manager-bundle/src/HttpKernel/ContaoKernel.php
public function getRootDir(): string
    {
        if (null === $this->rootDir) {
            $this->rootDir = $this->getProjectDir().'/app';
        }

        return $this->rootDir;
    }
public function getRootDir(): string
    {
        if (null === $this->rootDir) {
            $this->rootDir = $this->getProjectDir().'/app';
        }

        return $this->rootDir;
    }
{@inheritdoc}

@deprecated since Symfony 4.2, use getProjectDir() instead