1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
7 use core::ptr::{self, NonNull, Unique};
10 use crate::alloc::{handle_alloc_error, AllocErr, AllocRef, Global, Layout};
11 use crate::boxed::Box;
12 use crate::collections::TryReserveError::{self, *};
17 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
18 /// a buffer of memory on the heap without having to worry about all the corner cases
19 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
22 /// * Produces `Unique::empty()` on zero-sized types.
23 /// * Produces `Unique::empty()` on zero-length allocations.
24 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
25 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
26 /// * Guards against overflowing your length.
27 /// * Aborts on OOM or calls `handle_alloc_error` as applicable.
28 /// * Avoids freeing `Unique::empty()`.
29 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
31 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
32 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
33 /// to handle the actual things *stored* inside of a `RawVec`.
35 /// Note that a `RawVec` always forces its capacity to be `usize::MAX` for zero-sized types.
36 /// This enables you to use capacity-growing logic catch the overflows in your length
37 /// that might occur with zero-sized types.
39 /// The above means that you need to be careful when round-tripping this type with a
40 /// `Box<[T]>`, since `capacity()` won't yield the length. However, `with_capacity`,
41 /// `shrink_to_fit`, and `from_box` will actually set `RawVec`'s private capacity
42 /// field. This allows zero-sized types to not be special-cased by consumers of
44 #[allow(missing_debug_implementations)]
45 pub struct RawVec<T, A: AllocRef = Global> {
51 impl<T, A: AllocRef> RawVec<T, A> {
52 /// Like `new`, but parameterized over the choice of allocator for
53 /// the returned `RawVec`.
54 pub const fn new_in(a: A) -> Self {
55 let cap = if mem::size_of::<T>() == 0 { core::usize::MAX } else { 0 };
57 // `Unique::empty()` doubles as "unallocated" and "zero-sized allocation".
58 RawVec { ptr: Unique::empty(), cap, a }
61 /// Like `with_capacity`, but parameterized over the choice of
62 /// allocator for the returned `RawVec`.
64 pub fn with_capacity_in(capacity: usize, a: A) -> Self {
65 RawVec::allocate_in(capacity, false, a)
68 /// Like `with_capacity_zeroed`, but parameterized over the choice
69 /// of allocator for the returned `RawVec`.
71 pub fn with_capacity_zeroed_in(capacity: usize, a: A) -> Self {
72 RawVec::allocate_in(capacity, true, a)
75 fn allocate_in(mut capacity: usize, zeroed: bool, mut a: A) -> Self {
77 let elem_size = mem::size_of::<T>();
79 let alloc_size = capacity.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
80 alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
82 // Handles ZSTs and `capacity == 0` alike.
83 let ptr = if alloc_size == 0 {
84 NonNull::<T>::dangling()
86 let align = mem::align_of::<T>();
87 let layout = Layout::from_size_align(alloc_size, align).unwrap();
88 let result = if zeroed { a.alloc_zeroed(layout) } else { a.alloc(layout) };
91 capacity = size / elem_size;
94 Err(_) => handle_alloc_error(layout),
98 RawVec { ptr: ptr.into(), cap: capacity, a }
103 impl<T> RawVec<T, Global> {
104 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
105 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
107 /// If you change `RawVec<T>::new` or dependencies, please take care to not
108 /// introduce anything that would truly violate `min_const_fn`.
110 /// NOTE: We could avoid this hack and check conformance with some
111 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
112 /// with `min_const_fn` but does not necessarily allow calling it in
113 /// `stable(...) const fn` / user code not enabling `foo` when
114 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
115 pub const NEW: Self = Self::new();
117 /// Creates the biggest possible `RawVec` (on the system heap)
118 /// without allocating. If `T` has positive size, then this makes a
119 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
120 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
121 /// delayed allocation.
122 pub const fn new() -> Self {
126 /// Creates a `RawVec` (on the system heap) with exactly the
127 /// capacity and alignment requirements for a `[T; capacity]`. This is
128 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
129 /// zero-sized. Note that if `T` is zero-sized this means you will
130 /// *not* get a `RawVec` with the requested capacity.
134 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
135 /// * Panics on 32-bit platforms if the requested capacity exceeds
136 /// `isize::MAX` bytes.
142 pub fn with_capacity(capacity: usize) -> Self {
143 RawVec::allocate_in(capacity, false, Global)
146 /// Like `with_capacity`, but guarantees the buffer is zeroed.
148 pub fn with_capacity_zeroed(capacity: usize) -> Self {
149 RawVec::allocate_in(capacity, true, Global)
153 impl<T, A: AllocRef> RawVec<T, A> {
154 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
156 /// # Undefined Behavior
158 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
159 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
160 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
161 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
162 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a }
166 impl<T> RawVec<T, Global> {
167 /// Reconstitutes a `RawVec` from a pointer and capacity.
