library/core/src/ptr/non_null.rs

   1 use crate::cmp::Ordering;
   2 use crate::convert::From;
   3 use crate::fmt;
   4 use crate::hash;
   5 use crate::marker::Unsize;
   6 use crate::mem::{self, MaybeUninit};
   7 use crate::ops::{CoerceUnsized, DispatchFromDyn};
   8 use crate::ptr::Unique;
   9 use crate::slice::{self, SliceIndex};
  10
  11 /// `*mut T` but non-zero and covariant.
  12 ///
  13 /// This is often the correct thing to use when building data structures using
  14 /// raw pointers, but is ultimately more dangerous to use because of its additional
  15 /// properties. If you're not sure if you should use `NonNull<T>`, just use `*mut T`!
  16 ///
  17 /// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
  18 /// is never dereferenced. This is so that enums may use this forbidden value
  19 /// as a discriminant -- `Option<NonNull<T>>` has the same size as `*mut T`.
  20 /// However the pointer may still dangle if it isn't dereferenced.
  21 ///
  22 /// Unlike `*mut T`, `NonNull<T>` was chosen to be covariant over `T`. This makes it
  23 /// possible to use `NonNull<T>` when building covariant types, but introduces the
  24 /// risk of unsoundness if used in a type that shouldn't actually be covariant.
  25 /// (The opposite choice was made for `*mut T` even though technically the unsoundness
  26 /// could only be caused by calling unsafe functions.)
  27 ///
  28 /// Covariance is correct for most safe abstractions, such as `Box`, `Rc`, `Arc`, `Vec`,
  29 /// and `LinkedList`. This is the case because they provide a public API that follows the
  30 /// normal shared XOR mutable rules of Rust.
  31 ///
  32 /// If your type cannot safely be covariant, you must ensure it contains some
  33 /// additional field to provide invariance. Often this field will be a [`PhantomData`]
  34 /// type like `PhantomData<Cell<T>>` or `PhantomData<&'a mut T>`.
  35 ///
  36 /// Notice that `NonNull<T>` has a `From` instance for `&T`. However, this does
  37 /// not change the fact that mutating through a (pointer derived from a) shared
  38 /// reference is undefined behavior unless the mutation happens inside an
  39 /// [`UnsafeCell<T>`]. The same goes for creating a mutable reference from a shared
  40 /// reference. When using this `From` instance without an `UnsafeCell<T>`,
  41 /// it is your responsibility to ensure that `as_mut` is never called, and `as_ptr`
  42 /// is never used for mutation.
  43 ///
  44 /// [`PhantomData`]: crate::marker::PhantomData
  45 /// [`UnsafeCell<T>`]: crate::cell::UnsafeCell
  46 #[stable(feature = "nonnull", since = "1.25.0")]
  47 #[repr(transparent)]
  48 #[rustc_layout_scalar_valid_range_start(1)]
  49 #[rustc_nonnull_optimization_guaranteed]
  50 pub struct NonNull<T: ?Sized> {
  51     pointer: *const T,
  52 }
  53
  54 /// `NonNull` pointers are not `Send` because the data they reference may be aliased.
  55 // N.B., this impl is unnecessary, but should provide better error messages.
  56 #[stable(feature = "nonnull", since = "1.25.0")]
  57 impl<T: ?Sized> !Send for NonNull<T> {}
  58
  59 /// `NonNull` pointers are not `Sync` because the data they reference may be aliased.
  60 // N.B., this impl is unnecessary, but should provide better error messages.
  61 #[stable(feature = "nonnull", since = "1.25.0")]
  62 impl<T: ?Sized> !Sync for NonNull<T> {}
  63
  64 impl<T: Sized> NonNull<T> {
  65     /// Creates a new `NonNull` that is dangling, but well-aligned.
  66     ///
  67     /// This is useful for initializing types which lazily allocate, like
  68     /// `Vec::new` does.
  69     ///
  70     /// Note that the pointer value may potentially represent a valid pointer to
  71     /// a `T`, which means this must not be used as a "not yet initialized"
  72     /// sentinel value. Types that lazily allocate must track initialization by
  73     /// some other means.
