1 // SPDX-License-Identifier: Apache-2.0 OR MIT
3 #![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
5 use core::alloc::LayoutError;
8 use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
9 use core::ptr::{self, NonNull, Unique};
12 #[cfg(not(no_global_oom_handling))]
13 use crate::alloc::handle_alloc_error;
14 use crate::alloc::{Allocator, Global, Layout};
15 use crate::boxed::Box;
16 use crate::collections::TryReserveError;
17 use crate::collections::TryReserveErrorKind::*;
23 /// The contents of the new memory are uninitialized.
25 /// The new memory is guaranteed to be zeroed.
31 #[cfg_attr(target_pointer_width = "16", rustc_layout_scalar_valid_range_end(0x7fff))]
32 #[cfg_attr(target_pointer_width = "32", rustc_layout_scalar_valid_range_end(0x7fff_ffff))]
33 #[cfg_attr(target_pointer_width = "64", rustc_layout_scalar_valid_range_end(0x7fff_ffff_ffff_ffff))]
37 const ZERO: Cap = unsafe { Cap(0) };
40 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
41 /// a buffer of memory on the heap without having to worry about all the corner cases
42 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
45 /// * Produces `Unique::dangling()` on zero-sized types.
46 /// * Produces `Unique::dangling()` on zero-length allocations.
47 /// * Avoids freeing `Unique::dangling()`.
48 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
49 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
50 /// * Guards against overflowing your length.
51 /// * Calls `handle_alloc_error` for fallible allocations.
52 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
53 /// * Uses the excess returned from the allocator to use the largest available capacity.
55 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
56 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
57 /// to handle the actual things *stored* inside of a `RawVec`.
59 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
60 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
61 /// `Box<[T]>`, since `capacity()` won't yield the length.
62 #[allow(missing_debug_implementations)]
63 pub(crate) struct RawVec<T, A: Allocator = Global> {
65 /// Never used for ZSTs; it's `capacity()`'s responsibility to return usize::MAX in that case.
69 /// `cap` must be in the `0..=isize::MAX` range.
74 impl<T> RawVec<T, Global> {
75 /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
76 /// they cannot call `Self::new()`.
78 /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
79 /// that would truly const-call something unstable.
80 pub const NEW: Self = Self::new();
82 /// Creates the biggest possible `RawVec` (on the system heap)
83 /// without allocating. If `T` has positive size, then this makes a
84 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
85 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
86 /// delayed allocation.
88 pub const fn new() -> Self {
92 /// Creates a `RawVec` (on the system heap) with exactly the
93 /// capacity and alignment requirements for a `[T; capacity]`. This is
94 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
95 /// zero-sized. Note that if `T` is zero-sized this means you will
96 /// *not* get a `RawVec` with the requested capacity.
100 /// Panics if the requested capacity exceeds `isize::MAX` bytes.
105 #[cfg(not(any(no_global_oom_handling, test)))]
108 pub fn with_capacity(capacity: usize) -> Self {
109 Self::with_capacity_in(capacity, Global)
112 /// Like `with_capacity`, but guarantees the buffer is zeroed.
113 #[cfg(not(any(no_global_oom_handling, test)))]
116 pub fn with_capacity_zeroed(capacity: usize) -> Self {
117 Self::with_capacity_zeroed_in(capacity, Global)
121 impl<T, A: Allocator> RawVec<T, A> {
122 // Tiny Vecs are dumb. Skip to:
123 // - 8 if the element size is 1, because any heap allocators is likely
124 // to round up a request of less than 8 bytes to at least 8 bytes.
125 // - 4 if elements are moderate-sized (<= 1 KiB).
126 // - 1 otherwise, to avoid wasting too much space for very short Vecs.
127 pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
129 } else if mem::size_of::<T>() <= 1024 {
135 /// Like `new`, but parameterized over the choice of allocator for
136 /// the returned `RawVec`.
137 pub const fn new_in(alloc: A) -> Self {
138 // `cap: 0` means "unallocated". zero-sized types are ignored.
139 Self { ptr: Unique::dangling(), cap: Cap::ZERO, alloc }
142 /// Like `with_capacity`, but parameterized over the choice of
143 /// allocator for the returned `RawVec`.
