| 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)) } |
| } |
| } |