Returns closure which is FnOnce

vector.rs

fn test() -> Box<MFunBox<i32, Box<Vec<i32>>>> {
    let mut v = Box::new(vec![1, 2, 3]);
    return Box::new(move |x: i32|{ v.push(x); v});
}

trait MFunBox<A, B> {
    fn mcall(self: Box<Self>, arg: A) -> B;
}

impl<A, B, T:FnOnce(A) -> B> MFunBox<A, B> for T {
    fn mcall(self: Box<Self>, arg: A) -> B {
        (*self)(arg)
    }
}

fn main() {
    let f1 = test();
    let v1 = f1.mcall(4);
    println!("{:?}", v1);
    let f2 = test();
    let v2 = f2.mcall(5);
    println!("{:?}", v2);
}