message: Include rustc version in metadata
diff: @@ -482,6 +482,10 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
  unit.target.name().hash(&mut hasher);
  unit.target.kind().hash(&mut hasher);
  + if let Ok(ref rustc) = self.config.rustc() {
  + rustc.verbose_version.hash(&mut hasher);
  + }
  +
  Some(Metadata(hasher.finish()))
  }
  