144 #[cfg(not(no_global_oom_handling))]
146 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
147 Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
150 /// Like `try_with_capacity`, but parameterized over the choice of
151 /// allocator for the returned `RawVec`.
153 pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
154 Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc)
157 /// Like `with_capacity_zeroed`, but parameterized over the choice
158 /// of allocator for the returned `RawVec`.
159 #[cfg(not(no_global_oom_handling))]
161 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
162 Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
165 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
167 /// Note that this will correctly reconstitute any `cap` changes
168 /// that may have been performed. (See description of type for details.)
172 /// * `len` must be greater than or equal to the most recently requested capacity, and
173 /// * `len` must be less than or equal to `self.capacity()`.
175 /// Note, that the requested capacity and `self.capacity()` could differ, as
176 /// an allocator could overallocate and return a greater memory block than requested.
177 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
178 // Sanity-check one half of the safety requirement (we cannot check the other half).
180 len <= self.capacity(),
181 "`len` must be smaller than or equal to `self.capacity()`"
184 let me = ManuallyDrop::new(self);
186 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
187 Box::from_raw_in(slice, ptr::read(&me.alloc))
191 #[cfg(not(no_global_oom_handling))]
192 fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
193 // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
194 if T::IS_ZST || capacity == 0 {
197 // We avoid `unwrap_or_else` here because it bloats the amount of
198 // LLVM IR generated.
199 let layout = match Layout::array::<T>(capacity) {
200 Ok(layout) => layout,
201 Err(_) => capacity_overflow(),
203 match alloc_guard(layout.size()) {
205 Err(_) => capacity_overflow(),
207 let result = match init {
208 AllocInit::Uninitialized => alloc.allocate(layout),
209 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
211 let ptr = match result {
213 Err(_) => handle_alloc_error(layout),
216 // Allocators currently return a `NonNull<[u8]>` whose length
217 // matches the size requested. If that ever changes, the capacity
218 // here should change to `ptr.len() / mem::size_of::<T>()`.
220 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
221 cap: unsafe { Cap(capacity) },
227 fn try_allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Result<Self, TryReserveError> {
228 // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
229 if T::IS_ZST || capacity == 0 {
230 return Ok(Self::new_in(alloc));
233 let layout = Layout::array::<T>(capacity).map_err(|_| CapacityOverflow)?;
234 alloc_guard(layout.size())?;
235 let result = match init {
236 AllocInit::Uninitialized => alloc.allocate(layout),
237 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
239 let ptr = result.map_err(|_| AllocError { layout, non_exhaustive: () })?;
241 // Allocators currently return a `NonNull<[u8]>` whose length
242 // matches the size requested. If that ever changes, the capacity
243 // here should change to `ptr.len() / mem::size_of::<T>()`.
245 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
246 cap: unsafe { Cap(capacity) },
251 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
255 /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
257 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
258 /// systems). For ZSTs capacity is ignored.
259 /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
262 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
263 let cap = if T::IS_ZST { Cap::ZERO } else { unsafe { Cap(capacity) } };
264 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap, alloc }
267 /// Gets a raw pointer to the start of the allocation. Note that this is
268 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
271 pub fn ptr(&self) -> *mut T {
275 /// Gets the capacity of the allocation.
277 /// This will always be `usize::MAX` if `T` is zero-sized.
279 pub fn capacity(&self) -> usize {
280 if T::IS_ZST { usize::MAX } else { self.cap.0 }
283 /// Returns a shared reference to the allocator backing this `RawVec`.
284 pub fn allocator(&self) -> &A {
288 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
289 if T::IS_ZST || self.cap.0 == 0 {
292 // We could use Layout::array here which ensures the absence of isize and usize overflows
293 // and could hypothetically handle differences between stride and size, but this memory
294 // has already been allocated so we know it can't overflow and currently rust does not
295 // support such types. So we can do better by skipping some checks and avoid an unwrap.
296 let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
298 let align = mem::align_of::<T>();
299 let size = mem::size_of::<T>().unchecked_mul(self.cap.0);
300 let layout = Layout::from_size_align_unchecked(size, align);
301 Some((self.ptr.cast().into(), layout))
306 /// Ensures that the buffer contains at least enough space to hold `len +
307 /// additional` elements. If it doesn't already have enough capacity, will
308 /// reallocate enough space plus comfortable slack space to get amortized
309 /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
312 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
313 /// the requested space. This is not really unsafe, but the unsafe
314 /// code *you* write that relies on the behavior of this function may break.
