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

    // FIXME: talk about offset, copy_memory, copy_nonoverlapping_memory

    //! Raw, unsafe pointers, `*const T`, and `*mut T`.
    //!
    //! *[See also the pointer primitive types](../../std/primitive.pointer.html).*

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

    use convert::From;
    use intrinsics;
    use ops::CoerceUnsized;
    use fmt;
    use hash;
    use marker::{PhantomData, Unsize};
    use mem;
    use nonzero::NonZero;

    use cmp::Ordering::{self, Less, Equal, Greater};

    // FIXME #19649: intrinsic docs don't render, so these have no docs :(

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

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

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

    /// Executes the destructor (if any) of the pointed-to value.
    ///
    /// This has two use cases:
    ///
    /// * 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.
    ///
    /// # Undefined Behavior
    ///
    /// This has all the same safety problems as `ptr::read` with respect to
    /// invalid pointers, types, and double drops.
    #[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.
        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]
    #[stable(feature = "rust1", since = "1.0.0")]
    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]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub const fn null_mut<T>() -> *mut T { 0 as *mut T }

    /// Swaps the values at two mutable locations of the same type, without
    /// deinitializing either. They may overlap, unlike `mem::swap` which is
    /// otherwise equivalent.
    ///
    /// # Safety
    ///
    /// This function copies the memory through the raw pointers passed to it
    /// as arguments.
    ///
    /// Ensure that these pointers are valid before calling `swap`.
    #[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
        let mut tmp: T = mem::uninitialized();

        // Perform the swap
        copy_nonoverlapping(x, &mut tmp, 1);
        copy(y, x, 1); // `x` and `y` may overlap
        copy_nonoverlapping(&tmp, y, 1);

        // y and t now point to the same thing, but we need to completely forget `tmp`
        // because it's no longer relevant.
        mem::forget(tmp);
    }

    /// Swaps a sequence of values at two mutable locations of the same type.
    ///
    /// # Safety
    ///
    /// The two arguments must each point to the beginning of `count` locations
    /// of valid memory, and the two memory ranges must not overlap.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// #![feature(swap_nonoverlapping)]
    ///
    /// 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]
    #[unstable(feature = "swap_nonoverlapping", issue = "42818")]
    pub unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
        let x = x as *mut u8;
        let y = y as *mut u8;
        let len = mem::size_of::<T>() * count;
        swap_nonoverlapping_bytes(x, y, len)
    }

    #[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
        // It's also broken on big-endian powerpc64 and s390x.  #42778
        #[cfg_attr(not(any(target_os = "emscripten", target_os = "redox",
                           target_endian = "big")),
                   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: Block = mem::uninitialized();
            let t = &mut t as *mut _ as *mut u8;
            let x = x.offset(i as isize);
            let y = y.offset(i as isize);

            // 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: UnalignedBlock = mem::uninitialized();
            let rem = len - i;

            let t = &mut t as *mut _ as *mut u8;
            let x = x.offset(i as isize);
            let y = y.offset(i as isize);

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

    /// Replaces the value at `dest` with `src`, returning the old
    /// value, without dropping either.
    ///
    /// # Safety
    ///
    /// This is only unsafe because it accepts a raw pointer.
    /// Otherwise, this operation is identical to `mem::replace`.
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn replace<T>(dest: *mut T, mut src: T) -> T {
        mem::swap(&mut *dest, &mut src); // cannot overlap
        src
    }

    /// Reads the value from `src` without moving it. This leaves the
    /// memory in `src` unchanged.
    ///
    /// # Safety
    ///
    /// Beyond accepting a raw pointer, this is unsafe because it semantically
    /// moves the value out of `src` without preventing further usage of `src`.
    /// If `T` is not `Copy`, then care must be taken to ensure that the value at
    /// `src` is not used before the data is overwritten again (e.g. with `write`,
    /// `zero_memory`, or `copy_memory`). Note that `*src = foo` counts as a use
    /// because it will attempt to drop the value previously at `*src`.
    ///
    /// The pointer must be aligned; use `read_unaligned` if that is not the case.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let x = 12;
    /// let y = &x as *const i32;
    ///
    /// unsafe {
    ///     assert_eq!(std::ptr::read(y), 12);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn read<T>(src: *const T) -> T {
        let mut tmp: T = mem::uninitialized();
        copy_nonoverlapping(src, &mut tmp, 1);
        tmp
    }

