1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
4 use core::alloc::LayoutError;
7 use core::mem::{self, ManuallyDrop, MaybeUninit};
9 use core::ptr::{self, NonNull, Unique};
12 use crate::alloc::{handle_alloc_error, Allocator, Global, Layout};
13 use crate::boxed::Box;
14 use crate::collections::TryReserveError::{self, *};
20 /// The contents of the new memory are uninitialized.
22 /// The new memory is guaranteed to be zeroed.
26 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
27 /// a buffer of memory on the heap without having to worry about all the corner cases
28 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
31 /// * Produces `Unique::dangling()` on zero-sized types.
32 /// * Produces `Unique::dangling()` on zero-length allocations.
33 /// * Avoids freeing `Unique::dangling()`.
34 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
35 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
36 /// * Guards against overflowing your length.
37 /// * Calls `handle_alloc_error` for fallible allocations.
38 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
39 /// * Uses the excess returned from the allocator to use the largest available capacity.
41 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
42 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
43 /// to handle the actual things *stored* inside of a `RawVec`.
45 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
46 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
47 /// `Box<[T]>`, since `capacity()` won't yield the length.
48 #[allow(missing_debug_implementations)]
49 pub struct RawVec<T, A: Allocator = Global> {
55 impl<T> RawVec<T, Global> {
56 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
57 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
59 /// If you change `RawVec<T>::new` or dependencies, please take care to not
60 /// introduce anything that would truly violate `min_const_fn`.
62 /// NOTE: We could avoid this hack and check conformance with some
63 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
64 /// with `min_const_fn` but does not necessarily allow calling it in
65 /// `stable(...) const fn` / user code not enabling `foo` when
66 /// `#[rustc_const_unstable(feature = "foo", issue = "01234")]` is present.
67 pub const NEW: Self = Self::new();
69 /// Creates the biggest possible `RawVec` (on the system heap)
70 /// without allocating. If `T` has positive size, then this makes a
71 /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
72 /// `RawVec` with capacity `usize::MAX`. Useful for implementing
73 /// 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.
92 pub fn with_capacity(capacity: usize) -> Self {
93 Self::with_capacity_in(capacity, Global)
96 /// Like `with_capacity`, but guarantees the buffer is zeroed.
98 pub fn with_capacity_zeroed(capacity: usize) -> Self {
99 Self::with_capacity_zeroed_in(capacity, Global)
102 /// Reconstitutes a `RawVec` from a pointer and capacity.
106 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
107 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
108 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
109 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
111 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
112 unsafe { Self::from_raw_parts_in(ptr, capacity, Global) }
116 impl<T, A: Allocator> RawVec<T, A> {
117 // Tiny Vecs are dumb. Skip to:
118 // - 8 if the element size is 1, because any heap allocators is likely
119 // to round up a request of less than 8 bytes to at least 8 bytes.
120 // - 4 if elements are moderate-sized (<= 1 KiB).
121 // - 1 otherwise, to avoid wasting too much space for very short Vecs.
122 const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
124 } else if mem::size_of::<T>() <= 1024 {
130 /// Like `new`, but parameterized over the choice of allocator for
131 /// the returned `RawVec`.
132 #[rustc_allow_const_fn_unstable(const_fn)]
133 pub const fn new_in(alloc: A) -> Self {
134 // `cap: 0` means "unallocated". zero-sized types are ignored.
135 Self { ptr: Unique::dangling(), cap: 0, alloc }
138 /// Like `with_capacity`, but parameterized over the choice of
139 /// allocator for the returned `RawVec`.
141 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
142 Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
145 /// Like `with_capacity_zeroed`, but parameterized over the choice
146 /// of allocator for the returned `RawVec`.
148 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
149 Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
152 /// Converts a `Box<[T]>` into a `RawVec<T>`.
153 pub fn from_box(slice: Box<[T], A>) -> Self {
155 let (slice, alloc) = Box::into_raw_with_allocator(slice);
156 RawVec::from_raw_parts_in(slice.as_mut_ptr(), slice.len(), alloc)
160 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
162 /// Note that this will correctly reconstitute any `cap` changes
163 /// that may have been performed. (See description of type for details.)
167 /// * `len` must be greater than or equal to the most recently requested capacity, and
168 /// * `len` must be less than or equal to `self.capacity()`.
