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::intrinsics::assert_unsafe_precondition;
   6 use crate::marker::Unsize;
   7 use crate::mem::{self, MaybeUninit};
   8 use crate::num::NonZeroUsize;
   9 use crate::ops::{CoerceUnsized, DispatchFromDyn};
  10 use crate::ptr::Unique;
  11 use crate::slice::{self, SliceIndex};
  12
  13 /// `*mut T` but non-zero and [covariant].
  14 ///
  15 /// This is often the correct thing to use when building data structures using
  16 /// raw pointers, but is ultimately more dangerous to use because of its additional
  17 /// properties. If you're not sure if you should use `NonNull<T>`, just use `*mut T`!
  18 ///
  19 /// Unlike `*mut T`, the pointer must always be non-null, even if the pointer
  20 /// is never dereferenced. This is so that enums may use this forbidden value
  21 /// as a discriminant -- `Option<NonNull<T>>` has the same size as `*mut T`.
  22 /// However the pointer may still dangle if it isn't dereferenced.
  23 ///
  24 /// Unlike `*mut T`, `NonNull<T>` was chosen to be covariant over `T`. This makes it
  25 /// possible to use `NonNull<T>` when building covariant types, but introduces the
  26 /// risk of unsoundness if used in a type that shouldn't actually be covariant.
  27 /// (The opposite choice was made for `*mut T` even though technically the unsoundness
  28 /// could only be caused by calling unsafe functions.)
  29 ///
  30 /// Covariance is correct for most safe abstractions, such as `Box`, `Rc`, `Arc`, `Vec`,
  31 /// and `LinkedList`. This is the case because they provide a public API that follows the
  32 /// normal shared XOR mutable rules of Rust.
  33 ///
  34 /// If your type cannot safely be covariant, you must ensure it contains some
  35 /// additional field to provide invariance. Often this field will be a [`PhantomData`]
  36 /// type like `PhantomData<Cell<T>>` or `PhantomData<&'a mut T>`.
  37 ///
  38 /// Notice that `NonNull<T>` has a `From` instance for `&T`. However, this does
  39 /// not change the fact that mutating through a (pointer derived from a) shared
  40 /// reference is undefined behavior unless the mutation happens inside an
  41 /// [`UnsafeCell<T>`]. The same goes for creating a mutable reference from a shared
  42 /// reference. When using this `From` instance without an `UnsafeCell<T>`,
  43 /// it is your responsibility to ensure that `as_mut` is never called, and `as_ptr`
  44 /// is never used for mutation.
  45 ///
  46 /// # Representation
  47 ///
  48 /// Thanks to the [null pointer optimization],
  49 /// `NonNull<T>` and `Option<NonNull<T>>`
  50 /// are guaranteed to have the same size and alignment:
  51 ///
  52 /// ```
  53 /// # use std::mem::{size_of, align_of};
  54 /// use std::ptr::NonNull;
  55 ///
  56 /// assert_eq!(size_of::<NonNull<i16>>(), size_of::<Option<NonNull<i16>>>());
  57 /// assert_eq!(align_of::<NonNull<i16>>(), align_of::<Option<NonNull<i16>>>());
  58 ///
  59 /// assert_eq!(size_of::<NonNull<str>>(), size_of::<Option<NonNull<str>>>());
  60 /// assert_eq!(align_of::<NonNull<str>>(), align_of::<Option<NonNull<str>>>());
  61 /// ```
  62 ///
  63 /// [covariant]: https://doc.rust-lang.org/reference/subtyping.html
  64 /// [`PhantomData`]: crate::marker::PhantomData
  65 /// [`UnsafeCell<T>`]: crate::cell::UnsafeCell
  66 /// [null pointer optimization]: crate::option#representation
  67 #[stable(feature = "nonnull", since = "1.25.0")]
  68 #[repr(transparent)]
  69 #[rustc_layout_scalar_valid_range_start(1)]
  70 #[rustc_nonnull_optimization_guaranteed]
  71 pub struct NonNull<T: ?Sized> {
  72     pointer: *const T,
  73 }
  74
  75 /// `NonNull` pointers are not `Send` because the data they reference may be aliased.
  76 // N.B., this impl is unnecessary, but should provide better error messages.
  77 #[stable(feature = "nonnull", since = "1.25.0")]
  78 impl<T: ?Sized> !Send for NonNull<T> {}
  79
  80 /// `NonNull` pointers are not `Sync` because the data they reference may be aliased.
