Tests(deferred custom module): add more tests, dissallow nested imports

lib/types/default.nix

@@ -2,18 +2,24 @@
 {
   uniqueDeferredSerializableModule = lib.fix (
     self:
+
+    let
+      checkDef = loc: def: if def.value ? imports then throw "uniqueDeferredSerializableModule doesn't allow nested imports" else def;
+    in
     # Essentially the "raw" type, but with a custom name and check
     lib.mkOptionType {
       name = "deferredModule";
-      description = "deferred module that has custom check and merge behavior";
+      description = "deferred custom module. Must be JSON serializable.";
       descriptionClass = "noun";
       # Unfortunately, tryEval doesn't catch JSON errors
-      check = value: lib.seq (builtins.toJSON value) true;
+      check = value: lib.seq (builtins.toJSON value) (lib.isAttrs value);
       merge = lib.options.mergeUniqueOption {
         message = "------";
         merge = loc: defs: {
           imports = map (
-            def: lib.setDefaultModuleLocation "${def.file}, via option ${lib.showOption loc}" def.value
+            def:
+            lib.seq (checkDef loc def)
+            lib.setDefaultModuleLocation "${def.file}, via option ${lib.showOption loc}" def.value
           ) defs;
         };
       };

lib/types/tests.nix

@@ -14,7 +14,7 @@ let
     };
 in
 {
-  test_1 =
+  test_simple =
     let
       eval = evalSettingsModule {
         foo = { };
@@ -38,4 +38,53 @@ in
         ];
       };
     };
+
+  test_no_nested_imports =
+    let
+      eval = evalSettingsModule {
+        foo = {
+          imports = [];
+        };
+      };
+    in
+    {
+      inherit eval;
+      expr = eval.config.foo;
+      expectedError = {
+        type = "ThrownError";
+        message = "*nested imports";
+       };
+    };
+
+  test_no_function_modules =
+    let
+      eval = evalSettingsModule {
+        foo = {...}: {
+
+        };
+      };
+    in
+    {
+      inherit eval;
+      expr = eval.config.foo;
+      expectedError = {
+        type = "TypeError";
+        message = "cannot convert a function to JSON";
+      };
+    };
+
+  test_non_attrs_module =
+    let
+      eval = evalSettingsModule {
+        foo = "foo.nix";
+      };
+    in
+    {
+      inherit eval;
+      expr = eval.config.foo;
+      expectedError = {
+        type = "ThrownError";
+        message = ".*foo.* is not of type";
+      };
+    };
 }