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, Alloc, AllocErr, 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: Alloc = Global> {
51 impl<T, A: Alloc> 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(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) };
90 Ok(ptr) => ptr.cast(),
91 Err(_) => handle_alloc_error(layout),
95 RawVec { ptr: ptr.into(), cap: capacity, a }
100 impl<T> RawVec<T, Global> {
101 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
102 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
104 /// If you change `RawVec<T>::new` or dependencies, please take care to not
105 /// introduce anything that would truly violate `min_const_fn`.
107 /// NOTE: We could avoid this hack and check conformance with some
108 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
109 /// with `min_const_fn` but does not necessarily allow calling it in
110 /// `stable(...) const fn` / user code not enabling `foo` when
111 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
112 pub const NEW: Self = Self::new();
114 /// Creates the biggest possible `RawVec` (on the system heap)
115 /// without allocating. If `T` has positive size, then this makes a
116 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
117 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
118 /// delayed allocation.
119 pub const fn new() -> Self {
123 /// Creates a `RawVec` (on the system heap) with exactly the
124 /// capacity and alignment requirements for a `[T; capacity]`. This is
125 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
126 /// zero-sized. Note that if `T` is zero-sized this means you will
127 /// *not* get a `RawVec` with the requested capacity.
131 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
132 /// * Panics on 32-bit platforms if the requested capacity exceeds
133 /// `isize::MAX` bytes.
139 pub fn with_capacity(capacity: usize) -> Self {
140 RawVec::allocate_in(capacity, false, Global)
143 /// Like `with_capacity`, but guarantees the buffer is zeroed.
145 pub fn with_capacity_zeroed(capacity: usize) -> Self {
146 RawVec::allocate_in(capacity, true, Global)
150 impl<T, A: Alloc> RawVec<T, A> {
151 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
153 /// # Undefined Behavior
155 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
156 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
157 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
158 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
159 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a }
163 impl<T> RawVec<T, Global> {
164 /// Reconstitutes a `RawVec` from a pointer and capacity.
166 /// # Undefined Behavior
168 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
169 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
170 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
171 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
172 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a: Global }
175 /// Converts a `Box<[T]>` into a `RawVec<T>`.
176 pub fn from_box(mut slice: Box<[T]>) -> Self {
178 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
185 impl<T, A: Alloc> RawVec<T, A> {
186 /// Gets a raw pointer to the start of the allocation. Note that this is
187 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
189 pub fn ptr(&self) -> *mut T {
193 /// Gets the capacity of the allocation.
195 /// This will always be `usize::MAX` if `T` is zero-sized.
197 pub fn capacity(&self) -> usize {
198 if mem::size_of::<T>() == 0 { !0 } else { self.cap }
201 /// Returns a shared reference to the allocator backing this `RawVec`.
202 pub fn alloc(&self) -> &A {
206 /// Returns a mutable reference to the allocator backing this `RawVec`.
207 pub fn alloc_mut(&mut self) -> &mut A {
211 fn current_layout(&self) -> Option<Layout> {
215 // We have an allocated chunk of memory, so we can bypass runtime
216 // checks to get our current layout.
218 let align = mem::align_of::<T>();
219 let size = mem::size_of::<T>() * self.cap;
220 Some(Layout::from_size_align_unchecked(size, align))
225 /// Doubles the size of the type's backing allocation. This is common enough
226 /// to want to do that it's easiest to just have a dedicated method. Slightly
227 /// more efficient logic can be provided for this than the general case.
229 /// This function is ideal for when pushing elements one-at-a-time because
230 /// you don't need to incur the costs of the more general computations
231 /// reserve needs to do to guard against overflow. You do however need to
232 /// manually check if your `len == capacity`.
236 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
237 /// all `usize::MAX` slots in your imaginary buffer.
238 /// * Panics on 32-bit platforms if the requested capacity exceeds
239 /// `isize::MAX` bytes.
