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};
10 use core::ptr::{self, NonNull, Unique};
13 #[cfg(not(no_global_oom_handling))]
14 use crate::alloc::handle_alloc_error;
15 use crate::alloc::{Allocator, Global, Layout};
16 use crate::boxed::Box;
17 use crate::collections::TryReserveError;
18 use crate::collections::TryReserveErrorKind::*;
23 #[cfg(not(no_global_oom_handling))]
25 /// The contents of the new memory are uninitialized.
27 /// The new memory is guaranteed to be zeroed.
31 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
32 /// a buffer of memory on the heap without having to worry about all the corner cases
33 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
36 /// * Produces `Unique::dangling()` on zero-sized types.
37 /// * Produces `Unique::dangling()` on zero-length allocations.
38 /// * Avoids freeing `Unique::dangling()`.
39 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
40 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
41 /// * Guards against overflowing your length.
42 /// * Calls `handle_alloc_error` for fallible allocations.
43 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
44 /// * Uses the excess returned from the allocator to use the largest available capacity.
46 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
47 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
48 /// to handle the actual things *stored* inside of a `RawVec`.
50 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
51 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
52 /// `Box<[T]>`, since `capacity()` won't yield the length.
53 #[allow(missing_debug_implementations)]
54 pub(crate) struct RawVec<T, A: Allocator = Global> {
60 impl<T> RawVec<T, Global> {
61 /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
62 /// they cannot call `Self::new()`.
64 /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
65 /// that would truly const-call something unstable.
66 pub const NEW: Self = Self::new();
68 /// Creates the biggest possible `RawVec` (on the system heap)
69 /// without allocating. If `T` has positive size, then this makes a
70 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
71 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
72 /// delayed allocation.
74 pub const fn new() -> Self {
78 /// Creates a `RawVec` (on the system heap) with exactly the
79 /// capacity and alignment requirements for a `[T; capacity]`. This is
80 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
81 /// zero-sized. Note that if `T` is zero-sized this means you will
82 /// *not* get a `RawVec` with the requested capacity.
86 /// Panics if the requested capacity exceeds `isize::MAX` bytes.
91 #[cfg(not(any(no_global_oom_handling, test)))]
94 pub fn with_capacity(capacity: usize) -> Self {
95 Self::with_capacity_in(capacity, Global)
98 /// Like `with_capacity`, but guarantees the buffer is zeroed.
99 #[cfg(not(any(no_global_oom_handling, test)))]
102 pub fn with_capacity_zeroed(capacity: usize) -> Self {
103 Self::with_capacity_zeroed_in(capacity, Global)
107 impl<T, A: Allocator> RawVec<T, A> {
108 // Tiny Vecs are dumb. Skip to:
109 // - 8 if the element size is 1, because any heap allocators is likely
110 // to round up a request of less than 8 bytes to at least 8 bytes.
111 // - 4 if elements are moderate-sized (<= 1 KiB).
112 // - 1 otherwise, to avoid wasting too much space for very short Vecs.
113 pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
115 } else if mem::size_of::<T>() <= 1024 {
121 /// Like `new`, but parameterized over the choice of allocator for
122 /// the returned `RawVec`.
123 pub const fn new_in(alloc: A) -> Self {
124 // `cap: 0` means "unallocated". zero-sized types are ignored.
125 Self { ptr: Unique::dangling(), cap: 0, alloc }
128 /// Like `with_capacity`, but parameterized over the choice of
129 /// allocator for the returned `RawVec`.
130 #[cfg(not(no_global_oom_handling))]
132 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
133 Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
136 /// Like `with_capacity_zeroed`, but parameterized over the choice
137 /// of allocator for the returned `RawVec`.
138 #[cfg(not(no_global_oom_handling))]
140 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
141 Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
144 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
146 /// Note that this will correctly reconstitute any `cap` changes
147 /// that may have been performed. (See description of type for details.)
151 /// * `len` must be greater than or equal to the most recently requested capacity, and
152 /// * `len` must be less than or equal to `self.capacity()`.
154 /// Note, that the requested capacity and `self.capacity()` could differ, as
155 /// an allocator could overallocate and return a greater memory block than requested.
156 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
157 // Sanity-check one half of the safety requirement (we cannot check the other half).
159 len <= self.capacity(),
160 "`len` must be smaller than or equal to `self.capacity()`"
163 let me = ManuallyDrop::new(self);
165 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
166 Box::from_raw_in(slice, ptr::read(&me.alloc))
170 #[cfg(not(no_global_oom_handling))]
171 fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
172 // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
173 if mem::size_of::<T>() == 0 || capacity == 0 {
176 // We avoid `unwrap_or_else` here because it bloats the amount of
177 // LLVM IR generated.
