direct_deps.rs

        // This is a map of direct deps by name -> version -> packages that depend on it.
        let mut direct_deps: BTreeMap<&str, BTreeMap<&Version, Vec<&str>>> = BTreeMap::new();
        package_graph.query_workspace().resolve_with_fn(|_, link| {
            // Collect direct dependencies of workspace packages.
            let (from, to) = link.endpoints();
            if from.in_workspace() && !to.in_workspace() {
                direct_deps
                    .entry(to.name())
                    .or_default()
                    .entry(to.version())
                    .or_default()
                    .push(from.name());
            }
            // query_workspace + preventing further traversals will mean that only direct
            // dependencies are considered.
            false
        });
        for (direct_dep, versions) in direct_deps {
            if versions.len() > 1 {
                let mut msg = format!("duplicate direct dependency '{}':\n", direct_dep);
                for (version, packages) in versions {
                    msg.push_str(&format!("  * {} (", version));
                    msg.push_str(&packages.join(", "));
                    msg.push_str(")\n");
                }
                out.write(LintLevel::Error, msg);
            }
        }