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

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    // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! 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 clone;
    use cmp;
    use fmt;
    use hash;
    use intrinsics;
    use marker::{Copy, PhantomData, Sized};
    use ptr;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use intrinsics::transmute;

    /// Leaks a value: 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.
    ///
    /// 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 some heap memory by never deallocating it:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let heap_memory = Box::new(3);
    /// mem::forget(heap_memory);
    /// ```
    ///
    /// 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.
    ///
    /// ## Use case 1
    ///
    /// You have created an uninitialized value using [`mem::uninitialized`][uninit].
    /// You must either initialize or `forget` it on every computation path before
    /// Rust drops it automatically, like at the end of a scope or after a panic.
    /// Running the destructor on an uninitialized value would be [undefined behavior][ub].
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// # let some_condition = false;
    /// unsafe {
    ///     let mut uninit_vec: Vec<u32> = mem::uninitialized();
    ///
    ///     if some_condition {
    ///         // Initialize the variable.
    ///         ptr::write(&mut uninit_vec, Vec::new());
    ///     } else {
    ///         // Forget the uninitialized value so its destructor doesn't run.
    ///         mem::forget(uninit_vec);
    ///     }
    /// }
    /// ```
    ///
    /// ## Use case 2
    ///
    /// You have duplicated the bytes making up a value, without doing a proper
    /// [`Clone`][clone]. You need the value's destructor to run only once,
    /// because a double `free` is undefined behavior.
    ///
    /// An example is a possible implementation of [`mem::swap`][swap]:
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// # #[allow(dead_code)]
    /// fn swap<T>(x: &mut T, y: &mut T) {
    ///     unsafe {
    ///         // Give ourselves some scratch space to work with
    ///         let mut t: T = mem::uninitialized();
    ///
    ///         // Perform the swap, `&mut` pointers never alias
    ///         ptr::copy_nonoverlapping(&*x, &mut t, 1);
    ///         ptr::copy_nonoverlapping(&*y, x, 1);
    ///         ptr::copy_nonoverlapping(&t, y, 1);
    ///
    ///         // y and t now point to the same thing, but we need to completely
    ///         // forget `t` because we do not want to run the destructor for `T`
    ///         // on its value, which is still owned somewhere outside this function.
    ///         mem::forget(t);
    ///     }
    /// }
    /// ```
    ///
    /// ## Use case 3
    ///
    /// You are transferring ownership across a [FFI] boundary to code written in
    /// another language. You need to `forget` the value on the Rust side because Rust
    /// code is no longer responsible for it.
    ///
    /// ```no_run
    /// use std::mem;
    ///
    /// extern "C" {
    ///     fn my_c_function(x: *const u32);
    /// }
    ///
    /// let x: Box<u32> = Box::new(3);
    ///
    /// // Transfer ownership into C code.
    /// unsafe {
    ///     my_c_function(&*x);
    /// }
    /// mem::forget(x);
    /// ```
    ///
    /// In this case, C code must call back into Rust to free the object. Calling C's `free`
    /// function on a [`Box`][box] is *not* safe! Also, `Box` provides an [`into_raw`][into_raw]
    /// method which is the preferred way to do this in practice.
    ///
    /// [drop]: fn.drop.html
    /// [uninit]: fn.uninitialized.html
    /// [clone]: ../clone/trait.Clone.html
    /// [swap]: fn.swap.html
    /// [FFI]: ../../book/first-edition/ffi.html
    /// [box]: ../../std/boxed/struct.Box.html
    /// [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);
    }

    /// Returns the size of a type in bytes.
    ///
    /// More specifically, this is the offset in bytes between successive
    /// items of the same type, including alignment padding.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::size_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg(stage0)]
    pub fn size_of<T>() -> usize {
        unsafe { intrinsics::size_of::<T>() }
    }

    /// Returns the size of a type in bytes.
    ///
    /// More specifically, this is the offset in bytes between successive
    /// items of the same type, including alignment padding.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// assert_eq!(4, mem::size_of::<i32>());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg(not(stage0))]
    pub const fn size_of<T>() -> usize {
        unsafe { 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/first-edition/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 {
        unsafe { 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")]
    #[cfg(stage0)]
    pub fn align_of<T>() -> usize {
        unsafe { intrinsics::min_align_of::<T>() }
    }