    /// Reads the value from `src` without moving it. This leaves the
    /// memory in `src` unchanged.
    ///
    /// Unlike `read`, the pointer may be unaligned.
    ///
    /// # Safety
    ///
    /// Beyond accepting a raw pointer, this is unsafe because it semantically
    /// moves the value out of `src` without preventing further usage of `src`.
    /// If `T` is not `Copy`, then care must be taken to ensure that the value at
    /// `src` is not used before the data is overwritten again (e.g. with `write`,
    /// `zero_memory`, or `copy_memory`). Note that `*src = foo` counts as a use
    /// because it will attempt to drop the value previously at `*src`.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let x = 12;
    /// let y = &x as *const i32;
    ///
    /// unsafe {
    ///     assert_eq!(std::ptr::read_unaligned(y), 12);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "ptr_unaligned", since = "1.17.0")]
    pub unsafe fn read_unaligned<T>(src: *const T) -> T {
        let mut tmp: T = mem::uninitialized();
        copy_nonoverlapping(src as *const u8,
                            &mut tmp as *mut T as *mut u8,
                            mem::size_of::<T>());
        tmp
    }

    /// Overwrites a memory location with the given value without reading or
    /// dropping the old value.
    ///
    /// # Safety
    ///
    /// This operation is marked unsafe because it accepts a raw pointer.
    ///
    /// It does not drop the contents of `dst`. This is safe, but it could leak
    /// allocations or resources, so care must 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.
    ///
    /// The pointer must be aligned; use `write_unaligned` if that is not the case.
    ///
    /// # 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);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub unsafe fn write<T>(dst: *mut T, src: T) {
        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.
    ///
    /// # Safety
    ///
    /// This operation is marked unsafe because it accepts a raw pointer.
    ///
    /// It does not drop the contents of `dst`. This is safe, but it could leak
    /// allocations or resources, so care must 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.
    ///
    /// # Examples
    ///
    /// Basic usage:
    ///
    /// ```
    /// let mut x = 0;
    /// let y = &mut x as *mut i32;
    /// let z = 12;
    ///
    /// unsafe {
    ///     std::ptr::write_unaligned(y, z);
    ///     assert_eq!(std::ptr::read_unaligned(y), 12);
    /// }
    /// ```
    #[inline]
    #[stable(feature = "ptr_unaligned", since = "1.17.0")]
    pub unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
        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.
    ///
    /// # 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 no-ops
    /// and may be ignored.
    ///
    /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
    ///
    /// # Safety
    ///
    /// Beyond accepting a raw pointer, this is unsafe because it semantically
    /// moves the value out of `src` without preventing further usage of `src`.
    /// If `T` is not `Copy`, then care must be taken to ensure that the value at
    /// `src` is not used before the data is overwritten again (e.g. with `write`,
    /// `zero_memory`, or `copy_memory`). Note that `*src = foo` counts as a use
    /// because it will attempt to drop the value previously at `*src`.
    ///
    /// # 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 {
        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.
    ///
    /// # 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 no-ops
    /// and may be ignored.
    ///
    /// [c11]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
    ///
    /// # Safety
    ///
    /// This operation is marked unsafe because it accepts a raw pointer.
    ///
    /// It does not drop the contents of `dst`. This is safe, but it could leak
    /// allocations or resources, so care must be taken not to overwrite an object
    /// that should be dropped.
    ///
    /// This is appropriate for initializing uninitialized memory, or overwriting
    /// memory that has previously been `read` from.
    ///
    /// # 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) {
        intrinsics::volatile_store(dst, src);
    }

    #[lang = "const_ptr"]
    impl<T: ?Sized> *const T {
        /// Returns `true` if the pointer is null.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "Follow the rabbit";
        /// let ptr: *const u8 = s.as_ptr();
        /// assert!(!ptr.is_null());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn is_null(self) -> bool where T: Sized {
            self == null()
        }

        /// Returns `None` if the pointer is null, or else returns a reference to
        /// the value wrapped in `Some`.
        ///
        /// # Safety
        ///
        /// While this method and its mutable counterpart are useful for
        /// null-safety, it is important to note that this is still an unsafe
        /// operation because the returned value could be pointing to invalid
        /// memory.
        ///
        /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
        /// not necessarily reflect the actual lifetime of the data.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let ptr: *const u8 = &10u8 as *const u8;
        ///
        /// unsafe {
        ///     if let Some(val_back) = ptr.as_ref() {
        ///         println!("We got back the value: {}!", val_back);
        ///     }
        /// }
        /// ```
        #[stable(feature = "ptr_as_ref", since = "1.9.0")]
        #[inline]
        pub unsafe fn as_ref<'a>(self) -> Option<&'a T> where T: Sized {
            if self.is_null() {
                None
            } else {
                Some(&*self)
            }
        }

