workaround bug in macro expansion.

Author: pnkfelix

src/test/run-fail/explicit-panic-msg.rs

@@ -15,5 +15,6 @@
 fn main() {
     let mut a = 1;
     if 1 == 1 { a = 2; }
-    panic!(format!("woooo{}", "o"));
+    let msg = format!("woooo{}", "o");
+    panic!(msg);
 }