  81 // N.B., this impl is unnecessary, but should provide better error messages.
  82 #[stable(feature = "nonnull", since = "1.25.0")]
  83 impl<T: ?Sized> !Sync for NonNull<T> {}
  84
  85 impl<T: Sized> NonNull<T> {
  86     /// Creates a new `NonNull` that is dangling, but well-aligned.
  87     ///
  88     /// This is useful for initializing types which lazily allocate, like
  89     /// `Vec::new` does.
  90     ///
  91     /// Note that the pointer value may potentially represent a valid pointer to
  92     /// a `T`, which means this must not be used as a "not yet initialized"
  93     /// sentinel value. Types that lazily allocate must track initialization by
  94     /// some other means.
  95     ///
  96     /// # Examples
  97     ///
  98     /// ```
  99     /// use std::ptr::NonNull;
 100     ///
 101     /// let ptr = NonNull::<u32>::dangling();
 102     /// // Important: don't try to access the value of `ptr` without
 103     /// // initializing it first! The pointer is not null but isn't valid either!
 104     /// ```
 105     #[stable(feature = "nonnull", since = "1.25.0")]
 106     #[rustc_const_stable(feature = "const_nonnull_dangling", since = "1.36.0")]
 107     #[must_use]
 108     #[inline]
 109     pub const fn dangling() -> Self {
 110         // SAFETY: mem::align_of() returns a non-zero usize which is then casted
 111         // to a *mut T. Therefore, `ptr` is not null and the conditions for
 112         // calling new_unchecked() are respected.
 113         unsafe {
 114             let ptr = crate::ptr::invalid_mut::<T>(mem::align_of::<T>());
 115             NonNull::new_unchecked(ptr)
 116         }
 117     }
 118
 119     /// Returns a shared references to the value. In contrast to [`as_ref`], this does not require
 120     /// that the value has to be initialized.
 121     ///
 122     /// For the mutable counterpart see [`as_uninit_mut`].
 123     ///
 124     /// [`as_ref`]: NonNull::as_ref
 125     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
 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 "dereferenceable" 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, while this reference exists, the memory the pointer points to must
 138     ///   not get mutated (except inside `UnsafeCell`).
 139     ///
 140     /// This applies even if the result of this method is unused!
 141     ///
 142     /// [the module documentation]: crate::ptr#safety
 143     #[inline]
 144     #[must_use]
 145     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 146     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 147     pub const unsafe fn as_uninit_ref<'a>(self) -> &'a MaybeUninit<T> {
 148         // SAFETY: the caller must guarantee that `self` meets all the
 149         // requirements for a reference.
 150         unsafe { &*self.cast().as_ptr() }
 151     }
 152
 153     /// Returns a unique references to the value. In contrast to [`as_mut`], this does not require
 154     /// that the value has to be initialized.
 155     ///
 156     /// For the shared counterpart see [`as_uninit_ref`].
 157     ///
 158     /// [`as_mut`]: NonNull::as_mut
 159     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 160     ///
 161     /// # Safety
 162     ///
 163     /// When calling this method, you have to ensure that all of the following is true:
 164     ///
 165     /// * The pointer must be properly aligned.
 166     ///
 167     /// * It must be "dereferenceable" in the sense defined in [the module documentation].
 168     ///
 169     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 170     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 171     ///   In particular, while this reference exists, the memory the pointer points to must
 172     ///   not get accessed (read or written) through any other pointer.
 173     ///
 174     /// This applies even if the result of this method is unused!
 175     ///
 176     /// [the module documentation]: crate::ptr#safety
 177     #[inline]
 178     #[must_use]
 179     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 180     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 181     pub const unsafe fn as_uninit_mut<'a>(self) -> &'a mut MaybeUninit<T> {
 182         // SAFETY: the caller must guarantee that `self` meets all the
 183         // requirements for a reference.
 184         unsafe { &mut *self.cast().as_ptr() }
 185     }
 186 }
 187
 188 impl<T: ?Sized> NonNull<T> {
 189     /// Creates a new `NonNull`.
 190     ///
 191     /// # Safety
 192     ///
 193     /// `ptr` must be non-null.
