1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
4 use core::alloc::MemoryBlock;
6 use core::mem::{self, ManuallyDrop, MaybeUninit};
8 use core::ptr::{NonNull, Unique};
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.
105 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
106 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
107 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
108 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
110 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
111 Self::from_raw_parts_in(ptr, capacity, Global)
114 /// Converts a `Box<[T]>` into a `RawVec<T>`.
115 pub fn from_box(slice: Box<[T]>) -> Self {
117 let mut slice = ManuallyDrop::new(slice);
118 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: unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) },
155 cap: Self::capacity_from_bytes(memory.size),
161 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
165 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
166 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
167 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
168 /// 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 {
171 Self { ptr: Unique::new_unchecked(ptr), cap: capacity, alloc: a }
174 /// Gets a raw pointer to the start of the allocation. Note that this is
175 /// `Unique::empty()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
177 pub fn ptr(&self) -> *mut T {
181 /// Gets the capacity of the allocation.
183 /// This will always be `usize::MAX` if `T` is zero-sized.
185 pub fn capacity(&self) -> usize {
186 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
189 /// Returns a shared reference to the allocator backing this `RawVec`.
190 pub fn alloc(&self) -> &A {
194 /// Returns a mutable reference to the allocator backing this `RawVec`.
195 pub fn alloc_mut(&mut self) -> &mut A {
199 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
200 if mem::size_of::<T>() == 0 || self.cap == 0 {
203 // We have an allocated chunk of memory, so we can bypass runtime
204 // checks to get our current layout.
206 let align = mem::align_of::<T>();
207 let size = mem::size_of::<T>() * self.cap;
208 let layout = Layout::from_size_align_unchecked(size, align);
209 Some((self.ptr.cast().into(), layout))
214 /// Doubles the size of the type's backing allocation. This is common enough
215 /// to want to do that it's easiest to just have a dedicated method. Slightly
216 /// more efficient logic can be provided for this than the general case.
218 /// This function is ideal for when pushing elements one-at-a-time because
219 /// you don't need to incur the costs of the more general computations
220 /// reserve needs to do to guard against overflow. You do however need to
221 /// manually check if your `len == capacity`.
225 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
226 /// all `usize::MAX` slots in your imaginary buffer.
227 /// * Panics on 32-bit platforms if the requested capacity exceeds
228 /// `isize::MAX` bytes.
237 /// # #![feature(raw_vec_internals)]
238 /// # extern crate alloc;
240 /// # use alloc::raw_vec::RawVec;
241 /// struct MyVec<T> {
246 /// impl<T> MyVec<T> {
247 /// pub fn push(&mut self, elem: T) {
248 /// if self.len == self.buf.capacity() { self.buf.double(); }
249 /// // double would have aborted or panicked if the len exceeded
250 /// // `isize::MAX` so this is safe to do unchecked now.
252 /// ptr::write(self.buf.ptr().add(self.len), elem);
258 /// # let mut vec = MyVec { buf: RawVec::new(), len: 0 };
264 pub fn double(&mut self) {
265 match self.grow(Double, MayMove, Uninitialized) {
266 Err(CapacityOverflow) => capacity_overflow(),
267 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
268 Ok(()) => { /* yay */ }
272 /// Attempts to double the size of the type's backing allocation in place. This is common
273 /// enough to want to do that it's easiest to just have a dedicated method. Slightly
274 /// more efficient logic can be provided for this than the general case.
276 /// Returns `true` if the reallocation attempt has succeeded.
280 /// * Panics if `T` is zero-sized on the assumption that you managed to exhaust
281 /// all `usize::MAX` slots in your imaginary buffer.
282 /// * Panics on 32-bit platforms if the requested capacity exceeds
283 /// `isize::MAX` bytes.
286 pub fn double_in_place(&mut self) -> bool {
287 self.grow(Double, InPlace, Uninitialized).is_ok()
290 /// Ensures that the buffer contains at least enough space to hold
291 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
292 /// enough capacity, will reallocate enough space plus comfortable slack
293 /// space to get amortized `O(1)` behavior. Will limit this behavior
294 /// if it would needlessly cause itself to panic.
