modules/lua-loader: make enable option nullable

This avoids having the option always "defined".

This also avoids luaLoader configuration in extra files by default.
Earlier `vim.loader.disable()` was always added to configs produced
by `files` option, effectively disabling luaLoader even if it was
explicitly enabled in a top-level configuration.

modules/lua-loader.nix

@@ -1,14 +1,14 @@
-{ lib, config, ... }:
-with lib;
+{
+  lib,
+  config,
+  helpers,
+  ...
+}:
 let
   cfg = config.luaLoader;
 in
 {
-  options.luaLoader = {
-    enable = mkOption {
-      type = types.bool;
-      default = false;
-      description = ''
+  options.luaLoader.enable = helpers.mkNullOrOption lib.types.bool ''
     Whether to enable/disable the experimental lua loader:
 
     If `true`: Enables the experimental Lua module loader:
@@ -20,11 +20,11 @@ in
     If `false`: Disables the experimental Lua module loader:
       - removes the loaders
       - adds the default Neovim loader
-      '';
-    };
-  };
 
-  config = {
+    If `null`: Nothing is configured.
+  '';
+
+  config = helpers.mkIfNonNull' cfg.enable {
     extraConfigLuaPre = if cfg.enable then "vim.loader.enable()" else "vim.loader.disable()";
   };
 }

tests/test-sources/modules/lua-loader.nix

@@ -1,9 +1,59 @@
 {
-  enabled = {
-    luaLoader.enable = true;
+  # Test that nothing is configured by default
+  default.module =
+    { config, ... }:
+    {
+      files."files_test.lua" = { };
+
+      assertions = [
+        {
+          assertion = builtins.match ".*vim\.loader.*" config.content == null;
+          message = "No luaLoader configuration is expected in init.lua by default.";
+        }
+        {
+          assertion = builtins.match ".*vim\.loader.*" config.files."files_test.lua".content == null;
+          message = "No luaLoader configuration is expected in 'files' submodules.";
+        }
+      ];
     };
 
-  disabled = {
+  # Test lua loader enabled
+  enabled.module =
+    { config, ... }:
+    {
+      luaLoader.enable = true;
+
+      files."files_test.lua" = { };
+
+      assertions = [
+        {
+          assertion = builtins.match ".*vim\.loader\.enable\(\).*" config.content != null;
+          message = "luaLoader is expected to be explicitly enabled.";
+        }
+        {
+          assertion = builtins.match ".*vim\.loader.*" config.files."files_test.lua".content == null;
+          message = "No luaLoader configuration is expected in 'files' submodules.";
+        }
+      ];
+    };
+
+  # Test lua loader disabled
+  disabled.module =
+    { config, ... }:
+    {
       luaLoader.enable = false;
+
+      files."files_test.lua" = { };
+
+      assertions = [
+        {
+          assertion = builtins.match ".*vim\.loader\.disable\(\).*" config.content != null;
+          message = "luaLoader is expected to be explicitly disabled.";
+        }
+        {
+          assertion = builtins.match ".*vim\.loader.*" config.files."files_test.lua".content == null;
+          message = "No luaLoader configuration is expected in 'files' submodules.";
+        }
+      ];
     };
 }