nix: Test module

flake.nix

@@ -135,6 +135,8 @@
                   pname = "${prev.pname}-msrv";
                 }
               );
+
+            nixos-test = pkgs.nixosTest (import ./nixos-test.nix self');
           };
 
           devShells.default = pkgs.mkShell {

nixos-test.nix

@@ -0,0 +1,34 @@
+self': {
+  name = "bcachefs-nixos";
+
+  nodes.machine =
+    { config, ... }:
+    {
+      assertions = [
+        {
+          assertion =
+            config.boot.bcachefs.modulePackage or null == self'.packages.bcachefs-module-linux-latest;
+          message = "Local bcachefs module isn't being used; update nixpkgs?";
+        }
+      ];
+
+      virtualisation.emptyDiskImages = [
+        {
+          size = 4096;
+          driveConfig.deviceExtraOpts.serial = "test-disk";
+        }
+      ];
+
+      boot.supportedFilesystems.bcachefs = true;
+      boot.bcachefs.package = self'.packages.bcachefs-tools;
+    };
+
+  testScript = ''
+    machine.succeed(
+      "modinfo bcachefs | grep updates/src/fs/bcachefs > /dev/null",
+      "mkfs.bcachefs /dev/disk/by-id/virtio-test-disk",
+      "mkdir /mnt",
+      "mount /dev/disk/by-id/virtio-test-disk /mnt",
+    )
+  '';
+}