178 let layout = match Layout::array::<T>(capacity) {
179 Ok(layout) => layout,
180 Err(_) => capacity_overflow(),
182 match alloc_guard(layout.size()) {
184 Err(_) => capacity_overflow(),
186 let result = match init {
187 AllocInit::Uninitialized => alloc.allocate(layout),
188 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
190 let ptr = match result {
192 Err(_) => handle_alloc_error(layout),
195 // Allocators currently return a `NonNull<[u8]>` whose length
196 // matches the size requested. If that ever changes, the capacity
197 // here should change to `ptr.len() / mem::size_of::<T>()`.
199 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
206 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
210 /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
212 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
213 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
214 /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
217 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
218 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
221 /// Gets a raw pointer to the start of the allocation. Note that this is
222 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
225 pub fn ptr(&self) -> *mut T {
229 /// Gets the capacity of the allocation.
231 /// This will always be `usize::MAX` if `T` is zero-sized.
233 pub fn capacity(&self) -> usize {
234 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
237 /// Returns a shared reference to the allocator backing this `RawVec`.
238 pub fn allocator(&self) -> &A {
242 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
243 if mem::size_of::<T>() == 0 || self.cap == 0 {
246 // We have an allocated chunk of memory, so we can bypass runtime
247 // checks to get our current layout.
249 let layout = Layout::array::<T>(self.cap).unwrap_unchecked();
250 Some((self.ptr.cast().into(), layout))
255 /// Ensures that the buffer contains at least enough space to hold `len +
256 /// additional` elements. If it doesn't already have enough capacity, will
257 /// reallocate enough space plus comfortable slack space to get amortized
258 /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
261 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
262 /// the requested space. This is not really unsafe, but the unsafe
263 /// code *you* write that relies on the behavior of this function may break.
265 /// This is ideal for implementing a bulk-push operation like `extend`.
269 /// Panics if the new capacity exceeds `isize::MAX` bytes.
274 #[cfg(not(no_global_oom_handling))]
276 pub fn reserve(&mut self, len: usize, additional: usize) {
277 // Callers expect this function to be very cheap when there is already sufficient capacity.
278 // Therefore, we move all the resizing and error-handling logic from grow_amortized and
279 // handle_reserve behind a call, while making sure that this function is likely to be
280 // inlined as just a comparison and a call if the comparison fails.
282 fn do_reserve_and_handle<T, A: Allocator>(
283 slf: &mut RawVec<T, A>,
287 handle_reserve(slf.grow_amortized(len, additional));
290 if self.needs_to_grow(len, additional) {
291 do_reserve_and_handle(self, len, additional);
295 /// A specialized version of `reserve()` used only by the hot and
296 /// oft-instantiated `Vec::push()`, which does its own capacity check.
297 #[cfg(not(no_global_oom_handling))]
299 pub fn reserve_for_push(&mut self, len: usize) {
300 handle_reserve(self.grow_amortized(len, 1));
303 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
304 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
305 if self.needs_to_grow(len, additional) {
306 self.grow_amortized(len, additional)
312 /// The same as `reserve_for_push`, but returns on errors instead of panicking or aborting.
314 pub fn try_reserve_for_push(&mut self, len: usize) -> Result<(), TryReserveError> {
315 self.grow_amortized(len, 1)
318 /// Ensures that the buffer contains at least enough space to hold `len +
319 /// additional` elements. If it doesn't already, will reallocate the
320 /// minimum possible amount of memory necessary. Generally this will be
321 /// exactly the amount of memory necessary, but in principle the allocator
322 /// is free to give back more than we asked for.
324 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
325 /// the requested space. This is not really unsafe, but the unsafe code
326 /// *you* write that relies on the behavior of this function may break.
330 /// Panics if the new capacity exceeds `isize::MAX` bytes.
335 #[cfg(not(no_global_oom_handling))]
336 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
337 handle_reserve(self.try_reserve_exact(len, additional));
340 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
341 pub fn try_reserve_exact(
345 ) -> Result<(), TryReserveError> {
346 if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
349 /// Shrinks the buffer down to the specified capacity. If the given amount
350 /// is 0, actually completely deallocates.
354 /// Panics if the given amount is *larger* than the current capacity.
