alloc/raw_vec.rs
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#![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
use core::marker::PhantomData;
use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
use core::ptr::{self, NonNull, Unique};
use core::{cmp, hint};
#[cfg(not(no_global_oom_handling))]
use crate::alloc::handle_alloc_error;
use crate::alloc::{Allocator, Global, Layout};
use crate::boxed::Box;
use crate::collections::TryReserveError;
use crate::collections::TryReserveErrorKind::*;
#[cfg(test)]
mod tests;
// One central function responsible for reporting capacity overflows. This'll
// ensure that the code generation related to these panics is minimal as there's
// only one location which panics rather than a bunch throughout the module.
#[cfg(not(no_global_oom_handling))]
#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
fn capacity_overflow() -> ! {
panic!("capacity overflow");
}
enum AllocInit {
/// The contents of the new memory are uninitialized.
Uninitialized,
#[cfg(not(no_global_oom_handling))]
/// The new memory is guaranteed to be zeroed.
Zeroed,
}
#[repr(transparent)]
#[cfg_attr(target_pointer_width = "16", rustc_layout_scalar_valid_range_end(0x7fff))]
#[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0x7fff_ffff))]
#[cfg_attr(target_pointer_width = "64", rustc_layout_scalar_valid_range_end(0x7fff_ffff_ffff_ffff))]
struct Cap(usize);
impl Cap {
const ZERO: Cap = unsafe { Cap(0) };
/// `Cap(cap)`, except if `T` is a ZST then `Cap::ZERO`.
///
/// # Safety: cap must be <= `isize::MAX`.
unsafe fn new<T>(cap: usize) -> Self {
if T::IS_ZST { Cap::ZERO } else { unsafe { Self(cap) } }
}
}
/// A low-level utility for more ergonomically allocating, reallocating, and deallocating
/// a buffer of memory on the heap without having to worry about all the corner cases
/// involved. This type is excellent for building your own data structures like Vec and VecDeque.
/// In particular:
///
/// * Produces `Unique::dangling()` on zero-sized types.
/// * Produces `Unique::dangling()` on zero-length allocations.
/// * Avoids freeing `Unique::dangling()`.
/// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
/// * Guards against 32-bit systems allocating more than `isize::MAX` bytes.
/// * Guards against overflowing your length.
/// * Calls `handle_alloc_error` for fallible allocations.
/// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
/// * Uses the excess returned from the allocator to use the largest available capacity.
///
/// This type does not in anyway inspect the memory that it manages. When dropped it *will*
/// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
/// to handle the actual things *stored* inside of a `RawVec`.
///
/// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
/// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
/// `Box<[T]>`, since `capacity()` won't yield the length.
#[allow(missing_debug_implementations)]
pub(crate) struct RawVec<T, A: Allocator = Global> {
inner: RawVecInner<A>,
_marker: PhantomData<T>,
}
/// Like a `RawVec`, but only generic over the allocator, not the type.
///
/// As such, all the methods need the layout passed-in as a parameter.
///
/// Having this separation reduces the amount of code we need to monomorphize,
/// as most operations don't need the actual type, just its layout.
#[allow(missing_debug_implementations)]
struct RawVecInner<A: Allocator = Global> {
ptr: Unique<u8>,
/// Never used for ZSTs; it's `capacity()`'s responsibility to return usize::MAX in that case.
///
/// # Safety
///
/// `cap` must be in the `0..=isize::MAX` range.
cap: Cap,
alloc: A,
}
impl<T> RawVec<T, Global> {
/// Creates the biggest possible `RawVec` (on the system heap)
/// without allocating. If `T` has positive size, then this makes a
/// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
/// `RawVec` with capacity `usize::MAX`. Useful for implementing
/// delayed allocation.
#[must_use]
#[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
pub const fn new() -> Self {
Self::new_in(Global)
}
/// Creates a `RawVec` (on the system heap) with exactly the
/// capacity and alignment requirements for a `[T; capacity]`. This is
/// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
/// zero-sized. Note that if `T` is zero-sized this means you will
/// *not* get a `RawVec` with the requested capacity.
///
/// Non-fallible version of `try_with_capacity`
///
/// # Panics
///
/// Panics if the requested capacity exceeds `isize::MAX` bytes.
