backwards: Automatically garbage collect generations

hosts/backwards/modules/boot/default.nix

@@ -1,12 +1,38 @@
-{ ... }:
+{ inputs, lib, config, ... }:
+let
+  configurationLimit = config.mod.gc.configurationLimit;
+in
 {
+  imports = [ inputs.nix-gc-env.nixosModules.default ];
+
+  options = {
+    mod.gc = {
+      configurationLimit = lib.mkOption {
+        type = lib.types.int;
+        default = 10;
+        description = "number of configuration generations to keep";
+      };
+    };
+  };
+
+  config = {
+    nix.gc = {
+      automatic = true;
+      dates = "weekly";
+
+      # `delete_generations` added by nix-gc-env
+      delete_generations = "+${builtins.toString configurationLimit}";
+    };
+
     boot = {
       loader = {
         systemd-boot = {
           enable = true;
+          inherit configurationLimit;
         };
 
         efi.canTouchEfiVariables = true;
       };
     };
+  };
 }

hosts/backwards/modules/default.nix

@@ -8,6 +8,8 @@ in
 
   config = {
     mod = {
+      gc.configurationLimit = 10;
+
       ssh.enable = true;
       git.enable = true;
     };