split libcore::mem into multiple files

4 files changed, 1417 insertions, 1406 deletions

diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
deleted file mode 100644
index 40f4354213b..00000000000
--- a/src/libcore/mem.rs
+++ /dev/null
@@ -1,1406 +0,0 @@
-//! Basic functions for dealing with memory.
-//!
-//! This module contains functions for querying the size and alignment of
-//! types, initializing and manipulating memory.
-
-#![stable(feature = "rust1", since = "1.0.0")]
-
-use crate::clone;
-use crate::cmp;
-use crate::fmt;
-use crate::hash;
-use crate::intrinsics;
-use crate::marker::{Copy, PhantomData, Sized};
-use crate::ptr;
-use crate::ops::{Deref, DerefMut};
-
-#[stable(feature = "rust1", since = "1.0.0")]
-#[doc(inline)]
-pub use crate::intrinsics::transmute;
-
-/// Takes ownership and "forgets" about the value **without running its destructor**.
-///
-/// Any resources the value manages, such as heap memory or a file handle, will linger
-/// forever in an unreachable state. However, it does not guarantee that pointers
-/// to this memory will remain valid.
-///
-/// * If you want to leak memory, see [`Box::leak`][leak].
-/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
-/// * If you want to dispose of a value properly, running its destructor, see
-/// [`mem::drop`][drop].
-///
-/// # Safety
-///
-/// `forget` is not marked as `unsafe`, because Rust's safety guarantees
-/// do not include a guarantee that destructors will always run. For example,
-/// a program can create a reference cycle using [`Rc`][rc], or call
-/// [`process::exit`][exit] to exit without running destructors. Thus, allowing
-/// `mem::forget` from safe code does not fundamentally change Rust's safety
-/// guarantees.
-///
-/// That said, leaking resources such as memory or I/O objects is usually undesirable,
-/// so `forget` is only recommended for specialized use cases like those shown below.
-///
-/// Because forgetting a value is allowed, any `unsafe` code you write must
-/// allow for this possibility. You cannot return a value and expect that the
-/// caller will necessarily run the value's destructor.
-///
-/// [rc]: ../../std/rc/struct.Rc.html
-/// [exit]: ../../std/process/fn.exit.html
-///
-/// # Examples
-///
-/// Leak an I/O object, never closing the file:
-///
-/// ```no_run
-/// use std::mem;
-/// use std::fs::File;
-///
-/// let file = File::open("foo.txt").unwrap();
-/// mem::forget(file);
-/// ```
-///
-/// The practical use cases for `forget` are rather specialized and mainly come
-/// up in unsafe or FFI code.
-///
-/// [drop]: fn.drop.html
-/// [uninit]: fn.uninitialized.html
-/// [clone]: ../clone/trait.Clone.html
-/// [swap]: fn.swap.html
-/// [box]: ../../std/boxed/struct.Box.html
-/// [leak]: ../../std/boxed/struct.Box.html#method.leak
-/// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
-/// [ub]: ../../reference/behavior-considered-undefined.html
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn forget<T>(t: T) {
-    ManuallyDrop::new(t);
-}
-
-/// Like [`forget`], but also accepts unsized values.
-///
-/// This function is just a shim intended to be removed when the `unsized_locals` feature gets
-/// stabilized.
-///
-/// [`forget`]: fn.forget.html
-#[inline]
-#[unstable(feature = "forget_unsized", issue = "0")]
-pub fn forget_unsized<T: ?Sized>(t: T) {
-    unsafe { intrinsics::forget(t) }
-}
-
-/// Returns the size of a type in bytes.
-///
-/// More specifically, this is the offset in bytes between successive elements
-/// in an array with that item type including alignment padding. Thus, for any
-/// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
-///
-/// In general, the size of a type is not stable across compilations, but
-/// specific types such as primitives are.
-///
-/// The following table gives the size for primitives.
-///
-/// Type | size_of::\<Type>()
-/// ---- | ---------------
-/// () | 0
-/// bool | 1
-/// u8 | 1
-/// u16 | 2
-/// u32 | 4
-/// u64 | 8
-/// u128 | 16
-/// i8 | 1
-/// i16 | 2
-/// i32 | 4
-/// i64 | 8
-/// i128 | 16
-/// f32 | 4
-/// f64 | 8
-/// char | 4
-///
-/// Furthermore, `usize` and `isize` have the same size.
-///
-/// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
-/// the same size. If `T` is Sized, all of those types have the same size as `usize`.
-///
-/// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
-/// have the same size. Likewise for `*const T` and `*mut T`.
-///
-/// # Size of `#[repr(C)]` items
-///
-/// The `C` representation for items has a defined layout. With this layout,
-/// the size of items is also stable as long as all fields have a stable size.
-///
-/// ## Size of Structs
-///
-/// For `structs`, the size is determined by the following algorithm.
-///
-/// For each field in the struct ordered by declaration order:
-///
-/// 1. Add the size of the field.
-/// 2. Round up the current size to the nearest multiple of the next field's [alignment].
-///
-/// Finally, round the size of the struct to the nearest multiple of its [alignment].
-/// The alignment of the struct is usually the largest alignment of all its
-/// fields; this can be changed with the use of `repr(align(N))`.
-///
-/// Unlike `C`, zero sized structs are not rounded up to one byte in size.
-///
-/// ## Size of Enums
-///
-/// Enums that carry no data other than the discriminant have the same size as C enums
-/// on the platform they are compiled for.
