1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
#![unstable(feature = "raw_vec_internals", reason = "implementation detail", issue = "none")]
#![doc(hidden)]

use core::alloc::LayoutError;
use core::cmp;
use core::intrinsics;
use core::mem::{self, ManuallyDrop, MaybeUninit};
use core::ops::Drop;
use core::ptr::{self, NonNull, Unique};
use core::slice;

use crate::alloc::{handle_alloc_error, Allocator, Global, Layout};
use crate::boxed::Box;
use crate::collections::TryReserveError::{self, *};

#[cfg(test)]
mod tests;

enum AllocInit {
    /// The contents of the new memory are uninitialized.
    Uninitialized,
    /// The new memory is guaranteed to be zeroed.
    Zeroed,
}

/// A low-level utility for more ergonomically allocating, reallocating, and deallocating
/// a buffer of memory on the heap without having to worry about all the corner cases
/// involved. This type is excellent for building your own data structures like Vec and VecDeque.
/// In particular:
///
/// * Produces `Unique::dangling()` on zero-sized types.
/// * Produces `Unique::dangling()` on zero-length allocations.
/// * Avoids freeing `Unique::dangling()`.
/// * Catches all overflows in capacity computations (promotes them to "capacity overflow" panics).
/// * Guards against 32-bit systems allocating more than isize::MAX bytes.
/// * Guards against overflowing your length.
/// * Calls `handle_alloc_error` for fallible allocations.
/// * Contains a `ptr::Unique` and thus endows the user with all related benefits.
/// * Uses the excess returned from the allocator to use the largest available capacity.
///
/// This type does not in anyway inspect the memory that it manages. When dropped it *will*
/// free its memory, but it *won't* try to drop its contents. It is up to the user of `RawVec`
/// to handle the actual things *stored* inside of a `RawVec`.
///
/// Note that the excess of a zero-sized types is always infinite, so `capacity()` always returns
/// `usize::MAX`. This means that you need to be careful when round-tripping this type with a
/// `Box<[T]>`, since `capacity()` won't yield the length.
#[allow(missing_debug_implementations)]
pub struct RawVec<T, A: Allocator = Global> {
    ptr: Unique<T>,
    cap: usize,
    alloc: A,
}

impl<T> RawVec<T, Global> {
    /// HACK(Centril): This exists because stable `const fn` can only call stable `const fn`, so
    /// they cannot call `Self::new()`.
    ///
    /// If you change `RawVec<T>::new` or dependencies, please take care to not introduce anything
    /// that would truly const-call something unstable.
    pub const NEW: Self = Self::new();

    /// Creates the biggest possible `RawVec` (on the system heap)
    /// without allocating. If `T` has positive size, then this makes a
    /// `RawVec` with capacity `0`. If `T` is zero-sized, then it makes a
    /// `RawVec` with capacity `usize::MAX`. Useful for implementing
    /// delayed allocation.
    pub const fn new() -> Self {
        Self::new_in(Global)
    }

    /// Creates a `RawVec` (on the system heap) with exactly the
    /// capacity and alignment requirements for a `[T; capacity]`. This is
    /// equivalent to calling `RawVec::new` when `capacity` is `0` or `T` is
    /// zero-sized. Note that if `T` is zero-sized this means you will
    /// *not* get a `RawVec` with the requested capacity.
    ///
    /// # Panics
    ///
    /// Panics if the requested capacity exceeds `isize::MAX` bytes.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    #[inline]
    pub fn with_capacity(capacity: usize) -> Self {
        Self::with_capacity_in(capacity, Global)
    }

    /// Like `with_capacity`, but guarantees the buffer is zeroed.
    #[inline]
    pub fn with_capacity_zeroed(capacity: usize) -> Self {
        Self::with_capacity_zeroed_in(capacity, Global)
    }

