635 lines
21 KiB
Rust
635 lines
21 KiB
Rust
//! The arena, a fast but limited type of allocator.
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//!
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//! Arenas are a type of allocator that destroy the objects within, all at
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//! once, once the arena itself is destroyed. They do not support deallocation
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//! of individual objects while the arena itself is still alive. The benefit
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//! of an arena is very fast allocation; just a pointer bump.
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//!
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//! This crate implements several kinds of arena.
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#![doc(
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html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
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test(no_crate_inject, attr(deny(warnings)))
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)]
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#![feature(dropck_eyepatch)]
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#![feature(new_uninit)]
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#![feature(maybe_uninit_slice)]
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#![feature(min_specialization)]
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#![feature(decl_macro)]
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#![feature(rustc_attrs)]
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#![cfg_attr(test, feature(test))]
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#![feature(strict_provenance)]
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use smallvec::SmallVec;
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use std::alloc::Layout;
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use std::cell::{Cell, RefCell};
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use std::cmp;
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use std::marker::{PhantomData, Send};
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use std::mem::{self, MaybeUninit};
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use std::ptr;
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use std::slice;
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#[inline(never)]
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#[cold]
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fn cold_path<F: FnOnce() -> R, R>(f: F) -> R {
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f()
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}
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/// An arena that can hold objects of only one type.
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pub struct TypedArena<T> {
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/// A pointer to the next object to be allocated.
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ptr: Cell<*mut T>,
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/// A pointer to the end of the allocated area. When this pointer is
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/// reached, a new chunk is allocated.
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end: Cell<*mut T>,
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/// A vector of arena chunks.
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chunks: RefCell<Vec<ArenaChunk<T>>>,
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/// Marker indicating that dropping the arena causes its owned
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/// instances of `T` to be dropped.
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_own: PhantomData<T>,
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}
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struct ArenaChunk<T = u8> {
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/// The raw storage for the arena chunk.
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storage: Box<[MaybeUninit<T>]>,
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/// The number of valid entries in the chunk.
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entries: usize,
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}
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impl<T> ArenaChunk<T> {
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#[inline]
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unsafe fn new(capacity: usize) -> ArenaChunk<T> {
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ArenaChunk { storage: Box::new_uninit_slice(capacity), entries: 0 }
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}
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/// Destroys this arena chunk.
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#[inline]
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unsafe fn destroy(&mut self, len: usize) {
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// The branch on needs_drop() is an -O1 performance optimization.
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// Without the branch, dropping TypedArena<u8> takes linear time.
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if mem::needs_drop::<T>() {
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ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(&mut self.storage[..len]));
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}
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}
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// Returns a pointer to the first allocated object.
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#[inline]
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fn start(&mut self) -> *mut T {
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MaybeUninit::slice_as_mut_ptr(&mut self.storage)
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}
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// Returns a pointer to the end of the allocated space.
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#[inline]
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fn end(&mut self) -> *mut T {
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unsafe {
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if mem::size_of::<T>() == 0 {
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// A pointer as large as possible for zero-sized elements.
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ptr::invalid_mut(!0)
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} else {
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self.start().add(self.storage.len())
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}
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}
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}
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}
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// The arenas start with PAGE-sized chunks, and then each new chunk is twice as
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// big as its predecessor, up until we reach HUGE_PAGE-sized chunks, whereupon
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// we stop growing. This scales well, from arenas that are barely used up to
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// arenas that are used for 100s of MiBs. Note also that the chosen sizes match
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// the usual sizes of pages and huge pages on Linux.
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const PAGE: usize = 4096;
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const HUGE_PAGE: usize = 2 * 1024 * 1024;
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impl<T> Default for TypedArena<T> {
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/// Creates a new `TypedArena`.
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fn default() -> TypedArena<T> {
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TypedArena {
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// We set both `ptr` and `end` to 0 so that the first call to
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// alloc() will trigger a grow().
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ptr: Cell::new(ptr::null_mut()),
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end: Cell::new(ptr::null_mut()),
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chunks: Default::default(),
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_own: PhantomData,
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}
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}
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}
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trait IterExt<T> {
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fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T];
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}
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impl<I, T> IterExt<T> for I
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where
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I: IntoIterator<Item = T>,
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{
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// This default collects into a `SmallVec` and then allocates by copying
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// from it. The specializations below for types like `Vec` are more
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// efficient, copying directly without the intermediate collecting step.
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// This default could be made more efficient, like
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// `DroplessArena::alloc_from_iter`, but it's not hot enough to bother.
