pub fn default_feature_id(&self) -> FeatureId<'g> {
        if self.inner.has_default_feature {
            FeatureId::new(self.id(), FeatureLabel::Named("default"))
        } else {
            FeatureId::base(self.id())
        }
    }
Returns the `FeatureId` corresponding to the default feature.
guppy/src/graph/graph_impl.rs