rollup merge of #20657: alexcrichton/stabilize-macros

diff --cc src/libstd/lib.rs
Simple merge

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

diff --cc src/libsyntax/ext/deriving/mod.rs
Simple merge

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

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