    /// Reconstitutes a `RawVec` from a pointer and capacity.
    ///
    /// # Safety
    ///
    /// The `ptr` must be allocated (on the system heap), and with the given `capacity`.
    /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
    /// systems). ZST vectors may have a capacity up to `usize::MAX`.
    /// If the `ptr` and `capacity` come from a `RawVec`, then this is guaranteed.
    #[inline]
    pub unsafe fn from_raw_parts(ptr: *mut T, capacity: usize) -> Self {
        unsafe { Self::from_raw_parts_in(ptr, capacity, Global) }
    }
}

impl<T, A: Allocator> RawVec<T, A> {
    // Tiny Vecs are dumb. Skip to:
    // - 8 if the element size is 1, because any heap allocators is likely
    //   to round up a request of less than 8 bytes to at least 8 bytes.
    // - 4 if elements are moderate-sized (<= 1 KiB).
    // - 1 otherwise, to avoid wasting too much space for very short Vecs.
    const MIN_NON_ZERO_CAP: usize = if mem::size_of::<T>() == 1 {
        8
    } else if mem::size_of::<T>() <= 1024 {
        4
    } else {
        1
    };

    /// Like `new`, but parameterized over the choice of allocator for
    /// the returned `RawVec`.
    #[rustc_allow_const_fn_unstable(const_fn)]
    pub const fn new_in(alloc: A) -> Self {
        // `cap: 0` means "unallocated". zero-sized types are ignored.
        Self { ptr: Unique::dangling(), cap: 0, alloc }
    }

    /// Like `with_capacity`, but parameterized over the choice of
    /// allocator for the returned `RawVec`.
    #[inline]
    pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
        Self::allocate_in(capacity, AllocInit::Uninitialized, alloc)
    }

    /// Like `with_capacity_zeroed`, but parameterized over the choice
    /// of allocator for the returned `RawVec`.
    #[inline]
    pub fn with_capacity_zeroed_in(capacity: usize, alloc: A) -> Self {
        Self::allocate_in(capacity, AllocInit::Zeroed, alloc)
    }

    /// Converts a `Box<[T]>` into a `RawVec<T>`.
    pub fn from_box(slice: Box<[T], A>) -> Self {
        unsafe {
            let (slice, alloc) = Box::into_raw_with_allocator(slice);
            RawVec::from_raw_parts_in(slice.as_mut_ptr(), slice.len(), alloc)
        }
    }

    /// Converts the entire buffer into `Box<[MaybeUninit<T>]>` with the specified `len`.
    ///
    /// Note that this will correctly reconstitute any `cap` changes
    /// that may have been performed. (See description of type for details.)
    ///
    /// # Safety
    ///
    /// * `len` must be greater than or equal to the most recently requested capacity, and
    /// * `len` must be less than or equal to `self.capacity()`.
    ///
    /// Note, that the requested capacity and `self.capacity()` could differ, as
    /// an allocator could overallocate and return a greater memory block than requested.
    pub unsafe fn into_box(self, len: usize) -> Box<[MaybeUninit<T>], A> {
        // Sanity-check one half of the safety requirement (we cannot check the other half).
        debug_assert!(
            len <= self.capacity(),
            "`len` must be smaller than or equal to `self.capacity()`"
        );

        let me = ManuallyDrop::new(self);
        unsafe {
            let slice = slice::from_raw_parts_mut(me.ptr() as *mut MaybeUninit<T>, len);
            Box::from_raw_in(slice, ptr::read(&me.alloc))
        }
    }

    fn allocate_in(capacity: usize, init: AllocInit, alloc: A) -> Self {
        if mem::size_of::<T>() == 0 {
            Self::new_in(alloc)
        } else {
            // We avoid `unwrap_or_else` here because it bloats the amount of
            // LLVM IR generated.
            let layout = match Layout::array::<T>(capacity) {
                Ok(layout) => layout,
                Err(_) => capacity_overflow(),
            };
            match alloc_guard(layout.size()) {
                Ok(_) => {}
                Err(_) => capacity_overflow(),
            }
            let result = match init {
                AllocInit::Uninitialized => alloc.allocate(layout),
                AllocInit::Zeroed => alloc.allocate_zeroed(layout),
            };
            let ptr = match result {
                Ok(ptr) => ptr,
                Err(_) => handle_alloc_error(layout),
            };

