impermanence: Init module

modules/system/impermanence.nix

@@ -0,0 +1,39 @@
+{
+  config,
+  inputs,
+  lib,
+  ...
+}:
+let
+  cfg = config.custom.impermanence;
+
+  inherit (config.custom) services;
+in
+{
+  imports = [ inputs.impermanence.nixosModules.impermanence ];
+
+  options.custom.impermanence.enable = lib.mkEnableOption "";
+
+  config = lib.mkIf cfg.enable {
+    fileSystems."/persist".neededForBoot = true;
+
+    environment.persistence."/persist" = {
+      hideMounts = true;
+
+      # See https://nixos.org/manual/nixos/stable/#ch-system-state
+      directories = [
+        "/var/lib/nixos"
+        "/var/lib/systemd"
+        "/var/log/journal"
+
+        (lib.optionalString services.tailscale.enable "/var/lib/tailscale")
+      ];
+    
+      files = [
+        "/etc/machine-id"
+        "/etc/ssh/ssh_host_ed25519_key"
+        "/etc/ssh/ssh_host_ed25519_key.pub"
+      ];
+    };
+  };
+}