ```
/// use std::mem::MaybeUninit;
///
/// unsafe fn make_vec(out: *mut Vec<i32>) {
/// // `write` does not drop the old contents, which is important.
/// out.write(vec![1, 2, 3]);
/// }
///
/// let mut v = MaybeUninit::uninit();
/// unsafe { make_vec(v.as_mut_ptr()); }
/// // Now we know `v` is initialized! This also makes sure the vector gets
/// // properly dropped.
/// let v = unsafe { v.assume_init() };
/// assert_eq!(&v, &[1, 2, 3]);
/// ```
///
/// ## Initializing an array element-by-element
///
/// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
///
/// ```
/// use std::mem::{self, MaybeUninit};
///
/// let data = {
/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
/// // safe because the type we are claiming to have initialized here is a
/// // bunch of `MaybeUninit`s, which do not require initialization.
/// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
/// MaybeUninit::uninit().assume_init()
/// };
///
/// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop,
/// // we have a memory leak, but there is no memory safety issue.
/// for elem in &mut data[..] {
/// elem.write(vec![42]);
/// }
///
/// // Everything is initialized. Transmute the array to the
/// // initialized type.
/// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
/// };
///
/// assert_eq!(&data[0], &[42]);
/// ```
///
/// You can also work with partially initialized arrays, which could
/// be found in low-level datastructures.
///
/// ```
/// use std::mem::MaybeUninit;
///
/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
/// // safe because the type we are claiming to have initialized here is a
/// // bunch of `MaybeUninit`s, which do not require initialization.
/// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
/// // Count the number of elements we have assigned.
/// let mut data_len: usize = 0;
///
/// for elem in &mut data[0..500] {
/// elem.write(String::from("hello"));
/// data_len += 1;
/// }
///
/// // For each item in the array, drop if we allocated it.
/// for elem in &mut data[0..data_len] {
/// unsafe { elem.assume_init_drop(); }
/// }
/// ```
///
/// ## Initializing a struct field-by-field
///
/// You can use `MaybeUninit<T>`, and the [`std::ptr::addr_of_mut`] macro, to initialize structs field by field:
///
/// ```rust
/// use std::mem::MaybeUninit;
/// use std::ptr::addr_of_mut;
///
/// #[derive(Debug, PartialEq)]
/// pub struct Foo {
/// name: String,
/// list: Vec<u8>,
/// }
///
/// let foo = {
/// let mut uninit: MaybeUninit<Foo> = MaybeUninit::uninit();
/// let ptr = uninit.as_mut_ptr();
///
/// // Initializing the `name` field
/// // Using `write` instead of assignment via `=` to not call `drop` on the
/// // old, uninitialized value.
/// unsafe { addr_of_mut!((*ptr).name).write("Bob".to_string()); }
///
/// // Initializing the `list` field
/// // If there is a panic here, then the `String` in the `name` field leaks.
/// unsafe { addr_of_mut!((*ptr).list).write(vec![0, 1, 2]); }
///
/// // All the fields are initialized, so we call `assume_init` to get an initialized Foo.
/// unsafe { uninit.assume_init() }
/// };
///
/// assert_eq!(
/// foo,
/// Foo {
/// name: "Bob".to_string(),
/// list: vec![0, 1, 2]
/// }
/// );
/// ```
/// [`std::ptr::addr_of_mut`]: crate::ptr::addr_of_mut
/// [ub]: ../../reference/behavior-considered-undefined.html
///
/// # Layout
///
/// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
///
/// ```rust
/// use std::mem::{MaybeUninit, size_of, align_of};
/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
/// ```
///
/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
/// optimizations, potentially resulting in a larger size:
///
/// ```rust
/// # use std::mem::{MaybeUninit, size_of};
/// assert_eq!(size_of::<Option<bool>>(), 1);
/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
/// ```
///
/// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
///
/// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
/// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
/// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
/// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
/// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
/// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
/// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
/// guarantee may evolve.
#[stable(feature = "maybe_uninit", since = "1.36.0")]
// Lang item so we can wrap other types in it. This is useful for generators.
#[lang = "maybe_uninit"]
#[derive(Copy)]
#[repr(transparent)]
pub union MaybeUninit<T> {
uninit: (),
value: ManuallyDrop<T>,
}
#[stable(feature = "maybe_uninit", since = "1.36.0")]
impl<T: Copy> Clone for MaybeUninit<T> {
#[inline(always)]
fn clone(&self) -> Self {
// Not calling `T::clone()`, we cannot know if we are initialized enough for that.
