But Iterator's next() returns an Option, not a Result.

I don't see why you'd want to manipulate iterators by hand, unless you're implementing custom iterators. But, in any case:

    try!(iter.next().ok_or("end of sequence"))

EDIT: Sorry. I was just being dense.

Check out the code that goes along the "no try! for Option" comment in the OP: it is implementing a custom iterator.

Oh, my bad. I guess `try!` isn't a good enough replacement for properly treating `Option` and `Result` as monads.