1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
4 use core::alloc::MemoryBlock;
6 use core::mem::{self, MaybeUninit};
12 handle_alloc_error, AllocErr,
14 AllocRef, Global, Layout,
15 ReallocPlacement::{self, *},
17 use crate::boxed::Box;
18 use crate::collections::TryReserveError::{self, *};
23 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
24 /// a buffer of memory on the heap without having to worry about all the corner cases
25 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
28 /// * Produces `Unique::empty()` on zero-sized types.
29 /// * Produces `Unique::empty()` on zero-length allocations.
30 /// * Avoids freeing `Unique::empty()`.
31 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
32 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
33 /// * Guards against overflowing your length.
34 /// * Calls `handle_alloc_error` for fallible allocations.
35 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
36 /// * Uses the excess returned from the allocator to use the largest available capacity.
38 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
39 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
40 /// to handle the actual things *stored* inside of a `RawVec`.
42 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
43 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
44 /// `Box<[T]>`, since `capacity()` won't yield the length.
45 #[allow(missing_debug_implementations)]
46 pub struct RawVec<T, A: AllocRef = Global> {
52 impl<T> RawVec<T, Global> {
53 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
54 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
56 /// If you change `RawVec<T>::new` or dependencies, please take care to not
57 /// introduce anything that would truly violate `min_const_fn`.
59 /// NOTE: We could avoid this hack and check conformance with some
60 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
61 /// with `min_const_fn` but does not necessarily allow calling it in
62 /// `stable(...) const fn` / user code not enabling `foo` when
63 /// `#[rustc_const_unstable(feature = "foo", ..)]` is present.
64 pub const NEW: Self = Self::new();
66 /// Creates the biggest possible `RawVec` (on the system heap)
67 /// without allocating. If `T` has positive size, then this makes a
68 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
69 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
70 /// delayed allocation.
71 pub const fn new() -> Self {
75 /// Creates a `RawVec` (on the system heap) with exactly the
76 /// capacity and alignment requirements for a `[T; capacity]`. This is
77 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
78 /// zero-sized. Note that if `T` is zero-sized this means you will
79 /// *not* get a `RawVec` with the requested capacity.
83 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
84 /// * Panics on 32-bit platforms if the requested capacity exceeds
85 /// `isize::MAX` bytes.
91 pub fn with_capacity(capacity: usize) -> Self {
92 Self::with_capacity_in(capacity, Global)
95 /// Like `with_capacity`, but guarantees the buffer is zeroed.
97 pub fn with_capacity_zeroed(capacity: usize) -> Self {
98 Self::with_capacity_zeroed_in(capacity, Global)
101 /// Reconstitutes a `RawVec` from a pointer and capacity.
103 /// # Undefined Behavior
105 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
106 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
107 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
109 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
110 Self::from_raw_parts_in(ptr, capacity, Global)
113 /// Converts a `Box<[T]>` into a `RawVec<T>`.
114 pub fn from_box(mut slice: Box<[T]>) -> Self {
116 let result = RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len());
123 impl<T, A: AllocRef> RawVec<T, A> {
124 /// Like `new`, but parameterized over the choice of allocator for
125 /// the returned `RawVec`.
126 pub const fn new_in(alloc: A) -> Self {
127 // `cap: 0` means "unallocated". zero-sized types are ignored.
128 Self { ptr: Unique::empty(), cap: 0, alloc }
131 /// Like `with_capacity`, but parameterized over the choice of
132 /// allocator for the returned `RawVec`.
134 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
135 Self::allocate_in(capacity, Uninitialized, alloc)
138 /// Like `with_capacity_zeroed`, but parameterized over the choice
139 /// of allocator for the returned `RawVec`.
