How do you implement a builder pattern?

Question

Accepted Answer

**Builder** — construct complex objects step by step.

**Implementation:**
```rust
struct Server {
    host: String,
    port: u16,
    max_connections: usize,
}

struct ServerBuilder {
    host: String,
    port: u16,
    max_connections: usize,
}

impl ServerBuilder {
    fn new() -> Self {
        Self {
            host: "localhost".into(),
            port: 8080,
            max_connections: 100,
        }
    }

fn host(mut self, host: &str) -> Self {
        self.host = host.into();
        self
    }

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

fn build(self) -> Server {
        Server {
            host: self.host,
            port: self.port,
            max_connections: self.max_connections,
        }
    }
}

let server = ServerBuilder::new()
    .host("0.0.0.0")
    .port(3000)
    .build();
```

**Key:** Builder for complex construction.

Rust: How do you implement a builder pattern?