Yes, there is a simpler approach.
If your function returns Result<T, Box<Error>>, then you can use try! or ? operator to return any kind of error. It will be automatically boxed and converted into the generic Box<Error>.
This works for two reasons:
1) try! and ? don't simply return Err(err) on error case as one might think. They actually return Err(From::from(err)).
2) The standard library has: impl<'a, E: Error + 'a> From<E> for Box<Error + 'a> { ... }
Check out this article, starting from "The From trait": http://blog.burntsushi.net/rust-error-handling/
Another solution I hadn't considered is to simply impl std::convert::From<SomeError> for your custom error type for each SomeError that might be encountered.
Awesome, thanks. Does returning boxed errors have a performance impact?
Can someone with more experience than I tell me if there is a simpler approach?