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

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    //! 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(always)]
    #[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(always)]
    #[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 [`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.
    ///
    /// [`drop_in_place`]: ../ptr/fn.drop_in_place.html
    /// [`HashMap`]: ../../std/collections/struct.HashMap.html
    ///
    /// # 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>()
    }

    /// Returns the value of type `T` represented by the all-zero byte-pattern.
    ///
    /// This means that, for example, the padding byte in `(u8, u16)` is not
    /// necessarily zeroed.
    ///
    /// 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.
    ///
    /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
    /// It is useful for FFI sometimes, but should generally be avoided.
    ///
    /// [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.
    ///
    /// ```rust,no_run
    /// # #![allow(invalid_value)]
    /// use std::mem;
    ///
    /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow(deprecated_in_future)]
    #[allow(deprecated)]
    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 function 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 = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[allow(deprecated_in_future)]
    #[allow(deprecated)]
    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);
        }
    }

    /// Replace `dest` with the default value of `T`, and return the previous `dest` value.
    ///
    /// # Examples
    ///
    /// A simple example:
    ///
    /// ```
    /// use std::mem;
    ///
    /// let mut v: Vec<i32> = vec![1, 2];
    ///
    /// let old_v = mem::take(&mut v);
    /// assert_eq!(vec![1, 2], old_v);
    /// assert!(v.is_empty());
    /// ```
    ///
    /// `take` allows taking ownership of a struct field by replacing it with an "empty" value.
    /// Without `take` 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 `take` can be used to disassociate the original value of `self.buf` from
    /// `self`, allowing it to be returned:
    ///
    /// ```
    /// use std::mem;
    ///
    /// # struct Buffer<T> { buf: Vec<T> }
    /// impl<T> Buffer<T> {
    ///     fn get_and_reset(&mut self) -> Vec<T> {
    ///         mem::take(&mut self.buf)
    ///     }
    /// }
    ///
    /// let mut buffer = Buffer { buf: vec![0, 1] };
    /// assert_eq!(buffer.buf.len(), 2);
    ///
    /// assert_eq!(buffer.get_and_reset(), vec![0, 1]);
    /// assert_eq!(buffer.buf.len(), 0);
    /// ```
    ///
    /// [`Clone`]: ../../std/clone/trait.Clone.html
    #[inline]
    #[stable(feature = "mem_take", since = "1.40.0")]
    pub fn take<T: Default>(dest: &mut T) -> T {
        replace(dest, T::default())
    }

    /// 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 replace_index(&mut self, i: usize, v: T) -> T {
    ///         // error: cannot move out of dereference of `&mut`-pointer
    ///         let t = self.buf[i];
    ///         self.buf[i] = v;
    ///         t
    ///     }
    /// }
    /// ```
    ///
    /// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to
    /// avoid the move. But `replace` can be used to disassociate the original value at that index from
    /// `self`, allowing it to be returned:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// use std::mem;
    ///
    /// # struct Buffer<T> { buf: Vec<T> }
    /// impl<T> Buffer<T> {
    ///     fn replace_index(&mut self, i: usize, v: T) -> T {
    ///         mem::replace(&mut self.buf[i], v)
    ///     }
    /// }
    ///
    /// let mut buffer = Buffer { buf: vec![0, 1] };
    /// assert_eq!(buffer.buf[0], 0);
    ///
    /// assert_eq!(buffer.replace_index(0, 2), 0);
    /// assert_eq!(buffer.buf[0], 2);
    /// ```
    ///
    /// [`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_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));
    /// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));
    /// assert_ne!(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)
        }
    }