Function prebuild 

pub(crate) fn prebuild(
    args: &PrebuildArgs,
) -> Vec<(PathBuf, GeopResult<Counts>)>

Builds args.programs, args.jobs at once, each on a thread of its own in a kernel of its own — a kernel shares nothing with another thread — keeping what each builds beside its file (build): the cache files are what the threads share. Says, of each, what its build did, or why it failed.