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//@revisions: stack tree
//@[tree]compile-flags: -Zmiri-tree-borrows
#![feature(allocator_api)]
use std::alloc::{AllocError, Allocator, Layout};
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>, #[allow(dead_code)] 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();
}
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