Function dir 

pub fn dir() -> PathBuf

The cache directory, from the environment.

RUCC_CACHE_DIR first, because somebody who set it meant it. Then the platform’s own place for a cache that a user can delete without losing anything: XDG_CACHE_HOME or ~/.cache on a Unix, LOCALAPPDATA on Windows, which is where a Windows program is expected to put this and not where XDG_CACHE_HOME would put it.

A temporary directory is the last answer rather than a failure. A machine with no home directory is a build container, and a build container that cannot link because nothing set HOME is worse than one that downloads a sysroot again on its next run.