/usr/lib/systemd/user-environment-generators/30-systemd-environment-d-generator
systemd-environment-d-generator is a systemd.environment-generator(7) that reads environment configuration specified by environment.d(5) configuration files and passes it to the systemd(1) user manager instance.
systemd(1), systemctl(1), systemd.environment-generator(7), systemd.generator(7)