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

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    //! Manually manage memory through raw pointers.
    //!
    //! *[See also the pointer primitive types](../../std/primitive.pointer.html).*
    //!
    //! # Safety
    //!
    //! Many functions in this module take raw pointers as arguments and read from
    //! or write to them. For this to be safe, these pointers must be *valid*.
    //! Whether a pointer is valid depends on the operation it is used for
    //! (read or write), and the extent of the memory that is accessed (i.e.,
    //! how many bytes are read/written). Most functions use `*mut T` and `*const T`
    //! to access only a single value, in which case the documentation omits the size
    //! and implicitly assumes it to be `size_of::<T>()` bytes.
    //!
    //! The precise rules for validity are not determined yet. The guarantees that are
    //! provided at this point are very minimal:
    //!
    //! * A [null] pointer is *never* valid, not even for accesses of [size zero][zst].
    //! * All pointers (except for the null pointer) are valid for all operations of
    //!   [size zero][zst].
    //! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer
    //!   be *dereferenceable*: the memory range of the given size starting at the pointer must all be
    //!   within the bounds of a single allocated object. Note that in Rust,
    //!   every (stack-allocated) variable is considered a separate allocated object.
    //! * All accesses performed by functions in this module are *non-atomic* in the sense
    //!   of [atomic operations] used to synchronize between threads. This means it is
    //!   undefined behavior to perform two concurrent accesses to the same location from different
    //!   threads unless both accesses only read from memory. Notice that this explicitly
    //!   includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
    //!   be used for inter-thread synchronization.
    //! * The result of casting a reference to a pointer is valid for as long as the
    //!   underlying object is live and no reference (just raw pointers) is used to
    //!   access the same memory.
    //!
    //! These axioms, along with careful use of [`offset`] for pointer arithmetic,
    //! are enough to correctly implement many useful things in unsafe code. Stronger guarantees
    //! will be provided eventually, as the [aliasing] rules are being determined. For more
    //! information, see the [book] as well as the section in the reference devoted
    //! to [undefined behavior][ub].
    //!
    //! ## Alignment
    //!
    //! Valid raw pointers as defined above are not necessarily properly aligned (where
    //! "proper" alignment is defined by the pointee type, i.e., `*const T` must be
    //! aligned to `mem::align_of::<T>()`). However, most functions require their
    //! arguments to be properly aligned, and will explicitly state
    //! this requirement in their documentation. Notable exceptions to this are
    //! [`read_unaligned`] and [`write_unaligned`].
    //!
    //! When a function requires proper alignment, it does so even if the access
    //! has size 0, i.e., even if memory is not actually touched. Consider using
    //! [`NonNull::dangling`] in such cases.
    //!
    //! [aliasing]: ../../nomicon/aliasing.html
    //! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
    //! [ub]: ../../reference/behavior-considered-undefined.html
    //! [null]: ./fn.null.html
    //! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
    //! [atomic operations]: ../../std/sync/atomic/index.html
    //! [`copy`]: ../../std/ptr/fn.copy.html
    //! [`offset`]: ../../std/primitive.pointer.html#method.offset
    //! [`read_unaligned`]: ./fn.read_unaligned.html
    //! [`write_unaligned`]: ./fn.write_unaligned.html
    //! [`read_volatile`]: ./fn.read_volatile.html
    //! [`write_volatile`]: ./fn.write_volatile.html
    //! [`NonNull::dangling`]: ./struct.NonNull.html#method.dangling

    #![stable(feature = "rust1", since = "1.0.0")]

    use crate::cmp::Ordering;
    use crate::fmt;
    use crate::hash;
    use crate::intrinsics::{self, abort, is_aligned_and_not_null, is_nonoverlapping};
    use crate::mem::{self, MaybeUninit};

    #[stable(feature = "rust1", since = "1.0.0")]
    #[doc(inline)]
    pub use crate::intrinsics::copy_nonoverlapping;

    #[stable(feature = "rust1", since = "1.0.0")]
    #[doc(inline)]
    pub use crate::intrinsics::copy;

    #[stable(feature = "rust1", since = "1.0.0")]
    #[doc(inline)]
    pub use crate::intrinsics::write_bytes;

    mod non_null;
    #[stable(feature = "nonnull", since = "1.25.0")]
    pub use non_null::NonNull;

    mod unique;
    #[unstable(feature = "ptr_internals", issue = "none")]
    pub use unique::Unique;

    mod const_ptr;
    mod mut_ptr;

    /// Executes the destructor (if any) of the pointed-to value.
    ///
    /// This is semantically equivalent to calling [`ptr::read`] and discarding
    /// the result, but has the following advantages:
    ///
    /// * It is *required* to use `drop_in_place` to drop unsized types like
    ///   trait objects, because they can't be read out onto the stack and
    ///   dropped normally.
    ///
    /// * It is friendlier to the optimizer to do this over [`ptr::read`] when
    ///   dropping manually allocated memory (e.g., when writing Box/Rc/Vec),
    ///   as the compiler doesn't need to prove that it's sound to elide the
    ///   copy.
    ///
    /// * It can be used to drop [pinned] data when `T` is not `repr(packed)`
    ///   (pinned data must not be moved before it is dropped).
    ///
    /// Unaligned values cannot be dropped in place, they must be copied to an aligned
    /// location first using [`ptr::read_unaligned`]. For packed structs, this move is
    /// done automatically by the compiler. This means the fields of packed structs
    /// are not dropped in-place.
    ///
    /// [`ptr::read`]: ../ptr/fn.read.html
    /// [`ptr::read_unaligned`]: ../ptr/fn.read_unaligned.html
    /// [pinned]: ../pin/index.html
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `to_drop` must be [valid] for both reads and writes.
    ///
    /// * `to_drop` must be properly aligned.
    ///
    /// * The value `to_drop` points to must be valid for dropping, which may mean it must uphold
    ///   additional invariants - this is type-dependent.
    ///
    /// Additionally, if `T` is not [`Copy`], using the pointed-to value after
    /// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
    /// foo` counts as a use because it will cause the value to be dropped
    /// again. [`write`] can be used to overwrite data without causing it to be
    /// dropped.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    /// [`Copy`]: ../marker/trait.Copy.html
    /// [`write`]: ../ptr/fn.write.html
    ///
    /// # Examples
    ///
    /// Manually remove the last item from a vector:
    ///
    /// ```
    /// use std::ptr;
    /// use std::rc::Rc;
    ///
    /// let last = Rc::new(1);
    /// let weak = Rc::downgrade(&last);
    ///
    /// let mut v = vec![Rc::new(0), last];
    ///
    /// unsafe {
    ///     // Get a raw pointer to the last element in `v`.
    ///     let ptr = &mut v[1] as *mut _;
    ///     // Shorten `v` to prevent the last item from being dropped. We do that first,
    ///     // to prevent issues if the `drop_in_place` below panics.
    ///     v.set_len(1);
    ///     // Without a call `drop_in_place`, the last item would never be dropped,
    ///     // and the memory it manages would be leaked.
    ///     ptr::drop_in_place(ptr);
    /// }
    ///
    /// assert_eq!(v, &[0.into()]);
    ///
    /// // Ensure that the last item was dropped.
    /// assert!(weak.upgrade().is_none());
    /// ```
    ///
    /// Notice that the compiler performs this copy automatically when dropping packed structs,
    /// i.e., you do not usually have to worry about such issues unless you call `drop_in_place`
    /// manually.
    #[stable(feature = "drop_in_place", since = "1.8.0")]
    #[lang = "drop_in_place"]
    #[allow(unconditional_recursion)]
    pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
        // Code here does not matter - this is replaced by the
        // real drop glue by the compiler.