  74     #[stable(feature = "nonnull", since = "1.25.0")]
  75     #[rustc_const_stable(feature = "const_nonnull_dangling", since = "1.32.0")]
  76     #[inline]
  77     pub const fn dangling() -> Self {
  78         // SAFETY: mem::align_of() returns a non-zero usize which is then casted
  79         // to a *mut T. Therefore, `ptr` is not null and the conditions for
  80         // calling new_unchecked() are respected.
  81         unsafe {
  82             let ptr = mem::align_of::<T>() as *mut T;
  83             NonNull::new_unchecked(ptr)
  84         }
  85     }
  86
  87     /// Returns a shared references to the value. In contrast to [`as_ref`], this does not require
  88     /// that the value has to be initialized.
  89     ///
  90     /// For the mutable counterpart see [`as_uninit_mut`].
  91     ///
  92     /// [`as_ref`]: NonNull::as_ref
  93     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
  94     ///
  95     /// # Safety
  96     ///
  97     /// When calling this method, you have to ensure that all of the following is true:
  98     ///
  99     /// * The pointer must be properly aligned.
 100     ///
 101     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 102     ///
 103     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 104     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 105     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 106     ///   not get mutated (except inside `UnsafeCell`).
 107     ///
 108     /// This applies even if the result of this method is unused!
 109     ///
 110     /// [the module documentation]: crate::ptr#safety
 111     #[inline]
 112     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 113     pub unsafe fn as_uninit_ref(&self) -> &MaybeUninit<T> {
 114         // SAFETY: the caller must guarantee that `self` meets all the
 115         // requirements for a reference.
 116         unsafe { &*self.cast().as_ptr() }
 117     }
 118
 119     /// Returns a unique references to the value. In contrast to [`as_mut`], this does not require
 120     /// that the value has to be initialized.
 121     ///
 122     /// For the shared counterpart see [`as_uninit_ref`].
 123     ///
 124     /// [`as_mut`]: NonNull::as_mut
 125     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 126     ///
 127     /// # Safety
 128     ///
 129     /// When calling this method, you have to ensure that all of the following is true:
 130     ///
 131     /// * The pointer must be properly aligned.
 132     ///
 133     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 134     ///
 135     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 136     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 137     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 138     ///   not get accessed (read or written) through any other pointer.
 139     ///
 140     /// This applies even if the result of this method is unused!
 141     ///
 142     /// [the module documentation]: crate::ptr#safety
 143     #[inline]
 144     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 145     pub unsafe fn as_uninit_mut(&mut self) -> &mut MaybeUninit<T> {
 146         // SAFETY: the caller must guarantee that `self` meets all the
 147         // requirements for a reference.
 148         unsafe { &mut *self.cast().as_ptr() }
 149     }
 150 }
 151
 152 impl<T: ?Sized> NonNull<T> {
 153     /// Creates a new `NonNull`.
 154     ///
 155     /// # Safety
 156     ///
 157     /// `ptr` must be non-null.
 158     #[stable(feature = "nonnull", since = "1.25.0")]
 159     #[rustc_const_stable(feature = "const_nonnull_new_unchecked", since = "1.32.0")]
 160     #[inline]
 161     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
 162         // SAFETY: the caller must guarantee that `ptr` is non-null.
 163         unsafe { NonNull { pointer: ptr as _ } }
 164     }
 165
 166     /// Creates a new `NonNull` if `ptr` is non-null.
 167     #[stable(feature = "nonnull", since = "1.25.0")]
 168     #[inline]
 169     pub fn new(ptr: *mut T) -> Option<Self> {
 170         if !ptr.is_null() {
 171             // SAFETY: The pointer is already checked and is not null
 172             Some(unsafe { Self::new_unchecked(ptr) })
 173         } else {
 174             None
 175         }
 176     }
 177
 178     /// Performs the same functionality as [`std::ptr::from_raw_parts`], except that a
 179     /// `NonNull` pointer is returned, as opposed to a raw `*const` pointer.
