alloc/
raw_vec.rs

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
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
#![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]

use core::marker::PhantomData;
use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
use core::ptr::{self, NonNull, Unique};
use core::{cmp, hint};

#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
use crate::alloc::{Allocator, Global, Layout};
use crate::boxed::Box;
use crate::collections::TryReserveError;
use crate::collections::TryReserveErrorKind::*;

#[cfg(test)]
mod tests;

// One central function responsible for reporting capacity overflows. This'll
// ensure that the code generation related to these panics is minimal as there's
// only one location which panics rather than a bunch throughout the module.
#[cfg(not(no_global_oom_handling))]
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
fn capacity_overflow() -> ! {
    panic!("capacity overflow");
}

enum AllocInit {
    /// The contents of the new memory are uninitialized.
    Uninitialized,
    #[cfg(not(no_global_oom_handling))]
    /// The new memory is guaranteed to be zeroed.
    Zeroed,
}

#[repr(transparent)]
#[cfg_attr(target_pointer_width = "16", rustc_layout_scalar_valid_range_end(0x7fff))]
#[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0x7fff_ffff))]
#[cfg_attr(target_pointer_width = "64", rustc_layout_scalar_valid_range_end(0x7fff_ffff_ffff_ffff))]
struct Cap(usize);

impl Cap {
    const ZERO: Cap = unsafe { Cap(0) };

    /// `Cap(cap)`, except if `T` is a ZST then `Cap::ZERO`.
    ///
    /// # Safety: cap must be <= `isize::MAX`.
    unsafe fn new<T>(cap: usize) -> Self {
        if T::IS_ZST { Cap::ZERO } else { unsafe { Self(cap) } }
    }
}

/// A low-level utility for more ergonomically allocating, reallocating, and deallocating
/// a buffer of memory on the heap without having to worry about all the corner cases
/// involved. This type is excellent for building your own data structures like Vec and VecDeque.
/// In particular:
///
/// * Produces `Unique::dangling()` on zero-sized types.
/// * Produces `Unique::dangling()` on zero-length allocations.
/// * Avoids freeing `Unique::dangling()`.
/// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
/// * Guards against 32-bit systems allocating more than `isize::MAX` bytes.
/// * Guards against overflowing your length.
/// * Calls `handle_alloc_error` for fallible allocations.
/// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
/// * Uses the excess returned from the allocator to use the largest available capacity.
///
/// This type does not in anyway inspect the memory that it manages. When dropped it *will*
/// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
/// to handle the actual things *stored* inside of a `RawVec`.
///
/// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
/// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
/// `Box<[T]>`, since `capacity()` won't yield the length.
#[allow(missing_debug_implementations)]
pub(crate) struct RawVec<T, A: Allocator = Global> {
    inner: RawVecInner<A>,
    _marker: PhantomData<T>,
}

/// Like a `RawVec`, but only generic over the allocator, not the type.
///
/// As such, all the methods need the layout passed-in as a parameter.
///
/// Having this separation reduces the amount of code we need to monomorphize,
/// as most operations don't need the actual type, just its layout.
#[allow(missing_debug_implementations)]
struct RawVecInner<A: Allocator = Global> {
    ptr: Unique<u8>,
    /// Never used for ZSTs; it's `capacity()`'s responsibility to return usize::MAX in that case.
    ///
    /// # Safety
    ///
    /// `cap` must be in the `0..=isize::MAX` range.
    cap: Cap,
    alloc: A,
}

impl<T> RawVec<T, Global> {
    /// Creates the biggest possible `RawVec` (on the system heap)
    /// without allocating. If `T` has positive size, then this makes a
    /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
    /// `RawVec` with capacity `usize::MAX`. Useful for implementing
    /// delayed allocation.
    #[must_use]
    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
    pub const fn new() -> Self {
        Self::new_in(Global)
    }

    /// Creates a `RawVec` (on the system heap) with exactly the
    /// capacity and alignment requirements for a `[T; capacity]`. This is
    /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
    /// zero-sized. Note that if `T` is zero-sized this means you will
    /// *not* get a `RawVec` with the requested capacity.
    ///
    /// Non-fallible version of `try_with_capacity`
    ///
    /// # Panics
    ///
    /// Panics if the requested capacity exceeds `isize::MAX` bytes.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    #[cfg(not(any(no_global_oom_handling, test)))]
    #[must_use]
    #[inline]
    pub fn with_capacity(capacity: usize) -> Self {
        Self { inner: RawVecInner::with_capacity(capacity, T::LAYOUT), _marker: PhantomData }
    }

