Function resolve_root 

pub fn resolve_root(
    config: &CacheConfig,
    runner_root: Option<&Path>,
    fallback: Option<PathBuf>,
) -> Result<ResolvedRoot, String>

The cache root for a host whose effective runner root is runner_root.

fallback is used when <runner root>/_cache would contain whitespace (the macOS platform-default runner root does: Application Support); see platform_fallback_root. None when nothing usable resolves, with the reason.

Errors

The sentence an operator reads about why there is no cache root.