1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
14 use core::ptr::{self, NonNull, Unique};
17 use alloc::{Alloc, Layout, Global, oom};
18 use alloc::CollectionAllocErr;
19 use alloc::CollectionAllocErr::*;
22 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
23 /// a buffer of memory on the heap without having to worry about all the corner cases
24 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
27 /// * Produces Unique::empty() on zero-sized types
28 /// * Produces Unique::empty() on zero-length allocations
29 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics)
30 /// * Guards against 32-bit systems allocating more than isize::MAX bytes
31 /// * Guards against overflowing your length
33 /// * Avoids freeing Unique::empty()
34 /// * Contains a ptr::Unique and thus endows the user with all related benefits
36 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
37 /// free its memory, but it *won't* try to Drop its contents. It is up to the user of RawVec
38 /// to handle the actual things *stored* inside of a RawVec.
40 /// Note that a RawVec always forces its capacity to be usize::MAX for zero-sized types.
41 /// This enables you to use capacity growing logic catch the overflows in your length
42 /// that might occur with zero-sized types.
44 /// However this means that you need to be careful when roundtripping this type
45 /// with a `Box<[T]>`: `cap()` won't yield the len. However `with_capacity`,
46 /// `shrink_to_fit`, and `from_box` will actually set RawVec's private capacity
47 /// field. This allows zero-sized types to not be special-cased by consumers of
49 #[allow(missing_debug_implementations)]
50 pub struct RawVec<T, A: Alloc = Global> {
56 impl<T, A: Alloc> RawVec<T, A> {
57 /// Like `new` but parameterized over the choice of allocator for
58 /// the returned RawVec.
59 pub fn new_in(a: A) -> Self {
60 // !0 is usize::MAX. This branch should be stripped at compile time.
61 let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
63 // Unique::empty() doubles as "unallocated" and "zero-sized allocation"
71 /// Like `with_capacity` but parameterized over the choice of
72 /// allocator for the returned RawVec.
74 pub fn with_capacity_in(cap: usize, a: A) -> Self {
75 RawVec::allocate_in(cap, false, a)
78 /// Like `with_capacity_zeroed` but parameterized over the choice
79 /// of allocator for the returned RawVec.
81 pub fn with_capacity_zeroed_in(cap: usize, a: A) -> Self {
82 RawVec::allocate_in(cap, true, a)
85 fn allocate_in(cap: usize, zeroed: bool, mut a: A) -> Self {
87 let elem_size = mem::size_of::<T>();
89 let alloc_size = cap.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
90 alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
92 // handles ZSTs and `cap = 0` alike
93 let ptr = if alloc_size == 0 {
94 NonNull::<T>::dangling().as_opaque()
96 let align = mem::align_of::<T>();
97 let result = if zeroed {
98 a.alloc_zeroed(Layout::from_size_align(alloc_size, align).unwrap())
100 a.alloc(Layout::from_size_align(alloc_size, align).unwrap())
109 ptr: ptr.cast().into(),
117 impl<T> RawVec<T, Global> {
118 /// Creates the biggest possible RawVec (on the system heap)
119 /// without allocating. If T has positive size, then this makes a
120 /// RawVec with capacity 0. If T has 0 size, then it makes a
121 /// RawVec with capacity `usize::MAX`. Useful for implementing
122 /// delayed allocation.
123 pub fn new() -> Self {
127 /// Creates a RawVec (on the system heap) with exactly the
128 /// capacity and alignment requirements for a `[T; cap]`. This is
129 /// equivalent to calling RawVec::new when `cap` is 0 or T is
130 /// zero-sized. Note that if `T` is zero-sized this means you will
131 /// *not* get a RawVec with the requested capacity!
135 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
136 /// * Panics on 32-bit platforms if the requested capacity exceeds
137 /// `isize::MAX` bytes.
143 pub fn with_capacity(cap: usize) -> Self {
144 RawVec::allocate_in(cap, false, Global)
147 /// Like `with_capacity` but guarantees the buffer is zeroed.
149 pub fn with_capacity_zeroed(cap: usize) -> Self {
150 RawVec::allocate_in(cap, true, Global)
154 impl<T, A: Alloc> RawVec<T, A> {
155 /// Reconstitutes a RawVec from a pointer, capacity, and allocator.
157 /// # Undefined Behavior
159 /// The ptr must be allocated (via the given allocator `a`), and with the given capacity. The
160 /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
161 /// If the ptr and capacity come from a RawVec created via `a`, then this is guaranteed.