///
/// # Aborts
///
/// Aborts on OOM.
#[cfg(not(any(no_global_oom_handling, test)))]
#[must_use]
#[inline]
pub fn with_capacity(capacity: usize) -> Self {
Self { inner: RawVecInner::with_capacity(capacity, T::LAYOUT), _marker: PhantomData }
}
/// Like `with_capacity`, but guarantees the buffer is zeroed.
#[cfg(not(any(no_global_oom_handling, test)))]
#[must_use]
#[inline]
pub fn with_capacity_zeroed(capacity: usize) -> Self {
Self {
inner: RawVecInner::with_capacity_zeroed_in(capacity, Global, T::LAYOUT),
_marker: PhantomData,
}
}
}
impl RawVecInner<Global> {
#[cfg(not(any(no_global_oom_handling, test)))]
#[must_use]
#[inline]
fn with_capacity(capacity: usize, elem_layout: Layout) -> Self {
match Self::try_allocate_in(capacity, AllocInit::Uninitialized, Global, elem_layout) {
Ok(res) => res,
Err(err) => handle_error(err),
}
}
}
// Tiny Vecs are dumb. Skip to:
// - 8 if the element size is 1, because any heap allocators is likely
// to round up a request of less than 8 bytes to at least 8 bytes.
// - 4 if elements are moderate-sized (<= 1 KiB).
// - 1 otherwise, to avoid wasting too much space for very short Vecs.
const fn min_non_zero_cap(size: usize) -> usize {
if size == 1 {
8
} else if size <= 1024 {
4
} else {
1
}
}
impl<T, A: Allocator> RawVec<T, A> {
#[cfg(not(no_global_oom_handling))]
pub(crate) const MIN_NON_ZERO_CAP: usize = min_non_zero_cap(size_of::<T>());
/// Like `new`, but parameterized over the choice of allocator for
/// the returned `RawVec`.
#[inline]
#[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
pub const fn new_in(alloc: A) -> Self {
Self { inner: RawVecInner::new_in(alloc, align_of::<T>()), _marker: PhantomData }
}
/// Like `with_capacity`, but parameterized over the choice of
/// allocator for the returned `RawVec`.
#[cfg(not(no_global_oom_handling))]
#[inline]
pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
Self {
inner: RawVecInner::with_capacity_in(capacity, alloc, T::LAYOUT),
_marker: PhantomData,
}
}
/// Like `try_with_capacity`, but parameterized over the choice of
/// allocator for the returned `RawVec`.
#[inline]
pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
match RawVecInner::try_with_capacity_in(capacity, alloc, T::LAYOUT) {
Ok(inner) => Ok(Self { inner, _marker: PhantomData }),
Err(e) => Err(e),
}
}
/// Like `with_capacity_zeroed`, but parameterized over the choice
/// of allocator for the returned `RawVec`.
#[cfg(not(no_global_oom_handling))]
#[inline]
pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
Self {
inner: RawVecInner::with_capacity_zeroed_in(capacity, alloc, T::LAYOUT),
_marker: PhantomData,
}
}
/// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
///
/// Note that this will correctly reconstitute any `cap` changes
/// that may have been performed. (See description of type for details.)
///
/// # Safety
///
/// * `len` must be greater than or equal to the most recently requested capacity, and
/// * `len` must be less than or equal to `self.capacity()`.
///
/// Note, that the requested capacity and `self.capacity()` could differ, as
/// an allocator could overallocate and return a greater memory block than requested.
pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
// Sanity-check one half of the safety requirement (we cannot check the other half).
debug_assert!(
len <= self.capacity(),
"`len` must be smaller than or equal to `self.capacity()`"
);
let me = ManuallyDrop::new(self);
unsafe {
let slice = ptr::slice_from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
Box::from_raw_in(slice, ptr::read(&me.inner.alloc))
}
}
/// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
///
/// # Safety
///
/// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
/// `capacity`.
/// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
/// systems). For ZSTs capacity is ignored.
/// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
/// guaranteed.
#[inline]
pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
// SAFETY: Precondition passed to the caller
unsafe {
let ptr = ptr.cast();
let capacity = Cap::new::<T>(capacity);
Self {
inner: RawVecInner::from_raw_parts_in(ptr, capacity, alloc),
_marker: PhantomData,
}
}
}
/// A convenience method for hoisting the non-null precondition out of [`RawVec::from_raw_parts_in`].