    /// Like `with_capacity`, but guarantees the buffer is zeroed.
    #[cfg(not(any(no_global_oom_handling, test)))]
    #[must_use]
    #[inline]
    pub fn with_capacity_zeroed(capacity: usize) -> Self {
        Self {
            inner: RawVecInner::with_capacity_zeroed_in(capacity, Global, T::LAYOUT),
            _marker: PhantomData,
        }
    }
}

impl RawVecInner<Global> {
    #[cfg(not(any(no_global_oom_handling, test)))]
    #[must_use]
    #[inline]
    fn with_capacity(capacity: usize, elem_layout: Layout) -> Self {
        match Self::try_allocate_in(capacity, AllocInit::Uninitialized, Global, elem_layout) {
            Ok(res) => res,
            Err(err) => handle_error(err),
        }
    }
}

// Tiny Vecs are dumb. Skip to:
// - 8 if the element size is 1, because any heap allocators is likely
//   to round up a request of less than 8 bytes to at least 8 bytes.
// - 4 if elements are moderate-sized (<= 1 KiB).
// - 1 otherwise, to avoid wasting too much space for very short Vecs.
const fn min_non_zero_cap(size: usize) -> usize {
    if size == 1 {
        8
    } else if size <= 1024 {
        4
    } else {
        1
    }
}

impl<T, A: Allocator> RawVec<T, A> {
    #[cfg(not(no_global_oom_handling))]
    pub(crate) const MIN_NON_ZERO_CAP: usize = min_non_zero_cap(size_of::<T>());

    /// Like `new`, but parameterized over the choice of allocator for
    /// the returned `RawVec`.
    #[inline]
    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
    pub const fn new_in(alloc: A) -> Self {
        Self { inner: RawVecInner::new_in(alloc, align_of::<T>()), _marker: PhantomData }
    }

    /// Like `with_capacity`, but parameterized over the choice of
    /// allocator for the returned `RawVec`.
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
        Self {
            inner: RawVecInner::with_capacity_in(capacity, alloc, T::LAYOUT),
            _marker: PhantomData,
        }
    }

    /// Like `try_with_capacity`, but parameterized over the choice of
    /// allocator for the returned `RawVec`.
    #[inline]
    pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
        match RawVecInner::try_with_capacity_in(capacity, alloc, T::LAYOUT) {
            Ok(inner) => Ok(Self { inner, _marker: PhantomData }),
            Err(e) => Err(e),
        }
    }

    /// Like `with_capacity_zeroed`, but parameterized over the choice
    /// of allocator for the returned `RawVec`.
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
        Self {
            inner: RawVecInner::with_capacity_zeroed_in(capacity, alloc, T::LAYOUT),
            _marker: PhantomData,
        }
    }

    /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
    ///
    /// Note that this will correctly reconstitute any `cap` changes
    /// that may have been performed. (See description of type for details.)
    ///
    /// # Safety
    ///
    /// * `len` must be greater than or equal to the most recently requested capacity, and
    /// * `len` must be less than or equal to `self.capacity()`.
    ///
    /// Note, that the requested capacity and `self.capacity()` could differ, as
    /// an allocator could overallocate and return a greater memory block than requested.
    pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
        // Sanity-check one half of the safety requirement (we cannot check the other half).
        debug_assert!(
            len <= self.capacity(),
            "`len` must be smaller than or equal to `self.capacity()`"
        );

        let me = ManuallyDrop::new(self);
        unsafe {
            let slice = ptr::slice_from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
            Box::from_raw_in(slice, ptr::read(&me.inner.alloc))
        }
    }

    /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
    ///
    /// # Safety
    ///
    /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
    /// `capacity`.
    /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
    /// systems). For ZSTs capacity is ignored.
    /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
    /// guaranteed.
    #[inline]
    pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
        // SAFETY: Precondition passed to the caller
        unsafe {
            let ptr = ptr.cast();
            let capacity = Cap::new::<T>(capacity);
            Self {
                inner: RawVecInner::from_raw_parts_in(ptr, capacity, alloc),
                _marker: PhantomData,
            }
        }
    }