        // SAFETY: see comment above
        unsafe { drop_in_place(to_drop) }
    }

    /// Creates a null raw pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let p: *const i32 = ptr::null();
    /// assert!(p.is_null());
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_promotable]
    #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
    pub const fn null<T>() -> *const T {
        0 as *const T
    }

    /// Creates a null mutable raw pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let p: *mut i32 = ptr::null_mut();
    /// assert!(p.is_null());
    /// ```
    #[inline(always)]
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_promotable]
    #[rustc_const_stable(feature = "const_ptr_null", since = "1.32.0")]
    pub const fn null_mut<T>() -> *mut T {
        0 as *mut T
    }

    #[repr(C)]
    pub(crate) union Repr<T> {
        pub(crate) rust: *const [T],
        rust_mut: *mut [T],
        pub(crate) raw: FatPtr<T>,
    }

    #[repr(C)]
    pub(crate) struct FatPtr<T> {
        data: *const T,
        pub(crate) len: usize,
    }

    /// Forms a raw slice from a pointer and a length.
    ///
    /// The `len` argument is the number of **elements**, not the number of bytes.
    ///
    /// This function is safe, but actually using the return value is unsafe.
    /// See the documentation of [`from_raw_parts`] for slice safety requirements.
    ///
    /// [`from_raw_parts`]: ../../std/slice/fn.from_raw_parts.html
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::ptr;
    ///
    /// // create a slice pointer when starting out with a pointer to the first element
    /// let x = [5, 6, 7];
    /// let raw_pointer = x.as_ptr();
    /// let slice = ptr::slice_from_raw_parts(raw_pointer, 3);
    /// assert_eq!(unsafe { &*slice }[2], 7);
    /// ```
    #[inline]
    #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
    #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
    pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
        // SAFETY: Accessing the value from the `Repr` union is safe since *const [T]
        // and FatPtr have the same memory layouts. Only std can make this
        // guarantee.
        unsafe { Repr { raw: FatPtr { data, len } }.rust }
    }

    /// Performs the same functionality as [`slice_from_raw_parts`], except that a
    /// raw mutable slice is returned, as opposed to a raw immutable slice.
    ///
    /// See the documentation of [`slice_from_raw_parts`] for more details.
    ///
    /// This function is safe, but actually using the return value is unsafe.
    /// See the documentation of [`from_raw_parts_mut`] for slice safety requirements.
    ///
    /// [`slice_from_raw_parts`]: fn.slice_from_raw_parts.html
    /// [`from_raw_parts_mut`]: ../../std/slice/fn.from_raw_parts_mut.html
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::ptr;
    ///
    /// let x = &mut [5, 6, 7];
    /// let raw_pointer = x.as_mut_ptr();
    /// let slice = ptr::slice_from_raw_parts_mut(raw_pointer, 3);
    ///
    /// unsafe {
    ///     (*slice)[2] = 99; // assign a value at an index in the slice
    /// };
    ///
    /// assert_eq!(unsafe { &*slice }[2], 99);
    /// ```
    #[inline]
    #[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
    #[rustc_const_unstable(feature = "const_slice_from_raw_parts", issue = "67456")]
    pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
        // SAFETY: Accessing the value from the `Repr` union is safe since *mut [T]
        // and FatPtr have the same memory layouts
        unsafe { Repr { raw: FatPtr { data, len } }.rust_mut }
    }

    /// Swaps the values at two mutable locations of the same type, without
    /// deinitializing either.
    ///
    /// But for the following two exceptions, this function is semantically
    /// equivalent to [`mem::swap`]:
    ///
    /// * It operates on raw pointers instead of references. When references are
    ///   available, [`mem::swap`] should be preferred.
    ///
    /// * The two pointed-to values may overlap. If the values do overlap, then the
    ///   overlapping region of memory from `x` will be used. This is demonstrated
    ///   in the second example below.
    ///
    /// [`mem::swap`]: ../mem/fn.swap.html
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * Both `x` and `y` must be [valid] for both reads and writes.
    ///
    /// * Both `x` and `y` must be properly aligned.
    ///
    /// Note that even if `T` has size `0`, the pointers must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    ///
    /// # Examples
    ///
    /// Swapping two non-overlapping regions:
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let mut array = [0, 1, 2, 3];
    ///
    /// let x = array[0..].as_mut_ptr() as *mut [u32; 2]; // this is `array[0..2]`
    /// let y = array[2..].as_mut_ptr() as *mut [u32; 2]; // this is `array[2..4]`
    ///
    /// unsafe {
    ///     ptr::swap(x, y);
    ///     assert_eq!([2, 3, 0, 1], array);
    /// }
    /// ```
    ///
    /// Swapping two overlapping regions:
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let mut array = [0, 1, 2, 3];
    ///
    /// let x = array[0..].as_mut_ptr() as *mut [u32; 3]; // this is `array[0..3]`
    /// let y = array[1..].as_mut_ptr() as *mut [u32; 3]; // this is `array[1..4]`
    ///
    /// unsafe {
    ///     ptr::swap(x, y);
    ///     // The indices `1..3` of the slice overlap between `x` and `y`.
    ///     // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
    ///     // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
    ///     // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
    ///     // This implementation is defined to make the latter choice.
    ///     assert_eq!([1, 0, 1, 2], array);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
        // Give ourselves some scratch space to work with.
        // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
        let mut tmp = MaybeUninit::<T>::uninit();

