ptr.rs

// 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 intrinsics;
use ops::{CoerceUnsized, Deref};
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;

#[stable(feature = "drop_in_place", since = "1.8.0")]
pub use intrinsics::drop_in_place;

/// 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 is only unsafe because it accepts a raw pointer.
#[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);
}

/// 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`]). Note that `*src = foo`
/// counts as a use because it will attempt to drop the value previously at
/// `*src`.
///
/// [`Copy`]: ../marker/trait.Copy.html
/// [`write`]: fn.write.html
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let x = 12;
/// let y = &x as *const i32;
///
/// unsafe {
///     assert_eq!(std::ptr::read(y), 12);
/// }
/// ```
#[inline(always)]
#[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
}

/// 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.
///
/// 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(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)
}

/// 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].
///
/// [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`]). Note that `*src = foo`
/// counts as a use because it will attempt to drop the value previously at
/// `*src`.
///
/// [`Copy`]: ../marker/trait.Copy.html
/// [`write`]: fn.write.html
///
/// # 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].
///
/// [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:
    ///
    /// ```ignore
    /// let val: *const u8 = &10u8 as *const u8;
    ///
    /// unsafe {
    ///     if let Some(val_back) = val.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 * sizeof::<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)
    }
}

#[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:
    ///
    /// ```ignore
    /// let val: *mut u8 = &mut 10u8 as *mut u8;
    ///
    /// unsafe {
    ///     if let Some(val_back) = val.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 * sizeof::<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
    }

    /// 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)
        }
    }
}

// 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 {}

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

#[stable(feature = "rust1", since = "1.0.0")]
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")]
        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. This in turn implies that the
/// `Unique<T>` is `Send`/`Sync` if `T` is `Send`/`Sync`, unlike a raw
/// `*mut T` (which conveys no particular ownership semantics).  It
/// also implies that the referent of the pointer should not be
/// modified without a unique path to the `Unique` reference. Useful
/// for building abstractions like `Vec<T>` or `Box<T>`, which
/// internally use raw pointers to manage the memory that they own.
#[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`.
    ///
    /// # Safety
    ///
    /// `ptr` must be non-null.
    pub const unsafe fn new(ptr: *mut T) -> Unique<T> {
        Unique { pointer: NonZero::new(ptr), _marker: PhantomData }
    }

    /// Dereferences the content.
    pub unsafe fn get(&self) -> &T {
        &**self.pointer
    }

    /// Mutably dereferences the content.
    pub unsafe fn get_mut(&mut self) -> &mut T {
        &mut ***self
    }
}

#[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> Deref for Unique<T> {
    type Target = *mut T;

    #[inline]
    fn deref(&self) -> &*mut T {
        unsafe { mem::transmute(&*self.pointer) }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T> fmt::Pointer for Unique<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Pointer::fmt(&*self.pointer, f)
    }
}

/// A wrapper around a raw non-null `*mut T` that indicates that the possessor
/// of this wrapper has shared ownership of the referent. Useful for
/// building abstractions like `Rc<T>` or `Arc<T>`, which internally
/// use raw pointers to manage the memory that they own.
#[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`.
    ///
    /// # Safety
    ///
    /// `ptr` must be non-null.
    pub unsafe fn new(ptr: *mut T) -> Self {
        Shared { pointer: NonZero::new(ptr), _marker: PhantomData }
    }
}

#[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> Deref for Shared<T> {
    type Target = *mut T;

    #[inline]
    fn deref(&self) -> &*mut T {
        unsafe { mem::transmute(&*self.pointer) }
    }
}

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