library/alloc/src/raw_vec.rs

   1 #![unstable(feature = "raw_vec_internals", reason = "unstable const warnings", issue = "none")]
   2
   3 use core::alloc::LayoutError;
   4 use core::cmp;
   5 use core::intrinsics;
   6 use core::mem::{self, ManuallyDrop, MaybeUninit, SizedTypeProperties};
   7 use core::ops::Drop;
   8 use core::ptr::{self, NonNull, Unique};
   9 use core::slice;
  10
  11 #[cfg(not(no_global_oom_handling))]
  12 use crate::alloc::handle_alloc_error;
  13 use crate::alloc::{Allocator, Global, Layout};
  14 use crate::boxed::Box;
  15 use crate::collections::TryReserveError;
  16 use crate::collections::TryReserveErrorKind::*;
  17
  18 #[cfg(test)]
  19 mod tests;
  20
  21 #[cfg(not(no_global_oom_handling))]
  22 enum AllocInit {
  23     /// The contents of the new memory are uninitialized.
  24     Uninitialized,
  25     /// The new memory is guaranteed to be zeroed.
  26     Zeroed,
  27 }
  28
  29 /// A low-level utility for more ergonomically allocating, reallocating, and deallocating
  30 /// a buffer of memory on the heap without having to worry about all the corner cases
  31 /// involved. This type is excellent for building your own data structures like Vec and VecDeque.
  32 /// In particular:
  33 ///
  34 /// * Produces `Unique::dangling()` on zero-sized types.
  35 /// * Produces `Unique::dangling()` on zero-length allocations.
  36 /// * Avoids freeing `Unique::dangling()`.
  37 /// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
  38 /// * Guards against 32-bit systems allocating more than isize::MAX bytes.
  39 /// * Guards against overflowing your length.
  40 /// * Calls `handle_alloc_error` for fallible allocations.
  41 /// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
  42 /// * Uses the excess returned from the allocator to use the largest available capacity.
  43 ///
  44 /// This type does not in anyway inspect the memory that it manages. When dropped it *will*
  45 /// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
  46 /// to handle the actual things *stored* inside of a `RawVec`.
  47 ///
  48 /// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
  49 /// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
  50 /// `Box<[T]>`, since `capacity()` won't yield the length.
  51 #[allow(missing_debug_implementations)]
  52 pub(crate) struct RawVec<T, A: Allocator = Global> {
  53     ptr: Unique<T>,
  54     cap: usize,
  55     alloc: A,
  56 }
  57
  58 impl<T> RawVec<T, Global> {
  59     /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
  60     /// they cannot call `Self::new()`.
  61     ///
  62     /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
  63     /// that would truly const-call something unstable.
  64     pub const NEW: Self = Self::new();
  65
  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     #[must_use]
  72     pub const fn new() -> Self {
  73         Self::new_in(Global)
  74     }
  75
  76     /// Creates a `RawVec` (on the system heap) with exactly the
  77     /// capacity and alignment requirements for a `[T; capacity]`. This is
  78     /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
  79     /// zero-sized. Note that if `T` is zero-sized this means you will
  80     /// *not* get a `RawVec` with the requested capacity.
  81     ///
  82     /// # Panics
  83     ///
  84     /// Panics if the requested capacity exceeds `isize::MAX` bytes.
  85     ///
  86     /// # Aborts
  87     ///
  88     /// Aborts on OOM.
  89     #[cfg(not(any(no_global_oom_handling, test)))]
  90     #[must_use]
  91     #[inline]
  92     pub fn with_capacity(capacity: usize) -> Self {
  93         Self::with_capacity_in(capacity, Global)
  94     }
  95
  96     /// Like `with_capacity`, but guarantees the buffer is zeroed.