        /// Calculates the offset from a pointer. `count` is in units of T; e.g. a
        /// `count` of 3 represents a pointer offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// Both the starting and resulting pointer must be either in bounds or one
        /// byte past the end of an allocated object. If either pointer is out of
        /// bounds or arithmetic overflow occurs then
        /// any further use of the returned value will result in undefined behavior.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "123";
        /// let ptr: *const u8 = s.as_ptr();
        ///
        /// unsafe {
        ///     println!("{}", *ptr.offset(1) as char);
        ///     println!("{}", *ptr.offset(2) as char);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub unsafe fn offset(self, count: isize) -> *const T where T: Sized {
            intrinsics::offset(self, count)
        }

        /// Calculates the offset from a pointer using wrapping arithmetic.
        /// `count` is in units of T; e.g. a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The resulting pointer does not need to be in bounds, but it is
        /// potentially hazardous to dereference (which requires `unsafe`).
        ///
        /// Always use `.offset(count)` instead when possible, because `offset`
        /// allows the compiler to optimize better.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // Iterate using a raw pointer in increments of two elements
        /// let data = [1u8, 2, 3, 4, 5];
        /// let mut ptr: *const u8 = data.as_ptr();
        /// let step = 2;
        /// let end_rounded_up = ptr.wrapping_offset(6);
        ///
        /// // This loop prints "1, 3, 5, "
        /// while ptr != end_rounded_up {
        ///     unsafe {
        ///         print!("{}, ", *ptr);
        ///     }
        ///     ptr = ptr.wrapping_offset(step);
        /// }
        /// ```
        #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
        #[inline]
        pub fn wrapping_offset(self, count: isize) -> *const T where T: Sized {
            unsafe {
                intrinsics::arith_offset(self, count)
            }
        }

        /// Calculates the distance between two pointers. The returned value is in
        /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
        ///
        /// If the address different between the two pointers ia not a multiple of
        /// `mem::size_of::<T>()` then the result of the division is rounded towards
        /// zero.
        ///
        /// This function returns `None` if `T` is a zero-sized typed.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(offset_to)]
        ///
        /// fn main() {
        ///     let a = [0; 5];
        ///     let ptr1: *const i32 = &a[1];
        ///     let ptr2: *const i32 = &a[3];
        ///     assert_eq!(ptr1.offset_to(ptr2), Some(2));
        ///     assert_eq!(ptr2.offset_to(ptr1), Some(-2));
        ///     assert_eq!(unsafe { ptr1.offset(2) }, ptr2);
        ///     assert_eq!(unsafe { ptr2.offset(-2) }, ptr1);
        /// }
        /// ```
        #[unstable(feature = "offset_to", issue = "41079")]
        #[inline]
        pub fn offset_to(self, other: *const T) -> Option<isize> where T: Sized {
            let size = mem::size_of::<T>();
            if size == 0 {
                None
            } else {
                let diff = (other as isize).wrapping_sub(self as isize);
                Some(diff / size as isize)
            }
        }
    }

    #[lang = "mut_ptr"]
    impl<T: ?Sized> *mut T {
        /// Returns `true` if the pointer is null.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = [1, 2, 3];
        /// let ptr: *mut u32 = s.as_mut_ptr();
        /// assert!(!ptr.is_null());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn is_null(self) -> bool where T: Sized {
            self == null_mut()
        }

        /// Returns `None` if the pointer is null, or else returns a reference to
        /// the value wrapped in `Some`.
        ///
        /// # Safety
        ///
        /// While this method and its mutable counterpart are useful for
        /// null-safety, it is important to note that this is still an unsafe
        /// operation because the returned value could be pointing to invalid
        /// memory.
        ///
        /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
        /// not necessarily reflect the actual lifetime of the data.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let ptr: *mut u8 = &mut 10u8 as *mut u8;
        ///
        /// unsafe {
        ///     if let Some(val_back) = ptr.as_ref() {
        ///         println!("We got back the value: {}!", val_back);
        ///     }
        /// }
        /// ```
        #[stable(feature = "ptr_as_ref", since = "1.9.0")]
        #[inline]
        pub unsafe fn as_ref<'a>(self) -> Option<&'a T> where T: Sized {
            if self.is_null() {
                None
            } else {
                Some(&*self)
            }
        }