169 /// # Undefined Behavior
171 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
172 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
173 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
174 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
175 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a: Global }
178 /// Converts a `Box<[T]>` into a `RawVec<T>`.
179 pub fn from_box(mut slice: Box<[T]>) -> Self {
181 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
188 impl<T, A: AllocRef> RawVec<T, A> {
189 /// Gets a raw pointer to the start of the allocation. Note that this is
190 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
192 pub fn ptr(&self) -> *mut T {
196 /// Gets the capacity of the allocation.
198 /// This will always be `usize::MAX` if `T` is zero-sized.
200 pub fn capacity(&self) -> usize {
201 if mem::size_of::<T>() == 0 { !0 } else { self.cap }
204 /// Returns a shared reference to the allocator backing this `RawVec`.
205 pub fn alloc(&self) -> &A {
209 /// Returns a mutable reference to the allocator backing this `RawVec`.
210 pub fn alloc_mut(&mut self) -> &mut A {
214 fn current_layout(&self) -> Option<Layout> {
218 // We have an allocated chunk of memory, so we can bypass runtime
219 // checks to get our current layout.
221 let align = mem::align_of::<T>();
222 let size = mem::size_of::<T>() * self.cap;
223 Some(Layout::from_size_align_unchecked(size, align))
228 /// Doubles the size of the type's backing allocation. This is common enough
229 /// to want to do that it's easiest to just have a dedicated method. Slightly
230 /// more efficient logic can be provided for this than the general case.
232 /// This function is ideal for when pushing elements one-at-a-time because
233 /// you don't need to incur the costs of the more general computations
234 /// reserve needs to do to guard against overflow. You do however need to
235 /// manually check if your `len == capacity`.
239 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
240 /// all `usize::MAX` slots in your imaginary buffer.
241 /// * Panics on 32-bit platforms if the requested capacity exceeds
242 /// `isize::MAX` bytes.
251 /// # #![feature(raw_vec_internals)]
252 /// # extern crate alloc;
254 /// # use alloc::raw_vec::RawVec;
255 /// struct MyVec<T> {
260 /// impl<T> MyVec<T> {
261 /// pub fn push(&mut self, elem: T) {
262 /// if self.len == self.buf.capacity() { self.buf.double(); }
263 /// // double would have aborted or panicked if the len exceeded
264 /// // `isize::MAX` so this is safe to do unchecked now.
266 /// ptr::write(self.buf.ptr().add(self.len), elem);
272 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
278 pub fn double(&mut self) {
280 let elem_size = mem::size_of::<T>();
282 // Since we set the capacity to `usize::MAX` when `elem_size` is
283 // 0, getting to here necessarily means the `RawVec` is overfull.
284 assert!(elem_size != 0, "capacity overflow");
286 let (ptr, new_cap) = match self.current_layout() {
288 // Since we guarantee that we never allocate more than
289 // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
290 // a precondition, so this can't overflow. Additionally the
291 // alignment will never be too large as to "not be
292 // satisfiable", so `Layout::from_size_align` will always
295 // TL;DR, we bypass runtime checks due to dynamic assertions
296 // in this module, allowing us to use
297 // `from_size_align_unchecked`.
298 let new_cap = 2 * self.cap;
299 let new_size = new_cap * elem_size;
300 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
301 let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(), cur, new_size);
303 Ok((ptr, new_size)) => (ptr, new_size / elem_size),
304 Err(_) => handle_alloc_error(Layout::from_size_align_unchecked(
311 // Skip to 4 because tiny `Vec`'s are dumb; but not if that
312 // would cause overflow.
313 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
314 let layout = Layout::array::<T>(new_cap).unwrap();
315 match self.a.alloc(layout) {
316 Ok((ptr, new_size)) => (ptr, new_size / elem_size),
317 Err(_) => handle_alloc_error(layout),
321 self.ptr = ptr.cast().into();
326 /// Attempts to double the size of the type's backing allocation in place. This is common
327 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
328 /// more efficient logic can be provided for this than the general case.
330 /// Returns `true` if the reallocation attempt has succeeded.
334 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
335 /// all `usize::MAX` slots in your imaginary buffer.
336 /// * Panics on 32-bit platforms if the requested capacity exceeds
337 /// `isize::MAX` bytes.
340 pub fn double_in_place(&mut self) -> bool {
342 let elem_size = mem::size_of::<T>();
343 let old_layout = match self.current_layout() {
344 Some(layout) => layout,
345 None => return false, // nothing to double
348 // Since we set the capacity to `usize::MAX` when `elem_size` is
349 // 0, getting to here necessarily means the `RawVec` is overfull.