 194     ///
 195     /// # Examples
 196     ///
 197     /// ```
 198     /// use std::ptr::NonNull;
 199     ///
 200     /// let mut x = 0u32;
 201     /// let ptr = unsafe { NonNull::new_unchecked(&mut x as *mut _) };
 202     /// ```
 203     ///
 204     /// *Incorrect* usage of this function:
 205     ///
 206     /// ```rust,no_run
 207     /// use std::ptr::NonNull;
 208     ///
 209     /// // NEVER DO THAT!!! This is undefined behavior. ⚠️
 210     /// let ptr = unsafe { NonNull::<u32>::new_unchecked(std::ptr::null_mut()) };
 211     /// ```
 212     #[stable(feature = "nonnull", since = "1.25.0")]
 213     #[rustc_const_stable(feature = "const_nonnull_new_unchecked", since = "1.25.0")]
 214     #[inline]
 215     pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
 216         // SAFETY: the caller must guarantee that `ptr` is non-null.
 217         unsafe {
 218             assert_unsafe_precondition!("NonNull::new_unchecked requires that the pointer is non-null", [T: ?Sized](ptr: *mut T) => !ptr.is_null());
 219             NonNull { pointer: ptr as _ }
 220         }
 221     }
 222
 223     /// Creates a new `NonNull` if `ptr` is non-null.
 224     ///
 225     /// # Examples
 226     ///
 227     /// ```
 228     /// use std::ptr::NonNull;
 229     ///
 230     /// let mut x = 0u32;
 231     /// let ptr = NonNull::<u32>::new(&mut x as *mut _).expect("ptr is null!");
 232     ///
 233     /// if let Some(ptr) = NonNull::<u32>::new(std::ptr::null_mut()) {
 234     ///     unreachable!();
 235     /// }
 236     /// ```
 237     #[stable(feature = "nonnull", since = "1.25.0")]
 238     #[rustc_const_unstable(feature = "const_nonnull_new", issue = "93235")]
 239     #[inline]
 240     pub const fn new(ptr: *mut T) -> Option<Self> {
 241         if !ptr.is_null() {
 242             // SAFETY: The pointer is already checked and is not null
 243             Some(unsafe { Self::new_unchecked(ptr) })
 244         } else {
 245             None
 246         }
 247     }
 248
 249     /// Performs the same functionality as [`std::ptr::from_raw_parts`], except that a
 250     /// `NonNull` pointer is returned, as opposed to a raw `*const` pointer.
 251     ///
 252     /// See the documentation of [`std::ptr::from_raw_parts`] for more details.
 253     ///
 254     /// [`std::ptr::from_raw_parts`]: crate::ptr::from_raw_parts
 255     #[unstable(feature = "ptr_metadata", issue = "81513")]
 256     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 257     #[inline]
 258     pub const fn from_raw_parts(
 259         data_address: NonNull<()>,
 260         metadata: <T as super::Pointee>::Metadata,
 261     ) -> NonNull<T> {
 262         // SAFETY: The result of `ptr::from::raw_parts_mut` is non-null because `data_address` is.
 263         unsafe {
 264             NonNull::new_unchecked(super::from_raw_parts_mut(data_address.as_ptr(), metadata))
 265         }
 266     }
 267
 268     /// Decompose a (possibly wide) pointer into its address and metadata components.
 269     ///
 270     /// The pointer can be later reconstructed with [`NonNull::from_raw_parts`].
 271     #[unstable(feature = "ptr_metadata", issue = "81513")]
 272     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 273     #[must_use = "this returns the result of the operation, \
 274                   without modifying the original"]
 275     #[inline]
 276     pub const fn to_raw_parts(self) -> (NonNull<()>, <T as super::Pointee>::Metadata) {
 277         (self.cast(), super::metadata(self.as_ptr()))
 278     }
 279
 280     /// Gets the "address" portion of the pointer.
 281     ///
 282     /// For more details see the equivalent method on a raw pointer, [`pointer::addr`].
 283     ///
 284     /// This API and its claimed semantics are part of the Strict Provenance experiment,
 285     /// see the [`ptr` module documentation][crate::ptr].
 286     #[must_use]
 287     #[inline]
 288     #[unstable(feature = "strict_provenance", issue = "95228")]
 289     pub fn addr(self) -> NonZeroUsize {
 290         // SAFETY: The pointer is guaranteed by the type to be non-null,
 291         // meaning that the address will be non-zero.
 292         unsafe { NonZeroUsize::new_unchecked(self.pointer.addr()) }
 293     }
 294
 295     /// Creates a new pointer with the given address.
 296     ///
 297     /// For more details see the equivalent method on a raw pointer, [`pointer::with_addr`].
 298     ///
 299     /// This API and its claimed semantics are part of the Strict Provenance experiment,
 300     /// see the [`ptr` module documentation][crate::ptr].