170 /// Note, that the requested capacity and `self.capacity()` could differ, as
171 /// an allocator could overallocate and return a greater memory block than requested.
172 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
173 // Sanity-check one half of the safety requirement (we cannot check the other half).
175 len <= self.capacity(),
176 "`len` must be smaller than or equal to `self.capacity()`"
179 let me = ManuallyDrop::new(self);
181 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
182 Box::from_raw_in(slice, ptr::read(&me.alloc))
186 fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
187 if mem::size_of::<T>() == 0 {
190 // We avoid `unwrap_or_else` here because it bloats the amount of
191 // LLVM IR generated.
192 let layout = match Layout::array::<T>(capacity) {
193 Ok(layout) => layout,
194 Err(_) => capacity_overflow(),
196 match alloc_guard(layout.size()) {
198 Err(_) => capacity_overflow(),
200 let result = match init {
201 AllocInit::Uninitialized => alloc.allocate(layout),
202 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
204 let ptr = match result {
206 Err(_) => handle_alloc_error(layout),
210 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
211 cap: Self::capacity_from_bytes(ptr.len()),
217 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
221 /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
223 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
224 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
225 /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
228 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
229 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
232 /// Gets a raw pointer to the start of the allocation. Note that this is
233 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
236 pub fn ptr(&self) -> *mut T {
240 /// Gets the capacity of the allocation.
242 /// This will always be `usize::MAX` if `T` is zero-sized.
244 pub fn capacity(&self) -> usize {
245 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
248 /// Returns a shared reference to the allocator backing this `RawVec`.
249 pub fn allocator(&self) -> &A {
253 fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
254 if mem::size_of::<T>() == 0 || self.cap == 0 {
257 // We have an allocated chunk of memory, so we can bypass runtime
258 // checks to get our current layout.
260 let align = mem::align_of::<T>();
261 let size = mem::size_of::<T>() * self.cap;
262 let layout = Layout::from_size_align_unchecked(size, align);
263 Some((self.ptr.cast().into(), layout))
268 /// Ensures that the buffer contains at least enough space to hold `len +
269 /// additional` elements. If it doesn't already have enough capacity, will
270 /// reallocate enough space plus comfortable slack space to get amortized
271 /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
274 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
275 /// the requested space. This is not really unsafe, but the unsafe
276 /// code *you* write that relies on the behavior of this function may break.
278 /// This is ideal for implementing a bulk-push operation like `extend`.
282 /// Panics if the new capacity exceeds `isize::MAX` bytes.
291 /// # #![feature(raw_vec_internals)]
292 /// # extern crate alloc;
294 /// # use alloc::raw_vec::RawVec;
295 /// struct MyVec<T> {
300 /// impl<T: Clone> MyVec<T> {
301 /// pub fn push_all(&mut self, elems: &[T]) {
302 /// self.buf.reserve(self.len, elems.len());
303 /// // reserve would have aborted or panicked if the len exceeded
304 /// // `isize::MAX` so this is safe to do unchecked now.
307 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
314 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
315 /// # vector.push_all(&[1, 3, 5, 7, 9]);
318 pub fn reserve(&mut self, len: usize, additional: usize) {
319 handle_reserve(self.try_reserve(len, additional));
322 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
323 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
324 if self.needs_to_grow(len, additional) {
325 self.grow_amortized(len, additional)
331 /// Ensures that the buffer contains at least enough space to hold `len +
332 /// additional` elements. If it doesn't already, will reallocate the
333 /// minimum possible amount of memory necessary. Generally this will be
334 /// exactly the amount of memory necessary, but in principle the allocator
335 /// is free to give back more than we asked for.
337 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
338 /// the requested space. This is not really unsafe, but the unsafe code
339 /// *you* write that relies on the behavior of this function may break.
343 /// Panics if the new capacity exceeds `isize::MAX` bytes.
348 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
349 handle_reserve(self.try_reserve_exact(len, additional));
352 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
353 pub fn try_reserve_exact(
357 ) -> Result<(), TryReserveError> {
358 if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
361 /// Shrinks the allocation down to the specified amount. If the given amount
362 /// is 0, actually completely deallocates.
366 /// Panics if the given amount is *larger* than the current capacity.
