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1// SPDX-License-Identifier: Apache-2.0 OR MIT
2
3#![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
4
5use core::alloc::LayoutError;
6use core::cmp;
7use core::intrinsics;
8use core::mem::{self, ManuallyDrop, MaybeUninit};
9use core::ops::Drop;
10use core::ptr::{self, NonNull, Unique};
11use core::slice;
12
13#[cfg(not(no_global_oom_handling))]
14use crate::alloc::handle_alloc_error;
15use crate::alloc::{Allocator, Global, Layout};
16use crate::boxed::Box;
17use crate::collections::TryReserveError;
18use crate::collections::TryReserveErrorKind::*;
19
20#[cfg(test)]
21mod tests;
22
23enum AllocInit {
24 /// The contents of the new memory are uninitialized.
25 Uninitialized,
26 /// The new memory is guaranteed to be zeroed.
27 #[allow(dead_code)]
28 Zeroed,
29}
30
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.
34/// In particular:
35///
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.
45///
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`.
49///
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)]
54pub(crate) struct RawVec<T, A: Allocator = Global> {
55 ptr: Unique<T>,
56 cap: usize,
57 alloc: A,
58}
59
60impl<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()`.
63 ///
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();
67
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.
73 #[must_use]
74 pub const fn new() -> Self {
75 Self::new_in(Global)
76 }
77
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.
83 ///
84 /// # Panics
85 ///
86 /// Panics if the requested capacity exceeds `isize::MAX` bytes.
87 ///
88 /// # Aborts
89 ///
90 /// Aborts on OOM.
91 #[cfg(not(any(no_global_oom_handling, test)))]
92 #[must_use]
93 #[inline]
94 pub fn with_capacity(capacity: usize) -> Self {
95 Self::with_capacity_in(capacity, Global)
96 }
97
98 /// Like `with_capacity`, but guarantees the buffer is zeroed.
99 #[cfg(not(any(no_global_oom_handling, test)))]
100 #[must_use]
101 #[inline]
102 pub fn with_capacity_zeroed(capacity: usize) -> Self {
103 Self::with_capacity_zeroed_in(capacity, Global)
104 }
105}
106
107impl<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 {
114 8
115 } else if mem::size_of::<T>() <= 1024 {
116 4
117 } else {
118 1
119 };
120
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 }
126 }
127
128 /// Like `with_capacity`, but parameterized over the choice of
129 /// allocator for the returned `RawVec`.
130 #[cfg(not(no_global_oom_handling))]
131 #[inline]
132 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
133 Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
134 }
135
136 /// Like `try_with_capacity`, but parameterized over the choice of
137 /// allocator for the returned `RawVec`.
138 #[inline]
139 pub fn try_with_capacity_in(capacity: usize, alloc: A) -> Result<Self, TryReserveError> {
140 Self::try_allocate_in(capacity, AllocInit::Uninitialized, alloc)
141 }
142
143 /// Like `with_capacity_zeroed`, but parameterized over the choice
144 /// of allocator for the returned `RawVec`.
145 #[cfg(not(no_global_oom_handling))]
146 #[inline]
147 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
148 Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
149 }
150
151 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
152 ///
153 /// Note that this will correctly reconstitute any `cap` changes
154 /// that may have been performed. (See description of type for details.)
155 ///
156 /// # Safety
157 ///
158 /// * `len` must be greater than or equal to the most recently requested capacity, and
159 /// * `len` must be less than or equal to `self.capacity()`.
160 ///
161 /// Note, that the requested capacity and `self.capacity()` could differ, as
162 /// an allocator could overallocate and return a greater memory block than requested.
163 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
164 // Sanity-check one half of the safety requirement (we cannot check the other half).
165 debug_assert!(
166 len <= self.capacity(),
167 "`len` must be smaller than or equal to `self.capacity()`"
168 );
169
170 let me = ManuallyDrop::new(self);
171 unsafe {
172 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
173 Box::from_raw_in(slice, ptr::read(&me.alloc))
174 }
175 }
176
177 #[cfg(not(no_global_oom_handling))]
178 fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
179 // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
180 if mem::size_of::<T>() == 0 || capacity == 0 {
181 Self::new_in(alloc)
182 } else {
183 // We avoid `unwrap_or_else` here because it bloats the amount of
184 // LLVM IR generated.
