src/librustc_arena/lib.rs

   1 //! The arena, a fast but limited type of allocator.
   2 //!
   3 //! Arenas are a type of allocator that destroy the objects within, all at
   4 //! once, once the arena itself is destroyed. They do not support deallocation
   5 //! of individual objects while the arena itself is still alive. The benefit
   6 //! of an arena is very fast allocation; just a pointer bump.
   7 //!
   8 //! This crate implements several kinds of arena.
   9
  10 #![doc(
  11     html_root_url = "https://doc.rust-lang.org/nightly/",
  12     test(no_crate_inject, attr(deny(warnings)))
  13 )]
  14 #![feature(core_intrinsics)]
  15 #![feature(dropck_eyepatch)]
  16 #![feature(raw_vec_internals)]
  17 #![cfg_attr(test, feature(test))]
  18 #![allow(deprecated)]
  19
  20 extern crate alloc;
  21
  22 use rustc_data_structures::cold_path;
  23 use smallvec::SmallVec;
  24
  25 use std::alloc::Layout;
  26 use std::cell::{Cell, RefCell};
  27 use std::cmp;
  28 use std::intrinsics;
  29 use std::marker::{PhantomData, Send};
  30 use std::mem;
  31 use std::ptr;
  32 use std::slice;
  33
  34 use alloc::raw_vec::RawVec;
  35
  36 /// An arena that can hold objects of only one type.
  37 pub struct TypedArena<T> {
  38     /// A pointer to the next object to be allocated.
  39     ptr: Cell<*mut T>,
  40
  41     /// A pointer to the end of the allocated area. When this pointer is
  42     /// reached, a new chunk is allocated.
  43     end: Cell<*mut T>,
  44
  45     /// A vector of arena chunks.
  46     chunks: RefCell<Vec<TypedArenaChunk<T>>>,
  47
  48     /// Marker indicating that dropping the arena causes its owned
  49     /// instances of `T` to be dropped.
  50     _own: PhantomData<T>,
  51 }
  52
  53 struct TypedArenaChunk<T> {
  54     /// The raw storage for the arena chunk.
  55     storage: RawVec<T>,
  56     /// The number of valid entries in the chunk.
  57     entries: usize,
  58 }
  59
  60 impl<T> TypedArenaChunk<T> {
  61     #[inline]
  62     unsafe fn new(capacity: usize) -> TypedArenaChunk<T> {
  63         TypedArenaChunk { storage: RawVec::with_capacity(capacity), entries: 0 }
  64     }
  65
  66     /// Destroys this arena chunk.
  67     #[inline]
  68     unsafe fn destroy(&mut self, len: usize) {
  69         // The branch on needs_drop() is an -O1 performance optimization.
  70         // Without the branch, dropping TypedArena<u8> takes linear time.
  71         if mem::needs_drop::<T>() {
  72             let mut start = self.start();
  73             // Destroy all allocated objects.
  74             for _ in 0..len {
  75                 ptr::drop_in_place(start);
  76                 start = start.offset(1);
  77             }
  78         }
  79     }
  80
  81     // Returns a pointer to the first allocated object.
  82     #[inline]
  83     fn start(&self) -> *mut T {
  84         self.storage.ptr()
  85     }
  86
  87     // Returns a pointer to the end of the allocated space.
  88     #[inline]
  89     fn end(&self) -> *mut T {
  90         unsafe {
  91             if mem::size_of::<T>() == 0 {
  92                 // A pointer as large as possible for zero-sized elements.
  93                 !0 as *mut T
  94             } else {
  95                 self.start().add(self.storage.capacity())
  96             }
  97         }
  98     }
  99 }
 100
 101 // The arenas start with PAGE-sized chunks, and then each new chunk is twice as
 102 // big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
 103 // we stop growing. This scales well, from arenas that are barely used up to
 104 // arenas that are used for 100s of MiBs. Note also that the chosen sizes match
 105 // the usual sizes of pages and huge pages on Linux.
 106 const PAGE: usize = 4096;
 107 const HUGE_PAGE: usize = 2 * 1024 * 1024;
 108
 109 impl<T> Default for TypedArena<T> {
 110     /// Creates a new `TypedArena`.
 111     fn default() -> TypedArena<T> {
 112         TypedArena {
 113             // We set both `ptr` and `end` to 0 so that the first call to
 114             // alloc() will trigger a grow().