 301     #[must_use]
 302     #[inline]
 303     #[unstable(feature = "strict_provenance", issue = "95228")]
 304     pub fn with_addr(self, addr: NonZeroUsize) -> Self {
 305         // SAFETY: The result of `ptr::from::with_addr` is non-null because `addr` is guaranteed to be non-zero.
 306         unsafe { NonNull::new_unchecked(self.pointer.with_addr(addr.get()) as *mut _) }
 307     }
 308
 309     /// Creates a new pointer by mapping `self`'s address to a new one.
 310     ///
 311     /// For more details see the equivalent method on a raw pointer, [`pointer::map_addr`].
 312     ///
 313     /// This API and its claimed semantics are part of the Strict Provenance experiment,
 314     /// see the [`ptr` module documentation][crate::ptr].
 315     #[must_use]
 316     #[inline]
 317     #[unstable(feature = "strict_provenance", issue = "95228")]
 318     pub fn map_addr(self, f: impl FnOnce(NonZeroUsize) -> NonZeroUsize) -> Self {
 319         self.with_addr(f(self.addr()))
 320     }
 321
 322     /// Acquires the underlying `*mut` pointer.
 323     ///
 324     /// # Examples
 325     ///
 326     /// ```
 327     /// use std::ptr::NonNull;
 328     ///
 329     /// let mut x = 0u32;
 330     /// let ptr = NonNull::new(&mut x).expect("ptr is null!");
 331     ///
 332     /// let x_value = unsafe { *ptr.as_ptr() };
 333     /// assert_eq!(x_value, 0);
 334     ///
 335     /// unsafe { *ptr.as_ptr() += 2; }
 336     /// let x_value = unsafe { *ptr.as_ptr() };
 337     /// assert_eq!(x_value, 2);
 338     /// ```
 339     #[stable(feature = "nonnull", since = "1.25.0")]
 340     #[rustc_const_stable(feature = "const_nonnull_as_ptr", since = "1.32.0")]
 341     #[cfg_attr(not(bootstrap), rustc_never_returns_null_ptr)]
 342     #[must_use]
 343     #[inline(always)]
 344     pub const fn as_ptr(self) -> *mut T {
 345         self.pointer as *mut T
 346     }
 347
 348     /// Returns a shared reference to the value. If the value may be uninitialized, [`as_uninit_ref`]
 349     /// must be used instead.
 350     ///
 351     /// For the mutable counterpart see [`as_mut`].
 352     ///
 353     /// [`as_uninit_ref`]: NonNull::as_uninit_ref
 354     /// [`as_mut`]: NonNull::as_mut
 355     ///
 356     /// # Safety
 357     ///
 358     /// When calling this method, you have to ensure that all of the following is true:
 359     ///
 360     /// * The pointer must be properly aligned.
 361     ///
 362     /// * It must be "dereferenceable" in the sense defined in [the module documentation].
 363     ///
 364     /// * The pointer must point to an initialized instance of `T`.
 365     ///
 366     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 367     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 368     ///   In particular, while this reference exists, the memory the pointer points to must
 369     ///   not get mutated (except inside `UnsafeCell`).
 370     ///
 371     /// This applies even if the result of this method is unused!
 372     /// (The part about being initialized is not yet fully decided, but until
 373     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 374     ///
 375     /// # Examples
 376     ///
 377     /// ```
 378     /// use std::ptr::NonNull;
 379     ///
 380     /// let mut x = 0u32;
 381     /// let ptr = NonNull::new(&mut x as *mut _).expect("ptr is null!");
 382     ///
 383     /// let ref_x = unsafe { ptr.as_ref() };
 384     /// println!("{ref_x}");
 385     /// ```
 386     ///
 387     /// [the module documentation]: crate::ptr#safety
 388     #[stable(feature = "nonnull", since = "1.25.0")]
 389     #[rustc_const_stable(feature = "const_nonnull_as_ref", since = "1.73.0")]
 390     #[must_use]
 391     #[inline(always)]
 392     pub const unsafe fn as_ref<'a>(&self) -> &'a T {
 393         // SAFETY: the caller must guarantee that `self` meets all the
 394         // requirements for a reference.
 395         // `cast_const` avoids a mutable raw pointer deref.
 396         unsafe { &*self.as_ptr().cast_const() }
 397     }
 398
 399     /// Returns a unique reference to the value. If the value may be uninitialized, [`as_uninit_mut`]
 400     /// must be used instead.
