Using a builder pattern

Sometimes data structures have lots of or complicated fields, so that they need a number of constructors to effectively create them. Other languages would use method overloading or named arguments, but Rust doesn't have these. Instead, we can use the so-called builder pattern, which is used occasionally in Servo and the Rust standard library. The idea is that the instance construction is forwarded to an analogous Builder struct, which has a default new constructor, methods that change each of its properties, and a finish method that returns an instance of the original struct. Here is an example:

// see code in Chapter 6/code/builder_pattern.rs 
struct Alien { 
    name: &'static str, 
    health: u32, 
    damage: u32 
} 
 
struct AlienBuilder { 
    name: &'static str, 
    health: u32, 
    damage: u32 
} 
 
impl AlienBuilder { 
    fn new() -> Self { 
        AlienBuilder { name: "Walker", health: 100, damage: 10 } 
    } 
 
    fn name(&mut self...