*self
}
}
#[stable(feature = "maybe_uninit_debug", since = "1.41.0")]
impl<T> fmt::Debug for MaybeUninit<T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad(type_name::<Self>())
}
}
impl<T> MaybeUninit<T> {
/// Creates a new `MaybeUninit<T>` initialized with the given value.
/// It is safe to call [`assume_init`] on the return value of this function.
///
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
///
/// # Example
///
/// ```
/// use std::mem::MaybeUninit;
///
/// let v: MaybeUninit<Vec<u8>> = MaybeUninit::new(vec![42]);
/// ```
///
/// [`assume_init`]: MaybeUninit::assume_init
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
#[must_use = "use `forget` to avoid running Drop code"]
#[inline(always)]
pub const fn new(val: T) -> MaybeUninit<T> {
MaybeUninit { value: ManuallyDrop::new(val) }
}
/// Creates a new `MaybeUninit<T>` in an uninitialized state.
///
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
///
/// See the [type-level documentation][MaybeUninit] for some examples.
///
/// # Example
///
/// ```
/// use std::mem::MaybeUninit;
///
/// let v: MaybeUninit<String> = MaybeUninit::uninit();
/// ```
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
#[must_use]
#[inline(always)]
#[rustc_diagnostic_item = "maybe_uninit_uninit"]
pub const fn uninit() -> MaybeUninit<T> {
MaybeUninit { uninit: () }
}
/// Create a new array of `MaybeUninit<T>` items, in an uninitialized state.
///
/// Note: in a future Rust version this method may become unnecessary
/// when Rust allows
/// [inline const expressions](https://github.com/rust-lang/rust/issues/76001).
/// The example below could then use `let mut buf = [const { MaybeUninit::<u8>::uninit() }; 32];`.
///
/// # Examples
///
/// ```no_run
/// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
///
/// use std::mem::MaybeUninit;
///
/// extern "C" {
/// fn read_into_buffer(ptr: *mut u8, max_len: usize) -> usize;
/// }
///
/// /// Returns a (possibly smaller) slice of data that was actually read
/// fn read(buf: &mut [MaybeUninit<u8>]) -> &[u8] {
/// unsafe {
/// let len = read_into_buffer(buf.as_mut_ptr() as *mut u8, buf.len());
/// MaybeUninit::slice_assume_init_ref(&buf[..len])
/// }
/// }
///
/// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
/// let data = read(&mut buf);
/// ```
#[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
#[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")]
#[must_use]
#[inline(always)]
pub const fn uninit_array<const N: usize>() -> [Self; N] {
// SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() }
}
/// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
/// filled with `0` bytes. It depends on `T` whether that already makes for
/// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
/// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
/// be null.
///
/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
/// It is your responsibility to make sure `T` gets dropped if it got initialized.
///
/// # Example
///
/// Correct usage of this function: initializing a struct with zero, where all
/// fields of the struct can hold the bit-pattern 0 as a valid value.
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let x = MaybeUninit::<(u8, bool)>::zeroed();
/// let x = unsafe { x.assume_init() };
/// assert_eq!(x, (0, false));
/// ```
///
/// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
/// when `0` is not a valid bit-pattern for the type:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// enum NotZero { One = 1, Two = 2 }
///
/// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
/// let x = unsafe { x.assume_init() };
/// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
/// // This is undefined behavior. ⚠️
/// ```
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")]
#[must_use]
#[inline]
#[rustc_diagnostic_item = "maybe_uninit_zeroed"]
pub const fn zeroed() -> MaybeUninit<T> {
let mut u = MaybeUninit::<T>::uninit();
// SAFETY: `u.as_mut_ptr()` points to allocated memory.
unsafe {
u.as_mut_ptr().write_bytes(0u8, 1);
}
u
}
/// Sets the value of the `MaybeUninit<T>`.
///
/// This overwrites any previous value without dropping it, so be careful
/// not to use this twice unless you want to skip running the destructor.
/// For your convenience, this also returns a mutable reference to the
/// (now safely initialized) contents of `self`.
///
/// As the content is stored inside a `MaybeUninit`, the destructor is not
/// run for the inner data if the MaybeUninit leaves scope without a call to
/// [`assume_init`], [`assume_init_drop`], or similar. Code that receives
/// the mutable reference returned by this function needs to keep this in
/// mind. The safety model of Rust regards leaks as safe, but they are
/// usually still undesirable. This being said, the mutable reference
/// behaves like any other mutable reference would, so assigning a new value
/// to it will drop the old content.
