pub fn optional_deps(&self) -> impl Iterator<Item = &'g str> + '_ {
        // XXX: binary search?
        self.labels.iter().filter_map(|label| match label {
            FeatureLabel::OptionalDependency(dep_name) => Some(*dep_name),
            _ => None,
        })
    }
Returns an iterator containing all optional dependencies.

The iterator is guaranteed to be sorted and unique.
guppy/src/graph/feature/feature_list.rs