    /// A convenience method for hoisting the non-null precondition out of [`RawVec::from_raw_parts_in`].
    ///
    /// # Safety
    ///
    /// See [`RawVec::from_raw_parts_in`].
    #[inline]
    pub unsafe fn from_nonnull_in(ptr: NonNull<T>, capacity: usize, alloc: A) -> Self {
        // SAFETY: Precondition passed to the caller
        unsafe {
            let ptr = ptr.cast();
            let capacity = Cap::new::<T>(capacity);
            Self { inner: RawVecInner::from_nonnull_in(ptr, capacity, alloc), _marker: PhantomData }
        }
    }

    /// Gets a raw pointer to the start of the allocation. Note that this is
    /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
    /// be careful.
    #[inline]
    pub const fn ptr(&self) -> *mut T {
        self.inner.ptr()
    }

    #[inline]
    pub fn non_null(&self) -> NonNull<T> {
        self.inner.non_null()
    }

    /// Gets the capacity of the allocation.
    ///
    /// This will always be `usize::MAX` if `T` is zero-sized.
    #[inline]
    pub const fn capacity(&self) -> usize {
        self.inner.capacity(size_of::<T>())
    }

    /// Returns a shared reference to the allocator backing this `RawVec`.
    #[inline]
    pub fn allocator(&self) -> &A {
        self.inner.allocator()
    }

    /// Ensures that the buffer contains at least enough space to hold `len +
    /// additional` elements. If it doesn't already have enough capacity, will
    /// reallocate enough space plus comfortable slack space to get amortized
    /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
    /// itself to panic.
    ///
    /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
    /// the requested space. This is not really unsafe, but the unsafe
    /// code *you* write that relies on the behavior of this function may break.
    ///
    /// This is ideal for implementing a bulk-push operation like `extend`.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    pub fn reserve(&mut self, len: usize, additional: usize) {
        self.inner.reserve(len, additional, T::LAYOUT)
    }

    /// A specialized version of `self.reserve(len, 1)` which requires the
    /// caller to ensure `len == self.capacity()`.
    #[cfg(not(no_global_oom_handling))]
    #[inline(never)]
    pub fn grow_one(&mut self) {
        self.inner.grow_one(T::LAYOUT)
    }

    /// The same as `reserve`, but returns on errors instead of panicking or aborting.
    pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
        self.inner.try_reserve(len, additional, T::LAYOUT)
    }

    /// Ensures that the buffer contains at least enough space to hold `len +
    /// additional` elements. If it doesn't already, will reallocate the
    /// minimum possible amount of memory necessary. Generally this will be
    /// exactly the amount of memory necessary, but in principle the allocator
    /// is free to give back more than we asked for.
    ///
    /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
    /// the requested space. This is not really unsafe, but the unsafe code
    /// *you* write that relies on the behavior of this function may break.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    #[cfg(not(no_global_oom_handling))]
    pub fn reserve_exact(&mut self, len: usize, additional: usize) {
        self.inner.reserve_exact(len, additional, T::LAYOUT)
    }

    /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
    pub fn try_reserve_exact(
        &mut self,
        len: usize,
        additional: usize,
    ) -> Result<(), TryReserveError> {
        self.inner.try_reserve_exact(len, additional, T::LAYOUT)
    }

    /// Shrinks the buffer down to the specified capacity. If the given amount
    /// is 0, actually completely deallocates.
    ///
    /// # Panics
    ///
    /// Panics if the given amount is *larger* than the current capacity.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    #[cfg(not(no_global_oom_handling))]
    #[inline]
    pub fn shrink_to_fit(&mut self, cap: usize) {
        self.inner.shrink_to_fit(cap, T::LAYOUT)
    }
}

unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
    /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
    fn drop(&mut self) {
        // SAFETY: We are in a Drop impl, self.inner will not be used again.
        unsafe { self.inner.deallocate(T::LAYOUT) }
    }
}

impl<A: Allocator> RawVecInner<A> {
    #[inline]
    #[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
    const fn new_in(alloc: A, align: usize) -> Self {
        let ptr = unsafe { core::mem::transmute(align) };
        // `cap: 0` means "unallocated". zero-sized types are ignored.
        Self { ptr, cap: Cap::ZERO, alloc }
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn with_capacity_in(capacity: usize, alloc: A, elem_layout: Layout) -> Self {
        match Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc, elem_layout) {
            Ok(this) => {
                unsafe {
                    // Make it more obvious that a subsquent Vec::reserve(capacity) will not allocate.
                    hint::assert_unchecked(!this.needs_to_grow(0, capacity, elem_layout));
                }
                this
            }
            Err(err) => handle_error(err),
        }
    }