///
/// [`assume_init`]: Self::assume_init
/// [`assume_init_drop`]: Self::assume_init_drop
///
/// # Examples
///
/// Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Vec<u8>>::uninit();
///
/// {
/// let hello = x.write((&b"Hello, world!").to_vec());
/// // Setting hello does not leak prior allocations, but drops them
/// *hello = (&b"Hello").to_vec();
/// hello[0] = 'h' as u8;
/// }
/// // x is initialized now:
/// let s = unsafe { x.assume_init() };
/// assert_eq!(b"hello", s.as_slice());
/// ```
///
/// This usage of the method causes a leak:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<String>::uninit();
///
/// x.write("Hello".to_string());
/// // This leaks the contained string:
/// x.write("hello".to_string());
/// // x is initialized now:
/// let s = unsafe { x.assume_init() };
/// ```
///
/// This method can be used to avoid unsafe in some cases. The example below
/// shows a part of an implementation of a fixed sized arena that lends out
/// pinned references.
/// With `write`, we can avoid the need to write through a raw pointer:
///
/// ```rust
/// use core::pin::Pin;
/// use core::mem::MaybeUninit;
///
/// struct PinArena<T> {
/// memory: Box<[MaybeUninit<T>]>,
/// len: usize,
/// }
///
/// impl <T> PinArena<T> {
/// pub fn capacity(&self) -> usize {
/// self.memory.len()
/// }
/// pub fn push(&mut self, val: T) -> Pin<&mut T> {
/// if self.len >= self.capacity() {
/// panic!("Attempted to push to a full pin arena!");
/// }
/// let ref_ = self.memory[self.len].write(val);
/// self.len += 1;
/// unsafe { Pin::new_unchecked(ref_) }
/// }
/// }
/// ```
#[stable(feature = "maybe_uninit_write", since = "1.55.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_write", issue = "63567")]
#[inline(always)]
pub const fn write(&mut self, val: T) -> &mut T {
*self = MaybeUninit::new(val);
// SAFETY: We just initialized this value.
unsafe { self.assume_init_mut() }
}
/// Gets a pointer to the contained value. Reading from this pointer or turning it
/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
/// Writing to memory that this pointer (non-transitively) points to is undefined behavior
/// (except inside an `UnsafeCell<T>`).
///
/// # Examples
///
/// Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
/// x.write(vec![0, 1, 2]);
/// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
/// let x_vec = unsafe { &*x.as_ptr() };
/// assert_eq!(x_vec.len(), 3);
/// ```
///
/// *Incorrect* usage of this method:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let x = MaybeUninit::<Vec<u32>>::uninit();
/// let x_vec = unsafe { &*x.as_ptr() };
/// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
/// ```
///
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
/// until they are, it is advisable to avoid them.)
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
#[inline(always)]
pub const fn as_ptr(&self) -> *const T {
// `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
self as *const _ as *const T
}
/// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
///
/// # Examples
///
/// Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
/// x.write(vec![0, 1, 2]);
/// // Create a reference into the `MaybeUninit<Vec<u32>>`.
/// // This is okay because we initialized it.
/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
/// x_vec.push(3);
/// assert_eq!(x_vec.len(), 4);
/// ```
///
/// *Incorrect* usage of this method:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
/// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
/// ```
///
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
/// until they are, it is advisable to avoid them.)
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
#[inline(always)]
pub const fn as_mut_ptr(&mut self) -> *mut T {
// `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
self as *mut _ as *mut T
}
/// Extracts the value from the `MaybeUninit<T>` container. This is a great way
/// to ensure that the data will get dropped, because the resulting `T` is
/// subject to the usual drop handling.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
/// state. Calling this when the content is not yet fully initialized causes immediate undefined
/// behavior. The [type-level documentation][inv] contains more information about
/// this initialization invariant.
///
/// [inv]: #initialization-invariant
///
/// On top of that, remember that most types have additional invariants beyond merely
/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
/// is considered initialized (under the current implementation; this does not constitute
/// a stable guarantee) because the only requirement the compiler knows about it
/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
/// *immediate* undefined behavior, but will cause undefined behavior with most
/// safe operations (including dropping it).
///
/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
///
/// # Examples
///
/// Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<bool>::uninit();
/// x.write(true);
/// let x_init = unsafe { x.assume_init() };
/// assert_eq!(x_init, true);
/// ```
///
/// *Incorrect* usage of this method:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let x = MaybeUninit::<Vec<u32>>::uninit();
/// let x_init = unsafe { x.assume_init() };
/// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
/// ```
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_stable(feature = "const_maybe_uninit_assume_init_by_value", since = "1.59.0")]
#[inline(always)]
#[rustc_diagnostic_item = "assume_init"]
#[track_caller]
pub const unsafe fn assume_init(self) -> T {
// SAFETY: the caller must guarantee that `self` is initialized.
