1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "0")]
7 use core::ptr::{self, NonNull, Unique};
10 use crate::alloc::{Alloc, Layout, Global, AllocErr, handle_alloc_error};
11 use crate::collections::TryReserveError::{self, *};
12 use crate::boxed::Box;
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".
65 /// Like `with_capacity`, but parameterized over the choice of
66 /// allocator for the returned `RawVec`.
68 pub fn with_capacity_in(capacity: usize, a: A) -> Self {
69 RawVec::allocate_in(capacity, false, a)
72 /// Like `with_capacity_zeroed`, but parameterized over the choice
73 /// of allocator for the returned `RawVec`.
75 pub fn with_capacity_zeroed_in(capacity: usize, a: A) -> Self {
76 RawVec::allocate_in(capacity, true, a)
79 fn allocate_in(capacity: usize, zeroed: bool, mut a: A) -> Self {
81 let elem_size = mem::size_of::<T>();
83 let alloc_size = capacity.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
84 alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
86 // Handles ZSTs and `capacity == 0` alike.
87 let ptr = if alloc_size == 0 {
88 NonNull::<T>::dangling()
90 let align = mem::align_of::<T>();
91 let layout = Layout::from_size_align(alloc_size, align).unwrap();
92 let result = if zeroed {
93 a.alloc_zeroed(layout)
98 Ok(ptr) => ptr.cast(),
99 Err(_) => handle_alloc_error(layout),
112 impl<T> RawVec<T, Global> {
113 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
114 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
116 /// If you change `RawVec<T>::new` or dependencies, please take care to not
117 /// introduce anything that would truly violate `min_const_fn`.
119 /// NOTE: We could avoid this hack and check conformance with some
120 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
121 /// with `min_const_fn` but does not necessarily allow calling it in
122 /// `stable(...) const fn` / user code not enabling `foo` when
123 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
124 pub const NEW: Self = Self::new();
126 /// Creates the biggest possible `RawVec` (on the system heap)
127 /// without allocating. If `T` has positive size, then this makes a
128 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
129 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
130 /// delayed allocation.
131 pub const fn new() -> Self {
135 /// Creates a `RawVec` (on the system heap) with exactly the
136 /// capacity and alignment requirements for a `[T; capacity]`. This is
137 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
138 /// zero-sized. Note that if `T` is zero-sized this means you will
139 /// *not* get a `RawVec` with the requested capacity.
143 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
144 /// * Panics on 32-bit platforms if the requested capacity exceeds
145 /// `isize::MAX` bytes.
151 pub fn with_capacity(capacity: usize) -> Self {
152 RawVec::allocate_in(capacity, false, Global)
155 /// Like `with_capacity`, but guarantees the buffer is zeroed.
157 pub fn with_capacity_zeroed(capacity: usize) -> Self {
158 RawVec::allocate_in(capacity, true, Global)
162 impl<T, A: Alloc> RawVec<T, A> {
163 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
165 /// # Undefined Behavior
167 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
168 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
169 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
170 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
172 ptr: Unique::new_unchecked(ptr),
179 impl<T> RawVec<T, Global> {
180 /// Reconstitutes a `RawVec` from a pointer and capacity.
182 /// # Undefined Behavior
184 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
185 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
186 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
187 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
189 ptr: Unique::new_unchecked(ptr),
195 /// Converts a `Box<[T]>` into a `RawVec<T>`.
196 pub fn from_box(mut slice: Box<[T]>) -> Self {
198 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
205 impl<T, A: Alloc> RawVec<T, A> {
206 /// Gets a raw pointer to the start of the allocation. Note that this is
207 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
209 pub fn ptr(&self) -> *mut T {
213 /// Gets the capacity of the allocation.
215 /// This will always be `usize::MAX` if `T` is zero-sized.
217 pub fn capacity(&self) -> usize {
218 if mem::size_of::<T>() == 0 {
225 /// Returns a shared reference to the allocator backing this `RawVec`.
226 pub fn alloc(&self) -> &A {
230 /// Returns a mutable reference to the allocator backing this `RawVec`.
231 pub fn alloc_mut(&mut self) -> &mut A {
235 fn current_layout(&self) -> Option<Layout> {
239 // We have an allocated chunk of memory, so we can bypass runtime
240 // checks to get our current layout.