        /// Calculates the offset from a pointer. `count` is in units of T; e.g. a
        /// `count` of 3 represents a pointer offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The offset must be in-bounds of the object, or one-byte-past-the-end.
        /// Otherwise `offset` invokes Undefined Behavior, regardless of whether
        /// the pointer is used.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = [1, 2, 3];
        /// let ptr: *mut u32 = s.as_mut_ptr();
        ///
        /// unsafe {
        ///     println!("{}", *ptr.offset(1));
        ///     println!("{}", *ptr.offset(2));
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub unsafe fn offset(self, count: isize) -> *mut T where T: Sized {
            intrinsics::offset(self, count) as *mut T
        }

        /// Calculates the offset from a pointer using wrapping arithmetic.
        /// `count` is in units of T; e.g. a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The resulting pointer does not need to be in bounds, but it is
        /// potentially hazardous to dereference (which requires `unsafe`).
        ///
        /// Always use `.offset(count)` instead when possible, because `offset`
        /// allows the compiler to optimize better.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // Iterate using a raw pointer in increments of two elements
        /// let mut data = [1u8, 2, 3, 4, 5];
        /// let mut ptr: *mut u8 = data.as_mut_ptr();
        /// let step = 2;
        /// let end_rounded_up = ptr.wrapping_offset(6);
        ///
        /// while ptr != end_rounded_up {
        ///     unsafe {
        ///         *ptr = 0;
        ///     }
        ///     ptr = ptr.wrapping_offset(step);
        /// }
        /// assert_eq!(&data, &[0, 2, 0, 4, 0]);
        /// ```
        #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
        #[inline]
        pub fn wrapping_offset(self, count: isize) -> *mut T where T: Sized {
            unsafe {
                intrinsics::arith_offset(self, count) as *mut T
            }
        }

        /// Returns `None` if the pointer is null, or else returns a mutable
        /// reference to the value wrapped in `Some`.
        ///
        /// # Safety
        ///
        /// As with `as_ref`, this is unsafe because it cannot verify the validity
        /// of the returned pointer, nor can it ensure that the lifetime `'a`
        /// returned is indeed a valid lifetime for the contained data.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let mut s = [1, 2, 3];
        /// let ptr: *mut u32 = s.as_mut_ptr();
        /// let first_value = unsafe { ptr.as_mut().unwrap() };
        /// *first_value = 4;
        /// println!("{:?}", s); // It'll print: "[4, 2, 3]".
        /// ```
        #[stable(feature = "ptr_as_ref", since = "1.9.0")]
        #[inline]
        pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T> where T: Sized {
            if self.is_null() {
                None
            } else {
                Some(&mut *self)
            }
        }

        /// Calculates the distance between two pointers. The returned value is in
        /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
        ///
        /// If the address different between the two pointers ia not a multiple of
        /// `mem::size_of::<T>()` then the result of the division is rounded towards
        /// zero.
        ///
        /// This function returns `None` if `T` is a zero-sized typed.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(offset_to)]
        ///
        /// fn main() {
        ///     let mut a = [0; 5];
        ///     let ptr1: *mut i32 = &mut a[1];
        ///     let ptr2: *mut i32 = &mut a[3];
        ///     assert_eq!(ptr1.offset_to(ptr2), Some(2));
        ///     assert_eq!(ptr2.offset_to(ptr1), Some(-2));
        ///     assert_eq!(unsafe { ptr1.offset(2) }, ptr2);
        ///     assert_eq!(unsafe { ptr2.offset(-2) }, ptr1);
        /// }
        /// ```
        #[unstable(feature = "offset_to", issue = "41079")]
        #[inline]
        pub fn offset_to(self, other: *const T) -> Option<isize> where T: Sized {
            let size = mem::size_of::<T>();
            if size == 0 {
                None
            } else {
                let diff = (other as isize).wrapping_sub(self as isize);
                Some(diff / size as isize)
            }
        }
    }

    // Equality for pointers
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialEq for *const T {
        #[inline]
        fn eq(&self, other: &*const T) -> bool { *self == *other }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Eq for *const T {}