// This also means that `self` must be a `value` variant.
unsafe {
intrinsics::assert_inhabited::<T>();
ManuallyDrop::into_inner(self.value)
}
}
/// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
/// to the usual drop handling.
///
/// Whenever possible, it is preferable to use [`assume_init`] instead, which
/// prevents duplicating the content of the `MaybeUninit<T>`.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
/// state. Calling this when the content is not yet fully initialized causes undefined
/// behavior. The [type-level documentation][inv] contains more information about
/// this initialization invariant.
///
/// Moreover, similar to the [`ptr::read`] function, this function creates a
/// bitwise copy of the contents, regardless whether the contained type
/// implements the [`Copy`] trait or not. When using multiple copies of the
/// data (by calling `assume_init_read` multiple times, or first calling
/// `assume_init_read` and then [`assume_init`]), it is your responsibility
/// to ensure that data may indeed be duplicated.
///
/// [inv]: #initialization-invariant
/// [`assume_init`]: MaybeUninit::assume_init
///
/// # Examples
///
/// Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<u32>::uninit();
/// x.write(13);
/// let x1 = unsafe { x.assume_init_read() };
/// // `u32` is `Copy`, so we may read multiple times.
/// let x2 = unsafe { x.assume_init_read() };
/// assert_eq!(x1, x2);
///
/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
/// x.write(None);
/// let x1 = unsafe { x.assume_init_read() };
/// // Duplicating a `None` value is okay, so we may read multiple times.
/// let x2 = unsafe { x.assume_init_read() };
/// assert_eq!(x1, x2);
/// ```
///
/// *Incorrect* usage of this method:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
/// x.write(Some(vec![0, 1, 2]));
/// let x1 = unsafe { x.assume_init_read() };
/// let x2 = unsafe { x.assume_init_read() };
/// // We now created two copies of the same vector, leading to a double-free ⚠️ when
/// // they both get dropped!
/// ```
#[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_assume_init_read", issue = "63567")]
#[inline(always)]
#[track_caller]
pub const unsafe fn assume_init_read(&self) -> T {
// SAFETY: the caller must guarantee that `self` is initialized.
// Reading from `self.as_ptr()` is safe since `self` should be initialized.
unsafe {
intrinsics::assert_inhabited::<T>();
self.as_ptr().read()
}
}
/// Drops the contained value in place.
///
/// If you have ownership of the `MaybeUninit`, you can also use
/// [`assume_init`] as an alternative.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is
/// in an initialized state. Calling this when the content is not yet fully
/// initialized causes undefined behavior.
///
/// On top of that, all additional invariants of the type `T` must be
/// satisfied, as the `Drop` implementation of `T` (or its members) may
/// rely on this. For example, setting a [`Vec<T>`] to an invalid but
/// non-null address makes it initialized (under the current implementation;
/// this does not constitute a stable guarantee), because the only
/// requirement the compiler knows about it is that the data pointer must be
/// non-null. Dropping such a `Vec<T>` however will cause undefined
/// behaviour.
///
/// [`assume_init`]: MaybeUninit::assume_init
/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
#[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
pub unsafe fn assume_init_drop(&mut self) {
// SAFETY: the caller must guarantee that `self` is initialized and
// satisfies all invariants of `T`.
// Dropping the value in place is safe if that is the case.
unsafe { ptr::drop_in_place(self.as_mut_ptr()) }
}
/// Gets a shared reference to the contained value.
///
/// This can be useful when we want to access a `MaybeUninit` that has been
/// initialized but don't have ownership of the `MaybeUninit` (preventing the use
/// of `.assume_init()`).
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
/// is in an initialized state.
///
/// # Examples
///
/// ### Correct usage of this method:
///
/// ```rust
/// use std::mem::MaybeUninit;
///
/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
/// // Initialize `x`:
/// x.write(vec![1, 2, 3]);
/// // Now that our `MaybeUninit<_>` is known to be initialized, it is okay to
/// // create a shared reference to it:
/// let x: &Vec<u32> = unsafe {
/// // SAFETY: `x` has been initialized.
