gistfile1.txt

```rust
enum Thing<D> {
    AnItem(i32),
    C(D),
}

type A = Thing<()>;
type B = Thing<String>;

fn demo() {
    let items = vec![
        Thing::AnItem(32),
        Thing::C(()),
    ];
    let more_complicated_items: Vec<_> = items.into_iter().map(fill_in_payload).collect();
}

fn fill_in_payload(thing: A) -> B {
    match thing {
        Thing::AnItem(a) => Thing::AnItem(a), // can I avoid this line?
            // If there are lots of other variants in Thing it's tedious
        Thing::C(_) => Thing::C("payload".into())
    }
}
```