248 /// # #![feature(raw_vec_internals)]
249 /// # extern crate alloc;
251 /// # use alloc::raw_vec::RawVec;
252 /// struct MyVec<T> {
257 /// impl<T> MyVec<T> {
258 /// pub fn push(&mut self, elem: T) {
259 /// if self.len == self.buf.capacity() { self.buf.double(); }
260 /// // double would have aborted or panicked if the len exceeded
261 /// // `isize::MAX` so this is safe to do unchecked now.
263 /// ptr::write(self.buf.ptr().add(self.len), elem);
269 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
275 pub fn double(&mut self) {
277 let elem_size = mem::size_of::<T>();
279 // Since we set the capacity to `usize::MAX` when `elem_size` is
280 // 0, getting to here necessarily means the `RawVec` is overfull.
281 assert!(elem_size != 0, "capacity overflow");
283 let (new_cap, uniq) = match self.current_layout() {
285 // Since we guarantee that we never allocate more than
286 // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
287 // a precondition, so this can't overflow. Additionally the
288 // alignment will never be too large as to "not be
289 // satisfiable", so `Layout::from_size_align` will always
292 // TL;DR, we bypass runtime checks due to dynamic assertions
293 // in this module, allowing us to use
294 // `from_size_align_unchecked`.
295 let new_cap = 2 * self.cap;
296 let new_size = new_cap * elem_size;
297 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
298 let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(), cur, new_size);
300 Ok(ptr) => (new_cap, ptr.cast().into()),
301 Err(_) => handle_alloc_error(Layout::from_size_align_unchecked(
308 // Skip to 4 because tiny `Vec`'s are dumb; but not if that
309 // would cause overflow.
310 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
311 match self.a.alloc_array::<T>(new_cap) {
312 Ok(ptr) => (new_cap, ptr.into()),
313 Err(_) => handle_alloc_error(Layout::array::<T>(new_cap).unwrap()),
322 /// Attempts to double the size of the type's backing allocation in place. This is common
323 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
324 /// more efficient logic can be provided for this than the general case.
326 /// Returns `true` if the reallocation attempt has succeeded.
330 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
331 /// all `usize::MAX` slots in your imaginary buffer.
332 /// * Panics on 32-bit platforms if the requested capacity exceeds
333 /// `isize::MAX` bytes.
336 pub fn double_in_place(&mut self) -> bool {
338 let elem_size = mem::size_of::<T>();
339 let old_layout = match self.current_layout() {
340 Some(layout) => layout,
341 None => return false, // nothing to double
344 // Since we set the capacity to `usize::MAX` when `elem_size` is
345 // 0, getting to here necessarily means the `RawVec` is overfull.
346 assert!(elem_size != 0, "capacity overflow");
348 // Since we guarantee that we never allocate more than `isize::MAX`
349 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
350 // this can't overflow.
352 // Similarly to with `double` above, we can go straight to
353 // `Layout::from_size_align_unchecked` as we know this won't
354 // overflow and the alignment is sufficiently small.
355 let new_cap = 2 * self.cap;
356 let new_size = new_cap * elem_size;
357 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
358 match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
360 // We can't directly divide `size`.
369 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
370 pub fn try_reserve_exact(
372 used_capacity: usize,
373 needed_extra_capacity: usize,
374 ) -> Result<(), TryReserveError> {
375 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
378 /// Ensures that the buffer contains at least enough space to hold
379 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
380 /// will reallocate the minimum possible amount of memory necessary.
381 /// Generally this will be exactly the amount of memory necessary,
382 /// but in principle the allocator is free to give back more than
385 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
386 /// the requested space. This is not really unsafe, but the unsafe
387 /// code *you* write that relies on the behavior of this function may break.
391 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
392 /// * Panics on 32-bit platforms if the requested capacity exceeds
393 /// `isize::MAX` bytes.