 115             ptr: Cell::new(ptr::null_mut()),
 116             end: Cell::new(ptr::null_mut()),
 117             chunks: RefCell::new(vec![]),
 118             _own: PhantomData,
 119         }
 120     }
 121 }
 122
 123 impl<T> TypedArena<T> {
 124     /// Allocates an object in the `TypedArena`, returning a reference to it.
 125     #[inline]
 126     pub fn alloc(&self, object: T) -> &mut T {
 127         if self.ptr == self.end {
 128             self.grow(1)
 129         }
 130
 131         unsafe {
 132             if mem::size_of::<T>() == 0 {
 133                 self.ptr.set(intrinsics::arith_offset(self.ptr.get() as *mut u8, 1) as *mut T);
 134                 let ptr = mem::align_of::<T>() as *mut T;
 135                 // Don't drop the object. This `write` is equivalent to `forget`.
 136                 ptr::write(ptr, object);
 137                 &mut *ptr
 138             } else {
 139                 let ptr = self.ptr.get();
 140                 // Advance the pointer.
 141                 self.ptr.set(self.ptr.get().offset(1));
 142                 // Write into uninitialized memory.
 143                 ptr::write(ptr, object);
 144                 &mut *ptr
 145             }
 146         }
 147     }
 148
 149     #[inline]
 150     fn can_allocate(&self, additional: usize) -> bool {
 151         let available_bytes = self.end.get() as usize - self.ptr.get() as usize;
 152         let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap();
 153         available_bytes >= additional_bytes
 154     }
 155
 156     /// Ensures there's enough space in the current chunk to fit `len` objects.
 157     #[inline]
 158     fn ensure_capacity(&self, additional: usize) {
 159         if !self.can_allocate(additional) {
 160             self.grow(additional);
 161             debug_assert!(self.can_allocate(additional));
 162         }
 163     }
 164
 165     #[inline]
 166     unsafe fn alloc_raw_slice(&self, len: usize) -> *mut T {
 167         assert!(mem::size_of::<T>() != 0);
 168         assert!(len != 0);
 169
 170         self.ensure_capacity(len);
 171
 172         let start_ptr = self.ptr.get();
 173         self.ptr.set(start_ptr.add(len));
 174         start_ptr
 175     }
 176
 177     /// Allocates a slice of objects that are copied into the `TypedArena`, returning a mutable
 178     /// reference to it. Will panic if passed a zero-sized types.
 179     ///
 180     /// Panics:
 181     ///
 182     ///  - Zero-sized types
 183     ///  - Zero-length slices
 184     #[inline]
 185     pub fn alloc_slice(&self, slice: &[T]) -> &mut [T]
 186     where
 187         T: Copy,
 188     {
 189         unsafe {
 190             let len = slice.len();
 191             let start_ptr = self.alloc_raw_slice(len);
 192             slice.as_ptr().copy_to_nonoverlapping(start_ptr, len);
 193             slice::from_raw_parts_mut(start_ptr, len)
 194         }
 195     }
 196
 197     #[inline]
 198     pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
 199         assert!(mem::size_of::<T>() != 0);
 200         let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
 201         if vec.is_empty() {
 202             return &mut [];
 203         }
 204         // Move the content to the arena by copying it and then forgetting
 205         // the content of the SmallVec
 206         unsafe {
 207             let len = vec.len();
 208             let start_ptr = self.alloc_raw_slice(len);
 209             vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
 210             vec.set_len(0);
 211             slice::from_raw_parts_mut(start_ptr, len)
 212         }
 213     }
 214
 215     /// Grows the arena.
 216     #[inline(never)]
 217     #[cold]
 218     fn grow(&self, additional: usize) {
 219         unsafe {
 220             // We need the element size to convert chunk sizes (ranging from
 221             // PAGE to HUGE_PAGE bytes) to element counts.
 222             let elem_size = cmp::max(1, mem::size_of::<T>());
 223             let mut chunks = self.chunks.borrow_mut();
 224             let mut new_cap;
 225             if let Some(last_chunk) = chunks.last_mut() {
 226                 let used_bytes = self.ptr.get() as usize - last_chunk.start() as usize;
 227                 last_chunk.entries = used_bytes / mem::size_of::<T>();
 228
 229                 // If the previous chunk's capacity is less than HUGE_PAGE
 230                 // bytes, then this chunk will be least double the previous
 231                 // chunk's size.
