finish until lifetimes

solutions/16_lifetimes/lifetimes1.rs

@@ -1,4 +1,24 @@
-fn main() {
-    // DON'T EDIT THIS SOLUTION FILE!
-    // It will be automatically filled after you finish the exercise.
+// The Rust compiler needs to know how to check whether supplied references are
+// valid, so that it can let the programmer know if a reference is at risk of
+// going out of scope before it is used. Remember, references are borrows and do
+// not own their own data. What if their owner goes out of scope?
+
+fn longest<'a>(x: &'a str, y: &'a str) -> &'a str {
+    //    ^^^^     ^^          ^^          ^^
+    if x.len() > y.len() { x } else { y }
+}
+
+fn main() {
+    // You can optionally experiment here.
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_longest() {
+        assert_eq!(longest("abcd", "123"), "abcd");
+        assert_eq!(longest("abc", "1234"), "1234");
+    }
 }