            Self {
                ptr: unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) },
                cap: Self::capacity_from_bytes(ptr.len()),
                alloc,
            }
        }
    }

    /// Reconstitutes a `RawVec` from a pointer, capacity, and allocator.
    ///
    /// # Safety
    ///
    /// The `ptr` must be allocated (via the given allocator `alloc`), and with the given
    /// `capacity`.
    /// The `capacity` cannot exceed `isize::MAX` for sized types. (only a concern on 32-bit
    /// systems). ZST vectors may have a capacity up to `usize::MAX`.
    /// If the `ptr` and `capacity` come from a `RawVec` created via `alloc`, then this is
    /// guaranteed.
    #[inline]
    pub unsafe fn from_raw_parts_in(ptr: *mut T, capacity: usize, alloc: A) -> Self {
        Self { ptr: unsafe { Unique::new_unchecked(ptr) }, cap: capacity, alloc }
    }

    /// Gets a raw pointer to the start of the allocation. Note that this is
    /// `Unique::dangling()` if `capacity == 0` or `T` is zero-sized. In the former case, you must
    /// be careful.
    #[inline]
    pub fn ptr(&self) -> *mut T {
        self.ptr.as_ptr()
    }

    /// Gets the capacity of the allocation.
    ///
    /// This will always be `usize::MAX` if `T` is zero-sized.
    #[inline(always)]
    pub fn capacity(&self) -> usize {
        if mem::size_of::<T>() == 0 { usize::MAX } else { self.cap }
    }

    /// Returns a shared reference to the allocator backing this `RawVec`.
    pub fn allocator(&self) -> &A {
        &self.alloc
    }

    fn current_memory(&self) -> Option<(NonNull<u8>, Layout)> {
        if mem::size_of::<T>() == 0 || self.cap == 0 {
            None
        } else {
            // We have an allocated chunk of memory, so we can bypass runtime
            // checks to get our current layout.
            unsafe {
                let align = mem::align_of::<T>();
                let size = mem::size_of::<T>() * self.cap;
                let layout = Layout::from_size_align_unchecked(size, align);
                Some((self.ptr.cast().into(), layout))
            }
        }
    }

    /// Ensures that the buffer contains at least enough space to hold `len +
    /// additional` elements. If it doesn't already have enough capacity, will
    /// reallocate enough space plus comfortable slack space to get amortized
    /// *O*(1) behavior. Will limit this behavior if it would needlessly cause
    /// itself to panic.
    ///
    /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
    /// the requested space. This is not really unsafe, but the unsafe
    /// code *you* write that relies on the behavior of this function may break.
    ///
    /// This is ideal for implementing a bulk-push operation like `extend`.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    ///
    /// # Examples
    ///
    /// ```
    /// # #![feature(raw_vec_internals)]
    /// # extern crate alloc;
    /// # use std::ptr;
    /// # use alloc::raw_vec::RawVec;
    /// struct MyVec<T> {
    ///     buf: RawVec<T>,
    ///     len: usize,
    /// }
    ///
    /// impl<T: Clone> MyVec<T> {
    ///     pub fn push_all(&mut self, elems: &[T]) {
    ///         self.buf.reserve(self.len, elems.len());
    ///         // reserve would have aborted or panicked if the len exceeded
    ///         // `isize::MAX` so this is safe to do unchecked now.
    ///         for x in elems {
    ///             unsafe {
    ///                 ptr::write(self.buf.ptr().add(self.len), x.clone());
    ///             }
    ///             self.len += 1;
    ///         }
    ///     }
    /// }
    /// # fn main() {
    /// #   let mut vector = MyVec { buf: RawVec::new(), len: 0 };
    /// #   vector.push_all(&[1, 3, 5, 7, 9]);
    /// # }
    /// ```
    #[inline]
    pub fn reserve(&mut self, len: usize, additional: usize) {
        // Callers expect this function to be very cheap when there is already sufficient capacity.
        // Therefore, we move all the resizing and error-handling logic from grow_amortized and
        // handle_reserve behind a call, while making sure that the this function is likely to be
        // inlined as just a comparison and a call if the comparison fails.
        #[cold]
        fn do_reserve_and_handle<T, A: Allocator>(
            slf: &mut RawVec<T, A>,
            len: usize,
            additional: usize,
        ) {
            handle_reserve(slf.grow_amortized(len, additional));
        }

        if self.needs_to_grow(len, additional) {
            do_reserve_and_handle(self, len, additional);
        }
    }