-///
-/// ## Size of Unions
-///
-/// The size of a union is the size of its largest field.
-///
-/// Unlike `C`, zero sized unions are not rounded up to one byte in size.
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// // Some primitives
-/// assert_eq!(4, mem::size_of::<i32>());
-/// assert_eq!(8, mem::size_of::<f64>());
-/// assert_eq!(0, mem::size_of::<()>());
-///
-/// // Some arrays
-/// assert_eq!(8, mem::size_of::<[i32; 2]>());
-/// assert_eq!(12, mem::size_of::<[i32; 3]>());
-/// assert_eq!(0, mem::size_of::<[i32; 0]>());
-///
-///
-/// // Pointer size equality
-/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
-/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
-/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
-/// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
-/// ```
-///
-/// Using `#[repr(C)]`.
-///
-/// ```
-/// use std::mem;
-///
-/// #[repr(C)]
-/// struct FieldStruct {
-///     first: u8,
-///     second: u16,
-///     third: u8
-/// }
-///
-/// // The size of the first field is 1, so add 1 to the size. Size is 1.
-/// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
-/// // The size of the second field is 2, so add 2 to the size. Size is 4.
-/// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
-/// // The size of the third field is 1, so add 1 to the size. Size is 5.
-/// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
-/// // fields is 2), so add 1 to the size for padding. Size is 6.
-/// assert_eq!(6, mem::size_of::<FieldStruct>());
-///
-/// #[repr(C)]
-/// struct TupleStruct(u8, u16, u8);
-///
-/// // Tuple structs follow the same rules.
-/// assert_eq!(6, mem::size_of::<TupleStruct>());
-///
-/// // Note that reordering the fields can lower the size. We can remove both padding bytes
-/// // by putting `third` before `second`.
-/// #[repr(C)]
-/// struct FieldStructOptimized {
-///     first: u8,
-///     third: u8,
-///     second: u16
-/// }
-///
-/// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
-///
-/// // Union size is the size of the largest field.
-/// #[repr(C)]
-/// union ExampleUnion {
-///     smaller: u8,
-///     larger: u16
-/// }
-///
-/// assert_eq!(2, mem::size_of::<ExampleUnion>());
-/// ```
-///
-/// [alignment]: ./fn.align_of.html
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
-pub const fn size_of<T>() -> usize {
-    intrinsics::size_of::<T>()
-}
-
-/// Returns the size of the pointed-to value in bytes.
-///
-/// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
-/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
-/// then `size_of_val` can be used to get the dynamically-known size.
-///
-/// [slice]: ../../std/primitive.slice.html
-/// [trait object]: ../../book/ch17-02-trait-objects.html
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// assert_eq!(4, mem::size_of_val(&5i32));
-///
-/// let x: [u8; 13] = [0; 13];
-/// let y: &[u8] = &x;
-/// assert_eq!(13, mem::size_of_val(y));
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
-    unsafe { intrinsics::size_of_val(val) }
-}
-
-/// Returns the [ABI]-required minimum alignment of a type.
-///
-/// Every reference to a value of the type `T` must be a multiple of this number.
-///
-/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
-///
-/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
-///
-/// # Examples
-///
-/// ```
-/// # #![allow(deprecated)]
-/// use std::mem;
-///
-/// assert_eq!(4, mem::min_align_of::<i32>());
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
-pub fn min_align_of<T>() -> usize {
-    intrinsics::min_align_of::<T>()
-}
-
-/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
-///
-/// Every reference to a value of the type `T` must be a multiple of this number.
-///
-/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
-///
-/// # Examples
-///
-/// ```
-/// # #![allow(deprecated)]
-/// use std::mem;
-///
-/// assert_eq!(4, mem::min_align_of_val(&5i32));
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
-pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
-    unsafe { intrinsics::min_align_of_val(val) }
-}
-
-/// Returns the [ABI]-required minimum alignment of a type.
-///
-/// Every reference to a value of the type `T` must be a multiple of this number.
-///
-/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
-///
-/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// assert_eq!(4, mem::align_of::<i32>());
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_promotable]
-pub const fn align_of<T>() -> usize {
-    intrinsics::min_align_of::<T>()
-}
-
-/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
-///
-/// Every reference to a value of the type `T` must be a multiple of this number.
-///
-/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// assert_eq!(4, mem::align_of_val(&5i32));
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
-    unsafe { intrinsics::min_align_of_val(val) }
-}
-
-/// Returns `true` if dropping values of type `T` matters.
-///
-/// This is purely an optimization hint, and may be implemented conservatively:
-/// it may return `true` for types that don't actually need to be dropped.
-/// As such always returning `true` would be a valid implementation of
-/// this function. However if this function actually returns `false`, then you
-/// can be certain dropping `T` has no side effect.
-///
-/// Low level implementations of things like collections, which need to manually
-/// drop their data, should use this function to avoid unnecessarily
-/// trying to drop all their contents when they are destroyed. This might not
-/// make a difference in release builds (where a loop that has no side-effects
-/// is easily detected and eliminated), but is often a big win for debug builds.
-///
-/// Note that `ptr::drop_in_place` already performs this check, so if your workload
-/// can be reduced to some small number of drop_in_place calls, using this is
-/// unnecessary. In particular note that you can drop_in_place a slice, and that
-/// will do a single needs_drop check for all the values.
-///
-/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
-/// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
-/// values one at a time and should use this API.
