maybe_uninit.rs.html -- source
Description: Source to the Rust file `src\libcore\mem\maybe_uninit.rs`.
Keywords: rust, rustlang, rust-lang

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    use crate::any::type_name;
    use crate::fmt;
    use crate::intrinsics;
    use crate::mem::ManuallyDrop;

    // ignore-tidy-undocumented-unsafe

    /// 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 the compiler knows that
    /// it does not have a fixed value. This makes it undefined behavior to have
    /// uninitialized data in a variable even if that variable has an integer type,
    /// which otherwise can hold any *fixed* bit pattern:
    ///
    /// ```rust,no_run
    /// # #![allow(invalid_value)]
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let x: i32 = unsafe { mem::uninitialized() }; // undefined behavior! ⚠️
    /// // The equivalent code with `MaybeUninit<i32>`:
    /// let x: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
    /// ```
    /// (Notice that the rules around uninitialized integers are not finalized yet, but
    /// until they are, it is advisable to avoid them.)
    ///
    /// On top of that, remember that most types have additional invariants beyond merely
    /// being considered initialized at the type level. For example, a `1`-initialized [`Vec<T>`]
    /// is considered initialized (under the current implementation; this does not constitute
    /// a stable guarantee) because the only requirement the compiler knows about it
    /// is that the data pointer must be non-null. Creating such a `Vec<T>` does not cause
    /// *immediate* undefined behavior, but will cause undefined behavior with most
    /// safe operations (including dropping it).
    ///
    /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
    ///
    /// # Examples
    ///
    /// `MaybeUninit<T>` serves to enable unsafe code to deal with uninitialized data.
    /// It is a signal to the compiler indicating that the data here might *not*
    /// be initialized:
    ///
    /// ```rust
    /// use std::mem::MaybeUninit;
    ///
    /// // Create an explicitly uninitialized reference. The compiler knows that data inside
    /// // a `MaybeUninit<T>` may be invalid, and hence this is not UB:
    /// let mut x = MaybeUninit::<&i32>::uninit();
    /// // Set it to a valid value.
    /// unsafe { x.as_mut_ptr().write(&0); }
    /// // Extract the initialized data -- this is only allowed *after* properly
    /// // initializing `x`!
    /// let x = unsafe { x.assume_init() };
    /// ```
    ///
    /// The compiler then knows to not make any incorrect assumptions or optimizations on this code.
    ///
    /// You can think of `MaybeUninit<T>` as being a bit like `Option<T>` but without
    /// any of the run-time tracking and without any of the safety checks.
    ///
    /// ## out-pointers
    ///
    /// You can use `MaybeUninit<T>` to implement "out-pointers": instead of returning data
    /// from a function, pass it a pointer to some (uninitialized) memory to put the
    /// result into. This can be useful when it is important for the caller to control
    /// how the memory the result is stored in gets allocated, and you want to avoid
    /// unnecessary moves.
    ///
    /// ```
    /// use std::mem::MaybeUninit;
    ///
    /// unsafe fn make_vec(out: *mut Vec<i32>) {
    ///     // `write` does not drop the old contents, which is important.
    ///     out.write(vec![1, 2, 3]);
    /// }
    ///
    /// let mut v = MaybeUninit::uninit();
    /// unsafe { make_vec(v.as_mut_ptr()); }
    /// // Now we know `v` is initialized! This also makes sure the vector gets
    /// // properly dropped.
    /// let v = unsafe { v.assume_init() };
    /// assert_eq!(&v, &[1, 2, 3]);
    /// ```
    ///
    /// ## Initializing an array element-by-element
    ///
    /// `MaybeUninit<T>` can be used to initialize a large array element-by-element:
    ///
    /// ```
    /// use std::mem::{self, MaybeUninit};
    ///
    /// let data = {
    ///     // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    ///     // safe because the type we are claiming to have initialized here is a
    ///     // bunch of `MaybeUninit`s, which do not require initialization.
    ///     let mut data: [MaybeUninit<Vec<u32>>; 1000] = unsafe {
    ///         MaybeUninit::uninit().assume_init()
    ///     };
    ///
    ///     // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
    ///     // assignment instead of `ptr::write` does not cause the old
    ///     // uninitialized value to be dropped. Also if there is a panic during
    ///     // this loop, we have a memory leak, but there is no memory safety
    ///     // issue.
    ///     for elem in &mut data[..] {
    ///         *elem = MaybeUninit::new(vec![42]);
    ///     }
    ///
    ///     // Everything is initialized. Transmute the array to the
    ///     // initialized type.
    ///     unsafe { mem::transmute::<_, [Vec<u32>; 1000]>(data) }
    /// };
    ///
    /// assert_eq!(&data[0], &[42]);
    /// ```
    ///
    /// You can also work with partially initialized arrays, which could
    /// be found in low-level datastructures.
    ///
    /// ```
    /// use std::mem::MaybeUninit;
    /// use std::ptr;
    ///
    /// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
    /// // safe because the type we are claiming to have initialized here is a
    /// // bunch of `MaybeUninit`s, which do not require initialization.
    /// let mut data: [MaybeUninit<String>; 1000] = unsafe { MaybeUninit::uninit().assume_init() };
    /// // Count the number of elements we have assigned.
    /// let mut data_len: usize = 0;
    ///
    /// for elem in &mut data[0..500] {
    ///     *elem = MaybeUninit::new(String::from("hello"));
    ///     data_len += 1;
    /// }
    ///
    /// // For each item in the array, drop if we allocated it.
    /// for elem in &mut data[0..data_len] {
    ///     unsafe { ptr::drop_in_place(elem.as_mut_ptr()); }
    /// }
    /// ```
    ///
    /// ## Initializing a struct field-by-field
    ///
    /// There is currently no supported way to create a raw pointer or reference
    /// to a field of a struct inside `MaybeUninit<Struct>`. That means it is not possible
    /// to create a struct by calling `MaybeUninit::uninit::<Struct>()` and then writing
    /// to its fields.
    ///
    /// [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.
        ///
        /// [`assume_init`]: #method.assume_init
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
        #[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][type] for some examples.
        ///
        /// [type]: union.MaybeUninit.html
        #[stable(feature = "maybe_uninit", since = "1.36.0")]
        #[rustc_const_stable(feature = "const_maybe_uninit", since = "1.36.0")]
        #[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 array literal syntax allows
        /// [repeating const expressions](https://github.com/rust-lang/rust/issues/49147).
        /// The example below could then use `let mut buf = [MaybeUninit::<u8>::uninit(); 32];`.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice_assume_init)]
        ///
        /// 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_get_ref(&buf[..len])
        ///     }
        /// }
        ///
        /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
        /// let data = read(&mut buf);
        /// ```
        #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
        #[inline(always)]
        pub fn uninit_array<const LEN: usize>() -> [Self; LEN] {
            unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
        }