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#[inline]
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default fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
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let vec: SmallVec<[_; 8]> = self.into_iter().collect();
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vec.alloc_from_iter(arena)
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}
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}
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impl<T, const N: usize> IterExt<T> for std::array::IntoIter<T, N> {
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#[inline]
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fn alloc_from_iter(self, arena: &TypedArena<T>) -> &mut [T] {
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let len = self.len();
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if len == 0 {
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return &mut [];
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}
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// Move the content to the arena by copying and then forgetting it.
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unsafe {
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let start_ptr = arena.alloc_raw_slice(len);
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self.as_slice().as_ptr().copy_to_nonoverlapping(start_ptr, len);
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mem::forget(self);
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slice::from_raw_parts_mut(start_ptr, len)
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}
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}
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}
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impl<T> IterExt<T> for Vec<T> {
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#[inline]
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fn alloc_from_iter(mut self, arena: &TypedArena<T>) -> &mut [T] {
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let len = self.len();
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if len == 0 {
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return &mut [];
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}
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// Move the content to the arena by copying and then forgetting it.
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unsafe {
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let start_ptr = arena.alloc_raw_slice(len);
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self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
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self.set_len(0);
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slice::from_raw_parts_mut(start_ptr, len)
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}
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}
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}
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impl<A: smallvec::Array> IterExt<A::Item> for SmallVec<A> {
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#[inline]
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fn alloc_from_iter(mut self, arena: &TypedArena<A::Item>) -> &mut [A::Item] {
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let len = self.len();
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if len == 0 {
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return &mut [];
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}
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// Move the content to the arena by copying and then forgetting it.
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unsafe {
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let start_ptr = arena.alloc_raw_slice(len);
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self.as_ptr().copy_to_nonoverlapping(start_ptr, len);
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self.set_len(0);
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slice::from_raw_parts_mut(start_ptr, len)
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}
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}
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}
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impl<T> TypedArena<T> {
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/// Allocates an object in the `TypedArena`, returning a reference to it.
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#[inline]
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pub fn alloc(&self, object: T) -> &mut T {
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if self.ptr == self.end {
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self.grow(1)
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}
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unsafe {
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if mem::size_of::<T>() == 0 {
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self.ptr.set((self.ptr.get() as *mut u8).wrapping_offset(1) as *mut T);
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let ptr = ptr::NonNull::<T>::dangling().as_ptr();
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// Don't drop the object. This `write` is equivalent to `forget`.
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ptr::write(ptr, object);
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&mut *ptr
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} else {
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let ptr = self.ptr.get();
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// Advance the pointer.
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self.ptr.set(self.ptr.get().offset(1));
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// Write into uninitialized memory.
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ptr::write(ptr, object);
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&mut *ptr
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}
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}
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}
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#[inline]
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fn can_allocate(&self, additional: usize) -> bool {
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// FIXME: this should *likely* use `offset_from`, but more
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// investigation is needed (including running tests in miri).
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let available_bytes = self.end.get().addr() - self.ptr.get().addr();
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let additional_bytes = additional.checked_mul(mem::size_of::<T>()).unwrap();
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available_bytes >= additional_bytes
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}
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/// Ensures there's enough space in the current chunk to fit `len` objects.
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#[inline]
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fn ensure_capacity(&self, additional: usize) {
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if !self.can_allocate(additional) {
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self.grow(additional);
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debug_assert!(self.can_allocate(additional));
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}
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}
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#[inline]
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unsafe fn alloc_raw_slice(&self, len: usize) -> *mut T {
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assert!(mem::size_of::<T>() != 0);
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assert!(len != 0);
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self.ensure_capacity(len);
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let start_ptr = self.ptr.get();
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self.ptr.set(start_ptr.add(len));
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start_ptr
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}
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#[inline]
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pub fn alloc_from_iter<I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
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assert!(mem::size_of::<T>() != 0);
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iter.alloc_from_iter(self)
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}
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/// Grows the arena.
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#[inline(never)]
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#[cold]
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fn grow(&self, additional: usize) {
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unsafe {
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// We need the element size to convert chunk sizes (ranging from
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// PAGE to HUGE_PAGE bytes) to element counts.
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let elem_size = cmp::max(1, mem::size_of::<T>());
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let mut chunks = self.chunks.borrow_mut();
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let mut new_cap;
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if let Some(last_chunk) = chunks.last_mut() {
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// If a type is `!needs_drop`, we don't need to keep track of how many elements
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// the chunk stores - the field will be ignored anyway.
