builder 2 (no unwrap() needed)

builder2.rs

struct Unset;

struct PizzaBuilder<Dough, Topping> {
    dough: Dough,
    topping: Topping,
}

impl<T> PizzaBuilder<Unset, T> {
    fn with_dough(self, dough: &str) -> PizzaBuilder<String, T> {
        PizzaBuilder {
            dough: dough.to_string(),
            topping: self.topping,
        }
    }
}

impl<T> PizzaBuilder<T, Unset> {
    fn with_topping(self, topping: &str) -> PizzaBuilder<T, String> {
        PizzaBuilder {
            topping: topping.to_string(),
            dough: self.dough,
        }
    }
}

impl PizzaBuilder<String, String> {
    fn build(self) -> Pizza {
        Pizza {
            dough: self.dough,
            topping: self.topping,
        }
    }
}

#[derive(Debug)]
struct Pizza {
    #[allow(dead_code)]
    dough: String,
    #[allow(dead_code)]
    topping: String,
}

impl Pizza {
    fn builder() -> PizzaBuilder<Unset, Unset> {
        PizzaBuilder {
            dough: Unset,
            topping: Unset,
        }
    }
}

fn main() {
    let x = Pizza::builder()
        .with_topping("topping")
        .with_dough("dough")
        .build();
    print!("{x:?}");
}