Best way to convert between Option<T> and sentinel value for reading AND writing.

[Lehona]

Very similar to a previous question, but not quite answered by it.

I have a field in a binary struct that is a u32, but uses 0xFFFFFFFF as a sentinel value to mean None. I can deserialize this with the map attribute to Option<u32>, but I don't think this will work the other way around (None should be serialized as 0xFFFFFFFF). Is there any way I can achieve this without writing my own write implementation?

[wcampbell0x2a]

I think I would just use my own enum:

use deku::prelude::*;
use dbg_hex::dbg_hex;

#[derive(Debug, DekuWrite, DekuRead)]
#[deku(type = "u32")]
enum Value {
    #[deku(id = 0xffffffff)]
    None,
    #[deku(id_pat = "_")]
    Some(u32)
}

fn main() {
    let data = [0xff, 0xff, 0xff, 0xff];
    let v = Value::from_bytes((&data, 0)).unwrap().1;
    assert_eq!(data, &*v.to_bytes().unwrap());
    dbg_hex!(v);
    let data = [0x01, 0x02, 0x03, 0x04];
    let v = Value::from_bytes((&data, 0)).unwrap().1;
    assert_eq!(data, &*v.to_bytes().unwrap());
    dbg_hex!(v);
}

[examples/312.rs:17:5] v = None
[examples/312.rs:21:5] v = Some(
    0x4030201,
)