use std::hint::black_box;
enum Inner { A(u32), B(u32) }
struct Big { _pad: u64, inner: Inner }
struct Small { a: u16, b: u16, _f: fn() }
enum Checksum { X(Big), Y(Small) }
impl Checksum {
    fn finalize(self) -> u32 {
        match self {
            Checksum::X(h) => match h.inner { Inner::A(s) => s, Inner::B(s) => s },
            Checksum::Y(s) => (u32::from(s.b) << 16) | u32::from(s.a),
        }
    }
}
#[inline(never)]
fn run(c: Option<Checksum>) -> Option<u32> {
    c.map(|c| c.finalize())
}
fn main() { println!("{:?}", run(black_box(None))); }