A bit confused by the Either usage.

[Magicloud]

impl<'r, 'o: 'r, 't: 'o, 'e: 'o, T, E> Responder<'r, 'o> for Either<T, E>
Here, assuming E means error. Then this is similar as Result<T, E>.

impl<L, R> From<Result<R, L>> for Either<L, R>
But here, Either implies the first type as error (the second type in Result).

So this wouldn't work, if we say E in the first line means error.

fn handler() -> Either<(), Error> {
  (Ok(()) as Result<(), Error>).into()
}

Of course, right this does not cause issue since how to process left or right value depends on their types, not position.

[the10thWiz]

Rocket (and Either in general) does not treat either Left or Right as an error.

When you use Either as a responder, it simply requires both the Left and Right types to implement Responder.