// Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 or the MIT license // , at your // option. This file may not be copied, modified, or distributed // except according to those terms. #![unstable(feature = "raw_vec_internals", reason = "implemention detail", issue = "0")] #![doc(hidden)] use core::cmp; use core::mem; use core::ops::Drop; use core::ptr::{self, NonNull, Unique}; use core::slice; use alloc::{Alloc, Layout, Global, handle_alloc_error}; use collections::CollectionAllocErr; use 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 roundtripping 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 { ptr: Unique, cap: usize, a: A, } impl RawVec { /// Like `new` but parameterized over the choice of allocator for /// the returned RawVec. pub const fn new_in(a: A) -> Self { // !0 is usize::MAX. This branch should be stripped at compile time. // FIXME(mark-i-m): use this line when `if`s are allowed in `const` //let cap = if mem::size_of::() == 0 { !0 } else { 0 }; // Unique::empty() doubles as "unallocated" and "zero-sized allocation" RawVec { ptr: Unique::empty(), // FIXME(mark-i-m): use `cap` when ifs are allowed in const cap: [0, !0][(mem::size_of::() == 0) as usize], 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) -> 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) -> Self { RawVec::allocate_in(cap, true, a) } fn allocate_in(cap: usize, zeroed: bool, mut a: A) -> Self { unsafe { let elem_size = mem::size_of::(); 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::::dangling() } else { let align = mem::align_of::(); 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) => ptr.cast(), Err(_) => handle_alloc_error(layout), } }; RawVec { ptr: ptr.into(), cap, a, } } } } impl RawVec { /// 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 has 0 size, 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; cap]`. This is /// equivalent to calling RawVec::new when `cap` 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(cap: usize) -> Self { RawVec::allocate_in(cap, false, Global) } /// Like `with_capacity` but guarantees the buffer is zeroed. #[inline] pub fn with_capacity_zeroed(cap: usize) -> Self { RawVec::allocate_in(cap, true, Global) } } impl RawVec { /// 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) -> Self { RawVec { ptr: Unique::new_unchecked(ptr), cap, a, } } } impl RawVec { /// Reconstitutes a RawVec from a pointer, 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, cap: usize) -> Self { RawVec { ptr: Unique::new_unchecked(ptr), cap, a: Global, } } /// Converts a `Box<[T]>` into a `RawVec`. 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 RawVec { /// 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::() == 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 { 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::(); let size = mem::size_of::() * 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 /// /// ``` /// # #![feature(alloc, raw_vec_internals)] /// # extern crate alloc; /// # use std::ptr; /// # use alloc::raw_vec::RawVec; /// struct MyVec { /// buf: RawVec, /// len: usize, /// } /// /// impl MyVec { /// 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().offset(self.len as isize), 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::(); // 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(NonNull::from(self.ptr).cast(), cur, new_size); match ptr_res { Ok(ptr) => (new_cap, ptr.cast().into()), 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::(new_cap) { Ok(ptr) => (new_cap, ptr.into()), Err(_) => handle_alloc_error(Layout::array::(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::(); 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(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_cap: usize, needed_extra_cap: usize) -> Result<(), CollectionAllocErr> { self.reserve_internal(used_cap, needed_extra_cap, Fallible, 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) { match self.reserve_internal(used_cap, needed_extra_cap, Infallible, Exact) { Err(CapacityOverflow) => capacity_overflow(), Err(AllocErr) => unreachable!(), Ok(()) => { /* yay */ } } } /// 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 { // 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.reserve_internal(used_cap, needed_extra_cap, Fallible, 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 /// /// ``` /// # #![feature(alloc, raw_vec_internals)] /// # extern crate alloc; /// # use std::ptr; /// # use alloc::raw_vec::RawVec; /// struct MyVec { /// buf: RawVec, /// len: usize, /// } /// /// impl MyVec { /// 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().offset(self.len as isize), 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_cap: usize, needed_extra_cap: usize) { match self.reserve_internal(used_cap, needed_extra_cap, Infallible, Amortized) { Err(CapacityOverflow) => capacity_overflow(), Err(AllocErr) => unreachable!(), Ok(()) => { /* yay */ } } } /// 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::().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::(); // 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::(); 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(p) => self.ptr = p.cast().into(), Err(_) => handle_alloc_error( Layout::from_size_align_unchecked(new_size, align) ), } } self.cap = amount; } } } enum Fallibility { Fallible, Infallible, } use self::Fallibility::*; enum ReserveStrategy { Exact, Amortized, } use self::ReserveStrategy::*; impl RawVec { fn reserve_internal( &mut self, used_cap: usize, needed_extra_cap: usize, fallibility: Fallibility, strategy: ReserveStrategy, ) -> Result<(), CollectionAllocErr> { unsafe { use alloc::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. // Don't actually need any more capacity. // Wrapping in case they gave a bad `used_cap`. if self.cap().wrapping_sub(used_cap) >= needed_extra_cap { return Ok(()); } // 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::(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), }; match (&res, fallibility) { (Err(AllocErr), Infallible) => handle_alloc_error(new_layout), _ => {} } self.ptr = res?.cast().into(); self.cap = new_cap; Ok(()) } } } impl RawVec { /// Converts the entire buffer into `Box<[T]>`. /// /// While it is not *strictly* Undefined Behavior to call /// this procedure while some of the RawVec is uninitialized, /// it certainly makes it trivial to trigger it. /// /// Note that this will correctly reconstitute any `cap` changes /// that may have been performed. (see description of type for details) pub unsafe fn into_box(self) -> Box<[T]> { // NOTE: not calling `cap()` 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 RawVec { /// 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::(); 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: Alloc> Drop for RawVec { /// 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::() < 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 allocator_param() { use alloc::AllocErr; // Writing a test of integration between third-party // allocators and RawVec is a little tricky because the RawVec // API does not expose fallible allocation methods, so we // cannot check what happens when allocator is exhausted // (beyond detecting a panic). // // Instead, this just checks that the RawVec methods do at // least go through the Allocator API when it reserves // storage. // A dumb allocator that consumes a fixed amount of fuel // before allocation attempts start failing. struct BoundedAlloc { fuel: usize } unsafe impl Alloc for BoundedAlloc { unsafe fn alloc(&mut self, layout: Layout) -> Result, AllocErr> { let size = layout.size(); if size > self.fuel { return Err(AllocErr); } match Global.alloc(layout) { ok @ Ok(_) => { self.fuel -= size; ok } err @ Err(_) => err, } } unsafe fn dealloc(&mut self, ptr: NonNull, layout: Layout) { Global.dealloc(ptr, layout) } } let a = BoundedAlloc { fuel: 500 }; let mut v: RawVec = RawVec::with_capacity_in(50, a); assert_eq!(v.a.fuel, 450); v.reserve(50, 150); // (causes a realloc, thus using 50 + 150 = 200 units of fuel) assert_eq!(v.a.fuel, 250); } #[test] fn reserve_does_not_overallocate() { { let mut v: RawVec = RawVec::new(); // First `reserve` allocates like `reserve_exact` v.reserve(0, 9); assert_eq!(9, v.cap()); } { let mut v: RawVec = RawVec::new(); 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 = RawVec::new(); 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); } } }