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialEq for *mut T {
        #[inline]
        fn eq(&self, other: &*mut T) -> bool { *self == *other }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Eq for *mut T {}

    /// Compare 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!(five_ref == other_five_ref);
    ///
    /// assert!(ptr::eq(five_ref, same_five_ref));
    /// assert!(!ptr::eq(five_ref, other_five_ref));
    /// ```
    #[stable(feature = "ptr_eq", since = "1.17.0")]
    #[inline]
    pub fn eq<T: ?Sized>(a: *const T, b: *const T) -> bool {
        a == b
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg(stage0)]
    impl<T: ?Sized> Clone for *const T {
        #[inline]
        fn clone(&self) -> *const T {
            *self
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    #[cfg(stage0)]
    impl<T: ?Sized> Clone for *mut T {
        #[inline]
        fn clone(&self) -> *mut T {
            *self
        }
    }

    // Impls for function pointers
    macro_rules! fnptr_impls_safety_abi {
        ($FnTy: ty, $($Arg: ident),*) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            #[cfg(stage0)]
            impl<Ret, $($Arg),*> Clone for $FnTy {
                #[inline]
                fn clone(&self) -> Self {
                    *self
                }
            }

            #[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 {
                    fmt::Pointer::fmt(&(*self 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 {
                    fmt::Pointer::fmt(&(*self 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 }

    // Comparison for pointers
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Ord for *const T {
        #[inline]
        fn cmp(&self, other: &*const T) -> Ordering {
            if self < other {
                Less
            } else if self == other {
                Equal
            } else {
                Greater
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialOrd for *const T {
        #[inline]
        fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
            Some(self.cmp(other))
        }

        #[inline]
        fn lt(&self, other: &*const T) -> bool { *self < *other }

        #[inline]
        fn le(&self, other: &*const T) -> bool { *self <= *other }

        #[inline]
        fn gt(&self, other: &*const T) -> bool { *self > *other }

        #[inline]
        fn ge(&self, other: &*const T) -> bool { *self >= *other }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Ord for *mut T {
        #[inline]
        fn cmp(&self, other: &*mut T) -> Ordering {
            if self < other {
                Less
            } else if self == other {
                Equal
            } else {
                Greater
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialOrd for *mut T {
        #[inline]
        fn partial_cmp(&self, other: &*mut T) -> Option<Ordering> {
            Some(self.cmp(other))
        }

        #[inline]
        fn lt(&self, other: &*mut T) -> bool { *self < *other }

        #[inline]
        fn le(&self, other: &*mut T) -> bool { *self <= *other }

        #[inline]
        fn gt(&self, other: &*mut T) -> bool { *self > *other }

        #[inline]
        fn ge(&self, other: &*mut T) -> bool { *self >= *other }
    }

    /// A wrapper around a raw non-null `*mut T` that indicates that the possessor
    /// of this wrapper owns the referent. Useful for building abstractions like
    /// `Box<T>`, `Vec<T>`, `String`, and `HashMap<K, V>`.
    ///
    /// Unlike `*mut T`, `Unique<T>` behaves "as if" it were an instance of `T`.
    /// It implements `Send`/`Sync` if `T` is `Send`/`Sync`. It also implies
    /// the kind of strong aliasing guarantees an instance of `T` can expect:
    /// the referent of the pointer should not be modified without a unique path to
    /// its owning Unique.
    ///
    /// If you're uncertain of whether it's correct to use `Unique` for your purposes,
    /// consider using `Shared`, which has weaker semantics.
    ///
    /// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
    /// is never dereferenced. This is so that enums may use this forbidden value
    /// as a discriminant -- `Option<Unique<T>>` has the same size as `Unique<T>`.
    /// However the pointer may still dangle if it isn't dereferenced.
    ///
    /// Unlike `*mut T`, `Unique<T>` is covariant over `T`. This should always be correct
    /// for any type which upholds Unique's aliasing requirements.
    #[allow(missing_debug_implementations)]
    #[unstable(feature = "unique", reason = "needs an RFC to flesh out design",
               issue = "27730")]
    pub struct Unique<T: ?Sized> {
        pointer: NonZero<*const T>,
        // NOTE: this marker has no consequences for variance, but is necessary
        // for dropck to understand that we logically own a `T`.
        //
        // For details, see:
        // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
        _marker: PhantomData<T>,
    }

