      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

    #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
    #![doc(hidden)]

    use core::cmp;
    use core::mem;
    use core::ops::Drop;
    use core::ptr::{self, NonNull, Unique};
    use core::slice;

    use crate::alloc::{handle_alloc_error, AllocErr, AllocRef, Global, Layout};
    use crate::boxed::Box;
    use crate::collections::TryReserveError::{self, *};

    #[cfg(test)]
    mod tests;

    /// 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::empty()` on zero-sized types.
    /// * Produces `Unique::empty()` on zero-length allocations.
    /// * 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.
    /// * Aborts on OOM or calls `handle_alloc_error` as applicable.
    /// * Avoids freeing `Unique::empty()`.
    /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
    ///
    /// 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 a `RawVec` always forces its capacity to be `usize::MAX` for zero-sized types.
    /// This enables you to use capacity-growing logic catch the overflows in your length
    /// that might occur with zero-sized types.
    ///
    /// The above means that you need to be careful when round-tripping this type with a
    /// `Box<[T]>`, since `capacity()` won't yield the length. However, `with_capacity`,
    /// `shrink_to_fit`, and `from_box` will actually set `RawVec`'s private capacity
    /// field. This allows zero-sized types to not be special-cased by consumers of
    /// this type.
    #[allow(missing_debug_implementations)]
    pub struct RawVec<T, A: AllocRef = Global> {
        ptr: Unique<T>,
        cap: usize,
        a: A,
    }

    impl<T, A: AllocRef> RawVec<T, A> {
        /// Like `new`, but parameterized over the choice of allocator for
        /// the returned `RawVec`.
        pub const fn new_in(a: A) -> Self {
            let cap = if mem::size_of::<T>() == 0 { core::usize::MAX } else { 0 };

            // `Unique::empty()` doubles as "unallocated" and "zero-sized allocation".
            RawVec { ptr: Unique::empty(), cap, a }
        }

        /// Like `with_capacity`, but parameterized over the choice of
        /// allocator for the returned `RawVec`.
        #[inline]
        pub fn with_capacity_in(capacity: usize, a: A) -> Self {
            RawVec::allocate_in(capacity, false, a)
        }

        /// Like `with_capacity_zeroed`, but parameterized over the choice
        /// of allocator for the returned `RawVec`.
        #[inline]
        pub fn with_capacity_zeroed_in(capacity: usize, a: A) -> Self {
            RawVec::allocate_in(capacity, true, a)
        }

        fn allocate_in(mut capacity: usize, zeroed: bool, mut a: A) -> Self {
            let elem_size = mem::size_of::<T>();

            let alloc_size = capacity.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
            alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());

            // Handles ZSTs and `capacity == 0` alike.
            let ptr = if alloc_size == 0 {
                NonNull::<T>::dangling()
            } else {
                let align = mem::align_of::<T>();
                let layout = Layout::from_size_align(alloc_size, align).unwrap();
                let result = if zeroed { a.alloc_zeroed(layout) } else { a.alloc(layout) };
                match result {
                    Ok((ptr, size)) => {
                        capacity = size / elem_size;
                        ptr.cast()
                    }
                    Err(_) => handle_alloc_error(layout),
                }
            };

            RawVec { ptr: ptr.into(), cap: capacity, a }
        }
    }

    impl<T> RawVec<T, Global> {
        /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
        /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
        ///
        /// If you change `RawVec<T>::new` or dependencies, please take care to not
        /// introduce anything that would truly violate `min_const_fn`.
        ///
        /// NOTE: We could avoid this hack and check conformance with some
        /// `#[rustc_force_min_const_fn]` attribute which requires conformance
        /// with `min_const_fn` but does not necessarily allow calling it in
        /// `stable(...) const fn` / user code not enabling `foo` when
        /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
        pub const NEW: Self = Self::new();

        /// 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.
        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.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        #[inline]
        pub fn with_capacity(capacity: usize) -> Self {
            RawVec::allocate_in(capacity, false, Global)
        }

        /// Like `with_capacity`, but guarantees the buffer is zeroed.
        #[inline]
        pub fn with_capacity_zeroed(capacity: usize) -> Self {
            RawVec::allocate_in(capacity, true, Global)
        }
    }

    impl<T, A: AllocRef> RawVec<T, A> {
        /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
        ///
        /// # Undefined Behavior
        ///
        /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
        /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
        /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
        pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
            RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a }
        }
    }

    impl<T> RawVec<T, Global> {
        /// Reconstitutes a `RawVec` from a pointer and capacity.
        ///
        /// # Undefined Behavior
        ///
        /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
        /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
        /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
        pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
            RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a: Global }
        }

