Restore (and reword) the warning against passing invalid values to mem::forget.

[rust.git] / src / libcore / mem / mod.rs

//! 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.
/// The need comes up in some specialized use cases for FFI or unsafe code, but even
/// then, [`ManuallyDrop`] is typically preferred.
///
/// 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
///
/// The canonical safe use of `mem::forget` is to circumvent a value's destructor
/// implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim
/// the space taken by the variable but never close the underlying system resource:
///
/// ```no_run
/// use std::mem;
/// use std::fs::File;
///
/// let file = File::open("foo.txt").unwrap();
/// mem::forget(file);
/// ```
///
/// This is useful when the ownership of the underlying was previously
/// transferred to code outside of Rust, for example by transmitting the raw
/// file descriptor to C code.
///
/// # Relationship with `ManuallyDrop`
///
/// Using `mem::forget` to transmit memory ownership is error-prone and is best
/// replaced with `ManuallyDrop`. Consider, for example, this code:
///
/// ```
/// use std::mem;
///
/// let mut v = vec![65, 122];
/// // Build a `String` using the contents of `v`
/// let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), 2, v.capacity()) };
/// // leak `v` because its memory is now managed by `s`
/// mem::forget(v);  // ERROR - v is invalid and must not be passed to a function
/// assert_eq!(s, "Az");
/// // `s` is implicitly dropped and its memory deallocated.
/// ```
///
/// There are two issues with the above example:
///
/// * If more code were added between the construction of `String` and the invocation of
///   `mem::forget()`, a panic within it would cause a double free because the same memory
///   is handled by both `v` and `s`.
/// * After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`,
///   the `v` value is invalid. Although moving a value to `mem::forget` (which won't
///   inspect it) seems safe, some types have strict requirements on their values that
///   make them invalid when dangling or no longer owned.  Using invalid values in any
///   way, including passing them to or returning them from functions, constitutes
///   undefined behavior and may break the assumptions made by the compiler.
///
/// Switching to `ManuallyDrop` avoids both issues:
///
/// ```
/// use std::mem::ManuallyDrop;
///
/// let v = vec![65, 122];
/// // Before we disassemble `v` into its raw parts, make sure it
/// // does not get dropped!
/// let mut v = ManuallyDrop::new(v);
/// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
/// let ptr = v.as_mut_ptr();
/// let cap = v.capacity();
/// // Finally, build a `String`.
/// let s = unsafe { String::from_raw_parts(ptr, 2, cap) };
/// assert_eq!(s, "Az");
/// // `s` is implicitly dropped and its memory deallocated.
/// ```
///
/// `ManuallyDrop` robustly prevents double-free because we disable `v`'s destructor
/// before doing anything else. `mem::forget()` doesn't allow this because it consumes its
/// argument, forcing us to call it only after extracting anything we need from `v`.  Even
/// if a panic were introduced between construction of `ManuallyDrop` and building the
/// string (which cannot happen in the code as shown), it would result in a leak and not a
/// double free. In other words, `ManuallyDrop` errs on the side of leaking instead of
/// erring on the side of dropping.
///
/// Also, `ManuallyDrop` prevents us from having to "touch" `v` after transferring the
/// ownership to `s` - the final step of interacting with `v` to dispoe of it without
/// running its destructor is entirely avoided.
///
/// [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
/// [`ManuallyDrop`]: struct.ManuallyDrop.html
#[inline]
#[rustc_const_unstable(feature = "const_forget", issue = "69616")]
#[stable(feature = "rust1", since = "1.0.0")]
pub const 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 = "none")]
pub fn forget_unsized<T: ?Sized>(t: T) {
    // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since
    // we'll be implementing this function soon via `ManuallyDrop`
    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]
#[rustc_const_stable(feature = "const_size_of", since = "1.32.0")]
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 {
    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 {
    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]
#[rustc_const_stable(feature = "const_align_of", since = "1.32.0")]
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")]
#[allow(deprecated)]
pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
    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")]
#[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")]
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(always)]
#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated_in_future)]
#[allow(deprecated)]
#[rustc_diagnostic_item = "mem_zeroed"]
pub unsafe fn zeroed<T>() -> T {
    #[cfg(not(bootstrap))]
    intrinsics::assert_zero_valid::<T>();
    #[cfg(bootstrap)]
    intrinsics::panic_if_uninhabited::<T>();
    MaybeUninit::zeroed().assume_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: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
/// As the [`assume_init` documentation][assume_init] explains,
/// [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
/// [uninit]: union.MaybeUninit.html#method.uninit
/// [assume_init]: union.MaybeUninit.html#method.assume_init
/// [inv]: union.MaybeUninit.html#initialization-invariant
#[inline(always)]
#[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
#[stable(feature = "rust1", since = "1.0.0")]
#[allow(deprecated_in_future)]
#[allow(deprecated)]
#[rustc_diagnostic_item = "mem_uninitialized"]
pub unsafe fn uninitialized<T>() -> T {
    #[cfg(not(bootstrap))]
    intrinsics::assert_uninit_valid::<T>();
    #[cfg(bootstrap)]
    intrinsics::panic_if_uninhabited::<T>();
    MaybeUninit::uninit().assume_init()
}

/// 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) {
    // SAFETY: the raw pointers have been created from safe mutable references satisfying all the
    // constraints on `ptr::swap_nonoverlapping_one`
    unsafe {
        ptr::swap_nonoverlapping_one(x, y);
    }
}

/// Replaces `dest` with the default value of `T`, returning 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_array = [10u8];
///
/// unsafe {
///     // Copy the data from 'foo_array' and treat it as a 'Foo'
///     let mut foo_struct: Foo = mem::transmute_copy(&foo_array);
///     assert_eq!(foo_struct.bar, 10);
///
///     // Modify the copied data
///     foo_struct.bar = 20;
///     assert_eq!(foo_struct.bar, 20);
/// }
///
/// // The contents of 'foo_array' should not have changed
/// assert_eq!(foo_array, [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")]
#[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
    Discriminant(intrinsics::discriminant_value(v), PhantomData)
}