 232                 new_cap = last_chunk.storage.capacity();
 233                 if new_cap < HUGE_PAGE / elem_size {
 234                     new_cap = new_cap.checked_mul(2).unwrap();
 235                 }
 236             } else {
 237                 new_cap = PAGE / elem_size;
 238             }
 239             // Also ensure that this chunk can fit `additional`.
 240             new_cap = cmp::max(additional, new_cap);
 241
 242             let chunk = TypedArenaChunk::<T>::new(new_cap);
 243             self.ptr.set(chunk.start());
 244             self.end.set(chunk.end());
 245             chunks.push(chunk);
 246         }
 247     }
 248
 249     /// Clears the arena. Deallocates all but the longest chunk which may be reused.
 250     pub fn clear(&mut self) {
 251         unsafe {
 252             // Clear the last chunk, which is partially filled.
 253             let mut chunks_borrow = self.chunks.borrow_mut();
 254             if let Some(mut last_chunk) = chunks_borrow.last_mut() {
 255                 self.clear_last_chunk(&mut last_chunk);
 256                 let len = chunks_borrow.len();
 257                 // If `T` is ZST, code below has no effect.
 258                 for mut chunk in chunks_borrow.drain(..len - 1) {
 259                     chunk.destroy(chunk.entries);
 260                 }
 261             }
 262         }
 263     }
 264
 265     // Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
 266     // chunks.
 267     fn clear_last_chunk(&self, last_chunk: &mut TypedArenaChunk<T>) {
 268         // Determine how much was filled.
 269         let start = last_chunk.start() as usize;
 270         // We obtain the value of the pointer to the first uninitialized element.
 271         let end = self.ptr.get() as usize;
 272         // We then calculate the number of elements to be dropped in the last chunk,
 273         // which is the filled area's length.
 274         let diff = if mem::size_of::<T>() == 0 {
 275             // `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
 276             // the number of zero-sized values in the last and only chunk, just out of caution.
 277             // Recall that `end` was incremented for each allocated value.
 278             end - start
 279         } else {
 280             (end - start) / mem::size_of::<T>()
 281         };
 282         // Pass that to the `destroy` method.
 283         unsafe {
 284             last_chunk.destroy(diff);
 285         }
 286         // Reset the chunk.
 287         self.ptr.set(last_chunk.start());
 288     }
 289 }
 290
 291 unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
 292     fn drop(&mut self) {
 293         unsafe {
 294             // Determine how much was filled.
 295             let mut chunks_borrow = self.chunks.borrow_mut();
 296             if let Some(mut last_chunk) = chunks_borrow.pop() {
 297                 // Drop the contents of the last chunk.
 298                 self.clear_last_chunk(&mut last_chunk);
 299                 // The last chunk will be dropped. Destroy all other chunks.
 300                 for chunk in chunks_borrow.iter_mut() {
 301                     chunk.destroy(chunk.entries);
 302                 }
 303             }
 304             // RawVec handles deallocation of `last_chunk` and `self.chunks`.
 305         }
 306     }
 307 }
 308
 309 unsafe impl<T: Send> Send for TypedArena<T> {}
 310
 311 pub struct DroplessArena {
 312     /// A pointer to the next object to be allocated.
 313     ptr: Cell<*mut u8>,
 314
 315     /// A pointer to the end of the allocated area. When this pointer is
 316     /// reached, a new chunk is allocated.
 317     end: Cell<*mut u8>,
 318
 319     /// A vector of arena chunks.
 320     chunks: RefCell<Vec<TypedArenaChunk<u8>>>,
 321 }
 322
 323 unsafe impl Send for DroplessArena {}
 324
 325 impl Default for DroplessArena {
 326     #[inline]
 327     fn default() -> DroplessArena {
 328         DroplessArena {
 329             ptr: Cell::new(ptr::null_mut()),
 330             end: Cell::new(ptr::null_mut()),
 331             chunks: Default::default(),
 332         }
 333     }
 334 }
 335
 336 impl DroplessArena {
 337     #[inline(never)]
 338     #[cold]
 339     fn grow(&self, additional: usize) {
 340         unsafe {
 341             let mut chunks = self.chunks.borrow_mut();
 342             let mut new_cap;
 343             if let Some(last_chunk) = chunks.last_mut() {
 344                 // There is no need to update `last_chunk.entries` because that
 345                 // field isn't used by `DroplessArena`.
