Function index_root 

pub(super) fn index_root(cache_dir: &Path) -> PathBuf

cacache’s on-disk index root inside a cache directory.

cacache::list_sync reports a cache nothing has ever been written to as a NotFound walk error rather than as an empty iterator (its own list_sync test asserts exactly that), so telling “empty” apart from “unreadable” means knowing whether this directory exists. cacache does not expose the path, so the layout is mirrored here – INDEX_VERSION is 5 in cacache 13.1.0, the same way cache_tests::corrupt_stored_content mirrors the content layout. index_root_matches_the_cacache_layout fails if a cacache bump moves it.