use std::{io, rc::Rc};

type RawErr = Rc<Box<dyn std::error::Error>>;

fn io_interrupted() -> RawErr {
    Rc::new(Box::new(io::Error::from(io::ErrorKind::Interrupted)))
}
fn io_other() -> RawErr {
    Rc::new(Box::new(io::Error::from(io::ErrorKind::UnexpectedEof)))
}
fn fmt() -> RawErr {
    Rc::new(Box::new(std::fmt::Error))
}

fn downcast(r: &RawErr) -> &str {
    match r.downcast_ref::<io::Error>() {
        Some(r) if r.kind() == io::ErrorKind::Interrupted => "io-interrupted",
        Some(_) => "io-other",
        None => "unknown",
    }
}

fn main() {
    assert_eq!(downcast(&io_interrupted()), "io-interrupted");
    assert_eq!(downcast(&io_other()), "io-other");
    assert_eq!(downcast(&fmt()), "unknown");
}