non_null.rs

use crate::cmp::Ordering;
use crate::convert::From;
use crate::fmt;
use crate::hash;
use crate::intrinsics::assert_unsafe_precondition;
use crate::marker::Unsize;
use crate::mem::{self, MaybeUninit};
use crate::num::NonZeroUsize;
use crate::ops::{CoerceUnsized, DispatchFromDyn};
use crate::ptr::Unique;
use crate::slice::{self, SliceIndex};

/// `*mut T` but non-zero and [covariant].
///
/// This is often the correct thing to use when building data structures using
/// raw pointers, but is ultimately more dangerous to use because of its additional
/// properties. If you're not sure if you should use `NonNull<T>`, just use `*mut T`!
///
/// 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<NonNull<T>>` has the same size as `*mut T`.
/// However the pointer may still dangle if it isn't dereferenced.
///
/// Unlike `*mut T`, `NonNull<T>` was chosen to be covariant over `T`. This makes it
/// possible to use `NonNull<T>` when building covariant types, but introduces the
/// risk of unsoundness if used in a type that shouldn't actually be covariant.
/// (The opposite choice was made for `*mut T` even though technically the unsoundness
/// could only be caused by calling unsafe functions.)
///
/// Covariance is correct for most safe abstractions, such as `Box`, `Rc`, `Arc`, `Vec`,
/// and `LinkedList`. This is the case because they provide a public API that follows the
/// normal shared XOR mutable rules of Rust.
///
/// If your type cannot safely be covariant, you must ensure it contains some
/// additional field to provide invariance. Often this field will be a [`PhantomData`]
/// type like `PhantomData<Cell<T>>` or `PhantomData<&'a mut T>`.
///
/// Notice that `NonNull<T>` has a `From` instance for `&T`. However, this does
/// not change the fact that mutating through a (pointer derived from a) shared
/// reference is undefined behavior unless the mutation happens inside an
/// [`UnsafeCell<T>`]. The same goes for creating a mutable reference from a shared
/// reference. When using this `From` instance without an `UnsafeCell<T>`,
/// it is your responsibility to ensure that `as_mut` is never called, and `as_ptr`
/// is never used for mutation.
///
/// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
/// [`PhantomData`]: crate::marker::PhantomData
/// [`UnsafeCell<T>`]: crate::cell::UnsafeCell
#[stable(feature = "nonnull", since = "1.25.0")]
#[repr(transparent)]
#[rustc_layout_scalar_valid_range_start(1)]
#[rustc_nonnull_optimization_guaranteed]
pub struct NonNull<T: ?Sized> {
    pointer: *const T,
}

/// `NonNull` pointers are not `Send` because the data they reference may be aliased.
// N.B., this impl is unnecessary, but should provide better error messages.
#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> !Send for NonNull<T> {}

/// `NonNull` pointers are not `Sync` because the data they reference may be aliased.
// N.B., this impl is unnecessary, but should provide better error messages.
#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> !Sync for NonNull<T> {}

impl<T: Sized> NonNull<T> {
    /// Creates a new `NonNull` 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.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let ptr = NonNull::<u32>::dangling();
    /// // Important: don't try to access the value of `ptr` without
    /// // initializing it first! The pointer is not null but isn't valid either!
    /// ```
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_stable(feature = "const_nonnull_dangling", since = "1.36.0")]
    #[must_use]
    #[inline]
    pub const fn dangling() -> Self {
        // SAFETY: mem::align_of() returns a non-zero usize which is then casted
        // to a *mut T. Therefore, `ptr` is not null and the conditions for
        // calling new_unchecked() are respected.
        unsafe {
            let ptr = crate::ptr::invalid_mut::<T>(mem::align_of::<T>());
            NonNull::new_unchecked(ptr)
        }
    }

    /// Returns a shared references to the value. In contrast to [`as_ref`], this does not require
    /// that the value has to be initialized.
    ///
    /// For the mutable counterpart see [`as_uninit_mut`].
    ///
    /// [`as_ref`]: NonNull::as_ref
    /// [`as_uninit_mut`]: NonNull::as_uninit_mut
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be properly aligned.
    ///
    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get mutated (except inside `UnsafeCell`).
    ///
    /// This applies even if the result of this method is unused!
    ///
    /// [the module documentation]: crate::ptr#safety
    #[inline]
    #[must_use]
    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    pub const unsafe fn as_uninit_ref<'a>(self) -> &'a MaybeUninit<T> {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a reference.
        unsafe { &*self.cast().as_ptr() }
    }