296 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
297 /// the requested space. This is not really unsafe, but the unsafe
298 /// code *you* write that relies on the behavior of this function may break.
300 /// This is ideal for implementing a bulk-push operation like `extend`.
304 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
305 /// * Panics on 32-bit platforms if the requested capacity exceeds
306 /// `isize::MAX` bytes.
315 /// # #![feature(raw_vec_internals)]
316 /// # extern crate alloc;
318 /// # use alloc::raw_vec::RawVec;
319 /// struct MyVec<T> {
324 /// impl<T: Clone> MyVec<T> {
325 /// pub fn push_all(&mut self, elems: &[T]) {
326 /// self.buf.reserve(self.len, elems.len());
327 /// // reserve would have aborted or panicked if the len exceeded
328 /// // `isize::MAX` so this is safe to do unchecked now.
331 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
338 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
339 /// # vector.push_all(&[1, 3, 5, 7, 9]);
342 pub fn reserve(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
343 match self.try_reserve(used_capacity, needed_extra_capacity) {
344 Err(CapacityOverflow) => capacity_overflow(),
345 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
346 Ok(()) => { /* yay */ }
350 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
353 used_capacity: usize,
354 needed_extra_capacity: usize,
355 ) -> Result<(), TryReserveError> {
356 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
357 self.grow(Amortized { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
363 /// Attempts to ensure that the buffer contains at least enough space to hold
364 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already have
365 /// enough capacity, will reallocate in place enough space plus comfortable slack
366 /// space to get amortized `O(1)` behavior. Will limit this behaviour
367 /// if it would needlessly cause itself to panic.
369 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
370 /// the requested space. This is not really unsafe, but the unsafe
371 /// code *you* write that relies on the behavior of this function may break.
373 /// Returns `true` if the reallocation attempt has succeeded.
377 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
378 /// * Panics on 32-bit platforms if the requested capacity exceeds
379 /// `isize::MAX` bytes.
380 pub fn reserve_in_place(&mut self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
381 // This is more readable than putting this in one line:
382 // `!self.needs_to_grow(...) || self.grow(...).is_ok()`
383 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
384 self.grow(Amortized { used_capacity, needed_extra_capacity }, InPlace, Uninitialized)
391 /// Ensures that the buffer contains at least enough space to hold
392 /// `used_capacity + needed_extra_capacity` elements. If it doesn't already,
393 /// will reallocate the minimum possible amount of memory necessary.
394 /// Generally this will be exactly the amount of memory necessary,
395 /// but in principle the allocator is free to give back more than
398 /// If `used_capacity` exceeds `self.capacity()`, this may fail to actually allocate
399 /// the requested space. This is not really unsafe, but the unsafe
400 /// code *you* write that relies on the behavior of this function may break.
404 /// * Panics if the requested capacity exceeds `usize::MAX` bytes.
405 /// * Panics on 32-bit platforms if the requested capacity exceeds
406 /// `isize::MAX` bytes.
411 pub fn reserve_exact(&mut self, used_capacity: usize, needed_extra_capacity: usize) {
412 match self.try_reserve_exact(used_capacity, needed_extra_capacity) {
413 Err(CapacityOverflow) => capacity_overflow(),
414 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
415 Ok(()) => { /* yay */ }
419 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
420 pub fn try_reserve_exact(
422 used_capacity: usize,
423 needed_extra_capacity: usize,
424 ) -> Result<(), TryReserveError> {
425 if self.needs_to_grow(used_capacity, needed_extra_capacity) {
426 self.grow(Exact { used_capacity, needed_extra_capacity }, MayMove, Uninitialized)
432 /// Shrinks the allocation down to the specified amount. If the given amount
433 /// is 0, actually completely deallocates.
437 /// Panics if the given amount is *larger* than the current capacity.
