split libcore::mem into multiple files

Author: RalfJung

src/libcore/mem.rs

@@ -0,0 +1,146 @@
+use crate::ptr;
+use crate::ops::{Deref, DerefMut};
+
+/// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
+///
+/// This wrapper is 0-cost.
+///
+/// `ManuallyDrop<T>` is subject to the same layout optimizations as `T`.
+/// As a consequence, it has *no effect* on the assumptions that the compiler makes
+/// about all values being initialized at their type.  In particular, initializing
+/// a `ManuallyDrop<&mut T>` with [`mem::zeroed`] is undefined behavior.
+/// If you need to handle uninitialized data, use [`MaybeUninit<T>`] instead.
+///
+/// # Examples
+///
+/// This wrapper helps with explicitly documenting the drop order dependencies between fields of
+/// the type:
+///
+/// ```rust
+/// use std::mem::ManuallyDrop;
+/// struct Peach;
+/// struct Banana;
+/// struct Melon;
+/// struct FruitBox {
+///     // Immediately clear there’s something non-trivial going on with these fields.
+///     peach: ManuallyDrop<Peach>,
+///     melon: Melon, // Field that’s independent of the other two.
+///     banana: ManuallyDrop<Banana>,
+/// }
+///
+/// impl Drop for FruitBox {
+///     fn drop(&mut self) {
+///         unsafe {
+///             // Explicit ordering in which field destructors are run specified in the intuitive
+///             // location – the destructor of the structure containing the fields.
+///             // Moreover, one can now reorder fields within the struct however much they want.
+///             ManuallyDrop::drop(&mut self.peach);
+///             ManuallyDrop::drop(&mut self.banana);
+///         }
+///         // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
+///         // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
+///     }
+/// }
+/// ```
+///
+/// [`mem::zeroed`]: fn.zeroed.html
+/// [`MaybeUninit<T>`]: union.MaybeUninit.html
+#[stable(feature = "manually_drop", since = "1.20.0")]
+#[lang = "manually_drop"]
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+#[repr(transparent)]
+pub struct ManuallyDrop<T: ?Sized> {
+    value: T,
+}
+
+impl<T> ManuallyDrop<T> {
+    /// Wrap a value to be manually dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::mem::ManuallyDrop;
+    /// ManuallyDrop::new(Box::new(()));
+    /// ```
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[inline(always)]
+    pub const fn new(value: T) -> ManuallyDrop<T> {
+        ManuallyDrop { value }
+    }
+
+    /// Extracts the value from the `ManuallyDrop` container.
+    ///
+    /// This allows the value to be dropped again.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::mem::ManuallyDrop;
+    /// let x = ManuallyDrop::new(Box::new(()));
+    /// let _: Box<()> = ManuallyDrop::into_inner(x); // This drops the `Box`.
+    /// ```
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[inline(always)]
+    pub const fn into_inner(slot: ManuallyDrop<T>) -> T {
+        slot.value
+    }
+
+    /// Takes the contained value out.
+    ///
+    /// This method is primarily intended for moving out values in drop.
+    /// Instead of using [`ManuallyDrop::drop`] to manually drop the value,
+    /// you can use this method to take the value and use it however desired.
+    /// `Drop` will be invoked on the returned value following normal end-of-scope rules.
+    ///
+    /// If you have ownership of the container, you can use [`ManuallyDrop::into_inner`] instead.
+    ///
+    /// # Safety
+    ///
+    /// This function semantically moves out the contained value without preventing further usage.
+    /// It is up to the user of this method to ensure that this container is not used again.
+    ///
+    /// [`ManuallyDrop::drop`]: #method.drop
+    /// [`ManuallyDrop::into_inner`]: #method.into_inner
+    #[must_use = "if you don't need the value, you can use `ManuallyDrop::drop` instead"]
+    #[unstable(feature = "manually_drop_take", issue = "55422")]
+    #[inline]
+    pub unsafe fn take(slot: &mut ManuallyDrop<T>) -> T {
+        ManuallyDrop::into_inner(ptr::read(slot))
+    }
+}
+
+impl<T: ?Sized> ManuallyDrop<T> {
+    /// Manually drops the contained value.
+    ///
+    /// If you have ownership of the value, you can use [`ManuallyDrop::into_inner`] instead.
+    ///
+    /// # Safety
+    ///
+    /// This function runs the destructor of the contained value and thus the wrapped value
+    /// now represents uninitialized data. It is up to the user of this method to ensure the
+    /// uninitialized data is not actually used.
+    ///
+    /// [`ManuallyDrop::into_inner`]: #method.into_inner
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[inline]
+    pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
+        ptr::drop_in_place(&mut slot.value)
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized> Deref for ManuallyDrop<T> {
+    type Target = T;
+    #[inline(always)]
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
+    #[inline(always)]
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+}

src/libcore/mem/manually_drop.rs

@@ -0,0 +1,519 @@
+use crate::intrinsics;
+use crate::mem::ManuallyDrop;
+
+/// A wrapper type to construct uninitialized instances of `T`.
+///
+/// # Initialization invariant
+///
+/// The compiler, in general, assumes that variables are properly initialized
+/// at their respective 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
+/// 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
+/// 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
+/// 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 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};
+/// use std::ptr;
+///
+/// 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[..] {
+///         unsafe { ptr::write(elem.as_mut_ptr(), 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] {
+///     unsafe { ptr::write(elem.as_mut_ptr(), 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 and alignment 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);
+/// ```
+#[allow(missing_debug_implementations)]
+#[stable(feature = "maybe_uninit", since = "1.36.0")]
+#[derive(Copy)]
+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
+    }
+}
+
+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")]
+    #[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")]
+    #[inline(always)]
+    pub const fn uninit() -> MaybeUninit<T> {
+        MaybeUninit { 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]
+    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 = "53491")]
+    #[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
+    ///
+    /// # 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)]
+    pub unsafe fn assume_init(self) -> T {
+        intrinsics::panic_if_uninhabited::<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 preferrable 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 = "53491")]
+    #[inline(always)]
+    pub unsafe fn read(&self) -> T {
+        intrinsics::panic_if_uninhabited::<T>();
+        self.as_ptr().read()
+    }
+
+    /// Gets a reference to the contained value.
+    ///
+    /// # 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.
+    #[unstable(feature = "maybe_uninit_ref", issue = "53491")]
+    #[inline(always)]
+    pub unsafe fn get_ref(&self) -> &T {
+        &*self.value
+    }
+
+    /// Gets a mutable reference to the contained value.
+    ///
+    /// # 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.
+    // 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 = "53491")]
+    #[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 = "53491")]
+    #[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 = "53491")]
+    #[inline(always)]
+    pub fn first_ptr_mut(this: &mut [MaybeUninit<T>]) -> *mut T {
+        this as *mut [MaybeUninit<T>] as *mut T
+    }
+}