162 pub unsafe fn from_raw_parts_in(ptr: *mut T, cap: usize, a: A) -> Self {
164 ptr: Unique::new_unchecked(ptr),
171 impl<T> RawVec<T, Global> {
172 /// Reconstitutes a RawVec from a pointer, capacity.
174 /// # Undefined Behavior
176 /// The ptr must be allocated (on the system heap), and with the given capacity. The
177 /// capacity cannot exceed `isize::MAX` (only a concern on 32-bit systems).
178 /// If the ptr and capacity come from a RawVec, then this is guaranteed.
179 pub unsafe fn from_raw_parts(ptr: *mut T, cap: usize) -> Self {
181 ptr: Unique::new_unchecked(ptr),
187 /// Converts a `Box<[T]>` into a `RawVec<T>`.
188 pub fn from_box(mut slice: Box<[T]>) -> Self {
190 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
197 impl<T, A: Alloc> RawVec<T, A> {
198 /// Gets a raw pointer to the start of the allocation. Note that this is
199 /// Unique::empty() if `cap = 0` or T is zero-sized. In the former case, you must
201 pub fn ptr(&self) -> *mut T {
205 /// Gets the capacity of the allocation.
207 /// This will always be `usize::MAX` if `T` is zero-sized.
209 pub fn cap(&self) -> usize {
210 if mem::size_of::<T>() == 0 {
217 /// Returns a shared reference to the allocator backing this RawVec.
218 pub fn alloc(&self) -> &A {
222 /// Returns a mutable reference to the allocator backing this RawVec.
223 pub fn alloc_mut(&mut self) -> &mut A {
227 fn current_layout(&self) -> Option<Layout> {
231 // We have an allocated chunk of memory, so we can bypass runtime
232 // checks to get our current layout.
234 let align = mem::align_of::<T>();
235 let size = mem::size_of::<T>() * self.cap;
236 Some(Layout::from_size_align_unchecked(size, align))
241 /// Doubles the size of the type's backing allocation. This is common enough
242 /// to want to do that it's easiest to just have a dedicated method. Slightly
243 /// more efficient logic can be provided for this than the general case.
245 /// This function is ideal for when pushing elements one-at-a-time because
246 /// you don't need to incur the costs of the more general computations
247 /// reserve needs to do to guard against overflow. You do however need to
248 /// manually check if your `len == cap`.
252 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
253 /// all `usize::MAX` slots in your imaginary buffer.
254 /// * Panics on 32-bit platforms if the requested capacity exceeds
255 /// `isize::MAX` bytes.
264 /// # #![feature(alloc)]
265 /// # extern crate alloc;
267 /// # use alloc::raw_vec::RawVec;
268 /// struct MyVec<T> {
273 /// impl<T> MyVec<T> {
274 /// pub fn push(&mut self, elem: T) {
275 /// if self.len == self.buf.cap() { self.buf.double(); }
276 /// // double would have aborted or panicked if the len exceeded
277 /// // `isize::MAX` so this is safe to do unchecked now.
279 /// ptr::write(self.buf.ptr().offset(self.len as isize), elem);
285 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
291 pub fn double(&mut self) {
293 let elem_size = mem::size_of::<T>();
295 // since we set the capacity to usize::MAX when elem_size is
296 // 0, getting to here necessarily means the RawVec is overfull.
297 assert!(elem_size != 0, "capacity overflow");
299 let (new_cap, uniq) = match self.current_layout() {
301 // Since we guarantee that we never allocate more than
302 // isize::MAX bytes, `elem_size * self.cap <= isize::MAX` as
303 // a precondition, so this can't overflow. Additionally the
304 // alignment will never be too large as to "not be
305 // satisfiable", so `Layout::from_size_align` will always
308 // tl;dr; we bypass runtime checks due to dynamic assertions
309 // in this module, allowing us to use
310 // `from_size_align_unchecked`.
311 let new_cap = 2 * self.cap;
312 let new_size = new_cap * elem_size;
313 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
314 let ptr_res = self.a.realloc(NonNull::from(self.ptr).as_opaque(),
318 Ok(ptr) => (new_cap, ptr.cast().into()),
323 // skip to 4 because tiny Vec's are dumb; but not if that
324 // would cause overflow
325 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
326 match self.a.alloc_array::<T>(new_cap) {
327 Ok(ptr) => (new_cap, ptr.into()),
337 /// Attempts to double the size of the type's backing allocation in place. This is common
338 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
339 /// more efficient logic can be provided for this than the general case.
