lib.rs

pub enum Thing {
    Lol {
        name: String,
        other: u64,
    },
    Haha {
        name: String,
        whatever: i8,
    }
}

impl Thing {
    fn with_modification(&self) -> Thing {
        use Thing::*;
        let tweak = "hahahahaha";

        match self {
            lol @ Lol { .. } => {
                let new = lol.clone();
                new.name.push_str(tweak);
                new
            },
            haha @ Haha { .. } => {
                let new = haha.clone();
                new.name.push_str(tweak);
                new
            }
        }
    }
}