message: Only pass -Zincremental to nightly rustc.
  Fixes
diff: @@ -83,6 +83,14 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
  Err(_) => false,
  };
  + // -Z can only be used on nightly builds; other builds complain loudly.
  + // Since incremental builds only work on nightly anyway, we silently
  + // ignore CARGO_INCREMENTAL on anything but nightly. This allows users
  + // to always have CARGO_INCREMENTAL set without getting unexpected
  + // errors on stable/beta builds.
  + let incremental_enabled = incremental_enabled
  + && config.rustc()?.verbose_version.contains("nightly");
  +
  Ok(Context {
  ws: ws,
  host: host_layout,
  