///
/// # Safety
///
/// See [`RawVec::from_raw_parts_in`].
#[inline]
pub unsafe fn from_nonnull_in(ptr: NonNull<T>, capacity: usize, alloc: A) -> Self {
// SAFETY: Precondition passed to the caller
unsafe {
let ptr = ptr.cast();
let capacity = Cap::new::<T>(capacity);
Self { inner: RawVecInner::from_nonnull_in(ptr, capacity, alloc), _marker: PhantomData }
}
}
/// Gets a raw pointer to the start of the allocation. Note that this is
/// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
/// be careful.
#[inline]
pub const fn ptr(&self) -> *mut T {
self.inner.ptr()
}
#[inline]
pub fn non_null(&self) -> NonNull<T> {
self.inner.non_null()
}
/// Gets the capacity of the allocation.
///
/// This will always be `usize::MAX` if `T` is zero-sized.
#[inline]
pub const fn capacity(&self) -> usize {
self.inner.capacity(size_of::<T>())
}
/// Returns a shared reference to the allocator backing this `RawVec`.
#[inline]
pub fn allocator(&self) -> &A {
self.inner.allocator()
}
/// Ensures that the buffer contains at least enough space to hold `len +
/// additional` elements. If it doesn't already have enough capacity, will
/// reallocate enough space plus comfortable slack space to get amortized
/// *O*(1) behavior. Will limit this behavior if it would needlessly cause
/// itself to panic.
///
/// If `len` exceeds `self.capacity()`, this may fail to actually allocate
/// the requested space. This is not really unsafe, but the unsafe
/// code *you* write that relies on the behavior of this function may break.
///
/// This is ideal for implementing a bulk-push operation like `extend`.
///
/// # Panics
///
/// Panics if the new capacity exceeds `isize::MAX` _bytes_.
///
/// # Aborts
///
/// Aborts on OOM.
#[cfg(not(no_global_oom_handling))]
#[inline]
pub fn reserve(&mut self, len: usize, additional: usize) {
self.inner.reserve(len, additional, T::LAYOUT)
}
/// A specialized version of `self.reserve(len, 1)` which requires the
/// caller to ensure `len == self.capacity()`.
#[cfg(not(no_global_oom_handling))]
#[inline(never)]
pub fn grow_one(&mut self) {
self.inner.grow_one(T::LAYOUT)
}
/// The same as `reserve`, but returns on errors instead of panicking or aborting.
pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
self.inner.try_reserve(len, additional, T::LAYOUT)
}
/// Ensures that the buffer contains at least enough space to hold `len +
/// additional` elements. If it doesn't already, will reallocate the
/// minimum possible amount of memory necessary. Generally this will be
/// exactly the amount of memory necessary, but in principle the allocator
/// is free to give back more than we asked for.
///
/// If `len` exceeds `self.capacity()`, this may fail to actually allocate
/// the requested space. This is not really unsafe, but the unsafe code
/// *you* write that relies on the behavior of this function may break.
///
/// # Panics
///
/// Panics if the new capacity exceeds `isize::MAX` _bytes_.
///
/// # Aborts
///
/// Aborts on OOM.
#[cfg(not(no_global_oom_handling))]
pub fn reserve_exact(&mut self, len: usize, additional: usize) {
self.inner.reserve_exact(len, additional, T::LAYOUT)
}
/// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
pub fn try_reserve_exact(
&mut self,
len: usize,
additional: usize,
) -> Result<(), TryReserveError> {
self.inner.try_reserve_exact(len, additional, T::LAYOUT)
}
/// Shrinks the buffer down to the specified capacity. If the given amount
/// is 0, actually completely deallocates.
///
/// # Panics
///
/// Panics if the given amount is *larger* than the current capacity.
///
/// # Aborts
///
/// Aborts on OOM.
