(library
 (name main)
 (public_name main)
 (root_module root)
 (libraries unix))

  $ dune build
  Internal error, please report upstream including the contents of _build/log.
  Description:
    ("External.cm_dir",
    { t =
        { public_dir = In_build_dir "default"
        ; private_dir = None
        ; public_cmi_dir = None
        }
    })
  Raised at file "otherlibs/stdune-unstable/code_error.ml", line 11, characters
    30-62
  Called from file "src/dune_rules/install_rules.ml", line 148, characters
    19-69
  Called from file "src/dune_rules/install_rules.ml" (inlined), line 165,
    characters 23-31
  Called from file "src/dune_rules/install_rules.ml", line 180, characters
    49-63
  Called from file "list.ml", line 103, characters 22-25
  Called from file "otherlibs/stdune-unstable/list.ml", line 5, characters
    19-33
  ... etc. ...