Function suggest 

pub fn suggest(
    base: &str,
    discovered_exts: &BTreeSet<String>,
    handled_exts: &BTreeSet<String>,
    plugins_dir: &Path,
) -> Result<()>

treetags --suggest-plugins — list uninstalled plugins from the index that could handle files in the tree. Covers both file types nothing handles today (the plugin adds support) and natively-supported types (the plugin is an alternative/enhanced take), labelling each so the distinction is clear.

discovered_exts is every file extension seen; handled_exts (a subset) are those the current registry already resolves via a builtin, grammar, or installed plugin.