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     ///
  75     /// # Examples
  76     ///
  77     /// ```
  78     /// use std::ptr::NonNull;
  79     ///
  80     /// let ptr = NonNull::<u32>::dangling();
  81     /// // Important: don't try to access the value of `ptr` without
  82     /// // initializing it first! The pointer is not null but isn't valid either!
  83     /// ```
  84     #[stable(feature = "nonnull", since = "1.25.0")]
  85     #[rustc_const_stable(feature = "const_nonnull_dangling", since = "1.36.0")]
  86     #[must_use]
  87     #[inline]
  88     pub const fn dangling() -> Self {
  89         // SAFETY: mem::align_of() returns a non-zero usize which is then casted
  90         // to a *mut T. Therefore, `ptr` is not null and the conditions for
  91         // calling new_unchecked() are respected.
  92         unsafe {
  93             let ptr = mem::align_of::<T>() as *mut T;
  94             NonNull::new_unchecked(ptr)
  95         }
  96     }
  97
  98     /// Returns a shared references to the value. In contrast to [`as_ref`], this does not require
  99     /// that the value has to be initialized.
 100     ///
 101     /// For the mutable counterpart see [`as_uninit_mut`].
 102     ///
 103     /// [`as_ref`]: NonNull::as_ref
 104     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
 105     ///
 106     /// # Safety
 107     ///
 108     /// When calling this method, you have to ensure that all of the following is true:
 109     ///
 110     /// * The pointer must be properly aligned.
 111     ///
 112     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 113     ///
 114     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 115     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 116     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 117     ///   not get mutated (except inside `UnsafeCell`).
 118     ///
 119     /// This applies even if the result of this method is unused!
 120     ///
 121     /// [the module documentation]: crate::ptr#safety
 122     #[inline]
 123     #[must_use]
 124     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 125     pub unsafe fn as_uninit_ref<'a>(&self) -> &'a MaybeUninit<T> {
 126         // SAFETY: the caller must guarantee that `self` meets all the
 127         // requirements for a reference.
 128         unsafe { &*self.cast().as_ptr() }
 129     }
 130
 131     /// Returns a unique references to the value. In contrast to [`as_mut`], this does not require
 132     /// that the value has to be initialized.
 133     ///
 134     /// For the shared counterpart see [`as_uninit_ref`].
 135     ///
 136     /// [`as_mut`]: NonNull::as_mut
 137     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 138     ///
 139     /// # Safety
 140     ///
 141     /// When calling this method, you have to ensure that all of the following is true:
 142     ///
 143     /// * The pointer must be properly aligned.
 144     ///
 145     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 146     ///
 147     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 148     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 149     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 150     ///   not get accessed (read or written) through any other pointer.
 151     ///
 152     /// This applies even if the result of this method is unused!
 153     ///
 154     /// [the module documentation]: crate::ptr#safety
 155     #[inline]
 156     #[must_use]
 157     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 158     pub unsafe fn as_uninit_mut<'a>(&mut self) -> &'a mut MaybeUninit<T> {
 159         // SAFETY: the caller must guarantee that `self` meets all the
 160         // requirements for a reference.
 161         unsafe { &mut *self.cast().as_ptr() }
 162     }
 163 }
 164
 165 impl<T: ?Sized> NonNull<T> {
 166     /// Creates a new `NonNull`.
 167     ///
 168     /// # Safety
 169     ///
 170     /// `ptr` must be non-null.
 171     ///
 172     /// # Examples
 173     ///
 174     /// ```
 175     /// use std::ptr::NonNull;
 176     ///
 177     /// let mut x = 0u32;
 178     /// let ptr = unsafe { NonNull::new_unchecked(&mut x as *mut _) };
 179     /// ```
 180     ///
 181     /// *Incorrect* usage of this function:
 182     ///
 183     /// ```rust,no_run
 184     /// use std::ptr::NonNull;
 185     ///
 186     /// // NEVER DO THAT!!! This is undefined behavior. ⚠️
 187     /// let ptr = unsafe { NonNull::<u32>::new_unchecked(std::ptr::null_mut()) };
 188     /// ```
 189     #[stable(feature = "nonnull", since = "1.25.0")]
 190     #[rustc_const_stable(feature = "const_nonnull_new_unchecked", since = "1.25.0")]
 191     #[inline]
 192     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
 193         // SAFETY: the caller must guarantee that `ptr` is non-null.
