test.rs

trait Handler {
    fn handle(&mut self, &str) -> &str;
}


#[derive(Debug)]
struct TestHandle {
    uri: String,
    value: Option<String>,
}

impl Handler for TestHandle {
    fn handle(&mut self, s: &str) -> &str {
        self.value = Some(format!("via {}", s));
        self.value.as_ref().unwrap()
    }
}


struct A<S: Handler> {
    name: String,
    handler: S,
}

fn main() {
    let mut handler = TestHandle {
        uri: "/test".into(),
        value: None,
    };
    let mut cont = A {
        name: "runner".into(),
        handler: handler,
    };
    println!("{}", cont.handler.handle("abc"));
}