Using crate overrides

You can patch the individual crate derivations with crateOverrides, for example to add native library dependencies such as openssl!

NixOS comes with defaultCrateOverrides which specifies mostly some additional native buildInputs for various popular crates. If you are using a rust crate with native dependencies which is not yet covered, you can add additional buildInputs with the crateOverride parameter (similar to features):

let
  customBuildRustCrateForPkgs = pkgs: pkgs.buildRustCrate.override {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      funky-things = attrs: {
        buildInputs = [ pkgs.openssl ];
      };
    };
  };
  generatedBuild = callPackage ./crate2nix/Cargo.nix {
    buildRustCrateForPkgs = customBuildRustCrateForPkgs;
  };
in generatedBuild.rootCrate.build

Or obviously you can use the power of nix to add a dependency conditionally:

let
  customBuildRustCrateForPkgs = pkgs: pkgs.buildRustCrate.override {
    defaultCrateOverrides = pkgs.defaultCrateOverrides // {
      cssparser-macros = attrs: {
        buildInputs =
          lib.optionals
            pkgs.stdenv.isDarwin
            [ pkgs.darwin.apple_sdk.frameworks.Security ];
      };
    };
  };
  generatedBuild = callPackage ./crate2nix/Cargo.nix {
    buildRustCrateForPkgs = customBuildRustCrateForPkgs;
  };
in generatedBuild.rootCrate.build

crateOverrides are not restricted to buildInputs however. You should also be able to add patches and the like! (I didn’t try that, though.)

crateOverrides are a feature of the underlying buildRustCrate support in NixOS that crate2nix uses.