316 /// This is ideal for implementing a bulk-push operation like `extend`.
320 /// Panics if the new capacity exceeds `isize::MAX` bytes.
325 #[cfg(not(no_global_oom_handling))]
327 pub fn reserve(&mut self, len: usize, additional: usize) {
328 // Callers expect this function to be very cheap when there is already sufficient capacity.
329 // Therefore, we move all the resizing and error-handling logic from grow_amortized and
330 // handle_reserve behind a call, while making sure that this function is likely to be
331 // inlined as just a comparison and a call if the comparison fails.
333 fn do_reserve_and_handle<T, A: Allocator>(
334 slf: &mut RawVec<T, A>,
338 handle_reserve(slf.grow_amortized(len, additional));
341 if self.needs_to_grow(len, additional) {
342 do_reserve_and_handle(self, len, additional);
346 /// A specialized version of `reserve()` used only by the hot and
347 /// oft-instantiated `Vec::push()`, which does its own capacity check.
348 #[cfg(not(no_global_oom_handling))]
350 pub fn reserve_for_push(&mut self, len: usize) {
351 handle_reserve(self.grow_amortized(len, 1));
354 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
355 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
356 if self.needs_to_grow(len, additional) {
357 self.grow_amortized(len, additional)?;
360 // Inform the optimizer that the reservation has succeeded or wasn't needed
361 core::intrinsics::assume(!self.needs_to_grow(len, additional));
366 /// The same as `reserve_for_push`, but returns on errors instead of panicking or aborting.
368 pub fn try_reserve_for_push(&mut self, len: usize) -> Result<(), TryReserveError> {
369 self.grow_amortized(len, 1)
372 /// Ensures that the buffer contains at least enough space to hold `len +
373 /// additional` elements. If it doesn't already, will reallocate the
374 /// minimum possible amount of memory necessary. Generally this will be
375 /// exactly the amount of memory necessary, but in principle the allocator
376 /// is free to give back more than we asked for.
378 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
379 /// the requested space. This is not really unsafe, but the unsafe code
380 /// *you* write that relies on the behavior of this function may break.
384 /// Panics if the new capacity exceeds `isize::MAX` bytes.
389 #[cfg(not(no_global_oom_handling))]
390 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
391 handle_reserve(self.try_reserve_exact(len, additional));
394 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
395 pub fn try_reserve_exact(
399 ) -> Result<(), TryReserveError> {
400 if self.needs_to_grow(len, additional) {
401 self.grow_exact(len, additional)?;
404 // Inform the optimizer that the reservation has succeeded or wasn't needed
405 core::intrinsics::assume(!self.needs_to_grow(len, additional));
410 /// Shrinks the buffer down to the specified capacity. If the given amount
411 /// is 0, actually completely deallocates.
415 /// Panics if the given amount is *larger* than the current capacity.
420 #[cfg(not(no_global_oom_handling))]
421 pub fn shrink_to_fit(&mut self, cap: usize) {
422 handle_reserve(self.shrink(cap));
426 impl<T, A: Allocator> RawVec<T, A> {
427 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
428 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
429 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
430 additional > self.capacity().wrapping_sub(len)
435 /// `cap` must not exceed `isize::MAX`.
436 unsafe fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
437 // Allocators currently return a `NonNull<[u8]>` whose length matches
438 // the size requested. If that ever changes, the capacity here should
439 // change to `ptr.len() / mem::size_of::<T>()`.
440 self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
441 self.cap = unsafe { Cap(cap) };
444 // This method is usually instantiated many times. So we want it to be as
445 // small as possible, to improve compile times. But we also want as much of
446 // its contents to be statically computable as possible, to make the
447 // generated code run faster. Therefore, this method is carefully written
448 // so that all of the code that depends on `T` is within it, while as much
449 // of the code that doesn't depend on `T` as possible is in functions that
450 // are non-generic over `T`.
451 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
452 // This is ensured by the calling contexts.
