pub fn all_features_for(&self, package_id: &PackageId) -> Result<FeatureList<'g>, Error> {
        let package = self.package_graph.metadata(package_id)?;
        let dep_graph = self.dep_graph();
        let features = self
            .feature_ixs_for_package_ix(package.package_ix())
            .map(|feature_ix| FeatureId::node_to_feature(package, &dep_graph[feature_ix]));
        Ok(FeatureList::new(package, features))
    }
Returns all known features for a package.

Returns an error if the package ID was unknown.
guppy/src/graph/feature/graph_impl.rs