/// x.assume_init_ref()
/// };
/// assert_eq!(x, &vec![1, 2, 3]);
/// ```
///
/// ### *Incorrect* usages of this method:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let x = MaybeUninit::<Vec<u32>>::uninit();
/// let x_vec: &Vec<u32> = unsafe { x.assume_init_ref() };
/// // We have created a reference to an uninitialized vector! This is undefined behavior. ⚠️
/// ```
///
/// ```rust,no_run
/// use std::{cell::Cell, mem::MaybeUninit};
///
/// let b = MaybeUninit::<Cell<bool>>::uninit();
/// // Initialize the `MaybeUninit` using `Cell::set`:
/// unsafe {
/// b.assume_init_ref().set(true);
/// // ^^^^^^^^^^^^^^^
/// // Reference to an uninitialized `Cell<bool>`: UB!
/// }
/// ```
#[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
#[rustc_const_stable(feature = "const_maybe_uninit_assume_init_ref", since = "1.59.0")]
#[inline(always)]
pub const unsafe fn assume_init_ref(&self) -> &T {
// SAFETY: the caller must guarantee that `self` is initialized.
// This also means that `self` must be a `value` variant.
unsafe {
intrinsics::assert_inhabited::<T>();
&*self.as_ptr()
}
}
/// Gets a mutable (unique) reference to the contained value.
///
/// This can be useful when we want to access a `MaybeUninit` that has been
/// initialized but don't have ownership of the `MaybeUninit` (preventing the use
/// of `.assume_init()`).
///
/// # Safety
///
/// Calling this when the content is not yet fully initialized causes undefined
/// behavior: it is up to the caller to guarantee that the `MaybeUninit<T>` really
/// is in an initialized state. For instance, `.assume_init_mut()` cannot be used to
/// initialize a `MaybeUninit`.
///
/// # Examples
///
/// ### Correct usage of this method:
///
/// ```rust
/// # #![allow(unexpected_cfgs)]
/// use std::mem::MaybeUninit;
///
/// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }
/// # #[cfg(FALSE)]
/// extern "C" {
/// /// Initializes *all* the bytes of the input buffer.
/// fn initialize_buffer(buf: *mut [u8; 1024]);
/// }
///
/// let mut buf = MaybeUninit::<[u8; 1024]>::uninit();
///
/// // Initialize `buf`:
/// unsafe { initialize_buffer(buf.as_mut_ptr()); }
/// // Now we know that `buf` has been initialized, so we could `.assume_init()` it.
/// // However, using `.assume_init()` may trigger a `memcpy` of the 1024 bytes.
/// // To assert our buffer has been initialized without copying it, we upgrade
/// // the `&mut MaybeUninit<[u8; 1024]>` to a `&mut [u8; 1024]`:
/// let buf: &mut [u8; 1024] = unsafe {
/// // SAFETY: `buf` has been initialized.
/// buf.assume_init_mut()
/// };
///
/// // Now we can use `buf` as a normal slice:
/// buf.sort_unstable();
/// assert!(
/// buf.windows(2).all(|pair| pair[0] <= pair[1]),
/// "buffer is sorted",
/// );
/// ```
///
/// ### *Incorrect* usages of this method:
///
/// You cannot use `.assume_init_mut()` to initialize a value:
///
/// ```rust,no_run
/// use std::mem::MaybeUninit;
///
/// let mut b = MaybeUninit::<bool>::uninit();
/// unsafe {
/// *b.assume_init_mut() = true;
/// // We have created a (mutable) reference to an uninitialized `bool`!
/// // This is undefined behavior. ⚠️
/// }
/// ```
///
/// For instance, you cannot [`Read`] into an uninitialized buffer:
///
/// [`Read`]: ../../std/io/trait.Read.html
///
/// ```rust,no_run
/// use std::{io, mem::MaybeUninit};
///
/// fn read_chunk (reader: &'_ mut dyn io::Read) -> io::Result<[u8; 64]>
/// {
/// let mut buffer = MaybeUninit::<[u8; 64]>::uninit();
/// reader.read_exact(unsafe { buffer.assume_init_mut() })?;
/// // ^^^^^^^^^^^^^^^^^^^^^^^^
/// // (mutable) reference to uninitialized memory!
/// // This is undefined behavior.