  97     #[cfg(not(any(no_global_oom_handling, test)))]
  98     #[must_use]
  99     #[inline]
 100     pub fn with_capacity_zeroed(capacity: usize) -> Self {
 101         Self::with_capacity_zeroed_in(capacity, Global)
 102     }
 103 }
 104
 105 impl<T, A: Allocator> RawVec<T, A> {
 106     // Tiny Vecs are dumb. Skip to:
 107     // - 8 if the element size is 1, because any heap allocators is likely
 108     //   to round up a request of less than 8 bytes to at least 8 bytes.
 109     // - 4 if elements are moderate-sized (<= 1 KiB).
 110     // - 1 otherwise, to avoid wasting too much space for very short Vecs.
 111     pub(crate) const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
 112         8
 113     } else if mem::size_of::<T>() <= 1024 {
 114         4
 115     } else {
 116         1
 117     };
 118
 119     /// Like `new`, but parameterized over the choice of allocator for
 120     /// the returned `RawVec`.
 121     pub const fn new_in(alloc: A) -> Self {
 122         // `cap: 0` means "unallocated". zero-sized types are ignored.
 123         Self { ptr: Unique::dangling(), cap: 0, alloc }
 124     }
 125
 126     /// Like `with_capacity`, but parameterized over the choice of
 127     /// allocator for the returned `RawVec`.
 128     #[cfg(not(no_global_oom_handling))]
 129     #[inline]
 130     pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
 131         Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
 132     }
 133
 134     /// Like `with_capacity_zeroed`, but parameterized over the choice
 135     /// of allocator for the returned `RawVec`.
 136     #[cfg(not(no_global_oom_handling))]
 137     #[inline]
 138     pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
 139         Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
 140     }
 141
 142     /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
 143     ///
 144     /// Note that this will correctly reconstitute any `cap` changes
 145     /// that may have been performed. (See description of type for details.)
 146     ///
 147     /// # Safety
 148     ///
 149     /// * `len` must be greater than or equal to the most recently requested capacity, and
 150     /// * `len` must be less than or equal to `self.capacity()`.
 151     ///
 152     /// Note, that the requested capacity and `self.capacity()` could differ, as
 153     /// an allocator could overallocate and return a greater memory block than requested.
 154     pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
 155         // Sanity-check one half of the safety requirement (we cannot check the other half).
 156         debug_assert!(
 157             len <= self.capacity(),
 158             "`len` must be smaller than or equal to `self.capacity()`"
 159         );
 160
 161         let me = ManuallyDrop::new(self);
 162         unsafe {
 163             let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
 164             Box::from_raw_in(slice, ptr::read(&me.alloc))
 165         }
 166     }
 167
 168     #[cfg(not(no_global_oom_handling))]
 169     fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
 170         // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
 171         if T::IS_ZST || capacity == 0 {
 172             Self::new_in(alloc)
 173         } else {
 174             // We avoid `unwrap_or_else` here because it bloats the amount of
 175             // LLVM IR generated.
 176             let layout = match Layout::array::<T>(capacity) {
 177                 Ok(layout) => layout,
 178                 Err(_) => capacity_overflow(),
 179             };
 180             match alloc_guard(layout.size()) {
 181                 Ok(_) => {}
 182                 Err(_) => capacity_overflow(),
 183             }
 184             let result = match init {
 185                 AllocInit::Uninitialized => alloc.allocate(layout),
 186                 AllocInit::Zeroed => alloc.allocate_zeroed(layout),
 187             };
 188             let ptr = match result {
 189                 Ok(ptr) => ptr,
 190                 Err(_) => handle_alloc_error(layout),
 191             };
 192
 193             // Allocators currently return a `NonNull<[u8]>` whose length
 194             // matches the size requested. If that ever changes, the capacity
 195             // here should change to `ptr.len() / mem::size_of::<T>()`.
 196             Self {
 197                 ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
 198                 cap: capacity,
 199                 alloc,
 200             }
 201         }
 202     }
 203
 204     /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
 205     ///
 206     /// # Safety
 207     ///
 208     /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
 209     /// `capacity`.
 210     /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
 211     /// systems). ZST vectors may have a capacity up to `usize::MAX`.
 212     /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
 213     /// guaranteed.