185 let layout = match Layout::array::<T>(capacity) {
186 Ok(layout) => layout,
187 Err(_) => capacity_overflow(),
188 };
189 match alloc_guard(layout.size()) {
190 Ok(_) => {}
191 Err(_) => capacity_overflow(),
192 }
193 let result = match init {
194 AllocInit::Uninitialized => alloc.allocate(layout),
195 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
196 };
197 let ptr = match result {
198 Ok(ptr) => ptr,
199 Err(_) => handle_alloc_error(layout),
200 };
201
202 // Allocators currently return a `NonNull<[u8]>` whose length
203 // matches the size requested. If that ever changes, the capacity
204 // here should change to `ptr.len() / mem::size_of::<T>()`.
205 Self {
206 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
207 cap: capacity,
208 alloc,
209 }
210 }
211 }
212
213 fn try_allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Result<Self, TryReserveError> {
214 // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
215 if mem::size_of::<T>() == 0 || capacity == 0 {
216 return Ok(Self::new_in(alloc));
217 }
218
219 let layout = Layout::array::<T>(capacity).map_err(|_| CapacityOverflow)?;
220 alloc_guard(layout.size())?;
221 let result = match init {
222 AllocInit::Uninitialized => alloc.allocate(layout),
223 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
224 };
225 let ptr = result.map_err(|_| AllocError { layout, non_exhaustive: () })?;
226
227 // Allocators currently return a `NonNull<[u8]>` whose length
228 // matches the size requested. If that ever changes, the capacity
229 // here should change to `ptr.len() / mem::size_of::<T>()`.
230 Ok(Self {
231 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
232 cap: capacity,
233 alloc,
234 })
235 }
236
237 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
238 ///
239 /// # Safety
240 ///
241 /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
242 /// `capacity`.
243 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
244 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
245 /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
246 /// guaranteed.
247 #[inline]
248 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
249 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
250 }
251
252 /// Gets a raw pointer to the start of the allocation. Note that this is
253 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
254 /// be careful.
255 #[inline]
256 pub fn ptr(&self) -> *mut T {
257 self.ptr.as_ptr()
258 }
259
260 /// Gets the capacity of the allocation.
261 ///
262 /// This will always be `usize::MAX` if `T` is zero-sized.
263 #[inline(always)]
264 pub fn capacity(&self) -> usize {
265 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
266 }
267
268 /// Returns a shared reference to the allocator backing this `RawVec`.
269 pub fn allocator(&self) -> &A {
270 &self.alloc
271 }
272
273 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
274 if mem::size_of::<T>() == 0 || self.cap == 0 {
275 None
276 } else {
277 // We have an allocated chunk of memory, so we can bypass runtime
278 // checks to get our current layout.
279 unsafe {
280 let layout = Layout::array::<T>(self.cap).unwrap_unchecked();
281 Some((self.ptr.cast().into(), layout))
282 }
283 }
284 }
285
286 /// Ensures that the buffer contains at least enough space to hold `len +
287 /// additional` elements. If it doesn't already have enough capacity, will
288 /// reallocate enough space plus comfortable slack space to get amortized
289 /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
290 /// itself to panic.
291 ///
292 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
293 /// the requested space. This is not really unsafe, but the unsafe
294 /// code *you* write that relies on the behavior of this function may break.
295 ///
296 /// This is ideal for implementing a bulk-push operation like `extend`.
297 ///
298 /// # Panics
299 ///
300 /// Panics if the new capacity exceeds `isize::MAX` bytes.
301 ///
302 /// # Aborts
303 ///
304 /// Aborts on OOM.
305 #[cfg(not(no_global_oom_handling))]
306 #[inline]
307 pub fn reserve(&mut self, len: usize, additional: usize) {
308 // Callers expect this function to be very cheap when there is already sufficient capacity.
