auto merge of #12451 : edwardw/rust/ident-2-name, r=cmr

Closes #7743.

diff --cc src/librustc/metadata/decoder.rs
Simple merge

diff --cc src/librustc/metadata/encoder.rs
Simple merge

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

diff --cc src/libsyntax/ast.rs
Simple merge

diff --cc src/libsyntax/ext/build.rs
Simple merge

diff --cc src/libsyntax/print/pprust.rs
Simple merge