message: Add comment for unusual placement of add_sysroot_artifact.
diff: @@ -619,6 +619,19 @@ impl<'a, 'cfg> JobQueue<'a, 'cfg> {
  Artifact::Metadata => self.timings.unit_rmeta_finished(id, unlocked),
  }
  if unit.is_std && unit.kind == super::Kind::Target && !cx.bcx.build_config.build_plan {
  + // This is a bit of an unusual place to copy files around, and
  + // ideally this would be somewhere like the Work closure
  + // (`link_targets`). The tricky issue is handling rmeta files for
  + // pipelining. Since those are emitted asynchronously, the code
  + // path (like `on_stderr_line`) does not have enough information
  + // to know where the sysroot is, and that it is an std unit. If
  + // possible, it might be nice to eventually move this to the
  + // worker thread, but may be tricky to have the paths available.
  + // Another possibility is to disable pipelining between std ->
  + // non-std. The pipelining opportunities are small, and are not a
  + // huge win (in a full build, only proc_macro overlaps for 2
  + // seconds out of a 90s build on my system). Care must also be
  + // taken to properly copy these artifacts for Fresh units.
  let rmeta = artifact == Artifact::Metadata;
  standard_lib::add_sysroot_artifact(cx, unit, rmeta)?;
  }
  