trait Container {
    type Item;
    fn get(&self) -> Option<&Self::Item>;
    fn set(&mut self, item: Self::Item);
}

struct Single<T> {
    value: Option<T>,
}

impl<T> Container for Single<T> {
    type Item = T;

    fn get(&self) -> Option<&T> {
        self.value.as_ref()
    }

    fn set(&mut self, item: T) {
        self.value = Some(item);
    }
}

fn main() {
    let mut s = Single { value: None };
    s.set(42);
    println!("value: {:?}", s.get());
}