709 lines
27 KiB
Rust
709 lines
27 KiB
Rust
use crate::intrinsics;
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use crate::mem::ManuallyDrop;
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/// A wrapper type to construct uninitialized instances of `T`.
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///
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/// # Initialization invariant
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///
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/// The compiler, in general, assumes that a variable is properly initialized
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/// according to the requirements of the variable's type. For example, a variable of
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/// reference type must be aligned and non-NULL. This is an invariant that must
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/// *always* be upheld, even in unsafe code. As a consequence, zero-initializing a
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/// variable of reference type causes instantaneous [undefined behavior][ub],
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/// no matter whether that reference ever gets used to access memory:
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///
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/// ```rust,no_run
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/// # #![allow(invalid_value)]
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/// use std::mem::{self, MaybeUninit};
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///
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/// let x: &i32 = unsafe { mem::zeroed() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<&i32>`:
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/// let x: &i32 = unsafe { MaybeUninit::zeroed().assume_init() }; // undefined behavior!
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/// ```
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///
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/// This is exploited by the compiler for various optimizations, such as eliding
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/// run-time checks and optimizing `enum` layout.
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///
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/// Similarly, entirely uninitialized memory may have any content, while a `bool` must
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/// always be `true` or `false`. Hence, creating an uninitialized `bool` is undefined behavior:
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///
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/// ```rust,no_run
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/// # #![allow(invalid_value)]
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/// use std::mem::{self, MaybeUninit};
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///
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/// let b: bool = unsafe { mem::uninitialized() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<bool>`:
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/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
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/// ```
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///
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/// Moreover, uninitialized memory is special in that the compiler knows that
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/// it does not have a fixed value. This makes it undefined behavior to have
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/// uninitialized data in a variable even if that variable has an integer type,
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/// which otherwise can hold any *fixed* bit pattern:
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///
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/// ```rust,no_run
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/// # #![allow(invalid_value)]
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/// use std::mem::{self, MaybeUninit};
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///
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/// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior!
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/// // The equivalent code with `MaybeUninit<i32>`:
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/// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior!
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/// ```
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/// (Notice that the rules around uninitialized integers are not finalized yet, but
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/// until they are, it is advisable to avoid them.)
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///
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/// On top of that, remember that most types have additional invariants beyond merely
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/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
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/// is considered initialized (under the current implementation; this does not constitute
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/// a stable guarantee) because the only requirement the compiler knows about it
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/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
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/// *immediate* undefined behavior, but will cause undefined behavior with most
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/// safe operations (including dropping it).
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///
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/// [`Vec<T>`]: ../../std/vec/struct.Vec.html
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///
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/// # Examples
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///
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/// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
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/// It is a signal to the compiler indicating that the data here might *not*
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/// be initialized:
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// // Create an explicitly uninitialized reference. The compiler knows that data inside
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/// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
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/// let mut x = MaybeUninit::<&i32>::uninit();
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/// // Set it to a valid value.
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/// unsafe { x.as_mut_ptr().write(&0); }
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/// // Extract the initialized data -- this is only allowed *after* properly
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/// // initializing `x`!
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/// let x = unsafe { x.assume_init() };
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/// ```
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///
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/// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
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///
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/// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
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/// any of the run-time tracking and without any of the safety checks.
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///
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/// ## out-pointers
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///
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/// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
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/// from a function, pass it a pointer to some (uninitialized) memory to put the
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/// result into. This can be useful when it is important for the caller to control
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/// how the memory the result is stored in gets allocated, and you want to avoid
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/// unnecessary moves.
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///
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/// ```
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/// use std::mem::MaybeUninit;
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///
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/// unsafe fn make_vec(out: *mut Vec<i32>) {
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/// // `write` does not drop the old contents, which is important.
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/// out.write(vec![1, 2, 3]);
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/// }
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///
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/// let mut v = MaybeUninit::uninit();
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/// unsafe { make_vec(v.as_mut_ptr()); }
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/// // Now we know `v` is initialized! This also makes sure the vector gets
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/// // properly dropped.
