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 {
56 #[cfg(not(bootstrap))]
57 { if mem::size_of::<T>() == 0 { core::usize::MAX } else { 0 } }
60 [0, !0][(mem::size_of::<T>() == 0) as usize]
63 // `Unique::empty()` doubles as "unallocated" and "zero-sized allocation".
71 /// Like `with_capacity`, but parameterized over the choice of
72 /// allocator for the returned `RawVec`.
74 pub fn with_capacity_in(capacity: usize, a: A) -> Self {
75 RawVec::allocate_in(capacity, false, a)
78 /// Like `with_capacity_zeroed`, but parameterized over the choice
79 /// of allocator for the returned `RawVec`.
81 pub fn with_capacity_zeroed_in(capacity: usize, a: A) -> Self {
82 RawVec::allocate_in(capacity, true, a)
85 fn allocate_in(capacity: usize, zeroed: bool, mut a: A) -> Self {
87 let elem_size = mem::size_of::<T>();
89 let alloc_size = capacity.checked_mul(elem_size).unwrap_or_else(|| capacity_overflow());
90 alloc_guard(alloc_size).unwrap_or_else(|_| capacity_overflow());
92 // Handles ZSTs and `capacity == 0` alike.
93 let ptr = if alloc_size == 0 {
94 NonNull::<T>::dangling()
96 let align = mem::align_of::<T>();
97 let layout = Layout::from_size_align(alloc_size, align).unwrap();
98 let result = if zeroed {
99 a.alloc_zeroed(layout)
104 Ok(ptr) => ptr.cast(),
105 Err(_) => handle_alloc_error(layout),
118 impl<T> RawVec<T, Global> {
119 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
120 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
122 /// If you change `RawVec<T>::new` or dependencies, please take care to not
123 /// introduce anything that would truly violate `min_const_fn`.
125 /// NOTE: We could avoid this hack and check conformance with some
126 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
127 /// with `min_const_fn` but does not necessarily allow calling it in
128 /// `stable(...) const fn` / user code not enabling `foo` when
129 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
130 pub const NEW: Self = Self::new();
132 /// Creates the biggest possible `RawVec` (on the system heap)
133 /// without allocating. If `T` has positive size, then this makes a
134 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
135 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
136 /// delayed allocation.
137 pub const fn new() -> Self {
141 /// Creates a `RawVec` (on the system heap) with exactly the
142 /// capacity and alignment requirements for a `[T; capacity]`. This is
143 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
144 /// zero-sized. Note that if `T` is zero-sized this means you will
145 /// *not* get a `RawVec` with the requested capacity.
149 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
150 /// * Panics on 32-bit platforms if the requested capacity exceeds
151 /// `isize::MAX` bytes.
157 pub fn with_capacity(capacity: usize) -> Self {
158 RawVec::allocate_in(capacity, false, Global)
161 /// Like `with_capacity`, but guarantees the buffer is zeroed.
163 pub fn with_capacity_zeroed(capacity: usize) -> Self {
164 RawVec::allocate_in(capacity, true, Global)
168 impl<T, A: Alloc> RawVec<T, A> {
169 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
171 /// # Undefined Behavior
173 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
174 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
175 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
176 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
178 ptr: Unique::new_unchecked(ptr),
185 impl<T> RawVec<T, Global> {
186 /// Reconstitutes a `RawVec` from a pointer and capacity.
188 /// # Undefined Behavior
190 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
191 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
192 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
193 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
195 ptr: Unique::new_unchecked(ptr),
201 /// Converts a `Box<[T]>` into a `RawVec<T>`.
202 pub fn from_box(mut slice: Box<[T]>) -> Self {
204 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
211 impl<T, A: Alloc> RawVec<T, A> {
212 /// Gets a raw pointer to the start of the allocation. Note that this is
213 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
215 pub fn ptr(&self) -> *mut T {
219 /// Gets the capacity of the allocation.
221 /// This will always be `usize::MAX` if `T` is zero-sized.
223 pub fn capacity(&self) -> usize {
224 if mem::size_of::<T>() == 0 {
231 /// Returns a shared reference to the allocator backing this `RawVec`.
232 pub fn alloc(&self) -> &A {
236 /// Returns a mutable reference to the allocator backing this `RawVec`.
237 pub fn alloc_mut(&mut self) -> &mut A {
241 fn current_layout(&self) -> Option<Layout> {
245 // We have an allocated chunk of memory, so we can bypass runtime
246 // checks to get our current layout.
248 let align = mem::align_of::<T>();
249 let size = mem::size_of::<T>() * self.cap;
250 Some(Layout::from_size_align_unchecked(size, align))
255 /// Doubles the size of the type's backing allocation. This is common enough
256 /// to want to do that it's easiest to just have a dedicated method. Slightly
257 /// more efficient logic can be provided for this than the general case.
