3 #![feature(const_ptr_write)]
4 #![feature(const_mut_refs)]
6 // Or, equivalently: `MaybeUninit`.
7 pub union BagOfBits<T: Copy> {
12 pub const fn make_1u8_bag<T: Copy>() -> BagOfBits<T> {
13 assert!(core::mem::size_of::<T>() >= 1);
14 let mut bag = BagOfBits { uninit: () };
15 unsafe { (&mut bag as *mut _ as *mut u8).write(1); };
19 pub fn check_bag<T: Copy>(bag: &BagOfBits<T>) {
20 let val = unsafe { (bag as *const _ as *const u8).read() };
25 check_bag(&make_1u8_bag::<[usize; 1]>()); // Fine
26 check_bag(&make_1u8_bag::<usize>()); // Fine
28 const CONST_ARRAY_BAG: BagOfBits<[usize; 1]> = make_1u8_bag();
29 check_bag(&CONST_ARRAY_BAG); // Fine.
30 const CONST_USIZE_BAG: BagOfBits<usize> = make_1u8_bag();
32 // Used to panic since CTFE would make the entire `BagOfBits<usize>` uninit
33 check_bag(&CONST_USIZE_BAG);