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if mem::needs_drop::<T>() {
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// FIXME: this should *likely* use `offset_from`, but more
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// investigation is needed (including running tests in miri).
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let used_bytes = self.ptr.get().addr() - last_chunk.start().addr();
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last_chunk.entries = used_bytes / mem::size_of::<T>();
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}
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// If the previous chunk's len is less than HUGE_PAGE
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// bytes, then this chunk will be least double the previous
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// chunk's size.
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new_cap = last_chunk.storage.len().min(HUGE_PAGE / elem_size / 2);
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new_cap *= 2;
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} else {
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new_cap = PAGE / elem_size;
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}
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// Also ensure that this chunk can fit `additional`.
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new_cap = cmp::max(additional, new_cap);
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let mut chunk = ArenaChunk::<T>::new(new_cap);
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self.ptr.set(chunk.start());
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self.end.set(chunk.end());
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chunks.push(chunk);
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}
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}
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// Drops the contents of the last chunk. The last chunk is partially empty, unlike all other
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// chunks.
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fn clear_last_chunk(&self, last_chunk: &mut ArenaChunk<T>) {
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// Determine how much was filled.
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let start = last_chunk.start().addr();
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// We obtain the value of the pointer to the first uninitialized element.
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let end = self.ptr.get().addr();
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// We then calculate the number of elements to be dropped in the last chunk,
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// which is the filled area's length.
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let diff = if mem::size_of::<T>() == 0 {
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// `T` is ZST. It can't have a drop flag, so the value here doesn't matter. We get
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// the number of zero-sized values in the last and only chunk, just out of caution.
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// Recall that `end` was incremented for each allocated value.
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end - start
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} else {
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// FIXME: this should *likely* use `offset_from`, but more
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// investigation is needed (including running tests in miri).
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(end - start) / mem::size_of::<T>()
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};
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// Pass that to the `destroy` method.
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unsafe {
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last_chunk.destroy(diff);
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}
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// Reset the chunk.
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self.ptr.set(last_chunk.start());
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}
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}
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unsafe impl<#[may_dangle] T> Drop for TypedArena<T> {
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fn drop(&mut self) {
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unsafe {
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// Determine how much was filled.
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let mut chunks_borrow = self.chunks.borrow_mut();
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if let Some(mut last_chunk) = chunks_borrow.pop() {
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// Drop the contents of the last chunk.
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self.clear_last_chunk(&mut last_chunk);
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// The last chunk will be dropped. Destroy all other chunks.
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for chunk in chunks_borrow.iter_mut() {
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chunk.destroy(chunk.entries);
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}
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}
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// Box handles deallocation of `last_chunk` and `self.chunks`.
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}
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}
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}
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unsafe impl<T: Send> Send for TypedArena<T> {}
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/// An arena that can hold objects of multiple different types that impl `Copy`
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/// and/or satisfy `!mem::needs_drop`.
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pub struct DroplessArena {
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/// A pointer to the start of the free space.
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start: Cell<*mut u8>,
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/// A pointer to the end of free space.
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///
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/// The allocation proceeds downwards from the end of the chunk towards the
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/// start. (This is slightly simpler and faster than allocating upwards,
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/// see <https://fitzgeraldnick.com/2019/11/01/always-bump-downwards.html>.)
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/// When this pointer crosses the start pointer, a new chunk is allocated.
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end: Cell<*mut u8>,
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/// A vector of arena chunks.
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chunks: RefCell<Vec<ArenaChunk>>,
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}
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unsafe impl Send for DroplessArena {}
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impl Default for DroplessArena {
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#[inline]
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fn default() -> DroplessArena {
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DroplessArena {
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start: Cell::new(ptr::null_mut()),
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end: Cell::new(ptr::null_mut()),
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chunks: Default::default(),
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}
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}
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}
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impl DroplessArena {
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#[inline(never)]
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#[cold]
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fn grow(&self, additional: usize) {
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unsafe {
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let mut chunks = self.chunks.borrow_mut();
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let mut new_cap;
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if let Some(last_chunk) = chunks.last_mut() {
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// There is no need to update `last_chunk.entries` because that
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// field isn't used by `DroplessArena`.
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// If the previous chunk's len is less than HUGE_PAGE
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// bytes, then this chunk will be least double the previous
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// chunk's size.
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new_cap = last_chunk.storage.len().min(HUGE_PAGE / 2);
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new_cap *= 2;
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} else {
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new_cap = PAGE;
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}
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// Also ensure that this chunk can fit `additional`.