350 assert!(elem_size != 0, "capacity overflow");
352 // Since we guarantee that we never allocate more than `isize::MAX`
353 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
354 // this can't overflow.
356 // Similarly to with `double` above, we can go straight to
357 // `Layout::from_size_align_unchecked` as we know this won't
358 // overflow and the alignment is sufficiently small.
359 let new_cap = 2 * self.cap;
360 let new_size = new_cap * elem_size;
361 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
362 match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
364 // We can't directly divide `size`.
373 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
374 pub fn try_reserve_exact(
376 used_capacity: usize,
377 needed_extra_capacity: usize,
378 ) -> Result<(), TryReserveError> {
379 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
382 /// Ensures that the buffer contains at least enough space to hold
383 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
384 /// will reallocate the minimum possible amount of memory necessary.
385 /// Generally this will be exactly the amount of memory necessary,
386 /// but in principle the allocator is free to give back more than
389 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
390 /// the requested space. This is not really unsafe, but the unsafe
391 /// code *you* write that relies on the behavior of this function may break.
395 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
396 /// * Panics on 32-bit platforms if the requested capacity exceeds
397 /// `isize::MAX` bytes.
402 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
403 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
404 Err(CapacityOverflow) => capacity_overflow(),
405 Err(AllocError { .. }) => unreachable!(),
406 Ok(()) => { /* yay */ }
410 /// Calculates the buffer's new size given that it'll hold `used_capacity +
411 /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
412 /// Returns `(new_capacity, new_alloc_size)`.
413 fn amortized_new_size(
415 used_capacity: usize,
416 needed_extra_capacity: usize,
417 ) -> Result<usize, TryReserveError> {
418 // Nothing we can really do about these checks, sadly.
420 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
421 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
422 let double_cap = self.cap * 2;
423 // `double_cap` guarantees exponential growth.
424 Ok(cmp::max(double_cap, required_cap))
427 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
430 used_capacity: usize,
431 needed_extra_capacity: usize,
432 ) -> Result<(), TryReserveError> {
433 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
436 /// Ensures that the buffer contains at least enough space to hold
437 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
438 /// enough capacity, will reallocate enough space plus comfortable slack
439 /// space to get amortized `O(1)` behavior. Will limit this behavior
440 /// if it would needlessly cause itself to panic.
442 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
443 /// the requested space. This is not really unsafe, but the unsafe
444 /// code *you* write that relies on the behavior of this function may break.
446 /// This is ideal for implementing a bulk-push operation like `extend`.
450 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
451 /// * Panics on 32-bit platforms if the requested capacity exceeds
452 /// `isize::MAX` bytes.
461 /// # #![feature(raw_vec_internals)]
462 /// # extern crate alloc;
464 /// # use alloc::raw_vec::RawVec;
465 /// struct MyVec<T> {
470 /// impl<T: Clone> MyVec<T> {
471 /// pub fn push_all(&mut self, elems: &[T]) {
472 /// self.buf.reserve(self.len, elems.len());
473 /// // reserve would have aborted or panicked if the len exceeded
474 /// // `isize::MAX` so this is safe to do unchecked now.
477 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
484 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
485 /// # vector.push_all(&[1, 3, 5, 7, 9]);
488 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
489 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
490 Err(CapacityOverflow) => capacity_overflow(),
491 Err(AllocError { .. }) => unreachable!(),
492 Ok(()) => { /* yay */ }
495 /// Attempts to ensure that the buffer contains at least enough space to hold
496 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
497 /// enough capacity, will reallocate in place enough space plus comfortable slack
498 /// space to get amortized `O(1)` behavior. Will limit this behaviour
499 /// if it would needlessly cause itself to panic.
501 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
502 /// the requested space. This is not really unsafe, but the unsafe
503 /// code *you* write that relies on the behavior of this function may break.
505 /// Returns `true` if the reallocation attempt has succeeded.
509 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
510 /// * Panics on 32-bit platforms if the requested capacity exceeds
511 /// `isize::MAX` bytes.
512 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
514 // NOTE: we don't early branch on ZSTs here because we want this
515 // to actually catch "asking for more than usize::MAX" in that case.
516 // If we make it past the first branch then we are guaranteed to
519 // Don't actually need any more capacity. If the current `cap` is 0, we can't
520 // reallocate in place.
521 // Wrapping in case they give a bad `used_capacity`
522 let old_layout = match self.current_layout() {
523 Some(layout) => layout,
524 None => return false,
526 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
531 .amortized_new_size(used_capacity, needed_extra_capacity)
532 .unwrap_or_else(|_| capacity_overflow());
534 // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
535 // (regardless of whether `self.cap - used_capacity` wrapped).