141 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
142 Self::allocate_in(capacity, Zeroed, alloc)
145 fn allocate_in(capacity: usize, init: AllocInit, mut alloc: A) -> Self {
146 if mem::size_of::<T>() == 0 {
149 let layout = Layout::array::<T>(capacity).unwrap_or_else(|_| capacity_overflow());
150 alloc_guard(layout.size()).unwrap_or_else(|_| capacity_overflow());
152 let memory = alloc.alloc(layout, init).unwrap_or_else(|_| handle_alloc_error(layout));
154 ptr: memory.ptr().cast().into(),
155 cap: Self::capacity_from_bytes(memory.size()),
161 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
163 /// # Undefined Behavior
165 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
166 /// The `capacity` cannot exceed `isize::MAX` (only a concern on 32-bit systems).
167 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
169 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
170 Self { ptr: Unique::new_unchecked(ptr), cap: capacity, alloc: a }
173 /// Gets a raw pointer to the start of the allocation. Note that this is
174 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
176 pub fn ptr(&self) -> *mut T {
180 /// Gets the capacity of the allocation.
182 /// This will always be `usize::MAX` if `T` is zero-sized.
184 pub fn capacity(&self) -> usize {
185 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
188 /// Returns a shared reference to the allocator backing this `RawVec`.
189 pub fn alloc(&self) -> &A {
193 /// Returns a mutable reference to the allocator backing this `RawVec`.
194 pub fn alloc_mut(&mut self) -> &mut A {
198 fn current_memory(&self) -> Option<MemoryBlock> {
199 if mem::size_of::<T>() == 0 || self.cap == 0 {
202 // We have an allocated chunk of memory, so we can bypass runtime
203 // checks to get our current layout.
205 let align = mem::align_of::<T>();
206 let size = mem::size_of::<T>() * self.cap;
207 let layout = Layout::from_size_align_unchecked(size, align);
208 Some(MemoryBlock::new(self.ptr.cast().into(), layout))
213 /// Doubles the size of the type's backing allocation. This is common enough
214 /// to want to do that it's easiest to just have a dedicated method. Slightly
215 /// more efficient logic can be provided for this than the general case.
217 /// This function is ideal for when pushing elements one-at-a-time because
218 /// you don't need to incur the costs of the more general computations
219 /// reserve needs to do to guard against overflow. You do however need to
220 /// manually check if your `len == capacity`.
224 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
225 /// all `usize::MAX` slots in your imaginary buffer.
226 /// * Panics on 32-bit platforms if the requested capacity exceeds
227 /// `isize::MAX` bytes.
236 /// # #![feature(raw_vec_internals)]
237 /// # extern crate alloc;
239 /// # use alloc::raw_vec::RawVec;
240 /// struct MyVec<T> {
245 /// impl<T> MyVec<T> {
246 /// pub fn push(&mut self, elem: T) {
247 /// if self.len == self.buf.capacity() { self.buf.double(); }
248 /// // double would have aborted or panicked if the len exceeded
249 /// // `isize::MAX` so this is safe to do unchecked now.
251 /// ptr::write(self.buf.ptr().add(self.len), elem);
257 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
263 pub fn double(&mut self) {
264 match self.grow(Double, MayMove, Uninitialized) {
265 Err(CapacityOverflow) => capacity_overflow(),
266 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
267 Ok(()) => { /* yay */ }
271 /// Attempts to double the size of the type's backing allocation in place. This is common
272 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
273 /// more efficient logic can be provided for this than the general case.
275 /// Returns `true` if the reallocation attempt has succeeded.
279 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
280 /// all `usize::MAX` slots in your imaginary buffer.
281 /// * Panics on 32-bit platforms if the requested capacity exceeds
282 /// `isize::MAX` bytes.
285 pub fn double_in_place(&mut self) -> bool {
286 self.grow(Double, InPlace, Uninitialized).is_ok()
289 /// Ensures that the buffer contains at least enough space to hold
290 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
291 /// enough capacity, will reallocate enough space plus comfortable slack
292 /// space to get amortized `O(1)` behavior. Will limit this behavior
293 /// if it would needlessly cause itself to panic.
295 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
296 /// the requested space. This is not really unsafe, but the unsafe
297 /// code *you* write that relies on the behavior of this function may break.
