message: add rustflags to the metadata
diff: @@ -465,6 +465,15 @@ fn compute_metadata<'a, 'cfg>(
  args.hash(&mut hasher);
  }
  + // Throw in the rustflags we're compiling with.
  + // This helps when the target directory is a shared cache for projects with different cargo configs,
  + // or if the user is experimenting with different rustflags manually.
  + if unit.mode.is_doc() {
  + cx.bcx.rustdocflags_args(unit).hash(&mut hasher);
  + } else {
  + cx.bcx.rustflags_args(unit).hash(&mut hasher);
  + }
  +
  // Artifacts compiled for the host should have a different metadata
  // piece than those compiled for the target, so make sure we throw in
  // the unit's `kind` as well
  