341 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
345 /// * Panics if T is zero-sized on the assumption that you managed to exhaust
346 /// all `usize::MAX` slots in your imaginary buffer.
347 /// * Panics on 32-bit platforms if the requested capacity exceeds
348 /// `isize::MAX` bytes.
351 pub fn double_in_place(&mut self) -> bool {
353 let elem_size = mem::size_of::<T>();
354 let old_layout = match self.current_layout() {
355 Some(layout) => layout,
356 None => return false, // nothing to double
359 // since we set the capacity to usize::MAX when elem_size is
360 // 0, getting to here necessarily means the RawVec is overfull.
361 assert!(elem_size != 0, "capacity overflow");
363 // Since we guarantee that we never allocate more than isize::MAX
364 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
365 // this can't overflow.
367 // Similarly like with `double` above we can go straight to
368 // `Layout::from_size_align_unchecked` as we know this won't
369 // overflow and the alignment is sufficiently small.
370 let new_cap = 2 * self.cap;
371 let new_size = new_cap * elem_size;
372 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
373 match self.a.grow_in_place(NonNull::from(self.ptr).as_opaque(), old_layout, new_size) {
375 // We can't directly divide `size`.
386 /// Ensures that the buffer contains at least enough space to hold
387 /// `used_cap + needed_extra_cap` elements. If it doesn't already,
388 /// will reallocate the minimum possible amount of memory necessary.
389 /// Generally this will be exactly the amount of memory necessary,
390 /// but in principle the allocator is free to give back more than
393 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
394 /// the requested space. This is not really unsafe, but the unsafe
395 /// code *you* write that relies on the behavior of this function may break.
399 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
400 /// * Panics on 32-bit platforms if the requested capacity exceeds
401 /// `isize::MAX` bytes.
406 pub fn try_reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize)
407 -> Result<(), CollectionAllocErr> {
410 // NOTE: we don't early branch on ZSTs here because we want this
411 // to actually catch "asking for more than usize::MAX" in that case.
412 // If we make it past the first branch then we are guaranteed to
415 // Don't actually need any more capacity.
416 // Wrapping in case they gave a bad `used_cap`.
417 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
421 // Nothing we can really do about these checks :(
422 let new_cap = used_cap.checked_add(needed_extra_cap).ok_or(CapacityOverflow)?;
423 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
425 alloc_guard(new_layout.size())?;
427 let res = match self.current_layout() {
429 debug_assert!(new_layout.align() == layout.align());
430 self.a.realloc(NonNull::from(self.ptr).as_opaque(), layout, new_layout.size())
432 None => self.a.alloc(new_layout),
435 self.ptr = res?.cast().into();
442 pub fn reserve_exact(&mut self, used_cap: usize, needed_extra_cap: usize) {
443 match self.try_reserve_exact(used_cap, needed_extra_cap) {
444 Err(CapacityOverflow) => capacity_overflow(),
445 Err(AllocErr) => oom(),
446 Ok(()) => { /* yay */ }
450 /// Calculates the buffer's new size given that it'll hold `used_cap +
451 /// needed_extra_cap` elements. This logic is used in amortized reserve methods.
452 /// Returns `(new_capacity, new_alloc_size)`.
453 fn amortized_new_size(&self, used_cap: usize, needed_extra_cap: usize)
454 -> Result<usize, CollectionAllocErr> {
456 // Nothing we can really do about these checks :(
457 let required_cap = used_cap.checked_add(needed_extra_cap).ok_or(CapacityOverflow)?;
458 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
459 let double_cap = self.cap * 2;
460 // `double_cap` guarantees exponential growth.
461 Ok(cmp::max(double_cap, required_cap))
464 /// Ensures that the buffer contains at least enough space to hold
465 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
466 /// enough capacity, will reallocate enough space plus comfortable slack
467 /// space to get amortized `O(1)` behavior. Will limit this behavior
468 /// if it would needlessly cause itself to panic.
470 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
471 /// the requested space. This is not really unsafe, but the unsafe
472 /// code *you* write that relies on the behavior of this function may break.
474 /// This is ideal for implementing a bulk-push operation like `extend`.
478 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
479 /// * Panics on 32-bit platforms if the requested capacity exceeds
480 /// `isize::MAX` bytes.
489 /// # #![feature(alloc)]
490 /// # extern crate alloc;
492 /// # use alloc::raw_vec::RawVec;
493 /// struct MyVec<T> {
498 /// impl<T: Clone> MyVec<T> {
499 /// pub fn push_all(&mut self, elems: &[T]) {
500 /// self.buf.reserve(self.len, elems.len());
501 /// // reserve would have aborted or panicked if the len exceeded
502 /// // `isize::MAX` so this is safe to do unchecked now.