        // Perform the swap
        // SAFETY: the caller must guarantee that `x` and `y` are
        // valid for writes and properly aligned. `tmp` cannot be
        // overlapping either `x` or `y` because `tmp` was just allocated
        // on the stack as a separate allocated object.
        unsafe {
            copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
            copy(y, x, 1); // `x` and `y` may overlap
            copy_nonoverlapping(tmp.as_ptr(), y, 1);
        }
    }

    /// Swaps `count * size_of::<T>()` bytes between the two regions of memory
    /// beginning at `x` and `y`. The two regions must *not* overlap.
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * Both `x` and `y` must be [valid] for both reads and writes of `count *
    ///   size_of::<T>()` bytes.
    ///
    /// * Both `x` and `y` must be properly aligned.
    ///
    /// * The region of memory beginning at `x` with a size of `count *
    ///   size_of::<T>()` bytes must *not* overlap with the region of memory
    ///   beginning at `y` with the same size.
    ///
    /// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
    /// the pointers must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let mut x = [1, 2, 3, 4];
    /// let mut y = [7, 8, 9];
    ///
    /// unsafe {
    ///     ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
    /// }
    ///
    /// assert_eq!(x, [7, 8, 3, 4]);
    /// assert_eq!(y, [1, 2, 9]);
    /// ```
    #[inline]
    #[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
    pub unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
        if cfg!(debug_assertions)
            && !(is_aligned_and_not_null(x)
                && is_aligned_and_not_null(y)
                && is_nonoverlapping(x, y, count))
        {
            // Not panicking to keep codegen impact smaller.
            abort();
        }

        let x = x as *mut u8;
        let y = y as *mut u8;
        let len = mem::size_of::<T>() * count;
        // SAFETY: the caller must guarantee that `x` and `y` are
        // valid for writes and properly aligned.
        unsafe { swap_nonoverlapping_bytes(x, y, len) }
    }

    #[inline]
    pub(crate) unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
        // For types smaller than the block optimization below,
        // just swap directly to avoid pessimizing codegen.
        if mem::size_of::<T>() < 32 {
            // SAFETY: the caller must guarantee that `x` and `y` are valid
            // for writes, properly aligned, and non-overlapping.
            unsafe {
                let z = read(x);
                copy_nonoverlapping(y, x, 1);
                write(y, z);
            }
        } else {
            // SAFETY: the caller must uphold the safety contract for `swap_nonoverlapping`.
            unsafe { swap_nonoverlapping(x, y, 1) };
        }
    }

    #[inline]
    unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, len: usize) {
        // The approach here is to utilize simd to swap x & y efficiently. Testing reveals
        // that swapping either 32 bytes or 64 bytes at a time is most efficient for Intel
        // Haswell E processors. LLVM is more able to optimize if we give a struct a
        // #[repr(simd)], even if we don't actually use this struct directly.
        //
        // FIXME repr(simd) broken on emscripten and redox
        #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox")), repr(simd))]
        struct Block(u64, u64, u64, u64);
        struct UnalignedBlock(u64, u64, u64, u64);

        let block_size = mem::size_of::<Block>();

        // Loop through x & y, copying them `Block` at a time
        // The optimizer should unroll the loop fully for most types
        // N.B. We can't use a for loop as the `range` impl calls `mem::swap` recursively
        let mut i = 0;
        while i + block_size <= len {
            // Create some uninitialized memory as scratch space
            // Declaring `t` here avoids aligning the stack when this loop is unused
            let mut t = mem::MaybeUninit::<Block>::uninit();
            let t = t.as_mut_ptr() as *mut u8;

            // SAFETY: As `i < len`, and as the caller must guarantee that `x` and `y` are valid
            // for `len` bytes, `x + i` and `y + i` must be valid adresses, which fulfills the
            // safety contract for `add`.
            //
            // Also, the caller must guarantee that `x` and `y` are valid for writes, properly aligned,
            // and non-overlapping, which fulfills the safety contract for `copy_nonoverlapping`.
            unsafe {
                let x = x.add(i);
                let y = y.add(i);

                // Swap a block of bytes of x & y, using t as a temporary buffer
                // This should be optimized into efficient SIMD operations where available
                copy_nonoverlapping(x, t, block_size);
                copy_nonoverlapping(y, x, block_size);
                copy_nonoverlapping(t, y, block_size);
            }
            i += block_size;
        }

        if i < len {
            // Swap any remaining bytes
            let mut t = mem::MaybeUninit::<UnalignedBlock>::uninit();
            let rem = len - i;

            let t = t.as_mut_ptr() as *mut u8;

            // SAFETY: see previous safety comment.
            unsafe {
                let x = x.add(i);
                let y = y.add(i);

                copy_nonoverlapping(x, t, rem);
                copy_nonoverlapping(y, x, rem);
                copy_nonoverlapping(t, y, rem);
            }
        }
    }

    /// Moves `src` into the pointed `dst`, returning the previous `dst` value.
    ///
    /// Neither value is dropped.
    ///
    /// This function is semantically equivalent to [`mem::replace`] except that it
    /// operates on raw pointers instead of references. When references are
    /// available, [`mem::replace`] should be preferred.
    ///
    /// [`mem::replace`]: ../mem/fn.replace.html
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `dst` must be [valid] for both reads and writes.
    ///
    /// * `dst` must be properly aligned.
    ///
    /// * `dst` must point to a properly initialized value of type `T`.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let mut rust = vec!['b', 'u', 's', 't'];
    ///
    /// // `mem::replace` would have the same effect without requiring the unsafe
    /// // block.
    /// let b = unsafe {
    ///     ptr::replace(&mut rust[0], 'r')
    /// };
    ///
    /// assert_eq!(b, 'b');
    /// assert_eq!(rust, &['r', 'u', 's', 't']);
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
        // SAFETY: the caller must guarantee that `dst` is valid to be
        // cast to a mutable reference (valid for writes, aligned, initialized),
        // and cannot overlap `src` since `dst` must point to a distinct
        // allocated object.
        unsafe {
            mem::swap(&mut *dst, &mut src); // cannot overlap
        }
        src
    }

