R"foo(
{ derivations, manifest }:

derivation {
  name = "user-environment";
  system = "builtin";
  builder = "builtin:buildenv";

  inherit manifest;

  # !!! grmbl, need structured data for passing this in a clean way.
  derivations =
    map (d:
      [ (d.meta.active or "true")
        (d.meta.priority or 5)
        (builtins.length d.outputs)
      ] ++ map (output: builtins.getAttr output d) d.outputs)
      derivations;

  # Building user environments remotely just causes huge amounts of
  # network traffic, so don't do that.
  preferLocalBuild = true;

  # Also don't bother substituting.
  allowSubstitutes = false;
}
)foo"