I think a bigger one from what I have looked is the complexity of (en^de)code bytes around.

I suspect if you could use UNION better and be the same as Enum in Rust you could do us a favor! Also I wish I could do this easily:

    enum Tag {
        Int,
        Bool
    }

    enum Data {
        Int(Vec<i32>),
        Bool(Vec<bool>)
    }

    struct Val {
        tag:Tag,
        data:Data
    }

    let n = val.as_i32::<Tag::Int>(&self) -> Vec<i32>