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 {
76 let elem_size = mem::size_of::<T>();
78 let alloc_size = capacity.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
79 alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
81 // Handles ZSTs and `capacity == 0` alike.
82 let ptr = if alloc_size == 0 {
83 NonNull::<T>::dangling()
85 let align = mem::align_of::<T>();
86 let layout = Layout::from_size_align(alloc_size, align).unwrap();
87 let result = if zeroed { a.alloc_zeroed(layout) } else { a.alloc(layout) };
90 capacity = size / elem_size;
93 Err(_) => handle_alloc_error(layout),
97 RawVec { ptr: ptr.into(), cap: capacity, a }
101 impl<T> RawVec<T, Global> {
102 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
103 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
105 /// If you change `RawVec<T>::new` or dependencies, please take care to not
106 /// introduce anything that would truly violate `min_const_fn`.
108 /// NOTE: We could avoid this hack and check conformance with some
109 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
110 /// with `min_const_fn` but does not necessarily allow calling it in
111 /// `stable(...) const fn` / user code not enabling `foo` when
112 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
113 pub const NEW: Self = Self::new();
115 /// Creates the biggest possible `RawVec` (on the system heap)
116 /// without allocating. If `T` has positive size, then this makes a
117 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
118 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
119 /// delayed allocation.
120 pub const fn new() -> Self {
124 /// Creates a `RawVec` (on the system heap) with exactly the
125 /// capacity and alignment requirements for a `[T; capacity]`. This is
126 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
127 /// zero-sized. Note that if `T` is zero-sized this means you will
128 /// *not* get a `RawVec` with the requested capacity.
132 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
133 /// * Panics on 32-bit platforms if the requested capacity exceeds
134 /// `isize::MAX` bytes.
140 pub fn with_capacity(capacity: usize) -> Self {
141 RawVec::allocate_in(capacity, false, Global)
144 /// Like `with_capacity`, but guarantees the buffer is zeroed.
146 pub fn with_capacity_zeroed(capacity: usize) -> Self {
147 RawVec::allocate_in(capacity, true, Global)
151 impl<T, A: AllocRef> RawVec<T, A> {
152 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
154 /// # Undefined Behavior
156 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
157 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
158 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
159 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
160 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a }
164 impl<T> RawVec<T, Global> {
165 /// Reconstitutes a `RawVec` from a pointer and capacity.
167 /// # Undefined Behavior
169 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
170 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
171 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
172 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
173 RawVec { ptr: Unique::new_unchecked(ptr), cap: capacity, a: Global }
176 /// Converts a `Box<[T]>` into a `RawVec<T>`.
177 pub fn from_box(mut slice: Box<[T]>) -> Self {
179 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
186 impl<T, A: AllocRef> RawVec<T, A> {
187 /// Gets a raw pointer to the start of the allocation. Note that this is
188 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
190 pub fn ptr(&self) -> *mut T {
194 /// Gets the capacity of the allocation.
196 /// This will always be `usize::MAX` if `T` is zero-sized.
198 pub fn capacity(&self) -> usize {
199 if mem::size_of::<T>() == 0 { !0 } else { self.cap }
202 /// Returns a shared reference to the allocator backing this `RawVec`.
203 pub fn alloc(&self) -> &A {
207 /// Returns a mutable reference to the allocator backing this `RawVec`.
208 pub fn alloc_mut(&mut self) -> &mut A {
212 fn current_layout(&self) -> Option<Layout> {
216 // We have an allocated chunk of memory, so we can bypass runtime
217 // checks to get our current layout.
219 let align = mem::align_of::<T>();
220 let size = mem::size_of::<T>() * self.cap;
221 Some(Layout::from_size_align_unchecked(size, align))
226 /// Doubles the size of the type's backing allocation. This is common enough
227 /// to want to do that it's easiest to just have a dedicated method. Slightly
228 /// more efficient logic can be provided for this than the general case.
230 /// This function is ideal for when pushing elements one-at-a-time because
231 /// you don't need to incur the costs of the more general computations
232 /// reserve needs to do to guard against overflow. You do however need to
233 /// manually check if your `len == capacity`.