 180     ///
 181     /// See the documentation of [`std::ptr::from_raw_parts`] for more details.
 182     ///
 183     /// [`std::ptr::from_raw_parts`]: crate::ptr::from_raw_parts
 184     #[cfg(not(bootstrap))]
 185     #[unstable(feature = "ptr_metadata", issue = "81513")]
 186     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 187     #[inline]
 188     pub const fn from_raw_parts(
 189         data_address: NonNull<()>,
 190         metadata: <T as super::Pointee>::Metadata,
 191     ) -> NonNull<T> {
 192         // SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_address` is.
 193         unsafe {
 194             NonNull::new_unchecked(super::from_raw_parts_mut(data_address.as_ptr(), metadata))
 195         }
 196     }
 197
 198     /// Decompose a (possibly wide) pointer into is address and metadata components.
 199     ///
 200     /// The pointer can be later reconstructed with [`NonNull::from_raw_parts`].
 201     #[cfg(not(bootstrap))]
 202     #[unstable(feature = "ptr_metadata", issue = "81513")]
 203     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 204     #[inline]
 205     pub const fn to_raw_parts(self) -> (NonNull<()>, <T as super::Pointee>::Metadata) {
 206         (self.cast(), super::metadata(self.as_ptr()))
 207     }
 208
 209     /// Acquires the underlying `*mut` pointer.
 210     #[stable(feature = "nonnull", since = "1.25.0")]
 211     #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]
 212     #[inline]
 213     pub const fn as_ptr(self) -> *mut T {
 214         self.pointer as *mut T
 215     }
 216
 217     /// Returns a shared reference to the value. If the value may be uninitialized, [`as_uninit_ref`]
 218     /// must be used instead.
 219     ///
 220     /// For the mutable counterpart see [`as_mut`].
 221     ///
 222     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 223     /// [`as_mut`]: NonNull::as_mut
 224     ///
 225     /// # Safety
 226     ///
 227     /// When calling this method, you have to ensure that all of the following is true:
 228     ///
 229     /// * The pointer must be properly aligned.
 230     ///
 231     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 232     ///
 233     /// * The pointer must point to an initialized instance of `T`.
 234     ///
 235     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 236     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 237     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 238     ///   not get mutated (except inside `UnsafeCell`).
 239     ///
 240     /// This applies even if the result of this method is unused!
 241     /// (The part about being initialized is not yet fully decided, but until
 242     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 243     ///
 244     /// [the module documentation]: crate::ptr#safety
 245     #[stable(feature = "nonnull", since = "1.25.0")]
 246     #[inline]
 247     pub unsafe fn as_ref(&self) -> &T {
 248         // SAFETY: the caller must guarantee that `self` meets all the
 249         // requirements for a reference.
 250         unsafe { &*self.as_ptr() }
 251     }
 252
 253     /// Returns a unique reference to the value. If the value may be uninitialized, [`as_uninit_mut`]
 254     /// must be used instead.
 255     ///
 256     /// For the shared counterpart see [`as_ref`].
 257     ///
 258     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
 259     /// [`as_ref`]: NonNull::as_ref
 260     ///
 261     /// # Safety
 262     ///
 263     /// When calling this method, you have to ensure that all of the following is true:
 264     ///
 265     /// * The pointer must be properly aligned.
 266     ///
 267     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 268     ///
 269     /// * The pointer must point to an initialized instance of `T`.
 270     ///
 271     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 272     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 273     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 274     ///   not get accessed (read or written) through any other pointer.
 275     ///
 276     /// This applies even if the result of this method is unused!
 277     /// (The part about being initialized is not yet fully decided, but until
 278     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 279     ///
 280     /// [the module documentation]: crate::ptr#safety
 281     #[stable(feature = "nonnull", since = "1.25.0")]
 282     #[inline]
 283     pub unsafe fn as_mut(&mut self) -> &mut T {
 284         // SAFETY: the caller must guarantee that `self` meets all the
 285         // requirements for a mutable reference.
 286         unsafe { &mut *self.as_ptr() }
 287     }
 288
 289     /// Casts to a pointer of another type.