-///
-///
-/// # Examples
-///
-/// Here's an example of how a collection might make use of needs_drop:
-///
-/// ```
-/// use std::{mem, ptr};
-///
-/// pub struct MyCollection<T> {
-/// #   data: [T; 1],
-///     /* ... */
-/// }
-/// # impl<T> MyCollection<T> {
-/// #   fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
-/// #   fn free_buffer(&mut self) {}
-/// # }
-///
-/// impl<T> Drop for MyCollection<T> {
-///     fn drop(&mut self) {
-///         unsafe {
-///             // drop the data
-///             if mem::needs_drop::<T>() {
-///                 for x in self.iter_mut() {
-///                     ptr::drop_in_place(x);
-///                 }
-///             }
-///             self.free_buffer();
-///         }
-///     }
-/// }
-/// ```
-#[inline]
-#[stable(feature = "needs_drop", since = "1.21.0")]
-pub const fn needs_drop<T>() -> bool {
-    intrinsics::needs_drop::<T>()
-}
-
-/// Creates a value whose bytes are all zero.
-///
-/// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
-/// It is useful for FFI sometimes, but should generally be avoided.
-///
-/// There is no guarantee that an all-zero byte-pattern represents a valid value of
-/// some type `T`. For example, the all-zero byte-pattern is not a valid value
-/// for reference types (`&T` and `&mut T`). Using `zeroed` on such types
-/// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv]
-/// that there always is a valid value in a variable it considers initialized.
-///
-/// [zeroed]: union.MaybeUninit.html#method.zeroed
-/// [ub]: ../../reference/behavior-considered-undefined.html
-/// [inv]: union.MaybeUninit.html#initialization-invariant
-///
-/// # Examples
-///
-/// Correct usage of this function: initializing an integer with zero.
-///
-/// ```
-/// use std::mem;
-///
-/// let x: i32 = unsafe { mem::zeroed() };
-/// assert_eq!(0, x);
-/// ```
-///
-/// *Incorrect* usage of this function: initializing a reference with zero.
-///
-/// ```no_run
-/// use std::mem;
-///
-/// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn zeroed<T>() -> T {
-    intrinsics::panic_if_uninhabited::<T>();
-    intrinsics::init()
-}
-
-/// Bypasses Rust's normal memory-initialization checks by pretending to
-/// produce a value of type `T`, while doing nothing at all.
-///
-/// **This functon is deprecated.** Use [`MaybeUninit<T>`] instead.
-///
-/// The reason for deprecation is that the function basically cannot be used
-/// correctly: [the Rust compiler assumes][inv] that values are properly initialized.
-/// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
-/// undefined behavior for returning a `bool` that is not definitely either `true`
-/// or `false`. Worse, truly uninitialized memory like what gets returned here
-/// 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.
-/// (Notice that the rules around uninitialized integers are not finalized yet, but
-/// until they are, it is advisable to avoid them.)
-///
-/// [`MaybeUninit<T>`]: union.MaybeUninit.html
-/// [inv]: union.MaybeUninit.html#initialization-invariant
-#[inline]
-#[rustc_deprecated(since = "1.38.0", reason = "use `mem::MaybeUninit` instead")]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn uninitialized<T>() -> T {
-    intrinsics::panic_if_uninhabited::<T>();
-    intrinsics::uninit()
-}
-
-/// Swaps the values at two mutable locations, without deinitializing either one.
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// let mut x = 5;
-/// let mut y = 42;
-///
-/// mem::swap(&mut x, &mut y);
-///
-/// assert_eq!(42, x);
-/// assert_eq!(5, y);
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn swap<T>(x: &mut T, y: &mut T) {
-    unsafe {
-        ptr::swap_nonoverlapping_one(x, y);
-    }
-}
-
-/// Moves `src` into the referenced `dest`, returning the previous `dest` value.
-///
-/// Neither value is dropped.
-///
-/// # Examples
-///
-/// A simple example:
-///
-/// ```
-/// use std::mem;
-///
-/// let mut v: Vec<i32> = vec![1, 2];
-///
-/// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
-/// assert_eq!(vec![1, 2], old_v);
-/// assert_eq!(vec![3, 4, 5], v);
-/// ```
-///
-/// `replace` allows consumption of a struct field by replacing it with another value.
-/// Without `replace` you can run into issues like these:
-///
-/// ```compile_fail,E0507
-/// struct Buffer<T> { buf: Vec<T> }
-///
-/// impl<T> Buffer<T> {
-///     fn get_and_reset(&mut self) -> Vec<T> {
-///         // error: cannot move out of dereference of `&mut`-pointer
-///         let buf = self.buf;
-///         self.buf = Vec::new();
-///         buf
-///     }
-/// }
-/// ```
-///
-/// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
-/// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
-/// `self`, allowing it to be returned:
-///
-/// ```
-/// # #![allow(dead_code)]
-/// use std::mem;
-///
-/// # struct Buffer<T> { buf: Vec<T> }
-/// impl<T> Buffer<T> {
-///     fn get_and_reset(&mut self) -> Vec<T> {
-///         mem::replace(&mut self.buf, Vec::new())
-///     }
-/// }
-/// ```
-///
-/// [`Clone`]: ../../std/clone/trait.Clone.html
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn replace<T>(dest: &mut T, mut src: T) -> T {
-    swap(dest, &mut src);
-    src
-}
-
-/// Disposes of a value.
-///
-/// This does call the argument's implementation of [`Drop`][drop].
-///
-/// This effectively does nothing for types which implement `Copy`, e.g.
-/// integers. Such values are copied and _then_ moved into the function, so the
-/// value persists after this function call.
