extern crate failure;
use error::Error;
use std::fs::File;

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

mod error {
    use std::io::Error as IoError;
    use super::Result;

    #[derive(Debug, PartialEq/*<- ruh roh!*/)]
    pub enum Error {
        MyVariant(IoError),
    }

    impl Display...

    impl From<IoError> for Error...

struct Foo {}

impl Foo {
    fn example_fn(name: &str) -> Result<File> {
        match File::open(name)?
}

fn main() -> Result<()> {
    // ...
    Ok(())
}

#[test]
fn test() {
    use super::{Error, Foo};

    let result = Foo::example_fn("foo.txt");

    // Verify Err(Error::MyVariant(std::io::Error(Simple(std::io::ErrorKind::NotFound)))) was received
    assert!(result == ???);
}