 290     #[stable(feature = "nonnull_cast", since = "1.27.0")]
 291     #[rustc_const_stable(feature = "const_nonnull_cast", since = "1.32.0")]
 292     #[inline]
 293     pub const fn cast<U>(self) -> NonNull<U> {
 294         // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null
 295         unsafe { NonNull::new_unchecked(self.as_ptr() as *mut U) }
 296     }
 297 }
 298
 299 impl<T> NonNull<[T]> {
 300     /// Creates a non-null raw slice from a thin pointer and a length.
 301     ///
 302     /// The `len` argument is the number of **elements**, not the number of bytes.
 303     ///
 304     /// This function is safe, but dereferencing the return value is unsafe.
 305     /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
 306     ///
 307     /// # Examples
 308     ///
 309     /// ```rust
 310     /// #![feature(nonnull_slice_from_raw_parts)]
 311     ///
 312     /// use std::ptr::NonNull;
 313     ///
 314     /// // create a slice pointer when starting out with a pointer to the first element
 315     /// let mut x = [5, 6, 7];
 316     /// let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
 317     /// let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
 318     /// assert_eq!(unsafe { slice.as_ref()[2] }, 7);
 319     /// ```
 320     ///
 321     /// (Note that this example artificially demonstrates a use of this method,
 322     /// but `let slice = NonNull::from(&x[..]);` would be a better way to write code like this.)
 323     #[unstable(feature = "nonnull_slice_from_raw_parts", issue = "71941")]
 324     #[rustc_const_unstable(feature = "const_nonnull_slice_from_raw_parts", issue = "71941")]
 325     #[inline]
 326     pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self {
 327         // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
 328         unsafe { Self::new_unchecked(super::slice_from_raw_parts_mut(data.as_ptr(), len)) }
 329     }
 330
 331     /// Returns the length of a non-null raw slice.
 332     ///
 333     /// The returned value is the number of **elements**, not the number of bytes.
 334     ///
 335     /// This function is safe, even when the non-null raw slice cannot be dereferenced to a slice
 336     /// because the pointer does not have a valid address.
 337     ///
 338     /// # Examples
 339     ///
 340     /// ```rust
 341     /// #![feature(slice_ptr_len, nonnull_slice_from_raw_parts)]
 342     /// use std::ptr::NonNull;
 343     ///
 344     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 345     /// assert_eq!(slice.len(), 3);
 346     /// ```
 347     #[unstable(feature = "slice_ptr_len", issue = "71146")]
 348     #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
 349     #[inline]
 350     pub const fn len(self) -> usize {
 351         self.as_ptr().len()
 352     }
 353
 354     /// Returns a non-null pointer to the slice's buffer.
 355     ///
 356     /// # Examples
 357     ///
 358     /// ```rust
 359     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 360     /// use std::ptr::NonNull;
 361     ///
 362     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 363     /// assert_eq!(slice.as_non_null_ptr(), NonNull::new(1 as *mut i8).unwrap());
 364     /// ```
 365     #[inline]
 366     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 367     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 368     pub const fn as_non_null_ptr(self) -> NonNull<T> {
 369         // SAFETY: We know `self` is non-null.
 370         unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
 371     }
 372
 373     /// Returns a raw pointer to the slice's buffer.
 374     ///
 375     /// # Examples
 376     ///
 377     /// ```rust
 378     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 379     /// use std::ptr::NonNull;
 380     ///
 381     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 382     /// assert_eq!(slice.as_mut_ptr(), 1 as *mut i8);
 383     /// ```
 384     #[inline]
 385     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 386     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 387     pub const fn as_mut_ptr(self) -> *mut T {
 388         self.as_non_null_ptr().as_ptr()
 389     }
 390
 391     /// Returns a shared reference to a slice of possibly uninitialized values. In contrast to
 392     /// [`as_ref`], this does not require that the value has to be initialized.
 393     ///
 394     /// For the mutable counterpart see [`as_uninit_slice_mut`].