398 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
399 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
400 Err(CapacityOverflow) => capacity_overflow(),
401 Err(AllocError { .. }) => unreachable!(),
402 Ok(()) => { /* yay */ }
406 /// Calculates the buffer's new size given that it'll hold `used_capacity +
407 /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
408 /// Returns `(new_capacity, new_alloc_size)`.
409 fn amortized_new_size(
411 used_capacity: usize,
412 needed_extra_capacity: usize,
413 ) -> Result<usize, TryReserveError> {
414 // Nothing we can really do about these checks, sadly.
416 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
417 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
418 let double_cap = self.cap * 2;
419 // `double_cap` guarantees exponential growth.
420 Ok(cmp::max(double_cap, required_cap))
423 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
426 used_capacity: usize,
427 needed_extra_capacity: usize,
428 ) -> Result<(), TryReserveError> {
429 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
432 /// Ensures that the buffer contains at least enough space to hold
433 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
434 /// enough capacity, will reallocate enough space plus comfortable slack
435 /// space to get amortized `O(1)` behavior. Will limit this behavior
436 /// if it would needlessly cause itself to panic.
438 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
439 /// the requested space. This is not really unsafe, but the unsafe
440 /// code *you* write that relies on the behavior of this function may break.
442 /// This is ideal for implementing a bulk-push operation like `extend`.
446 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
447 /// * Panics on 32-bit platforms if the requested capacity exceeds
448 /// `isize::MAX` bytes.
457 /// # #![feature(raw_vec_internals)]
458 /// # extern crate alloc;
460 /// # use alloc::raw_vec::RawVec;
461 /// struct MyVec<T> {
466 /// impl<T: Clone> MyVec<T> {
467 /// pub fn push_all(&mut self, elems: &[T]) {
468 /// self.buf.reserve(self.len, elems.len());
469 /// // reserve would have aborted or panicked if the len exceeded
470 /// // `isize::MAX` so this is safe to do unchecked now.
473 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
480 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
481 /// # vector.push_all(&[1, 3, 5, 7, 9]);
484 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
485 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
486 Err(CapacityOverflow) => capacity_overflow(),
487 Err(AllocError { .. }) => unreachable!(),
488 Ok(()) => { /* yay */ }
491 /// Attempts to ensure that the buffer contains at least enough space to hold
492 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
493 /// enough capacity, will reallocate in place enough space plus comfortable slack
494 /// space to get amortized `O(1)` behavior. Will limit this behaviour
495 /// if it would needlessly cause itself to panic.
497 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
498 /// the requested space. This is not really unsafe, but the unsafe
499 /// code *you* write that relies on the behavior of this function may break.
501 /// Returns `true` if the reallocation attempt has succeeded.
505 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
506 /// * Panics on 32-bit platforms if the requested capacity exceeds
507 /// `isize::MAX` bytes.
508 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
510 // NOTE: we don't early branch on ZSTs here because we want this
511 // to actually catch "asking for more than usize::MAX" in that case.
512 // If we make it past the first branch then we are guaranteed to
515 // Don't actually need any more capacity. If the current `cap` is 0, we can't
516 // reallocate in place.
517 // Wrapping in case they give a bad `used_capacity`
518 let old_layout = match self.current_layout() {
519 Some(layout) => layout,
520 None => return false,
522 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
527 .amortized_new_size(used_capacity, needed_extra_capacity)
528 .unwrap_or_else(|_| capacity_overflow());
530 // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
531 // (regardless of whether `self.cap - used_capacity` wrapped).
532 // Therefore, we can safely call `grow_in_place`.
534 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
535 // FIXME: may crash and burn on over-reserve
536 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
537 match self.a.grow_in_place(
538 NonNull::from(self.ptr).cast(),
551 /// Shrinks the allocation down to the specified amount. If the given amount
552 /// is 0, actually completely deallocates.
556 /// Panics if the given amount is *larger* than the current capacity.
