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    use crate::convert::From;
    use crate::fmt;
    use crate::marker::{PhantomData, Unsize};
    use crate::mem;
    use crate::ops::{CoerceUnsized, DispatchFromDyn};
    use crate::ptr::NonNull;

    // ignore-tidy-undocumented-unsafe

    /// 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 = "0",
        reason = "use `NonNull` instead and consider `PhantomData<T>` \
                  (if you also use `#[may_dangle]`), `Send`, and/or `Sync`"
    )]
    #[doc(hidden)]
    #[repr(transparent)]
    #[rustc_layout_scalar_valid_range_start(1)]
    pub struct Unique<T: ?Sized> {
        pointer: *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 = "ptr_internals", issue = "0")]
    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 = "0")]
    unsafe impl<T: Sync + ?Sized> Sync for Unique<T> {}

    #[unstable(feature = "ptr_internals", issue = "0")]
    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.
        // FIXME: rename to dangling() to match NonNull?
        #[inline]
        pub const fn empty() -> Self {
            unsafe { Unique::new_unchecked(mem::align_of::<T>() as *mut T) }
        }
    }

    #[unstable(feature = "ptr_internals", issue = "0")]
    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 {
            Unique { pointer: ptr as _, _marker: PhantomData }
        }

        /// Creates a new `Unique` if `ptr` is non-null.
        #[inline]
        pub fn new(ptr: *mut T) -> Option<Self> {
            if !ptr.is_null() {
                Some(unsafe { Unique { pointer: ptr as _, _marker: PhantomData } })
            } else {
                None
            }
        }

        /// Acquires the underlying `*mut` pointer.
        #[inline]
        pub const fn as_ptr(self) -> *mut T {
            self.pointer 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.as_ptr()`.
        #[inline]
        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.as_ptr()`.
        #[inline]
        pub unsafe fn as_mut(&mut self) -> &mut T {
            &mut *self.as_ptr()
        }

        /// Casts to a pointer of another type.
        #[inline]
        pub const fn cast<U>(self) -> Unique<U> {
            unsafe { Unique::new_unchecked(self.as_ptr() as *mut U) }
        }
    }

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized> Clone for Unique<T> {
        #[inline]
        fn clone(&self) -> Self {
            *self
        }
    }

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized> Copy for Unique<T> {}

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized, U: ?Sized> CoerceUnsized<Unique<U>> for Unique<T> where T: Unsize<U> {}

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized, U: ?Sized> DispatchFromDyn<Unique<U>> for Unique<T> where T: Unsize<U> {}

    #[unstable(feature = "ptr_internals", issue = "0")]
    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 = "0")]
    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 = "0")]
    impl<T: ?Sized> From<&mut T> for Unique<T> {
        #[inline]
        fn from(reference: &mut T) -> Self {
            unsafe { Unique { pointer: reference as *mut T, _marker: PhantomData } }
        }
    }

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized> From<&T> for Unique<T> {
        #[inline]
        fn from(reference: &T) -> Self {
            unsafe { Unique { pointer: reference as *const T, _marker: PhantomData } }
        }
    }

    #[unstable(feature = "ptr_internals", issue = "0")]
    impl<T: ?Sized> From<NonNull<T>> for Unique<T> {
        #[inline]
        fn from(p: NonNull<T>) -> Self {
            unsafe { Unique::new_unchecked(p.as_ptr()) }
        }
    }