    /// 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")]
    #[cfg(not(stage0))]
    pub const fn align_of<T>() -> usize {
        unsafe { 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 whether dropping values of type `T` matters.
    ///
    /// This is purely an optimization hint, and may be implemented conservatively.
    /// For instance, always returning `true` would be a valid implementation of
    /// this function.
    ///
    /// 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 fn needs_drop<T>() -> bool {
        unsafe { intrinsics::needs_drop::<T>() }
    }

    /// Creates a value whose bytes are all zero.
    ///
    /// This has the same effect as allocating space with
    /// [`mem::uninitialized`][uninit] and then zeroing it out. 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`. If `T` has a destructor and the value is destroyed (due to
    /// a panic or the end of a scope) before being initialized, then the destructor
    /// will run on zeroed data, likely leading to [undefined behavior][ub].
    ///
    /// See also the documentation for [`mem::uninitialized`][uninit], which has
    /// many of the same caveats.
    ///
    /// [uninit]: fn.uninitialized.html
    /// [FFI]: ../../book/first-edition/ffi.html
    /// [ub]: ../../reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::mem;
    ///
    /// let x: i32 = unsafe { mem::zeroed() };
    /// assert_eq!(0, x);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn zeroed<T>() -> 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 is incredibly dangerous and should not be done lightly. Deeply
    /// consider initializing your memory with a default value instead.**
    ///
    /// This is useful for [FFI] functions and initializing arrays sometimes,
    /// but should generally be avoided.
    ///
    /// [FFI]: ../../book/first-edition/ffi.html
    ///
    /// # Undefined behavior
    ///
    /// It is [undefined behavior][ub] to read uninitialized memory, even just an
    /// uninitialized boolean. For instance, if you branch on the value of such
    /// a boolean, your program may take one, both, or neither of the branches.
    ///
    /// Writing to the uninitialized value is similarly dangerous. Rust believes the
    /// value is initialized, and will therefore try to [`Drop`] the uninitialized
    /// value and its fields if you try to overwrite it in a normal manner. The only way
    /// to safely initialize an uninitialized value is with [`ptr::write`][write],
    /// [`ptr::copy`][copy], or [`ptr::copy_nonoverlapping`][copy_no].
    ///
    /// If the value does implement [`Drop`], it must be initialized before
    /// it goes out of scope (and therefore would be dropped). Note that this
    /// includes a `panic` occurring and unwinding the stack suddenly.
    ///
    /// # Examples
    ///
    /// Here's how to safely initialize an array of [`Vec`]s.
    ///
    /// ```
    /// use std::mem;
    /// use std::ptr;
    ///
    /// // Only declare the array. This safely leaves it
    /// // uninitialized in a way that Rust will track for us.
    /// // However we can't initialize it element-by-element
    /// // safely, and we can't use the `[value; 1000]`
    /// // constructor because it only works with `Copy` data.
    /// let mut data: [Vec<u32>; 1000];
    ///
    /// unsafe {
    ///     // So we need to do this to initialize it.
    ///     data = mem::uninitialized();
    ///
    ///     // DANGER ZONE: if anything panics or otherwise
    ///     // incorrectly reads the array here, we will have
    ///     // Undefined Behavior.
    ///
    ///     // It's ok to mutably iterate the data, since this
    ///     // doesn't involve reading it at all.
    ///     // (ptr and len are statically known for arrays)
    ///     for elem in &mut data[..] {
    ///         // *elem = Vec::new() would try to drop the
    ///         // uninitialized memory at `elem` -- bad!
    ///         //
    ///         // Vec::new doesn't allocate or do really
    ///         // anything. It's only safe to call here
    ///         // because we know it won't panic.
    ///         ptr::write(elem, Vec::new());
    ///     }
    ///
    ///     // SAFE ZONE: everything is initialized.
    /// }
    ///
    /// println!("{:?}", &data[0]);
    /// ```
    ///
    /// This example emphasizes exactly how delicate and dangerous using `mem::uninitialized`
    /// can be. Note that the [`vec!`] macro *does* let you initialize every element with a
    /// value that is only [`Clone`], so the following is semantically equivalent and
    /// vastly less dangerous, as long as you can live with an extra heap
    /// allocation:
    ///
    /// ```
    /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
    /// println!("{:?}", &data[0]);
    /// ```
    ///
    /// [`Vec`]: ../../std/vec/struct.Vec.html
    /// [`vec!`]: ../../std/macro.vec.html
    /// [`Clone`]: ../../std/clone/trait.Clone.html
    /// [ub]: ../../reference/behavior-considered-undefined.html
    /// [write]: ../ptr/fn.write.html
    /// [copy]: ../intrinsics/fn.copy.html
    /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html
    /// [`Drop`]: ../ops/trait.Drop.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn uninitialized<T>() -> 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(x, y, 1);
        }
    }