561 pub fn shrink_to_fit(&mut self, amount: usize) {
562 let elem_size = mem::size_of::<T>();
564 // Set the `cap` because they might be about to promote to a `Box<[T]>`
570 // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
571 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
574 // We want to create a new zero-length vector within the
575 // same allocator. We use `ptr::write` to avoid an
576 // erroneous attempt to drop the contents, and we use
577 // `ptr::read` to sidestep condition against destructuring
578 // types that implement Drop.
581 let a = ptr::read(&self.a as *const A);
582 self.dealloc_buffer();
583 ptr::write(self, RawVec::new_in(a));
585 } else if self.cap != amount {
587 // We know here that our `amount` is greater than zero. This
588 // implies, via the assert above, that capacity is also greater
589 // than zero, which means that we've got a current layout that
592 // We also know that `self.cap` is greater than `amount`, and
593 // consequently we don't need runtime checks for creating either
595 let old_size = elem_size * self.cap;
596 let new_size = elem_size * amount;
597 let align = mem::align_of::<T>();
598 let old_layout = Layout::from_size_align_unchecked(old_size, align);
599 match self.a.realloc(NonNull::from(self.ptr).cast(), old_layout, new_size) {
600 Ok(p) => self.ptr = p.cast().into(),
602 handle_alloc_error(Layout::from_size_align_unchecked(new_size, align))
618 enum ReserveStrategy {
623 use ReserveStrategy::*;
625 impl<T, A: Alloc> RawVec<T, A> {
628 used_capacity: usize,
629 needed_extra_capacity: usize,
630 fallibility: Fallibility,
631 strategy: ReserveStrategy,
632 ) -> Result<(), TryReserveError> {
634 // NOTE: we don't early branch on ZSTs here because we want this
635 // to actually catch "asking for more than usize::MAX" in that case.
636 // If we make it past the first branch then we are guaranteed to
639 // Don't actually need any more capacity.
640 // Wrapping in case they gave a bad `used_capacity`.
641 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
645 // Nothing we can really do about these checks, sadly.
646 let new_cap = match strategy {
648 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?
650 Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
652 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
654 alloc_guard(new_layout.size())?;
656 let res = match self.current_layout() {
658 debug_assert!(new_layout.align() == layout.align());
659 self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
661 None => self.a.alloc(new_layout),
664 let ptr = match (res, fallibility) {
665 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
666 (Err(AllocErr), Fallible) => {
667 return Err(TryReserveError::AllocError {
675 self.ptr = ptr.cast().into();
683 impl<T> RawVec<T, Global> {
684 /// Converts the entire buffer into `Box<[T]>`.
686 /// Note that this will correctly reconstitute any `cap` changes
687 /// that may have been performed. (See description of type for details.)
689 /// # Undefined Behavior
691 /// All elements of `RawVec<T, Global>` must be initialized. Notice that
692 /// the rules around uninitialized boxed values are not finalized yet,
693 /// but until they are, it is advisable to avoid them.
694 pub unsafe fn into_box(self) -> Box<[T]> {
695 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
696 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
697 let output: Box<[T]> = Box::from_raw(slice);
703 impl<T, A: Alloc> RawVec<T, A> {
704 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
705 pub unsafe fn dealloc_buffer(&mut self) {
706 let elem_size = mem::size_of::<T>();
708 if let Some(layout) = self.current_layout() {
709 self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
715 unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
716 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
719 self.dealloc_buffer();
724 // We need to guarantee the following:
725 // * We don't ever allocate `> isize::MAX` byte-size objects.
726 // * We don't overflow `usize::MAX` and actually allocate too little.
728 // On 64-bit we just need to check for overflow since trying to allocate
729 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
730 // an extra guard for this in case we're running on a platform which can use
731 // all 4GB in user-space, e.g., PAE or x32.
734 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
735 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
736 Err(CapacityOverflow)
742 // One central function responsible for reporting capacity overflows. This'll
743 // ensure that the code generation related to these panics is minimal as there's
744 // only one location which panics rather than a bunch throughout the module.
745 fn capacity_overflow() -> ! {
746 panic!("capacity overflow");