    /// Returns a unique references to the value. In contrast to [`as_mut`], this does not require
    /// that the value has to be initialized.
    ///
    /// For the shared counterpart see [`as_uninit_ref`].
    ///
    /// [`as_mut`]: NonNull::as_mut
    /// [`as_uninit_ref`]: NonNull::as_uninit_ref
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be properly aligned.
    ///
    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get accessed (read or written) through any other pointer.
    ///
    /// This applies even if the result of this method is unused!
    ///
    /// [the module documentation]: crate::ptr#safety
    #[inline]
    #[must_use]
    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    pub const unsafe fn as_uninit_mut<'a>(self) -> &'a mut MaybeUninit<T> {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a reference.
        unsafe { &mut *self.cast().as_ptr() }
    }
}

impl<T: ?Sized> NonNull<T> {
    /// Creates a new `NonNull`.
    ///
    /// # Safety
    ///
    /// `ptr` must be non-null.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let ptr = unsafe { NonNull::new_unchecked(&mut x as *mut _) };
    /// ```
    ///
    /// *Incorrect* usage of this function:
    ///
    /// ```rust,no_run
    /// use std::ptr::NonNull;
    ///
    /// // NEVER DO THAT!!! This is undefined behavior. ⚠️
    /// let ptr = unsafe { NonNull::<u32>::new_unchecked(std::ptr::null_mut()) };
    /// ```
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_stable(feature = "const_nonnull_new_unchecked", since = "1.25.0")]
    #[inline]
    pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
        // SAFETY: the caller must guarantee that `ptr` is non-null.
        unsafe {
            assert_unsafe_precondition!("NonNull::new_unchecked requires that the pointer is non-null", [T: ?Sized](ptr: *mut T) => !ptr.is_null());
            NonNull { pointer: ptr as _ }
        }
    }

    /// Creates a new `NonNull` if `ptr` is non-null.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let ptr = NonNull::<u32>::new(&mut x as *mut _).expect("ptr is null!");
    ///
    /// if let Some(ptr) = NonNull::<u32>::new(std::ptr::null_mut()) {
    ///     unreachable!();
    /// }
    /// ```
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_unstable(feature = "const_nonnull_new", issue = "93235")]
    #[inline]
    pub const fn new(ptr: *mut T) -> Option<Self> {
        if !ptr.is_null() {
            // SAFETY: The pointer is already checked and is not null
            Some(unsafe { Self::new_unchecked(ptr) })
        } else {
            None
        }
    }

    /// Performs the same functionality as [`std::ptr::from_raw_parts`], except that a
    /// `NonNull` pointer is returned, as opposed to a raw `*const` pointer.
    ///
    /// See the documentation of [`std::ptr::from_raw_parts`] for more details.
    ///
    /// [`std::ptr::from_raw_parts`]: crate::ptr::from_raw_parts
    #[unstable(feature = "ptr_metadata", issue = "81513")]
    #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
    #[inline]
    pub const fn from_raw_parts(
        data_address: NonNull<()>,
        metadata: <T as super::Pointee>::Metadata,
    ) -> NonNull<T> {
        // SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_address` is.
        unsafe {
            NonNull::new_unchecked(super::from_raw_parts_mut(data_address.as_ptr(), metadata))
        }
    }

    /// Decompose a (possibly wide) pointer into its address and metadata components.
    ///
    /// The pointer can be later reconstructed with [`NonNull::from_raw_parts`].
    #[unstable(feature = "ptr_metadata", issue = "81513")]
    #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
    #[must_use = "this returns the result of the operation, \
                  without modifying the original"]
    #[inline]
    pub const fn to_raw_parts(self) -> (NonNull<()>, <T as super::Pointee>::Metadata) {
        (self.cast(), super::metadata(self.as_ptr()))
    }

    /// Gets the "address" portion of the pointer.
    ///
    /// For more details see the equivalent method on a raw pointer, [`pointer::addr`].
    ///
    /// This API and its claimed semantics are part of the Strict Provenance experiment,
    /// see the [`ptr` module documentation][crate::ptr].
    #[must_use]
    #[inline]
    #[unstable(feature = "strict_provenance", issue = "95228")]
    pub fn addr(self) -> NonZeroUsize {
        // SAFETY: The pointer is guaranteed by the type to be non-null,
        // meaning that the address will be non-zero.
        unsafe { NonZeroUsize::new_unchecked(self.pointer.addr()) }
    }