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/// let v = unsafe { v.assume_init() };
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/// assert_eq!(&v, &[1, 2, 3]);
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/// ```
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///
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/// ## Initializing an array element-by-element
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///
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/// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
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///
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/// ```
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/// use std::mem::{self, MaybeUninit};
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///
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/// let data = {
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/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
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/// // safe because the type we are claiming to have initialized here is a
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/// // bunch of `MaybeUninit`s, which do not require initialization.
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/// let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
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/// MaybeUninit::uninit().assume_init()
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/// };
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///
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/// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
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/// // assignment instead of `ptr::write` does not cause the old
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/// // uninitialized value to be dropped. Also if there is a panic during
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/// // this loop, we have a memory leak, but there is no memory safety
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/// // issue.
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/// for elem in &mut data[..] {
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/// *elem = MaybeUninit::new(vec![42]);
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/// }
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///
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/// // Everything is initialized. Transmute the array to the
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/// // initialized type.
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/// unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
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/// };
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///
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/// assert_eq!(&data[0], &[42]);
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/// ```
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///
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/// You can also work with partially initialized arrays, which could
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/// be found in low-level datastructures.
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///
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/// ```
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/// use std::mem::MaybeUninit;
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/// use std::ptr;
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///
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/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
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/// // safe because the type we are claiming to have initialized here is a
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/// // bunch of `MaybeUninit`s, which do not require initialization.
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/// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
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/// // Count the number of elements we have assigned.
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/// let mut data_len: usize = 0;
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///
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/// for elem in &mut data[0..500] {
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/// *elem = MaybeUninit::new(String::from("hello"));
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/// data_len += 1;
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/// }
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///
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/// // For each item in the array, drop if we allocated it.
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/// for elem in &mut data[0..data_len] {
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/// unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
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/// }
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/// ```
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///
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/// ## Initializing a struct field-by-field
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///
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/// There is currently no supported way to create a raw pointer or reference
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/// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible
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/// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
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/// to its fields.
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///
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/// [ub]: ../../reference/behavior-considered-undefined.html
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///
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/// # Layout
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///
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/// `MaybeUninit<T>` is guaranteed to have the same size, alignment, and ABI as `T`:
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///
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/// ```rust
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/// use std::mem::{MaybeUninit, size_of, align_of};
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/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
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/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
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/// ```
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///
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/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
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/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
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/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
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/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
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/// optimizations, potentially resulting in a larger size:
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///
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/// ```rust
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/// # use std::mem::{MaybeUninit, size_of};
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/// assert_eq!(size_of::<Option<bool>>(), 1);
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/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
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/// ```
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///
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/// If `T` is FFI-safe, then so is `MaybeUninit<T>`.
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///
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/// While `MaybeUninit` is `#[repr(transparent)]` (indicating it guarantees the same size,
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/// alignment, and ABI as `T`), this does *not* change any of the previous caveats. `Option<T>` and
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/// `Option<MaybeUninit<T>>` may still have different sizes, and types containing a field of type
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/// `T` may be laid out (and sized) differently than if that field were `MaybeUninit<T>`.
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/// `MaybeUninit` is a union type, and `#[repr(transparent)]` on unions is unstable (see [the
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/// tracking issue](https://github.com/rust-lang/rust/issues/60405)). Over time, the exact
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/// guarantees of `#[repr(transparent)]` on unions may evolve, and `MaybeUninit` may or may not
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/// remain `#[repr(transparent)]`. That said, `MaybeUninit<T>` will *always* guarantee that it has
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/// the same size, alignment, and ABI as `T`; it's just that the way `MaybeUninit` implements that
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/// guarantee may evolve.
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#[allow(missing_debug_implementations)]
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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// Lang item so we can wrap other types in it. This is useful for generators.