299 /// This is ideal for implementing a bulk-push operation like `extend`.
303 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
304 /// * Panics on 32-bit platforms if the requested capacity exceeds
305 /// `isize::MAX` bytes.
314 /// # #![feature(raw_vec_internals)]
315 /// # extern crate alloc;
317 /// # use alloc::raw_vec::RawVec;
318 /// struct MyVec<T> {
323 /// impl<T: Clone> MyVec<T> {
324 /// pub fn push_all(&mut self, elems: &[T]) {
325 /// self.buf.reserve(self.len, elems.len());
326 /// // reserve would have aborted or panicked if the len exceeded
327 /// // `isize::MAX` so this is safe to do unchecked now.
330 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
337 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
338 /// # vector.push_all(&[1, 3, 5, 7, 9]);
341 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
342 match self.try_reserve(used_capacity, needed_extra_capacity) {
343 Err(CapacityOverflow) => capacity_overflow(),
344 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
345 Ok(()) => { /* yay */ }
349 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
352 used_capacity: usize,
353 needed_extra_capacity: usize,
354 ) -> Result<(), TryReserveError> {
355 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
356 self.grow(Amortized { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
362 /// Attempts to ensure that the buffer contains at least enough space to hold
363 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
364 /// enough capacity, will reallocate in place enough space plus comfortable slack
365 /// space to get amortized `O(1)` behavior. Will limit this behaviour
366 /// if it would needlessly cause itself to panic.
368 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
369 /// the requested space. This is not really unsafe, but the unsafe
370 /// code *you* write that relies on the behavior of this function may break.
372 /// Returns `true` if the reallocation attempt has succeeded.
376 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
377 /// * Panics on 32-bit platforms if the requested capacity exceeds
378 /// `isize::MAX` bytes.
379 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
380 // This is more readable than putting this in one line:
381 // `!self.needs_to_grow(...) || self.grow(...).is_ok()`
382 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
383 self.grow(Amortized { used_capacity, needed_extra_capacity }, InPlace, Uninitialized)
390 /// Ensures that the buffer contains at least enough space to hold
391 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
392 /// will reallocate the minimum possible amount of memory necessary.
393 /// Generally this will be exactly the amount of memory necessary,
394 /// but in principle the allocator is free to give back more than
397 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
398 /// the requested space. This is not really unsafe, but the unsafe
399 /// code *you* write that relies on the behavior of this function may break.
403 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
404 /// * Panics on 32-bit platforms if the requested capacity exceeds
405 /// `isize::MAX` bytes.
410 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
411 match self.try_reserve_exact(used_capacity, needed_extra_capacity) {
412 Err(CapacityOverflow) => capacity_overflow(),
413 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
414 Ok(()) => { /* yay */ }
418 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
419 pub fn try_reserve_exact(
421 used_capacity: usize,
422 needed_extra_capacity: usize,
423 ) -> Result<(), TryReserveError> {
424 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
425 self.grow(Exact { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
431 /// Shrinks the allocation down to the specified amount. If the given amount
432 /// is 0, actually completely deallocates.
436 /// Panics if the given amount is *larger* than the current capacity.
441 pub fn shrink_to_fit(&mut self, amount: usize) {
442 match self.shrink(amount, MayMove) {
443 Err(CapacityOverflow) => capacity_overflow(),
444 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
445 Ok(()) => { /* yay */ }
450 #[derive(Copy, Clone)]
453 Amortized { used_capacity: usize, needed_extra_capacity: usize },
454 Exact { used_capacity: usize, needed_extra_capacity: usize },
458 impl<T, A: AllocRef> RawVec<T, A> {
459 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
460 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
461 fn needs_to_grow(&self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
462 mem::size_of::<T>() != 0
463 && needed_extra_capacity > self.capacity().wrapping_sub(used_capacity)
466 fn capacity_from_bytes(excess: usize) -> usize {
467 debug_assert_ne!(mem::size_of::<T>(), 0);
468 excess / mem::size_of::<T>()
471 fn set_memory(&mut self, memory: MemoryBlock) {
472 self.ptr = memory.ptr().cast().into();
473 self.cap = Self::capacity_from_bytes(memory.size());
477 /// Single method to handle all possibilities of growing the buffer.
