Allow unfree packages with nix shell

[GuillaumeDesforges]

Issue description

Currently, it's not possible to use unfree packages with nix shell

e.g.

nix shell nixpkgs#python3Packages.cupy

a current workaround is to use --impure, but one can want both unfree and not impure.

Possible fix

My guess is that the legacyPackages is loaded with inherited system.

nixpkgs/flake.nix

Line 43 in c618329

legacyPackages = forAllSystems (system: import ./. { inherit system; });

There is no way to provide an argument to flakes (see NixOS/nix#3843 (comment)) and pass it to the instanciation, and most likely won't ever be.

Therefore, a suggestion would be to add

legacyPackages.withUnfree = forAllSystems (system: import ./. { inherit system; config.allowUnfree = true; });

The downside is that we get all packages twice, e.g. in commands such as nix search.

Note

We could think of being able to pass an argument such as --allow-unfree to the Nix command, but in my opinion it is too focused on nixpkgs and most likely to be rejected.

[con-f-use]

So is there a solution or workaround for this? I'd like have a devShell with unfree packages.

[GuillaumeDesforges]

For now a workaround is to use

$ NIXPKGS_ALLOW_UNFREE=1 nix shell --impure nixpkgs#yourpackage

[con-f-use]

Norbert (NobbZ) suggest the following on discord, which seems to work well:

inputs = {
  nixunstable.url = "github:NixOS/nixpkgs/nixos-unstable";
};

outputs = { nixunstable }: {
  devShells.x86_64-linux.default = let
      pkgs = import nixunstable { system = "x86_64-linux"; config.allowUnfree = true; };
  in pkgs.callPackage ./myshelll.nix {};
};

[Artturin]

the error message already contains

a) To temporarily allow unfree packages, you can use an environment variable
   for a single invocation of the nix tools.

     $ export NIXPKGS_ALLOW_UNFREE=1

 Note: For `nix shell`, `nix build`, `nix develop` or any other Nix 2.4+
 (Flake) command, `--impure` must be passed in order to read this
 environment variable.

anything else should be discussed in nix instead of nixpkgs

legacyPackages.withUnfree would probably cause a performance impact

[guissalustiano]

You also could add in nxpkg import

{ pkgs ? (import <nixpkgs> { 
    config.allowUnfree = true;
    config.segger-jlink.acceptLicense = true; 
}), ... }:
pkgs.mkShell {
  packages = with pkgs; [
    nrfconnect
  ];
}