    /// Creates a new pointer with the given address.
    ///
    /// For more details see the equivalent method on a raw pointer, [`pointer::with_addr`].
    ///
    /// This API and its claimed semantics are part of the Strict Provenance experiment,
    /// see the [`ptr` module documentation][crate::ptr].
    #[must_use]
    #[inline]
    #[unstable(feature = "strict_provenance", issue = "95228")]
    pub fn with_addr(self, addr: NonZeroUsize) -> Self {
        // SAFETY: The result of `ptr::from::with_addr` is non-null because `addr` is guaranteed to be non-zero.
        unsafe { NonNull::new_unchecked(self.pointer.with_addr(addr.get()) as *mut _) }
    }

    /// Creates a new pointer by mapping `self`'s address to a new one.
    ///
    /// For more details see the equivalent method on a raw pointer, [`pointer::map_addr`].
    ///
    /// This API and its claimed semantics are part of the Strict Provenance experiment,
    /// see the [`ptr` module documentation][crate::ptr].
    #[must_use]
    #[inline]
    #[unstable(feature = "strict_provenance", issue = "95228")]
    pub fn map_addr(self, f: impl FnOnce(NonZeroUsize) -> NonZeroUsize) -> Self {
        self.with_addr(f(self.addr()))
    }

    /// Acquires the underlying `*mut` pointer.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let ptr = NonNull::new(&mut x).expect("ptr is null!");
    ///
    /// let x_value = unsafe { *ptr.as_ptr() };
    /// assert_eq!(x_value, 0);
    ///
    /// unsafe { *ptr.as_ptr() += 2; }
    /// let x_value = unsafe { *ptr.as_ptr() };
    /// assert_eq!(x_value, 2);
    /// ```
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]
    #[must_use]
    #[inline(always)]
    pub const fn as_ptr(self) -> *mut T {
        self.pointer as *mut T
    }

    /// Returns a shared reference to the value. If the value may be uninitialized, [`as_uninit_ref`]
    /// must be used instead.
    ///
    /// For the mutable counterpart see [`as_mut`].
    ///
    /// [`as_uninit_ref`]: NonNull::as_uninit_ref
    /// [`as_mut`]: NonNull::as_mut
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be properly aligned.
    ///
    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
    ///
    /// * The pointer must point to an initialized instance of `T`.
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get mutated (except inside `UnsafeCell`).
    ///
    /// This applies even if the result of this method is unused!
    /// (The part about being initialized is not yet fully decided, but until
    /// it is, the only safe approach is to ensure that they are indeed initialized.)
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let ptr = NonNull::new(&mut x as *mut _).expect("ptr is null!");
    ///
    /// let ref_x = unsafe { ptr.as_ref() };
    /// println!("{ref_x}");
    /// ```
    ///
    /// [the module documentation]: crate::ptr#safety
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    #[must_use]
    #[inline(always)]
    pub const unsafe fn as_ref<'a>(&self) -> &'a T {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a reference.
        unsafe { &*self.as_ptr() }
    }

    /// Returns a unique reference to the value. If the value may be uninitialized, [`as_uninit_mut`]
    /// must be used instead.
    ///
    /// For the shared counterpart see [`as_ref`].
    ///
    /// [`as_uninit_mut`]: NonNull::as_uninit_mut
    /// [`as_ref`]: NonNull::as_ref
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be properly aligned.
    ///
    /// * It must be "dereferenceable" in the sense defined in [the module documentation].
    ///
    /// * The pointer must point to an initialized instance of `T`.
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get accessed (read or written) through any other pointer.
    ///
    /// This applies even if the result of this method is unused!
    /// (The part about being initialized is not yet fully decided, but until
    /// it is, the only safe approach is to ensure that they are indeed initialized.)
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let mut ptr = NonNull::new(&mut x).expect("null pointer");
    ///
    /// let x_ref = unsafe { ptr.as_mut() };
    /// assert_eq!(*x_ref, 0);
    /// *x_ref += 2;
    /// assert_eq!(*x_ref, 2);
    /// ```
    ///
    /// [the module documentation]: crate::ptr#safety
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    #[must_use]
    #[inline(always)]
    pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a mutable reference.
        unsafe { &mut *self.as_ptr() }
    }