    #[inline]
    fn try_with_capacity_in(
        capacity: usize,
        alloc: A,
        elem_layout: Layout,
    ) -> Result<Self, TryReserveError> {
        Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc, elem_layout)
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn with_capacity_zeroed_in(capacity: usize, alloc: A, elem_layout: Layout) -> Self {
        match Self::try_allocate_in(capacity, AllocInit::Zeroed, alloc, elem_layout) {
            Ok(res) => res,
            Err(err) => handle_error(err),
        }
    }

    fn try_allocate_in(
        capacity: usize,
        init: AllocInit,
        alloc: A,
        elem_layout: Layout,
    ) -> Result<Self, TryReserveError> {
        // We avoid `unwrap_or_else` here because it bloats the amount of
        // LLVM IR generated.
        let layout = match layout_array(capacity, elem_layout) {
            Ok(layout) => layout,
            Err(_) => return Err(CapacityOverflow.into()),
        };

        // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
        if layout.size() == 0 {
            return Ok(Self::new_in(alloc, elem_layout.align()));
        }

        if let Err(err) = alloc_guard(layout.size()) {
            return Err(err);
        }

        let result = match init {
            AllocInit::Uninitialized => alloc.allocate(layout),
            #[cfg(not(no_global_oom_handling))]
            AllocInit::Zeroed => alloc.allocate_zeroed(layout),
        };
        let ptr = match result {
            Ok(ptr) => ptr,
            Err(_) => return Err(AllocError { layout, non_exhaustive: () }.into()),
        };

        // Allocators currently return a `NonNull<[u8]>` whose length
        // matches the size requested. If that ever changes, the capacity
        // here should change to `ptr.len() / mem::size_of::<T>()`.
        Ok(Self { ptr: Unique::from(ptr.cast()), cap: unsafe { Cap(capacity) }, alloc })
    }

    #[inline]
    unsafe fn from_raw_parts_in(ptr: *mut u8, cap: Cap, alloc: A) -> Self {
        Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap, alloc }
    }

    #[inline]
    unsafe fn from_nonnull_in(ptr: NonNull<u8>, cap: Cap, alloc: A) -> Self {
        Self { ptr: Unique::from(ptr), cap, alloc }
    }

    #[inline]
    const fn ptr<T>(&self) -> *mut T {
        self.non_null::<T>().as_ptr()
    }

    #[inline]
    const fn non_null<T>(&self) -> NonNull<T> {
        self.ptr.cast().as_non_null_ptr()
    }

    #[inline]
    const fn capacity(&self, elem_size: usize) -> usize {
        if elem_size == 0 { usize::MAX } else { self.cap.0 }
    }

    #[inline]
    fn allocator(&self) -> &A {
        &self.alloc
    }

    #[inline]
    fn current_memory(&self, elem_layout: Layout) -> Option<(NonNull<u8>, Layout)> {
        if elem_layout.size() == 0 || self.cap.0 == 0 {
            None
        } else {
            // We could use Layout::array here which ensures the absence of isize and usize overflows
            // and could hypothetically handle differences between stride and size, but this memory
            // has already been allocated so we know it can't overflow and currently Rust does not
            // support such types. So we can do better by skipping some checks and avoid an unwrap.
            unsafe {
                let alloc_size = elem_layout.size().unchecked_mul(self.cap.0);
                let layout = Layout::from_size_align_unchecked(alloc_size, elem_layout.align());
                Some((self.ptr.into(), layout))
            }
        }
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn reserve(&mut self, len: usize, additional: usize, elem_layout: Layout) {
        // Callers expect this function to be very cheap when there is already sufficient capacity.
        // Therefore, we move all the resizing and error-handling logic from grow_amortized and
        // handle_reserve behind a call, while making sure that this function is likely to be
        // inlined as just a comparison and a call if the comparison fails.
        #[cold]
        fn do_reserve_and_handle<A: Allocator>(
            slf: &mut RawVecInner<A>,
            len: usize,
            additional: usize,
            elem_layout: Layout,
        ) {
            if let Err(err) = slf.grow_amortized(len, additional, elem_layout) {
                handle_error(err);
            }
        }