/// Ok(unsafe { buffer.assume_init() })
/// }
/// ```
///
/// Nor can you use direct field access to do field-by-field gradual initialization:
///
/// ```rust,no_run
/// use std::{mem::MaybeUninit, ptr};
///
/// struct Foo {
/// a: u32,
/// b: u8,
/// }
///
/// let foo: Foo = unsafe {
/// let mut foo = MaybeUninit::<Foo>::uninit();
/// ptr::write(&mut foo.assume_init_mut().a as *mut u32, 1337);
/// // ^^^^^^^^^^^^^^^^^^^^^
/// // (mutable) reference to uninitialized memory!
/// // This is undefined behavior.
/// ptr::write(&mut foo.assume_init_mut().b as *mut u8, 42);
/// // ^^^^^^^^^^^^^^^^^^^^^
/// // (mutable) reference to uninitialized memory!
/// // This is undefined behavior.
/// foo.assume_init()
/// };
/// ```
#[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
#[inline(always)]
pub const unsafe fn assume_init_mut(&mut self) -> &mut T {
// SAFETY: the caller must guarantee that `self` is initialized.
// This also means that `self` must be a `value` variant.
unsafe {
intrinsics::assert_inhabited::<T>();
&mut *self.as_mut_ptr()
}
}
/// Extracts the values from an array of `MaybeUninit` containers.
///
/// # Safety
///
/// It is up to the caller to guarantee that all elements of the array are
/// in an initialized state.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_uninit_array)]
/// #![feature(maybe_uninit_array_assume_init)]
/// use std::mem::MaybeUninit;
///
/// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
/// array[0].write(0);
/// array[1].write(1);
/// array[2].write(2);
///
/// // SAFETY: Now safe as we initialised all elements
/// let array = unsafe {
/// MaybeUninit::array_assume_init(array)
/// };
///
/// assert_eq!(array, [0, 1, 2]);
/// ```
#[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
#[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")]
#[inline(always)]
#[track_caller]
pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
// SAFETY:
// * The caller guarantees that all elements of the array are initialized
// * `MaybeUninit<T>` and T are guaranteed to have the same layout
// * `MaybeUninit` does not drop, so there are no double-frees
// And thus the conversion is safe
let ret = unsafe {
intrinsics::assert_inhabited::<[T; N]>();
(&array as *const _ as *const [T; N]).read()
};
// FIXME: required to avoid `~const Destruct` bound
super::forget(array);
ret
}
/// Assuming all the elements are initialized, get a slice to them.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
/// really are in an initialized state.
/// Calling this when the content is not yet fully initialized causes undefined behavior.
///
/// See [`assume_init_ref`] for more details and examples.
///
/// [`assume_init_ref`]: MaybeUninit::assume_init_ref
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
// SAFETY: casting `slice` to a `*const [T]` is safe since the caller guarantees that
// `slice` is initialized, and `MaybeUninit` is guaranteed to have the same layout as `T`.
// The pointer obtained is valid since it refers to memory owned by `slice` which is a
// reference and thus guaranteed to be valid for reads.
unsafe { &*(slice as *const [Self] as *const [T]) }
}
/// Assuming all the elements are initialized, get a mutable slice to them.
///
/// # Safety
///
/// It is up to the caller to guarantee that the `MaybeUninit<T>` elements
/// really are in an initialized state.
/// Calling this when the content is not yet fully initialized causes undefined behavior.
///
/// See [`assume_init_mut`] for more details and examples.
///
/// [`assume_init_mut`]: MaybeUninit::assume_init_mut
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
#[inline(always)]
pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
// SAFETY: similar to safety notes for `slice_get_ref`, but we have a
// mutable reference which is also guaranteed to be valid for writes.
unsafe { &mut *(slice as *mut [Self] as *mut [T]) }
}
/// Gets a pointer to the first element of the array.
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
this.as_ptr() as *const T
}
/// Gets a mutable pointer to the first element of the array.
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
this.as_mut_ptr() as *mut T
}
/// Copies the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
///
/// If `T` does not implement `Copy`, use [`write_slice_cloned`]
///
/// This is similar to [`slice::copy_from_slice`].
///
/// # Panics
///
/// This function will panic if the two slices have different lengths.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_write_slice)]
/// use std::mem::MaybeUninit;
///
/// let mut dst = [MaybeUninit::uninit(); 32];
/// let src = [0; 32];
///
/// let init = MaybeUninit::write_slice(&mut dst, &src);
///
/// assert_eq!(init, src);
/// ```
///
/// ```
/// #![feature(maybe_uninit_write_slice)]
/// use std::mem::MaybeUninit;
///
/// let mut vec = Vec::with_capacity(32);
/// let src = [0; 16];
///
/// MaybeUninit::write_slice(&mut vec.spare_capacity_mut()[..src.len()], &src);
///
/// // SAFETY: we have just copied all the elements of len into the spare capacity
/// // the first src.len() elements of the vec are valid now.