        /// Converts a `Box<[T]>` into a `RawVec<T>`.
        pub fn from_box(mut slice: Box<[T]>) -> Self {
            unsafe {
                let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
                mem::forget(slice);
                result
            }
        }
    }

    impl<T, A: AllocRef> RawVec<T, A> {
        /// Gets a raw pointer to the start of the allocation. Note that this is
        /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
        /// be careful.
        pub fn ptr(&self) -> *mut T {
            self.ptr.as_ptr()
        }

        /// Gets the capacity of the allocation.
        ///
        /// This will always be `usize::MAX` if `T` is zero-sized.
        #[inline(always)]
        pub fn capacity(&self) -> usize {
            if mem::size_of::<T>() == 0 { !0 } else { self.cap }
        }

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

        /// Returns a mutable reference to the allocator backing this `RawVec`.
        pub fn alloc_mut(&mut self) -> &mut A {
            &mut self.a
        }

        fn current_layout(&self) -> Option<Layout> {
            if self.cap == 0 {
                None
            } else {
                // We have an allocated chunk of memory, so we can bypass runtime
                // checks to get our current layout.
                unsafe {
                    let align = mem::align_of::<T>();
                    let size = mem::size_of::<T>() * self.cap;
                    Some(Layout::from_size_align_unchecked(size, align))
                }
            }
        }

        /// Doubles the size of the type's backing allocation. This is common enough
        /// to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// This function is ideal for when pushing elements one-at-a-time because
        /// you don't need to incur the costs of the more general computations
        /// reserve needs to do to guard against overflow. You do however need to
        /// manually check if your `len == capacity`.
        ///
        /// # Panics
        ///
        /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(raw_vec_internals)]
        /// # extern crate alloc;
        /// # use std::ptr;
        /// # use alloc::raw_vec::RawVec;
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T> MyVec<T> {
        ///     pub fn push(&mut self, elem: T) {
        ///         if self.len == self.buf.capacity() { self.buf.double(); }
        ///         // double would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         unsafe {
        ///             ptr::write(self.buf.ptr().add(self.len), elem);
        ///         }
        ///         self.len += 1;
        ///     }
        /// }
        /// # fn main() {
        /// #   let mut vec = MyVec { buf: RawVec::new(), len: 0 };
        /// #   vec.push(1);
        /// # }
        /// ```
        #[inline(never)]
        #[cold]
        pub fn double(&mut self) {
            unsafe {
                let elem_size = mem::size_of::<T>();

                // Since we set the capacity to `usize::MAX` when `elem_size` is
                // 0, getting to here necessarily means the `RawVec` is overfull.
                assert!(elem_size != 0, "capacity overflow");

                let (ptr, new_cap) = match self.current_layout() {
                    Some(cur) => {
                        // Since we guarantee that we never allocate more than
                        // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
                        // a precondition, so this can't overflow. Additionally the
                        // alignment will never be too large as to "not be
                        // satisfiable", so `Layout::from_size_align` will always
                        // return `Some`.
                        //
                        // TL;DR, we bypass runtime checks due to dynamic assertions
                        // in this module, allowing us to use
                        // `from_size_align_unchecked`.
                        let new_cap = 2 * self.cap;
                        let new_size = new_cap * elem_size;
                        alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
                        let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(), cur, new_size);
                        match ptr_res {
                            Ok((ptr, new_size)) => (ptr, new_size / elem_size),
                            Err(_) => handle_alloc_error(Layout::from_size_align_unchecked(
                                new_size,
                                cur.align(),
                            )),
                        }
                    }
                    None => {
                        // Skip to 4 because tiny `Vec`'s are dumb; but not if that
                        // would cause overflow.
                        let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
                        let layout = Layout::array::<T>(new_cap).unwrap();
                        match self.a.alloc(layout) {
                            Ok((ptr, new_size)) => (ptr, new_size / elem_size),
                            Err(_) => handle_alloc_error(layout),
                        }
                    }
                };
                self.ptr = ptr.cast().into();
                self.cap = new_cap;
            }
        }

        /// Attempts to double the size of the type's backing allocation in place. This is common
        /// enough to want to do that it's easiest to just have a dedicated method. Slightly
        /// more efficient logic can be provided for this than the general case.
        ///
        /// Returns `true` if the reallocation attempt has succeeded.
        ///
        /// # Panics
        ///
        /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
        ///   all `usize::MAX` slots in your imaginary buffer.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        #[inline(never)]
        #[cold]
        pub fn double_in_place(&mut self) -> bool {
            unsafe {
                let elem_size = mem::size_of::<T>();
                let old_layout = match self.current_layout() {
                    Some(layout) => layout,
                    None => return false, // nothing to double
                };

                // Since we set the capacity to `usize::MAX` when `elem_size` is
                // 0, getting to here necessarily means the `RawVec` is overfull.
                assert!(elem_size != 0, "capacity overflow");

