rust-lang/rust — pull request #16728 by bluss (2014-08-24 13:52:26+00:00)
Title: libcore: Simplify Enumerate, Zip::next_back

Use ExactSize::len() and defer to its decisions about overly defensive
assertions. Remove the length double-check and simply put a failure
case if the Zip finds an uneven end in .next_back().

Fixing this up since I think I wrote this, and it's been known to
confuse rusties (PR#15886).