Some unit tests for the RelativeIndexable methods

src/characters.rs

@@ -117,3 +117,38 @@ impl RelativeIndexable for str {
         index
     }
 }
+
+#[test]
+fn index_before_is_well_behaved_for_ascii() {
+    let s = "abc";
+
+    // Sanity
+    assert_eq!(s.index_before(0), 0);
+    assert_eq!(s.index_before(2), 1);
+
+    // An index beyond the string bounds returns the index of the last character in the string.
+    {
+        let idx = s.index_before(4);
+        assert_eq!(idx, 2);
+        assert!(s.is_char_boundary(idx));
+        let last_char = &s[idx ..];
+        assert_eq!(last_char.len(), 1);
+        assert_eq!(last_char.chars().nth(0), Some('c'));
+    }
+}
+
+#[test]
+fn index_after_is_well_behaved_for_ascii() {
+    let s = "abc";
+
+    // Sanity
+    assert_eq!(s.index_after(0), 1);
+    assert_eq!(s.index_after(2), 3);
+
+    // An index beyond the string bounds returns the length of the string
+    {
+        let idx = s.index_after(4);
+        assert_eq!(idx, s.len());
+        assert!(s.is_char_boundary(idx));
+    }
+}