Add exercise traits6 practicing extension traits

exercises/15_traits/traits6.rs

@@ -0,0 +1,33 @@
+// TODO: Design a trait that enables you to calculate the digit sum over both an
+// integer as well as a string.
+
+// TODO: Change this function signature so it works with both integers and
+// strings, based on your trait.
+fn calculate_digit_sum(number: ???) -> usize {
+    // TODO: Calculate the digit sum of `number`.
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn integer() {
+        assert_eq!(calculate_digit_sum(0), 1);
+        assert_eq!(calculate_digit_sum(1), 1);
+        assert_eq!(calculate_digit_sum(1235), 11);
+        assert_eq!(calculate_digit_sum(789), 24);
+    }
+
+    #[test]
+    fn string() {
+        assert_eq!(calculate_digit_sum(""), 1);
+        assert_eq!(calculate_digit_sum("1"), 1);
+        assert_eq!(calculate_digit_sum("1235"), 11);
+        assert_eq!(calculate_digit_sum("789"), 24);
+    }
+}