1 #![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
4 use core::alloc::LayoutErr;
6 use core::mem::{self, ManuallyDrop, MaybeUninit};
8 use core::ptr::{NonNull, Unique};
11 use crate::alloc::{handle_alloc_error, AllocRef, Global, Layout};
12 use crate::boxed::Box;
13 use crate::collections::TryReserveError::{self, *};
19 /// The contents of the new memory are uninitialized.
21 /// The new memory is guaranteed to be zeroed.
25 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
26 /// a buffer of memory on the heap without having to worry about all the corner cases
27 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
30 /// * Produces `Unique::dangling()` on zero-sized types.
31 /// * Produces `Unique::dangling()` on zero-length allocations.
32 /// * Avoids freeing `Unique::dangling()`.
33 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
34 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
35 /// * Guards against overflowing your length.
36 /// * Calls `handle_alloc_error` for fallible allocations.
37 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
38 /// * Uses the excess returned from the allocator to use the largest available capacity.
40 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
41 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
42 /// to handle the actual things *stored* inside of a `RawVec`.
44 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
45 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
46 /// `Box<[T]>`, since `capacity()` won't yield the length.
47 #[allow(missing_debug_implementations)]
48 pub struct RawVec<T, A: AllocRef = Global> {
54 impl<T> RawVec<T, Global> {
55 /// HACK(Centril): This exists because `#[unstable]` `const fn`s needn't conform
56 /// to `min_const_fn` and so they cannot be called in `min_const_fn`s either.
58 /// If you change `RawVec<T>::new` or dependencies, please take care to not
59 /// introduce anything that would truly violate `min_const_fn`.
61 /// NOTE: We could avoid this hack and check conformance with some
62 /// `#[rustc_force_min_const_fn]` attribute which requires conformance
63 /// with `min_const_fn` but does not necessarily allow calling it in
64 /// `stable(...) const fn` / user code not enabling `foo` when
65 /// `#[rustc_const_unstable(feature = "foo", issue = "01234")]` is present.
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.
73 pub const fn new() -> Self {
77 /// Creates a `RawVec` (on the system heap) with exactly the
78 /// capacity and alignment requirements for a `[T; capacity]`. This is
79 /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
80 /// zero-sized. Note that if `T` is zero-sized this means you will
81 /// *not* get a `RawVec` with the requested capacity.
85 /// Panics if the requested capacity exceeds `isize::MAX` bytes.
91 pub fn with_capacity(capacity: usize) -> Self {
92 Self::with_capacity_in(capacity, Global)
95 /// Like `with_capacity`, but guarantees the buffer is zeroed.
97 pub fn with_capacity_zeroed(capacity: usize) -> Self {
98 Self::with_capacity_zeroed_in(capacity, Global)
101 /// Reconstitutes a `RawVec` from a pointer and capacity.
105 /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
106 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
107 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
108 /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
110 pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
111 unsafe { Self::from_raw_parts_in(ptr, capacity, Global) }
114 /// Converts a `Box<[T]>` into a `RawVec<T>`.
115 pub fn from_box(slice: Box<[T]>) -> Self {
117 let mut slice = ManuallyDrop::new(slice);
118 RawVec::from_raw_parts(slice.as_mut_ptr(), slice.len())
122 /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
124 /// Note that this will correctly reconstitute any `cap` changes
125 /// that may have been performed. (See description of type for details.)
129 /// * `len` must be greater than or equal to the most recently requested capacity, and
130 /// * `len` must be less than or equal to `self.capacity()`.
132 /// Note, that the requested capacity and `self.capacity()` could differ, as
133 /// an allocator could overallocate and return a greater memory block than requested.
134 pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>]> {
135 // Sanity-check one half of the safety requirement (we cannot check the other half).
137 len <= self.capacity(),
138 "`len` must be smaller than or equal to `self.capacity()`"
141 let me = ManuallyDrop::new(self);
143 let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
149 impl<T, A: AllocRef> RawVec<T, A> {
150 /// Like `new`, but parameterized over the choice of allocator for
151 /// the returned `RawVec`.
152 pub const fn new_in(alloc: A) -> Self {
153 // `cap: 0` means "unallocated". zero-sized types are ignored.
154 Self { ptr: Unique::dangling(), cap: 0, alloc }
157 /// Like `with_capacity`, but parameterized over the choice of
158 /// allocator for the returned `RawVec`.
160 pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
161 Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
164 /// Like `with_capacity_zeroed`, but parameterized over the choice
165 /// of allocator for the returned `RawVec`.
167 pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
168 Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
171 fn allocate_in(capacity: usize, init: AllocInit, mut alloc: A) -> Self {
172 if mem::size_of::<T>() == 0 {
175 // We avoid `unwrap_or_else` here because it bloats the amount of
176 // LLVM IR generated.