 346
 347                 // If the previous chunk's capacity is less than HUGE_PAGE
 348                 // bytes, then this chunk will be least double the previous
 349                 // chunk's size.
 350                 new_cap = last_chunk.storage.capacity();
 351                 if new_cap < HUGE_PAGE {
 352                     new_cap = new_cap.checked_mul(2).unwrap();
 353                 }
 354             } else {
 355                 new_cap = PAGE;
 356             }
 357             // Also ensure that this chunk can fit `additional`.
 358             new_cap = cmp::max(additional, new_cap);
 359
 360             let chunk = TypedArenaChunk::<u8>::new(new_cap);
 361             self.ptr.set(chunk.start());
 362             self.end.set(chunk.end());
 363             chunks.push(chunk);
 364         }
 365     }
 366
 367     /// Allocates a byte slice with specified layout from the current memory
 368     /// chunk. Returns `None` if there is no free space left to satisfy the
 369     /// request.
 370     #[inline]
 371     fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> {
 372         let ptr = self.ptr.get() as usize;
 373         let end = self.end.get() as usize;
 374         let align = layout.align();
 375         let bytes = layout.size();
 376         // The allocation request fits into the current chunk iff:
 377         //
 378         // let aligned = align_to(ptr, align);
 379         // ptr <= aligned && aligned + bytes <= end
 380         //
 381         // Except that we work with fixed width integers and need to be careful
 382         // about potential overflow in the calcuation. If the overflow does
 383         // happen, then we definitely don't have enough free and need to grow
 384         // the arena.
 385         let aligned = ptr.checked_add(align - 1)? & !(align - 1);
 386         let new_ptr = aligned.checked_add(bytes)?;
 387         if new_ptr <= end {
 388             self.ptr.set(new_ptr as *mut u8);
 389             Some(aligned as *mut u8)
 390         } else {
 391             None
 392         }
 393     }
 394
 395     #[inline]
 396     pub fn alloc_raw(&self, layout: Layout) -> *mut u8 {
 397         assert!(layout.size() != 0);
 398         loop {
 399             if let Some(a) = self.alloc_raw_without_grow(layout) {
 400                 break a;
 401             }
 402             // No free space left. Allocate a new chunk to satisfy the request.
 403             // On failure the grow will panic or abort.
 404             self.grow(layout.size());
 405         }
 406     }
 407
 408     #[inline]
 409     pub fn alloc<T>(&self, object: T) -> &mut T {
 410         assert!(!mem::needs_drop::<T>());
 411
 412         let mem = self.alloc_raw(Layout::for_value::<T>(&object)) as *mut T;
 413
 414         unsafe {
 415             // Write into uninitialized memory.
 416             ptr::write(mem, object);
 417             &mut *mem
 418         }
 419     }
 420
 421     /// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
 422     /// reference to it. Will panic if passed a zero-sized type.
 423     ///
 424     /// Panics:
 425     ///
 426     ///  - Zero-sized types
 427     ///  - Zero-length slices
 428     #[inline]
 429     pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
 430     where
 431         T: Copy,
 432     {
 433         assert!(!mem::needs_drop::<T>());
 434         assert!(mem::size_of::<T>() != 0);
 435         assert!(!slice.is_empty());
 436
 437         let mem = self.alloc_raw(Layout::for_value::<[T]>(slice)) as *mut T;
 438
 439         unsafe {
 440             mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len());
 441             slice::from_raw_parts_mut(mem, slice.len())
 442         }
 443     }
 444
 445     #[inline]
 446     unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
 447         &self,
 448         mut iter: I,
 449         len: usize,
 450         mem: *mut T,
 451     ) -> &mut [T] {
 452         let mut i = 0;
 453         // Use a manual loop since LLVM manages to optimize it better for
 454         // slice iterators
 455         loop {
 456             let value = iter.next();
 457             if i >= len || value.is_none() {
 458                 // We only return as many items as the iterator gave us, even
 459                 // though it was supposed to give us `len`
 460                 return slice::from_raw_parts_mut(mem, i);
 461             }
 462             ptr::write(mem.add(i), value.unwrap());
 463             i += 1;
 464         }
 465     }
 466
 467     #[inline]
 468     pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
 469         let iter = iter.into_iter();
 470         assert!(mem::size_of::<T>() != 0);
 471         assert!(!mem::needs_drop::<T>());
 472
 473         let size_hint = iter.size_hint();
 474
 475         match size_hint {
 476             (min, Some(max)) if min == max => {
 477                 // We know the exact number of elements the iterator will produce here
 478                 let len = min;
 479
 480                 if len == 0 {
 481                     return &mut [];
 482                 }
 483
 484                 let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
 485                 unsafe { self.write_from_iter(iter, len, mem) }
 486             }
 487             (_, _) => {
 488                 cold_path(move || -> &mut [T] {
 489                     let mut vec: SmallVec<[_; 8]> = iter.collect();
 490                     if vec.is_empty() {
 491                         return &mut [];
 492                     }
 493                     // Move the content to the arena by copying it and then forgetting
 494                     // the content of the SmallVec
 495                     unsafe {
 496                         let len = vec.len();
 497                         let start_ptr =
 498                             self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
 499                         vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
 500                         vec.set_len(0);
 501                         slice::from_raw_parts_mut(start_ptr, len)
 502                     }
 503                 })
 504             }
 505         }
 506     }
 507 }
 508
 509 /// Calls the destructor for an object when dropped.
