add rule nix_unfree

README.md

@@ -368,6 +368,7 @@ The following rules are enabled by default on specific platforms only:
 * `dnf_no_such_command` – fixes mistyped DNF commands;
 * `nixos_cmd_not_found` – installs apps on NixOS;
 * `nix_shell` – re-runs your command using `nix run`;
+* `nix_unfree` – re-runs your command with `NIXPKGS_ALLOW_UNFREE=1`;
 * `pacman` – installs app with `pacman` if it is not installed (uses `yay`, `pikaur` or `yaourt` if available);
 * `pacman_invalid_option` – replaces lowercase `pacman` options with uppercase.
 * `pacman_not_found` – fixes package name with `pacman`, `yay`, `pikaur` or `yaourt`.

thefuck/rules/nix_unfree.py

@@ -0,0 +1,21 @@
+from thefuck.specific.nix import nix_available
+import subprocess
+
+enabled_by_default = nix_available
+
+# Set the priority just ahead of `fix_file` rule, which can generate low quality matches due
+# to the sheer amount of paths in the nix store.
+priority = 999
+
+
+def match(command):
+    return (
+        "NIXPKGS_ALLOW_UNFREE=1" in command.output
+    )
+
+
+def get_new_command(command):
+    commands = [
+        'NIXPKGS_ALLOW_UNFREE=1 {}'.format(command.script)
+    ]
+    return commands