242 let align = mem::align_of::<T>();
243 let size = mem::size_of::<T>() * self.cap;
244 Some(Layout::from_size_align_unchecked(size, align))
249 /// Doubles the size of the type's backing allocation. This is common enough
250 /// to want to do that it's easiest to just have a dedicated method. Slightly
251 /// more efficient logic can be provided for this than the general case.
253 /// This function is ideal for when pushing elements one-at-a-time because
254 /// you don't need to incur the costs of the more general computations
255 /// reserve needs to do to guard against overflow. You do however need to
256 /// manually check if your `len == capacity`.
260 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
261 /// all `usize::MAX` slots in your imaginary buffer.
262 /// * Panics on 32-bit platforms if the requested capacity exceeds
263 /// `isize::MAX` bytes.
272 /// # #![feature(raw_vec_internals)]
273 /// # extern crate alloc;
275 /// # use alloc::raw_vec::RawVec;
276 /// struct MyVec<T> {
281 /// impl<T> MyVec<T> {
282 /// pub fn push(&mut self, elem: T) {
283 /// if self.len == self.buf.capacity() { self.buf.double(); }
284 /// // double would have aborted or panicked if the len exceeded
285 /// // `isize::MAX` so this is safe to do unchecked now.
287 /// ptr::write(self.buf.ptr().add(self.len), elem);
293 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
299 pub fn double(&mut self) {
301 let elem_size = mem::size_of::<T>();
303 // Since we set the capacity to `usize::MAX` when `elem_size` is
304 // 0, getting to here necessarily means the `RawVec` is overfull.
305 assert!(elem_size != 0, "capacity overflow");
307 let (new_cap, uniq) = match self.current_layout() {
309 // Since we guarantee that we never allocate more than
310 // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
311 // a precondition, so this can't overflow. Additionally the
312 // alignment will never be too large as to "not be
313 // satisfiable", so `Layout::from_size_align` will always
316 // TL;DR, we bypass runtime checks due to dynamic assertions
317 // in this module, allowing us to use
318 // `from_size_align_unchecked`.
319 let new_cap = 2 * self.cap;
320 let new_size = new_cap * elem_size;
321 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
322 let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(),
326 Ok(ptr) => (new_cap, ptr.cast().into()),
327 Err(_) => handle_alloc_error(
328 Layout::from_size_align_unchecked(new_size, cur.align())
333 // Skip to 4 because tiny `Vec`'s are dumb; but not if that
334 // would cause overflow.
335 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
336 match self.a.alloc_array::<T>(new_cap) {
337 Ok(ptr) => (new_cap, ptr.into()),
338 Err(_) => handle_alloc_error(Layout::array::<T>(new_cap).unwrap()),
347 /// Attempts to double the size of the type's backing allocation in place. This is common
348 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
349 /// more efficient logic can be provided for this than the general case.
351 /// Returns `true` if the reallocation attempt has succeeded.
355 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
356 /// all `usize::MAX` slots in your imaginary buffer.
357 /// * Panics on 32-bit platforms if the requested capacity exceeds
358 /// `isize::MAX` bytes.
361 pub fn double_in_place(&mut self) -> bool {
363 let elem_size = mem::size_of::<T>();
364 let old_layout = match self.current_layout() {
365 Some(layout) => layout,
366 None => return false, // nothing to double
369 // Since we set the capacity to `usize::MAX` when `elem_size` is
370 // 0, getting to here necessarily means the `RawVec` is overfull.
371 assert!(elem_size != 0, "capacity overflow");
373 // Since we guarantee that we never allocate more than `isize::MAX`
374 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
375 // this can't overflow.
377 // Similarly to with `double` above, we can go straight to
378 // `Layout::from_size_align_unchecked` as we know this won't
379 // overflow and the alignment is sufficiently small.
380 let new_cap = 2 * self.cap;
381 let new_size = new_cap * elem_size;
382 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
383 match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
385 // We can't directly divide `size`.
396 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
397 pub fn try_reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize)
398 -> Result<(), TryReserveError> {
400 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
403 /// Ensures that the buffer contains at least enough space to hold
404 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
405 /// will reallocate the minimum possible amount of memory necessary.
