1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
4 use core::alloc::{LayoutErr, MemoryBlock};
6 use core::mem::{self, ManuallyDrop, MaybeUninit};
8 use core::ptr::{NonNull, Unique};
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::dangling()` on zero-sized types.
29 /// * Produces `Unique::dangling()` on zero-length allocations.
30 /// * Avoids freeing `Unique::dangling()`.
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", issue = "01234")]` 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 `isize::MAX` bytes.
89 pub fn with_capacity(capacity: usize) -> Self {
90 Self::with_capacity_in(capacity, Global)
93 /// Like `with_capacity`, but guarantees the buffer is zeroed.
95 pub fn with_capacity_zeroed(capacity: usize) -> Self {
96 Self::with_capacity_zeroed_in(capacity, Global)
99 /// Reconstitutes a `RawVec` from a pointer and capacity.
103 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
104 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
105 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
106 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
108 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
109 unsafe { Self::from_raw_parts_in(ptr, capacity, Global) }
112 /// Converts a `Box<[T]>` into a `RawVec<T>`.
113 pub fn from_box(slice: Box<[T]>) -> Self {
115 let mut slice = ManuallyDrop::new(slice);
116 RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len())
120 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
122 /// Note that this will correctly reconstitute any `cap` changes
123 /// that may have been performed. (See description of type for details.)
127 /// * `len` must be greater than or equal to the most recently requested capacity, and
128 /// * `len` must be less than or equal to `self.capacity()`.
130 /// Note, that the requested capacity and `self.capacity()` could differ, as
131 /// an allocator could overallocate and return a greater memory block than requested.
132 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> {
133 // Sanity-check one half of the safety requirement (we cannot check the other half).
135 len <= self.capacity(),
136 "`len` must be smaller than or equal to `self.capacity()`"
139 let me = ManuallyDrop::new(self);
141 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
147 impl<T, A: AllocRef> RawVec<T, A> {
148 /// Like `new`, but parameterized over the choice of allocator for
149 /// the returned `RawVec`.
150 pub const fn new_in(alloc: A) -> Self {
151 // `cap: 0` means "unallocated". zero-sized types are ignored.
152 Self { ptr: Unique::dangling(), cap: 0, alloc }
155 /// Like `with_capacity`, but parameterized over the choice of
156 /// allocator for the returned `RawVec`.
158 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
159 Self::allocate_in(capacity, Uninitialized, alloc)
162 /// Like `with_capacity_zeroed`, but parameterized over the choice
163 /// of allocator for the returned `RawVec`.
165 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
166 Self::allocate_in(capacity, Zeroed, alloc)
169 fn allocate_in(capacity: usize, init: AllocInit, mut alloc: A) -> Self {
170 if mem::size_of::<T>() == 0 {
173 // We avoid `unwrap_or_else` here because it bloats the amount of
174 // LLVM IR generated.
175 let layout = match Layout::array::<T>(capacity) {
176 Ok(layout) => layout,
177 Err(_) => capacity_overflow(),
179 match alloc_guard(layout.size()) {
181 Err(_) => capacity_overflow(),
183 let memory = match alloc.alloc(layout, init) {
184 Ok(memory) => memory,
185 Err(_) => handle_alloc_error(layout),
189 ptr: unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) },
190 cap: Self::capacity_from_bytes(memory.size),
196 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
200 /// The `ptr` must be allocated (via the given allocator `a`), and with the given `capacity`.
201 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
202 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
203 /// If the `ptr` and `capacity` come from a `RawVec` created via `a`, then this is guaranteed.
205 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, a: A) -> Self {
206 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc: a }
209 /// Gets a raw pointer to the start of the allocation. Note that this is
210 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
212 pub fn ptr(&self) -> *mut T {
216 /// Gets the capacity of the allocation.
218 /// This will always be `usize::MAX` if `T` is zero-sized.
220 pub fn capacity(&self) -> usize {
221 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
224 /// Returns a shared reference to the allocator backing this `RawVec`.
225 pub fn alloc(&self) -> &A {
229 /// Returns a mutable reference to the allocator backing this `RawVec`.
