fn make_named_feature_cross_edge(
        link: &PackageLink<'_>,
        weak_index: Option<WeakIndex>,
    ) -> FeatureEdge {
        // This edge is enabled if the feature is enabled, which means the union of (required,
        // optional) build conditions.
        FeatureEdge::NamedFeatureWithSlash {
            link: Self::make_full_conditional_link_impl(link),
            weak_index,
        }
    }
Creates the cross link for situations like:

```toml
[features]
a = ["dep/foo"]
```

(a link (`from`, `a`) to (`dep`, `foo`) is created.

If `dep` is optional, the edge (`from`, `a`) to (`from`, `dep`) is also a
`NamedFeatureWithSlash` edge.
guppy/src/graph/feature/build.rs