nix: skip check completely when checkConfig=false

modules/misc/nix.nix

@@ -74,6 +74,7 @@ let
         ${mkKeyValuePairs cfg.settings}
         ${cfg.extraOptions}
       '';
+      passthru.doCheck = cfg.checkConfig;
       checkPhase =
         if pkgs.stdenv.hostPlatform != pkgs.stdenv.buildPlatform then ''
           echo "Ignoring validation for cross-compilation"
@@ -92,9 +93,13 @@ let
                 "--no-net --option experimental-features nix-command"
               } \
               |& sed -e 's/^warning:/error:/' \
-              | (! grep '${
-                if cfg.checkConfig then "^error:" else "^error: unknown setting"
-              }')
+              | ${
+                if cfg.allowUnknownSettings then
+                  "grep -v '^error: unknown setting'"
+                else
+                  "cat"
+              } \
+              | (! grep '^error:')
             set -o pipefail
           '';
     };
@@ -241,7 +246,16 @@ in {
       default = true;
       description = ''
         If enabled (the default), checks for data type mismatches and that Nix
-        can parse the generated nix.conf.
+        can parse the generated {file}`nix.conf`.
+      '';
+    };
+
+    allowUnknownSettings = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        If enabled, unknown setting errors will be allowed when checking the
+        generated {file}`nix.conf`. This has no effect if {option}`checkConfig` is false.
       '';
     };
 

tests/modules/misc/nix/allow-unknown-settings-expected.conf

@@ -0,0 +1,6 @@
+# WARNING: this file is generated from the nix.settings option in
+# your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
+# Do not edit it!
+and-another-one = hello
+some-arbitrary-setting = true
+

tests/modules/misc/nix/allow-unknown-settings.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }: {
+  nix = {
+    package = config.lib.test.mkStubPackage {
+      version = lib.getVersion pkgs.nixVersions.stable;
+      buildScript = ''
+        target=$out/bin/nix
+        mkdir -p "$(dirname "$target")"
+
+        echo -n "true" > "$target"
+
+        chmod +x "$target"
+      '';
+    };
+
+    settings = {
+      some-arbitrary-setting = true;
+      and-another-one = "hello";
+    };
+    checkConfig = false;
+  };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/.config/nix/nix.conf \
+      ${./allow-unknown-settings-expected.conf}
+  '';
+}

tests/modules/misc/nix/default.nix

@@ -5,4 +5,6 @@
   nix-keep-old-nix-path = ./keep-old-nix-path.nix;
   nix-example-channels = ./example-channels.nix;
   nix-example-channels-xdg = ./example-channels-xdg.nix;
+  nix-skip-check-settings = ./skip-check-settings.nix;
+  nix-allow-unknown-settings = ./allow-unknown-settings.nix;
 }

tests/modules/misc/nix/skip-check-settings-expected.conf

@@ -0,0 +1,7 @@
+# WARNING: this file is generated from the nix.settings option in
+# your Home Manager configuration at $XDG_CONFIG_HOME/nix/nix.conf.
+# Do not edit it!
+
+some! nonsense {
+  which should fail validation
+

tests/modules/misc/nix/skip-check-settings.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }: {
+  nix = {
+    package = config.lib.test.mkStubPackage {
+      version = lib.getVersion pkgs.nixVersions.stable;
+      buildScript = ''
+        target=$out/bin/nix
+        mkdir -p "$(dirname "$target")"
+
+        echo -n "true" > "$target"
+
+        chmod +x "$target"
+      '';
+    };
+
+    extraOptions = ''
+      some! nonsense {
+        which should fail validation
+    '';
+    checkConfig = false;
+  };
+
+  nmt.script = ''
+    assertFileContent \
+      home-files/.config/nix/nix.conf \
+      ${./skip-check-settings-expected.conf}
+  '';
+}