address with_target_names review comment

src/dataset/impl_dataset.rs

@@ -173,14 +173,8 @@ impl<L, R: Records, T: AsTargets<Elem = L>> DatasetBase<R, T> {
     /// Returns target names
     ///
     /// A target name gives a human-readable string describing the purpose of a single target.
-    pub fn target_names(&self) -> Vec<String> {
-        if !self.target_names.is_empty() {
-            self.target_names.clone()
-        } else {
-            (0..self.ntargets())
-                .map(|idx| format!("class-{}", idx))
-                .collect()
-        }
+    pub fn target_names(&self) -> &[String] {
+        &self.target_names
     }
 
     /// Return the number of targets in the dataset

src/dataset/mod.rs

@@ -342,6 +342,12 @@ mod tests {
         assert_eq!(dataset.target_names, vec!["test"]);
     }
 
+    #[test]
+    fn empty_target_name() {
+        let dataset = Dataset::new(array![[1., 2.], [1., 2.]], array![[0., 1.], [2., 3.]]);
+        assert_eq!(dataset.target_names, Vec::<String>::new());
+    }
+
     #[test]
     fn dataset_implements_required_methods() {
         let mut rng = SmallRng::seed_from_u64(42);