 401     ///
 402     /// For the shared counterpart see [`as_ref`].
 403     ///
 404     /// [`as_uninit_mut`]: NonNull::as_uninit_mut
 405     /// [`as_ref`]: NonNull::as_ref
 406     ///
 407     /// # Safety
 408     ///
 409     /// When calling this method, you have to ensure that all of the following is true:
 410     ///
 411     /// * The pointer must be properly aligned.
 412     ///
 413     /// * It must be "dereferenceable" in the sense defined in [the module documentation].
 414     ///
 415     /// * The pointer must point to an initialized instance of `T`.
 416     ///
 417     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 418     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 419     ///   In particular, while this reference exists, the memory the pointer points to must
 420     ///   not get accessed (read or written) through any other pointer.
 421     ///
 422     /// This applies even if the result of this method is unused!
 423     /// (The part about being initialized is not yet fully decided, but until
 424     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 425     /// # Examples
 426     ///
 427     /// ```
 428     /// use std::ptr::NonNull;
 429     ///
 430     /// let mut x = 0u32;
 431     /// let mut ptr = NonNull::new(&mut x).expect("null pointer");
 432     ///
 433     /// let x_ref = unsafe { ptr.as_mut() };
 434     /// assert_eq!(*x_ref, 0);
 435     /// *x_ref += 2;
 436     /// assert_eq!(*x_ref, 2);
 437     /// ```
 438     ///
 439     /// [the module documentation]: crate::ptr#safety
 440     #[stable(feature = "nonnull", since = "1.25.0")]
 441     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 442     #[must_use]
 443     #[inline(always)]
 444     pub const unsafe fn as_mut<'a>(&mut self) -> &'a mut T {
 445         // SAFETY: the caller must guarantee that `self` meets all the
 446         // requirements for a mutable reference.
 447         unsafe { &mut *self.as_ptr() }
 448     }
 449
 450     /// Casts to a pointer of another type.
 451     ///
 452     /// # Examples
 453     ///
 454     /// ```
 455     /// use std::ptr::NonNull;
 456     ///
 457     /// let mut x = 0u32;
 458     /// let ptr = NonNull::new(&mut x as *mut _).expect("null pointer");
 459     ///
 460     /// let casted_ptr = ptr.cast::<i8>();
 461     /// let raw_ptr: *mut i8 = casted_ptr.as_ptr();
 462     /// ```
 463     #[stable(feature = "nonnull_cast", since = "1.27.0")]
 464     #[rustc_const_stable(feature = "const_nonnull_cast", since = "1.36.0")]
 465     #[must_use = "this returns the result of the operation, \
 466                   without modifying the original"]
 467     #[inline]
 468     pub const fn cast<U>(self) -> NonNull<U> {
 469         // SAFETY: `self` is a `NonNull` pointer which is necessarily non-null
 470         unsafe { NonNull::new_unchecked(self.as_ptr() as *mut U) }
 471     }
 472
 473     /// See [`pointer::add`] for semantics and safety requirements.
 474     #[inline]
 475     pub(crate) const unsafe fn add(self, delta: usize) -> Self
 476     where
 477         T: Sized,
 478     {
 479         // SAFETY: We require that the delta stays in-bounds of the object, and
 480         // thus it cannot become null, as that would require wrapping the
 481         // address space, which no legal objects are allowed to do.
 482         // And the caller promised the `delta` is sound to add.
 483         unsafe { NonNull { pointer: self.pointer.add(delta) } }
 484     }
 485
 486     /// See [`pointer::sub`] for semantics and safety requirements.
 487     #[inline]
 488     pub(crate) const unsafe fn sub(self, delta: usize) -> Self
 489     where
 490         T: Sized,
 491     {
 492         // SAFETY: We require that the delta stays in-bounds of the object, and
 493         // thus it cannot become null, as no legal objects can be allocated
 494         // in such as way that the null address is part of them.
 495         // And the caller promised the `delta` is sound to subtract.
 496         unsafe { NonNull { pointer: self.pointer.sub(delta) } }
 497     }
 498
 499     /// See [`pointer::sub_ptr`] for semantics and safety requirements.
 500     #[inline]
 501     pub(crate) const unsafe fn sub_ptr(self, subtrahend: Self) -> usize
 502     where
 503         T: Sized,
 504     {
 505         // SAFETY: The caller promised that this is safe to do, and
 506         // the non-nullness is irrelevant to the operation.
