macro_rules! spread {
    (@inner $vec:ident) => { $vec };
    (@inner $vec:ident ...$item:expr) => {{
        $vec.extend($item.iter());
        $vec
    }};
    (@inner $vec:ident ...$item:expr, $($rest:tt)*) => {{
        $vec.extend($item.iter());
        spread![@inner $vec $($rest)*]
    }};
    (@inner $vec:ident $item:expr) => {{
        $vec.push($item);
        $vec
    }};
    (@inner $vec:ident $item:expr, $($rest:tt)*) => {{
        $vec.push($item);
        spread![@inner $vec $($rest)*]
    }};
    ($($rest:tt)*) => {{
        let mut items = Vec::new();
        spread![@inner items $($rest)*]
    }};
}

fn main() {
    let a = [1, 2, 3];
    let b = [6, 7, 8];
    let items = spread![0, ...a, 4, 5, ...b, 9];
    println!("{:?}", items);
}