259 /// This function is ideal for when pushing elements one-at-a-time because
260 /// you don't need to incur the costs of the more general computations
261 /// reserve needs to do to guard against overflow. You do however need to
262 /// manually check if your `len == capacity`.
266 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
267 /// all `usize::MAX` slots in your imaginary buffer.
268 /// * Panics on 32-bit platforms if the requested capacity exceeds
269 /// `isize::MAX` bytes.
278 /// # #![feature(raw_vec_internals)]
279 /// # extern crate alloc;
281 /// # use alloc::raw_vec::RawVec;
282 /// struct MyVec<T> {
287 /// impl<T> MyVec<T> {
288 /// pub fn push(&mut self, elem: T) {
289 /// if self.len == self.buf.capacity() { self.buf.double(); }
290 /// // double would have aborted or panicked if the len exceeded
291 /// // `isize::MAX` so this is safe to do unchecked now.
293 /// ptr::write(self.buf.ptr().add(self.len), elem);
299 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
305 pub fn double(&mut self) {
307 let elem_size = mem::size_of::<T>();
309 // Since we set the capacity to `usize::MAX` when `elem_size` is
310 // 0, getting to here necessarily means the `RawVec` is overfull.
311 assert!(elem_size != 0, "capacity overflow");
313 let (new_cap, uniq) = match self.current_layout() {
315 // Since we guarantee that we never allocate more than
316 // `isize::MAX` bytes, `elem_size * self.cap <= isize::MAX` as
317 // a precondition, so this can't overflow. Additionally the
318 // alignment will never be too large as to "not be
319 // satisfiable", so `Layout::from_size_align` will always
322 // TL;DR, we bypass runtime checks due to dynamic assertions
323 // in this module, allowing us to use
324 // `from_size_align_unchecked`.
325 let new_cap = 2 * self.cap;
326 let new_size = new_cap * elem_size;
327 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
328 let ptr_res = self.a.realloc(NonNull::from(self.ptr).cast(),
332 Ok(ptr) => (new_cap, ptr.cast().into()),
333 Err(_) => handle_alloc_error(
334 Layout::from_size_align_unchecked(new_size, cur.align())
339 // Skip to 4 because tiny `Vec`'s are dumb; but not if that
340 // would cause overflow.
341 let new_cap = if elem_size > (!0) / 8 { 1 } else { 4 };
342 match self.a.alloc_array::<T>(new_cap) {
343 Ok(ptr) => (new_cap, ptr.into()),
344 Err(_) => handle_alloc_error(Layout::array::<T>(new_cap).unwrap()),
353 /// Attempts to double the size of the type's backing allocation in place. This is common
354 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
355 /// more efficient logic can be provided for this than the general case.
357 /// Returns `true` if the reallocation attempt has succeeded.
361 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
362 /// all `usize::MAX` slots in your imaginary buffer.
363 /// * Panics on 32-bit platforms if the requested capacity exceeds
364 /// `isize::MAX` bytes.
367 pub fn double_in_place(&mut self) -> bool {
369 let elem_size = mem::size_of::<T>();
370 let old_layout = match self.current_layout() {
371 Some(layout) => layout,
372 None => return false, // nothing to double
375 // Since we set the capacity to `usize::MAX` when `elem_size` is
376 // 0, getting to here necessarily means the `RawVec` is overfull.
377 assert!(elem_size != 0, "capacity overflow");
379 // Since we guarantee that we never allocate more than `isize::MAX`
380 // bytes, `elem_size * self.cap <= isize::MAX` as a precondition, so
381 // this can't overflow.
383 // Similarly to with `double` above, we can go straight to
384 // `Layout::from_size_align_unchecked` as we know this won't
385 // overflow and the alignment is sufficiently small.
386 let new_cap = 2 * self.cap;
387 let new_size = new_cap * elem_size;
388 alloc_guard(new_size).unwrap_or_else(|_| capacity_overflow());
389 match self.a.grow_in_place(NonNull::from(self.ptr).cast(), old_layout, new_size) {
391 // We can't directly divide `size`.
402 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
403 pub fn try_reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize)
404 -> Result<(), TryReserveError> {
406 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Exact)
409 /// Ensures that the buffer contains at least enough space to hold
410 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
411 /// will reallocate the minimum possible amount of memory necessary.
412 /// Generally this will be exactly the amount of memory necessary,
413 /// but in principle the allocator is free to give back more than
416 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
417 /// the requested space. This is not really unsafe, but the unsafe
418 /// code *you* write that relies on the behavior of this function may break.
422 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
423 /// * Panics on 32-bit platforms if the requested capacity exceeds
424 /// `isize::MAX` bytes.