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#[lang = "maybe_uninit"]
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#[derive(Copy)]
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#[repr(transparent)]
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pub union MaybeUninit<T> {
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uninit: (),
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value: ManuallyDrop<T>,
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}
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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impl<T: Copy> Clone for MaybeUninit<T> {
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#[inline(always)]
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fn clone(&self) -> Self {
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// Not calling `T::clone()`, we cannot know if we are initialized enough for that.
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*self
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}
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}
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impl<T> MaybeUninit<T> {
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/// Creates a new `MaybeUninit<T>` initialized with the given value.
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/// It is safe to call [`assume_init`] on the return value of this function.
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///
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/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
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/// It is your responsibility to make sure `T` gets dropped if it got initialized.
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///
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/// [`assume_init`]: #method.assume_init
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline(always)]
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pub const fn new(val: T) -> MaybeUninit<T> {
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MaybeUninit { value: ManuallyDrop::new(val) }
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}
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/// Creates a new `MaybeUninit<T>` in an uninitialized state.
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///
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/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
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/// It is your responsibility to make sure `T` gets dropped if it got initialized.
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///
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/// See the [type-level documentation][type] for some examples.
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///
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/// [type]: union.MaybeUninit.html
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline(always)]
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pub const fn uninit() -> MaybeUninit<T> {
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MaybeUninit { uninit: () }
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}
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/// A promotable constant, equivalent to `uninit()`.
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#[unstable(feature = "internal_uninit_const", issue = "0",
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reason = "hack to work around promotability")]
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pub const UNINIT: Self = Self::uninit();
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/// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
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/// filled with `0` bytes. It depends on `T` whether that already makes for
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/// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,
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/// but `MaybeUninit<&'static i32>::zeroed()` is not because references must not
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/// be null.
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///
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/// Note that dropping a `MaybeUninit<T>` will never call `T`'s drop code.
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/// It is your responsibility to make sure `T` gets dropped if it got initialized.
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///
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/// # Example
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///
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/// Correct usage of this function: initializing a struct with zero, where all
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/// fields of the struct can hold the bit-pattern 0 as a valid value.
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// let x = MaybeUninit::<(u8, bool)>::zeroed();
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/// let x = unsafe { x.assume_init() };
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/// assert_eq!(x, (0, false));
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/// ```
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///
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/// *Incorrect* usage of this function: initializing a struct with zero, where some fields
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/// cannot hold 0 as a valid value.
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///
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/// ```rust,no_run
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/// use std::mem::MaybeUninit;
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///
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/// enum NotZero { One = 1, Two = 2 };
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///
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/// let x = MaybeUninit::<(u8, NotZero)>::zeroed();
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/// let x = unsafe { x.assume_init() };
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/// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
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/// // This is undefined behavior.
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/// ```
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline]
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pub fn zeroed() -> MaybeUninit<T> {
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let mut u = MaybeUninit::<T>::uninit();
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unsafe {
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u.as_mut_ptr().write_bytes(0u8, 1);
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}
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u
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}
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/// Sets the value of the `MaybeUninit<T>`. This overwrites any previous value
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/// without dropping it, so be careful not to use this twice unless you want to
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/// skip running the destructor. For your convenience, this also returns a mutable
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/// reference to the (now safely initialized) contents of `self`.
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#[unstable(feature = "maybe_uninit_extra", issue = "63567")]
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#[inline(always)]
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pub fn write(&mut self, val: T) -> &mut T {
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unsafe {
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self.value = ManuallyDrop::new(val);
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self.get_mut()
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}
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}
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/// Gets a pointer to the contained value. Reading from this pointer or turning it
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/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
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/// Writing to memory that this pointer (non-transitively) points to is undefined behavior
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/// (except inside an `UnsafeCell<T>`).
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///
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/// # Examples
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///
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/// Correct usage of this method:
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
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/// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
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/// // Create a reference into the `MaybeUninit<T>`. This is okay because we initialized it.
