You could instead do:

impl Classes {
    pub fn get_a(&self) -> Option<&A> {
        match self {
            Classes::A(o) => Some(o),
            _ => None,
        }
    }
}

fn main() -> Result<(), String> {
    let obj: Rc<RefCell<Classes>> = Rc::new(RefCell::new(Classes::A(A::default())));
    let a = obj.borrow().get_a().ok_or("not A".to_string())?;
    Ok(())
}

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