This specific issue (iterables over mutable references) is discussed in these two blog posts. The second one includes the Best Practices way to implement such an iterator:

http://smallcultfollowing.com/babysteps/blog/2013/10/24/iterators-yielding-mutable-references/
http://smallcultfollowing.com/babysteps/blog/2013/10/24/iterators-yielding-mutable-references-take-2/