Added unit test

src/query/scorer.rs

@@ -51,7 +51,8 @@ impl DocSet for EmptyScorer {
     }
 
     fn doc(&self) -> DocId {
-        DocId::max_value()
+        panic!("You may not call .doc() on a scorer \
+        where the last call to advance() did not return true.");
     }
 
     fn size_hint(&self) -> u32 {
@@ -122,3 +123,21 @@ impl<TDocSet: DocSet + 'static> Scorer for ConstScorer<TDocSet> {
         1f32
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::EmptyScorer;
+    use DocSet;
+
+    #[test]
+    fn test_empty_scorer() {
+        let mut empty_scorer = EmptyScorer;
+        assert!(!empty_scorer.advance());
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_empty_scorer_panic_on_doc_call() {
+        EmptyScorer.doc();
+    }
+}