environment: add darwinConfig option to set darwin-config in NIX_PATH

This simplifies using a non default configuration.nix without requiring
a symlink or redefining NIX_PATH.

The nix.nixPath option only defines a default which means defining a value
will drop all of the defaults. This is because correct merge behaviour
isn't very clear for named entries.

1 files changed, 13 insertions, 0 deletions

diff --git a/modules/environment/default.nix b/modules/environment/default.nix
index 3ab0756..6d77cd6 100644
--- a/modules/environment/default.nix
+++ b/modules/environment/default.nix
@@ -63,6 +63,19 @@ in {
       description = "List of directories to be symlinked in <filename>/run/current-system/sw</filename>.";
     };
 
+    environment.darwinConfig = mkOption {
+      type = types.either types.path types.str;
+      default = "$HOME/.nixpkgs/darwin-configuration.nix";
+      description = ''
+        The path of the darwin configuration.nix used to configure the system,
+        this updates the default darwin-config entry in NIX_PATH. Since this
+        changes an environment variable it will only apply to new shells.
+
+        NOTE: Changing this requires running <command>darwin-rebuild switch -I darwin-config=/path/to/configuration.nix</command>
+        the first time to make darwin-rebuild aware of the custom location.
+      '';
+    };
+
     environment.loginShell = mkOption {
       type = types.str;
       default = "$SHELL";