40 files declare their use of Extension in Drupal 9

advisory_feed_test.module in core/modules/system/tests/modules/advisory_feed_test/advisory_feed_test.module

Module for testing the display of security advisories.

config_translation_test.module in core/modules/config_translation/tests/modules/config_translation_test/config_translation_test.module

Configuration Translation Test module.

DrupalKernel.php in core/lib/Drupal/Core/DrupalKernel.php

ExtensionDiscoveryTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionDiscoveryTest.php

ExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionListTest.php

ExtensionSerializationTest.php in core/tests/Drupal/Tests/Core/Extension/ExtensionSerializationTest.php

FieldDefinitionIntegrityTest.php in core/modules/field/tests/src/Kernel/FieldDefinitionIntegrityTest.php

GenerateTheme.php in core/lib/Drupal/Core/Command/GenerateTheme.php

InstallerNonDefaultDatabaseDriverTest.php in core/tests/Drupal/FunctionalTests/Installer/InstallerNonDefaultDatabaseDriverTest.php

InstallStorage.php in core/lib/Drupal/Core/Config/InstallStorage.php

LayoutPluginManagerTest.php in core/tests/Drupal/Tests/Core/Layout/LayoutPluginManagerTest.php

LinkRelationTypeManager.php in core/lib/Drupal/Core/Http/LinkRelationTypeManager.php

locale_test.module in core/modules/locale/tests/modules/locale_test/locale_test.module

Simulate a custom module with a local po file.

locale_test_development_release.module in core/modules/locale/tests/modules/locale_test_development_release/locale_test_development_release.module

Simulate a Drupal version.

locale_test_translate.module in core/modules/locale/tests/modules/locale_test_translate/locale_test_translate.module

Simulates a custom module with a local po file.

ModulesListForm.php in core/modules/system/src/Form/ModulesListForm.php

module_required_test.module in core/modules/system/tests/modules/module_required_test/module_required_test.module

Test module.

module_test.module in core/modules/system/tests/modules/module_test/module_test.module

Test module.

nyan_cat.engine in core/modules/system/tests/themes/engines/nyan_cat/nyan_cat.engine

Handles integration of Nyan cat templates because we love kittens.

PermissionHandlerTest.php in core/modules/user/tests/src/Unit/PermissionHandlerTest.php

Contains \Drupal\Tests\user\Unit\PermissionHandlerTest.

ProjectInfo.php in core/lib/Drupal/Core/Utility/ProjectInfo.php

ResourceResponseValidatorTest.php in core/modules/jsonapi/tests/src/Unit/EventSubscriber/ResourceResponseValidatorTest.php

SecurityAdvisoriesFetcherTest.php in core/modules/system/tests/src/Kernel/SecurityAdvisories/SecurityAdvisoriesFetcherTest.php

system.module in core/modules/system/system.module

Configuration system that lets administrators modify the workings of the site.

SystemLocalTasksTest.php in core/modules/system/tests/src/Unit/Menu/SystemLocalTasksTest.php

system_test.module in core/modules/system/tests/modules/system_test/system_test.module

Test module.

TestDiscoveryTest.php in core/tests/Drupal/Tests/Core/Test/TestDiscoveryTest.php

TestRunnerKernel.php in core/lib/Drupal/Core/Test/TestRunnerKernel.php

TestRunnerKernelTest.php in core/tests/Drupal/Tests/Core/Test/TestRunnerKernelTest.php

test_module_required_by_theme.module in core/modules/system/tests/themes/test_theme_depending_on_modules/test_module_required_by_theme/test_module_required_by_theme.module

This file provides testing functionality for update.php.

ThemeExtensionListTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeExtensionListTest.php

ThemeHandlerTest.php in core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php

Contains \Drupal\Tests\Core\Extension\ThemeHandlerTest.

ThemeInitialization.php in core/lib/Drupal/Core/Theme/ThemeInitialization.php

ThemeInitializationInterface.php in core/lib/Drupal/Core/Theme/ThemeInitializationInterface.php

theme_page_test.module in core/modules/system/tests/modules/theme_page_test/theme_page_test.module

Test module.

theme_test.module in core/modules/system/tests/modules/theme_test/theme_test.module

Test module.

twig.engine in core/themes/engines/twig/twig.engine

Handles integration of Twig templates with the Drupal theme system.

UpdateRegistry.php in core/lib/Drupal/Core/Update/UpdateRegistry.php

update_script_test.module in core/modules/system/tests/modules/update_script_test/update_script_test.module

This file provides testing functionality for update.php.

update_test.module in core/modules/update/tests/modules/update_test/update_test.module

Module for testing Update Manager functionality.