481 placement: ReallocPlacement,
483 ) -> Result<(), TryReserveError> {
484 let elem_size = mem::size_of::<T>();
486 // Since we return a capacity of `usize::MAX` when `elem_size` is
487 // 0, getting to here necessarily means the `RawVec` is overfull.
488 return Err(CapacityOverflow);
490 let layout = match strategy {
492 // Since we guarantee that we never allocate more than `isize::MAX` bytes,
493 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow.
494 // Additionally the alignment will never be too large as to "not be satisfiable",
495 // so `Layout::from_size_align` will always return `Some`.
497 // TL;DR, we bypass runtime checks due to dynamic assertions in this module,
498 // allowing us to use `from_size_align_unchecked`.
499 let cap = if self.cap == 0 {
500 // Skip to 4 because tiny `Vec`'s are dumb; but not if that would cause overflow.
501 if elem_size > usize::MAX / 8 { 1 } else { 4 }
505 Layout::from_size_align_unchecked(cap * elem_size, mem::align_of::<T>())
507 Amortized { used_capacity, needed_extra_capacity } => {
508 // Nothing we can really do about these checks, sadly.
510 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
511 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
512 let double_cap = self.cap * 2;
513 // `double_cap` guarantees exponential growth.
514 let cap = cmp::max(double_cap, required_cap);
515 Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
517 Exact { used_capacity, needed_extra_capacity } => {
519 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
520 Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
524 let memory = if let Some(mut memory) = self.current_memory() {
525 debug_assert_eq!(memory.align(), layout.align());
528 .grow(&mut memory, layout.size(), placement, init)
529 .map_err(|_| AllocError { layout, non_exhaustive: () })?
534 MayMove => self.alloc.alloc(layout, init),
535 InPlace => Err(AllocErr),
537 .map_err(|_| AllocError { layout, non_exhaustive: () })?
540 self.set_memory(memory);
547 placement: ReallocPlacement,
548 ) -> Result<(), TryReserveError> {
549 assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
551 let mut memory = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
552 let new_size = amount * mem::size_of::<T>();
555 self.alloc.shrink(&mut memory, new_size, placement).map_err(|_| {
556 TryReserveError::AllocError {
557 layout: Layout::from_size_align_unchecked(new_size, memory.align()),
563 self.set_memory(memory);
568 impl<T> RawVec<T, Global> {
569 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
571 /// Note that this will correctly reconstitute any `cap` changes
572 /// that may have been performed. (See description of type for details.)
576 /// * `len` must be smaller than or equal to `self.capacity()`
577 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> {
579 len <= self.capacity(),
580 "`len` must be smaller than or equal to `self.capacity()`"
583 // NOTE: not calling `capacity()` here; actually using the real `cap` field!
584 let slice = slice::from_raw_parts_mut(self.ptr() as *mut MaybeUninit<T>, len);
585 let output = Box::from_raw(slice);
591 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
592 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
594 if let Some(memory) = self.current_memory() {
595 unsafe { self.alloc.dealloc(memory) }
600 // We need to guarantee the following:
601 // * We don't ever allocate `> isize::MAX` byte-size objects.
602 // * We don't overflow `usize::MAX` and actually allocate too little.
604 // On 64-bit we just need to check for overflow since trying to allocate
605 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
606 // an extra guard for this in case we're running on a platform which can use
607 // all 4GB in user-space, e.g., PAE or x32.
610 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
611 if mem::size_of::<usize>() < 8 && alloc_size > core::isize::MAX as usize {
612 Err(CapacityOverflow)
618 // One central function responsible for reporting capacity overflows. This'll
619 // ensure that the code generation related to these panics is minimal as there's
620 // only one location which panics rather than a bunch throughout the module.
621 fn capacity_overflow() -> ! {
622 panic!("capacity overflow");