trait Collection<T>: Sized {
    fn take_items(self) -> Vec<T>;
}

struct AList<T> {
    items: Vec<T>,
}

impl<T> AList<T> {
    pub fn new() -> Self {
        Self { items: Vec::new() }
    }
}

impl<T> Collection<T> for AList<T> {
    fn take_items(self) -> Vec<T> {
        self.items
    }
}

struct BList<T> {
    items: Vec<T>,
}

impl<T> Collection<T> for BList<T> {
    fn take_items(self) -> Vec<T> {
        self.items
    }
}

fn map_a_to_b(coll: Box<Collection<i64>>) -> Box<Collection<i64>> {
    Box::new(BList {
        items: coll.take_items(),
    })
}

fn main() {
    let mut coll: Box<Collection<i64>> = Box::new(AList::new());
    coll = map_a_to_b(coll);
}