-///
-/// This function is not magic; it is literally defined as
-///
-/// ```
-/// pub fn drop<T>(_x: T) { }
-/// ```
-///
-/// Because `_x` is moved into the function, it is automatically dropped before
-/// the function returns.
-///
-/// [drop]: ../ops/trait.Drop.html
-///
-/// # Examples
-///
-/// Basic usage:
-///
-/// ```
-/// let v = vec![1, 2, 3];
-///
-/// drop(v); // explicitly drop the vector
-/// ```
-///
-/// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
-/// release a [`RefCell`] borrow:
-///
-/// ```
-/// use std::cell::RefCell;
-///
-/// let x = RefCell::new(1);
-///
-/// let mut mutable_borrow = x.borrow_mut();
-/// *mutable_borrow = 1;
-///
-/// drop(mutable_borrow); // relinquish the mutable borrow on this slot
-///
-/// let borrow = x.borrow();
-/// println!("{}", *borrow);
-/// ```
-///
-/// Integers and other types implementing [`Copy`] are unaffected by `drop`.
-///
-/// ```
-/// #[derive(Copy, Clone)]
-/// struct Foo(u8);
-///
-/// let x = 1;
-/// let y = Foo(2);
-/// drop(x); // a copy of `x` is moved and dropped
-/// drop(y); // a copy of `y` is moved and dropped
-///
-/// println!("x: {}, y: {}", x, y.0); // still available
-/// ```
-///
-/// [`RefCell`]: ../../std/cell/struct.RefCell.html
-/// [`Copy`]: ../../std/marker/trait.Copy.html
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub fn drop<T>(_x: T) { }
-
-/// Interprets `src` as having type `&U`, and then reads `src` without moving
-/// the contained value.
-///
-/// This function will unsafely assume the pointer `src` is valid for
-/// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
-/// the `&U`. It will also unsafely create a copy of the contained value instead of
-/// moving out of `src`.
-///
-/// It is not a compile-time error if `T` and `U` have different sizes, but it
-/// is highly encouraged to only invoke this function where `T` and `U` have the
-/// same size. This function triggers [undefined behavior][ub] if `U` is larger than
-/// `T`.
-///
-/// [ub]: ../../reference/behavior-considered-undefined.html
-/// [size_of]: fn.size_of.html
-///
-/// # Examples
-///
-/// ```
-/// use std::mem;
-///
-/// #[repr(packed)]
-/// struct Foo {
-///     bar: u8,
-/// }
-///
-/// let foo_slice = [10u8];
-///
-/// unsafe {
-///     // Copy the data from 'foo_slice' and treat it as a 'Foo'
-///     let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
-///     assert_eq!(foo_struct.bar, 10);
-///
-///     // Modify the copied data
-///     foo_struct.bar = 20;
-///     assert_eq!(foo_struct.bar, 20);
-/// }
-///
-/// // The contents of 'foo_slice' should not have changed
-/// assert_eq!(foo_slice, [10]);
-/// ```
-#[inline]
-#[stable(feature = "rust1", since = "1.0.0")]
-pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
-    ptr::read_unaligned(src as *const T as *const U)
-}
-
-/// Opaque type representing the discriminant of an enum.
-///
-/// See the [`discriminant`] function in this module for more information.
-///
-/// [`discriminant`]: fn.discriminant.html
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
-
-// N.B. These trait implementations cannot be derived because we don't want any bounds on T.
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> Copy for Discriminant<T> {}
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> clone::Clone for Discriminant<T> {
-    fn clone(&self) -> Self {
-        *self
-    }
-}
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> cmp::PartialEq for Discriminant<T> {
-    fn eq(&self, rhs: &Self) -> bool {
-        self.0 == rhs.0
-    }
-}
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> cmp::Eq for Discriminant<T> {}
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> hash::Hash for Discriminant<T> {
-    fn hash<H: hash::Hasher>(&self, state: &mut H) {
-        self.0.hash(state);
-    }
-}
-
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-impl<T> fmt::Debug for Discriminant<T> {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt.debug_tuple("Discriminant")
-           .field(&self.0)
-           .finish()
-    }
-}
-
-/// Returns a value uniquely identifying the enum variant in `v`.
-///
-/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
-/// return value is unspecified.
-///
-/// # Stability
-///
-/// The discriminant of an enum variant may change if the enum definition changes. A discriminant
-/// of some variant will not change between compilations with the same compiler.
-///
-/// # Examples
-///
-/// This can be used to compare enums that carry data, while disregarding
-/// the actual data:
-///
-/// ```
-/// use std::mem;
-///
-/// enum Foo { A(&'static str), B(i32), C(i32) }
-///
-/// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
-/// assert!(mem::discriminant(&Foo::B(1))     == mem::discriminant(&Foo::B(2)));
-/// assert!(mem::discriminant(&Foo::B(3))     != mem::discriminant(&Foo::C(3)));
-/// ```
-#[stable(feature = "discriminant_value", since = "1.21.0")]
-pub fn discriminant<T>(v: &T) -> Discriminant<T> {
-    unsafe {
-        Discriminant(intrinsics::discriminant_value(v), PhantomData)
-    }
-}
-
-/// 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
-    }
-}
-
-/// 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
-    }
-}
diff --git a/src/libcore/mem/manually_drop.rs b/src/libcore/mem/manually_drop.rs
new file mode 100644
index 00000000000..3ad1223e331
--- /dev/null
+++ b/src/libcore/mem/manually_drop.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
+    }
+}
diff --git a/src/libcore/mem/maybe_uninit.rs b/src/libcore/mem/maybe_uninit.rs
new file mode 100644
index 00000000000..eeff9d0303a
--- /dev/null
+++ b/src/libcore/mem/maybe_uninit.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
+    }
+}
diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
new file mode 100644
index 00000000000..91449f09936
--- /dev/null
+++ b/src/libcore/mem/mod.rs
@@ -0,0 +1,752 @@
+//! Basic functions for dealing with memory.