    /// Reads the value from `src` without moving it. This leaves the
    /// memory in `src` unchanged.
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `src` must be [valid] for reads.
    ///
    /// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
    ///   case.
    ///
    /// * `src` must point to a properly initialized value of type `T`.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let x = 12;
    /// let y = &x as *const i32;
    ///
    /// unsafe {
    ///     assert_eq!(std::ptr::read(y), 12);
    /// }
    /// ```
    ///
    /// Manually implement [`mem::swap`]:
    ///
    /// ```
    /// use std::ptr;
    ///
    /// fn swap<T>(a: &mut T, b: &mut T) {
    ///     unsafe {
    ///         // Create a bitwise copy of the value at `a` in `tmp`.
    ///         let tmp = ptr::read(a);
    ///
    ///         // Exiting at this point (either by explicitly returning or by
    ///         // calling a function which panics) would cause the value in `tmp` to
    ///         // be dropped while the same value is still referenced by `a`. This
    ///         // could trigger undefined behavior if `T` is not `Copy`.
    ///
    ///         // Create a bitwise copy of the value at `b` in `a`.
    ///         // This is safe because mutable references cannot alias.
    ///         ptr::copy_nonoverlapping(b, a, 1);
    ///
    ///         // As above, exiting here could trigger undefined behavior because
    ///         // the same value is referenced by `a` and `b`.
    ///
    ///         // Move `tmp` into `b`.
    ///         ptr::write(b, tmp);
    ///
    ///         // `tmp` has been moved (`write` takes ownership of its second argument),
    ///         // so nothing is dropped implicitly here.
    ///     }
    /// }
    ///
    /// let mut foo = "foo".to_owned();
    /// let mut bar = "bar".to_owned();
    ///
    /// swap(&mut foo, &mut bar);
    ///
    /// assert_eq!(foo, "bar");
    /// assert_eq!(bar, "foo");
    /// ```
    ///
    /// ## Ownership of the Returned Value
    ///
    /// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
    /// If `T` is not [`Copy`], using both the returned value and the value at
    /// `*src` can violate memory safety. Note that assigning to `*src` counts as a
    /// use because it will attempt to drop the value at `*src`.
    ///
    /// [`write`] can be used to overwrite data without causing it to be dropped.
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let mut s = String::from("foo");
    /// unsafe {
    ///     // `s2` now points to the same underlying memory as `s`.
    ///     let mut s2: String = ptr::read(&s);
    ///
    ///     assert_eq!(s2, "foo");
    ///
    ///     // Assigning to `s2` causes its original value to be dropped. Beyond
    ///     // this point, `s` must no longer be used, as the underlying memory has
    ///     // been freed.
    ///     s2 = String::default();
    ///     assert_eq!(s2, "");
    ///
    ///     // Assigning to `s` would cause the old value to be dropped again,
    ///     // resulting in undefined behavior.
    ///     // s = String::from("bar"); // ERROR
    ///
    ///     // `ptr::write` can be used to overwrite a value without dropping it.
    ///     ptr::write(&mut s, String::from("bar"));
    /// }
    ///
    /// assert_eq!(s, "bar");
    /// ```
    ///
    /// [`mem::swap`]: ../mem/fn.swap.html
    /// [valid]: ../ptr/index.html#safety
    /// [`Copy`]: ../marker/trait.Copy.html
    /// [`read_unaligned`]: ./fn.read_unaligned.html
    /// [`write`]: ./fn.write.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn read<T>(src: *const T) -> T {
        // `copy_nonoverlapping` takes care of debug_assert.
        let mut tmp = MaybeUninit::<T>::uninit();
        // SAFETY: the caller must guarantee that `src` is valid for reads.
        // `src` cannot overlap `tmp` because `tmp` was just allocated on
        // the stack as a separate allocated object.
        //
        // Also, since we just wrote a valid value into `tmp`, it is guaranteed
        // to be properly initialized.
        unsafe {
            copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
            tmp.assume_init()
        }
    }

    /// Reads the value from `src` without moving it. This leaves the
    /// memory in `src` unchanged.
    ///
    /// Unlike [`read`], `read_unaligned` works with unaligned pointers.
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `src` must be [valid] for reads.
    ///
    /// * `src` must point to a properly initialized value of type `T`.
    ///
    /// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
    /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
    /// value and the value at `*src` can [violate memory safety][read-ownership].
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL.
    ///
    /// [`Copy`]: ../marker/trait.Copy.html
    /// [`read`]: ./fn.read.html
    /// [`write_unaligned`]: ./fn.write_unaligned.html
    /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
    /// [valid]: ../ptr/index.html#safety
    ///
    /// ## On `packed` structs
    ///
    /// It is currently impossible to create raw pointers to unaligned fields
    /// of a packed struct.
    ///
    /// Attempting to create a raw pointer to an `unaligned` struct field with
    /// an expression such as `&packed.unaligned as *const FieldType` creates an
    /// intermediate unaligned reference before converting that to a raw pointer.
    /// That this reference is temporary and immediately cast is inconsequential
    /// as the compiler always expects references to be properly aligned.
    /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
    /// *undefined behavior* in your program.
    ///
    /// An example of what not to do and how this relates to `read_unaligned` is:
    ///
    /// ```no_run
    /// #[repr(packed, C)]
    /// struct Packed {
    ///     _padding: u8,
    ///     unaligned: u32,
    /// }
    ///
    /// let packed = Packed {
    ///     _padding: 0x00,
    ///     unaligned: 0x01020304,
    /// };
    ///
    /// let v = unsafe {
    ///     // Here we attempt to take the address of a 32-bit integer which is not aligned.
    ///     let unaligned =
    ///         // A temporary unaligned reference is created here which results in
    ///         // undefined behavior regardless of whether the reference is used or not.
    ///         &packed.unaligned
    ///         // Casting to a raw pointer doesn't help; the mistake already happened.
    ///         as *const u32;
    ///
    ///     let v = std::ptr::read_unaligned(unaligned);
    ///
    ///     v
    /// };
    /// ```
    ///
    /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
    // FIXME: Update docs based on outcome of RFC #2582 and friends.
    ///
    /// # Examples
    ///
    /// Read an usize value from a byte buffer:
    ///
    /// ```
    /// use std::mem;
    ///
    /// fn read_usize(x: &[u8]) -> usize {
    ///     assert!(x.len() >= mem::size_of::<usize>());
    ///
    ///     let ptr = x.as_ptr() as *const usize;
    ///
    ///     unsafe { ptr.read_unaligned() }
    /// }
    /// ```
    #[inline]
    #[stable(feature = "ptr_unaligned", since = "1.17.0")]
    pub unsafe fn read_unaligned<T>(src: *const T) -> T {
        // `copy_nonoverlapping` takes care of debug_assert.
        let mut tmp = MaybeUninit::<T>::uninit();
        // SAFETY: the caller must guarantee that `src` is valid for reads.
        // `src` cannot overlap `tmp` because `tmp` was just allocated on
        // the stack as a separate allocated object.
        //
        // Also, since we just wrote a valid value into `tmp`, it is guaranteed
        // to be properly initialized.
        unsafe {
            copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
            tmp.assume_init()
        }
    }