237 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
238 /// all `usize::MAX` slots in your imaginary buffer.
239 /// * Panics on 32-bit platforms if the requested capacity exceeds
240 /// `isize::MAX` bytes.
249 /// # #![feature(raw_vec_internals)]
250 /// # extern crate alloc;
252 /// # use alloc::raw_vec::RawVec;
253 /// struct MyVec<T> {
258 /// impl<T> MyVec<T> {
259 /// pub fn push(&mut self, elem: T) {
260 /// if self.len == self.buf.capacity() { self.buf.double(); }
261 /// // double would have aborted or panicked if the len exceeded
262 /// // `isize::MAX` so this is safe to do unchecked now.
264 /// ptr::write(self.buf.ptr().add(self.len), elem);
270 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
276 pub fn double(&mut self) {
278 let elem_size = mem::size_of::<T>();
280 // Since we set the capacity to `usize::MAX` when `elem_size` is
281 // 0, getting to here necessarily means the `RawVec` is overfull.
282 assert!(elem_size != 0, "capacity overflow");
284 let (ptr, new_cap) = match self.current_layout() {
286 // Since we guarantee that we never allocate more than
287 // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
288 // a precondition, so this can't overflow. Additionally the
289 // alignment will never be too large as to "not be
290 // satisfiable", so `Layout::from_size_align` will always
293 // TL;DR, we bypass runtime checks due to dynamic assertions
294 // in this module, allowing us to use
295 // `from_size_align_unchecked`.
296 let new_cap = 2 * self.cap;
297 let new_size = new_cap * elem_size;
298 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
299 let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(), cur, new_size);
301 Ok((ptr, new_size)) => (ptr, new_size / elem_size),
302 Err(_) => handle_alloc_error(Layout::from_size_align_unchecked(
309 // Skip to 4 because tiny `Vec`'s are dumb; but not if that
310 // would cause overflow.
311 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
312 let layout = Layout::array::<T>(new_cap).unwrap();
313 match self.a.alloc(layout) {
314 Ok((ptr, new_size)) => (ptr, new_size / elem_size),
315 Err(_) => handle_alloc_error(layout),
319 self.ptr = ptr.cast().into();
324 /// Attempts to double the size of the type's backing allocation in place. This is common
325 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
326 /// more efficient logic can be provided for this than the general case.
328 /// Returns `true` if the reallocation attempt has succeeded.
332 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
333 /// all `usize::MAX` slots in your imaginary buffer.
334 /// * Panics on 32-bit platforms if the requested capacity exceeds
335 /// `isize::MAX` bytes.
338 pub fn double_in_place(&mut self) -> bool {
340 let elem_size = mem::size_of::<T>();
341 let old_layout = match self.current_layout() {
342 Some(layout) => layout,
343 None => return false, // nothing to double
346 // Since we set the capacity to `usize::MAX` when `elem_size` is
347 // 0, getting to here necessarily means the `RawVec` is overfull.
348 assert!(elem_size != 0, "capacity overflow");
350 // Since we guarantee that we never allocate more than `isize::MAX`
351 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
352 // this can't overflow.
354 // Similarly to with `double` above, we can go straight to
355 // `Layout::from_size_align_unchecked` as we know this won't
356 // overflow and the alignment is sufficiently small.
357 let new_cap = 2 * self.cap;
358 let new_size = new_cap * elem_size;
359 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
360 match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
362 // We can't directly divide `size`.
371 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
372 pub fn try_reserve_exact(
374 used_capacity: usize,
375 needed_extra_capacity: usize,
376 ) -> Result<(), TryReserveError> {
377 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
380 /// Ensures that the buffer contains at least enough space to hold
381 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
382 /// will reallocate the minimum possible amount of memory necessary.
383 /// Generally this will be exactly the amount of memory necessary,
384 /// but in principle the allocator is free to give back more than
387 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
388 /// the requested space. This is not really unsafe, but the unsafe
389 /// code *you* write that relies on the behavior of this function may break.
393 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
394 /// * Panics on 32-bit platforms if the requested capacity exceeds
395 /// `isize::MAX` bytes.