 194         unsafe { NonNull { pointer: ptr as _ } }
 195     }
 196
 197     /// Creates a new `NonNull` if `ptr` is non-null.
 198     ///
 199     /// # Examples
 200     ///
 201     /// ```
 202     /// use std::ptr::NonNull;
 203     ///
 204     /// let mut x = 0u32;
 205     /// let ptr = NonNull::<u32>::new(&mut x as *mut _).expect("ptr is null!");
 206     ///
 207     /// if let Some(ptr) = NonNull::<u32>::new(std::ptr::null_mut()) {
 208     ///     unreachable!();
 209     /// }
 210     /// ```
 211     #[stable(feature = "nonnull", since = "1.25.0")]
 212     #[inline]
 213     pub fn new(ptr: *mut T) -> Option<Self> {
 214         if !ptr.is_null() {
 215             // SAFETY: The pointer is already checked and is not null
 216             Some(unsafe { Self::new_unchecked(ptr) })
 217         } else {
 218             None
 219         }
 220     }
 221
 222     /// Performs the same functionality as [`std::ptr::from_raw_parts`], except that a
 223     /// `NonNull` pointer is returned, as opposed to a raw `*const` pointer.
 224     ///
 225     /// See the documentation of [`std::ptr::from_raw_parts`] for more details.
 226     ///
 227     /// [`std::ptr::from_raw_parts`]: crate::ptr::from_raw_parts
 228     #[unstable(feature = "ptr_metadata", issue = "81513")]
 229     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 230     #[inline]
 231     pub const fn from_raw_parts(
 232         data_address: NonNull<()>,
 233         metadata: <T as super::Pointee>::Metadata,
 234     ) -> NonNull<T> {
 235         // SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_address` is.
 236         unsafe {
 237             NonNull::new_unchecked(super::from_raw_parts_mut(data_address.as_ptr(), metadata))
 238         }
 239     }
 240
 241     /// Decompose a (possibly wide) pointer into its address and metadata components.
 242     ///
 243     /// The pointer can be later reconstructed with [`NonNull::from_raw_parts`].
 244     #[unstable(feature = "ptr_metadata", issue = "81513")]
 245     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 246     #[must_use = "this returns the result of the operation, \
 247                   without modifying the original"]
 248     #[inline]
 249     pub const fn to_raw_parts(self) -> (NonNull<()>, <T as super::Pointee>::Metadata) {
 250         (self.cast(), super::metadata(self.as_ptr()))
 251     }
 252
 253     /// Acquires the underlying `*mut` pointer.
 254     ///
 255     /// # Examples
 256     ///
 257     /// ```
 258     /// use std::ptr::NonNull;
 259     ///
 260     /// let mut x = 0u32;
 261     /// let ptr = NonNull::new(&mut x).expect("ptr is null!");
 262     ///
 263     /// let x_value = unsafe { *ptr.as_ptr() };
 264     /// assert_eq!(x_value, 0);
 265     ///
 266     /// unsafe { *ptr.as_ptr() += 2; }
 267     /// let x_value = unsafe { *ptr.as_ptr() };
 268     /// assert_eq!(x_value, 2);
 269     /// ```
 270     #[stable(feature = "nonnull", since = "1.25.0")]
 271     #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]
 272     #[must_use]
 273     #[inline]
 274     pub const fn as_ptr(self) -> *mut T {
 275         self.pointer as *mut T
 276     }
 277
 278     /// Returns a shared reference to the value. If the value may be uninitialized, [`as_uninit_ref`]
 279     /// must be used instead.
 280     ///
 281     /// For the mutable counterpart see [`as_mut`].
