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Diffstat (limited to 'library/core/src/mem/maybe_uninit.rs')
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diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs new file mode 100644 index 000000000..b4ea53608 --- /dev/null +++ b/library/core/src/mem/maybe_uninit.rs @@ -0,0 +1,1292 @@ +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! ⚠️ +/// ``` +/// (Notice that the rules around uninitialized integers are not finalized yet, but +/// until they are, it is advisable to avoid them.) +/// +/// 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. Thus using raw pointer +/// // assignment instead of `ptr::write` does not cause the old +/// // uninitialized value to be dropped. Also 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; +/// use std::ptr; +/// +/// // 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 { ptr::drop_in_place(elem.as_mut_ptr()); } +/// } +/// ``` +/// +/// ## 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 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 = 0x12345678i32; + /// 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 = 0x12345678i32; + /// 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>(), + ) + } + } +} |