        /// A promotable constant, equivalent to `uninit()`.
        #[unstable(
            feature = "internal_uninit_const",
            issue = "none",
            reason = "hack to work around promotability"
        )]
        pub const UNINIT: Self = Self::uninit();

        /// 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: initializing a struct with zero, where some fields
        /// cannot hold 0 as a valid value.
        ///
        /// ```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")]
        #[inline]
        #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
        pub fn zeroed() -> MaybeUninit<T> {
            let mut u = MaybeUninit::<T>::uninit();
            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`.
        #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
        #[inline(always)]
        pub fn write(&mut self, val: T) -> &mut T {
            unsafe {
                self.value = ManuallyDrop::new(val);
                self.get_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();
        /// unsafe { x.as_mut_ptr().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")]
        #[inline(always)]
        pub fn as_ptr(&self) -> *const T {
            unsafe { &*self.value 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();
        /// unsafe { x.as_mut_ptr().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")]
        #[inline(always)]
        pub fn as_mut_ptr(&mut self) -> *mut T {
            unsafe { &mut *self.value 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).
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<bool>::uninit();
        /// unsafe { x.as_mut_ptr().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")]
        #[inline(always)]
        #[rustc_diagnostic_item = "assume_init"]
        pub 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, this leaves a copy of the same data behind in the `MaybeUninit<T>`. When using
        /// multiple copies of the data (by calling `read` multiple times, or first
        /// calling `read` and then [`assume_init`]), it is your responsibility
        /// to ensure that that data may indeed be duplicated.
        ///
        /// [inv]: #initialization-invariant
        /// [`assume_init`]: #method.assume_init
        ///
        /// # Examples
        ///
        /// Correct usage of this method:
        ///
        /// ```rust
        /// #![feature(maybe_uninit_extra)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<u32>::uninit();
        /// x.write(13);
        /// let x1 = unsafe { x.read() };
        /// // `u32` is `Copy`, so we may read multiple times.
        /// let x2 = unsafe { x.read() };
        /// assert_eq!(x1, x2);
        ///
        /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
        /// x.write(None);
        /// let x1 = unsafe { x.read() };
        /// // Duplicating a `None` value is okay, so we may read multiple times.
        /// let x2 = unsafe { x.read() };
        /// assert_eq!(x1, x2);
        /// ```
        ///
        /// *Incorrect* usage of this method:
        ///
        /// ```rust,no_run
        /// #![feature(maybe_uninit_extra)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
        /// x.write(Some(vec![0,1,2]));
        /// let x1 = unsafe { x.read() };
        /// let x2 = unsafe { x.read() };
        /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
        /// // they both get dropped!
        /// ```
        #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
        #[inline(always)]
        pub unsafe fn 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()
            }
        }

        /// 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
        /// #![feature(maybe_uninit_ref)]
        /// use std::mem::MaybeUninit;
        ///
        /// let mut x = MaybeUninit::<Vec<u32>>::uninit();
        /// // Initialize `x`:
        /// unsafe { x.as_mut_ptr().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.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 {
            // 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.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.windows(2).all(|pair| pair[0] <= pair[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 {
            // 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.value
            }
        }

        /// 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.
        #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
        #[inline(always)]
        pub unsafe fn slice_get_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.
        #[unstable(feature = "maybe_uninit_slice_assume_init", issue = "none")]
        #[inline(always)]
        pub unsafe fn slice_get_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")]
        #[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
        }
    }