trait A {
    fn foo(&self, _x: &Self);
}

impl A for () {
    fn foo(&self, _x: &Self) {}
}

fn main() {                                                 
    let _x: &A;
}

This code passes, while the codes like let x = &() as &A; cause compile error because the trait A is not object-safe. 0255-object-safety

Is there any reason for type &A to exist? If not, the declaration of variables of type &A should be invalid.