more review stuff

src/backend/actual/rust.rs

@@ -31,6 +31,10 @@ impl Backend for Rust {
         self.get_all_installed_packages()
             .context("getting all installed packages")
     }
+
+    fn supports_assigning_packages_as_dependency(&self) -> bool {
+        false
+    }
 }
 
 fn extract_packages(json: Value) -> Result<HashSet<Package>> {