pub fn is_default_feature<'a>(
        &self,
        feature_id: impl Into<FeatureId<'a>>,
    ) -> Result<bool, Error> {
        let feature_id = feature_id.into();
        let default_ix = self.feature_ix(
            self.package_graph
                .metadata(feature_id.package_id())?
                .default_feature_id(),
        )?;
        let feature_ix = self.feature_ix(feature_id)?;
        // Do not follow conditional links.
        Ok(self.feature_ix_depends_on_no_conditional(default_ix, feature_ix))
    }
Returns true if this feature is included in a package's build by default.

Returns an error if this feature ID is unknown.

## Cycles

A cyclic dev-dependency may cause additional features to be turned on. This computation
does *not* follow conditional links and will *not* return true for such additional
features.
guppy/src/graph/feature/graph_impl.rs