fn main() {
    let a: Top = Top {
        map_from: vec![Map {
            labels: vec![
                Label {
                    name: 1,
                },
                Label {
                    name: 2,
                },
                Label {
                    name: 3
                },
            ],
        }],
    };

    let test: Vec<_> = a
        .map_from
        .iter()
        .flat_map(|lmf| &lmf.labels)
        .map(|l| l.name)
        .collect();

    assert_eq!(test, vec![1,2,3]);
}

#[derive(Debug)]
struct Map {
    labels: Vec<Label>,
}

#[derive(Debug)]
struct Top {
    map_from: Vec<Map>,
}

#[derive(Debug)]
struct Label {
    name: usize,
}