    /// Casts to a pointer of another type.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let mut x = 0u32;
    /// let ptr = NonNull::new(&mut x as *mut _).expect("null pointer");
    ///
    /// let casted_ptr = ptr.cast::<i8>();
    /// let raw_ptr: *mut i8 = casted_ptr.as_ptr();
    /// ```
    #[stable(feature = "nonnull_cast", since = "1.27.0")]
    #[rustc_const_stable(feature = "const_nonnull_cast", since = "1.36.0")]
    #[must_use = "this returns the result of the operation, \
                  without modifying the original"]
    #[inline]
    pub const fn cast<U>(self) -> NonNull<U> {
        // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null
        unsafe { NonNull::new_unchecked(self.as_ptr() as *mut U) }
    }
}

impl<T> NonNull<[T]> {
    /// Creates a non-null raw slice from a thin pointer and a length.
    ///
    /// The `len` argument is the number of **elements**, not the number of bytes.
    ///
    /// This function is safe, but dereferencing the return value is unsafe.
    /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::ptr::NonNull;
    ///
    /// // create a slice pointer when starting out with a pointer to the first element
    /// let mut x = [5, 6, 7];
    /// let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
    /// let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
    /// assert_eq!(unsafe { slice.as_ref()[2] }, 7);
    /// ```
    ///
    /// (Note that this example artificially demonstrates a use of this method,
    /// but `let slice = NonNull::from(&x[..]);` would be a better way to write code like this.)
    #[stable(feature = "nonnull_slice_from_raw_parts", since = "CURRENT_RUSTC_VERSION")]
    #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
    #[must_use]
    #[inline]
    pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self {
        // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
        unsafe { Self::new_unchecked(super::slice_from_raw_parts_mut(data.as_ptr(), len)) }
    }

    /// Returns the length of a non-null raw slice.
    ///
    /// The returned value is the number of **elements**, not the number of bytes.
    ///
    /// This function is safe, even when the non-null raw slice cannot be dereferenced to a slice
    /// because the pointer does not have a valid address.
    ///
    /// # Examples
    ///
    /// ```rust
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.len(), 3);
    /// ```
    #[stable(feature = "slice_ptr_len_nonnull", since = "1.63.0")]
    #[rustc_const_stable(feature = "const_slice_ptr_len_nonnull", since = "1.63.0")]
    #[rustc_allow_const_fn_unstable(const_slice_ptr_len)]
    #[must_use]
    #[inline]
    pub const fn len(self) -> usize {
        self.as_ptr().len()
    }

    /// Returns a non-null pointer to the slice's buffer.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(slice_ptr_get)]
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.as_non_null_ptr(), NonNull::<i8>::dangling());
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
    pub const fn as_non_null_ptr(self) -> NonNull<T> {
        // SAFETY: We know `self` is non-null.
        unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
    }

    /// Returns a raw pointer to the slice's buffer.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(slice_ptr_get)]
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.as_mut_ptr(), NonNull::<i8>::dangling().as_ptr());
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
    pub const fn as_mut_ptr(self) -> *mut T {
        self.as_non_null_ptr().as_ptr()
    }

    /// Returns a shared reference to a slice of possibly uninitialized values. In contrast to
    /// [`as_ref`], this does not require that the value has to be initialized.
    ///
    /// For the mutable counterpart see [`as_uninit_slice_mut`].
    ///
    /// [`as_ref`]: NonNull::as_ref
    /// [`as_uninit_slice_mut`]: NonNull::as_uninit_slice_mut
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
    ///   and it must be properly aligned. This means in particular:
    ///
    ///     * The entire memory range of this slice must be contained within a single allocated object!
    ///       Slices can never span across multiple allocated objects.
    ///
    ///     * The pointer must be aligned even for zero-length slices. One
    ///       reason for this is that enum layout optimizations may rely on references
    ///       (including slices of any length) being aligned and non-null to distinguish
    ///       them from other data. You can obtain a pointer that is usable as `data`
    ///       for zero-length slices using [`NonNull::dangling()`].
    ///
    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
    ///   See the safety documentation of [`pointer::offset`].
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get mutated (except inside `UnsafeCell`).
    ///
    /// This applies even if the result of this method is unused!
    ///
    /// See also [`slice::from_raw_parts`].
    ///
    /// [valid]: crate::ptr#safety
    #[inline]
    #[must_use]
    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    pub const unsafe fn as_uninit_slice<'a>(self) -> &'a [MaybeUninit<T>] {
        // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
        unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
    }