400 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
401 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
402 Err(CapacityOverflow) => capacity_overflow(),
403 Err(AllocError { .. }) => unreachable!(),
404 Ok(()) => { /* yay */ }
408 /// Calculates the buffer's new size given that it'll hold `used_capacity +
409 /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
410 /// Returns `(new_capacity, new_alloc_size)`.
411 fn amortized_new_size(
413 used_capacity: usize,
414 needed_extra_capacity: usize,
415 ) -> Result<usize, TryReserveError> {
416 // Nothing we can really do about these checks, sadly.
418 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
419 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
420 let double_cap = self.cap * 2;
421 // `double_cap` guarantees exponential growth.
422 Ok(cmp::max(double_cap, required_cap))
425 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
428 used_capacity: usize,
429 needed_extra_capacity: usize,
430 ) -> Result<(), TryReserveError> {
431 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
434 /// Ensures that the buffer contains at least enough space to hold
435 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
436 /// enough capacity, will reallocate enough space plus comfortable slack
437 /// space to get amortized `O(1)` behavior. Will limit this behavior
438 /// if it would needlessly cause itself to panic.
440 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
441 /// the requested space. This is not really unsafe, but the unsafe
442 /// code *you* write that relies on the behavior of this function may break.
444 /// This is ideal for implementing a bulk-push operation like `extend`.
448 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
449 /// * Panics on 32-bit platforms if the requested capacity exceeds
450 /// `isize::MAX` bytes.
459 /// # #![feature(raw_vec_internals)]
460 /// # extern crate alloc;
462 /// # use alloc::raw_vec::RawVec;
463 /// struct MyVec<T> {
468 /// impl<T: Clone> MyVec<T> {
469 /// pub fn push_all(&mut self, elems: &[T]) {
470 /// self.buf.reserve(self.len, elems.len());
471 /// // reserve would have aborted or panicked if the len exceeded
472 /// // `isize::MAX` so this is safe to do unchecked now.
475 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
482 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
483 /// # vector.push_all(&[1, 3, 5, 7, 9]);
486 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
487 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
488 Err(CapacityOverflow) => capacity_overflow(),
489 Err(AllocError { .. }) => unreachable!(),
490 Ok(()) => { /* yay */ }
493 /// Attempts to ensure that the buffer contains at least enough space to hold
494 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
495 /// enough capacity, will reallocate in place enough space plus comfortable slack
496 /// space to get amortized `O(1)` behavior. Will limit this behaviour
497 /// if it would needlessly cause itself to panic.
499 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
500 /// the requested space. This is not really unsafe, but the unsafe
501 /// code *you* write that relies on the behavior of this function may break.
503 /// Returns `true` if the reallocation attempt has succeeded.
507 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
508 /// * Panics on 32-bit platforms if the requested capacity exceeds
509 /// `isize::MAX` bytes.
510 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
512 // NOTE: we don't early branch on ZSTs here because we want this
513 // to actually catch "asking for more than usize::MAX" in that case.
514 // If we make it past the first branch then we are guaranteed to
517 // Don't actually need any more capacity. If the current `cap` is 0, we can't
518 // reallocate in place.
519 // Wrapping in case they give a bad `used_capacity`
520 let old_layout = match self.current_layout() {
521 Some(layout) => layout,
522 None => return false,
524 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
529 .amortized_new_size(used_capacity, needed_extra_capacity)
530 .unwrap_or_else(|_| capacity_overflow());
532 // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
533 // (regardless of whether `self.cap - used_capacity` wrapped).
534 // Therefore, we can safely call `grow_in_place`.
536 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
537 // FIXME: may crash and burn on over-reserve
538 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
539 match self.a.grow_in_place(
540 NonNull::from(self.ptr).cast(),
553 /// Shrinks the allocation down to the specified amount. If the given amount
554 /// is 0, actually completely deallocates.
558 /// Panics if the given amount is *larger* than the current capacity.