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/// let x_vec = unsafe { &*x.as_ptr() };
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/// assert_eq!(x_vec.len(), 3);
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/// ```
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///
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/// *Incorrect* usage of this method:
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///
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/// ```rust,no_run
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/// use std::mem::MaybeUninit;
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///
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/// let x = MaybeUninit::<Vec<u32>>::uninit();
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/// let x_vec = unsafe { &*x.as_ptr() };
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/// // We have created a reference to an uninitialized vector! This is undefined behavior.
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/// ```
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///
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/// (Notice that the rules around references to uninitialized data are not finalized yet, but
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/// until they are, it is advisable to avoid them.)
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline(always)]
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pub fn as_ptr(&self) -> *const T {
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unsafe { &*self.value as *const T }
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}
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/// Gets a mutable pointer to the contained value. Reading from this pointer or turning it
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/// into a reference is undefined behavior unless the `MaybeUninit<T>` is initialized.
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///
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/// # Examples
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///
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/// Correct usage of this method:
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
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/// unsafe { x.as_mut_ptr().write(vec![0,1,2]); }
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/// // Create a reference into the `MaybeUninit<Vec<u32>>`.
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/// // This is okay because we initialized it.
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/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
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/// x_vec.push(3);
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/// assert_eq!(x_vec.len(), 4);
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/// ```
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///
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/// *Incorrect* usage of this method:
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///
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/// ```rust,no_run
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
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/// let x_vec = unsafe { &mut *x.as_mut_ptr() };
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/// // We have created a reference to an uninitialized vector! This is undefined behavior.
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/// ```
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///
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/// (Notice that the rules around references to uninitialized data are not finalized yet, but
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/// until they are, it is advisable to avoid them.)
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline(always)]
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pub fn as_mut_ptr(&mut self) -> *mut T {
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unsafe { &mut *self.value as *mut T }
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}
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/// Extracts the value from the `MaybeUninit<T>` container. This is a great way
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/// to ensure that the data will get dropped, because the resulting `T` is
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/// subject to the usual drop handling.
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///
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/// # Safety
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///
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/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
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/// state. Calling this when the content is not yet fully initialized causes immediate undefined
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/// behavior. The [type-level documentation][inv] contains more information about
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/// this initialization invariant.
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///
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/// [inv]: #initialization-invariant
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///
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/// On top of that, remember that most types have additional invariants beyond merely
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/// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
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/// is considered initialized (under the current implementation; this does not constitute
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/// a stable guarantee) because the only requirement the compiler knows about it
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/// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
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/// *immediate* undefined behavior, but will cause undefined behavior with most
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/// safe operations (including dropping it).
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///
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/// # Examples
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///
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/// Correct usage of this method:
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///
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/// ```rust
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<bool>::uninit();
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/// unsafe { x.as_mut_ptr().write(true); }
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/// let x_init = unsafe { x.assume_init() };
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/// assert_eq!(x_init, true);
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/// ```
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///
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/// *Incorrect* usage of this method:
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///
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/// ```rust,no_run
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/// use std::mem::MaybeUninit;
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///
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/// let x = MaybeUninit::<Vec<u32>>::uninit();
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/// let x_init = unsafe { x.assume_init() };
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/// // `x` had not been initialized yet, so this last line caused undefined behavior.
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/// ```
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#[stable(feature = "maybe_uninit", since = "1.36.0")]
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#[inline(always)]
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pub unsafe fn assume_init(self) -> T {
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intrinsics::panic_if_uninhabited::<T>();
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ManuallyDrop::into_inner(self.value)
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}
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/// Reads the value from the `MaybeUninit<T>` container. The resulting `T` is subject
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/// to the usual drop handling.
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///
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/// Whenever possible, it is preferable to use [`assume_init`] instead, which
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/// prevents duplicating the content of the `MaybeUninit<T>`.
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///
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/// # Safety
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///
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/// It is up to the caller to guarantee that the `MaybeUninit<T>` really is in an initialized
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/// state. Calling this when the content is not yet fully initialized causes undefined
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/// behavior. The [type-level documentation][inv] contains more information about
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/// this initialization invariant.
