add libcore from 2016-04-11 nightly

1 files changed, 744 insertions, 0 deletions

diff --git a/libcore/ptr.rs b/libcore/ptr.rs
new file mode 100644
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--- /dev/null
+++ b/libcore/ptr.rs
@@ -0,0 +1,744 @@
+// 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 clone::Clone;
+use intrinsics;
+use ops::{CoerceUnsized, Deref};
+use fmt;
+use hash;
+use option::Option::{self, Some, None};
+use marker::{Copy, PhantomData, Send, Sized, Sync, Unsize};
+use mem;
+use nonzero::NonZero;
+
+use cmp::{PartialEq, Eq, Ord, PartialOrd};
+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`,
+/// `zero_memory`, or `copy_memory`). Note that `*src = foo` counts as a use
+/// because it will attempt to drop the value previously at `*src`.
+#[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
+}
+
+#[allow(missing_docs)]
+#[inline(always)]
+#[unstable(feature = "filling_drop",
+           reason = "may play a larger role in std::ptr future extensions",
+           issue = "5016")]
+pub unsafe fn read_and_drop<T>(dest: *mut T) -> T {
+    // Copy the data out from `dest`:
+    let tmp = read(&*dest);
+
+    // Now mark `dest` as dropped:
+    write_bytes(dest, mem::POST_DROP_U8, 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.
+#[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. See the LLVM documentation on [[volatile]].
+///
+/// [volatile]: http://llvm.org/docs/LangRef.html#volatile-memory-accesses
+///
+/// # 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`.
+#[inline]
+#[unstable(feature = "volatile", reason = "recently added", issue = "31756")]
+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. See the LLVM documentation on [[volatile]].
+///
+/// [volatile]: http://llvm.org/docs/LangRef.html#volatile-memory-accesses
+///
+/// # 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.
+#[inline]
+#[unstable(feature = "volatile", reason = "recently added", issue = "31756")]
+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.
+    ///
+    /// # 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);
+    ///     }
+    /// }
+    /// ```
+    #[unstable(feature = "ptr_as_ref",
+               reason = "Option is not clearly the right return type, and we \
+                         may want to tie the return lifetime to a borrow of \
+                         the raw pointer",
+               issue = "27780")]
+    #[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.
+    ///
+    /// # 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);
+    ///     }
+    /// }
+    /// ```
+    #[unstable(feature = "ptr_as_ref",
+               reason = "Option is not clearly the right return type, and we \
+                         may want to tie the return lifetime to a borrow of \
+                         the raw pointer",
+               issue = "27780")]
+    #[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.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let mut s = [1, 2, 3];
+    /// let ptr: *mut u32 = s.as_mut_ptr();
+    /// ```
+    #[unstable(feature = "ptr_as_ref",
+               reason = "return value does not necessarily convey all possible \
+                         information",
+               issue = "27780")]
+    #[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! { unsafe extern "Rust" fn($($Arg),*) -> Ret, $($Arg),* }
+        fnptr_impls_safety_abi! { unsafe extern "C" fn($($Arg),*) -> Ret, $($Arg),* }
+    }
+}
+
+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)
+    }
+}