use std::io; struct MyError(()); // doesn't implement Debug #[derive(Debug)] struct MyErrorWithParam<T> { x: T } fn main() { let res: Result<i32, ()> = Ok(0); let _ = res.unwrap(); res.ok().expect("disaster!"); // the following should not warn, since `expect` isn't implemented unless // the error type implements `Debug` let res2: Result<i32, MyError> = Ok(0); res2.ok().expect("oh noes!"); let res3: Result<u32, MyErrorWithParam<u8>>= Ok(0); res3.ok().expect("whoof"); let res4: Result<u32, io::Error> = Ok(0); res4.ok().expect("argh"); let res5: io::Result<u32> = Ok(0); res5.ok().expect("oops"); let res6: Result<u32, &str> = Ok(0); res6.ok().expect("meh"); }