177 let layout = match Layout::array::<T>(capacity) {
178 Ok(layout) => layout,
179 Err(_) => capacity_overflow(),
181 match alloc_guard(layout.size()) {
183 Err(_) => capacity_overflow(),
185 let result = match init {
186 AllocInit::Uninitialized => alloc.alloc(layout),
187 AllocInit::Zeroed => alloc.alloc_zeroed(layout),
189 let ptr = match result {
191 Err(_) => handle_alloc_error(layout),
195 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
196 cap: Self::capacity_from_bytes(ptr.len()),
202 /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
206 /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
208 /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
209 /// systems). ZST vectors may have a capacity up to `usize::MAX`.
210 /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
213 pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
214 Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
217 /// Gets a raw pointer to the start of the allocation. Note that this is
218 /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
220 pub fn ptr(&self) -> *mut T {
224 /// Gets the capacity of the allocation.
226 /// This will always be `usize::MAX` if `T` is zero-sized.
228 pub fn capacity(&self) -> usize {
229 if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
232 /// Returns a shared reference to the allocator backing this `RawVec`.
233 pub fn alloc(&self) -> &A {
237 /// Returns a mutable reference to the allocator backing this `RawVec`.
238 pub fn alloc_mut(&mut self) -> &mut 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 align = mem::align_of::<T>();
250 let size = mem::size_of::<T>() * self.cap;
251 let layout = Layout::from_size_align_unchecked(size, align);
252 Some((self.ptr.cast().into(), layout))
257 /// Ensures that the buffer contains at least enough space to hold `len +
258 /// additional` elements. If it doesn't already have enough capacity, will
259 /// reallocate enough space plus comfortable slack space to get amortized
260 /// `O(1)` behavior. Will limit this behavior if it would needlessly cause
263 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
264 /// the requested space. This is not really unsafe, but the unsafe
265 /// code *you* write that relies on the behavior of this function may break.
267 /// This is ideal for implementing a bulk-push operation like `extend`.
271 /// Panics if the new capacity exceeds `isize::MAX` bytes.
280 /// # #![feature(raw_vec_internals)]
281 /// # extern crate alloc;
283 /// # use alloc::raw_vec::RawVec;
284 /// struct MyVec<T> {
289 /// impl<T: Clone> MyVec<T> {
290 /// pub fn push_all(&mut self, elems: &[T]) {
291 /// self.buf.reserve(self.len, elems.len());
292 /// // reserve would have aborted or panicked if the len exceeded
293 /// // `isize::MAX` so this is safe to do unchecked now.
296 /// ptr::write(self.buf.ptr().add(self.len), x.clone());
303 /// # let mut vector = MyVec { buf: RawVec::new(), len: 0 };
304 /// # vector.push_all(&[1, 3, 5, 7, 9]);
307 pub fn reserve(&mut self, len: usize, additional: usize) {
308 match self.try_reserve(len, additional) {
309 Err(CapacityOverflow) => capacity_overflow(),
310 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
311 Ok(()) => { /* yay */ }
315 /// The same as `reserve`, but returns on errors instead of panicking or aborting.
316 pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
317 if self.needs_to_grow(len, additional) {
318 self.grow_amortized(len, additional)
324 /// Ensures that the buffer contains at least enough space to hold `len +
325 /// additional` elements. If it doesn't already, will reallocate the
326 /// minimum possible amount of memory necessary. Generally this will be
327 /// exactly the amount of memory necessary, but in principle the allocator
328 /// is free to give back more than we asked for.
330 /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
331 /// the requested space. This is not really unsafe, but the unsafe code
332 /// *you* write that relies on the behavior of this function may break.
336 /// Panics if the new capacity exceeds `isize::MAX` bytes.
341 pub fn reserve_exact(&mut self, len: usize, additional: usize) {
342 match self.try_reserve_exact(len, additional) {
343 Err(CapacityOverflow) => capacity_overflow(),
344 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
345 Ok(()) => { /* yay */ }
349 /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
350 pub fn try_reserve_exact(
354 ) -> Result<(), TryReserveError> {
355 if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
358 /// Shrinks the allocation down to the specified amount. If the given amount
359 /// is 0, actually completely deallocates.
363 /// Panics if the given amount is *larger* than the current capacity.
