pub fn directly_depends_on<'a>(
        &self,
        feature_a: impl Into<FeatureId<'a>>,
        feature_b: impl Into<FeatureId<'a>>,
    ) -> Result<bool, Error> {
        let feature_a = feature_a.into();
        let feature_b = feature_b.into();
        let a_ix = self.feature_ix(feature_a)?;
        let b_ix = self.feature_ix(feature_b)?;
        Ok(self.dep_graph().contains_edge(a_ix, b_ix))
    }
Returns true if `feature_a` directly depends on `feature_b`.

In other words, this returns true if `feature_a` is a direct dependency of `feature_b`.

This returns false if `feature_a` is the same as `feature_b`.
guppy/src/graph/feature/graph_impl.rs