    /// Overwrites a memory location with the given value without reading or
    /// dropping the old value.
    ///
    /// `write` does not drop the contents of `dst`. This is safe, but it could leak
    /// allocations or resources, so care should be taken not to overwrite an object
    /// that should be dropped.
    ///
    /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
    /// location pointed to by `dst`.
    ///
    /// This is appropriate for initializing uninitialized memory, or overwriting
    /// memory that has previously been [`read`] from.
    ///
    /// [`read`]: ./fn.read.html
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `dst` must be [valid] for writes.
    ///
    /// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
    ///   case.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    /// [`write_unaligned`]: ./fn.write_unaligned.html
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut x = 0;
    /// let y = &mut x as *mut i32;
    /// let z = 12;
    ///
    /// unsafe {
    ///     std::ptr::write(y, z);
    ///     assert_eq!(std::ptr::read(y), 12);
    /// }
    /// ```
    ///
    /// Manually implement [`mem::swap`]:
    ///
    /// ```
    /// use std::ptr;
    ///
    /// fn swap<T>(a: &mut T, b: &mut T) {
    ///     unsafe {
    ///         // Create a bitwise copy of the value at `a` in `tmp`.
    ///         let tmp = ptr::read(a);
    ///
    ///         // Exiting at this point (either by explicitly returning or by
    ///         // calling a function which panics) would cause the value in `tmp` to
    ///         // be dropped while the same value is still referenced by `a`. This
    ///         // could trigger undefined behavior if `T` is not `Copy`.
    ///
    ///         // Create a bitwise copy of the value at `b` in `a`.
    ///         // This is safe because mutable references cannot alias.
    ///         ptr::copy_nonoverlapping(b, a, 1);
    ///
    ///         // As above, exiting here could trigger undefined behavior because
    ///         // the same value is referenced by `a` and `b`.
    ///
    ///         // Move `tmp` into `b`.
    ///         ptr::write(b, tmp);
    ///
    ///         // `tmp` has been moved (`write` takes ownership of its second argument),
    ///         // so nothing is dropped implicitly here.
    ///     }
    /// }
    ///
    /// let mut foo = "foo".to_owned();
    /// let mut bar = "bar".to_owned();
    ///
    /// swap(&mut foo, &mut bar);
    ///
    /// assert_eq!(foo, "bar");
    /// assert_eq!(bar, "foo");
    /// ```
    ///
    /// [`mem::swap`]: ../mem/fn.swap.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn write<T>(dst: *mut T, src: T) {
        if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
            // Not panicking to keep codegen impact smaller.
            abort();
        }
        // SAFETY: the caller must uphold the safety contract for `move_val_init`.
        unsafe { intrinsics::move_val_init(&mut *dst, src) }
    }

    /// Overwrites a memory location with the given value without reading or
    /// dropping the old value.
    ///
    /// Unlike [`write`], the pointer may be unaligned.
    ///
    /// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
    /// could leak allocations or resources, so care should be taken not to overwrite
    /// an object that should be dropped.
    ///
    /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
    /// location pointed to by `dst`.
    ///
    /// This is appropriate for initializing uninitialized memory, or overwriting
    /// memory that has previously been read with [`read_unaligned`].
    ///
    /// [`write`]: ./fn.write.html
    /// [`read_unaligned`]: ./fn.read_unaligned.html
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `dst` must be [valid] for writes.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL.
    ///
    /// [valid]: ../ptr/index.html#safety
    ///
    /// ## On `packed` structs
    ///
    /// It is currently impossible to create raw pointers to unaligned fields
    /// of a packed struct.
    ///
    /// Attempting to create a raw pointer to an `unaligned` struct field with
    /// an expression such as `&packed.unaligned as *const FieldType` creates an
    /// intermediate unaligned reference before converting that to a raw pointer.
    /// That this reference is temporary and immediately cast is inconsequential
    /// as the compiler always expects references to be properly aligned.
    /// As a result, using `&packed.unaligned as *const FieldType` causes immediate
    /// *undefined behavior* in your program.
    ///
    /// An example of what not to do and how this relates to `write_unaligned` is:
    ///
    /// ```no_run
    /// #[repr(packed, C)]
    /// struct Packed {
    ///     _padding: u8,
    ///     unaligned: u32,
    /// }
    ///
    /// let v = 0x01020304;
    /// let mut packed: Packed = unsafe { std::mem::zeroed() };
    ///
    /// let v = unsafe {
    ///     // Here we attempt to take the address of a 32-bit integer which is not aligned.
    ///     let unaligned =
    ///         // A temporary unaligned reference is created here which results in
    ///         // undefined behavior regardless of whether the reference is used or not.
    ///         &mut packed.unaligned
    ///         // Casting to a raw pointer doesn't help; the mistake already happened.
    ///         as *mut u32;
    ///
    ///     std::ptr::write_unaligned(unaligned, v);
    ///
    ///     v
    /// };
    /// ```
    ///
    /// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
    // FIXME: Update docs based on outcome of RFC #2582 and friends.
    ///
    /// # Examples
    ///
    /// Write an usize value to a byte buffer:
    ///
    /// ```
    /// use std::mem;
    ///
    /// fn write_usize(x: &mut [u8], val: usize) {
    ///     assert!(x.len() >= mem::size_of::<usize>());
    ///
    ///     let ptr = x.as_mut_ptr() as *mut usize;
    ///
    ///     unsafe { ptr.write_unaligned(val) }
    /// }
    /// ```
    #[inline]
    #[stable(feature = "ptr_unaligned", since = "1.17.0")]
    pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
        // SAFETY: the caller must guarantee that `dst` is valid for writes.
        // `dst` cannot overlap `src` because the caller has mutable access
        // to `dst` while `src` is owned by this function.
        unsafe {
            // `copy_nonoverlapping` takes care of debug_assert.
            copy_nonoverlapping(&src as *const T as *const u8, dst as *mut u8, mem::size_of::<T>());
        }
        mem::forget(src);
    }