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new_cap = cmp::max(additional, new_cap);
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let mut chunk = ArenaChunk::new(new_cap);
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self.start.set(chunk.start());
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self.end.set(chunk.end());
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chunks.push(chunk);
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}
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}
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/// Allocates a byte slice with specified layout from the current memory
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/// chunk. Returns `None` if there is no free space left to satisfy the
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/// request.
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#[inline]
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fn alloc_raw_without_grow(&self, layout: Layout) -> Option<*mut u8> {
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let start = self.start.get().addr();
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let old_end = self.end.get();
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let end = old_end.addr();
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let align = layout.align();
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let bytes = layout.size();
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let new_end = end.checked_sub(bytes)? & !(align - 1);
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if start <= new_end {
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let new_end = old_end.with_addr(new_end);
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self.end.set(new_end);
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Some(new_end)
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} else {
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None
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}
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}
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#[inline]
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pub fn alloc_raw(&self, layout: Layout) -> *mut u8 {
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assert!(layout.size() != 0);
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loop {
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if let Some(a) = self.alloc_raw_without_grow(layout) {
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break a;
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}
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// No free space left. Allocate a new chunk to satisfy the request.
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// On failure the grow will panic or abort.
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self.grow(layout.size());
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}
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}
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#[inline]
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pub fn alloc<T>(&self, object: T) -> &mut T {
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assert!(!mem::needs_drop::<T>());
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let mem = self.alloc_raw(Layout::for_value::<T>(&object)) as *mut T;
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unsafe {
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// Write into uninitialized memory.
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ptr::write(mem, object);
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&mut *mem
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}
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}
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/// Allocates a slice of objects that are copied into the `DroplessArena`, returning a mutable
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/// reference to it. Will panic if passed a zero-sized type.
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///
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/// Panics:
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///
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/// - Zero-sized types
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/// - Zero-length slices
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#[inline]
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pub fn alloc_slice<T>(&self, slice: &[T]) -> &mut [T]
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where
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T: Copy,
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{
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assert!(!mem::needs_drop::<T>());
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assert!(mem::size_of::<T>() != 0);
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assert!(!slice.is_empty());
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let mem = self.alloc_raw(Layout::for_value::<[T]>(slice)) as *mut T;
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unsafe {
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mem.copy_from_nonoverlapping(slice.as_ptr(), slice.len());
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slice::from_raw_parts_mut(mem, slice.len())
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}
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}
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#[inline]
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unsafe fn write_from_iter<T, I: Iterator<Item = T>>(
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&self,
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mut iter: I,
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len: usize,
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mem: *mut T,
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) -> &mut [T] {
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let mut i = 0;
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// Use a manual loop since LLVM manages to optimize it better for
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// slice iterators
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loop {
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let value = iter.next();
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if i >= len || value.is_none() {
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// We only return as many items as the iterator gave us, even
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// though it was supposed to give us `len`
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return slice::from_raw_parts_mut(mem, i);
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}
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ptr::write(mem.add(i), value.unwrap());
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i += 1;
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}
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}
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#[inline]
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pub fn alloc_from_iter<T, I: IntoIterator<Item = T>>(&self, iter: I) -> &mut [T] {
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let iter = iter.into_iter();
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assert!(mem::size_of::<T>() != 0);
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assert!(!mem::needs_drop::<T>());
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let size_hint = iter.size_hint();
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match size_hint {
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|
(min, Some(max)) if min == max => {
|
|
// We know the exact number of elements the iterator will produce here
|
|
let len = min;
|
|
|
|
if len == 0 {
|
|
return &mut [];
|
|
}
|
|
|
|
let mem = self.alloc_raw(Layout::array::<T>(len).unwrap()) as *mut T;
|
|
unsafe { self.write_from_iter(iter, len, mem) }
|
|
}
|
|
(_, _) => {
|
|
cold_path(move || -> &mut [T] {
|
|
let mut vec: SmallVec<[_; 8]> = iter.collect();
|
|
if vec.is_empty() {
|
|
return &mut [];
|
|
}
|
|
// Move the content to the arena by copying it and then forgetting
|
|
// the content of the SmallVec
|
|
unsafe {
|
|
let len = vec.len();
|
|
let start_ptr =
|
|
self.alloc_raw(Layout::for_value::<[T]>(vec.as_slice())) as *mut T;
|
|
vec.as_ptr().copy_to_nonoverlapping(start_ptr, len);
|
|
vec.set_len(0);
|
|
slice::from_raw_parts_mut(start_ptr, len)
|
|
}
|
|
})
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
/// Declare an `Arena` containing one dropless arena and many typed arenas (the
|
|
/// types of the typed arenas are specified by the arguments).
