DO NOT MERGE: This fails to compile

This is meant to demonstrate why `&'static str` of #239 is very
restrictive.

Author: petertseng

exercises/parallel-letter-frequency/tests/parallel-letter-frequency.rs

@@ -40,6 +40,13 @@ fn test_no_texts() {
     assert_eq!(frequency::frequency(&[], 4), HashMap::new());
 }
 
+#[test]
+#[ignore]
+fn test_hello_world() {
+    let hi = format!("Hello, {}!", "world");
+    frequency::frequency(&[&hi], 4);
+}
+
 #[test]
 #[ignore]
 fn test_one_letter() {