88 lines
2.5 KiB
Rust
88 lines
2.5 KiB
Rust
#![allow(incomplete_features)] // for trait upcasting
|
|
#![feature(allocator_api, trait_upcasting)]
|
|
|
|
use std::alloc::Layout;
|
|
use std::alloc::{AllocError, Allocator};
|
|
use std::cell::Cell;
|
|
use std::mem::MaybeUninit;
|
|
use std::ptr::{self, NonNull};
|
|
|
|
struct OnceAlloc<'a> {
|
|
space: Cell<&'a mut [MaybeUninit<u8>]>,
|
|
}
|
|
|
|
unsafe impl<'shared, 'a: 'shared> Allocator for &'shared OnceAlloc<'a> {
|
|
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
|
|
let space = self.space.replace(&mut []);
|
|
|
|
let (ptr, len) = (space.as_mut_ptr(), space.len());
|
|
|
|
if ptr.align_offset(layout.align()) != 0 || len < layout.size() {
|
|
return Err(AllocError);
|
|
}
|
|
|
|
let slice_ptr = ptr::slice_from_raw_parts_mut(ptr as *mut u8, len);
|
|
unsafe { Ok(NonNull::new_unchecked(slice_ptr)) }
|
|
}
|
|
|
|
unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {}
|
|
}
|
|
|
|
trait MyTrait {
|
|
fn hello(&self) -> u8;
|
|
}
|
|
|
|
impl MyTrait for [u8; 1] {
|
|
fn hello(&self) -> u8 {
|
|
self[0]
|
|
}
|
|
}
|
|
|
|
trait TheTrait: MyTrait {}
|
|
|
|
impl TheTrait for [u8; 1] {}
|
|
|
|
/// `Box<T, G>` is a `ScalarPair` where the 2nd component is the allocator.
|
|
fn test1() {
|
|
let mut space = vec![MaybeUninit::new(0); 1];
|
|
let once_alloc = OnceAlloc { space: Cell::new(&mut space[..]) };
|
|
|
|
let boxed = Box::new_in([42u8; 1], &once_alloc);
|
|
let _val = *boxed;
|
|
let with_dyn: Box<dyn TheTrait, &OnceAlloc> = boxed;
|
|
assert_eq!(42, with_dyn.hello());
|
|
let with_dyn: Box<dyn MyTrait, &OnceAlloc> = with_dyn; // upcast
|
|
assert_eq!(42, with_dyn.hello());
|
|
}
|
|
|
|
// Make the allocator itself so big that the Box is not even a ScalarPair any more.
|
|
struct OnceAllocRef<'s, 'a>(&'s OnceAlloc<'a>, u64);
|
|
|
|
unsafe impl<'shared, 'a: 'shared> Allocator for OnceAllocRef<'shared, 'a> {
|
|
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
|
|
self.0.allocate(layout)
|
|
}
|
|
|
|
unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
|
|
self.0.deallocate(ptr, layout)
|
|
}
|
|
}
|
|
|
|
/// `Box<T, G>` is an `Aggregate`.
|
|
fn test2() {
|
|
let mut space = vec![MaybeUninit::new(0); 1];
|
|
let once_alloc = OnceAlloc { space: Cell::new(&mut space[..]) };
|
|
|
|
let boxed = Box::new_in([42u8; 1], OnceAllocRef(&once_alloc, 0));
|
|
let _val = *boxed;
|
|
let with_dyn: Box<dyn TheTrait, OnceAllocRef> = boxed;
|
|
assert_eq!(42, with_dyn.hello());
|
|
let with_dyn: Box<dyn MyTrait, OnceAllocRef> = with_dyn; // upcast
|
|
assert_eq!(42, with_dyn.hello());
|
|
}
|
|
|
|
fn main() {
|
|
test1();
|
|
test2();
|
|
}
|