368 pub fn shrink_to_fit(&mut self, amount: usize) {
369 match self.shrink(amount) {
370 Err(CapacityOverflow) => capacity_overflow(),
371 Err(AllocError { layout, .. }) => handle_alloc_error(layout),
372 Ok(()) => { /* yay */ }
377 impl<T, A: AllocRef> RawVec<T, A> {
378 /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
379 /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
380 fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
381 additional > self.capacity().wrapping_sub(len)
384 fn capacity_from_bytes(excess: usize) -> usize {
385 debug_assert_ne!(mem::size_of::<T>(), 0);
386 excess / mem::size_of::<T>()
389 fn set_ptr(&mut self, ptr: NonNull<[u8]>) {
390 self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
391 self.cap = Self::capacity_from_bytes(ptr.len());
394 // This method is usually instantiated many times. So we want it to be as
395 // small as possible, to improve compile times. But we also want as much of
396 // its contents to be statically computable as possible, to make the
397 // generated code run faster. Therefore, this method is carefully written
398 // so that all of the code that depends on `T` is within it, while as much
399 // of the code that doesn't depend on `T` as possible is in functions that
400 // are non-generic over `T`.
401 fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
402 // This is ensured by the calling contexts.
403 debug_assert!(additional > 0);
405 if mem::size_of::<T>() == 0 {
406 // Since we return a capacity of `usize::MAX` when `elem_size` is
407 // 0, getting to here necessarily means the `RawVec` is overfull.
408 return Err(CapacityOverflow);
411 // Nothing we can really do about these checks, sadly.
412 let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
414 // This guarantees exponential growth. The doubling cannot overflow
415 // because `cap <= isize::MAX` and the type of `cap` is `usize`.
416 let cap = cmp::max(self.cap * 2, required_cap);
418 // Tiny Vecs are dumb. Skip to:
419 // - 8 if the element size is 1, because any heap allocators is likely
420 // to round up a request of less than 8 bytes to at least 8 bytes.
421 // - 4 if elements are moderate-sized (<= 1 KiB).
422 // - 1 otherwise, to avoid wasting too much space for very short Vecs.
423 // Note that `min_non_zero_cap` is computed statically.
424 let elem_size = mem::size_of::<T>();
425 let min_non_zero_cap = if elem_size == 1 {
427 } else if elem_size <= 1024 {
432 let cap = cmp::max(min_non_zero_cap, cap);
434 let new_layout = Layout::array::<T>(cap);
436 // `finish_grow` is non-generic over `T`.
437 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
442 // The constraints on this method are much the same as those on
443 // `grow_amortized`, but this method is usually instantiated less often so
444 // it's less critical.
445 fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
446 if mem::size_of::<T>() == 0 {
447 // Since we return a capacity of `usize::MAX` when the type size is
448 // 0, getting to here necessarily means the `RawVec` is overfull.
449 return Err(CapacityOverflow);
452 let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
453 let new_layout = Layout::array::<T>(cap);
455 // `finish_grow` is non-generic over `T`.
456 let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
461 fn shrink(&mut self, amount: usize) -> Result<(), TryReserveError> {
462 assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");
464 let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
465 let new_size = amount * mem::size_of::<T>();
468 self.alloc.shrink(ptr, layout, new_size).map_err(|_| TryReserveError::AllocError {
469 layout: Layout::from_size_align_unchecked(new_size, layout.align()),
478 // This function is outside `RawVec` to minimize compile times. See the comment
479 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
480 // significant, because the number of different `A` types seen in practice is
481 // much smaller than the number of `T` types.)
483 new_layout: Result<Layout, LayoutErr>,
484 current_memory: Option<(NonNull<u8>, Layout)>,
486 ) -> Result<NonNull<[u8]>, TryReserveError>
490 // Check for the error here to minimize the size of `RawVec::grow_*`.
491 let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
493 alloc_guard(new_layout.size())?;
495 let memory = if let Some((ptr, old_layout)) = current_memory {
496 debug_assert_eq!(old_layout.align(), new_layout.align());
497 unsafe { alloc.grow(ptr, old_layout, new_layout.size()) }
499 alloc.alloc(new_layout)
501 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?;
506 unsafe impl<#[may_dangle] T, A: AllocRef> Drop for RawVec<T, A> {
507 /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
509 if let Some((ptr, layout)) = self.current_memory() {
510 unsafe { self.alloc.dealloc(ptr, layout) }
515 // We need to guarantee the following:
516 // * We don't ever allocate `> isize::MAX` byte-size objects.
517 // * We don't overflow `usize::MAX` and actually allocate too little.
519 // On 64-bit we just need to check for overflow since trying to allocate
520 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
521 // an extra guard for this in case we're running on a platform which can use
522 // all 4GB in user-space, e.g., PAE or x32.
525 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
526 if mem::size_of::<usize>() < 8 && alloc_size > isize::MAX as usize {
527 Err(CapacityOverflow)
533 // One central function responsible for reporting capacity overflows. This'll
534 // ensure that the code generation related to these panics is minimal as there's
535 // only one location which panics rather than a bunch throughout the module.
536 fn capacity_overflow() -> ! {
537 panic!("capacity overflow");