453 debug_assert!(additional > 0);
456 // Since we return a capacity of `usize::MAX` when `elem_size` is
457 // 0, getting to here necessarily means the `RawVec` is overfull.
458 return Err(CapacityOverflow.into());
461 // Nothing we can really do about these checks, sadly.
462 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
464 // This guarantees exponential growth. The doubling cannot overflow
465 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
466 let cap = cmp::max(self.cap.0 * 2, required_cap);
467 let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
469 let new_layout = Layout::array::<T>(cap);
471 // `finish_grow` is non-generic over `T`.
472 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
473 // SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than isize::MAX items
474 unsafe { self.set_ptr_and_cap(ptr, cap) };
478 // The constraints on this method are much the same as those on
479 // `grow_amortized`, but this method is usually instantiated less often so
480 // it's less critical.
481 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
483 // Since we return a capacity of `usize::MAX` when the type size is
484 // 0, getting to here necessarily means the `RawVec` is overfull.
485 return Err(CapacityOverflow.into());
488 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
489 let new_layout = Layout::array::<T>(cap);
491 // `finish_grow` is non-generic over `T`.
492 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
493 // SAFETY: finish_grow would have resulted in a capacity overflow if we tried to allocate more than isize::MAX items
495 self.set_ptr_and_cap(ptr, cap);
500 #[cfg(not(no_global_oom_handling))]
501 fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
502 assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
504 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
505 // See current_memory() why this assert is here
506 let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
508 // If shrinking to 0, deallocate the buffer. We don't reach this point
509 // for the T::IS_ZST case since current_memory() will have returned
512 unsafe { self.alloc.deallocate(ptr, layout) };
513 self.ptr = Unique::dangling();
514 self.cap = Cap::ZERO;
517 // `Layout::array` cannot overflow here because it would have
518 // overflowed earlier when capacity was larger.
519 let new_size = mem::size_of::<T>().unchecked_mul(cap);
520 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
522 .shrink(ptr, layout, new_layout)
523 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
525 // SAFETY: if the allocation is valid, then the capacity is too
527 self.set_ptr_and_cap(ptr, cap);
534 // This function is outside `RawVec` to minimize compile times. See the comment
535 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
536 // significant, because the number of different `A` types seen in practice is
537 // much smaller than the number of `T` types.)
540 new_layout: Result<Layout, LayoutError>,
541 current_memory: Option<(NonNull<u8>, Layout)>,
543 ) -> Result<NonNull<[u8]>, TryReserveError>
547 // Check for the error here to minimize the size of `RawVec::grow_*`.
548 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
550 alloc_guard(new_layout.size())?;
552 let memory = if let Some((ptr, old_layout)) = current_memory {
553 debug_assert_eq!(old_layout.align(), new_layout.align());
555 // The allocator checks for alignment equality
556 intrinsics::assume(old_layout.align() == new_layout.align());
557 alloc.grow(ptr, old_layout, new_layout)
560 alloc.allocate(new_layout)
563 memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
566 unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
567 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
569 if let Some((ptr, layout)) = self.current_memory() {
570 unsafe { self.alloc.deallocate(ptr, layout) }
575 // Central function for reserve error handling.
576 #[cfg(not(no_global_oom_handling))]
578 fn handle_reserve(result: Result<(), TryReserveError>) {
579 match result.map_err(|e| e.kind()) {
580 Err(CapacityOverflow) => capacity_overflow(),
581 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
582 Ok(()) => { /* yay */ }
586 // We need to guarantee the following:
587 // * We don't ever allocate `> isize::MAX` byte-size objects.
588 // * We don't overflow `usize::MAX` and actually allocate too little.
590 // On 64-bit we just need to check for overflow since trying to allocate
591 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
592 // an extra guard for this in case we're running on a platform which can use
593 // all 4GB in user-space, e.g., PAE or x32.
596 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
597 if usize::BITS < 64 && alloc_size > isize::MAX as usize {
598 Err(CapacityOverflow.into())
604 // One central function responsible for reporting capacity overflows. This'll
605 // ensure that the code generation related to these panics is minimal as there's
606 // only one location which panics rather than a bunch throughout the module.
607 #[cfg(not(no_global_oom_handling))]
608 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
609 fn capacity_overflow() -> ! {
610 panic!("capacity overflow");