+//!
+//! This module contains functions for querying the size and alignment of
+//! types, initializing and manipulating memory.
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+use crate::clone;
+use crate::cmp;
+use crate::fmt;
+use crate::hash;
+use crate::intrinsics;
+use crate::marker::{Copy, PhantomData, Sized};
+use crate::ptr;
+
+mod manually_drop;
+#[stable(feature = "manually_drop", since = "1.20.0")]
+pub use manually_drop::ManuallyDrop;
+
+mod maybe_uninit;
+#[stable(feature = "maybe_uninit", since = "1.36.0")]
+pub use maybe_uninit::MaybeUninit;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+#[doc(inline)]
+pub use crate::intrinsics::transmute;
+
+/// Takes ownership and "forgets" about the value **without running its destructor**.
+///
+/// Any resources the value manages, such as heap memory or a file handle, will linger
+/// forever in an unreachable state. However, it does not guarantee that pointers
+/// to this memory will remain valid.
+///
+/// * If you want to leak memory, see [`Box::leak`][leak].
+/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
+/// * If you want to dispose of a value properly, running its destructor, see
+/// [`mem::drop`][drop].
+///
+/// # Safety
+///
+/// `forget` is not marked as `unsafe`, because Rust's safety guarantees
+/// do not include a guarantee that destructors will always run. For example,
+/// a program can create a reference cycle using [`Rc`][rc], or call
+/// [`process::exit`][exit] to exit without running destructors. Thus, allowing
+/// `mem::forget` from safe code does not fundamentally change Rust's safety
+/// guarantees.
+///
+/// That said, leaking resources such as memory or I/O objects is usually undesirable,
+/// so `forget` is only recommended for specialized use cases like those shown below.
+///
+/// Because forgetting a value is allowed, any `unsafe` code you write must
+/// allow for this possibility. You cannot return a value and expect that the
+/// caller will necessarily run the value's destructor.
+///
+/// [rc]: ../../std/rc/struct.Rc.html
+/// [exit]: ../../std/process/fn.exit.html
+///
+/// # Examples
+///
+/// Leak an I/O object, never closing the file:
+///
+/// ```no_run
+/// use std::mem;
+/// use std::fs::File;
+///
+/// let file = File::open("foo.txt").unwrap();
+/// mem::forget(file);
+/// ```
+///
+/// The practical use cases for `forget` are rather specialized and mainly come
+/// up in unsafe or FFI code.
+///
+/// [drop]: fn.drop.html
+/// [uninit]: fn.uninitialized.html
+/// [clone]: ../clone/trait.Clone.html
+/// [swap]: fn.swap.html
+/// [box]: ../../std/boxed/struct.Box.html
+/// [leak]: ../../std/boxed/struct.Box.html#method.leak
+/// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
+/// [ub]: ../../reference/behavior-considered-undefined.html
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn forget<T>(t: T) {
+    ManuallyDrop::new(t);
+}
+
+/// Like [`forget`], but also accepts unsized values.
+///
+/// This function is just a shim intended to be removed when the `unsized_locals` feature gets
+/// stabilized.
+///
+/// [`forget`]: fn.forget.html
+#[inline]
+#[unstable(feature = "forget_unsized", issue = "0")]
+pub fn forget_unsized<T: ?Sized>(t: T) {
+    unsafe { intrinsics::forget(t) }
+}
+
+/// Returns the size of a type in bytes.
+///
+/// More specifically, this is the offset in bytes between successive elements
+/// in an array with that item type including alignment padding. Thus, for any
+/// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
+///
+/// In general, the size of a type is not stable across compilations, but
+/// specific types such as primitives are.
+///
+/// The following table gives the size for primitives.
+///
+/// Type | size_of::\<Type>()
+/// ---- | ---------------
+/// () | 0
+/// bool | 1
+/// u8 | 1
+/// u16 | 2
+/// u32 | 4
+/// u64 | 8
+/// u128 | 16
+/// i8 | 1
+/// i16 | 2
+/// i32 | 4
+/// i64 | 8
+/// i128 | 16
+/// f32 | 4
+/// f64 | 8
+/// char | 4
+///
+/// Furthermore, `usize` and `isize` have the same size.
+///
+/// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
+/// the same size. If `T` is Sized, all of those types have the same size as `usize`.
+///
+/// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
+/// have the same size. Likewise for `*const T` and `*mut T`.
+///
+/// # Size of `#[repr(C)]` items
+///
+/// The `C` representation for items has a defined layout. With this layout,
+/// the size of items is also stable as long as all fields have a stable size.
+///
+/// ## Size of Structs
+///
+/// For `structs`, the size is determined by the following algorithm.
+///
+/// For each field in the struct ordered by declaration order:
+///
+/// 1. Add the size of the field.
+/// 2. Round up the current size to the nearest multiple of the next field's [alignment].
+///
+/// Finally, round the size of the struct to the nearest multiple of its [alignment].
