these 4 derivations will be built:
  /nix/store/snbzcijg7iwvjy25ipm3zh86qbsbrd0q-Cargo.toml.drv
  /nix/store/aq3l5yr1w9zf3f2dsj2h59qv31kiz3xa-source.drv
  /nix/store/wji6snl5fg1b9g2c95ia7zidg64pd73k-iets-deps-0.2.0.drv
  /nix/store/2y9a9hg84k3mps0dq2yb7h8g9kb885wz-iets-doctest-0.2.0.drv