6 files declare their use of StringInterface in Zircon Profile 8.0

ConfigSchemaTest.php in core/modules/config/src/Tests/ConfigSchemaTest.php

Contains \Drupal\config\Tests\ConfigSchemaTest.

Email.php in core/lib/Drupal/Core/TypedData/Plugin/DataType/Email.php

Contains \Drupal\Core\TypedData\Plugin\DataType\Email.

EntityFieldTest.php in core/modules/system/src/Tests/Entity/EntityFieldTest.php

Contains \Drupal\system\Tests\Entity\EntityFieldTest.

PrimitiveTypeConstraintValidator.php in core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/PrimitiveTypeConstraintValidator.php

Contains \Drupal\Core\Validation\Plugin\Validation\Constraint\PrimitiveTypeConstraintValidator.

SchemaCheckTrait.php in core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php

Contains \Drupal\Core\Config\Schema\SchemaCheckTrait.

StringData.php in core/lib/Drupal/Core/TypedData/Plugin/DataType/StringData.php

Contains \Drupal\Core\TypedData\Plugin\DataType\StringData.