This is type inference not being able to infer through a wrapping type—Option<_> in your case—when the wrapped type needs to be coerced in some way.
Besides your [..] workaround,
assert_eq!(iter.next().unwrap(), &[1, 1, 1]);
also works or using the as _ trick to nudge type inference a bit:
assert_eq!(iter.next(), Some(&[1, 1, 1] as _));