                // Since we guarantee that we never allocate more than `isize::MAX`
                // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
                // this can't overflow.
                //
                // Similarly to with `double` above, we can go straight to
                // `Layout::from_size_align_unchecked` as we know this won't
                // overflow and the alignment is sufficiently small.
                let new_cap = 2 * self.cap;
                let new_size = new_cap * elem_size;
                alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
                match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
                    Ok(_) => {
                        // We can't directly divide `size`.
                        self.cap = new_cap;
                        true
                    }
                    Err(_) => false,
                }
            }
        }

        /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
        pub fn try_reserve_exact(
            &mut self,
            used_capacity: usize,
            needed_extra_capacity: usize,
        ) -> Result<(), TryReserveError> {
            self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_capacity + needed_extra_capacity` 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 `used_capacity` 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 requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
            match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
                Err(CapacityOverflow) => capacity_overflow(),
                Err(AllocError { .. }) => unreachable!(),
                Ok(()) => { /* yay */ }
            }
        }

        /// Calculates the buffer's new size given that it'll hold `used_capacity +
        /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
        /// Returns `(new_capacity, new_alloc_size)`.
        fn amortized_new_size(
            &self,
            used_capacity: usize,
            needed_extra_capacity: usize,
        ) -> Result<usize, TryReserveError> {
            // Nothing we can really do about these checks, sadly.
            let required_cap =
                used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
            // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
            let double_cap = self.cap * 2;
            // `double_cap` guarantees exponential growth.
            Ok(cmp::max(double_cap, required_cap))
        }

        /// The same as `reserve`, but returns on errors instead of panicking or aborting.
        pub fn try_reserve(
            &mut self,
            used_capacity: usize,
            needed_extra_capacity: usize,
        ) -> Result<(), TryReserveError> {
            self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
        }

        /// Ensures that the buffer contains at least enough space to hold
        /// `used_capacity + needed_extra_capacity` 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 `used_capacity` 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 requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        ///
        /// # Aborts
        ///
        /// Aborts on OOM.
        ///
        /// # Examples
        ///
        /// ```
        /// # #![feature(raw_vec_internals)]
        /// # extern crate alloc;
        /// # use std::ptr;
        /// # use alloc::raw_vec::RawVec;
        /// struct MyVec<T> {
        ///     buf: RawVec<T>,
        ///     len: usize,
        /// }
        ///
        /// impl<T: Clone> MyVec<T> {
        ///     pub fn push_all(&mut self, elems: &[T]) {
        ///         self.buf.reserve(self.len, elems.len());
        ///         // reserve would have aborted or panicked if the len exceeded
        ///         // `isize::MAX` so this is safe to do unchecked now.
        ///         for x in elems {
        ///             unsafe {
        ///                 ptr::write(self.buf.ptr().add(self.len), x.clone());
        ///             }
        ///             self.len += 1;
        ///         }
        ///     }
        /// }
        /// # fn main() {
        /// #   let mut vector = MyVec { buf: RawVec::new(), len: 0 };
        /// #   vector.push_all(&[1, 3, 5, 7, 9]);
        /// # }
        /// ```
        pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
            match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
                Err(CapacityOverflow) => capacity_overflow(),
                Err(AllocError { .. }) => unreachable!(),
                Ok(()) => { /* yay */ }
            }
        }
        /// Attempts to ensure that the buffer contains at least enough space to hold
        /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
        /// enough capacity, will reallocate in place enough space plus comfortable slack
        /// space to get amortized `O(1)` behavior. Will limit this behaviour
        /// if it would needlessly cause itself to panic.
        ///
        /// If `used_capacity` 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.
        ///
        /// Returns `true` if the reallocation attempt has succeeded.
        ///
        /// # Panics
        ///
        /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
        /// * Panics on 32-bit platforms if the requested capacity exceeds
        ///   `isize::MAX` bytes.
        pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
            unsafe {
                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity. If the current `cap` is 0, we can't
                // reallocate in place.
                // Wrapping in case they give a bad `used_capacity`
                let old_layout = match self.current_layout() {
                    Some(layout) => layout,
                    None => return false,
                };
                if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
                    return false;
                }

                let new_cap = self
                    .amortized_new_size(used_capacity, needed_extra_capacity)
                    .unwrap_or_else(|_| capacity_overflow());

                // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
                // (regardless of whether `self.cap - used_capacity` wrapped).
                // Therefore, we can safely call `grow_in_place`.

                let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
                // FIXME: may crash and burn on over-reserve
                alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
                match self.a.grow_in_place(
                    NonNull::from(self.ptr).cast(),
                    old_layout,
                    new_layout.size(),
                ) {
                    Ok(_) => {
                        self.cap = new_cap;
                        true
                    }
                    Err(_) => false,
                }
            }
        }