442 pub fn shrink_to_fit(&mut self, amount: usize) {
443 match self.shrink(amount, MayMove) {
444 Err(CapacityOverflow) => capacity_overflow(),
445 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
446 Ok(()) => { /* yay */ }
451 #[derive(Copy, Clone)]
454 Amortized { used_capacity: usize, needed_extra_capacity: usize },
455 Exact { used_capacity: usize, needed_extra_capacity: usize },
459 impl<T, A: AllocRef> RawVec<T, A> {
460 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
461 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
462 fn needs_to_grow(&self, used_capacity: usize, needed_extra_capacity: usize) -> bool {
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 = unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) };
473 self.cap = Self::capacity_from_bytes(memory.size);
476 /// Single method to handle all possibilities of growing the buffer.
480 placement: ReallocPlacement,
482 ) -> Result<(), TryReserveError> {
483 let elem_size = mem::size_of::<T>();
485 // Since we return a capacity of `usize::MAX` when `elem_size` is
486 // 0, getting to here necessarily means the `RawVec` is overfull.
487 return Err(CapacityOverflow);
489 let new_layout = match strategy {
491 // Since we guarantee that we never allocate more than `isize::MAX` bytes,
492 // `elem_size * self.cap <= isize::MAX` as a precondition, so this can't overflow.
493 // Additionally the alignment will never be too large as to "not be satisfiable",
494 // so `Layout::from_size_align` will always return `Some`.
496 // TL;DR, we bypass runtime checks due to dynamic assertions in this module,
497 // allowing us to use `from_size_align_unchecked`.
498 let cap = if self.cap == 0 {
499 // Skip to 4 because tiny `Vec`'s are dumb; but not if that would cause overflow.
500 if elem_size > usize::MAX / 8 { 1 } else { 4 }
504 Layout::from_size_align_unchecked(cap * elem_size, mem::align_of::<T>())
506 Amortized { used_capacity, needed_extra_capacity } => {
507 // Nothing we can really do about these checks, sadly.
509 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
510 // Cannot overflow, because `cap <= isize::MAX`, and type of `cap` is `usize`.
511 let double_cap = self.cap * 2;
512 // `double_cap` guarantees exponential growth.
513 let cap = cmp::max(double_cap, required_cap);
514 Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
516 Exact { used_capacity, needed_extra_capacity } => {
518 used_capacity.checked_add(needed_extra_capacity).ok_or(CapacityOverflow)?;
519 Layout::array::<T>(cap).map_err(|_| CapacityOverflow)?
522 alloc_guard(new_layout.size())?;
524 let memory = if let Some((ptr, old_layout)) = self.current_memory() {
525 debug_assert_eq!(old_layout.align(), new_layout.align());
528 .grow(ptr, old_layout, new_layout.size(), placement, init)
529 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
533 MayMove => self.alloc.alloc(new_layout, init),
534 InPlace => Err(AllocErr),
536 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
538 self.set_memory(memory);
545 placement: ReallocPlacement,
546 ) -> Result<(), TryReserveError> {
547 assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
549 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
550 let new_size = amount * mem::size_of::<T>();
552 let memory = unsafe {
553 self.alloc.shrink(ptr, layout, new_size, placement).map_err(|_| {
554 TryReserveError::AllocError {
555 layout: Layout::from_size_align_unchecked(new_size, layout.align()),
560 self.set_memory(memory);
565 impl<T> RawVec<T, Global> {
566 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
568 /// Note that this will correctly reconstitute any `cap` changes
569 /// that may have been performed. (See description of type for details.)
573 /// * `len` must be greater than or equal to the most recently requested capacity, and
574 /// * `len` must be less than or equal to `self.capacity()`.
576 /// Note, that the requested capacity and `self.capacity()` could differ, as
577 /// an allocator could overallocate and return a greater memory block than requested.
578 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> {
579 // Sanity-check one half of the safety requirement (we cannot check the other half).
581 len <= self.capacity(),
582 "`len` must be smaller than or equal to `self.capacity()`"
585 let me = ManuallyDrop::new(self);
586 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
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((ptr, layout)) = self.current_memory() {
595 unsafe { self.alloc.dealloc(ptr, layout) }
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 > 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");