 282     ///
 283     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 284     /// [`as_mut`]: NonNull::as_mut
 285     ///
 286     /// # Safety
 287     ///
 288     /// When calling this method, you have to ensure that all of the following is true:
 289     ///
 290     /// * The pointer must be properly aligned.
 291     ///
 292     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 293     ///
 294     /// * The pointer must point to an initialized instance of `T`.
 295     ///
 296     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 297     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 298     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 299     ///   not get mutated (except inside `UnsafeCell`).
 300     ///
 301     /// This applies even if the result of this method is unused!
 302     /// (The part about being initialized is not yet fully decided, but until
 303     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 304     ///
 305     /// # Examples
 306     ///
 307     /// ```
 308     /// use std::ptr::NonNull;
 309     ///
 310     /// let mut x = 0u32;
 311     /// let ptr = NonNull::new(&mut x as *mut _).expect("ptr is null!");
 312     ///
 313     /// let ref_x = unsafe { ptr.as_ref() };
 314     /// println!("{}", ref_x);
 315     /// ```
 316     ///
 317     /// [the module documentation]: crate::ptr#safety
 318     #[stable(feature = "nonnull", since = "1.25.0")]
 319     #[must_use]
 320     #[inline]
 321     pub unsafe fn as_ref<'a>(&self) -> &'a T {
 322         // SAFETY: the caller must guarantee that `self` meets all the
 323         // requirements for a reference.
 324         unsafe { &*self.as_ptr() }
 325     }
 326
 327     /// Returns a unique reference to the value. If the value may be uninitialized, [`as_uninit_mut`]
 328     /// must be used instead.
 329     ///
 330     /// For the shared counterpart see [`as_ref`].
 331     ///
 332     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
 333     /// [`as_ref`]: NonNull::as_ref
 334     ///
 335     /// # Safety
 336     ///
 337     /// When calling this method, you have to ensure that all of the following is true:
 338     ///
 339     /// * The pointer must be properly aligned.
 340     ///
 341     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 342     ///
 343     /// * The pointer must point to an initialized instance of `T`.
 344     ///
 345     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 346     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 347     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 348     ///   not get accessed (read or written) through any other pointer.
 349     ///
 350     /// This applies even if the result of this method is unused!
 351     /// (The part about being initialized is not yet fully decided, but until
 352     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 353     /// # Examples
 354     ///
 355     /// ```
 356     /// use std::ptr::NonNull;
 357     ///
 358     /// let mut x = 0u32;
 359     /// let mut ptr = NonNull::new(&mut x).expect("null pointer");
 360     ///
 361     /// let x_ref = unsafe { ptr.as_mut() };
 362     /// assert_eq!(*x_ref, 0);
 363     /// *x_ref += 2;
 364     /// assert_eq!(*x_ref, 2);
 365     /// ```
 366     ///
 367     /// [the module documentation]: crate::ptr#safety
 368     #[stable(feature = "nonnull", since = "1.25.0")]
 369     #[must_use]
 370     #[inline]
 371     pub unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
 372         // SAFETY: the caller must guarantee that `self` meets all the
 373         // requirements for a mutable reference.
 374         unsafe { &mut *self.as_ptr() }
 375     }
 376
 377     /// Casts to a pointer of another type.
 378     ///
 379     /// # Examples
 380     ///
 381     /// ```
 382     /// use std::ptr::NonNull;
 383     ///
 384     /// let mut x = 0u32;
 385     /// let ptr = NonNull::new(&mut x as *mut _).expect("null pointer");
 386     ///
 387     /// let casted_ptr = ptr.cast::<i8>();
 388     /// let raw_ptr: *mut i8 = casted_ptr.as_ptr();
 389     /// ```
 390     #[stable(feature = "nonnull_cast", since = "1.27.0")]
 391     #[rustc_const_stable(feature = "const_nonnull_cast", since = "1.36.0")]
 392     #[must_use = "this returns the result of the operation, \
 393                   without modifying the original"]
 394     #[inline]
 395     pub const fn cast<U>(self) -> NonNull<U> {
 396         // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null
 397         unsafe { NonNull::new_unchecked(self.as_ptr() as *mut U) }
 398     }
 399 }
 400
 401 impl<T> NonNull<[T]> {
 402     /// Creates a non-null raw slice from a thin pointer and a length.
