nixos-rebuild-ng: add --diff argument

This displays the package updates, additions, and removals to the user.

pkgs/by-name/ni/nixos-rebuild-ng/nixos-rebuild.8.scd

@@ -308,6 +308,14 @@ It must be one of the following:
 	option, it is possible to build non-flake NixOS configurations even if
 	the current NixOS systems uses flakes.
 
+*--diff*
+	show the diff between the system closure in /run/current-system
+	and the newly built system closure.
+	(avaliable for actions: build, boot, test, switch)
+
+	This is similar to running:
+	"nix store diff-closures /run/current-system result" after build
+
 In addition, *nixos-rebuild* accepts following options from nix commands that
 the tool calls:
 

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/__init__.py

@@ -197,6 +197,12 @@ def get_parser() -> tuple[argparse.ArgumentParser, dict[str, argparse.ArgumentPa
         help="Selects an image variant to build from the "
         "config.system.build.images attribute of the given configuration",
     )
+    main_parser.add_argument(
+        "--diff",
+        action="store_true",
+        help="prints out the diff between the current system "
+        "and the newly built one using nix store diff-closures"
+    )
     main_parser.add_argument("action", choices=Action.values(), nargs="?")
 
     return main_parser, sub_parsers
@@ -259,6 +265,14 @@ def parse_args(
     if args.no_build_nix:
         parser_warn("--no-build-nix is deprecated, we do not build nix anymore")
 
+    if args.diff and args.action not in (
+        Action.SWITCH.value,
+        Action.BOOT.value,
+        Action.BUILD.value,
+        Action.TEST.value
+    ):
+        parser_warn(f"--diff is a no-op with '{args.action}'")
+
     if args.action == Action.EDIT.value and (args.file or args.attr):
         parser.error("--file and --attr are not supported with 'edit'")
 

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py

@@ -537,6 +537,18 @@ def list_generations(profile: Profile) -> list[GenerationJson]:
             reverse=True,
         )
 
+def diff_closures(path_to_config: Path):
+    run_wrapper(
+        [
+            "nix",
+            *FLAKE_FLAGS,
+            "store",
+            "diff-closures",
+            "/run/current-system",
+            path_to_config
+        ]
+    )
+
 
 def repl(build_attr: BuildAttr, nix_flags: Args | None = None) -> None:
     run_args = ["nix", "repl", "--file", build_attr.path]

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/services.py

@@ -321,6 +321,14 @@ def build_and_activate_system(
             grouped_nix_args=grouped_nix_args,
         )
 
+    if args.diff and args.action in (
+        Action.SWITCH.value,
+        Action.BOOT.value,
+        Action.BUILD.value,
+        Action.TEST.value
+    ):
+        nix.diff_closures(path_to_config)
+
     _activate_system(
         path_to_config=path_to_config,
         action=action,