+/// The alignment of the struct is usually the largest alignment of all its
+/// fields; this can be changed with the use of `repr(align(N))`.
+///
+/// Unlike `C`, zero sized structs are not rounded up to one byte in size.
+///
+/// ## Size of Enums
+///
+/// Enums that carry no data other than the discriminant have the same size as C enums
+/// on the platform they are compiled for.
+///
+/// ## Size of Unions
+///
+/// The size of a union is the size of its largest field.
+///
+/// Unlike `C`, zero sized unions are not rounded up to one byte in size.
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// // Some primitives
+/// assert_eq!(4, mem::size_of::<i32>());
+/// assert_eq!(8, mem::size_of::<f64>());
+/// assert_eq!(0, mem::size_of::<()>());
+///
+/// // Some arrays
+/// assert_eq!(8, mem::size_of::<[i32; 2]>());
+/// assert_eq!(12, mem::size_of::<[i32; 3]>());
+/// assert_eq!(0, mem::size_of::<[i32; 0]>());
+///
+///
+/// // Pointer size equality
+/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
+/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
+/// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
+/// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
+/// ```
+///
+/// Using `#[repr(C)]`.
+///
+/// ```
+/// use std::mem;
+///
+/// #[repr(C)]
+/// struct FieldStruct {
+///     first: u8,
+///     second: u16,
+///     third: u8
+/// }
+///
+/// // The size of the first field is 1, so add 1 to the size. Size is 1.
+/// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
+/// // The size of the second field is 2, so add 2 to the size. Size is 4.
+/// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
+/// // The size of the third field is 1, so add 1 to the size. Size is 5.
+/// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
+/// // fields is 2), so add 1 to the size for padding. Size is 6.
+/// assert_eq!(6, mem::size_of::<FieldStruct>());
+///
+/// #[repr(C)]
+/// struct TupleStruct(u8, u16, u8);
+///
+/// // Tuple structs follow the same rules.
+/// assert_eq!(6, mem::size_of::<TupleStruct>());
+///
+/// // Note that reordering the fields can lower the size. We can remove both padding bytes
+/// // by putting `third` before `second`.
+/// #[repr(C)]
+/// struct FieldStructOptimized {
+///     first: u8,
+///     third: u8,
+///     second: u16
+/// }
+///
+/// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
+///
+/// // Union size is the size of the largest field.
+/// #[repr(C)]
+/// union ExampleUnion {
+///     smaller: u8,
+///     larger: u16
+/// }
+///
+/// assert_eq!(2, mem::size_of::<ExampleUnion>());
+/// ```
+///
+/// [alignment]: ./fn.align_of.html
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_promotable]
+pub const fn size_of<T>() -> usize {
+    intrinsics::size_of::<T>()
+}
+
+/// Returns the size of the pointed-to value in bytes.
+///
+/// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
+/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
+/// then `size_of_val` can be used to get the dynamically-known size.
+///
+/// [slice]: ../../std/primitive.slice.html
+/// [trait object]: ../../book/ch17-02-trait-objects.html
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// assert_eq!(4, mem::size_of_val(&5i32));
+///
+/// let x: [u8; 13] = [0; 13];
+/// let y: &[u8] = &x;
+/// assert_eq!(13, mem::size_of_val(y));
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
+    unsafe { intrinsics::size_of_val(val) }
+}
+
+/// Returns the [ABI]-required minimum alignment of a type.
+///
+/// Every reference to a value of the type `T` must be a multiple of this number.
+///
+/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
+///
+/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
+///
+/// # Examples
+///
+/// ```
+/// # #![allow(deprecated)]
+/// use std::mem;
+///
+/// assert_eq!(4, mem::min_align_of::<i32>());
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
+pub fn min_align_of<T>() -> usize {
+    intrinsics::min_align_of::<T>()
+}
+
+/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
+///
+/// Every reference to a value of the type `T` must be a multiple of this number.
+///
+/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
+///
+/// # Examples
+///
+/// ```
+/// # #![allow(deprecated)]
+/// use std::mem;
+///
+/// assert_eq!(4, mem::min_align_of_val(&5i32));
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
+pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
+    unsafe { intrinsics::min_align_of_val(val) }
+}
+
+/// Returns the [ABI]-required minimum alignment of a type.
+///
+/// Every reference to a value of the type `T` must be a multiple of this number.
+///
+/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
+///
+/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// assert_eq!(4, mem::align_of::<i32>());
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_promotable]
+pub const fn align_of<T>() -> usize {
+    intrinsics::min_align_of::<T>()
+}
+
+/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
+///
+/// Every reference to a value of the type `T` must be a multiple of this number.
+///
+/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// assert_eq!(4, mem::align_of_val(&5i32));
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
+    unsafe { intrinsics::min_align_of_val(val) }
+}
+
+/// Returns `true` if dropping values of type `T` matters.
+///
+/// This is purely an optimization hint, and may be implemented conservatively:
+/// it may return `true` for types that don't actually need to be dropped.
+/// As such always returning `true` would be a valid implementation of
+/// this function. However if this function actually returns `false`, then you
+/// can be certain dropping `T` has no side effect.
+///
+/// Low level implementations of things like collections, which need to manually
+/// drop their data, should use this function to avoid unnecessarily
+/// trying to drop all their contents when they are destroyed. This might not
+/// make a difference in release builds (where a loop that has no side-effects
+/// is easily detected and eliminated), but is often a big win for debug builds.