 403     ///
 404     /// The `len` argument is the number of **elements**, not the number of bytes.
 405     ///
 406     /// This function is safe, but dereferencing the return value is unsafe.
 407     /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
 408     ///
 409     /// # Examples
 410     ///
 411     /// ```rust
 412     /// #![feature(nonnull_slice_from_raw_parts)]
 413     ///
 414     /// use std::ptr::NonNull;
 415     ///
 416     /// // create a slice pointer when starting out with a pointer to the first element
 417     /// let mut x = [5, 6, 7];
 418     /// let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
 419     /// let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
 420     /// assert_eq!(unsafe { slice.as_ref()[2] }, 7);
 421     /// ```
 422     ///
 423     /// (Note that this example artificially demonstrates a use of this method,
 424     /// but `let slice = NonNull::from(&x[..]);` would be a better way to write code like this.)
 425     #[unstable(feature = "nonnull_slice_from_raw_parts", issue = "71941")]
 426     #[rustc_const_unstable(feature = "const_nonnull_slice_from_raw_parts", issue = "71941")]
 427     #[must_use]
 428     #[inline]
 429     pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self {
 430         // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
 431         unsafe { Self::new_unchecked(super::slice_from_raw_parts_mut(data.as_ptr(), len)) }
 432     }
 433
 434     /// Returns the length of a non-null raw slice.
 435     ///
 436     /// The returned value is the number of **elements**, not the number of bytes.
 437     ///
 438     /// This function is safe, even when the non-null raw slice cannot be dereferenced to a slice
 439     /// because the pointer does not have a valid address.
 440     ///
 441     /// # Examples
 442     ///
 443     /// ```rust
 444     /// #![feature(slice_ptr_len, nonnull_slice_from_raw_parts)]
 445     /// use std::ptr::NonNull;
 446     ///
 447     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 448     /// assert_eq!(slice.len(), 3);
 449     /// ```
 450     #[unstable(feature = "slice_ptr_len", issue = "71146")]
 451     #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
 452     #[must_use]
 453     #[inline]
 454     pub const fn len(self) -> usize {
 455         self.as_ptr().len()
 456     }
 457
 458     /// Returns a non-null pointer to the slice's buffer.
 459     ///
 460     /// # Examples
 461     ///
 462     /// ```rust
 463     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 464     /// use std::ptr::NonNull;
 465     ///
 466     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 467     /// assert_eq!(slice.as_non_null_ptr(), NonNull::new(1 as *mut i8).unwrap());
 468     /// ```
 469     #[inline]
 470     #[must_use]
 471     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 472     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 473     pub const fn as_non_null_ptr(self) -> NonNull<T> {
 474         // SAFETY: We know `self` is non-null.
 475         unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
 476     }
 477
 478     /// Returns a raw pointer to the slice's buffer.
 479     ///
 480     /// # Examples
 481     ///
 482     /// ```rust
 483     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 484     /// use std::ptr::NonNull;
 485     ///
 486     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 487     /// assert_eq!(slice.as_mut_ptr(), 1 as *mut i8);
 488     /// ```
 489     #[inline]
 490     #[must_use]
 491     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 492     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 493     pub const fn as_mut_ptr(self) -> *mut T {
 494         self.as_non_null_ptr().as_ptr()
 495     }
 496
 497     /// Returns a shared reference to a slice of possibly uninitialized values. In contrast to
 498     /// [`as_ref`], this does not require that the value has to be initialized.
 499     ///
 500     /// For the mutable counterpart see [`as_uninit_slice_mut`].
 501     ///
 502     /// [`as_ref`]: NonNull::as_ref
 503     /// [`as_uninit_slice_mut`]: NonNull::as_uninit_slice_mut
 504     ///
 505     /// # Safety
 506     ///
 507     /// When calling this method, you have to ensure that all of the following is true:
 508     ///
 509     /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
 510     ///   and it must be properly aligned. This means in particular:
 511     ///
 512     ///     * The entire memory range of this slice must be contained within a single allocated object!