    /// The same as `reserve`, but returns on errors instead of panicking or aborting.
    pub fn try_reserve(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
        if self.needs_to_grow(len, additional) {
            self.grow_amortized(len, additional)
        } else {
            Ok(())
        }
    }

    /// Ensures that the buffer contains at least enough space to hold `len +
    /// additional` elements. If it doesn't already, will reallocate the
    /// minimum possible amount of memory necessary. Generally this will be
    /// exactly the amount of memory necessary, but in principle the allocator
    /// is free to give back more than we asked for.
    ///
    /// If `len` exceeds `self.capacity()`, this may fail to actually allocate
    /// the requested space. This is not really unsafe, but the unsafe code
    /// *you* write that relies on the behavior of this function may break.
    ///
    /// # Panics
    ///
    /// Panics if the new capacity exceeds `isize::MAX` bytes.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    pub fn reserve_exact(&mut self, len: usize, additional: usize) {
        handle_reserve(self.try_reserve_exact(len, additional));
    }

    /// The same as `reserve_exact`, but returns on errors instead of panicking or aborting.
    pub fn try_reserve_exact(
        &mut self,
        len: usize,
        additional: usize,
    ) -> Result<(), TryReserveError> {
        if self.needs_to_grow(len, additional) { self.grow_exact(len, additional) } else { Ok(()) }
    }

    /// Shrinks the allocation down to the specified amount. If the given amount
    /// is 0, actually completely deallocates.
    ///
    /// # Panics
    ///
    /// Panics if the given amount is *larger* than the current capacity.
    ///
    /// # Aborts
    ///
    /// Aborts on OOM.
    pub fn shrink_to_fit(&mut self, amount: usize) {
        handle_reserve(self.shrink(amount));
    }
}

impl<T, A: Allocator> RawVec<T, A> {
    /// Returns if the buffer needs to grow to fulfill the needed extra capacity.
    /// Mainly used to make inlining reserve-calls possible without inlining `grow`.
    fn needs_to_grow(&self, len: usize, additional: usize) -> bool {
        additional > self.capacity().wrapping_sub(len)
    }

    fn capacity_from_bytes(excess: usize) -> usize {
        debug_assert_ne!(mem::size_of::<T>(), 0);
        excess / mem::size_of::<T>()
    }

    fn set_ptr(&mut self, ptr: NonNull<[u8]>) {
        self.ptr = unsafe { Unique::new_unchecked(ptr.cast().as_ptr()) };
        self.cap = Self::capacity_from_bytes(ptr.len());
    }

    // This method is usually instantiated many times. So we want it to be as
    // small as possible, to improve compile times. But we also want as much of
    // its contents to be statically computable as possible, to make the
    // generated code run faster. Therefore, this method is carefully written
    // so that all of the code that depends on `T` is within it, while as much
    // of the code that doesn't depend on `T` as possible is in functions that
    // are non-generic over `T`.
    fn grow_amortized(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
        // This is ensured by the calling contexts.
        debug_assert!(additional > 0);

        if mem::size_of::<T>() == 0 {
            // Since we return a capacity of `usize::MAX` when `elem_size` is
            // 0, getting to here necessarily means the `RawVec` is overfull.
            return Err(CapacityOverflow);
        }

        // Nothing we can really do about these checks, sadly.
        let required_cap = len.checked_add(additional).ok_or(CapacityOverflow)?;

        // This guarantees exponential growth. The doubling cannot overflow
        // because `cap <= isize::MAX` and the type of `cap` is `usize`.
        let cap = cmp::max(self.cap * 2, required_cap);
        let cap = cmp::max(Self::MIN_NON_ZERO_CAP, cap);

        let new_layout = Layout::array::<T>(cap);

