mod m {
    #[readonly::make]
    pub struct S {
        pub n: i32,
    }
    impl S {
        pub fn demo(&mut self) {
            // Can read and write from inside the same module.
            println!("{}", self.n);
            self.n += 1;
        }
    }
}
fn demo(s: &mut m::S) {
    // From outside the module, can only read.
    println!("{}", s.n);
    // Does not compile:
    //s.n += 1;
}

#[readonly::make]
pub struct S {
    // This field can be read (but not written) by super.
    #[readonly]
    pub(super) readable: i32,
    // This field can be neither read nor written by other modules.
    private: i32,
}