1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
|
// run-pass
#![feature(const_ptr_write)]
#![feature(const_mut_refs)]
// Or, equivalently: `MaybeUninit`.
pub union BagOfBits<T: Copy> {
uninit: (),
_storage: T,
}
pub const fn make_1u8_bag<T: Copy>() -> BagOfBits<T> {
assert!(core::mem::size_of::<T>() >= 1);
let mut bag = BagOfBits { uninit: () };
unsafe { (&mut bag as *mut _ as *mut u8).write(1); };
bag
}
pub fn check_bag<T: Copy>(bag: &BagOfBits<T>) {
let val = unsafe { (bag as *const _ as *const u8).read() };
assert_eq!(val, 1);
}
fn main() {
check_bag(&make_1u8_bag::<[usize; 1]>()); // Fine
check_bag(&make_1u8_bag::<usize>()); // Fine
const CONST_ARRAY_BAG: BagOfBits<[usize; 1]> = make_1u8_bag();
check_bag(&CONST_ARRAY_BAG); // Fine.
const CONST_USIZE_BAG: BagOfBits<usize> = make_1u8_bag();
// Used to panic since CTFE would make the entire `BagOfBits<usize>` uninit
check_bag(&CONST_USIZE_BAG);
}
|