    /// `Unique` pointers are `Send` if `T` is `Send` because the data they
    /// reference is unaliased. Note that this aliasing invariant is
    /// unenforced by the type system; the abstraction using the
    /// `Unique` must enforce it.
    #[unstable(feature = "unique", issue = "27730")]
    unsafe impl<T: Send + ?Sized> Send for Unique<T> { }

    /// `Unique` pointers are `Sync` if `T` is `Sync` because the data they
    /// reference is unaliased. Note that this aliasing invariant is
    /// unenforced by the type system; the abstraction using the
    /// `Unique` must enforce it.
    #[unstable(feature = "unique", issue = "27730")]
    unsafe impl<T: Sync + ?Sized> Sync for Unique<T> { }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: Sized> Unique<T> {
        /// Creates a new `Unique` that is dangling, but well-aligned.
        ///
        /// This is useful for initializing types which lazily allocate, like
        /// `Vec::new` does.
        pub fn empty() -> Self {
            unsafe {
                let ptr = mem::align_of::<T>() as *mut T;
                Unique::new_unchecked(ptr)
            }
        }
    }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: ?Sized> Unique<T> {
        /// Creates a new `Unique`.
        ///
        /// # Safety
        ///
        /// `ptr` must be non-null.
        pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
            Unique { pointer: NonZero::new_unchecked(ptr), _marker: PhantomData }
        }

        /// Creates a new `Unique` if `ptr` is non-null.
        pub fn new(ptr: *mut T) -> Option<Self> {
            NonZero::new(ptr as *const T).map(|nz| Unique { pointer: nz, _marker: PhantomData })
        }

        /// Acquires the underlying `*mut` pointer.
        pub fn as_ptr(self) -> *mut T {
            self.pointer.get() as *mut T
        }

        /// Dereferences the content.
        ///
        /// The resulting lifetime is bound to self so this behaves "as if"
        /// it were actually an instance of T that is getting borrowed. If a longer
        /// (unbound) lifetime is needed, use `&*my_ptr.ptr()`.
        pub unsafe fn as_ref(&self) -> &T {
            &*self.as_ptr()
        }

        /// Mutably dereferences the content.
        ///
        /// The resulting lifetime is bound to self so this behaves "as if"
        /// it were actually an instance of T that is getting borrowed. If a longer
        /// (unbound) lifetime is needed, use `&mut *my_ptr.ptr()`.
        pub unsafe fn as_mut(&mut self) -> &mut T {
            &mut *self.as_ptr()
        }
    }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: ?Sized> Clone for Unique<T> {
        fn clone(&self) -> Self {
            *self
        }
    }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: ?Sized> Copy for Unique<T> { }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> { }

    #[unstable(feature = "unique", issue = "27730")]
    impl<T: ?Sized> fmt::Pointer for Unique<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Pointer::fmt(&self.as_ptr(), f)
        }
    }

    #[unstable(feature = "unique", issue = "27730")]
    impl<'a, T: ?Sized> From<&'a mut T> for Unique<T> {
        fn from(reference: &'a mut T) -> Self {
            Unique { pointer: NonZero::from(reference), _marker: PhantomData }
        }
    }

    #[unstable(feature = "unique", issue = "27730")]
    impl<'a, T: ?Sized> From<&'a T> for Unique<T> {
        fn from(reference: &'a T) -> Self {
            Unique { pointer: NonZero::from(reference), _marker: PhantomData }
        }
    }

    /// A wrapper around a raw `*mut T` that indicates that the possessor
    /// of this wrapper has shared ownership of the referent. Useful for
    /// building abstractions like `Rc<T>`, `Arc<T>`, or doubly-linked lists, which
    /// internally use aliased raw pointers to manage the memory that they own.
    ///
    /// This is similar to `Unique`, except that it doesn't make any aliasing
    /// guarantees, and doesn't derive Send and Sync. Note that unlike `&T`,
    /// Shared has no special mutability requirements. Shared may mutate data
    /// aliased by other Shared pointers. More precise rules require Rust to
    /// develop an actual aliasing model.
    ///
    /// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
    /// is never dereferenced. This is so that enums may use this forbidden value
    /// as a discriminant -- `Option<Shared<T>>` has the same size as `Shared<T>`.
    /// However the pointer may still dangle if it isn't dereferenced.
    ///
    /// Unlike `*mut T`, `Shared<T>` is covariant over `T`. If this is incorrect
    /// for your use case, you should include some PhantomData in your type to
    /// provide invariance, such as `PhantomData<Cell<T>>` or `PhantomData<&'a mut T>`.
    /// Usually this won't be necessary; covariance is correct for Rc, Arc, and LinkedList
    /// because they provide a public API that follows the normal shared XOR mutable
    /// rules of Rust.
    #[allow(missing_debug_implementations)]
    #[unstable(feature = "shared", reason = "needs an RFC to flesh out design",
               issue = "27730")]
    pub struct Shared<T: ?Sized> {
        pointer: NonZero<*const T>,
        // NOTE: this marker has no consequences for variance, but is necessary
        // for dropck to understand that we logically own a `T`.
        //
        // For details, see:
        // https://github.com/rust-lang/rfcs/blob/master/text/0769-sound-generic-drop.md#phantom-data
        _marker: PhantomData<T>,
    }