309 // Therefore, we move all the resizing and error-handling logic from grow_amortized and
310 // handle_reserve behind a call, while making sure that this function is likely to be
311 // inlined as just a comparison and a call if the comparison fails.
312 #[cold]
313 fn do_reserve_and_handle<T, A: Allocator>(
314 slf: &mut RawVec<T, A>,
315 len: usize,
316 additional: usize,
317 ) {
318 handle_reserve(slf.grow_amortized(len, additional));
319 }
320
321 if self.needs_to_grow(len, additional) {
322 do_reserve_and_handle(self, len, additional);
323 }
324 }
325
326 /// A specialized version of `reserve()` used only by the hot and
327 /// oft-instantiated `Vec::push()`, which does its own capacity check.
328 #[cfg(not(no_global_oom_handling))]
329 #[inline(never)]
330 pub fn reserve_for_push(&mut self, len: usize) {
331 handle_reserve(self.grow_amortized(len, 1));
332 }
333
334 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
335 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
336 if self.needs_to_grow(len, additional) {
337 self.grow_amortized(len, additional)
338 } else {
339 Ok(())
340 }
341 }
342
343 /// The same as `reserve_for_push`, but returns on errors instead of panicking or aborting.
344 #[inline(never)]
345 pub fn try_reserve_for_push(&mut self, len: usize) -> Result<(), TryReserveError> {
346 self.grow_amortized(len, 1)
347 }
348
349 /// Ensures that the buffer contains at least enough space to hold `len +
350 /// additional` elements. If it doesn't already, will reallocate the
351 /// minimum possible amount of memory necessary. Generally this will be
352 /// exactly the amount of memory necessary, but in principle the allocator
353 /// is free to give back more than we asked for.
354 ///
355 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
356 /// the requested space. This is not really unsafe, but the unsafe code
357 /// *you* write that relies on the behavior of this function may break.
358 ///
359 /// # Panics
360 ///
361 /// Panics if the new capacity exceeds `isize::MAX` bytes.
362 ///
363 /// # Aborts
364 ///
365 /// Aborts on OOM.
366 #[cfg(not(no_global_oom_handling))]
367 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
368 handle_reserve(self.try_reserve_exact(len, additional));
369 }
370
371 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
372 pub fn try_reserve_exact(
373 &mut self,
374 len: usize,
375 additional: usize,
376 ) -> Result<(), TryReserveError> {
377 if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
378 }
379
380 /// Shrinks the buffer down to the specified capacity. If the given amount
381 /// is 0, actually completely deallocates.
382 ///
383 /// # Panics
384 ///
385 /// Panics if the given amount is *larger* than the current capacity.
386 ///
387 /// # Aborts
388 ///
389 /// Aborts on OOM.
390 #[cfg(not(no_global_oom_handling))]
391 pub fn shrink_to_fit(&mut self, cap: usize) {
392 handle_reserve(self.shrink(cap));
393 }
394}
395
396impl<T, A: Allocator> RawVec<T, A> {
397 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
398 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
399 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
400 additional > self.capacity().wrapping_sub(len)
401 }
402
403 fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
404 // Allocators currently return a `NonNull<[u8]>` whose length matches
405 // the size requested. If that ever changes, the capacity here should
406 // change to `ptr.len() / mem::size_of::<T>()`.
407 self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
408 self.cap = cap;
409 }
410
411 // This method is usually instantiated many times. So we want it to be as
412 // small as possible, to improve compile times. But we also want as much of
413 // its contents to be statically computable as possible, to make the
414 // generated code run faster. Therefore, this method is carefully written
415 // so that all of the code that depends on `T` is within it, while as much
416 // of the code that doesn't depend on `T` as possible is in functions that
417 // are non-generic over `T`.
418 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
419 // This is ensured by the calling contexts.
420 debug_assert!(additional > 0);
421
422 if mem::size_of::<T>() == 0 {
423 // Since we return a capacity of `usize::MAX` when `elem_size` is
424 // 0, getting to here necessarily means the `RawVec` is overfull.
425 return Err(CapacityOverflow.into());
426 }
427
428 // Nothing we can really do about these checks, sadly.