#[cfg(not(no_global_oom_handling))]
#[inline]
pub fn shrink_to_fit(&mut self, cap: usize) {
self.inner.shrink_to_fit(cap, T::LAYOUT)
}
}
unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
/// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
fn drop(&mut self) {
// SAFETY: We are in a Drop impl, self.inner will not be used again.
unsafe { self.inner.deallocate(T::LAYOUT) }
}
}
impl<A: Allocator> RawVecInner<A> {
#[inline]
#[rustc_const_stable(feature = "raw_vec_internals_const", since = "1.81")]
const fn new_in(alloc: A, align: usize) -> Self {
let ptr = unsafe { core::mem::transmute(align) };
// `cap: 0` means "unallocated". zero-sized types are ignored.
Self { ptr, cap: Cap::ZERO, alloc }
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn with_capacity_in(capacity: usize, alloc: A, elem_layout: Layout) -> Self {
match Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc, elem_layout) {
Ok(this) => {
unsafe {
// Make it more obvious that a subsquent Vec::reserve(capacity) will not allocate.
hint::assert_unchecked(!this.needs_to_grow(0, capacity, elem_layout));
}
this
}
Err(err) => handle_error(err),
}
}
#[inline]
fn try_with_capacity_in(
capacity: usize,
alloc: A,
elem_layout: Layout,
) -> Result<Self, TryReserveError> {
Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc, elem_layout)
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn with_capacity_zeroed_in(capacity: usize, alloc: A, elem_layout: Layout) -> Self {
match Self::try_allocate_in(capacity, AllocInit::Zeroed, alloc, elem_layout) {
Ok(res) => res,
Err(err) => handle_error(err),
}
}
fn try_allocate_in(
capacity: usize,
init: AllocInit,
alloc: A,
elem_layout: Layout,
) -> Result<Self, TryReserveError> {
// We avoid `unwrap_or_else` here because it bloats the amount of
// LLVM IR generated.
let layout = match layout_array(capacity, elem_layout) {
Ok(layout) => layout,
Err(_) => return Err(CapacityOverflow.into()),
};
// Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
if layout.size() == 0 {
return Ok(Self::new_in(alloc, elem_layout.align()));
}
if let Err(err) = alloc_guard(layout.size()) {
return Err(err);
}
let result = match init {
AllocInit::Uninitialized => alloc.allocate(layout),
#[cfg(not(no_global_oom_handling))]
AllocInit::Zeroed => alloc.allocate_zeroed(layout),
};
let ptr = match result {
Ok(ptr) => ptr,
Err(_) => return Err(AllocError { layout, non_exhaustive: () }.into()),
};
// Allocators currently return a `NonNull<[u8]>` whose length
// matches the size requested. If that ever changes, the capacity
// here should change to `ptr.len() / mem::size_of::<T>()`.
Ok(Self { ptr: Unique::from(ptr.cast()), cap: unsafe { Cap(capacity) }, alloc })
}
#[inline]
unsafe fn from_raw_parts_in(ptr: *mut u8, cap: Cap, alloc: A) -> Self {
Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap, alloc }
}
#[inline]
unsafe fn from_nonnull_in(ptr: NonNull<u8>, cap: Cap, alloc: A) -> Self {
Self { ptr: Unique::from(ptr), cap, alloc }
}
#[inline]
const fn ptr<T>(&self) -> *mut T {
self.non_null::<T>().as_ptr()
}
#[inline]
const fn non_null<T>(&self) -> NonNull<T> {
self.ptr.cast().as_non_null_ptr()
}
#[inline]
const fn capacity(&self, elem_size: usize) -> usize {
if elem_size == 0 { usize::MAX } else { self.cap.0 }
}
#[inline]
fn allocator(&self) -> &A {
&self.alloc
}
#[inline]
fn current_memory(&self, elem_layout: Layout) -> Option<(NonNull<u8>, Layout)> {
if elem_layout.size() == 0 || self.cap.0 == 0 {
None
} else {
// We could use Layout::array here which ensures the absence of isize and usize overflows
// and could hypothetically handle differences between stride and size, but this memory
// has already been allocated so we know it can't overflow and currently Rust does not
// support such types. So we can do better by skipping some checks and avoid an unwrap.
unsafe {
let alloc_size = elem_layout.size().unchecked_mul(self.cap.0);
let layout = Layout::from_size_align_unchecked(alloc_size, elem_layout.align());
Some((self.ptr.into(), layout))
}
}
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn reserve(&mut self, len: usize, additional: usize, elem_layout: Layout) {
// Callers expect this function to be very cheap when there is already sufficient capacity.
