main.rs

#[macro_use] extern crate derive_builder;

#[derive(Debug, Default, Builder)]
struct SomeOptions {
    allow_a: Option<bool>,
}

impl SomeOptions {
    pub fn allowed(&self) -> bool {
        self.allow_a
    }
}

fn main() {
    let options = SomeOptions::default().allow_a(true);

    println!("{:#?}", options);
}