Function compose_tools_path 

pub fn compose_tools_path(
    existing: &str,
    bundled: Option<&Path>,
    user: Option<&Path>,
    user_precedence: UserToolsPrecedence,
) -> Option<String>

existing with the given tool dirs added (bundled first, user per user_precedence). None when neither dir is given, so the caller can leave PATH untouched.