use std::ops::Deref;
struct Foo {

}

impl Deref for Foo {
    type Target = i32;
    fn deref(&self) -> &Self::Target {
        unimplemented!()
    }
}


impl DerefMut for Foo {
    fn deref_mut(&mut self) -> &mut Self::Target {
        unimplemented!()
    }
}


fn main() {
    let f = Foo{};
    *f = 3;
}