+///
+/// Note that `ptr::drop_in_place` already performs this check, so if your workload
+/// can be reduced to some small number of drop_in_place calls, using this is
+/// unnecessary. In particular note that you can drop_in_place a slice, and that
+/// will do a single needs_drop check for all the values.
+///
+/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
+/// needs_drop explicitly. Types like HashMap, on the other hand, have to drop
+/// values one at a time and should use this API.
+///
+///
+/// # Examples
+///
+/// Here's an example of how a collection might make use of needs_drop:
+///
+/// ```
+/// use std::{mem, ptr};
+///
+/// pub struct MyCollection<T> {
+/// #   data: [T; 1],
+///     /* ... */
+/// }
+/// # impl<T> MyCollection<T> {
+/// #   fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
+/// #   fn free_buffer(&mut self) {}
+/// # }
+///
+/// impl<T> Drop for MyCollection<T> {
+///     fn drop(&mut self) {
+///         unsafe {
+///             // drop the data
+///             if mem::needs_drop::<T>() {
+///                 for x in self.iter_mut() {
+///                     ptr::drop_in_place(x);
+///                 }
+///             }
+///             self.free_buffer();
+///         }
+///     }
+/// }
+/// ```
+#[inline]
+#[stable(feature = "needs_drop", since = "1.21.0")]
+pub const fn needs_drop<T>() -> bool {
+    intrinsics::needs_drop::<T>()
+}
+
+/// Creates a value whose bytes are all zero.
+///
+/// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
+/// It is useful for FFI sometimes, but should generally be avoided.
+///
+/// There is no guarantee that an all-zero byte-pattern represents a valid value of
+/// some type `T`. For example, the all-zero byte-pattern is not a valid value
+/// for reference types (`&T` and `&mut T`). Using `zeroed` on such types
+/// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv]
+/// that there always is a valid value in a variable it considers initialized.
+///
+/// [zeroed]: union.MaybeUninit.html#method.zeroed
+/// [ub]: ../../reference/behavior-considered-undefined.html
+/// [inv]: union.MaybeUninit.html#initialization-invariant
+///
+/// # Examples
+///
+/// Correct usage of this function: initializing an integer with zero.
+///
+/// ```
+/// use std::mem;
+///
+/// let x: i32 = unsafe { mem::zeroed() };
+/// assert_eq!(0, x);
+/// ```
+///
+/// *Incorrect* usage of this function: initializing a reference with zero.
+///
+/// ```no_run
+/// use std::mem;
+///
+/// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub unsafe fn zeroed<T>() -> T {
+    intrinsics::panic_if_uninhabited::<T>();
+    intrinsics::init()
+}
+
+/// Bypasses Rust's normal memory-initialization checks by pretending to
+/// produce a value of type `T`, while doing nothing at all.
+///
+/// **This functon is deprecated.** Use [`MaybeUninit<T>`] instead.
+///
+/// The reason for deprecation is that the function basically cannot be used
+/// correctly: [the Rust compiler assumes][inv] that values are properly initialized.
+/// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
+/// undefined behavior for returning a `bool` that is not definitely either `true`
+/// or `false`. Worse, truly uninitialized memory like what gets returned here
+/// 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.
+/// (Notice that the rules around uninitialized integers are not finalized yet, but
+/// until they are, it is advisable to avoid them.)
+///
+/// [`MaybeUninit<T>`]: union.MaybeUninit.html
+/// [inv]: union.MaybeUninit.html#initialization-invariant
+#[inline]
+#[rustc_deprecated(since = "1.38.0", reason = "use `mem::MaybeUninit` instead")]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub unsafe fn uninitialized<T>() -> T {
+    intrinsics::panic_if_uninhabited::<T>();
+    intrinsics::uninit()
+}
+
+/// Swaps the values at two mutable locations, without deinitializing either one.
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// let mut x = 5;
+/// let mut y = 42;
+///
+/// mem::swap(&mut x, &mut y);
+///
+/// assert_eq!(42, x);
+/// assert_eq!(5, y);
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn swap<T>(x: &mut T, y: &mut T) {
+    unsafe {
+        ptr::swap_nonoverlapping_one(x, y);
+    }
+}
+
+/// Moves `src` into the referenced `dest`, returning the previous `dest` value.
+///
+/// Neither value is dropped.
+///
+/// # Examples
+///
+/// A simple example:
+///
+/// ```
+/// use std::mem;
+///
+/// let mut v: Vec<i32> = vec![1, 2];
+///
+/// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
+/// assert_eq!(vec![1, 2], old_v);
+/// assert_eq!(vec![3, 4, 5], v);
+/// ```
+///
+/// `replace` allows consumption of a struct field by replacing it with another value.
+/// Without `replace` you can run into issues like these:
+///
+/// ```compile_fail,E0507
+/// struct Buffer<T> { buf: Vec<T> }
+///
+/// impl<T> Buffer<T> {
+///     fn get_and_reset(&mut self) -> Vec<T> {
+///         // error: cannot move out of dereference of `&mut`-pointer
+///         let buf = self.buf;
+///         self.buf = Vec::new();
+///         buf
+///     }
+/// }
+/// ```
+///
+/// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
+/// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
+/// `self`, allowing it to be returned:
+///
+/// ```
+/// # #![allow(dead_code)]
+/// use std::mem;
+///
+/// # struct Buffer<T> { buf: Vec<T> }
+/// impl<T> Buffer<T> {
+///     fn get_and_reset(&mut self) -> Vec<T> {
+///         mem::replace(&mut self.buf, Vec::new())
+///     }
+/// }
+/// ```
+///
+/// [`Clone`]: ../../std/clone/trait.Clone.html
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn replace<T>(dest: &mut T, mut src: T) -> T {
+    swap(dest, &mut src);
+    src
+}
+
+/// Disposes of a value.