406 /// Generally this will be exactly the amount of memory necessary,
407 /// but in principle the allocator is free to give back more than
410 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
411 /// the requested space. This is not really unsafe, but the unsafe
412 /// code *you* write that relies on the behavior of this function may break.
416 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
417 /// * Panics on 32-bit platforms if the requested capacity exceeds
418 /// `isize::MAX` bytes.
423 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
424 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
425 Err(CapacityOverflow) => capacity_overflow(),
426 Err(AllocError { .. }) => unreachable!(),
427 Ok(()) => { /* yay */ }
431 /// Calculates the buffer's new size given that it'll hold `used_capacity +
432 /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
433 /// Returns `(new_capacity, new_alloc_size)`.
434 fn amortized_new_size(&self, used_capacity: usize, needed_extra_capacity: usize)
435 -> Result<usize, TryReserveError> {
437 // Nothing we can really do about these checks, sadly.
438 let required_cap = used_capacity.checked_add(needed_extra_capacity)
439 .ok_or(CapacityOverflow)?;
440 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
441 let double_cap = self.cap * 2;
442 // `double_cap` guarantees exponential growth.
443 Ok(cmp::max(double_cap, required_cap))
446 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
447 pub fn try_reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize)
448 -> Result<(), TryReserveError> {
449 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
452 /// Ensures that the buffer contains at least enough space to hold
453 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
454 /// enough capacity, will reallocate enough space plus comfortable slack
455 /// space to get amortized `O(1)` behavior. Will limit this behavior
456 /// if it would needlessly cause itself to panic.
458 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
459 /// the requested space. This is not really unsafe, but the unsafe
460 /// code *you* write that relies on the behavior of this function may break.
462 /// This is ideal for implementing a bulk-push operation like `extend`.
466 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
467 /// * Panics on 32-bit platforms if the requested capacity exceeds
468 /// `isize::MAX` bytes.
477 /// # #![feature(raw_vec_internals)]
478 /// # extern crate alloc;
480 /// # use alloc::raw_vec::RawVec;
481 /// struct MyVec<T> {
486 /// impl<T: Clone> MyVec<T> {
487 /// pub fn push_all(&mut self, elems: &[T]) {
488 /// self.buf.reserve(self.len, elems.len());
489 /// // reserve would have aborted or panicked if the len exceeded
490 /// // `isize::MAX` so this is safe to do unchecked now.
493 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
500 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
501 /// # vector.push_all(&[1, 3, 5, 7, 9]);
504 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
505 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
506 Err(CapacityOverflow) => capacity_overflow(),
507 Err(AllocError { .. }) => unreachable!(),
508 Ok(()) => { /* yay */ }
511 /// Attempts to ensure that the buffer contains at least enough space to hold
512 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
513 /// enough capacity, will reallocate in place enough space plus comfortable slack
514 /// space to get amortized `O(1)` behavior. Will limit this behaviour
515 /// if it would needlessly cause itself to panic.
517 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
518 /// the requested space. This is not really unsafe, but the unsafe
519 /// code *you* write that relies on the behavior of this function may break.
521 /// Returns `true` if the reallocation attempt has succeeded.
525 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
526 /// * Panics on 32-bit platforms if the requested capacity exceeds
527 /// `isize::MAX` bytes.
528 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
530 // NOTE: we don't early branch on ZSTs here because we want this
531 // to actually catch "asking for more than usize::MAX" in that case.
532 // If we make it past the first branch then we are guaranteed to
535 // Don't actually need any more capacity. If the current `cap` is 0, we can't
536 // reallocate in place.
537 // Wrapping in case they give a bad `used_capacity`
538 let old_layout = match self.current_layout() {
539 Some(layout) => layout,
540 None => return false,
542 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
546 let new_cap = self.amortized_new_size(used_capacity, needed_extra_capacity)
547 .unwrap_or_else(|_| capacity_overflow());
549 // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
550 // (regardless of whether `self.cap - used_capacity` wrapped).
551 // Therefore, we can safely call `grow_in_place`.
553 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
554 // FIXME: may crash and burn on over-reserve
555 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
556 match self.a.grow_in_place(
557 NonNull::from(self.ptr).cast(), old_layout, new_layout.size(),
570 /// Shrinks the allocation down to the specified amount. If the given amount
571 /// is 0, actually completely deallocates.
