Function create_index 

pub fn create_index(
    env: &BuildEnvironment,
    rel_uri: &dyn Fn(&str) -> Option<String>,
    messages: &mut Vec<IndexMessage>,
) -> Vec<IndexGroup>

IndexEntries.create_index(builder, group_entries=True) (environment/adapters/indexentries.py:54-197).

rel_uri(docname) is builder.get_relative_uri('genindex', docname); None is Sphinx’s NoUri, which keeps the entry but drops its link.