429 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
430 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Exact) {
431 Err(CapacityOverflow) => capacity_overflow(),
432 Err(AllocError { .. }) => unreachable!(),
433 Ok(()) => { /* yay */ }
437 /// Calculates the buffer's new size given that it'll hold `used_capacity +
438 /// needed_extra_capacity` elements. This logic is used in amortized reserve methods.
439 /// Returns `(new_capacity, new_alloc_size)`.
440 fn amortized_new_size(&self, used_capacity: usize, needed_extra_capacity: usize)
441 -> Result<usize, TryReserveError> {
443 // Nothing we can really do about these checks, sadly.
444 let required_cap = used_capacity.checked_add(needed_extra_capacity)
445 .ok_or(CapacityOverflow)?;
446 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
447 let double_cap = self.cap * 2;
448 // `double_cap` guarantees exponential growth.
449 Ok(cmp::max(double_cap, required_cap))
452 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
453 pub fn try_reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize)
454 -> Result<(), TryReserveError> {
455 self.reserve_internal(used_capacity, needed_extra_capacity, Fallible, Amortized)
458 /// Ensures that the buffer contains at least enough space to hold
459 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
460 /// enough capacity, will reallocate enough space plus comfortable slack
461 /// space to get amortized `O(1)` behavior. Will limit this behavior
462 /// if it would needlessly cause itself to panic.
464 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
465 /// the requested space. This is not really unsafe, but the unsafe
466 /// code *you* write that relies on the behavior of this function may break.
468 /// This is ideal for implementing a bulk-push operation like `extend`.
472 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
473 /// * Panics on 32-bit platforms if the requested capacity exceeds
474 /// `isize::MAX` bytes.
483 /// # #![feature(raw_vec_internals)]
484 /// # extern crate alloc;
486 /// # use alloc::raw_vec::RawVec;
487 /// struct MyVec<T> {
492 /// impl<T: Clone> MyVec<T> {
493 /// pub fn push_all(&mut self, elems: &[T]) {
494 /// self.buf.reserve(self.len, elems.len());
495 /// // reserve would have aborted or panicked if the len exceeded
496 /// // `isize::MAX` so this is safe to do unchecked now.
499 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
506 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
507 /// # vector.push_all(&[1, 3, 5, 7, 9]);
510 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
511 match self.reserve_internal(used_capacity, needed_extra_capacity, Infallible, Amortized) {
512 Err(CapacityOverflow) => capacity_overflow(),
513 Err(AllocError { .. }) => unreachable!(),
514 Ok(()) => { /* yay */ }
517 /// Attempts to ensure that the buffer contains at least enough space to hold
518 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
519 /// enough capacity, will reallocate in place enough space plus comfortable slack
520 /// space to get amortized `O(1)` behavior. Will limit this behaviour
521 /// if it would needlessly cause itself to panic.
523 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
524 /// the requested space. This is not really unsafe, but the unsafe
525 /// code *you* write that relies on the behavior of this function may break.
527 /// Returns `true` if the reallocation attempt has succeeded.
531 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
532 /// * Panics on 32-bit platforms if the requested capacity exceeds
533 /// `isize::MAX` bytes.
534 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
536 // NOTE: we don't early branch on ZSTs here because we want this
537 // to actually catch "asking for more than usize::MAX" in that case.
538 // If we make it past the first branch then we are guaranteed to
541 // Don't actually need any more capacity. If the current `cap` is 0, we can't
542 // reallocate in place.
543 // Wrapping in case they give a bad `used_capacity`
544 let old_layout = match self.current_layout() {
545 Some(layout) => layout,
546 None => return false,
548 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
552 let new_cap = self.amortized_new_size(used_capacity, needed_extra_capacity)
553 .unwrap_or_else(|_| capacity_overflow());
555 // Here, `cap < used_capacity + needed_extra_capacity <= new_cap`
556 // (regardless of whether `self.cap - used_capacity` wrapped).
557 // Therefore, we can safely call `grow_in_place`.
559 let new_layout = Layout::new::<T>().repeat(new_cap).unwrap().0;
560 // FIXME: may crash and burn on over-reserve
561 alloc_guard(new_layout.size()).unwrap_or_else(|_| capacity_overflow());
562 match self.a.grow_in_place(
563 NonNull::from(self.ptr).cast(), old_layout, new_layout.size(),
576 /// Shrinks the allocation down to the specified amount. If the given amount
577 /// is 0, actually completely deallocates.
581 /// Panics if the given amount is *larger* than the current capacity.
586 pub fn shrink_to_fit(&mut self, amount: usize) {
587 let elem_size = mem::size_of::<T>();
589 // Set the `cap` because they might be about to promote to a `Box<[T]>`
595 // This check is my waterloo; it's the only thing `Vec` wouldn't have to do.