        if self.needs_to_grow(len, additional, elem_layout) {
            do_reserve_and_handle(self, len, additional, elem_layout);
        }
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn grow_one(&mut self, elem_layout: Layout) {
        if let Err(err) = self.grow_amortized(self.cap.0, 1, elem_layout) {
            handle_error(err);
        }
    }

    fn try_reserve(
        &mut self,
        len: usize,
        additional: usize,
        elem_layout: Layout,
    ) -> Result<(), TryReserveError> {
        if self.needs_to_grow(len, additional, elem_layout) {
            self.grow_amortized(len, additional, elem_layout)?;
        }
        unsafe {
            // Inform the optimizer that the reservation has succeeded or wasn't needed
            hint::assert_unchecked(!self.needs_to_grow(len, additional, elem_layout));
        }
        Ok(())
    }

    #[cfg(not(no_global_oom_handling))]
    fn reserve_exact(&mut self, len: usize, additional: usize, elem_layout: Layout) {
        if let Err(err) = self.try_reserve_exact(len, additional, elem_layout) {
            handle_error(err);
        }
    }

    fn try_reserve_exact(
        &mut self,
        len: usize,
        additional: usize,
        elem_layout: Layout,
    ) -> Result<(), TryReserveError> {
        if self.needs_to_grow(len, additional, elem_layout) {
            self.grow_exact(len, additional, elem_layout)?;
        }
        unsafe {
            // Inform the optimizer that the reservation has succeeded or wasn't needed
            hint::assert_unchecked(!self.needs_to_grow(len, additional, elem_layout));
        }
        Ok(())
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn shrink_to_fit(&mut self, cap: usize, elem_layout: Layout) {
        if let Err(err) = self.shrink(cap, elem_layout) {
            handle_error(err);
        }
    }

    #[inline]
    fn needs_to_grow(&self, len: usize, additional: usize, elem_layout: Layout) -> bool {
        additional > self.capacity(elem_layout.size()).wrapping_sub(len)
    }

    #[inline]
    unsafe fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
        // Allocators currently return a `NonNull<[u8]>` whose length matches
        // the size requested. If that ever changes, the capacity here should
        // change to `ptr.len() / mem::size_of::<T>()`.
        self.ptr = Unique::from(ptr.cast());
        self.cap = unsafe { Cap(cap) };
    }

    fn grow_amortized(
        &mut self,
        len: usize,
        additional: usize,
        elem_layout: Layout,
    ) -> Result<(), TryReserveError> {
        // This is ensured by the calling contexts.
        debug_assert!(additional > 0);

        if elem_layout.size() == 0 {
            // Since we return a capacity of `usize::MAX` when `elem_size` is
            // 0, getting to here necessarily means the `RawVec` is overfull.
            return Err(CapacityOverflow.into());
        }

        // Nothing we can really do about these checks, sadly.
        let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;

        // This guarantees exponential growth. The doubling cannot overflow
        // because `cap <= isize::MAX` and the type of `cap` is `usize`.
        let cap = cmp::max(self.cap.0 * 2, required_cap);
        let cap = cmp::max(min_non_zero_cap(elem_layout.size()), cap);

        let new_layout = layout_array(cap, elem_layout)?;

        let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
        // SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items

        unsafe { self.set_ptr_and_cap(ptr, cap) };
        Ok(())
    }

    fn grow_exact(
        &mut self,
        len: usize,
        additional: usize,
        elem_layout: Layout,
    ) -> Result<(), TryReserveError> {
        if elem_layout.size() == 0 {
            // Since we return a capacity of `usize::MAX` when the type size is
            // 0, getting to here necessarily means the `RawVec` is overfull.
            return Err(CapacityOverflow.into());
        }

        let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
        let new_layout = layout_array(cap, elem_layout)?;

        let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
        // SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
        unsafe {
            self.set_ptr_and_cap(ptr, cap);
        }
        Ok(())
    }

