Code shared from the Rust Playground

playground.rs

use futures;
use std::char;
use std::num;
use futures::{Future, future};

#[derive(PartialEq, Debug)]
pub enum MyError {
    Utf16Error(char::DecodeUtf16Error),
    ParseError(num::ParseIntError)
}

impl From<char::DecodeUtf16Error> for MyError {
    fn from(e: char::DecodeUtf16Error) -> Self {
        MyError::Utf16Error(e)
    }
}
impl From<num::ParseIntError> for MyError {
    fn from(e: num::ParseIntError) -> Self {
        MyError::ParseError(e)
    }
}
#[test]
fn main() {
    // We define our base futures
    let future1 = future::ok::<(), char::DecodeUtf16Error>(());
    let future2 = future::ok::<u8, num::ParseIntError>(3);
    // We then cast them
    let future1_casted = future1.from_err::<MyError>();
    let future2_casted = future2.from_err::<MyError>();
    let combined = future2_casted.join(future1_casted);
    assert_eq!(combined.wait(), Ok((3, ())));
}