UniqueFieldConstraint.php

<?php

namespace Drupal\Core\Validation\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * Checks if an entity field has a unique value.
 */
class UniqueFieldConstraint extends SymfonyConstraint {
    public $message = 'A @entity_type with @field_name %value already exists.';
    
    /**
     * Returns the name of the class that validates this constraint.
     *
     * @return string
     */
    public function validatedBy() {
        return '\\Drupal\\Core\\Validation\\Plugin\\Validation\\Constraint\\UniqueFieldValueValidator';
    }

}