 510 struct DropType {
 511     drop_fn: unsafe fn(*mut u8),
 512     obj: *mut u8,
 513 }
 514
 515 unsafe fn drop_for_type<T>(to_drop: *mut u8) {
 516     std::ptr::drop_in_place(to_drop as *mut T)
 517 }
 518
 519 impl Drop for DropType {
 520     fn drop(&mut self) {
 521         unsafe { (self.drop_fn)(self.obj) }
 522     }
 523 }
 524
 525 /// An arena which can be used to allocate any type.
 526 /// Allocating in this arena is unsafe since the type system
 527 /// doesn't know which types it contains. In order to
 528 /// allocate safely, you must store a PhantomData<T>
 529 /// alongside this arena for each type T you allocate.
 530 #[derive(Default)]
 531 pub struct DropArena {
 532     /// A list of destructors to run when the arena drops.
 533     /// Ordered so `destructors` gets dropped before the arena
 534     /// since its destructor can reference memory in the arena.
 535     destructors: RefCell<Vec<DropType>>,
 536     arena: DroplessArena,
 537 }
 538
 539 impl DropArena {
 540     #[inline]
 541     pub unsafe fn alloc<T>(&self, object: T) -> &mut T {
 542         let mem = self.arena.alloc_raw(Layout::new::<T>()) as *mut T;
 543         // Write into uninitialized memory.
 544         ptr::write(mem, object);
 545         let result = &mut *mem;
 546         // Record the destructor after doing the allocation as that may panic
 547         // and would cause `object`'s destuctor to run twice if it was recorded before
 548         self.destructors
 549             .borrow_mut()
 550             .push(DropType { drop_fn: drop_for_type::<T>, obj: result as *mut T as *mut u8 });
 551         result
 552     }
 553
 554     #[inline]
 555     pub unsafe fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
 556         let mut vec: SmallVec<[_; 8]> = iter.into_iter().collect();
 557         if vec.is_empty() {
 558             return &mut [];
 559         }
 560         let len = vec.len();
 561
 562         let start_ptr = self.arena.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
 563
 564         let mut destructors = self.destructors.borrow_mut();
 565         // Reserve space for the destructors so we can't panic while adding them
 566         destructors.reserve(len);
 567
 568         // Move the content to the arena by copying it and then forgetting
 569         // the content of the SmallVec
 570         vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
 571         mem::forget(vec.drain(..));
 572
 573         // Record the destructors after doing the allocation as that may panic
 574         // and would cause `object`'s destuctor to run twice if it was recorded before
 575         for i in 0..len {
 576             destructors.push(DropType {
 577                 drop_fn: drop_for_type::<T>,
 578                 obj: start_ptr.offset(i as isize) as *mut u8,
 579             });
 580         }
 581
 582         slice::from_raw_parts_mut(start_ptr, len)
 583     }
 584 }
 585
 586 #[macro_export]
 587 macro_rules! arena_for_type {
 588     ([][$ty:ty]) => {
 589         $crate::TypedArena<$ty>
 590     };
 591     ([few $(, $attrs:ident)*][$ty:ty]) => {
 592         ::std::marker::PhantomData<$ty>
 593     };
 594     ([$ignore:ident $(, $attrs:ident)*]$args:tt) => {
 595         $crate::arena_for_type!([$($attrs),*]$args)
 596     };
 597 }
 598
 599 #[macro_export]
 600 macro_rules! which_arena_for_type {
 601     ([][$arena:expr]) => {
 602         ::std::option::Option::Some($arena)
 603     };
 604     ([few$(, $attrs:ident)*][$arena:expr]) => {
 605         ::std::option::Option::None
 606     };
 607     ([$ignore:ident$(, $attrs:ident)*]$args:tt) => {
 608         $crate::which_arena_for_type!([$($attrs),*]$args)
 609     };
 610 }
 611
 612 #[macro_export]