563 pub fn shrink_to_fit(&mut self, amount: usize) {
564 let elem_size = mem::size_of::<T>();
566 // Set the `cap` because they might be about to promote to a `Box<[T]>`
572 // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
573 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
576 // We want to create a new zero-length vector within the
577 // same allocator. We use `ptr::write` to avoid an
578 // erroneous attempt to drop the contents, and we use
579 // `ptr::read` to sidestep condition against destructuring
580 // types that implement Drop.
583 let a = ptr::read(&self.a as *const A);
584 self.dealloc_buffer();
585 ptr::write(self, RawVec::new_in(a));
587 } else if self.cap != amount {
589 // We know here that our `amount` is greater than zero. This
590 // implies, via the assert above, that capacity is also greater
591 // than zero, which means that we've got a current layout that
594 // We also know that `self.cap` is greater than `amount`, and
595 // consequently we don't need runtime checks for creating either
597 let old_size = elem_size * self.cap;
598 let new_size = elem_size * amount;
599 let align = mem::align_of::<T>();
600 let old_layout = Layout::from_size_align_unchecked(old_size, align);
601 match self.a.realloc(NonNull::from(self.ptr).cast(), old_layout, new_size) {
602 Ok((ptr, _)) => self.ptr = ptr.cast().into(),
604 handle_alloc_error(Layout::from_size_align_unchecked(new_size, align))
620 enum ReserveStrategy {
625 use ReserveStrategy::*;
627 impl<T, A: AllocRef> RawVec<T, A> {
630 used_capacity: usize,
631 needed_extra_capacity: usize,
632 fallibility: Fallibility,
633 strategy: ReserveStrategy,
634 ) -> Result<(), TryReserveError> {
635 let elem_size = mem::size_of::<T>();
638 // NOTE: we don't early branch on ZSTs here because we want this
639 // to actually catch "asking for more than usize::MAX" in that case.
640 // If we make it past the first branch then we are guaranteed to
643 // Don't actually need any more capacity.
644 // Wrapping in case they gave a bad `used_capacity`.
645 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
649 // Nothing we can really do about these checks, sadly.
650 let new_cap = match strategy {
652 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?
654 Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
656 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
658 alloc_guard(new_layout.size())?;
660 let res = match self.current_layout() {
662 debug_assert!(new_layout.align() == layout.align());
663 self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
665 None => self.a.alloc(new_layout),
668 let (ptr, new_cap) = match (res, fallibility) {
669 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
670 (Err(AllocErr), Fallible) => {
671 return Err(TryReserveError::AllocError {
676 (Ok((ptr, new_size)), _) => (ptr, new_size / elem_size),
679 self.ptr = ptr.cast().into();
687 impl<T> RawVec<T, Global> {
688 /// Converts the entire buffer into `Box<[T]>`.
690 /// Note that this will correctly reconstitute any `cap` changes
691 /// that may have been performed. (See description of type for details.)
693 /// # Undefined Behavior
695 /// All elements of `RawVec<T, Global>` must be initialized. Notice that
696 /// the rules around uninitialized boxed values are not finalized yet,
697 /// but until they are, it is advisable to avoid them.
698 pub unsafe fn into_box(self) -> Box<[T]> {
699 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
700 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
701 let output: Box<[T]> = Box::from_raw(slice);
707 impl<T, A: AllocRef> RawVec<T, A> {
708 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
709 pub unsafe fn dealloc_buffer(&mut self) {
710 let elem_size = mem::size_of::<T>();
712 if let Some(layout) = self.current_layout() {
713 self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
719 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
720 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
723 self.dealloc_buffer();
728 // We need to guarantee the following:
729 // * We don't ever allocate `> isize::MAX` byte-size objects.
730 // * We don't overflow `usize::MAX` and actually allocate too little.
732 // On 64-bit we just need to check for overflow since trying to allocate
733 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
734 // an extra guard for this in case we're running on a platform which can use
735 // all 4GB in user-space, e.g., PAE or x32.
738 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
739 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
740 Err(CapacityOverflow)
746 // One central function responsible for reporting capacity overflows. This'll
747 // ensure that the code generation related to these panics is minimal as there's
748 // only one location which panics rather than a bunch throughout the module.
749 fn capacity_overflow() -> ! {
750 panic!("capacity overflow");