Print error message if --arg is used with a flake

[matthewbauer]

Describe the bug

When you try to build something unfree, you get a message like this:

$ nix build nixpkgs#cudatoolkit
error: --- ThrownError --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
Package ‘cudatoolkit-10.2.89’ in /nix/store/9xsn9g209bvbl324kska98xzsnr4ls95-source/pkgs/development/compilers/cudatoolkit/common.nix:214 has an unfree license (‘unfree’), refusing to evaluate.

a) For `nixos-rebuild` you can set
  { nixpkgs.config.allowUnfree = true; }
in configuration.nix to override this.

b) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
  { allowUnfree = true; }
to ~/.config/nixpkgs/config.nix.

(use '--show-trace' to show detailed location information)

Previously, you could pass an arg like this:

$ nix build nixpkgs#cudatoolkit --arg config '{ allowUnfree = true; }'

But now I get:

cached failure of attribute 'legacyPackages.x86_64-linux.cudatoolkit.drvPath'

I think we may need to rethink how args work w.r.t. flakes. It's unclear whether "arg" applies to the flake itself, or to the flake + attr path. In addition, there's a few things that we could fix in the new command line that have existed previously in nix-build:

• Nix doesn't error when the arg doesn't actually exist.
• Nested attribute names like --arg config.allowUnfree false don't work.
• No completions for --arg names.

Version: nix-2.4pre20200622_334e26b

[edolstra]

The reason for not supporting arguments in flakes is discussed here: #3843 (comment)

[matthewbauer]

Maybe --arg should be an error on flakes then?