auto merge of #20412 : nikomatsakis/rust/assoc-types, r=aturon

These changes fix various problems encountered getting japaric's `at-iter` branch to work. This branch converts the `Iterator` trait to use an associated type.

diff --cc src/librustc/middle/expr_use_visitor.rs
Simple merge

diff --cc src/librustc/middle/ty.rs
Simple merge

diff --cc src/librustc_typeck/check/mod.rs
Simple merge