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