[Nix-dev] Why does my system rebuild rustc and cargo?

[stewart mackenzie]

Matthias, this default.nix is for my rust setup I use everyday and do
not have this problem.

default.nix:

 with import <nixpkgs> {};
   {
     rustfbpEnv = myEnvFun {
       name = "rustfbp";
       buildInputs = [ stdenv rustcMaster cargo nanomsg];
     };
   }

Would you mind sharing your default.nix to compare please.