        /// Shrinks the allocation down to the specified amount. 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.
        pub fn shrink_to_fit(&mut self, amount: usize) {
            let elem_size = mem::size_of::<T>();

            // Set the `cap` because they might be about to promote to a `Box<[T]>`
            if elem_size == 0 {
                self.cap = amount;
                return;
            }

            // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
            assert!(self.cap >= amount, "Tried to shrink to a larger capacity");

            if amount == 0 {
                // We want to create a new zero-length vector within the
                // same allocator. We use `ptr::write` to avoid an
                // erroneous attempt to drop the contents, and we use
                // `ptr::read` to sidestep condition against destructuring
                // types that implement Drop.

                unsafe {
                    let a = ptr::read(&self.a as *const A);
                    self.dealloc_buffer();
                    ptr::write(self, RawVec::new_in(a));
                }
            } else if self.cap != amount {
                unsafe {
                    // We know here that our `amount` is greater than zero. This
                    // implies, via the assert above, that capacity is also greater
                    // than zero, which means that we've got a current layout that
                    // "fits"
                    //
                    // We also know that `self.cap` is greater than `amount`, and
                    // consequently we don't need runtime checks for creating either
                    // layout.
                    let old_size = elem_size * self.cap;
                    let new_size = elem_size * amount;
                    let align = mem::align_of::<T>();
                    let old_layout = Layout::from_size_align_unchecked(old_size, align);
                    match self.a.realloc(NonNull::from(self.ptr).cast(), old_layout, new_size) {
                        Ok((ptr, _)) => self.ptr = ptr.cast().into(),
                        Err(_) => {
                            handle_alloc_error(Layout::from_size_align_unchecked(new_size, align))
                        }
                    }
                }
                self.cap = amount;
            }
        }
    }

    enum Fallibility {
        Fallible,
        Infallible,
    }

    use Fallibility::*;

    enum ReserveStrategy {
        Exact,
        Amortized,
    }

    use ReserveStrategy::*;

    impl<T, A: AllocRef> RawVec<T, A> {
        fn reserve_internal(
            &mut self,
            used_capacity: usize,
            needed_extra_capacity: usize,
            fallibility: Fallibility,
            strategy: ReserveStrategy,
        ) -> Result<(), TryReserveError> {
            let elem_size = mem::size_of::<T>();

            unsafe {
                // NOTE: we don't early branch on ZSTs here because we want this
                // to actually catch "asking for more than usize::MAX" in that case.
                // If we make it past the first branch then we are guaranteed to
                // panic.

                // Don't actually need any more capacity.
                // Wrapping in case they gave a bad `used_capacity`.
                if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
                    return Ok(());
                }

                // Nothing we can really do about these checks, sadly.
                let new_cap = match strategy {
                    Exact => {
                        used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?
                    }
                    Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
                };
                let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;

                alloc_guard(new_layout.size())?;

                let res = match self.current_layout() {
                    Some(layout) => {
                        debug_assert!(new_layout.align() == layout.align());
                        self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
                    }
                    None => self.a.alloc(new_layout),
                };

                let (ptr, new_cap) = match (res, fallibility) {
                    (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
                    (Err(AllocErr), Fallible) => {
                        return Err(TryReserveError::AllocError {
                            layout: new_layout,
                            non_exhaustive: (),
                        });
                    }
                    (Ok((ptr, new_size)), _) => (ptr, new_size / elem_size),
                };

                self.ptr = ptr.cast().into();
                self.cap = new_cap;

                Ok(())
            }
        }
    }

    impl<T> RawVec<T, Global> {
        /// Converts the entire buffer into `Box<[T]>`.
        ///
        /// Note that this will correctly reconstitute any `cap` changes
        /// that may have been performed. (See description of type for details.)
        ///
        /// # Undefined Behavior
        ///
        /// All elements of `RawVec<T, Global>` must be initialized. Notice that
        /// the rules around uninitialized boxed values are not finalized yet,
        /// but until they are, it is advisable to avoid them.
        pub unsafe fn into_box(self) -> Box<[T]> {
            // NOTE: not calling `capacity()` here; actually using the real `cap` field!
            let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
            let output: Box<[T]> = Box::from_raw(slice);
            mem::forget(self);
            output
        }
    }

    impl<T, A: AllocRef> RawVec<T, A> {
        /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
        pub unsafe fn dealloc_buffer(&mut self) {
            let elem_size = mem::size_of::<T>();
            if elem_size != 0 {
                if let Some(layout) = self.current_layout() {
                    self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
                }
            }
        }
    }

    unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
        /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
        fn drop(&mut self) {
            unsafe {
                self.dealloc_buffer();
            }
        }
    }

    // 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 mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
            Err(CapacityOverflow)
        } else {
            Ok(())
        }
    }

    // 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.
    fn capacity_overflow() -> ! {
        panic!("capacity overflow");
    }