    /// Replaces the value at a mutable location with a new one, returning the old value, without
    /// deinitializing either one.
    ///
    /// # 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!(2, old_v.len());
    /// assert_eq!(3, v.len());
    /// ```
    ///
    /// `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.
    ///
    /// While this does call the argument's implementation of [`Drop`][drop],
    /// it will not release any borrows, as borrows are based on lexical scope.
    ///
    /// This effectively does nothing for
    /// [types which implement `Copy`](../../book/first-edition/ownership.html#copy-types),
    /// 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
    /// ```
    ///
    /// Borrows are based on lexical scope, so this produces an error:
    ///
    /// ```compile_fail,E0502
    /// let mut v = vec![1, 2, 3];
    /// let x = &v[0];
    ///
    /// drop(x); // explicitly drop the reference, but the borrow still exists
    ///
    /// v.push(4); // error: cannot borrow `v` as mutable because it is also
    ///            // borrowed as immutable
    /// ```
    ///
    /// An inner scope is needed to fix this:
    ///
    /// ```
    /// let mut v = vec![1, 2, 3];
    ///
    /// {
    ///     let x = &v[0];
    ///
    ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
    /// }
    ///
    /// v.push(4); // no problems
    /// ```
    ///
    /// 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(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.
    #[stable(feature = "discriminant_value", since = "1.21.0")]
    pub struct Discriminant<T>(u64, PhantomData<*const 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.
    ///
    /// # 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`.
    ///     }
    /// }
    /// ```
    #[stable(feature = "manually_drop", since = "1.20.0")]
    #[allow(unions_with_drop_fields)]
    pub union ManuallyDrop<T>{ 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]
        pub fn new(value: T) -> ManuallyDrop<T> {
            ManuallyDrop { value: value }
        }

        /// Extract the value from the ManuallyDrop container.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::mem::ManuallyDrop;
        /// let x = ManuallyDrop::new(Box::new(()));
        /// let _: Box<()> = ManuallyDrop::into_inner(x);
        /// ```
        #[stable(feature = "manually_drop", since = "1.20.0")]
        #[inline]
        pub fn into_inner(slot: ManuallyDrop<T>) -> T {
            unsafe {
                slot.value
            }
        }

        /// Manually drops the contained value.
        ///
        /// # 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.
        #[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> ::ops::Deref for ManuallyDrop<T> {
        type Target = T;
        #[inline]
        fn deref(&self) -> &Self::Target {
            unsafe {
                &self.value
            }
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T> ::ops::DerefMut for ManuallyDrop<T> {
        #[inline]
        fn deref_mut(&mut self) -> &mut Self::Target {
            unsafe {
                &mut self.value
            }
        }
    }

    #[stable(feature = "manually_drop", since = "1.20.0")]
    impl<T: ::fmt::Debug> ::fmt::Debug for ManuallyDrop<T> {
        fn fmt(&self, fmt: &mut ::fmt::Formatter) -> ::fmt::Result {
            unsafe {
                fmt.debug_tuple("ManuallyDrop").field(&self.value).finish()
            }
        }
    }

    /// Tells LLVM that this point in the code is not reachable, enabling further
    /// optimizations.
    ///
    /// NB: This is very different from the `unreachable!()` macro: Unlike the
    /// macro, which panics when it is executed, it is *undefined behavior* to
    /// reach code marked with this function.
    #[inline]
    #[unstable(feature = "unreachable", issue = "43751")]
    pub unsafe fn unreachable() -> ! {
        intrinsics::unreachable()
    }

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