    /// Performs a volatile read of the value from `src` without moving it. This
    /// leaves the memory in `src` unchanged.
    ///
    /// Volatile operations are intended to act on I/O memory, and are guaranteed
    /// to not be elided or reordered by the compiler across other volatile
    /// operations.
    ///
    /// [`write_volatile`]: ./fn.write_volatile.html
    ///
    /// # Notes
    ///
    /// Rust does not currently have a rigorously and formally defined memory model,
    /// so the precise semantics of what "volatile" means here is subject to change
    /// over time. That being said, the semantics will almost always end up pretty
    /// similar to [C11's definition of volatile][c11].
    ///
    /// The compiler shouldn't change the relative order or number of volatile
    /// memory operations. However, volatile memory operations on zero-sized types
    /// (e.g., if a zero-sized type is passed to `read_volatile`) are noops
    /// and may be ignored.
    ///
    /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `src` must be [valid] for reads.
    ///
    /// * `src` must be properly aligned.
    ///
    /// * `src` must point to a properly initialized value of type `T`.
    ///
    /// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of
    /// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
    /// value and the value at `*src` can [violate memory safety][read-ownership].
    /// However, storing non-[`Copy`] types in volatile memory is almost certainly
    /// incorrect.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    /// [`Copy`]: ../marker/trait.Copy.html
    /// [`read`]: ./fn.read.html
    /// [read-ownership]: ./fn.read.html#ownership-of-the-returned-value
    ///
    /// Just like in C, whether an operation is volatile has no bearing whatsoever
    /// on questions involving concurrent access from multiple threads. Volatile
    /// accesses behave exactly like non-atomic accesses in that regard. In particular,
    /// a race between a `read_volatile` and any write operation to the same location
    /// is undefined behavior.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let x = 12;
    /// let y = &x as *const i32;
    ///
    /// unsafe {
    ///     assert_eq!(std::ptr::read_volatile(y), 12);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "volatile", since = "1.9.0")]
    pub unsafe fn read_volatile<T>(src: *const T) -> T {
        if cfg!(debug_assertions) && !is_aligned_and_not_null(src) {
            // Not panicking to keep codegen impact smaller.
            abort();
        }
        // SAFETY: the caller must uphold the safety contract for `volatile_load`.
        unsafe { intrinsics::volatile_load(src) }
    }

    /// Performs a volatile write of a memory location with the given value without
    /// reading or dropping the old value.
    ///
    /// Volatile operations are intended to act on I/O memory, and are guaranteed
    /// to not be elided or reordered by the compiler across other volatile
    /// operations.
    ///
    /// `write_volatile` does not drop the contents of `dst`. This is safe, but it
    /// could leak allocations or resources, so care should be taken not to overwrite
    /// an object that should be dropped.
    ///
    /// Additionally, it does not drop `src`. Semantically, `src` is moved into the
    /// location pointed to by `dst`.
    ///
    /// [`read_volatile`]: ./fn.read_volatile.html
    ///
    /// # Notes
    ///
    /// Rust does not currently have a rigorously and formally defined memory model,
    /// so the precise semantics of what "volatile" means here is subject to change
    /// over time. That being said, the semantics will almost always end up pretty
    /// similar to [C11's definition of volatile][c11].
    ///
    /// The compiler shouldn't change the relative order or number of volatile
    /// memory operations. However, volatile memory operations on zero-sized types
    /// (e.g., if a zero-sized type is passed to `write_volatile`) are noops
    /// and may be ignored.
    ///
    /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
    ///
    /// # Safety
    ///
    /// Behavior is undefined if any of the following conditions are violated:
    ///
    /// * `dst` must be [valid] for writes.
    ///
    /// * `dst` must be properly aligned.
    ///
    /// Note that even if `T` has size `0`, the pointer must be non-NULL and properly aligned.
    ///
    /// [valid]: ../ptr/index.html#safety
    ///
    /// Just like in C, whether an operation is volatile has no bearing whatsoever
    /// on questions involving concurrent access from multiple threads. Volatile
    /// accesses behave exactly like non-atomic accesses in that regard. In particular,
    /// a race between a `write_volatile` and any other operation (reading or writing)
    /// on the same location is undefined behavior.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut x = 0;
    /// let y = &mut x as *mut i32;
    /// let z = 12;
    ///
    /// unsafe {
    ///     std::ptr::write_volatile(y, z);
    ///     assert_eq!(std::ptr::read_volatile(y), 12);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "volatile", since = "1.9.0")]
    pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
        if cfg!(debug_assertions) && !is_aligned_and_not_null(dst) {
            // Not panicking to keep codegen impact smaller.
            abort();
        }
        // SAFETY: the caller must uphold the safety contract for `volatile_store`.
        unsafe {
            intrinsics::volatile_store(dst, src);
        }
    }