 507         unsafe { self.pointer.sub_ptr(subtrahend.pointer) }
 508     }
 509 }
 510
 511 impl<T> NonNull<[T]> {
 512     /// Creates a non-null raw slice from a thin pointer and a length.
 513     ///
 514     /// The `len` argument is the number of **elements**, not the number of bytes.
 515     ///
 516     /// This function is safe, but dereferencing the return value is unsafe.
 517     /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
 518     ///
 519     /// # Examples
 520     ///
 521     /// ```rust
 522     /// use std::ptr::NonNull;
 523     ///
 524     /// // create a slice pointer when starting out with a pointer to the first element
 525     /// let mut x = [5, 6, 7];
 526     /// let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
 527     /// let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
 528     /// assert_eq!(unsafe { slice.as_ref()[2] }, 7);
 529     /// ```
 530     ///
 531     /// (Note that this example artificially demonstrates a use of this method,
 532     /// but `let slice = NonNull::from(&x[..]);` would be a better way to write code like this.)
 533     #[stable(feature = "nonnull_slice_from_raw_parts", since = "1.70.0")]
 534     #[rustc_const_unstable(feature = "const_slice_from_raw_parts_mut", issue = "67456")]
 535     #[must_use]
 536     #[inline]
 537     pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self {
 538         // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
 539         unsafe { Self::new_unchecked(super::slice_from_raw_parts_mut(data.as_ptr(), len)) }
 540     }
 541
 542     /// Returns the length of a non-null raw slice.
 543     ///
 544     /// The returned value is the number of **elements**, not the number of bytes.
 545     ///
 546     /// This function is safe, even when the non-null raw slice cannot be dereferenced to a slice
 547     /// because the pointer does not have a valid address.
 548     ///
 549     /// # Examples
 550     ///
 551     /// ```rust
 552     /// use std::ptr::NonNull;
 553     ///
 554     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 555     /// assert_eq!(slice.len(), 3);
 556     /// ```
 557     #[stable(feature = "slice_ptr_len_nonnull", since = "1.63.0")]
 558     #[rustc_const_stable(feature = "const_slice_ptr_len_nonnull", since = "1.63.0")]
 559     #[rustc_allow_const_fn_unstable(const_slice_ptr_len)]
 560     #[must_use]
 561     #[inline]
 562     pub const fn len(self) -> usize {
 563         self.as_ptr().len()
 564     }
 565
 566     /// Returns a non-null pointer to the slice's buffer.
 567     ///
 568     /// # Examples
 569     ///
 570     /// ```rust
 571     /// #![feature(slice_ptr_get)]
 572     /// use std::ptr::NonNull;
 573     ///
 574     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 575     /// assert_eq!(slice.as_non_null_ptr(), NonNull::<i8>::dangling());
 576     /// ```
 577     #[inline]
 578     #[must_use]
 579     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 580     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 581     pub const fn as_non_null_ptr(self) -> NonNull<T> {
 582         // SAFETY: We know `self` is non-null.
 583         unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
 584     }
 585
 586     /// Returns a raw pointer to the slice's buffer.
 587     ///
 588     /// # Examples
 589     ///
 590     /// ```rust
 591     /// #![feature(slice_ptr_get)]
 592     /// use std::ptr::NonNull;
 593     ///
 594     /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
 595     /// assert_eq!(slice.as_mut_ptr(), NonNull::<i8>::dangling().as_ptr());
 596     /// ```
 597     #[inline]
 598     #[must_use]
 599     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 600     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 601     #[cfg_attr(not(bootstrap), rustc_never_returns_null_ptr)]
 602     pub const fn as_mut_ptr(self) -> *mut T {
 603         self.as_non_null_ptr().as_ptr()
 604     }
 605
 606     /// Returns a shared reference to a slice of possibly uninitialized values. In contrast to
 607     /// [`as_ref`], this does not require that the value has to be initialized.
 608     ///
 609     /// For the mutable counterpart see [`as_uninit_slice_mut`].
 610     ///
 611     /// [`as_ref`]: NonNull::as_ref
 612     /// [`as_uninit_slice_mut`]: NonNull::as_uninit_slice_mut
 613     ///
 614     /// # Safety
 615     ///
 616     /// When calling this method, you have to ensure that all of the following is true:
 617     ///
 618     /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
 619     ///   and it must be properly aligned. This means in particular:
 620     ///
 621     ///     * The entire memory range of this slice must be contained within a single allocated object!
 622     ///       Slices can never span across multiple allocated objects.