        // `finish_grow` is non-generic over `T`.
        let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
        self.set_ptr(ptr);
        Ok(())
    }

    // The constraints on this method are much the same as those on
    // `grow_amortized`, but this method is usually instantiated less often so
    // it's less critical.
    fn grow_exact(&mut self, len: usize, additional: usize) -> Result<(), TryReserveError> {
        if mem::size_of::<T>() == 0 {
            // Since we return a capacity of `usize::MAX` when the type size is
            // 0, getting to here necessarily means the `RawVec` is overfull.
            return Err(CapacityOverflow);
        }

        let cap = len.checked_add(additional).ok_or(CapacityOverflow)?;
        let new_layout = Layout::array::<T>(cap);

        // `finish_grow` is non-generic over `T`.
        let ptr = finish_grow(new_layout, self.current_memory(), &mut self.alloc)?;
        self.set_ptr(ptr);
        Ok(())
    }

    fn shrink(&mut self, amount: usize) -> Result<(), TryReserveError> {
        assert!(amount <= self.capacity(), "Tried to shrink to a larger capacity");

        let (ptr, layout) = if let Some(mem) = self.current_memory() { mem } else { return Ok(()) };
        let new_size = amount * mem::size_of::<T>();

        let ptr = unsafe {
            let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
            self.alloc.shrink(ptr, layout, new_layout).map_err(|_| TryReserveError::AllocError {
                layout: new_layout,
                non_exhaustive: (),
            })?
        };
        self.set_ptr(ptr);
        Ok(())
    }
}

// This function is outside `RawVec` to minimize compile times. See the comment
// above `RawVec::grow_amortized` for details. (The `A` parameter isn't
// significant, because the number of different `A` types seen in practice is
// much smaller than the number of `T` types.)
#[inline(never)]
fn finish_grow<A>(
    new_layout: Result<Layout, LayoutError>,
    current_memory: Option<(NonNull<u8>, Layout)>,
    alloc: &mut A,
) -> Result<NonNull<[u8]>, TryReserveError>
where
    A: Allocator,
{
    // Check for the error here to minimize the size of `RawVec::grow_*`.
    let new_layout = new_layout.map_err(|_| CapacityOverflow)?;

    alloc_guard(new_layout.size())?;

    let memory = if let Some((ptr, old_layout)) = current_memory {
        debug_assert_eq!(old_layout.align(), new_layout.align());
        unsafe {
            // The allocator checks for alignment equality
            intrinsics::assume(old_layout.align() == new_layout.align());
            alloc.grow(ptr, old_layout, new_layout)
        }
    } else {
        alloc.allocate(new_layout)
    };

    memory.map_err(|_| AllocError { layout: new_layout, non_exhaustive: () })
}

unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
    /// Frees the memory owned by the `RawVec` *without* trying to drop its contents.
    fn drop(&mut self) {
        if let Some((ptr, layout)) = self.current_memory() {
            unsafe { self.alloc.deallocate(ptr, layout) }
        }
    }
}

// Central function for reserve error handling.
#[inline]
fn handle_reserve(result: Result<(), TryReserveError>) {
    match result {
        Err(CapacityOverflow) => capacity_overflow(),
        Err(AllocError { layout, .. }) => handle_alloc_error(layout),
        Ok(()) => { /* yay */ }
    }
}

// We need to guarantee the following:
// * We don't ever allocate `> isize::MAX` byte-size objects.
// * We don't overflow `usize::MAX` and actually allocate too little.
//
// On 64-bit we just need to check for overflow since trying to allocate
// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
// an extra guard for this in case we're running on a platform which can use
// all 4GB in user-space, e.g., PAE or x32.

#[inline]
fn alloc_guard(alloc_size: usize) -> Result<(), TryReserveError> {
    if usize::BITS < 64 && alloc_size > isize::MAX as usize {
        Err(CapacityOverflow)
    } else {
        Ok(())
    }
}

// One central function responsible for reporting capacity overflows. This'll
// ensure that the code generation related to these panics is minimal as there's
// only one location which panics rather than a bunch throughout the module.
fn capacity_overflow() -> ! {
    panic!("capacity overflow");
}