Once breaks if moved

[bkragl]

Here's a test that illustrates the bug:

#[test]
fn once_test() {
    crate::check_dfs(
        || {
            let once = Once::new();
            once.call_once(|| {});
            assert_eq!(is_completed_borrowed(&once), true);
            assert_eq!(is_completed_owned(once), true); // FAILS
        },
        None,
    );
}

fn is_completed_borrowed(once: &Once) -> bool {
    once.is_completed()
}

fn is_completed_owned(once: Once) -> bool {
    once.is_completed()
}

The issue is that we use the address of the Once to compute a storage key, which changes when the Once is moved.

shuttle/src/sync/once.rs

Lines 166 to 170 in 2ac1fae

	impl From<&Once> for StorageKey {
	fn from(once: &Once) -> Self {
	StorageKey(once as *const _ as usize, 0x2)
	}
	}

[jamesbornholt]

Hmm, yeah, that's pretty silly. Once should just use a static counter or something to generate unique storage keys. Or I guess a Box<usize> if we wanted to keep using addresses for some reason.