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///
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/// Moreover, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using
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/// multiple copies of the data (by calling `read` multiple times, or first
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/// calling `read` and then [`assume_init`]), it is your responsibility
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/// to ensure that that data may indeed be duplicated.
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///
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/// [inv]: #initialization-invariant
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/// [`assume_init`]: #method.assume_init
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///
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/// # Examples
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///
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/// Correct usage of this method:
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///
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/// ```rust
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/// #![feature(maybe_uninit_extra)]
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<u32>::uninit();
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/// x.write(13);
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/// let x1 = unsafe { x.read() };
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/// // `u32` is `Copy`, so we may read multiple times.
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/// let x2 = unsafe { x.read() };
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/// assert_eq!(x1, x2);
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///
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/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
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/// x.write(None);
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/// let x1 = unsafe { x.read() };
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/// // Duplicating a `None` value is okay, so we may read multiple times.
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/// let x2 = unsafe { x.read() };
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/// assert_eq!(x1, x2);
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/// ```
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///
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/// *Incorrect* usage of this method:
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///
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|
/// ```rust,no_run
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/// #![feature(maybe_uninit_extra)]
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/// use std::mem::MaybeUninit;
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///
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/// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
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/// x.write(Some(vec![0,1,2]));
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/// let x1 = unsafe { x.read() };
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/// let x2 = unsafe { x.read() };
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/// // We now created two copies of the same vector, leading to a double-free when
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/// // they both get dropped!
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/// ```
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#[unstable(feature = "maybe_uninit_extra", issue = "63567")]
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#[inline(always)]
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pub unsafe fn read(&self) -> T {
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intrinsics::panic_if_uninhabited::<T>();
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self.as_ptr().read()
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}
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/// Gets a shared reference to the contained value.
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///
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|
/// This can be useful when we want to access a `MaybeUninit` that has been
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|
/// initialized but don't have ownership of the `MaybeUninit` (preventing the use
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|
/// of `.assume_init()`).
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|
///
|
|
/// # 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.
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|
///
|
|
/// # Examples
|
|
///
|
|
/// ### Correct usage of this method:
|
|
///
|
|
/// ```rust
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|
/// #![feature(maybe_uninit_ref)]
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|
/// use ::std::mem::MaybeUninit;
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|
///
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/// let mut x = MaybeUninit::<Vec<u32>>::uninit();
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|
/// // Initialize `x`:
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|
/// unsafe { x.as_mut_ptr().write(vec![1, 2, 3]); }
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|
/// /* The above line can also be done without unsafe:
|
|
/// x = MaybeUninit::new(vec![1, 2, 3]); // */
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|
/// // 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
|
|
/// //
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|
/// // - `x` has been initialized.
|
|
/// x.get_ref()
|
|
/// };
|
|
/// assert_eq!(x, &vec![1, 2, 3]);
|
|
/// ```
|
|
///
|
|
/// ### *Incorrect* usages of this method:
|
|
///
|
|
/// ```rust,no_run
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// use std::mem::MaybeUninit;
|
|
///
|
|
/// let x = MaybeUninit::<Vec<u32>>::uninit();
|
|
/// let x_vec: &Vec<u32> = unsafe { x.get_ref() };
|
|
/// // We have created a reference to an uninitialized vector! This is undefined behavior.
|
|
/// ```
|
|
///
|
|
/// ```rust,no_run
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// use std::{cell::Cell, mem::MaybeUninit};
|
|
///
|
|
/// let b = MaybeUninit::<Cell<bool>>::uninit();
|
|
/// // Initialize the `MaybeUninit` using `Cell::set`:
|
|
/// unsafe {
|
|
/// b.get_ref().set(true);
|
|
/// // ^^^^^^^^^^^
|
|
/// // Reference to an uninitialized `Cell<bool>`: UB!