 513     ///       Slices can never span across multiple allocated objects.
 514     ///
 515     ///     * The pointer must be aligned even for zero-length slices. One
 516     ///       reason for this is that enum layout optimizations may rely on references
 517     ///       (including slices of any length) being aligned and non-null to distinguish
 518     ///       them from other data. You can obtain a pointer that is usable as `data`
 519     ///       for zero-length slices using [`NonNull::dangling()`].
 520     ///
 521     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 522     ///   See the safety documentation of [`pointer::offset`].
 523     ///
 524     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 525     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 526     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 527     ///   not get mutated (except inside `UnsafeCell`).
 528     ///
 529     /// This applies even if the result of this method is unused!
 530     ///
 531     /// See also [`slice::from_raw_parts`].
 532     ///
 533     /// [valid]: crate::ptr#safety
 534     #[inline]
 535     #[must_use]
 536     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 537     pub unsafe fn as_uninit_slice<'a>(&self) -> &'a [MaybeUninit<T>] {
 538         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
 539         unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
 540     }
 541
 542     /// Returns a unique reference to a slice of possibly uninitialized values. In contrast to
 543     /// [`as_mut`], this does not require that the value has to be initialized.
 544     ///
 545     /// For the shared counterpart see [`as_uninit_slice`].
 546     ///
 547     /// [`as_mut`]: NonNull::as_mut
 548     /// [`as_uninit_slice`]: NonNull::as_uninit_slice
 549     ///
 550     /// # Safety
 551     ///
 552     /// When calling this method, you have to ensure that all of the following is true:
 553     ///
 554     /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
 555     ///   many bytes, and it must be properly aligned. This means in particular:
 556     ///
 557     ///     * The entire memory range of this slice must be contained within a single allocated object!
 558     ///       Slices can never span across multiple allocated objects.
 559     ///
 560     ///     * The pointer must be aligned even for zero-length slices. One
 561     ///       reason for this is that enum layout optimizations may rely on references
 562     ///       (including slices of any length) being aligned and non-null to distinguish
 563     ///       them from other data. You can obtain a pointer that is usable as `data`
 564     ///       for zero-length slices using [`NonNull::dangling()`].
 565     ///
 566     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 567     ///   See the safety documentation of [`pointer::offset`].
 568     ///
 569     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 570     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 571     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 572     ///   not get accessed (read or written) through any other pointer.
 573     ///
 574     /// This applies even if the result of this method is unused!
 575     ///
 576     /// See also [`slice::from_raw_parts_mut`].
 577     ///
 578     /// [valid]: crate::ptr#safety
 579     ///
 580     /// # Examples
 581     ///
 582     /// ```rust
 583     /// #![feature(allocator_api, ptr_as_uninit)]
 584     ///
 585     /// use std::alloc::{Allocator, Layout, Global};
 586     /// use std::mem::MaybeUninit;
 587     /// use std::ptr::NonNull;
 588     ///
 589     /// let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
 590     /// // This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
 591     /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
 592     /// # #[allow(unused_variables)]
 593     /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
 594     /// # Ok::<_, std::alloc::AllocError>(())
 595     /// ```
 596     #[inline]
 597     #[must_use]
 598     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 599     pub unsafe fn as_uninit_slice_mut<'a>(&self) -> &'a mut [MaybeUninit<T>] {
 600         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
 601         unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
 602     }
 603
 604     /// Returns a raw pointer to an element or subslice, without doing bounds
 605     /// checking.
 606     ///
 607     /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
 608     /// is *[undefined behavior]* even if the resulting pointer is not used.
