Builder pattern for complex struct initialization

                  pub struct Server {
    host: String,
    port: u16,
    workers: usize,
}

pub struct ServerBuilder {
    host: Option<String>,
    port: Option<u16>,
    workers: usize,
}

impl ServerBuilder {
    pub fn new() -> Self {
        Self { host: None, port: None, workers: 4 }
    }

    pub fn host(mut self, host: String) -> Self {
        self.host = Some(host);
        self
    }

    pub fn port(mut self, port: u16) -> Self {
        self.port = Some(port);
        self
    }

    pub fn build(self) -> Result<Server, &'static str> {
        Ok(Server {
            host: self.host.ok_or("host is required")?,
            port: self.port.ok_or("port is required")?,
            workers: self.workers,
        })
    }
}