 613 macro_rules! declare_arena {
 614     ([], [$($a:tt $name:ident: $ty:ty,)*], $tcx:lifetime) => {
 615         #[derive(Default)]
 616         pub struct Arena<$tcx> {
 617             pub dropless: $crate::DroplessArena,
 618             drop: $crate::DropArena,
 619             $($name: $crate::arena_for_type!($a[$ty]),)*
 620         }
 621
 622         pub trait ArenaAllocatable<'tcx, T = Self>: Sized {
 623             fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self;
 624             fn allocate_from_iter<'a>(
 625                 arena: &'a Arena<'tcx>,
 626                 iter: impl ::std::iter::IntoIterator<Item = Self>,
 627             ) -> &'a mut [Self];
 628         }
 629
 630         impl<'tcx, T: Copy> ArenaAllocatable<'tcx, ()> for T {
 631             #[inline]
 632             fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
 633                 arena.dropless.alloc(self)
 634             }
 635             #[inline]
 636             fn allocate_from_iter<'a>(
 637                 arena: &'a Arena<'tcx>,
 638                 iter: impl ::std::iter::IntoIterator<Item = Self>,
 639             ) -> &'a mut [Self] {
 640                 arena.dropless.alloc_from_iter(iter)
 641             }
 642
 643         }
 644         $(
 645             impl<$tcx> ArenaAllocatable<$tcx, $ty> for $ty {
 646                 #[inline]
 647                 fn allocate_on<'a>(self, arena: &'a Arena<$tcx>) -> &'a mut Self {
 648                     if !::std::mem::needs_drop::<Self>() {
 649                         return arena.dropless.alloc(self);
 650                     }
 651                     match $crate::which_arena_for_type!($a[&arena.$name]) {
 652                         ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
 653                             ty_arena.alloc(self)
 654                         }
 655                         ::std::option::Option::None => unsafe { arena.drop.alloc(self) },
 656                     }
 657                 }
 658
 659                 #[inline]
 660                 fn allocate_from_iter<'a>(
 661                     arena: &'a Arena<$tcx>,
 662                     iter: impl ::std::iter::IntoIterator<Item = Self>,
 663                 ) -> &'a mut [Self] {
 664                     if !::std::mem::needs_drop::<Self>() {
 665                         return arena.dropless.alloc_from_iter(iter);
 666                     }
 667                     match $crate::which_arena_for_type!($a[&arena.$name]) {
 668                         ::std::option::Option::<&$crate::TypedArena<Self>>::Some(ty_arena) => {
 669                             ty_arena.alloc_from_iter(iter)
 670                         }
 671                         ::std::option::Option::None => unsafe { arena.drop.alloc_from_iter(iter) },
 672                     }
 673                 }
 674             }
 675         )*
 676
 677         impl<'tcx> Arena<'tcx> {
 678             #[inline]
 679             pub fn alloc<T: ArenaAllocatable<'tcx, U>, U>(&self, value: T) -> &mut T {
 680                 value.allocate_on(self)
 681             }
 682
 683             #[inline]
 684             pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
 685                 if value.is_empty() {
 686                     return &mut [];
 687                 }
 688                 self.dropless.alloc_slice(value)
 689             }
 690
 691             pub fn alloc_from_iter<'a, T: ArenaAllocatable<'tcx, U>, U>(
 692                 &'a self,
 693                 iter: impl ::std::iter::IntoIterator<Item = T>,
 694             ) -> &'a mut [T] {
 695                 T::allocate_from_iter(self, iter)
 696             }
 697         }
 698     }
 699 }
 700
 701 #[cfg(test)]
 702 mod tests;