message: remove unused extern crate
diff: extern crate log;
  // From rustc.
  +extern crate syntax;
  #[macro_use]
  extern crate rustc;
  extern crate rustc_data_structures;
  extern crate rustc_mir;
  extern crate rustc_target;
  -extern crate rustc_driver;
  -extern crate syntax;
  use std::collections::HashMap;
  use std::borrow::Cow;
  