|
|
///
|
|
/// There are three cases of interest.
|
|
/// - Types that are `Copy`: these need not be specified in the arguments. They
|
|
/// will use the `DroplessArena`.
|
|
/// - Types that are `!Copy` and `!Drop`: these must be specified in the
|
|
/// arguments. An empty `TypedArena` will be created for each one, but the
|
|
/// `DroplessArena` will always be used and the `TypedArena` will stay empty.
|
|
/// This is odd but harmless, because an empty arena allocates no memory.
|
|
/// - Types that are `!Copy` and `Drop`: these must be specified in the
|
|
/// arguments. The `TypedArena` will be used for them.
|
|
///
|
|
#[rustc_macro_transparency = "semitransparent"]
|
|
pub macro declare_arena([$($a:tt $name:ident: $ty:ty,)*]) {
|
|
#[derive(Default)]
|
|
pub struct Arena<'tcx> {
|
|
pub dropless: $crate::DroplessArena,
|
|
$($name: $crate::TypedArena<$ty>,)*
|
|
}
|
|
|
|
pub trait ArenaAllocatable<'tcx, C = rustc_arena::IsNotCopy>: Sized {
|
|
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self;
|
|
fn allocate_from_iter<'a>(
|
|
arena: &'a Arena<'tcx>,
|
|
iter: impl ::std::iter::IntoIterator<Item = Self>,
|
|
) -> &'a mut [Self];
|
|
}
|
|
|
|
// Any type that impls `Copy` can be arena-allocated in the `DroplessArena`.
|
|
impl<'tcx, T: Copy> ArenaAllocatable<'tcx, rustc_arena::IsCopy> for T {
|
|
#[inline]
|
|
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
|
|
arena.dropless.alloc(self)
|
|
}
|
|
#[inline]
|
|
fn allocate_from_iter<'a>(
|
|
arena: &'a Arena<'tcx>,
|
|
iter: impl ::std::iter::IntoIterator<Item = Self>,
|
|
) -> &'a mut [Self] {
|
|
arena.dropless.alloc_from_iter(iter)
|
|
}
|
|
}
|
|
$(
|
|
impl<'tcx> ArenaAllocatable<'tcx, rustc_arena::IsNotCopy> for $ty {
|
|
#[inline]
|
|
fn allocate_on<'a>(self, arena: &'a Arena<'tcx>) -> &'a mut Self {
|
|
if !::std::mem::needs_drop::<Self>() {
|
|
arena.dropless.alloc(self)
|
|
} else {
|
|
arena.$name.alloc(self)
|
|
}
|
|
}
|
|
|
|
#[inline]
|
|
fn allocate_from_iter<'a>(
|
|
arena: &'a Arena<'tcx>,
|
|
iter: impl ::std::iter::IntoIterator<Item = Self>,
|
|
) -> &'a mut [Self] {
|
|
if !::std::mem::needs_drop::<Self>() {
|
|
arena.dropless.alloc_from_iter(iter)
|
|
} else {
|
|
arena.$name.alloc_from_iter(iter)
|
|
}
|
|
}
|
|
}
|
|
)*
|
|
|
|
impl<'tcx> Arena<'tcx> {
|
|
#[inline]
|
|
pub fn alloc<T: ArenaAllocatable<'tcx, C>, C>(&self, value: T) -> &mut T {
|
|
value.allocate_on(self)
|
|
}
|
|
|
|
// Any type that impls `Copy` can have slices be arena-allocated in the `DroplessArena`.
|
|
#[inline]
|
|
pub fn alloc_slice<T: ::std::marker::Copy>(&self, value: &[T]) -> &mut [T] {
|
|
if value.is_empty() {
|
|
return &mut [];
|
|
}
|
|
self.dropless.alloc_slice(value)
|
|
}
|
|
|
|
pub fn alloc_from_iter<'a, T: ArenaAllocatable<'tcx, C>, C>(
|
|
&'a self,
|
|
iter: impl ::std::iter::IntoIterator<Item = T>,
|
|
) -> &'a mut [T] {
|
|
T::allocate_from_iter(self, iter)
|
|
}
|
|
}
|
|
}
|
|
|
|
// Marker types that let us give different behaviour for arenas allocating
|
|
// `Copy` types vs `!Copy` types.
|
|
pub struct IsCopy;
|
|
pub struct IsNotCopy;
|
|
|
|
#[cfg(test)]
|
|
mod tests;
|