 214     #[inline]
 215     pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
 216         Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
 217     }
 218
 219     /// Gets a raw pointer to the start of the allocation. Note that this is
 220     /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
 221     /// be careful.
 222     #[inline]
 223     pub fn ptr(&self) -> *mut T {
 224         self.ptr.as_ptr()
 225     }
 226
 227     /// Gets the capacity of the allocation.
 228     ///
 229     /// This will always be `usize::MAX` if `T` is zero-sized.
 230     #[inline(always)]
 231     pub fn capacity(&self) -> usize {
 232         if T::IS_ZST { usize::MAX } else { self.cap }
 233     }
 234
 235     /// Returns a shared reference to the allocator backing this `RawVec`.
 236     pub fn allocator(&self) -> &A {
 237         &self.alloc
 238     }
 239
 240     fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
 241         if T::IS_ZST || self.cap == 0 {
 242             None
 243         } else {
 244             // We could use Layout::array here which ensures the absence of isize and usize overflows
 245             // and could hypothetically handle differences between stride and size, but this memory
 246             // has already been allocated so we know it can't overflow and currently rust does not
 247             // support such types. So we can do better by skipping some checks and avoid an unwrap.
 248             let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
 249             unsafe {
 250                 let align = mem::align_of::<T>();
 251                 let size = mem::size_of::<T>().unchecked_mul(self.cap);
 252                 let layout = Layout::from_size_align_unchecked(size, align);
 253                 Some((self.ptr.cast().into(), layout))
 254             }
 255         }
 256     }
 257
 258     /// Ensures that the buffer contains at least enough space to hold `len +
 259     /// additional` elements. If it doesn't already have enough capacity, will
 260     /// reallocate enough space plus comfortable slack space to get amortized
 261     /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
 262     /// itself to panic.
 263     ///
 264     /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
 265     /// the requested space. This is not really unsafe, but the unsafe
 266     /// code *you* write that relies on the behavior of this function may break.
 267     ///
 268     /// This is ideal for implementing a bulk-push operation like `extend`.
 269     ///
 270     /// # Panics
 271     ///
 272     /// Panics if the new capacity exceeds `isize::MAX` bytes.
 273     ///
 274     /// # Aborts
 275     ///
 276     /// Aborts on OOM.
 277     #[cfg(not(no_global_oom_handling))]
 278     #[inline]
 279     pub fn reserve(&mut self, len: usize, additional: usize) {
 280         // Callers expect this function to be very cheap when there is already sufficient capacity.
 281         // Therefore, we move all the resizing and error-handling logic from grow_amortized and
 282         // handle_reserve behind a call, while making sure that this function is likely to be
 283         // inlined as just a comparison and a call if the comparison fails.
 284         #[cold]
 285         fn do_reserve_and_handle<T, A: Allocator>(
 286             slf: &mut RawVec<T, A>,
 287             len: usize,
 288             additional: usize,
 289         ) {
 290             handle_reserve(slf.grow_amortized(len, additional));
 291         }
 292
 293         if self.needs_to_grow(len, additional) {
 294             do_reserve_and_handle(self, len, additional);
 295         }
 296     }
 297
 298     /// A specialized version of `reserve()` used only by the hot and
 299     /// oft-instantiated `Vec::push()`, which does its own capacity check.
 300     #[cfg(not(no_global_oom_handling))]
 301     #[inline(never)]
 302     pub fn reserve_for_push(&mut self, len: usize) {
 303         handle_reserve(self.grow_amortized(len, 1));
 304     }
 305
 306     /// The same as `reserve`, but returns on errors instead of panicking or aborting.
 307     pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
 308         if self.needs_to_grow(len, additional) {
 309             self.grow_amortized(len, additional)
 310         } else {
 311             Ok(())
 312         }
 313     }
 314
 315     /// Ensures that the buffer contains at least enough space to hold `len +
 316     /// additional` elements. If it doesn't already, will reallocate the
 317     /// minimum possible amount of memory necessary. Generally this will be
 318     /// exactly the amount of memory necessary, but in principle the allocator
 319     /// is free to give back more than we asked for.