505 /// ptr::write(self.buf.ptr().offset(self.len as isize), x.clone());
512 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
513 /// # vector.push_all(&[1, 3, 5, 7, 9]);
516 pub fn try_reserve(&mut self, used_cap: usize, needed_extra_cap: usize)
517 -> Result<(), CollectionAllocErr> {
519 // NOTE: we don't early branch on ZSTs here because we want this
520 // to actually catch "asking for more than usize::MAX" in that case.
521 // If we make it past the first branch then we are guaranteed to
524 // Don't actually need any more capacity.
525 // Wrapping in case they give a bad `used_cap`
526 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
530 let new_cap = self.amortized_new_size(used_cap, needed_extra_cap)?;
531 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
533 // FIXME: may crash and burn on over-reserve
534 alloc_guard(new_layout.size())?;
536 let res = match self.current_layout() {
538 debug_assert!(new_layout.align() == layout.align());
539 self.a.realloc(NonNull::from(self.ptr).as_opaque(), layout, new_layout.size())
541 None => self.a.alloc(new_layout),
544 self.ptr = res?.cast().into();
551 /// The same as try_reserve, but errors are lowered to a call to oom().
552 pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
553 match self.try_reserve(used_cap, needed_extra_cap) {
554 Err(CapacityOverflow) => capacity_overflow(),
555 Err(AllocErr) => oom(),
556 Ok(()) => { /* yay */ }
559 /// Attempts to ensure that the buffer contains at least enough space to hold
560 /// `used_cap + needed_extra_cap` elements. If it doesn't already have
561 /// enough capacity, will reallocate in place enough space plus comfortable slack
562 /// space to get amortized `O(1)` behavior. Will limit this behaviour
563 /// if it would needlessly cause itself to panic.
565 /// If `used_cap` exceeds `self.cap()`, this may fail to actually allocate
566 /// the requested space. This is not really unsafe, but the unsafe
567 /// code *you* write that relies on the behavior of this function may break.
569 /// Returns true if the reallocation attempt has succeeded, or false otherwise.
573 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
574 /// * Panics on 32-bit platforms if the requested capacity exceeds
575 /// `isize::MAX` bytes.
576 pub fn reserve_in_place(&mut self, used_cap: usize, needed_extra_cap: usize) -> bool {
578 // NOTE: we don't early branch on ZSTs here because we want this
579 // to actually catch "asking for more than usize::MAX" in that case.
580 // If we make it past the first branch then we are guaranteed to
583 // Don't actually need any more capacity. If the current `cap` is 0, we can't
584 // reallocate in place.
585 // Wrapping in case they give a bad `used_cap`
586 let old_layout = match self.current_layout() {
587 Some(layout) => layout,
588 None => return false,
590 if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
594 let new_cap = self.amortized_new_size(used_cap, needed_extra_cap)
595 .unwrap_or_else(|_| capacity_overflow());
597 // Here, `cap < used_cap + needed_extra_cap <= new_cap`
598 // (regardless of whether `self.cap - used_cap` wrapped).
599 // Therefore we can safely call grow_in_place.
601 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
602 // FIXME: may crash and burn on over-reserve
603 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
604 match self.a.grow_in_place(
605 NonNull::from(self.ptr).as_opaque(), old_layout, new_layout.size(),
618 /// Shrinks the allocation down to the specified amount. If the given amount
619 /// is 0, actually completely deallocates.
623 /// Panics if the given amount is *larger* than the current capacity.
628 pub fn shrink_to_fit(&mut self, amount: usize) {
629 let elem_size = mem::size_of::<T>();
631 // Set the `cap` because they might be about to promote to a `Box<[T]>`
637 // This check is my waterloo; it's the only thing Vec wouldn't have to do.
638 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
641 // We want to create a new zero-length vector within the
642 // same allocator. We use ptr::write to avoid an
643 // erroneous attempt to drop the contents, and we use
644 // ptr::read to sidestep condition against destructuring
645 // types that implement Drop.
