lib/modules: Throw earlier when module function does not return attrs

`m` must always be an attrset at this point. It is basically always
evaluated. This will make it throw when any of the attrs is accessed,
rather than just `config`. We assume that this will improve the error
message in more scenarios.

lib/modules.nix

@@ -462,14 +462,13 @@ rec {
           config = addFreeformType (addMeta (m.config or {}));
         }
     else
+      lib.throwIfNot (isAttrs m) "module ${file} (${key}) does not look like a module."
       { _file = toString m._file or file;
         key = toString m.key or key;
         disabledModules = m.disabledModules or [];
         imports = m.require or [] ++ m.imports or [];
         options = {};
-        config =
-          lib.throwIfNot (isAttrs m) "module ${file} (${key}) does not look like a module."
-          addFreeformType (addMeta (removeAttrs m ["_file" "key" "disabledModules" "require" "imports" "freeformType"]));
+        config = addFreeformType (addMeta (removeAttrs m ["_file" "key" "disabledModules" "require" "imports" "freeformType"]));
       };
 
   applyModuleArgsIfFunction = key: f: args@{ config, options, lib, ... }: if isFunction f then

lib/tests/modules.sh

@@ -199,6 +199,7 @@ checkConfigOutput '^true$' config.submodule.enable ./declare-submoduleWith-path.
 checkConfigOutput '"beta"' config.nodes.foo.settingsDict.c ./deferred-module.nix
 # errors from the default module are reported with accurate location
 checkConfigError 'In `the-file-that-contains-the-bad-config.nix, via option default'\'': "bogus"' config.nodes.foo.bottom ./deferred-module.nix
+checkConfigError '.*lib/tests/modules/deferred-module-error.nix, via option deferred [(]:anon-1:anon-1:anon-1[)] does not look like a module.' config.result ./deferred-module-error.nix
 
 # Check the file location information is propagated into submodules
 checkConfigOutput the-file.nix config.submodule.internalFiles.0 ./submoduleFiles.nix

lib/tests/modules/deferred-module-error.nix

@@ -0,0 +1,20 @@
+{ config, lib, ... }:
+let
+  inherit (lib) types mkOption setDefaultModuleLocation evalModules;
+  inherit (types) deferredModule lazyAttrsOf submodule str raw enum;
+in
+{
+  options = {
+    deferred = mkOption {
+      type = deferredModule;
+    };
+    result = mkOption {
+      default = (evalModules { modules = [ config.deferred ]; }).config.result;
+    };
+  };
+  config = {
+    deferred = { ... }:
+      # this should be an attrset, so this fails
+      true;
+  };
+}