+///
+/// This does call the argument's implementation of [`Drop`][drop].
+///
+/// This effectively does nothing for types which implement `Copy`, e.g.
+/// integers. Such values are copied and _then_ moved into the function, so the
+/// value persists after this function call.
+///
+/// This function is not magic; it is literally defined as
+///
+/// ```
+/// pub fn drop<T>(_x: T) { }
+/// ```
+///
+/// Because `_x` is moved into the function, it is automatically dropped before
+/// the function returns.
+///
+/// [drop]: ../ops/trait.Drop.html
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// let v = vec![1, 2, 3];
+///
+/// drop(v); // explicitly drop the vector
+/// ```
+///
+/// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
+/// release a [`RefCell`] borrow:
+///
+/// ```
+/// use std::cell::RefCell;
+///
+/// let x = RefCell::new(1);
+///
+/// let mut mutable_borrow = x.borrow_mut();
+/// *mutable_borrow = 1;
+///
+/// drop(mutable_borrow); // relinquish the mutable borrow on this slot
+///
+/// let borrow = x.borrow();
+/// println!("{}", *borrow);
+/// ```
+///
+/// Integers and other types implementing [`Copy`] are unaffected by `drop`.
+///
+/// ```
+/// #[derive(Copy, Clone)]
+/// struct Foo(u8);
+///
+/// let x = 1;
+/// let y = Foo(2);
+/// drop(x); // a copy of `x` is moved and dropped
+/// drop(y); // a copy of `y` is moved and dropped
+///
+/// println!("x: {}, y: {}", x, y.0); // still available
+/// ```
+///
+/// [`RefCell`]: ../../std/cell/struct.RefCell.html
+/// [`Copy`]: ../../std/marker/trait.Copy.html
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn drop<T>(_x: T) { }
+
+/// Interprets `src` as having type `&U`, and then reads `src` without moving
+/// the contained value.
+///
+/// This function will unsafely assume the pointer `src` is valid for
+/// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
+/// the `&U`. It will also unsafely create a copy of the contained value instead of
+/// moving out of `src`.
+///
+/// It is not a compile-time error if `T` and `U` have different sizes, but it
+/// is highly encouraged to only invoke this function where `T` and `U` have the
+/// same size. This function triggers [undefined behavior][ub] if `U` is larger than
+/// `T`.
+///
+/// [ub]: ../../reference/behavior-considered-undefined.html
+/// [size_of]: fn.size_of.html
+///
+/// # Examples
+///
+/// ```
+/// use std::mem;
+///
+/// #[repr(packed)]
+/// struct Foo {
+///     bar: u8,
+/// }
+///
+/// let foo_slice = [10u8];
+///
+/// unsafe {
+///     // Copy the data from 'foo_slice' and treat it as a 'Foo'
+///     let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
+///     assert_eq!(foo_struct.bar, 10);
+///
+///     // Modify the copied data
+///     foo_struct.bar = 20;
+///     assert_eq!(foo_struct.bar, 20);
+/// }
+///
+/// // The contents of 'foo_slice' should not have changed
+/// assert_eq!(foo_slice, [10]);
+/// ```
+#[inline]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
+    ptr::read_unaligned(src as *const T as *const U)
+}
+
+/// Opaque type representing the discriminant of an enum.
+///
+/// See the [`discriminant`] function in this module for more information.
+///
+/// [`discriminant`]: fn.discriminant.html
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
+
+// N.B. These trait implementations cannot be derived because we don't want any bounds on T.
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> Copy for Discriminant<T> {}
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> clone::Clone for Discriminant<T> {
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> cmp::PartialEq for Discriminant<T> {
+    fn eq(&self, rhs: &Self) -> bool {
+        self.0 == rhs.0
+    }
+}
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> cmp::Eq for Discriminant<T> {}
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> hash::Hash for Discriminant<T> {
+    fn hash<H: hash::Hasher>(&self, state: &mut H) {
+        self.0.hash(state);
+    }
+}
+
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+impl<T> fmt::Debug for Discriminant<T> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_tuple("Discriminant")
+           .field(&self.0)
+           .finish()
+    }
+}
+
+/// Returns a value uniquely identifying the enum variant in `v`.
+///
+/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
+/// return value is unspecified.
+///
+/// # Stability
+///
+/// The discriminant of an enum variant may change if the enum definition changes. A discriminant
+/// of some variant will not change between compilations with the same compiler.
+///
+/// # Examples
+///
+/// This can be used to compare enums that carry data, while disregarding
+/// the actual data:
+///
+/// ```
+/// use std::mem;
+///
+/// enum Foo { A(&'static str), B(i32), C(i32) }
+///
+/// assert!(mem::discriminant(&Foo::A("bar")) == mem::discriminant(&Foo::A("baz")));
+/// assert!(mem::discriminant(&Foo::B(1))     == mem::discriminant(&Foo::B(2)));
+/// assert!(mem::discriminant(&Foo::B(3))     != mem::discriminant(&Foo::C(3)));
+/// ```
+#[stable(feature = "discriminant_value", since = "1.21.0")]
+pub fn discriminant<T>(v: &T) -> Discriminant<T> {
+    unsafe {
+        Discriminant(intrinsics::discriminant_value(v), PhantomData)
+    }
+}