 320     ///
 321     /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
 322     /// the requested space. This is not really unsafe, but the unsafe code
 323     /// *you* write that relies on the behavior of this function may break.
 324     ///
 325     /// # Panics
 326     ///
 327     /// Panics if the new capacity exceeds `isize::MAX` bytes.
 328     ///
 329     /// # Aborts
 330     ///
 331     /// Aborts on OOM.
 332     #[cfg(not(no_global_oom_handling))]
 333     pub fn reserve_exact(&mut self, len: usize, additional: usize) {
 334         handle_reserve(self.try_reserve_exact(len, additional));
 335     }
 336
 337     /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
 338     pub fn try_reserve_exact(
 339         &mut self,
 340         len: usize,
 341         additional: usize,
 342     ) -> Result<(), TryReserveError> {
 343         if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
 344     }
 345
 346     /// Shrinks the buffer down to the specified capacity. If the given amount
 347     /// is 0, actually completely deallocates.
 348     ///
 349     /// # Panics
 350     ///
 351     /// Panics if the given amount is *larger* than the current capacity.
 352     ///
 353     /// # Aborts
 354     ///
 355     /// Aborts on OOM.
 356     #[cfg(not(no_global_oom_handling))]
 357     pub fn shrink_to_fit(&mut self, cap: usize) {
 358         handle_reserve(self.shrink(cap));
 359     }
 360 }
 361
 362 impl<T, A: Allocator> RawVec<T, A> {
 363     /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
 364     /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
 365     fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
 366         additional > self.capacity().wrapping_sub(len)
 367     }
 368
 369     fn set_ptr_and_cap(&mut self, ptr: NonNull<[u8]>, cap: usize) {
 370         // Allocators currently return a `NonNull<[u8]>` whose length matches
 371         // the size requested. If that ever changes, the capacity here should
 372         // change to `ptr.len() / mem::size_of::<T>()`.
 373         self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
 374         self.cap = cap;
 375     }
 376
 377     // This method is usually instantiated many times. So we want it to be as
 378     // small as possible, to improve compile times. But we also want as much of
 379     // its contents to be statically computable as possible, to make the
 380     // generated code run faster. Therefore, this method is carefully written
 381     // so that all of the code that depends on `T` is within it, while as much
 382     // of the code that doesn't depend on `T` as possible is in functions that
 383     // are non-generic over `T`.
 384     fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
 385         // This is ensured by the calling contexts.
 386         debug_assert!(additional > 0);
 387
 388         if T::IS_ZST {
 389             // Since we return a capacity of `usize::MAX` when `elem_size` is
 390             // 0, getting to here necessarily means the `RawVec` is overfull.
 391             return Err(CapacityOverflow.into());
 392         }
 393
 394         // Nothing we can really do about these checks, sadly.
 395         let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
 396
 397         // This guarantees exponential growth. The doubling cannot overflow
 398         // because `cap <= isize::MAX` and the type of `cap` is `usize`.
 399         let cap = cmp::max(self.cap * 2, required_cap);
 400         let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);
 401
 402         let new_layout = Layout::array::<T>(cap);
 403
 404         // `finish_grow` is non-generic over `T`.
 405         let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
 406         self.set_ptr_and_cap(ptr, cap);
 407         Ok(())
 408     }
 409
 410     // The constraints on this method are much the same as those on
 411     // `grow_amortized`, but this method is usually instantiated less often so
 412     // it's less critical.
 413     fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
 414         if T::IS_ZST {
 415             // Since we return a capacity of `usize::MAX` when the type size is
 416             // 0, getting to here necessarily means the `RawVec` is overfull.
 417             return Err(CapacityOverflow.into());
 418         }
 419
 420         let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
 421         let new_layout = Layout::array::<T>(cap);
 422
 423         // `finish_grow` is non-generic over `T`.