/// unsafe {
/// vec.set_len(src.len());
/// }
///
/// assert_eq!(vec, src);
/// ```
///
/// [`write_slice_cloned`]: MaybeUninit::write_slice_cloned
#[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
pub fn write_slice<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
where
T: Copy,
{
// SAFETY: &[T] and &[MaybeUninit<T>] have the same layout
let uninit_src: &[MaybeUninit<T>] = unsafe { super::transmute(src) };
this.copy_from_slice(uninit_src);
// SAFETY: Valid elements have just been copied into `this` so it is initialized
unsafe { MaybeUninit::slice_assume_init_mut(this) }
}
/// Clones the elements from `src` to `this`, returning a mutable reference to the now initialized contents of `this`.
/// Any already initialized elements will not be dropped.
///
/// If `T` implements `Copy`, use [`write_slice`]
///
/// This is similar to [`slice::clone_from_slice`] but does not drop existing elements.
///
/// # Panics
///
/// This function will panic if the two slices have different lengths, or if the implementation of `Clone` panics.
///
/// If there is a panic, the already cloned elements will be dropped.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_write_slice)]
/// use std::mem::MaybeUninit;
///
/// let mut dst = [MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit(), MaybeUninit::uninit()];
/// let src = ["wibbly".to_string(), "wobbly".to_string(), "timey".to_string(), "wimey".to_string(), "stuff".to_string()];
///
/// let init = MaybeUninit::write_slice_cloned(&mut dst, &src);
///
/// assert_eq!(init, src);
/// ```
///
/// ```
/// #![feature(maybe_uninit_write_slice)]
/// use std::mem::MaybeUninit;
///
/// let mut vec = Vec::with_capacity(32);
/// let src = ["rust", "is", "a", "pretty", "cool", "language"];
///
/// MaybeUninit::write_slice_cloned(&mut vec.spare_capacity_mut()[..src.len()], &src);
///
/// // SAFETY: we have just cloned all the elements of len into the spare capacity
/// // the first src.len() elements of the vec are valid now.
/// unsafe {
/// vec.set_len(src.len());
/// }
///
/// assert_eq!(vec, src);
/// ```
///
/// [`write_slice`]: MaybeUninit::write_slice
#[unstable(feature = "maybe_uninit_write_slice", issue = "79995")]
pub fn write_slice_cloned<'a>(this: &'a mut [MaybeUninit<T>], src: &[T]) -> &'a mut [T]
where
T: Clone,
{
// unlike copy_from_slice this does not call clone_from_slice on the slice
// this is because `MaybeUninit<T: Clone>` does not implement Clone.
struct Guard<'a, T> {
slice: &'a mut [MaybeUninit<T>],
initialized: usize,
}
impl<'a, T> Drop for Guard<'a, T> {
fn drop(&mut self) {
let initialized_part = &mut self.slice[..self.initialized];
// SAFETY: this raw slice will contain only initialized objects
// that's why, it is allowed to drop it.
unsafe {
crate::ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(initialized_part));
}
}
}
assert_eq!(this.len(), src.len(), "destination and source slices have different lengths");
// NOTE: We need to explicitly slice them to the same length
// for bounds checking to be elided, and the optimizer will
// generate memcpy for simple cases (for example T = u8).
