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// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// option. This file may not be copied, modified, or distributed
// except according to those terms.
#![allow(unstable_name_collisions)]
#![allow(dead_code)]
use crate::Bump;
use core::cmp;
use core::mem;
use core::ptr::{self, NonNull};
use crate::alloc::{handle_alloc_error, Alloc, Layout, UnstableLayoutMethods};
use crate::collections::CollectionAllocErr;
use crate::collections::CollectionAllocErr::*;
// use boxed::Box;
/// 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
/// * 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.
///
/// However this means that you need to be careful when round-tripping this type
/// with a `Box<[T]>`: `cap()` won't yield the len. 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<'a, T> {
ptr: NonNull<T>,
cap: usize,
a: &'a Bump,
}
impl<'a, T> RawVec<'a, T> {
/// Like `new` but parameterized over the choice of allocator for
/// the returned RawVec.
pub fn new_in(a: &'a Bump) -> Self {
// `cap: 0` means "unallocated". zero-sized types are ignored.
RawVec {
ptr: NonNull::dangling(),
cap: 0,
a,
}
}
/// Like `with_capacity` but parameterized over the choice of
/// allocator for the returned RawVec.
#[inline]
pub fn with_capacity_in(cap: usize, a: &'a Bump) -> Self {
RawVec::allocate_in(cap, false, a)
}
/// Like `with_capacity_zeroed` but parameterized over the choice
/// of allocator for the returned RawVec.
#[inline]
pub fn with_capacity_zeroed_in(cap: usize, a: &'a Bump) -> Self {
RawVec::allocate_in(cap, true, a)
}
fn allocate_in(cap: usize, zeroed: bool, mut a: &'a Bump) -> Self {
unsafe {
let elem_size = mem::size_of::<T>();
let alloc_size = cap
.checked_mul(elem_size)
.unwrap_or_else(|| capacity_overflow());
alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
// handles ZSTs and `cap = 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 {
Alloc::alloc(&mut a, layout)
};
match result {
Ok(ptr) => ptr.cast(),
Err(_) => handle_alloc_error(layout),
}
};
RawVec { ptr, cap, a }
}
}
}
impl<'a, T> RawVec<'a, T> {
/// 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, cap: usize, a: &'a Bump) -> Self {
RawVec {
ptr: NonNull::new_unchecked(ptr),
cap,
a,
}
}
}
impl<'a, T> RawVec<'a, T> {
/// Gets a raw pointer to the start of the allocation. Note that this is
/// Unique::empty() if `cap = 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 cap(&self) -> usize {
if mem::size_of::<T>() == 0 {
!0
} else {
self.cap
}
}
/// Returns a shared reference to the allocator backing this RawVec.
pub fn bump(&self) -> &'a Bump {
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 == cap`.
///
/// # 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
///
/// ```ignore
/// # #![feature(alloc, 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.cap() { 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 (new_cap, uniq) = 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(self.ptr.cast(), cur, new_size);
match ptr_res {
Ok(ptr) => (new_cap, ptr.cast()),
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 };
match self.a.alloc_array::<T>(new_cap) {
Ok(ptr) => (new_cap, ptr),
Err(_) => handle_alloc_error(Layout::array::<T>(new_cap).unwrap()),
}
}
};
self.ptr = uniq;
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, or false otherwise.
///
/// # 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 like 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(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_cap: usize,
needed_extra_cap: usize,
) -> Result<(), CollectionAllocErr> {
self.fallible_reserve_internal(used_cap, needed_extra_cap, Exact)
}
/// Ensures that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` 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_cap` exceeds `self.cap()`, 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_cap: usize, needed_extra_cap: usize) {
self.infallible_reserve_internal(used_cap, needed_extra_cap, Exact)
}
/// Calculates the buffer's new size given that it'll hold `used_cap +
/// needed_extra_cap` elements. This logic is used in amortized reserve methods.
/// Returns `(new_capacity, new_alloc_size)`.
fn amortized_new_size(
&self,
used_cap: usize,
needed_extra_cap: usize,
) -> Result<usize, CollectionAllocErr> {
// Nothing we can really do about these checks :(
let required_cap = used_cap
.checked_add(needed_extra_cap)
.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_cap: usize,
needed_extra_cap: usize,
) -> Result<(), CollectionAllocErr> {
self.fallible_reserve_internal(used_cap, needed_extra_cap, Amortized)
}
/// Ensures that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` 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_cap` exceeds `self.cap()`, 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
///
/// ```ignore
/// # #![feature(alloc, 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]);
/// # }
/// ```
#[inline(always)]
pub fn reserve(&mut self, used_cap: usize, needed_extra_cap: usize) {
self.infallible_reserve_internal(used_cap, needed_extra_cap, Amortized)
}
/// Attempts to ensure that the buffer contains at least enough space to hold
/// `used_cap + needed_extra_cap` 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_cap` exceeds `self.cap()`, 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, or false otherwise.
