feat: Add traits4.rs exercise

exercises/traits/traits4.rs

@@ -0,0 +1,44 @@
+// traits4.rs
+//
+// Your task is to replace the '??' sections so the code compiles.
+// Don't change any line other than 21.
+
+// I AM NOT DONE
+
+pub trait Licensed {
+    fn licensing_info(&self) -> String {
+        "some information".to_string()
+    }
+}
+
+struct SomeSoftware {}
+
+struct OtherSoftware {}
+
+impl Licensed for SomeSoftware {}
+impl Licensed for OtherSoftware {}
+
+fn compare_license_types(software: ??, software_two: ??) -> bool {
+    software.licensing_info() == software_two.licensing_info()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn compare_license_information() {
+        let some_software = SomeSoftware {};
+        let other_software = OtherSoftware {};
+
+        assert!(compare_license_types(some_software, other_software));
+    }
+
+    #[test]
+    fn compare_license_information_backwards() {
+        let some_software = SomeSoftware {};
+        let other_software = OtherSoftware {};
+
+        assert!(compare_license_types(other_software, some_software));
+    }
+}