 623     ///
 624     ///     * The pointer must be aligned even for zero-length slices. One
 625     ///       reason for this is that enum layout optimizations may rely on references
 626     ///       (including slices of any length) being aligned and non-null to distinguish
 627     ///       them from other data. You can obtain a pointer that is usable as `data`
 628     ///       for zero-length slices using [`NonNull::dangling()`].
 629     ///
 630     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 631     ///   See the safety documentation of [`pointer::offset`].
 632     ///
 633     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 634     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 635     ///   In particular, while this reference exists, the memory the pointer points to must
 636     ///   not get mutated (except inside `UnsafeCell`).
 637     ///
 638     /// This applies even if the result of this method is unused!
 639     ///
 640     /// See also [`slice::from_raw_parts`].
 641     ///
 642     /// [valid]: crate::ptr#safety
 643     #[inline]
 644     #[must_use]
 645     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 646     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 647     pub const unsafe fn as_uninit_slice<'a>(self) -> &'a [MaybeUninit<T>] {
 648         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
 649         unsafe { slice::from_raw_parts(self.cast().as_ptr(), self.len()) }
 650     }
 651
 652     /// Returns a unique reference to a slice of possibly uninitialized values. In contrast to
 653     /// [`as_mut`], this does not require that the value has to be initialized.
 654     ///
 655     /// For the shared counterpart see [`as_uninit_slice`].
 656     ///
 657     /// [`as_mut`]: NonNull::as_mut
 658     /// [`as_uninit_slice`]: NonNull::as_uninit_slice
 659     ///
 660     /// # Safety
 661     ///
 662     /// When calling this method, you have to ensure that all of the following is true:
 663     ///
 664     /// * The pointer must be [valid] for reads and writes for `ptr.len() * mem::size_of::<T>()`
 665     ///   many bytes, and it must be properly aligned. This means in particular:
 666     ///
 667     ///     * The entire memory range of this slice must be contained within a single allocated object!
 668     ///       Slices can never span across multiple allocated objects.
 669     ///
 670     ///     * The pointer must be aligned even for zero-length slices. One
 671     ///       reason for this is that enum layout optimizations may rely on references
 672     ///       (including slices of any length) being aligned and non-null to distinguish
 673     ///       them from other data. You can obtain a pointer that is usable as `data`
 674     ///       for zero-length slices using [`NonNull::dangling()`].
 675     ///
 676     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 677     ///   See the safety documentation of [`pointer::offset`].
 678     ///
 679     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 680     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 681     ///   In particular, while this reference exists, the memory the pointer points to must
 682     ///   not get accessed (read or written) through any other pointer.
 683     ///
 684     /// This applies even if the result of this method is unused!
 685     ///
 686     /// See also [`slice::from_raw_parts_mut`].
 687     ///
 688     /// [valid]: crate::ptr#safety
 689     ///
 690     /// # Examples
 691     ///
 692     /// ```rust
 693     /// #![feature(allocator_api, ptr_as_uninit)]
 694     ///
 695     /// use std::alloc::{Allocator, Layout, Global};
 696     /// use std::mem::MaybeUninit;
 697     /// use std::ptr::NonNull;
 698     ///
 699     /// let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
 700     /// // This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
 701     /// // Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
 702     /// # #[allow(unused_variables)]
 703     /// let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };
 704     /// # Ok::<_, std::alloc::AllocError>(())
 705     /// ```
 706     #[inline]
 707     #[must_use]
 708     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 709     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 710     pub const unsafe fn as_uninit_slice_mut<'a>(self) -> &'a mut [MaybeUninit<T>] {
 711         // SAFETY: the caller must uphold the safety contract for `as_uninit_slice_mut`.
 712         unsafe { slice::from_raw_parts_mut(self.cast().as_ptr(), self.len()) }
 713     }
 714
 715     /// Returns a raw pointer to an element or subslice, without doing bounds
 716     /// checking.
 717     ///
 718     /// Calling this method with an out-of-bounds index or when `self` is not dereferenceable
 719     /// is *[undefined behavior]* even if the resulting pointer is not used.
 720     ///
 721     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 722     ///
 723     /// # Examples
 724     ///
 725     /// ```
 726     /// #![feature(slice_ptr_get)]
 727     /// use std::ptr::NonNull;
 728     ///
 729     /// let x = &mut [1, 2, 4];
 730     /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
 731     ///
 732     /// unsafe {
 733     ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
 734     /// }
 735     /// ```
 736     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 737     #[inline]
 738     pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
 739     where
 740         I: SliceIndex<[T]>,
 741     {
 742         // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
 743         // As a consequence, the resulting pointer cannot be null.
