message: Add missing extern crate rustc_driver
diff: @@ -54,6 +54,11 @@ extern crate rustc_session;
  extern crate rustc_span;
  extern crate rustc_target;
  +// Necessary to pull in object code as the rest of the rustc crates are shipped only as rmeta
  +// files.
  +#[allow(unused_extern_crates)]
  +extern crate rustc_driver;
  +
  mod borrow_tracker;
  mod clock;
  mod concurrency;
  