596 assert!(self.cap >= amount, "Tried to shrink to a larger capacity");
599 // We want to create a new zero-length vector within the
600 // same allocator. We use `ptr::write` to avoid an
601 // erroneous attempt to drop the contents, and we use
602 // `ptr::read` to sidestep condition against destructuring
603 // types that implement Drop.
606 let a = ptr::read(&self.a as *const A);
607 self.dealloc_buffer();
608 ptr::write(self, RawVec::new_in(a));
610 } else if self.cap != amount {
612 // We know here that our `amount` is greater than zero. This
613 // implies, via the assert above, that capacity is also greater
614 // than zero, which means that we've got a current layout that
617 // We also know that `self.cap` is greater than `amount`, and
618 // consequently we don't need runtime checks for creating either
620 let old_size = elem_size * self.cap;
621 let new_size = elem_size * amount;
622 let align = mem::align_of::<T>();
623 let old_layout = Layout::from_size_align_unchecked(old_size, align);
624 match self.a.realloc(NonNull::from(self.ptr).cast(),
627 Ok(p) => self.ptr = p.cast().into(),
628 Err(_) => handle_alloc_error(
629 Layout::from_size_align_unchecked(new_size, align)
645 enum ReserveStrategy {
650 use ReserveStrategy::*;
652 impl<T, A: Alloc> RawVec<T, A> {
655 used_capacity: usize,
656 needed_extra_capacity: usize,
657 fallibility: Fallibility,
658 strategy: ReserveStrategy,
659 ) -> Result<(), TryReserveError> {
661 // NOTE: we don't early branch on ZSTs here because we want this
662 // to actually catch "asking for more than usize::MAX" in that case.
663 // If we make it past the first branch then we are guaranteed to
666 // Don't actually need any more capacity.
667 // Wrapping in case they gave a bad `used_capacity`.
668 if self.capacity().wrapping_sub(used_capacity) >= needed_extra_capacity {
672 // Nothing we can really do about these checks, sadly.
673 let new_cap = match strategy {
674 Exact => used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?,
675 Amortized => self.amortized_new_size(used_capacity, needed_extra_capacity)?,
677 let new_layout = Layout::array::<T>(new_cap).map_err(|_| CapacityOverflow)?;
679 alloc_guard(new_layout.size())?;
681 let res = match self.current_layout() {
683 debug_assert!(new_layout.align() == layout.align());
684 self.a.realloc(NonNull::from(self.ptr).cast(), layout, new_layout.size())
686 None => self.a.alloc(new_layout),
689 let ptr = match (res, fallibility) {
690 (Err(AllocErr), Infallible) => handle_alloc_error(new_layout),
691 (Err(AllocErr), Fallible) => return Err(TryReserveError::AllocError {
698 self.ptr = ptr.cast().into();
707 impl<T> RawVec<T, Global> {
708 /// Converts the entire buffer into `Box<[T]>`.
710 /// Note that this will correctly reconstitute any `cap` changes
711 /// that may have been performed. (See description of type for details.)
713 /// # Undefined Behavior
715 /// All elements of `RawVec<T, Global>` must be initialized. Notice that
716 /// the rules around uninitialized boxed values are not finalized yet,
717 /// but until they are, it is advisable to avoid them.
718 pub unsafe fn into_box(self) -> Box<[T]> {
719 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
720 let slice = slice::from_raw_parts_mut(self.ptr(), self.cap);
721 let output: Box<[T]> = Box::from_raw(slice);
727 impl<T, A: Alloc> RawVec<T, A> {
728 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
729 pub unsafe fn dealloc_buffer(&mut self) {
730 let elem_size = mem::size_of::<T>();
732 if let Some(layout) = self.current_layout() {
733 self.a.dealloc(NonNull::from(self.ptr).cast(), layout);
739 unsafe impl<#[may_dangle] T, A: Alloc> Drop for RawVec<T, A> {
740 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
742 unsafe { self.dealloc_buffer(); }
746 // We need to guarantee the following:
747 // * We don't ever allocate `> isize::MAX` byte-size objects.
748 // * We don't overflow `usize::MAX` and actually allocate too little.
750 // On 64-bit we just need to check for overflow since trying to allocate
751 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
752 // an extra guard for this in case we're running on a platform which can use
753 // all 4GB in user-space, e.g., PAE or x32.
756 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
757 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
758 Err(CapacityOverflow)
764 // One central function responsible for reporting capacity overflows. This'll
765 // ensure that the code generation related to these panics is minimal as there's
766 // only one location which panics rather than a bunch throughout the module.
767 fn capacity_overflow() -> ! {
768 panic!("capacity overflow");