// Therefore, we move all the resizing and error-handling logic from grow_amortized and
// handle_reserve behind a call, while making sure that this function is likely to be
// inlined as just a comparison and a call if the comparison fails.
#[cold]
fn do_reserve_and_handle<A: Allocator>(
slf: &mut RawVecInner<A>,
len: usize,
additional: usize,
elem_layout: Layout,
) {
if let Err(err) = slf.grow_amortized(len, additional, elem_layout) {
handle_error(err);
}
}
if self.needs_to_grow(len, additional, elem_layout) {
do_reserve_and_handle(self, len, additional, elem_layout);
}
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn grow_one(&mut self, elem_layout: Layout) {
if let Err(err) = self.grow_amortized(self.cap.0, 1, elem_layout) {
handle_error(err);
}
}
fn try_reserve(
&mut self,
len: usize,
additional: usize,
elem_layout: Layout,
) -> Result<(), TryReserveError> {
if self.needs_to_grow(len, additional, elem_layout) {
self.grow_amortized(len, additional, elem_layout)?;
}
unsafe {
// Inform the optimizer that the reservation has succeeded or wasn't needed
hint::assert_unchecked(!self.needs_to_grow(len, additional, elem_layout));
}
Ok(())
}
#[cfg(not(no_global_oom_handling))]
fn reserve_exact(&mut self, len: usize, additional: usize, elem_layout: Layout) {
if let Err(err) = self.try_reserve_exact(len, additional, elem_layout) {
handle_error(err);
}
}
fn try_reserve_exact(
&mut self,
len: usize,
additional: usize,
elem_layout: Layout,
) -> Result<(), TryReserveError> {
if self.needs_to_grow(len, additional, elem_layout) {
self.grow_exact(len, additional, elem_layout)?;
}
unsafe {
// Inform the optimizer that the reservation has succeeded or wasn't needed
hint::assert_unchecked(!self.needs_to_grow(len, additional, elem_layout));
}
Ok(())
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn shrink_to_fit(&mut self, cap: usize, elem_layout: Layout) {
if let Err(err) = self.shrink(cap, elem_layout) {
handle_error(err);
}
}
#[inline]
fn needs_to_grow(&self, len: usize, additional: usize, elem_layout: Layout) -> bool {
additional > self.capacity(elem_layout.size()).wrapping_sub(len)
}
#[inline]
unsafe fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
// Allocators currently return a `NonNull<[u8]>` whose length matches
// the size requested. If that ever changes, the capacity here should
// change to `ptr.len() / mem::size_of::<T>()`.
self.ptr = Unique::from(ptr.cast());
self.cap = unsafe { Cap(cap) };
}
fn grow_amortized(
&mut self,
len: usize,
additional: usize,
elem_layout: Layout,
) -> Result<(), TryReserveError> {
// This is ensured by the calling contexts.
debug_assert!(additional > 0);
if elem_layout.size() == 0 {
// Since we return a capacity of `usize::MAX` when `elem_size` is
// 0, getting to here necessarily means the `RawVec` is overfull.
return Err(CapacityOverflow.into());
}
// Nothing we can really do about these checks, sadly.
let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
// This guarantees exponential growth. The doubling cannot overflow
// because `cap <= isize::MAX` and the type of `cap` is `usize`.
let cap = cmp::max(self.cap.0 * 2, required_cap);
let cap = cmp::max(min_non_zero_cap(elem_layout.size()), cap);
let new_layout = layout_array(cap, elem_layout)?;
let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
// SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
unsafe { self.set_ptr_and_cap(ptr, cap) };
Ok(())
}
fn grow_exact(
&mut self,
len: usize,
additional: usize,
elem_layout: Layout,
) -> Result<(), TryReserveError> {
if elem_layout.size() == 0 {
// Since we return a capacity of `usize::MAX` when the type size is
// 0, getting to here necessarily means the `RawVec` is overfull.
return Err(CapacityOverflow.into());
}
let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
let new_layout = layout_array(cap, elem_layout)?;
let ptr = finish_grow(new_layout, self.current_memory(elem_layout), &mut self.alloc)?;
// SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than `isize::MAX` items
unsafe {
self.set_ptr_and_cap(ptr, cap);
}
Ok(())
}
#[cfg(not(no_global_oom_handling))]
#[inline]
fn shrink(&mut self, cap: usize, elem_layout: Layout) -> Result<(), TryReserveError> {
assert!(cap <= self.capacity(elem_layout.size()), "Tried to shrink to a larger capacity");
// SAFETY: Just checked this isn't trying to grow
unsafe { self.shrink_unchecked(cap, elem_layout) }
}
/// `shrink`, but without the capacity check.
