1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
use crate::cmp::Ordering;
use crate::convert::From;
use crate::fmt;
use crate::hash;
use crate::marker::Unsize;
use crate::mem::{self, MaybeUninit};
use crate::ops::{CoerceUnsized, DispatchFromDyn};
use crate::ptr::Unique;
use crate::slice::{self, SliceIndex};

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

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

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

impl<T: Sized> NonNull<T> {
    /// Creates a new `NonNull` that is dangling, but well-aligned.
    ///
    /// This is useful for initializing types which lazily allocate, like
    /// `Vec::new` does.
    ///
    /// Note that the pointer value may potentially represent a valid pointer to
    /// a `T`, which means this must not be used as a "not yet initialized"
    /// sentinel value. Types that lazily allocate must track initialization by
    /// some other means.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::ptr::NonNull;
    ///
    /// let ptr = NonNull::<u32>::dangling();
    /// // Important: don't try to access the value of `ptr` without
    /// // initializing it first! The pointer is not null but isn't valid either!
    /// ```
    #[stable(feature = "nonnull", since = "1.25.0")]
    #[rustc_const_stable(feature = "const_nonnull_dangling", since = "1.36.0")]
    #[must_use]
    #[inline]
    pub const fn dangling() -> Self {
        // SAFETY: mem::align_of() returns a non-zero usize which is then casted
        // to a *mut T. Therefore, `ptr` is not null and the conditions for
        // calling new_unchecked() are respected.
        unsafe {
            let ptr = mem::align_of::<T>() as *mut T;
            NonNull::new_unchecked(ptr)
        }
    }

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

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

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

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

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

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

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

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

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

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

impl<T> NonNull<[T]> {
    /// Creates a non-null raw slice from a thin pointer and a length.
    ///
    /// The `len` argument is the number of **elements**, not the number of bytes.
    ///
    /// This function is safe, but dereferencing the return value is unsafe.
    /// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(nonnull_slice_from_raw_parts)]
    ///
    /// use std::ptr::NonNull;
    ///
    /// // create a slice pointer when starting out with a pointer to the first element
    /// let mut x = [5, 6, 7];
    /// let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
    /// let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
    /// assert_eq!(unsafe { slice.as_ref()[2] }, 7);
    /// ```
    ///
    /// (Note that this example artificially demonstrates a use of this method,
    /// but `let slice = NonNull::from(&x[..]);` would be a better way to write code like this.)
    #[unstable(feature = "nonnull_slice_from_raw_parts", issue = "71941")]
    #[rustc_const_unstable(feature = "const_nonnull_slice_from_raw_parts", issue = "71941")]
    #[must_use]
    #[inline]
    pub const fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self {
        // SAFETY: `data` is a `NonNull` pointer which is necessarily non-null
        unsafe { Self::new_unchecked(super::slice_from_raw_parts_mut(data.as_ptr(), len)) }
    }

    /// Returns the length of a non-null raw slice.
    ///
    /// The returned value is the number of **elements**, not the number of bytes.
    ///
    /// This function is safe, even when the non-null raw slice cannot be dereferenced to a slice
    /// because the pointer does not have a valid address.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(slice_ptr_len, nonnull_slice_from_raw_parts)]
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.len(), 3);
    /// ```
    #[unstable(feature = "slice_ptr_len", issue = "71146")]
    #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
    #[must_use]
    #[inline]
    pub const fn len(self) -> usize {
        self.as_ptr().len()
    }

    /// Returns a non-null pointer to the slice's buffer.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.as_non_null_ptr(), NonNull::new(1 as *mut i8).unwrap());
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
    pub const fn as_non_null_ptr(self) -> NonNull<T> {
        // SAFETY: We know `self` is non-null.
        unsafe { NonNull::new_unchecked(self.as_ptr().as_mut_ptr()) }
    }

    /// Returns a raw pointer to the slice's buffer.
    ///
    /// # Examples
    ///
    /// ```rust
    /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
    /// use std::ptr::NonNull;
    ///
    /// let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
    /// assert_eq!(slice.as_mut_ptr(), 1 as *mut i8);
    /// ```
    #[inline]
    #[must_use]
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
    pub const fn as_mut_ptr(self) -> *mut T {
        self.as_non_null_ptr().as_ptr()
    }

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

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

    /// Returns a raw pointer to an element or subslice, without doing bounds
    /// checking.
    ///
    /// Calling this method with an out-of-bounds index or when `self` is not dereferenceable
    /// is *[undefined behavior]* even if the resulting pointer is not used.
    ///
    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(slice_ptr_get, nonnull_slice_from_raw_parts)]
    /// use std::ptr::NonNull;
    ///
    /// let x = &mut [1, 2, 4];
    /// let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());
    ///
    /// unsafe {
    ///     assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
    /// }
    /// ```
    #[unstable(feature = "slice_ptr_get", issue = "74265")]
    #[inline]
    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
    where
        I: SliceIndex<[T]>,
    {
        // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
        // As a consequence, the resulting pointer cannot be null.
        unsafe { NonNull::new_unchecked(self.as_ptr().get_unchecked_mut(index)) }
    }
}

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

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

#[unstable(feature = "coerce_unsized", issue = "27732")]
impl<T: ?Sized, U: ?Sized> CoerceUnsized<NonNull<U>> for NonNull<T> where T: Unsize<U> {}

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

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

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

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

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

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

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

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

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

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

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