    #[cfg(not(no_global_oom_handling))]
    #[inline]
    fn shrink(&mut self, cap: usize, elem_layout: Layout) -> Result<(), TryReserveError> {
        assert!(cap <= self.capacity(elem_layout.size()), "Tried to shrink to a larger capacity");
        // SAFETY: Just checked this isn't trying to grow
        unsafe { self.shrink_unchecked(cap, elem_layout) }
    }

    /// `shrink`, but without the capacity check.
    ///
    /// This is split out so that `shrink` can inline the check, since it
    /// optimizes out in things like `shrink_to_fit`, without needing to
    /// also inline all this code, as doing that ends up failing the
    /// `vec-shrink-panic` codegen test when `shrink_to_fit` ends up being too
    /// big for LLVM to be willing to inline.
    ///
    /// # Safety
    /// `cap <= self.capacity()`
    #[cfg(not(no_global_oom_handling))]
    unsafe fn shrink_unchecked(
        &mut self,
        cap: usize,
        elem_layout: Layout,
    ) -> Result<(), TryReserveError> {
        let (ptr, layout) =
            if let Some(mem) = self.current_memory(elem_layout) { mem } else { return Ok(()) };

        // If shrinking to 0, deallocate the buffer. We don't reach this point
        // for the T::IS_ZST case since current_memory() will have returned
        // None.
        if cap == 0 {
            unsafe { self.alloc.deallocate(ptr, layout) };
            self.ptr =
                unsafe { Unique::new_unchecked(ptr::without_provenance_mut(elem_layout.align())) };
            self.cap = Cap::ZERO;
        } else {
            let ptr = unsafe {
                // Layout cannot overflow here because it would have
                // overflowed earlier when capacity was larger.
                let new_size = elem_layout.size().unchecked_mul(cap);
                let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
                self.alloc
                    .shrink(ptr, layout, new_layout)
                    .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
            };
            // SAFETY: if the allocation is valid, then the capacity is too
            unsafe {
                self.set_ptr_and_cap(ptr, cap);
            }
        }
        Ok(())
    }

    /// # Safety
    ///
    /// This function deallocates the owned allocation, but does not update `ptr` or `cap` to
    /// prevent double-free or use-after-free. Essentially, do not do anything with the caller
    /// after this function returns.
    /// Ideally this function would take `self` by move, but it cannot because it exists to be
    /// called from a `Drop` impl.
    unsafe fn deallocate(&mut self, elem_layout: Layout) {
        if let Some((ptr, layout)) = self.current_memory(elem_layout) {
            unsafe {
                self.alloc.deallocate(ptr, layout);
            }
        }
    }
}

#[inline(never)]
fn finish_grow<A>(
    new_layout: Layout,
    current_memory: Option<(NonNull<u8>, Layout)>,
    alloc: &mut A,
) -> Result<NonNull<[u8]>, TryReserveError>
where
    A: Allocator,
{
    alloc_guard(new_layout.size())?;

    let memory = if let Some((ptr, old_layout)) = current_memory {
        debug_assert_eq!(old_layout.align(), new_layout.align());
        unsafe {
            // The allocator checks for alignment equality
            hint::assert_unchecked(old_layout.align() == new_layout.align());
            alloc.grow(ptr, old_layout, new_layout)
        }
    } else {
        alloc.allocate(new_layout)
    };

    memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
}

// Central function for reserve error handling.
#[cfg(not(no_global_oom_handling))]
#[cold]
#[optimize(size)]
fn handle_error(e: TryReserveError) -> ! {
    match e.kind() {
        CapacityOverflow => capacity_overflow(),
        AllocError { layout, .. } => handle_alloc_error(layout),
    }
}

// We need to guarantee the following:
// * We don't ever allocate `> isize::MAX` byte-size objects.
// * We don't overflow `usize::MAX` and actually allocate too little.
//
// On 64-bit we just need to check for overflow since trying to allocate
// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
// an extra guard for this in case we're running on a platform which can use
// all 4GB in user-space, e.g., PAE or x32.
#[inline]
fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
    if usize::BITS < 64 && alloc_size > isize::MAX as usize {
        Err(CapacityOverflow.into())
    } else {
        Ok(())
    }
}

#[inline]
fn layout_array(cap: usize, elem_layout: Layout) -> Result<Layout, TryReserveError> {
    elem_layout.repeat(cap).map(|(layout, _pad)| layout).map_err(|_| CapacityOverflow.into())
}