#[derive(Debug)]
struct Bar {
    x: i16
}

struct BarBuilder {
    init_x: i16
}

impl BarBuilder {
    fn inc(&mut self) -> &mut BarBuilder {
        self.init_x = self.init_x + 1;
        self
    }

    fn build(self) -> Bar {
        Bar {x: self.init_x}
    }
}

#[derive(Debug)]
struct Foo {
    bar: Bar
}

impl Foo {
    fn set_bar<T, F>(&mut self, f: F) -> &mut Foo
    where
        F: FnOnce(&mut BarBuilder) -> T,
    {
        let mut bar_builder = BarBuilder { init_x: 1 };
        f(&mut bar_builder);
        self.bar = bar_builder.build();
        self
    }
}

fn main() {
    let mut foo = Foo { bar: Bar { x: 0 }};
    let bar = foo.set_bar(|ref mut bar| bar.inc() );
    println!("{:?}", bar);
}