536 // Therefore, we can safely call `grow_in_place`.
538 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
539 // FIXME: may crash and burn on over-reserve
540 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
541 match self.a.grow_in_place(
542 NonNull::from(self.ptr).cast(),
555 /// Shrinks the allocation down to the specified amount. If the given amount
556 /// is 0, actually completely deallocates.
560 /// Panics if the given amount is *larger* than the current capacity.
565 pub fn shrink_to_fit(&mut self, amount: usize) {
566 let elem_size = mem::size_of::<T>();
568 // Set the `cap` because they might be about to promote to a `Box<[T]>`
574 // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
575 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
578 // We want to create a new zero-length vector within the
579 // same allocator. We use `ptr::write` to avoid an
580 // erroneous attempt to drop the contents, and we use
581 // `ptr::read` to sidestep condition against destructuring
582 // types that implement Drop.
585 let a = ptr::read(&self.a as *const A);
586 self.dealloc_buffer();
587 ptr::write(self, RawVec::new_in(a));
589 } else if self.cap != amount {
591 // We know here that our `amount` is greater than zero. This
592 // implies, via the assert above, that capacity is also greater
593 // than zero, which means that we've got a current layout that
596 // We also know that `self.cap` is greater than `amount`, and
597 // consequently we don't need runtime checks for creating either
599 let old_size = elem_size * self.cap;
600 let new_size = elem_size * amount;
601 let align = mem::align_of::<T>();
602 let old_layout = Layout::from_size_align_unchecked(old_size, align);
603 match self.a.realloc(NonNull::from(self.ptr).cast(), old_layout, new_size) {
604 Ok((ptr, _)) => self.ptr = ptr.cast().into(),
606 handle_alloc_error(Layout::from_size_align_unchecked(new_size, align))
622 enum ReserveStrategy {
627 use ReserveStrategy::*;
629 impl<T, A: AllocRef> RawVec<T, A> {
632 used_capacity: usize,
633 needed_extra_capacity: usize,
634 fallibility: Fallibility,
635 strategy: ReserveStrategy,
636 ) -> Result<(), TryReserveError> {
637 let elem_size = mem::size_of::<T>();
640 // NOTE: we don't early branch on ZSTs here because we want this
641 // to actually catch "asking for more than usize::MAX" in that case.
642 // If we make it past the first branch then we are guaranteed to
645 // Don't actually need any more capacity.
646 // Wrapping in case they gave a bad `used_capacity`.
647 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
651 // Nothing we can really do about these checks, sadly.
652 let new_cap = match strategy {
654 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?
656 Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
658 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
660 alloc_guard(new_layout.size())?;
662 let res = match self.current_layout() {
664 debug_assert!(new_layout.align() == layout.align());
665 self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
667 None => self.a.alloc(new_layout),
670 let (ptr, new_cap) = match (res, fallibility) {
671 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
672 (Err(AllocErr), Fallible) => {
673 return Err(TryReserveError::AllocError {
678 (Ok((ptr, new_size)), _) => (ptr, new_size / elem_size),
681 self.ptr = ptr.cast().into();
689 impl<T> RawVec<T, Global> {
690 /// Converts the entire buffer into `Box<[T]>`.
692 /// Note that this will correctly reconstitute any `cap` changes
693 /// that may have been performed. (See description of type for details.)
695 /// # Undefined Behavior
697 /// All elements of `RawVec<T, Global>` must be initialized. Notice that
698 /// the rules around uninitialized boxed values are not finalized yet,
699 /// but until they are, it is advisable to avoid them.
700 pub unsafe fn into_box(self) -> Box<[T]> {
701 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
702 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
703 let output: Box<[T]> = Box::from_raw(slice);
709 impl<T, A: AllocRef> RawVec<T, A> {
710 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
711 pub unsafe fn dealloc_buffer(&mut self) {
712 let elem_size = mem::size_of::<T>();
714 if let Some(layout) = self.current_layout() {
715 self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
721 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
722 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
725 self.dealloc_buffer();
730 // We need to guarantee the following:
731 // * We don't ever allocate `> isize::MAX` byte-size objects.
732 // * We don't overflow `usize::MAX` and actually allocate too little.
734 // On 64-bit we just need to check for overflow since trying to allocate
735 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
736 // an extra guard for this in case we're running on a platform which can use
737 // all 4GB in user-space, e.g., PAE or x32.
740 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
741 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
742 Err(CapacityOverflow)
748 // One central function responsible for reporting capacity overflows. This'll
749 // ensure that the code generation related to these panics is minimal as there's
750 // only one location which panics rather than a bunch throughout the module.
751 fn capacity_overflow() -> ! {
752 panic!("capacity overflow");