359 #[cfg(not(no_global_oom_handling))]
360 pub fn shrink_to_fit(&mut self, cap: usize) {
361 handle_reserve(self.shrink(cap));
365 impl<T, A: Allocator> RawVec<T, A> {
366 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
367 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
368 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
369 additional > self.capacity().wrapping_sub(len)
372 fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
373 // Allocators currently return a `NonNull<[u8]>` whose length matches
374 // the size requested. If that ever changes, the capacity here should
375 // change to `ptr.len() / mem::size_of::<T>()`.
376 self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
380 // This method is usually instantiated many times. So we want it to be as
381 // small as possible, to improve compile times. But we also want as much of
382 // its contents to be statically computable as possible, to make the
383 // generated code run faster. Therefore, this method is carefully written
384 // so that all of the code that depends on `T` is within it, while as much
385 // of the code that doesn't depend on `T` as possible is in functions that
386 // are non-generic over `T`.
387 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
388 // This is ensured by the calling contexts.
389 debug_assert!(additional > 0);
391 if mem::size_of::<T>() == 0 {
392 // Since we return a capacity of `usize::MAX` when `elem_size` is
393 // 0, getting to here necessarily means the `RawVec` is overfull.
394 return Err(CapacityOverflow.into());
397 // Nothing we can really do about these checks, sadly.
398 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
400 // This guarantees exponential growth. The doubling cannot overflow
401 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
402 let cap = cmp::max(self.cap * 2, required_cap);
403 let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
405 let new_layout = Layout::array::<T>(cap);
407 // `finish_grow` is non-generic over `T`.
408 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
409 self.set_ptr_and_cap(ptr, cap);
413 // The constraints on this method are much the same as those on
414 // `grow_amortized`, but this method is usually instantiated less often so
415 // it's less critical.
416 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
417 if mem::size_of::<T>() == 0 {
418 // Since we return a capacity of `usize::MAX` when the type size is
419 // 0, getting to here necessarily means the `RawVec` is overfull.
420 return Err(CapacityOverflow.into());
423 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
424 let new_layout = Layout::array::<T>(cap);
426 // `finish_grow` is non-generic over `T`.
427 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
428 self.set_ptr_and_cap(ptr, cap);
433 fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
434 assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
436 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
439 // `Layout::array` cannot overflow here because it would have
440 // overflowed earlier when capacity was larger.
441 let new_layout = Layout::array::<T>(cap).unwrap_unchecked();
443 .shrink(ptr, layout, new_layout)
444 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
446 self.set_ptr_and_cap(ptr, cap);
451 // This function is outside `RawVec` to minimize compile times. See the comment
452 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
453 // significant, because the number of different `A` types seen in practice is
454 // much smaller than the number of `T` types.)
457 new_layout: Result<Layout, LayoutError>,
458 current_memory: Option<(NonNull<u8>, Layout)>,
460 ) -> Result<NonNull<[u8]>, TryReserveError>
464 // Check for the error here to minimize the size of `RawVec::grow_*`.
465 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
467 alloc_guard(new_layout.size())?;
469 let memory = if let Some((ptr, old_layout)) = current_memory {
470 debug_assert_eq!(old_layout.align(), new_layout.align());
472 // The allocator checks for alignment equality
473 intrinsics::assume(old_layout.align() == new_layout.align());
474 alloc.grow(ptr, old_layout, new_layout)
477 alloc.allocate(new_layout)
480 memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
483 unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
484 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
486 if let Some((ptr, layout)) = self.current_memory() {
487 unsafe { self.alloc.deallocate(ptr, layout) }
492 // Central function for reserve error handling.
493 #[cfg(not(no_global_oom_handling))]
495 fn handle_reserve(result: Result<(), TryReserveError>) {
496 match result.map_err(|e| e.kind()) {
497 Err(CapacityOverflow) => capacity_overflow(),
498 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
499 Ok(()) => { /* yay */ }
503 // We need to guarantee the following:
504 // * We don't ever allocate `> isize::MAX` byte-size objects.
505 // * We don't overflow `usize::MAX` and actually allocate too little.
507 // On 64-bit we just need to check for overflow since trying to allocate
508 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
509 // an extra guard for this in case we're running on a platform which can use
510 // all 4GB in user-space, e.g., PAE or x32.
513 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
514 if usize::BITS < 64 && alloc_size > isize::MAX as usize {
515 Err(CapacityOverflow.into())
521 // One central function responsible for reporting capacity overflows. This'll
522 // ensure that the code generation related to these panics is minimal as there's
523 // only one location which panics rather than a bunch throughout the module.
524 #[cfg(not(no_global_oom_handling))]
525 fn capacity_overflow() -> ! {
526 panic!("capacity overflow");