 424         let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
 425         self.set_ptr_and_cap(ptr, cap);
 426         Ok(())
 427     }
 428
 429     #[cfg(not(no_global_oom_handling))]
 430     fn shrink(&mut self, cap: usize) -> Result<(), TryReserveError> {
 431         assert!(cap <= self.capacity(), "Tried to shrink to a larger capacity");
 432
 433         let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
 434         // See current_memory() why this assert is here
 435         let _: () = const { assert!(mem::size_of::<T>() % mem::align_of::<T>() == 0) };
 436         let ptr = unsafe {
 437             // `Layout::array` cannot overflow here because it would have
 438             // overflowed earlier when capacity was larger.
 439             let new_size = mem::size_of::<T>().unchecked_mul(cap);
 440             let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
 441             self.alloc
 442                 .shrink(ptr, layout, new_layout)
 443                 .map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })?
 444         };
 445         self.set_ptr_and_cap(ptr, cap);
 446         Ok(())
 447     }
 448 }
 449
 450 // This function is outside `RawVec` to minimize compile times. See the comment
 451 // above `RawVec::grow_amortized` for details. (The `A` parameter isn't
 452 // significant, because the number of different `A` types seen in practice is
 453 // much smaller than the number of `T` types.)
 454 #[inline(never)]
 455 fn finish_grow<A>(
 456     new_layout: Result<Layout, LayoutError>,
 457     current_memory: Option<(NonNull<u8>, Layout)>,
 458     alloc: &mut A,
 459 ) -> Result<NonNull<[u8]>, TryReserveError>
 460 where
 461     A: Allocator,
 462 {
 463     // Check for the error here to minimize the size of `RawVec::grow_*`.
 464     let new_layout = new_layout.map_err(|_| CapacityOverflow)?;
 465
 466     alloc_guard(new_layout.size())?;
 467
 468     let memory = if let Some((ptr, old_layout)) = current_memory {
 469         debug_assert_eq!(old_layout.align(), new_layout.align());
 470         unsafe {
 471             // The allocator checks for alignment equality
 472             intrinsics::assume(old_layout.align() == new_layout.align());
 473             alloc.grow(ptr, old_layout, new_layout)
 474         }
 475     } else {
 476         alloc.allocate(new_layout)
 477     };
 478
 479     memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () }.into())
 480 }
 481
 482 unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
 483     /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
 484     fn drop(&mut self) {
 485         if let Some((ptr, layout)) = self.current_memory() {
 486             unsafe { self.alloc.deallocate(ptr, layout) }
 487         }
 488     }
 489 }
 490
 491 // Central function for reserve error handling.
 492 #[cfg(not(no_global_oom_handling))]
 493 #[inline]
 494 fn handle_reserve(result: Result<(), TryReserveError>) {
 495     match result.map_err(|e| e.kind()) {
 496         Err(CapacityOverflow) => capacity_overflow(),
 497         Err(AllocError { layout, .. }) => handle_alloc_error(layout),
 498         Ok(()) => { /* yay */ }
 499     }
 500 }
 501
 502 // We need to guarantee the following:
 503 // * We don't ever allocate `> isize::MAX` byte-size objects.
 504 // * We don't overflow `usize::MAX` and actually allocate too little.
 505 //
 506 // On 64-bit we just need to check for overflow since trying to allocate
 507 // `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
 508 // an extra guard for this in case we're running on a platform which can use
 509 // all 4GB in user-space, e.g., PAE or x32.
 510
 511 #[inline]
 512 fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
 513     if usize::BITS < 64 && alloc_size > isize::MAX as usize {
 514         Err(CapacityOverflow.into())
 515     } else {
 516         Ok(())
 517     }
 518 }
 519
 520 // One central function responsible for reporting capacity overflows. This'll
 521 // ensure that the code generation related to these panics is minimal as there's
 522 // only one location which panics rather than a bunch throughout the module.
 523 #[cfg(not(no_global_oom_handling))]
 524 fn capacity_overflow() -> ! {
 525     panic!("capacity overflow");
 526 }