230 pub fn alloc_mut(&mut self) -> &mut A {
234 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
235 if mem::size_of::<T>() == 0 || self.cap == 0 {
238 // We have an allocated chunk of memory, so we can bypass runtime
239 // checks to get our current layout.
241 let align = mem::align_of::<T>();
242 let size = mem::size_of::<T>() * self.cap;
243 let layout = Layout::from_size_align_unchecked(size, align);
244 Some((self.ptr.cast().into(), layout))
249 /// Ensures that the buffer contains at least enough space to hold `len +
250 /// additional` elements. If it doesn't already have enough capacity, will
251 /// reallocate enough space plus comfortable slack space to get amortized
252 /// `O(1)` behavior. Will limit this behavior if it would needlessly cause
255 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
256 /// the requested space. This is not really unsafe, but the unsafe
257 /// code *you* write that relies on the behavior of this function may break.
259 /// This is ideal for implementing a bulk-push operation like `extend`.
263 /// Panics if the new capacity exceeds `isize::MAX` bytes.
272 /// # #![feature(raw_vec_internals)]
273 /// # extern crate alloc;
275 /// # use alloc::raw_vec::RawVec;
276 /// struct MyVec<T> {
281 /// impl<T: Clone> MyVec<T> {
282 /// pub fn push_all(&mut self, elems: &[T]) {
283 /// self.buf.reserve(self.len, elems.len());
284 /// // reserve would have aborted or panicked if the len exceeded
285 /// // `isize::MAX` so this is safe to do unchecked now.
288 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
295 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
296 /// # vector.push_all(&[1, 3, 5, 7, 9]);
299 pub fn reserve(&mut self, len: usize, additional: usize) {
300 match self.try_reserve(len, additional) {
301 Err(CapacityOverflow) => capacity_overflow(),
302 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
303 Ok(()) => { /* yay */ }
307 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
308 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
309 if self.needs_to_grow(len, additional) {
310 self.grow_amortized(len, additional)
316 /// Ensures that the buffer contains at least enough space to hold `len +
317 /// additional` elements. If it doesn't already, will reallocate the
318 /// minimum possible amount of memory necessary. Generally this will be
319 /// exactly the amount of memory necessary, but in principle the allocator
320 /// is free to give back more than we asked for.
322 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
323 /// the requested space. This is not really unsafe, but the unsafe code
324 /// *you* write that relies on the behavior of this function may break.
328 /// Panics if the new capacity exceeds `isize::MAX` bytes.
333 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
334 match self.try_reserve_exact(len, additional) {
335 Err(CapacityOverflow) => capacity_overflow(),
336 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
337 Ok(()) => { /* yay */ }
341 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
342 pub fn try_reserve_exact(
346 ) -> Result<(), TryReserveError> {
347 if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
350 /// Shrinks the allocation down to the specified amount. If the given amount
351 /// is 0, actually completely deallocates.
355 /// Panics if the given amount is *larger* than the current capacity.
360 pub fn shrink_to_fit(&mut self, amount: usize) {
361 match self.shrink(amount, MayMove) {
362 Err(CapacityOverflow) => capacity_overflow(),
363 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
364 Ok(()) => { /* yay */ }
369 impl<T, A: AllocRef> RawVec<T, A> {
370 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
371 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
372 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
373 additional > self.capacity().wrapping_sub(len)
376 fn capacity_from_bytes(excess: usize) -> usize {
377 debug_assert_ne!(mem::size_of::<T>(), 0);
378 excess / mem::size_of::<T>()
381 fn set_memory(&mut self, memory: MemoryBlock) {
382 self.ptr = unsafe { Unique::new_unchecked(memory.ptr.cast().as_ptr()) };
383 self.cap = Self::capacity_from_bytes(memory.size);
386 // This method is usually instantiated many times. So we want it to be as
387 // small as possible, to improve compile times. But we also want as much of
388 // its contents to be statically computable as possible, to make the
389 // generated code run faster. Therefore, this method is carefully written
390 // so that all of the code that depends on `T` is within it, while as much
391 // of the code that doesn't depend on `T` as possible is in functions that
392 // are non-generic over `T`.
