fix: avoid putting empty keys in the config file

Author: zimbatm

module-options.nix

@@ -23,6 +23,15 @@ let
 
   configFormat = pkgs.formats.toml { };
 
+  # Remove keys in the setting that are "empty" to keep the config file lean
+  emptySettingsKeys =
+    lib.optional (config.settings.excludes == []) "excludes"
+    ++ lib.optional (config.settings.on-unmatched == null) "on-unmatched";
+
+  settingsData = builtins.removeAttrs config.settings emptySettingsKeys;
+
+  configFile = configFormat.generate "treefmt.toml" settingsData;
+
   # The schema of the treefmt.toml data structure.
   configSchema = mkOption {
     default = { };
@@ -36,16 +45,17 @@ let
           default = [ ];
           example = [ "node_modules/*" ];
         };
+
         on-unmatched = mkOption {
           description = "Log paths that did not match any formatters at the specified log level.";
-          type = types.enum [
+          type = types.nullOr (types.enum [
             "debug"
             "info"
             "warn"
             "error"
             "fatal"
-          ];
-          default = "warn";
+          ]);
+          default = null;
         };
 
         formatter = mkOption {
@@ -324,7 +334,7 @@ in
 
   # Config
   config.build = {
-    configFile = configFormat.generate "treefmt.toml" config.settings;
+    inherit configFile;
     devShell = pkgs.mkShell {
       nativeBuildInputs = [ config.build.wrapper ] ++ (lib.attrValues config.build.programs);
     };