auto merge of #15440 : pcwalton/rust/struct-aliases, r=brson

Closes #4508.

r? @nick29581

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

diff --cc src/librustc/middle/trans/_match.rs
Simple merge

diff --cc src/librustc/middle/typeck/check/mod.rs
Simple merge

diff --cc src/librustc/util/ppaux.rs
Simple merge