    /// Returns a unique reference to a slice of possibly uninitialized values. In contrast to
    /// [`as_mut`], this does not require that the value has to be initialized.
    ///
    /// For the shared counterpart see [`as_uninit_slice`].
    ///
    /// [`as_mut`]: NonNull::as_mut
    /// [`as_uninit_slice`]: NonNull::as_uninit_slice
    ///
    /// # Safety
    ///
    /// When calling this method, you have to ensure that all of the following is true:
    ///
    /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
    ///   many bytes, and it must be properly aligned. This means in particular:
    ///
    ///     * The entire memory range of this slice must be contained within a single allocated object!
    ///       Slices can never span across multiple allocated objects.
    ///
    ///     * The pointer must be aligned even for zero-length slices. One
    ///       reason for this is that enum layout optimizations may rely on references
    ///       (including slices of any length) being aligned and non-null to distinguish
    ///       them from other data. You can obtain a pointer that is usable as `data`
    ///       for zero-length slices using [`NonNull::dangling()`].
    ///
    /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
    ///   See the safety documentation of [`pointer::offset`].
    ///
    /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
    ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
    ///   In particular, while this reference exists, the memory the pointer points to must
    ///   not get accessed (read or written) through any other pointer.
    ///
    /// This applies even if the result of this method is unused!
    ///
    /// See also [`slice::from_raw_parts_mut`].
    ///
    /// [valid]: crate::ptr#safety
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(allocator_api, ptr_as_uninit)]
    ///
    /// use std::alloc::{Allocator, Layout, Global};
    /// use std::mem::MaybeUninit;
    /// use std::ptr::NonNull;
    ///
    /// let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
    /// // This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
    /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
    /// # #[allow(unused_variables)]
    /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
    /// # Ok::<_, std::alloc::AllocError>(())
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "ptr_as_uninit", issue = "75402")]
    #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
    pub const unsafe fn as_uninit_slice_mut<'a>(self) -> &'a mut [MaybeUninit<T>] {
        // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
        unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
    }

    /// Returns a raw pointer to an element or subslice, without doing bounds
    /// checking.
    ///
    /// Calling this method with an out-of-bounds index or when `self` is not dereferenceable
    /// is *[undefined behavior]* even if the resulting pointer is not used.
    ///
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_ptr_get)]
    /// use std::ptr::NonNull;
    ///
    /// let x = &mut [1, 2, 4];
    /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
    ///
    /// unsafe {
    ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
    /// }
    /// ```
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
    #[inline]
    pub const unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
    where
        I: ~const SliceIndex<[T]>,
    {
        // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
        // As a consequence, the resulting pointer cannot be null.
        unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
#[rustc_const_unstable(feature = "const_clone", issue = "91805")]
impl<T: ?Sized> const Clone for NonNull<T> {
    #[inline(always)]
    fn clone(&self) -> Self {
        *self
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> Copy for NonNull<T> {}

#[unstable(feature = "coerce_unsized", issue = "18598")]
impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}

#[unstable(feature = "dispatch_from_dyn", issue = "none")]
impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> fmt::Debug for NonNull<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Pointer::fmt(&self.as_ptr(), f)
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> fmt::Pointer for NonNull<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Pointer::fmt(&self.as_ptr(), f)
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> Eq for NonNull<T> {}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> PartialEq for NonNull<T> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        self.as_ptr() == other.as_ptr()
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> Ord for NonNull<T> {
    #[inline]
    fn cmp(&self, other: &Self) -> Ordering {
        self.as_ptr().cmp(&other.as_ptr())
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> PartialOrd for NonNull<T> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
        self.as_ptr().partial_cmp(&other.as_ptr())
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
impl<T: ?Sized> hash::Hash for NonNull<T> {
    #[inline]
    fn hash<H: hash::Hasher>(&self, state: &mut H) {
        self.as_ptr().hash(state)
    }
}

#[unstable(feature = "ptr_internals", issue = "none")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T: ?Sized> const From<Unique<T>> for NonNull<T> {
    #[inline]
    fn from(unique: Unique<T>) -> Self {
        // SAFETY: A Unique pointer cannot be null, so the conditions for
        // new_unchecked() are respected.
        unsafe { NonNull::new_unchecked(unique.as_ptr()) }
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T: ?Sized> const From<&mut T> for NonNull<T> {
    /// Converts a `&mut T` to a `NonNull<T>`.
    ///
    /// This conversion is safe and infallible since references cannot be null.
    #[inline]
    fn from(reference: &mut T) -> Self {
        // SAFETY: A mutable reference cannot be null.
        unsafe { NonNull { pointer: reference as *mut T } }
    }
}

#[stable(feature = "nonnull", since = "1.25.0")]
#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
impl<T: ?Sized> const From<&T> for NonNull<T> {
    /// Converts a `&T` to a `NonNull<T>`.
    ///
    /// This conversion is safe and infallible since references cannot be null.
    #[inline]
    fn from(reference: &T) -> Self {
        // SAFETY: A reference cannot be null, so the conditions for
        // new_unchecked() are respected.
        unsafe { NonNull { pointer: reference as *const T } }
    }
}