use std::fs::File;

type Result<T> = std::result::Result<T, Box<std::error::Error>>;

struct Foo {}

impl Foo {
    fn example_fn(name: &str) -> Result<File> {
        match File::open(name) {
            Ok(v) => Ok(v),
            Err(e) => Err(Box::new(e)),
        }
    }
}


#[test]
fn test() {
    let result = Foo::example_fn("foo.txt");
    let err = result.unwrap_err().downcast::<std::io::Error>().unwrap();
    assert_eq!(err.kind(), std::io::ErrorKind::NotFound);
}