    /// Align pointer `p`.
    ///
    /// Calculate offset (in terms of elements of `stride` stride) that has to be applied
    /// to pointer `p` so that pointer `p` would get aligned to `a`.
    ///
    /// Note: This implementation has been carefully tailored to not panic. It is UB for this to panic.
    /// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
    /// constants.
    ///
    /// If we ever decide to make it possible to call the intrinsic with `a` that is not a
    /// power-of-two, it will probably be more prudent to just change to a naive implementation rather
    /// than trying to adapt this to accommodate that change.
    ///
    /// Any questions go to @nagisa.
    #[lang = "align_offset"]
    pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
        /// Calculate multiplicative modular inverse of `x` modulo `m`.
        ///
        /// This implementation is tailored for align_offset and has following preconditions:
        ///
        /// * `m` is a power-of-two;
        /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
        ///
        /// Implementation of this function shall not panic. Ever.
        #[inline]
        fn mod_inv(x: usize, m: usize) -> usize {
            /// Multiplicative modular inverse table modulo 2⁴ = 16.
            ///
            /// Note, that this table does not contain values where inverse does not exist (i.e., for
            /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
            const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
            /// Modulo for which the `INV_TABLE_MOD_16` is intended.
            const INV_TABLE_MOD: usize = 16;
            /// INV_TABLE_MOD²
            const INV_TABLE_MOD_SQUARED: usize = INV_TABLE_MOD * INV_TABLE_MOD;

            let table_inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
            if m <= INV_TABLE_MOD {
                table_inverse & (m - 1)
            } else {
                // We iterate "up" using the following formula:
                //
                // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
                //
                // until 2²ⁿ ≥ m. Then we can reduce to our desired `m` by taking the result `mod m`.
                let mut inverse = table_inverse;
                let mut going_mod = INV_TABLE_MOD_SQUARED;
                loop {
                    // y = y * (2 - xy) mod n
                    //
                    // Note, that we use wrapping operations here intentionally – the original formula
                    // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
                    // usize::MAX` instead, because we take the result `mod n` at the end
                    // anyway.
                    inverse = inverse.wrapping_mul(2usize.wrapping_sub(x.wrapping_mul(inverse)));
                    if going_mod >= m {
                        return inverse & (m - 1);
                    }
                    going_mod = going_mod.wrapping_mul(going_mod);
                }
            }
        }

        let stride = mem::size_of::<T>();
        let a_minus_one = a.wrapping_sub(1);
        let pmoda = p as usize & a_minus_one;

        if pmoda == 0 {
            // Already aligned. Yay!
            return 0;
        }

        if stride <= 1 {
            return if stride == 0 {
                // If the pointer is not aligned, and the element is zero-sized, then no amount of
                // elements will ever align the pointer.
                !0
            } else {
                a.wrapping_sub(pmoda)
            };
        }

        let smoda = stride & a_minus_one;
        // SAFETY: a is power-of-two so cannot be 0. stride = 0 is handled above.
        let gcdpow = unsafe { intrinsics::cttz_nonzero(stride).min(intrinsics::cttz_nonzero(a)) };
        let gcd = 1usize << gcdpow;

        if p as usize & (gcd.wrapping_sub(1)) == 0 {
            // This branch solves for the following linear congruence equation:
            //
            // ` p + so = 0 mod a `
            //
            // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
            // requested alignment.
            //
            // With `g = gcd(a, s)`, and the above asserting that `p` is also divisible by `g`, we can
            // denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
            //
            // ` p' + s'o = 0 mod a' `
            // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
            //
            // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the second
            // term is "how does incrementing `p` by `s` bytes change the relative alignment of `p`" (again
            // divided by `g`).
            // Division by `g` is necessary to make the inverse well formed if `a` and `s` are not
            // co-prime.
            //
            // Furthermore, the result produced by this solution is not "minimal", so it is necessary
            // to take the result `o mod lcm(s, a)`. We can replace `lcm(s, a)` with just a `a'`.
            let a2 = a >> gcdpow;
            let a2minus1 = a2.wrapping_sub(1);
            let s2 = smoda >> gcdpow;
            let minusp2 = a2.wrapping_sub(pmoda >> gcdpow);
            return (minusp2.wrapping_mul(mod_inv(s2, a2))) & a2minus1;
        }

        // Cannot be aligned at all.
        usize::MAX
    }

    /// Compares raw pointers for equality.
    ///
    /// This is the same as using the `==` operator, but less generic:
    /// the arguments have to be `*const T` raw pointers,
    /// not anything that implements `PartialEq`.
    ///
    /// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
    /// by their address rather than comparing the values they point to
    /// (which is what the `PartialEq for &T` implementation does).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr;
    ///
    /// let five = 5;
    /// let other_five = 5;
    /// let five_ref = &five;
    /// let same_five_ref = &five;
    /// let other_five_ref = &other_five;
    ///
    /// assert!(five_ref == same_five_ref);
    /// assert!(ptr::eq(five_ref, same_five_ref));
    ///
    /// assert!(five_ref == other_five_ref);
    /// assert!(!ptr::eq(five_ref, other_five_ref));
    /// ```
    ///
    /// Slices are also compared by their length (fat pointers):
    ///
    /// ```
    /// let a = [1, 2, 3];
    /// assert!(std::ptr::eq(&a[..3], &a[..3]));
    /// assert!(!std::ptr::eq(&a[..2], &a[..3]));
    /// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
    /// ```
    ///
    /// Traits are also compared by their implementation:
    ///
    /// ```
    /// #[repr(transparent)]
    /// struct Wrapper { member: i32 }
    ///
    /// trait Trait {}
    /// impl Trait for Wrapper {}
    /// impl Trait for i32 {}
    ///
    /// let wrapper = Wrapper { member: 10 };
    ///
    /// // Pointers have equal addresses.
    /// assert!(std::ptr::eq(
    ///     &wrapper as *const Wrapper as *const u8,
    ///     &wrapper.member as *const i32 as *const u8
    /// ));
    ///
    /// // Objects have equal addresses, but `Trait` has different implementations.
    /// assert!(!std::ptr::eq(
    ///     &wrapper as &dyn Trait,
    ///     &wrapper.member as &dyn Trait,
    /// ));
    /// assert!(!std::ptr::eq(
    ///     &wrapper as &dyn Trait as *const dyn Trait,
    ///     &wrapper.member as &dyn Trait as *const dyn Trait,
    /// ));
    ///
    /// // Converting the reference to a `*const u8` compares by address.
    /// assert!(std::ptr::eq(
    ///     &wrapper as &dyn Trait as *const dyn Trait as *const u8,
    ///     &wrapper.member as &dyn Trait as *const dyn Trait as *const u8,
    /// ));
    /// ```
    #[stable(feature = "ptr_eq", since = "1.17.0")]
    #[inline]
    pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
        a == b
    }

