Code shared from the Rust Playground

playground.rs

#[derive(Debug)]
struct Config {
    host: String,
    target: String,
    port: i32,
    timeout: i32,
}

impl Default for Config {
    fn default() -> Self {
        Self {
            host: "0.0.0.0".to_string(),
            target: "8.8.8.8".to_string(),
            port: 0,
            timeout: 1000,
        }
    }
}
type Builder = Box<dyn Fn(&mut Config) -> ()>;

impl Config {
    fn new(v: &[Builder]) -> Self {
        let mut ret = Default::default();
        for c in v {
            c(&mut ret)
        }
        ret
    }
}

fn host<S>(s: S) -> Builder
where
    S: Into<String>,
{
    let host = s.into();
    Box::new(move |config: &mut Config| config.host = host.clone())
}

fn target<S>(s: S) -> Builder
where
    S: Into<String>,
{
    let target = s.into();
    Box::new(move |config: &mut Config| config.target = target.clone())
}
fn port(n: i32) -> Builder {
    Box::new(move |config: &mut Config| config.port = n)
}

fn timeout(n: i32) -> Builder {
    Box::new(move |config: &mut Config| config.timeout = n)
}

fn main() {
    let config = Config::new(&[
        port(25),
        host("localhost"),
        target("example.com"),
        timeout(2000),
    ]);
    println!("config = {:?}", config);
}