these 4 derivations will be built:
  /nix/store/rvmrdgsm2idzbyv8xirp43yyss0ys46k-crate-rand-0.10.3.tar.gz.drv
  /nix/store/4fqbarpqnanszhqsvvmm7y6rlxsqcsds-rand-0.10.3.drv
  /nix/store/n1ddj1v4bcqp07zqznr1fwiczs42vg38-cargo-vendor-dir.drv
  /nix/store/ywqkp0zbxn6xp9vp970pcmgi2xdpxl0r-fast-nix-gc-proptest-0.1.0.drv