    /// Hash a raw pointer.
    ///
    /// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
    /// by its address rather than the value it points to
    /// (which is what the `Hash for &T` implementation does).
    ///
    /// # Examples
    ///
    /// ```
    /// use std::collections::hash_map::DefaultHasher;
    /// use std::hash::{Hash, Hasher};
    /// use std::ptr;
    ///
    /// let five = 5;
    /// let five_ref = &five;
    ///
    /// let mut hasher = DefaultHasher::new();
    /// ptr::hash(five_ref, &mut hasher);
    /// let actual = hasher.finish();
    ///
    /// let mut hasher = DefaultHasher::new();
    /// (five_ref as *const i32).hash(&mut hasher);
    /// let expected = hasher.finish();
    ///
    /// assert_eq!(actual, expected);
    /// ```
    #[stable(feature = "ptr_hash", since = "1.35.0")]
    pub fn hash<T: ?Sized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
        use crate::hash::Hash;
        hashee.hash(into);
    }

    // Impls for function pointers
    macro_rules! fnptr_impls_safety_abi {
        ($FnTy: ty, $($Arg: ident),*) => {
            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> PartialEq for $FnTy {
                #[inline]
                fn eq(&self, other: &Self) -> bool {
                    *self as usize == *other as usize
                }
            }

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> Eq for $FnTy {}

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> PartialOrd for $FnTy {
                #[inline]
                fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
                    (*self as usize).partial_cmp(&(*other as usize))
                }
            }

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> Ord for $FnTy {
                #[inline]
                fn cmp(&self, other: &Self) -> Ordering {
                    (*self as usize).cmp(&(*other as usize))
                }
            }

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> hash::Hash for $FnTy {
                fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
                    state.write_usize(*self as usize)
                }
            }

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> fmt::Pointer for $FnTy {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    // HACK: The intermediate cast as usize is required for AVR
                    // so that the address space of the source function pointer
                    // is preserved in the final function pointer.
                    //
                    // https://github.com/avr-rust/rust/issues/143
                    fmt::Pointer::fmt(&(*self as usize as *const ()), f)
                }
            }

            #[stable(feature = "fnptr_impls", since = "1.4.0")]
            impl<Ret, $($Arg),*> fmt::Debug for $FnTy {
                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                    // HACK: The intermediate cast as usize is required for AVR
                    // so that the address space of the source function pointer
                    // is preserved in the final function pointer.
                    //
                    // https://github.com/avr-rust/rust/issues/143
                    fmt::Pointer::fmt(&(*self as usize as *const ()), f)
                }
            }
        }
    }

    macro_rules! fnptr_impls_args {
        ($($Arg: ident),+) => {
            fnptr_impls_safety_abi! { extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
            fnptr_impls_safety_abi! { extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
            fnptr_impls_safety_abi! { extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
            fnptr_impls_safety_abi! { unsafe extern "Rust" fn($($Arg),+) -> Ret, $($Arg),+ }
            fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+) -> Ret, $($Arg),+ }
            fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),+ , ...) -> Ret, $($Arg),+ }
        };
        () => {
            // No variadic functions with 0 parameters
            fnptr_impls_safety_abi! { extern "Rust" fn() -> Ret, }
            fnptr_impls_safety_abi! { extern "C" fn() -> Ret, }
            fnptr_impls_safety_abi! { unsafe extern "Rust" fn() -> Ret, }
            fnptr_impls_safety_abi! { unsafe extern "C" fn() -> Ret, }
        };
    }

    fnptr_impls_args! {}
    fnptr_impls_args! { A }
    fnptr_impls_args! { A, B }
    fnptr_impls_args! { A, B, C }
    fnptr_impls_args! { A, B, C, D }
    fnptr_impls_args! { A, B, C, D, E }
    fnptr_impls_args! { A, B, C, D, E, F }
    fnptr_impls_args! { A, B, C, D, E, F, G }
    fnptr_impls_args! { A, B, C, D, E, F, G, H }
    fnptr_impls_args! { A, B, C, D, E, F, G, H, I }
    fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J }
    fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K }
    fnptr_impls_args! { A, B, C, D, E, F, G, H, I, J, K, L }

    /// Create a `const` raw pointer to a place, without creating an intermediate reference.
    ///
    /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
    /// and points to initialized data. For cases where those requirements do not hold,
    /// raw pointers should be used instead. However, `&expr as *const _` creates a reference
    /// before casting it to a raw pointer, and that reference is subject to the same rules
    /// as all other references. This macro can create a raw pointer *without* creating
    /// a reference first.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(raw_ref_macros)]
    /// use std::ptr;
    ///
    /// #[repr(packed)]
    /// struct Packed {
    ///     f1: u8,
    ///     f2: u16,
    /// }
    ///
    /// let packed = Packed { f1: 1, f2: 2 };
    /// // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
    /// let raw_f2 = ptr::raw_const!(packed.f2);
    /// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
    /// ```
    #[unstable(feature = "raw_ref_macros", issue = "73394")]
    #[rustc_macro_transparency = "semitransparent"]
    #[allow_internal_unstable(raw_ref_op)]
    pub macro raw_const($e:expr) {
        &raw const $e
    }

    /// Create a `mut` raw pointer to a place, without creating an intermediate reference.
    ///
    /// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
    /// and points to initialized data. For cases where those requirements do not hold,
    /// raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference
    /// before casting it to a raw pointer, and that reference is subject to the same rules
    /// as all other references. This macro can create a raw pointer *without* creating
    /// a reference first.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(raw_ref_macros)]
    /// use std::ptr;
    ///
    /// #[repr(packed)]
    /// struct Packed {
    ///     f1: u8,
    ///     f2: u16,
    /// }
    ///
    /// let mut packed = Packed { f1: 1, f2: 2 };
    /// // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
    /// let raw_f2 = ptr::raw_mut!(packed.f2);
    /// unsafe { raw_f2.write_unaligned(42); }
    /// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
    /// ```
    #[unstable(feature = "raw_ref_macros", issue = "73394")]
    #[rustc_macro_transparency = "semitransparent"]
    #[allow_internal_unstable(raw_ref_op)]
    pub macro raw_mut($e:expr) {
        &raw mut $e
    }