 744         unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
 745     }
 746 }
 747
 748 #[stable(feature = "nonnull", since = "1.25.0")]
 749 impl<T: ?Sized> Clone for NonNull<T> {
 750     #[inline(always)]
 751     fn clone(&self) -> Self {
 752         *self
 753     }
 754 }
 755
 756 #[stable(feature = "nonnull", since = "1.25.0")]
 757 impl<T: ?Sized> Copy for NonNull<T> {}
 758
 759 #[unstable(feature = "coerce_unsized", issue = "18598")]
 760 impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 761
 762 #[unstable(feature = "dispatch_from_dyn", issue = "none")]
 763 impl<T: ?Sized, U: ?Sized> DispatchFromDyn<NonNull<U>> for NonNull<T> where T: Unsize<U> {}
 764
 765 #[stable(feature = "nonnull", since = "1.25.0")]
 766 impl<T: ?Sized> fmt::Debug for NonNull<T> {
 767     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 768         fmt::Pointer::fmt(&self.as_ptr(), f)
 769     }
 770 }
 771
 772 #[stable(feature = "nonnull", since = "1.25.0")]
 773 impl<T: ?Sized> fmt::Pointer for NonNull<T> {
 774     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 775         fmt::Pointer::fmt(&self.as_ptr(), f)
 776     }
 777 }
 778
 779 #[stable(feature = "nonnull", since = "1.25.0")]
 780 impl<T: ?Sized> Eq for NonNull<T> {}
 781
 782 #[stable(feature = "nonnull", since = "1.25.0")]
 783 impl<T: ?Sized> PartialEq for NonNull<T> {
 784     #[inline]
 785     fn eq(&self, other: &Self) -> bool {
 786         self.as_ptr() == other.as_ptr()
 787     }
 788 }
 789
 790 #[stable(feature = "nonnull", since = "1.25.0")]
 791 impl<T: ?Sized> Ord for NonNull<T> {
 792     #[inline]
 793     fn cmp(&self, other: &Self) -> Ordering {
 794         self.as_ptr().cmp(&other.as_ptr())
 795     }
 796 }
 797
 798 #[stable(feature = "nonnull", since = "1.25.0")]
 799 impl<T: ?Sized> PartialOrd for NonNull<T> {
 800     #[inline]
 801     fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
 802         self.as_ptr().partial_cmp(&other.as_ptr())
 803     }
 804 }
 805
 806 #[stable(feature = "nonnull", since = "1.25.0")]
 807 impl<T: ?Sized> hash::Hash for NonNull<T> {
 808     #[inline]
 809     fn hash<H: hash::Hasher>(&self, state: &mut H) {
 810         self.as_ptr().hash(state)
 811     }
 812 }
 813
 814 #[unstable(feature = "ptr_internals", issue = "none")]
 815 impl<T: ?Sized> From<Unique<T>> for NonNull<T> {
 816     #[inline]
 817     fn from(unique: Unique<T>) -> Self {
 818         // SAFETY: A Unique pointer cannot be null, so the conditions for
 819         // new_unchecked() are respected.
 820         unsafe { NonNull::new_unchecked(unique.as_ptr()) }
 821     }
 822 }
 823
 824 #[stable(feature = "nonnull", since = "1.25.0")]
 825 impl<T: ?Sized> From<&mut T> for NonNull<T> {
 826     /// Converts a `&mut T` to a `NonNull<T>`.
 827     ///
 828     /// This conversion is safe and infallible since references cannot be null.
 829     #[inline]
 830     fn from(reference: &mut T) -> Self {
 831         // SAFETY: A mutable reference cannot be null.
 832         unsafe { NonNull { pointer: reference as *mut T } }
 833     }
 834 }
 835
 836 #[stable(feature = "nonnull", since = "1.25.0")]
 837 impl<T: ?Sized> From<&T> for NonNull<T> {
 838     /// Converts a `&T` to a `NonNull<T>`.
 839     ///
 840     /// This conversion is safe and infallible since references cannot be null.
 841     #[inline]
 842     fn from(reference: &T) -> Self {
 843         // SAFETY: A reference cannot be null, so the conditions for
 844         // new_unchecked() are respected.
 845         unsafe { NonNull { pointer: reference as *const T } }
 846     }
 847 }