43 files declare their use of UserInterface

BlockContent.php in core/modules/block_content/src/Entity/BlockContent.php
comment.module in core/modules/comment/comment.module
Enables users to comment on published content.
CommentManager.php in core/modules/comment/src/CommentManager.php
ContentTranslationMetadataWrapper.php in core/modules/content_translation/src/ContentTranslationMetadataWrapper.php
ContentTranslationMetadataWrapperInterface.php in core/modules/content_translation/src/ContentTranslationMetadataWrapperInterface.php
ContentTranslationWorkflowsTest.php in core/modules/content_translation/tests/src/Functional/ContentTranslationWorkflowsTest.php
EntityApiTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityApiTest.php
EntityReferenceFieldTest.php in core/tests/Drupal/KernelTests/Core/Entity/EntityReferenceFieldTest.php
EntityTest.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTest.php
EntityUser.php in core/modules/user/src/Plugin/migrate/destination/EntityUser.php
MigrateUserConfigsTest.php in core/modules/user/tests/src/Kernel/Migrate/d6/MigrateUserConfigsTest.php
MigrateUserSettingsTest.php in core/modules/user/tests/src/Kernel/Migrate/d7/MigrateUserSettingsTest.php
MigrateUserTest.php in core/modules/user/tests/src/Kernel/Migrate/d7/MigrateUserTest.php
RegisterAccessCheck.php in core/modules/user/src/Access/RegisterAccessCheck.php
RestRegisterUserTest.php in core/modules/hal/tests/src/Functional/user/RestRegisterUserTest.php
RevisionableContentEntityBaseTest.php in core/tests/Drupal/KernelTests/Core/Entity/RevisionableContentEntityBaseTest.php
RevisionLogEntityTrait.php in core/lib/Drupal/Core/Entity/RevisionLogEntityTrait.php
RevisionLogInterface.php in core/lib/Drupal/Core/Entity/RevisionLogInterface.php
SiteConfigureForm.php in core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php
TestContent.php in core/modules/system/tests/modules/router_test_directory/src/TestContent.php
TestContextAwareBlock.php in core/modules/block/tests/modules/block_test/src/Plugin/Block/TestContextAwareBlock.php
TestControllers.php in core/modules/system/tests/modules/router_test_directory/src/TestControllers.php
user.api.php in core/modules/user/user.api.php
Hooks provided by the User module.
user.module in core/modules/user/user.module
Enables the user registration and login system.
User.php in core/modules/user/src/Plugin/views/argument_default/User.php
User.php in core/modules/user/src/Entity/User.php
UserAdminTest.php in core/modules/user/tests/src/Functional/UserAdminTest.php
UserAuthenticationController.php in core/modules/user/src/Controller/UserAuthenticationController.php
UserBulkForm.php in core/modules/user/src/Plugin/views/field/UserBulkForm.php
UserController.php in core/modules/user/src/Controller/UserController.php
UserEditedOwnAccountTest.php in core/modules/user/tests/src/Functional/UserEditedOwnAccountTest.php
UserLoginBlock.php in core/modules/user/src/Plugin/Block/UserLoginBlock.php
UserLoginForm.php in core/modules/user/src/Form/UserLoginForm.php
UserLoginTest.php in core/modules/user/tests/src/Functional/UserLoginTest.php
UserMailRequiredValidatorTest.php in core/modules/user/tests/src/Unit/Plugin/Validation/Constraint/UserMailRequiredValidatorTest.php
UserNameConstraintValidator.php in core/modules/user/src/Plugin/Validation/Constraint/UserNameConstraintValidator.php
UserPasswordForm.php in core/modules/user/src/Form/UserPasswordForm.php
UserPasswordResetTest.php in core/modules/user/tests/src/Functional/UserPasswordResetTest.php
UserRegistrationResource.php in core/modules/user/src/Plugin/rest/resource/UserRegistrationResource.php
UserRegistrationResourceTest.php in core/modules/user/tests/src/Unit/UserRegistrationResourceTest.php
UserRegistrationRestTest.php in core/modules/user/tests/src/Functional/UserRegistrationRestTest.php
UserRegistrationTest.php in core/modules/user/tests/src/Functional/UserRegistrationTest.php
UserValidationTest.php in core/modules/user/tests/src/Kernel/UserValidationTest.php

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