macro derive_builder! {
    struct $name:ident { $($field:ident : $ty:ty),* } => {
        @{
            let builder_name = format_ident!("{}_Builder", name.to_string());
        }
        T~{
            pub struct ${ builder_name } {
                for field in $fields =>
                    pub $field.name: Option<$field.ty>,
            }
            impl ${ builder_name } {
                pub fn build(self) -> $name {
                    $name {
                        for field in $fields =>
                            $field.name: self.$field.name.unwrap(),
                    }
                }
            }
        }
    }
}