648 let a = ptr::read(&self.a as *const A);
649 self.dealloc_buffer();
650 ptr::write(self, RawVec::new_in(a));
652 } else if self.cap != amount {
654 // We know here that our `amount` is greater than zero. This
655 // implies, via the assert above, that capacity is also greater
656 // than zero, which means that we've got a current layout that
659 // We also know that `self.cap` is greater than `amount`, and
660 // consequently we don't need runtime checks for creating either
662 let old_size = elem_size * self.cap;
663 let new_size = elem_size * amount;
664 let align = mem::align_of::<T>();
665 let old_layout = Layout::from_size_align_unchecked(old_size, align);
666 match self.a.realloc(NonNull::from(self.ptr).as_opaque(),
669 Ok(p) => self.ptr = p.cast().into(),
678 impl<T> RawVec<T, Global> {
679 /// Converts the entire buffer into `Box<[T]>`.
681 /// While it is not *strictly* Undefined Behavior to call
682 /// this procedure while some of the RawVec is uninitialized,
683 /// it certainly makes it trivial to trigger it.
685 /// Note that this will correctly reconstitute any `cap` changes
686 /// that may have been performed. (see description of type for details)
687 pub unsafe fn into_box(self) -> Box<[T]> {
688 // NOTE: not calling `cap()` here, actually using the real `cap` field!
689 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
690 let output: Box<[T]> = Box::from_raw(slice);
696 impl<T, A: Alloc> RawVec<T, A> {
697 /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
698 pub unsafe fn dealloc_buffer(&mut self) {
699 let elem_size = mem::size_of::<T>();
701 if let Some(layout) = self.current_layout() {
702 self.a.dealloc(NonNull::from(self.ptr).as_opaque(), layout);
708 unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
709 /// Frees the memory owned by the RawVec *without* trying to Drop its contents.
711 unsafe { self.dealloc_buffer(); }
717 // We need to guarantee the following:
718 // * We don't ever allocate `> isize::MAX` byte-size objects
719 // * We don't overflow `usize::MAX` and actually allocate too little
721 // On 64-bit we just need to check for overflow since trying to allocate
722 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
723 // an extra guard for this in case we're running on a platform which can use
724 // all 4GB in user-space. e.g. PAE or x32
727 fn alloc_guard(alloc_size: usize) -> Result<(), CollectionAllocErr> {
728 if mem::size_of::<usize>() < 8 && alloc_size > ::core::isize::MAX as usize {
729 Err(CapacityOverflow)
735 // One central function responsible for reporting capacity overflows. This'll
736 // ensure that the code generation related to these panics is minimal as there's
737 // only one location which panics rather than a bunch throughout the module.
738 fn capacity_overflow() -> ! {
739 panic!("capacity overflow")
748 fn allocator_param() {
749 use allocator::{Alloc, AllocErr};
751 // Writing a test of integration between third-party
752 // allocators and RawVec is a little tricky because the RawVec
753 // API does not expose fallible allocation methods, so we
754 // cannot check what happens when allocator is exhausted
755 // (beyond detecting a panic).
757 // Instead, this just checks that the RawVec methods do at
758 // least go through the Allocator API when it reserves
761 // A dumb allocator that consumes a fixed amount of fuel
762 // before allocation attempts start failing.
763 struct BoundedAlloc { fuel: usize }
764 unsafe impl Alloc for BoundedAlloc {
765 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<Opaque>, AllocErr> {
766 let size = layout.size();
767 if size > self.fuel {
768 return Err(AllocErr);
770 match Global.alloc(layout) {
771 ok @ Ok(_) => { self.fuel -= size; ok }
775 unsafe fn dealloc(&mut self, ptr: NonNull<Opaque>, layout: Layout) {
776 Global.dealloc(ptr, layout)
780 let a = BoundedAlloc { fuel: 500 };
781 let mut v: RawVec<u8, _> = RawVec::with_capacity_in(50, a);
782 assert_eq!(v.a.fuel, 450);
783 v.reserve(50, 150); // (causes a realloc, thus using 50 + 150 = 200 units of fuel)
784 assert_eq!(v.a.fuel, 250);
788 fn reserve_does_not_overallocate() {
790 let mut v: RawVec<u32> = RawVec::new();
791 // First `reserve` allocates like `reserve_exact`
793 assert_eq!(9, v.cap());
797 let mut v: RawVec<u32> = RawVec::new();
799 assert_eq!(7, v.cap());
800 // 97 if more than double of 7, so `reserve` should work
801 // like `reserve_exact`.
803 assert_eq!(97, v.cap());
807 let mut v: RawVec<u32> = RawVec::new();
809 assert_eq!(12, v.cap());
811 // 3 is less than half of 12, so `reserve` must grow
812 // exponentially. At the time of writing this test grow
813 // factor is 2, so new capacity is 24, however, grow factor
814 // of 1.5 is OK too. Hence `>= 18` in assert.
815 assert!(v.cap() >= 12 + 12 / 2);