auto merge of #14696 : jakub-/rust/dead-struct-fields, r=alexcrichton

This uncovered some dead code, most notably in middle/liveness.rs, which I think suggests there must be something fishy with that part of the code.

The #[allow(dead_code)] annotations on some of the fields I am not super happy about but as I understand, marker type may disappear at some point.

diff --cc src/libnative/io/c_win32.rs
Simple merge

diff --cc src/libnative/io/mod.rs
Simple merge

diff --cc src/librustc/front/std_inject.rs
Simple merge

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

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

diff --cc src/libstd/collections/hashmap.rs
Simple merge

diff --cc src/libstd/rt/backtrace.rs
Simple merge

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

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