371 pub fn shrink_to_fit(&mut self, amount: usize) {
372 handle_reserve(self.shrink(amount));
376 impl<T, A: Allocator> RawVec<T, A> {
377 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
378 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
379 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
380 additional > self.capacity().wrapping_sub(len)
383 fn capacity_from_bytes(excess: usize) -> usize {
384 debug_assert_ne!(mem::size_of::<T>(), 0);
385 excess / mem::size_of::<T>()
388 fn set_ptr(&mut self, ptr: NonNull<[u8]>) {
389 self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
390 self.cap = Self::capacity_from_bytes(ptr.len());
393 // This method is usually instantiated many times. So we want it to be as
394 // small as possible, to improve compile times. But we also want as much of
395 // its contents to be statically computable as possible, to make the
396 // generated code run faster. Therefore, this method is carefully written
397 // so that all of the code that depends on `T` is within it, while as much
398 // of the code that doesn't depend on `T` as possible is in functions that
399 // are non-generic over `T`.
400 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
401 // This is ensured by the calling contexts.
402 debug_assert!(additional > 0);
404 if mem::size_of::<T>() == 0 {
405 // Since we return a capacity of `usize::MAX` when `elem_size` is
406 // 0, getting to here necessarily means the `RawVec` is overfull.
407 return Err(CapacityOverflow);
410 // Nothing we can really do about these checks, sadly.
411 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
413 // This guarantees exponential growth. The doubling cannot overflow
414 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
415 let cap = cmp::max(self.cap * 2, required_cap);
416 let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
418 let new_layout = Layout::array::<T>(cap);
420 // `finish_grow` is non-generic over `T`.
421 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
426 // The constraints on this method are much the same as those on
427 // `grow_amortized`, but this method is usually instantiated less often so
428 // it's less critical.
429 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
430 if mem::size_of::<T>() == 0 {
431 // Since we return a capacity of `usize::MAX` when the type size is
432 // 0, getting to here necessarily means the `RawVec` is overfull.
433 return Err(CapacityOverflow);
436 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
437 let new_layout = Layout::array::<T>(cap);
439 // `finish_grow` is non-generic over `T`.
440 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
445 fn shrink(&mut self, amount: usize) -> Result<(), TryReserveError> {
446 assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
448 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
449 let new_size = amount * mem::size_of::<T>();
452 let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
453 self.alloc.shrink(ptr, layout, new_layout).map_err(|_| TryReserveError::AllocError {
463 // This function is outside `RawVec` to minimize compile times. See the comment
464 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
465 // significant, because the number of different `A` types seen in practice is
466 // much smaller than the number of `T` types.)
469 new_layout: Result<Layout, LayoutError>,
470 current_memory: Option<(NonNull<u8>, Layout)>,
472 ) -> Result<NonNull<[u8]>, TryReserveError>
476 // Check for the error here to minimize the size of `RawVec::grow_*`.
477 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
479 alloc_guard(new_layout.size())?;
481 let memory = if let Some((ptr, old_layout)) = current_memory {
482 debug_assert_eq!(old_layout.align(), new_layout.align());
484 // The allocator checks for alignment equality
485 intrinsics::assume(old_layout.align() == new_layout.align());
486 alloc.grow(ptr, old_layout, new_layout)
489 alloc.allocate(new_layout)
492 memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })
495 unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
496 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
498 if let Some((ptr, layout)) = self.current_memory() {
499 unsafe { self.alloc.deallocate(ptr, layout) }
504 // Central function for reserve error handling.
506 fn handle_reserve(result: Result<(), TryReserveError>) {
508 Err(CapacityOverflow) => capacity_overflow(),
509 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
510 Ok(()) => { /* yay */ }
514 // We need to guarantee the following:
515 // * We don't ever allocate `> isize::MAX` byte-size objects.
516 // * We don't overflow `usize::MAX` and actually allocate too little.
518 // On 64-bit we just need to check for overflow since trying to allocate
519 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
520 // an extra guard for this in case we're running on a platform which can use
521 // all 4GB in user-space, e.g., PAE or x32.
524 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
525 if usize::BITS < 64 && alloc_size > isize::MAX as usize {
526 Err(CapacityOverflow)
532 // One central function responsible for reporting capacity overflows. This'll
533 // ensure that the code generation related to these panics is minimal as there's
534 // only one location which panics rather than a bunch throughout the module.
535 fn capacity_overflow() -> ! {
536 panic!("capacity overflow");