Function check_engine_dirs 

pub fn check_engine_dirs(
    data_dir: Option<&Path>,
    env: &[(String, String)],
) -> Result<()>

Engine launches have no workspace: judge the data directory and the environment values that name data, home, profile or config directories.

This judges only what it is given. A launch that names no directory passes it, so engine launches go through check_engine_launch, which also refuses a launch with no isolated data directory.