575 /// Panics if the given amount is *larger* than the current capacity.
580 pub fn shrink_to_fit(&mut self, amount: usize) {
581 let elem_size = mem::size_of::<T>();
583 // Set the `cap` because they might be about to promote to a `Box<[T]>`
589 // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
590 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
593 // We want to create a new zero-length vector within the
594 // same allocator. We use `ptr::write` to avoid an
595 // erroneous attempt to drop the contents, and we use
596 // `ptr::read` to sidestep condition against destructuring
597 // types that implement Drop.
600 let a = ptr::read(&self.a as *const A);
601 self.dealloc_buffer();
602 ptr::write(self, RawVec::new_in(a));
604 } else if self.cap != amount {
606 // We know here that our `amount` is greater than zero. This
607 // implies, via the assert above, that capacity is also greater
608 // than zero, which means that we've got a current layout that
611 // We also know that `self.cap` is greater than `amount`, and
612 // consequently we don't need runtime checks for creating either
614 let old_size = elem_size * self.cap;
615 let new_size = elem_size * amount;
616 let align = mem::align_of::<T>();
617 let old_layout = Layout::from_size_align_unchecked(old_size, align);
618 match self.a.realloc(NonNull::from(self.ptr).cast(),
621 Ok(p) => self.ptr = p.cast().into(),
622 Err(_) => handle_alloc_error(
623 Layout::from_size_align_unchecked(new_size, align)
639 enum ReserveStrategy {
644 use ReserveStrategy::*;
646 impl<T, A: Alloc> RawVec<T, A> {
649 used_capacity: usize,
650 needed_extra_capacity: usize,
651 fallibility: Fallibility,
652 strategy: ReserveStrategy,
653 ) -> Result<(), TryReserveError> {
655 // NOTE: we don't early branch on ZSTs here because we want this
656 // to actually catch "asking for more than usize::MAX" in that case.
657 // If we make it past the first branch then we are guaranteed to
660 // Don't actually need any more capacity.
661 // Wrapping in case they gave a bad `used_capacity`.
662 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
666 // Nothing we can really do about these checks, sadly.
667 let new_cap = match strategy {
668 Exact => used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?,
669 Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
671 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
673 alloc_guard(new_layout.size())?;
675 let res = match self.current_layout() {
677 debug_assert!(new_layout.align() == layout.align());
678 self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
680 None => self.a.alloc(new_layout),
683 let ptr = match (res, fallibility) {
684 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
685 (Err(AllocErr), Fallible) => return Err(TryReserveError::AllocError {
692 self.ptr = ptr.cast().into();
701 impl<T> RawVec<T, Global> {
702 /// Converts the entire buffer into `Box<[T]>`.
704 /// Note that this will correctly reconstitute any `cap` changes
705 /// that may have been performed. (See description of type for details.)
707 /// # Undefined Behavior
709 /// All elements of `RawVec<T, Global>` must be initialized. Notice that
710 /// the rules around uninitialized boxed values are not finalized yet,
711 /// but until they are, it is advisable to avoid them.
712 pub unsafe fn into_box(self) -> Box<[T]> {
713 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
714 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
715 let output: Box<[T]> = Box::from_raw(slice);
721 impl<T, A: Alloc> RawVec<T, A> {
722 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
723 pub unsafe fn dealloc_buffer(&mut self) {
724 let elem_size = mem::size_of::<T>();
726 if let Some(layout) = self.current_layout() {
727 self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
733 unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
734 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
736 unsafe { self.dealloc_buffer(); }
740 // We need to guarantee the following:
741 // * We don't ever allocate `> isize::MAX` byte-size objects.
742 // * We don't overflow `usize::MAX` and actually allocate too little.
744 // On 64-bit we just need to check for overflow since trying to allocate
745 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
746 // an extra guard for this in case we're running on a platform which can use
747 // all 4GB in user-space, e.g., PAE or x32.
750 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
751 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
752 Err(CapacityOverflow)
758 // One central function responsible for reporting capacity overflows. This'll
759 // ensure that the code generation related to these panics is minimal as there's
760 // only one location which panics rather than a bunch throughout the module.
761 fn capacity_overflow() -> ! {
762 panic!("capacity overflow");