#[derive(Debug)]
pub struct S {
    field: Option<u8>,
}

impl S {
    fn field(&mut self) -> &mut u8 {
        self.field.as_mut().expect("blop blop")
    }

    fn method(&mut self) {
        *self.field() = 1;
        *self.field() = 2;
    }
}

fn main() {
    let mut s = S { field: Some(0) };
    s.method();
    println!("{:?}", s);
}