use Symfony\Component\Security\Core\User\UserInterface;

class User implements UserInterface
{
    /** @var non-empty-string */
    private string $username;

    public function setUsername(string $username): self
    {
        $this->username = mb_convert_case($username, MB_CASE_LOWER, mb_detect_encoding($username); // should return non-empty-string but returns string.

        return $this;
    }

    /**
     * @return non-empty-string
     */
    public function getUserIdentifier(): string
    {
        return $this->username;
    }
}

use Symfony\Component\Security\Core\User\UserInterface;
use Symfony\Component\Uid\Uuid;

class User implements UserInterface
{
    /** @var non-empty-string */
    private string $hash;

    public function __construct()
    {
        $this->hash = Uuid::v4()->toRfc4122(); // $hash (non-empty-string) does not accept string.
    }

    /**
     * @return non-empty-string
     */
    public function getUserIdentifier(): string
    {
        return $this->hash;
    }
}