 609     ///
 610     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 611     ///
 612     /// # Examples
 613     ///
 614     /// ```
 615     /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
 616     /// use std::ptr::NonNull;
 617     ///
 618     /// let x = &mut [1, 2, 4];
 619     /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
 620     ///
 621     /// unsafe {
 622     ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
 623     /// }
 624     /// ```
 625     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 626     #[inline]
 627     pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
 628     where
 629         I: SliceIndex<[T]>,
 630     {
 631         // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
 632         // As a consequence, the resulting pointer cannot be null.
 633         unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
 634     }
 635 }
 636
 637 #[stable(feature = "nonnull", since = "1.25.0")]
 638 impl<T: ?Sized> Clone for NonNull<T> {
 639     #[inline]
 640     fn clone(&self) -> Self {
 641         *self
 642     }
 643 }
 644
 645 #[stable(feature = "nonnull", since = "1.25.0")]
 646 impl<T: ?Sized> Copy for NonNull<T> {}
 647
 648 #[unstable(feature = "coerce_unsized", issue = "27732")]
 649 impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 650
 651 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 652 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 653
 654 #[stable(feature = "nonnull", since = "1.25.0")]
 655 impl<T: ?Sized> fmt::Debug for NonNull<T> {
 656     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 657         fmt::Pointer::fmt(&self.as_ptr(), f)
 658     }
 659 }
 660
 661 #[stable(feature = "nonnull", since = "1.25.0")]
 662 impl<T: ?Sized> fmt::Pointer for NonNull<T> {
 663     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 664         fmt::Pointer::fmt(&self.as_ptr(), f)
 665     }
 666 }
 667
 668 #[stable(feature = "nonnull", since = "1.25.0")]
 669 impl<T: ?Sized> Eq for NonNull<T> {}
 670
 671 #[stable(feature = "nonnull", since = "1.25.0")]
 672 impl<T: ?Sized> PartialEq for NonNull<T> {
 673     #[inline]
 674     fn eq(&self, other: &Self) -> bool {
 675         self.as_ptr() == other.as_ptr()
 676     }
 677 }
 678
 679 #[stable(feature = "nonnull", since = "1.25.0")]
 680 impl<T: ?Sized> Ord for NonNull<T> {
 681     #[inline]
 682     fn cmp(&self, other: &Self) -> Ordering {
 683         self.as_ptr().cmp(&other.as_ptr())
 684     }
 685 }
 686
 687 #[stable(feature = "nonnull", since = "1.25.0")]
 688 impl<T: ?Sized> PartialOrd for NonNull<T> {
 689     #[inline]
 690     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
 691         self.as_ptr().partial_cmp(&other.as_ptr())
 692     }
 693 }
 694
 695 #[stable(feature = "nonnull", since = "1.25.0")]
 696 impl<T: ?Sized> hash::Hash for NonNull<T> {
 697     #[inline]
 698     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 699         self.as_ptr().hash(state)
 700     }
 701 }
 702
 703 #[unstable(feature = "ptr_internals", issue = "none")]
 704 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 705 impl<T: ?Sized> const From<Unique<T>> for NonNull<T> {
 706     #[inline]
 707     fn from(unique: Unique<T>) -> Self {
 708         // SAFETY: A Unique pointer cannot be null, so the conditions for
 709         // new_unchecked() are respected.
 710         unsafe { NonNull::new_unchecked(unique.as_ptr()) }
 711     }
 712 }
 713
 714 #[stable(feature = "nonnull", since = "1.25.0")]
 715 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 716 impl<T: ?Sized> const From<&mut T> for NonNull<T> {
 717     #[inline]
 718     fn from(reference: &mut T) -> Self {
 719         // SAFETY: A mutable reference cannot be null.
 720         unsafe { NonNull { pointer: reference as *mut T } }
 721     }
 722 }
 723
 724 #[stable(feature = "nonnull", since = "1.25.0")]
 725 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
 726 impl<T: ?Sized> const From<&T> for NonNull<T> {
 727     #[inline]
 728     fn from(reference: &T) -> Self {
 729         // SAFETY: A reference cannot be null, so the conditions for
 730         // new_unchecked() are respected.
 731         unsafe { NonNull { pointer: reference as *const T } }
 732     }
 733 }