let len = this.len();
let src = &src[..len];
// guard is needed b/c panic might happen during a clone
let mut guard = Guard { slice: this, initialized: 0 };
for i in 0..len {
guard.slice[i].write(src[i].clone());
guard.initialized += 1;
}
super::forget(guard);
// SAFETY: Valid elements have just been written into `this` so it is initialized
unsafe { MaybeUninit::slice_assume_init_mut(this) }
}
/// Returns the contents of this `MaybeUninit` as a slice of potentially uninitialized bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_as_bytes, maybe_uninit_slice)]
/// use std::mem::MaybeUninit;
///
/// let val = 0x12345678_i32;
/// let uninit = MaybeUninit::new(val);
/// let uninit_bytes = uninit.as_bytes();
/// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(uninit_bytes) };
/// assert_eq!(bytes, val.to_ne_bytes());
/// ```
#[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
pub fn as_bytes(&self) -> &[MaybeUninit<u8>] {
// SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
unsafe {
slice::from_raw_parts(self.as_ptr() as *const MaybeUninit<u8>, mem::size_of::<T>())
}
}
/// Returns the contents of this `MaybeUninit` as a mutable slice of potentially uninitialized
/// bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_as_bytes)]
/// use std::mem::MaybeUninit;
///
/// let val = 0x12345678_i32;
/// let mut uninit = MaybeUninit::new(val);
/// let uninit_bytes = uninit.as_bytes_mut();
/// if cfg!(target_endian = "little") {
/// uninit_bytes[0].write(0xcd);
/// } else {
/// uninit_bytes[3].write(0xcd);
/// }
/// let val2 = unsafe { uninit.assume_init() };
/// assert_eq!(val2, 0x123456cd);
/// ```
#[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>] {
// SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
unsafe {
slice::from_raw_parts_mut(
self.as_mut_ptr() as *mut MaybeUninit<u8>,
mem::size_of::<T>(),
)
}
}
/// Returns the contents of this slice of `MaybeUninit` as a slice of potentially uninitialized
/// bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
/// use std::mem::MaybeUninit;
///
/// let uninit = [MaybeUninit::new(0x1234u16), MaybeUninit::new(0x5678u16)];
/// let uninit_bytes = MaybeUninit::slice_as_bytes(&uninit);
/// let bytes = unsafe { MaybeUninit::slice_assume_init_ref(&uninit_bytes) };
/// let val1 = u16::from_ne_bytes(bytes[0..2].try_into().unwrap());
/// let val2 = u16::from_ne_bytes(bytes[2..4].try_into().unwrap());
/// assert_eq!(&[val1, val2], &[0x1234u16, 0x5678u16]);
/// ```
#[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>] {
// SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
unsafe {
slice::from_raw_parts(
this.as_ptr() as *const MaybeUninit<u8>,
this.len() * mem::size_of::<T>(),
)
}
}
/// Returns the contents of this mutable slice of `MaybeUninit` as a mutable slice of
/// potentially uninitialized bytes.
///
/// Note that even if the contents of a `MaybeUninit` have been initialized, the value may still
/// contain padding bytes which are left uninitialized.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_as_bytes, maybe_uninit_write_slice, maybe_uninit_slice)]
/// use std::mem::MaybeUninit;
///
/// let mut uninit = [MaybeUninit::<u16>::uninit(), MaybeUninit::<u16>::uninit()];
/// let uninit_bytes = MaybeUninit::slice_as_bytes_mut(&mut uninit);
/// MaybeUninit::write_slice(uninit_bytes, &[0x12, 0x34, 0x56, 0x78]);
/// let vals = unsafe { MaybeUninit::slice_assume_init_ref(&uninit) };
/// if cfg!(target_endian = "little") {
/// assert_eq!(vals, &[0x3412u16, 0x7856u16]);
/// } else {
/// assert_eq!(vals, &[0x1234u16, 0x5678u16]);
/// }
/// ```
#[unstable(feature = "maybe_uninit_as_bytes", issue = "93092")]
pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>] {
// SAFETY: MaybeUninit<u8> is always valid, even for padding bytes
unsafe {
slice::from_raw_parts_mut(
this.as_mut_ptr() as *mut MaybeUninit<u8>,
this.len() * mem::size_of::<T>(),
)
}
}
}
impl<T, const N: usize> MaybeUninit<[T; N]> {
/// Transposes a `MaybeUninit<[T; N]>` into a `[MaybeUninit<T>; N]`.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_uninit_array_transpose)]
/// # use std::mem::MaybeUninit;
///
/// let data: [MaybeUninit<u8>; 1000] = MaybeUninit::uninit().transpose();
/// ```
#[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
#[inline]
pub const fn transpose(self) -> [MaybeUninit<T>; N] {
// SAFETY: T and MaybeUninit<T> have the same layout
unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
}
}
impl<T, const N: usize> [MaybeUninit<T>; N] {
/// Transposes a `[MaybeUninit<T>; N]` into a `MaybeUninit<[T; N]>`.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_uninit_array_transpose)]
/// # use std::mem::MaybeUninit;
///
/// let data = [MaybeUninit::<u8>::uninit(); 1000];
/// let data: MaybeUninit<[u8; 1000]> = data.transpose();
/// ```
#[unstable(feature = "maybe_uninit_uninit_array_transpose", issue = "96097")]
#[inline]
pub const fn transpose(self) -> MaybeUninit<[T; N]> {
// SAFETY: T and MaybeUninit<T> have the same layout
unsafe { super::transmute_copy(&ManuallyDrop::new(self)) }
}
}