 395     ///
 396     /// [`as_ref`]: NonNull::as_ref
 397     /// [`as_uninit_slice_mut`]: NonNull::as_uninit_slice_mut
 398     ///
 399     /// # Safety
 400     ///
 401     /// When calling this method, you have to ensure that all of the following is true:
 402     ///
 403     /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
 404     ///   and it must be properly aligned. This means in particular:
 405     ///
 406     ///     * The entire memory range of this slice must be contained within a single allocated object!
 407     ///       Slices can never span across multiple allocated objects.
 408     ///
 409     ///     * The pointer must be aligned even for zero-length slices. One
 410     ///       reason for this is that enum layout optimizations may rely on references
 411     ///       (including slices of any length) being aligned and non-null to distinguish
 412     ///       them from other data. You can obtain a pointer that is usable as `data`
 413     ///       for zero-length slices using [`NonNull::dangling()`].
 414     ///
 415     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 416     ///   See the safety documentation of [`pointer::offset`].
 417     ///
 418     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 419     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 420     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 421     ///   not get mutated (except inside `UnsafeCell`).
 422     ///
 423     /// This applies even if the result of this method is unused!
 424     ///
 425     /// See also [`slice::from_raw_parts`].
 426     ///
 427     /// [valid]: crate::ptr#safety
 428     #[inline]
 429     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 430     pub unsafe fn as_uninit_slice(&self) -> &[MaybeUninit<T>] {
 431         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
 432         unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
 433     }
 434
 435     /// Returns a unique reference to a slice of possibly uninitialized values. In contrast to
 436     /// [`as_mut`], this does not require that the value has to be initialized.
 437     ///
 438     /// For the shared counterpart see [`as_uninit_slice`].
 439     ///
 440     /// [`as_mut`]: NonNull::as_mut
 441     /// [`as_uninit_slice`]: NonNull::as_uninit_slice
 442     ///
 443     /// # Safety
 444     ///
 445     /// When calling this method, you have to ensure that all of the following is true:
 446     ///
 447     /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
 448     ///   many bytes, and it must be properly aligned. This means in particular:
 449     ///
 450     ///     * The entire memory range of this slice must be contained within a single allocated object!
 451     ///       Slices can never span across multiple allocated objects.
 452     ///
 453     ///     * The pointer must be aligned even for zero-length slices. One
 454     ///       reason for this is that enum layout optimizations may rely on references
 455     ///       (including slices of any length) being aligned and non-null to distinguish
 456     ///       them from other data. You can obtain a pointer that is usable as `data`
 457     ///       for zero-length slices using [`NonNull::dangling()`].
 458     ///
 459     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 460     ///   See the safety documentation of [`pointer::offset`].
 461     ///
 462     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 463     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 464     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 465     ///   not get accessed (read or written) through any other pointer.
 466     ///
 467     /// This applies even if the result of this method is unused!
 468     ///
 469     /// See also [`slice::from_raw_parts_mut`].
 470     ///
 471     /// [valid]: crate::ptr#safety
 472     ///
 473     /// # Examples
 474     ///
 475     /// ```rust
 476     /// #![feature(allocator_api, ptr_as_uninit)]
 477     ///
 478     /// use std::alloc::{Allocator, Layout, Global};
 479     /// use std::mem::MaybeUninit;
 480     /// use std::ptr::NonNull;
 481     ///
 482     /// let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
 483     /// // This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
 484     /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
 485     /// # #[allow(unused_variables)]
 486     /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
 487     /// # Ok::<_, std::alloc::AllocError>(())
 488     /// ```
 489     #[inline]
 490     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 491     pub unsafe fn as_uninit_slice_mut(&self) -> &mut [MaybeUninit<T>] {
 492         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
 493         unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
 494     }
 495
 496     /// Returns a raw pointer to an element or subslice, without doing bounds
 497     /// checking.
 498     ///
 499     /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
 500     /// is *[undefined behavior]* even if the resulting pointer is not used.