///
/// This is split out so that `shrink` can inline the check, since it
/// optimizes out in things like `shrink_to_fit`, without needing to
/// also inline all this code, as doing that ends up failing the
/// `vec-shrink-panic` codegen test when `shrink_to_fit` ends up being too
/// big for LLVM to be willing to inline.
///
/// # Safety
/// `cap <= self.capacity()`
#[cfg(not(no_global_oom_handling))]
unsafe fn shrink_unchecked(
&mut self,
cap: usize,
elem_layout: Layout,
) -> Result<(), TryReserveError> {
let (ptr, layout) =
if let Some(mem) = self.current_memory(elem_layout) { mem } else { return Ok(()) };
// If shrinking to 0, deallocate the buffer. We don't reach this point
// for the T::IS_ZST case since current_memory() will have returned
// None.
if cap == 0 {
unsafe { self.alloc.deallocate(ptr, layout) };
self.ptr =
unsafe { Unique::new_unchecked(ptr::without_provenance_mut(elem_layout.align())) };
self.cap = Cap::ZERO;
} else {
let ptr = unsafe {
// Layout cannot overflow here because it would have
// overflowed earlier when capacity was larger.
let new_size = elem_layout.size().unchecked_mul(cap);
let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
self.alloc
.shrink(ptr, layout, new_layout)
.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
};
// SAFETY: if the allocation is valid, then the capacity is too
unsafe {
self.set_ptr_and_cap(ptr, cap);
}
}
Ok(())
}
/// # Safety
///
/// This function deallocates the owned allocation, but does not update `ptr` or `cap` to
/// prevent double-free or use-after-free. Essentially, do not do anything with the caller
/// after this function returns.
/// Ideally this function would take `self` by move, but it cannot because it exists to be
/// called from a `Drop` impl.
unsafe fn deallocate(&mut self, elem_layout: Layout) {
if let Some((ptr, layout)) = self.current_memory(elem_layout) {
unsafe {
self.alloc.deallocate(ptr, layout);
}
}
}
}
#[inline(never)]
fn finish_grow<A>(
new_layout: Layout,
current_memory: Option<(NonNull<u8>, Layout)>,
alloc: &mut A,
) -> Result<NonNull<[u8]>, TryReserveError>
where
A: Allocator,
{
alloc_guard(new_layout.size())?;
let memory = if let Some((ptr, old_layout)) = current_memory {
debug_assert_eq!(old_layout.align(), new_layout.align());
unsafe {
// The allocator checks for alignment equality
hint::assert_unchecked(old_layout.align() == new_layout.align());
alloc.grow(ptr, old_layout, new_layout)
}
} else {
alloc.allocate(new_layout)
};
memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
}
// Central function for reserve error handling.
#[cfg(not(no_global_oom_handling))]
#[cold]
#[optimize(size)]
fn handle_error(e: TryReserveError) -> ! {
match e.kind() {
CapacityOverflow => capacity_overflow(),
AllocError { layout, .. } => handle_alloc_error(layout),
}
}
// We need to guarantee the following:
// * We don't ever allocate `> isize::MAX` byte-size objects.
// * We don't overflow `usize::MAX` and actually allocate too little.
//
// On 64-bit we just need to check for overflow since trying to allocate
// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
// an extra guard for this in case we're running on a platform which can use
// all 4GB in user-space, e.g., PAE or x32.
#[inline]
fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
if usize::BITS < 64 && alloc_size > isize::MAX as usize {
Err(CapacityOverflow.into())
} else {
Ok(())
}
}
#[inline]
fn layout_array(cap: usize, elem_layout: Layout) -> Result<Layout, TryReserveError> {
elem_layout.repeat(cap).map(|(layout, _pad)| layout).map_err(|_| CapacityOverflow.into())
}