|
|
/// }
|
|
/// ```
|
|
#[unstable(feature = "maybe_uninit_ref", issue = "63568")]
|
|
#[inline(always)]
|
|
pub unsafe fn get_ref(&self) -> &T {
|
|
&*self.value
|
|
}
|
|
|
|
/// 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, `.get_mut()` cannot be used to
|
|
/// initialize a `MaybeUninit`.
|
|
///
|
|
/// # Examples
|
|
///
|
|
/// ### Correct usage of this method:
|
|
///
|
|
/// ```rust
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// use ::std::mem::MaybeUninit;
|
|
///
|
|
/// # unsafe extern "C" fn initialize_buffer (buf: *mut [u8; 2048]) { *buf = [0; 2048] }
|
|
/// # #[cfg(FALSE)]
|
|
/// extern "C" {
|
|
/// /// Initializes *all* the bytes of the input buffer.
|
|
/// fn initialize_buffer (buf: *mut [u8; 2048]);
|
|
/// }
|
|
///
|
|
/// let mut buf = MaybeUninit::<[u8; 2048]>::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 2048 bytes.
|
|
/// // To assert our buffer has been initialized without copying it, we upgrade
|
|
/// // the `&mut MaybeUninit<[u8; 2048]>` to a `&mut [u8; 2048]`:
|
|
/// let buf: &mut [u8; 2048] = unsafe {
|
|
/// // # Safety
|
|
/// //
|
|
/// // - `buf` has been initialized.
|
|
/// buf.get_mut()
|
|
/// };
|
|
///
|
|
/// // Now we can use `buf` as a normal slice:
|
|
/// buf.sort_unstable();
|
|
/// assert!(
|
|
/// buf.chunks(2).all(|chunk| chunk[0] <= chunk[1]),
|
|
/// "buffer is sorted",
|
|
/// );
|
|
/// ```
|
|
///
|
|
/// ### *Incorrect* usages of this method:
|
|
///
|
|
/// You cannot use `.get_mut()` to initialize a value:
|
|
///
|
|
/// ```rust,no_run
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// use std::mem::MaybeUninit;
|
|
///
|
|
/// let mut b = MaybeUninit::<bool>::uninit();
|
|
/// unsafe {
|
|
/// *b.get_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`]: https://doc.rust-lang.org/std/io/trait.Read.html
|
|
///
|
|
/// ```rust,no_run
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// 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.get_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
|
|
/// #![feature(maybe_uninit_ref)]
|
|
/// 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.get_mut().a as *mut u32, 1337);
|
|
/// // ^^^^^^^^^^^^^
|
|
/// // (mutable) reference to uninitialized memory!
|
|
/// // This is undefined behavior.
|
|
/// ptr::write(&mut foo.get_mut().b as *mut u8, 42);
|
|
/// // ^^^^^^^^^^^^^
|
|
/// // (mutable) reference to uninitialized memory!
|
|
/// // This is undefined behavior.
|
|
/// foo.assume_init()
|
|
/// };
|
|
/// ```
|
|
// FIXME(#53491): We currently rely on the above being incorrect, i.e., we have references
|
|
// to uninitialized data (e.g., in `libcore/fmt/float.rs`). We should make
|
|
// a final decision about the rules before stabilization.
|
|
#[unstable(feature = "maybe_uninit_ref", issue = "63568")]
|
|
#[inline(always)]
|
|
pub unsafe fn get_mut(&mut self) -> &mut T {
|
|
&mut *self.value
|
|
}
|
|
|
|
/// Gets a pointer to the first element of the array.
|
|
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
|
|
#[inline(always)]
|
|
pub fn first_ptr(this: &[MaybeUninit<T>]) -> *const T {
|
|
this as *const [MaybeUninit<T>] as *const T
|
|
}
|
|
|
|
/// Gets a mutable pointer to the first element of the array.
|
|
#[unstable(feature = "maybe_uninit_slice", issue = "63569")]
|
|
#[inline(always)]
|
|
pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T {
|
|
this as *mut [MaybeUninit<T>] as *mut T
|
|
}
|
|
}
|