 501     ///
 502     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 503     ///
 504     /// # Examples
 505     ///
 506     /// ```
 507     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 508     /// use std::ptr::NonNull;
 509     ///
 510     /// let x = &mut [1, 2, 4];
 511     /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
 512     ///
 513     /// unsafe {
 514     ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
 515     /// }
 516     /// ```
 517     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 518     #[inline]
 519     pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
 520     where
 521         I: SliceIndex<[T]>,
 522     {
 523         // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
 524         // As a consequence, the resulting pointer cannot be NULL.
 525         unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
 526     }
 527 }
 528
 529 #[stable(feature = "nonnull", since = "1.25.0")]
 530 impl<T: ?Sized> Clone for NonNull<T> {
 531     #[inline]
 532     fn clone(&self) -> Self {
 533         *self
 534     }
 535 }
 536
 537 #[stable(feature = "nonnull", since = "1.25.0")]
 538 impl<T: ?Sized> Copy for NonNull<T> {}
 539
 540 #[unstable(feature = "coerce_unsized", issue = "27732")]
 541 impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 542
 543 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 544 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 545
 546 #[stable(feature = "nonnull", since = "1.25.0")]
 547 impl<T: ?Sized> fmt::Debug for NonNull<T> {
 548     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 549         fmt::Pointer::fmt(&self.as_ptr(), f)
 550     }
 551 }
 552
 553 #[stable(feature = "nonnull", since = "1.25.0")]
 554 impl<T: ?Sized> fmt::Pointer for NonNull<T> {
 555     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 556         fmt::Pointer::fmt(&self.as_ptr(), f)
 557     }
 558 }
 559
 560 #[stable(feature = "nonnull", since = "1.25.0")]
 561 impl<T: ?Sized> Eq for NonNull<T> {}
 562
 563 #[stable(feature = "nonnull", since = "1.25.0")]
 564 impl<T: ?Sized> PartialEq for NonNull<T> {
 565     #[inline]
 566     fn eq(&self, other: &Self) -> bool {
 567         self.as_ptr() == other.as_ptr()
 568     }
 569 }
 570
 571 #[stable(feature = "nonnull", since = "1.25.0")]
 572 impl<T: ?Sized> Ord for NonNull<T> {
 573     #[inline]
 574     fn cmp(&self, other: &Self) -> Ordering {
 575         self.as_ptr().cmp(&other.as_ptr())
 576     }
 577 }
 578
 579 #[stable(feature = "nonnull", since = "1.25.0")]
 580 impl<T: ?Sized> PartialOrd for NonNull<T> {
 581     #[inline]
 582     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
 583         self.as_ptr().partial_cmp(&other.as_ptr())
 584     }
 585 }
 586
 587 #[stable(feature = "nonnull", since = "1.25.0")]
 588 impl<T: ?Sized> hash::Hash for NonNull<T> {
 589     #[inline]
 590     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 591         self.as_ptr().hash(state)
 592     }
 593 }
 594
 595 #[unstable(feature = "ptr_internals", issue = "none")]
 596 impl<T: ?Sized> From<Unique<T>> for NonNull<T> {
 597     #[inline]
 598     fn from(unique: Unique<T>) -> Self {
 599         // SAFETY: A Unique pointer cannot be null, so the conditions for
 600         // new_unchecked() are respected.
 601         unsafe { NonNull::new_unchecked(unique.as_ptr()) }
 602     }
 603 }
 604
 605 #[stable(feature = "nonnull", since = "1.25.0")]
 606 impl<T: ?Sized> From<&mut T> for NonNull<T> {
 607     #[inline]
 608     fn from(reference: &mut T) -> Self {
 609         // SAFETY: A mutable reference cannot be null.
 610         unsafe { NonNull { pointer: reference as *mut T } }
 611     }
 612 }
 613
 614 #[stable(feature = "nonnull", since = "1.25.0")]
 615 impl<T: ?Sized> From<&T> for NonNull<T> {
 616     #[inline]
 617     fn from(reference: &T) -> Self {
 618         // SAFETY: A reference cannot be null, so the conditions for
 619         // new_unchecked() are respected.
 620         unsafe { NonNull { pointer: reference as *const T } }
 621     }
 622 }