393 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
394 // This is ensured by the calling contexts.
395 debug_assert!(additional > 0);
397 if mem::size_of::<T>() == 0 {
398 // Since we return a capacity of `usize::MAX` when `elem_size` is
399 // 0, getting to here necessarily means the `RawVec` is overfull.
400 return Err(CapacityOverflow);
403 // Nothing we can really do about these checks, sadly.
404 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
406 // This guarantees exponential growth. The doubling cannot overflow
407 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
408 let cap = cmp::max(self.cap * 2, required_cap);
410 // Tiny Vecs are dumb. Skip to:
411 // - 8 if the element size is 1, because any heap allocators is likely
412 // to round up a request of less than 8 bytes to at least 8 bytes.
413 // - 4 if elements are moderate-sized (<= 1 KiB).
414 // - 1 otherwise, to avoid wasting too much space for very short Vecs.
415 // Note that `min_non_zero_cap` is computed statically.
416 let elem_size = mem::size_of::<T>();
417 let min_non_zero_cap = if elem_size == 1 {
419 } else if elem_size <= 1024 {
424 let cap = cmp::max(min_non_zero_cap, cap);
426 let new_layout = Layout::array::<T>(cap);
428 // `finish_grow` is non-generic over `T`.
429 let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
430 self.set_memory(memory);
434 // The constraints on this method are much the same as those on
435 // `grow_amortized`, but this method is usually instantiated less often so
436 // it's less critical.
437 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
438 if mem::size_of::<T>() == 0 {
439 // Since we return a capacity of `usize::MAX` when the type size is
440 // 0, getting to here necessarily means the `RawVec` is overfull.
441 return Err(CapacityOverflow);
444 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
445 let new_layout = Layout::array::<T>(cap);
447 // `finish_grow` is non-generic over `T`.
448 let memory = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
449 self.set_memory(memory);
456 placement: ReallocPlacement,
457 ) -> Result<(), TryReserveError> {
458 assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
460 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
461 let new_size = amount * mem::size_of::<T>();
463 let memory = unsafe {
464 self.alloc.shrink(ptr, layout, new_size, placement).map_err(|_| {
465 TryReserveError::AllocError {
466 layout: Layout::from_size_align_unchecked(new_size, layout.align()),
471 self.set_memory(memory);
476 // This function is outside `RawVec` to minimize compile times. See the comment
477 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
478 // significant, because the number of different `A` types seen in practice is
479 // much smaller than the number of `T` types.)
481 new_layout: Result<Layout, LayoutErr>,
482 current_memory: Option<(NonNull<u8>, Layout)>,
484 ) -> Result<MemoryBlock, TryReserveError>
488 // Check for the error here to minimize the size of `RawVec::grow_*`.
489 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
491 alloc_guard(new_layout.size())?;
493 let memory = if let Some((ptr, old_layout)) = current_memory {
494 debug_assert_eq!(old_layout.align(), new_layout.align());
495 unsafe { alloc.grow(ptr, old_layout, new_layout.size(), MayMove, Uninitialized) }
497 alloc.alloc(new_layout, Uninitialized)
499 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?;
504 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
505 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
507 if let Some((ptr, layout)) = self.current_memory() {
508 unsafe { self.alloc.dealloc(ptr, layout) }
513 // We need to guarantee the following:
514 // * We don't ever allocate `> isize::MAX` byte-size objects.
515 // * We don't overflow `usize::MAX` and actually allocate too little.
517 // On 64-bit we just need to check for overflow since trying to allocate
518 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
519 // an extra guard for this in case we're running on a platform which can use
520 // all 4GB in user-space, e.g., PAE or x32.
523 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
524 if mem::size_of::<usize>() < 8 && alloc_size > isize::MAX as usize {
525 Err(CapacityOverflow)
531 // One central function responsible for reporting capacity overflows. This'll
532 // ensure that the code generation related to these panics is minimal as there's
533 // only one location which panics rather than a bunch throughout the module.
534 fn capacity_overflow() -> ! {
535 panic!("capacity overflow");