///
/// # 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_cap: usize, needed_extra_cap: 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_cap`
let old_layout = match self.current_layout() {
Some(layout) => layout,
None => return false,
};
if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
return false;
}
let new_cap = self
.amortized_new_size(used_cap, needed_extra_cap)
.unwrap_or_else(|_| capacity_overflow());
// Here, `cap < used_cap + needed_extra_cap <= new_cap`
// (regardless of whether `self.cap - used_cap` 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(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 = self.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(self.ptr.cast(), old_layout, new_size) {
Ok(p) => self.ptr = p.cast(),
Err(_) => {
handle_alloc_error(Layout::from_size_align_unchecked(new_size, align))
}
}
}
self.cap = amount;
}
}
}
#[cfg(feature = "boxed")]
impl<'a, T> RawVec<'a, T> {
/// 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>` 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) -> crate::boxed::Box<'a, [T]> {
use crate::boxed::Box;
// NOTE: not calling `cap()` here; actually using the real `cap` field!
let slice = core::slice::from_raw_parts_mut(self.ptr(), self.cap);
let output: Box<'a, [T]> = Box::from_raw(slice);
mem::forget(self);
output
}
}
enum Fallibility {
Fallible,
Infallible,
}
use self::Fallibility::*;
enum ReserveStrategy {
Exact,
Amortized,
}
use self::ReserveStrategy::*;
impl<'a, T> RawVec<'a, T> {
#[inline(always)]
fn fallible_reserve_internal(
&mut self,
used_cap: usize,
needed_extra_cap: usize,
strategy: ReserveStrategy,
) -> Result<(), CollectionAllocErr> {
// This portion of the method should always be inlined.
if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
return Ok(());
}
// This portion of the method should never be inlined, and will only be called when
// the check above has confirmed that it is necessary.
self.reserve_internal_or_error(used_cap, needed_extra_cap, Fallible, strategy)
}
#[inline(always)]
fn infallible_reserve_internal(
&mut self,
used_cap: usize,
needed_extra_cap: usize,
strategy: ReserveStrategy,
) {
// This portion of the method should always be inlined.
if self.cap().wrapping_sub(used_cap) >= needed_extra_cap {
return;
}
// This portion of the method should never be inlined, and will only be called when
// the check above has confirmed that it is necessary.
self.reserve_internal_or_panic(used_cap, needed_extra_cap, strategy)
}
#[inline(never)]
fn reserve_internal_or_panic(
&mut self,
used_cap: usize,
needed_extra_cap: usize,
strategy: ReserveStrategy,
) {
// Delegates the call to `reserve_internal_or_error` and panics in the event of an error.
// This allows the method to have a return type of `()`, simplifying the assembly at the
// call site.
match self.reserve_internal(used_cap, needed_extra_cap, Infallible, strategy) {
Err(CapacityOverflow) => capacity_overflow(),
Err(AllocErr) => unreachable!(),
Ok(()) => { /* yay */ }
}
}
#[inline(never)]
fn reserve_internal_or_error(
&mut self,
used_cap: usize,
needed_extra_cap: usize,
fallibility: Fallibility,
strategy: ReserveStrategy,)-> Result<(), CollectionAllocErr> {
// Delegates the call to `reserve_internal`, which can be inlined.
self.reserve_internal(used_cap, needed_extra_cap, fallibility, strategy)
}
/// Helper method to reserve additional space, reallocating the backing memory.
/// The caller is responsible for confirming that there is not already enough space available.
fn reserve_internal(
&mut self,
used_cap: usize,
needed_extra_cap: usize,
fallibility: Fallibility,
strategy: ReserveStrategy,
) -> Result<(), CollectionAllocErr> {
unsafe {
use crate::AllocErr;
// 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.
// Nothing we can really do about these checks :(
let new_cap = match strategy {
Exact => used_cap
.checked_add(needed_extra_cap)
.ok_or(CapacityOverflow)?,
Amortized => self.amortized_new_size(used_cap, needed_extra_cap)?,
};
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(self.ptr.cast(), layout, new_layout.size())
}
None => Alloc::alloc(&mut self.a, new_layout),
};
if let (Err(AllocErr), Infallible) = (&res, fallibility) {
handle_alloc_error(new_layout);
}
self.ptr = res?.cast();
self.cap = new_cap;
Ok(())
}
}
}
impl<'a, T> RawVec<'a, T> {
/// 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(self.ptr.cast(), layout);
}
}
}
}
impl<'a, T> Drop for RawVec<'a, T> {
/// 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<(), CollectionAllocErr> {
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")
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn reserve_does_not_overallocate() {
let bump = Bump::new();
{
let mut v: RawVec<u32> = RawVec::new_in(&bump);
// First `reserve` allocates like `reserve_exact`
v.reserve(0, 9);
assert_eq!(9, v.cap());
}
{
let mut v: RawVec<u32> = RawVec::new_in(&bump);
v.reserve(0, 7);
assert_eq!(7, v.cap());
// 97 if more than double of 7, so `reserve` should work
// like `reserve_exact`.
v.reserve(7, 90);
assert_eq!(97, v.cap());
}
{
let mut v: RawVec<u32> = RawVec::new_in(&bump);
v.reserve(0, 12);
assert_eq!(12, v.cap());
v.reserve(12, 3);
// 3 is less than half of 12, so `reserve` must grow
// exponentially. At the time of writing this test grow
// factor is 2, so new capacity is 24, however, grow factor
// of 1.5 is OK too. Hence `>= 18` in assert.
assert!(v.cap() >= 12 + 12 / 2);
}
}
}