    /// `Shared` pointers are not `Send` because the data they reference may be aliased.
    // NB: This impl is unnecessary, but should provide better error messages.
    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> !Send for Shared<T> { }

    /// `Shared` pointers are not `Sync` because the data they reference may be aliased.
    // NB: This impl is unnecessary, but should provide better error messages.
    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> !Sync for Shared<T> { }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: Sized> Shared<T> {
        /// Creates a new `Shared` that is dangling, but well-aligned.
        ///
        /// This is useful for initializing types which lazily allocate, like
        /// `Vec::new` does.
        pub fn empty() -> Self {
            unsafe {
                let ptr = mem::align_of::<T>() as *mut T;
                Shared::new_unchecked(ptr)
            }
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> Shared<T> {
        /// Creates a new `Shared`.
        ///
        /// # Safety
        ///
        /// `ptr` must be non-null.
        pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
            Shared { pointer: NonZero::new_unchecked(ptr), _marker: PhantomData }
        }

        /// Creates a new `Shared` if `ptr` is non-null.
        pub fn new(ptr: *mut T) -> Option<Self> {
            NonZero::new(ptr as *const T).map(|nz| Shared { pointer: nz, _marker: PhantomData })
        }

        /// Acquires the underlying `*mut` pointer.
        pub fn as_ptr(self) -> *mut T {
            self.pointer.get() as *mut T
        }

        /// Dereferences the content.
        ///
        /// The resulting lifetime is bound to self so this behaves "as if"
        /// it were actually an instance of T that is getting borrowed. If a longer
        /// (unbound) lifetime is needed, use `&*my_ptr.ptr()`.
        pub unsafe fn as_ref(&self) -> &T {
            &*self.as_ptr()
        }

        /// Mutably dereferences the content.
        ///
        /// The resulting lifetime is bound to self so this behaves "as if"
        /// it were actually an instance of T that is getting borrowed. If a longer
        /// (unbound) lifetime is needed, use `&mut *my_ptr.ptr_mut()`.
        pub unsafe fn as_mut(&mut self) -> &mut T {
            &mut *self.as_ptr()
        }

        /// Acquires the underlying pointer as a `*mut` pointer.
        #[rustc_deprecated(since = "1.19", reason = "renamed to `as_ptr` for ergonomics/consistency")]
        #[unstable(feature = "shared", issue = "27730")]
        pub unsafe fn as_mut_ptr(&self) -> *mut T {
            self.as_ptr()
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> Clone for Shared<T> {
        fn clone(&self) -> Self {
            *self
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> Copy for Shared<T> { }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized, U: ?Sized> CoerceUnsized<Shared<U>> for Shared<T> where T: Unsize<U> { }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> fmt::Pointer for Shared<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Pointer::fmt(&self.as_ptr(), f)
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<T: ?Sized> From<Unique<T>> for Shared<T> {
        fn from(unique: Unique<T>) -> Self {
            Shared { pointer: unique.pointer, _marker: PhantomData }
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<'a, T: ?Sized> From<&'a mut T> for Shared<T> {
        fn from(reference: &'a mut T) -> Self {
            Shared { pointer: NonZero::from(reference), _marker: PhantomData }
        }
    }

    #[unstable(feature = "shared", issue = "27730")]
    impl<'a, T: ?Sized> From<&'a T> for Shared<T> {
        fn from(reference: &'a T) -> Self {
            Shared { pointer: NonZero::from(reference), _marker: PhantomData }
        }
    }

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