1 files changed, 193 insertions, 0 deletions
diff --git a/library/core/src/ptr/unique.rs b/library/core/src/ptr/unique.rs
new file mode 100644
index 000000000..64616142b
--- /dev/null
+++ b/library/core/src/ptr/unique.rs
@@ -0,0 +1,193 @@
+use crate::convert::From;
+use crate::fmt;
+use crate::marker::{PhantomData, Unsize};
+use crate::ops::{CoerceUnsized, DispatchFromDyn};
+use crate::ptr::NonNull;
+
+/// 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 `NonNull`, 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.
+#[unstable(
+    feature = "ptr_internals",
+    issue = "none",
+    reason = "use `NonNull` instead and consider `PhantomData<T>` \
+              (if you also use `#[may_dangle]`), `Send`, and/or `Sync`"
+)]
+#[doc(hidden)]
+#[repr(transparent)]
+pub struct Unique<T: ?Sized> {
+    pointer: NonNull<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 = "ptr_internals", issue = "none")]
+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 = "ptr_internals", issue = "none")]
+unsafe impl<T: Sync + ?Sized> Sync for Unique<T> {}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+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.
+    ///
+    /// Note that the pointer value may potentially represent a valid pointer to
+    /// a `T`, which means this must not be used as a "not yet initialized"
+    /// sentinel value. Types that lazily allocate must track initialization by
+    /// some other means.
+    #[must_use]
+    #[inline]
+    pub const fn dangling() -> Self {
+        Self::from(NonNull::dangling())
+    }
+}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized> Unique<T> {
+    /// Creates a new `Unique`.
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must be non-null.
+    #[inline]
+    pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
+        // SAFETY: the caller must guarantee that `ptr` is non-null.
+        unsafe { Unique { pointer: NonNull::new_unchecked(ptr), _marker: PhantomData } }
+    }
+
+    /// Creates a new `Unique` if `ptr` is non-null.
+    #[inline]
+    pub const fn new(ptr: *mut T) -> Option<Self> {
+        if let Some(pointer) = NonNull::new(ptr) {
+            Some(Unique { pointer, _marker: PhantomData })
+        } else {
+            None
+        }
+    }
+
+    /// Acquires the underlying `*mut` pointer.
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[inline]
+    pub const fn as_ptr(self) -> *mut T {
+        self.pointer.as_ptr()
+    }
+
+    /// 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.as_ptr()`.
+    #[must_use]
+    #[inline]
+    pub const unsafe fn as_ref(&self) -> &T {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a reference.
+        unsafe { self.pointer.as_ref() }
+    }
+
+    /// 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.as_ptr()`.
+    #[must_use]
+    #[inline]
+    pub const unsafe fn as_mut(&mut self) -> &mut T {
+        // SAFETY: the caller must guarantee that `self` meets all the
+        // requirements for a mutable reference.
+        unsafe { self.pointer.as_mut() }
+    }
+
+    /// Casts to a pointer of another type.
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[inline]
+    pub const fn cast<U>(self) -> Unique<U> {
+        Unique::from(self.pointer.cast())
+    }
+}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
+impl<T: ?Sized> const Clone for Unique<T> {
+    #[inline]
+    fn clone(&self) -> Self {
+        *self
+    }
+}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized> Copy for Unique<T> {}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized> fmt::Debug for Unique<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Pointer::fmt(&self.as_ptr(), f)
+    }
+}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+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 = "ptr_internals", issue = "none")]
+impl<T: ?Sized> const From<&mut T> for Unique<T> {
+    /// Converts a `&mut T` to a `Unique<T>`.
+    ///
+    /// This conversion is infallible since references cannot be null.
+    #[inline]
+    fn from(reference: &mut T) -> Self {
+        Self::from(NonNull::from(reference))
+    }
+}
+
+#[unstable(feature = "ptr_internals", issue = "none")]
+impl<T: ?Sized> const From<NonNull<T>> for Unique<T> {
+    /// Converts a `NonNull<T>` to a `Unique<T>`.
+    ///
+    /// This conversion is infallible since `NonNull` cannot be null.
+    #[inline]
+    fn from(pointer: NonNull<T>) -> Self {
+        Unique { pointer, _marker: PhantomData }
+    }
+}