429 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
430
431 // This guarantees exponential growth. The doubling cannot overflow
432 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
433 let cap = cmp::max(self.cap * 2, required_cap);
434 let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
435
436 let new_layout = Layout::array::<T>(cap);
437
438 // `finish_grow` is non-generic over `T`.
439 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
440 self.set_ptr_and_cap(ptr, cap);
441 Ok(())
442 }
443
444 // The constraints on this method are much the same as those on
445 // `grow_amortized`, but this method is usually instantiated less often so
446 // it's less critical.
447 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
448 if mem::size_of::<T>() == 0 {
449 // Since we return a capacity of `usize::MAX` when the type size is
450 // 0, getting to here necessarily means the `RawVec` is overfull.
451 return Err(CapacityOverflow.into());
452 }
453
454 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
455 let new_layout = Layout::array::<T>(cap);
456
457 // `finish_grow` is non-generic over `T`.
458 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
459 self.set_ptr_and_cap(ptr, cap);
460 Ok(())
461 }
462
463 #[allow(dead_code)]
464 fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
465 assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
466
467 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
468
469 let ptr = unsafe {
470 // `Layout::array` cannot overflow here because it would have
471 // overflowed earlier when capacity was larger.
472 let new_layout = Layout::array::<T>(cap).unwrap_unchecked();
473 self.alloc
474 .shrink(ptr, layout, new_layout)
475 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
476 };
477 self.set_ptr_and_cap(ptr, cap);
478 Ok(())
479 }
480}
481
482// This function is outside `RawVec` to minimize compile times. See the comment
483// above `RawVec::grow_amortized` for details. (The `A` parameter isn't
484// significant, because the number of different `A` types seen in practice is
485// much smaller than the number of `T` types.)
486#[inline(never)]
487fn finish_grow<A>(
488 new_layout: Result<Layout, LayoutError>,
489 current_memory: Option<(NonNull<u8>, Layout)>,
490 alloc: &mut A,
491) -> Result<NonNull<[u8]>, TryReserveError>
492where
493 A: Allocator,
494{
495 // Check for the error here to minimize the size of `RawVec::grow_*`.
496 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
497
498 alloc_guard(new_layout.size())?;
499
500 let memory = if let Some((ptr, old_layout)) = current_memory {
501 debug_assert_eq!(old_layout.align(), new_layout.align());
502 unsafe {
503 // The allocator checks for alignment equality
504 intrinsics::assume(old_layout.align() == new_layout.align());
505 alloc.grow(ptr, old_layout, new_layout)
506 }
507 } else {
508 alloc.allocate(new_layout)
509 };
510
511 memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
512}
513
514unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
515 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
516 fn drop(&mut self) {
517 if let Some((ptr, layout)) = self.current_memory() {
518 unsafe { self.alloc.deallocate(ptr, layout) }
519 }
520 }
521}
522
523// Central function for reserve error handling.
524#[cfg(not(no_global_oom_handling))]
525#[inline]
526fn handle_reserve(result: Result<(), TryReserveError>) {
527 match result.map_err(|e| e.kind()) {
528 Err(CapacityOverflow) => capacity_overflow(),
529 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
530 Ok(()) => { /* yay */ }
531 }
532}
533
534// We need to guarantee the following:
535// * We don't ever allocate `> isize::MAX` byte-size objects.
536// * We don't overflow `usize::MAX` and actually allocate too little.
537//
538// On 64-bit we just need to check for overflow since trying to allocate
539// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
540// an extra guard for this in case we're running on a platform which can use
541// all 4GB in user-space, e.g., PAE or x32.
542
543#[inline]
544fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
545 if usize::BITS < 64 && alloc_size > isize::MAX as usize {
546 Err(CapacityOverflow.into())
547 } else {
548 Ok(())
549 }
550}
551
552// One central function responsible for reporting capacity overflows. This'll
553// ensure that the code generation related to these panics is minimal as there's
554// only one location which panics rather than a bunch throughout the module.
555#[cfg(not(no_global_oom_handling))]
556fn capacity_overflow() -> ! {
557 panic!("capacity overflow");
558}