make assert_stderr_contains print its contents on panic

Author: Oneirical

src/tools/run-make-support/src/command.rs

@@ -127,14 +127,14 @@ impl CompletedProcess {
 
     #[track_caller]
     pub fn assert_stderr_contains<S: AsRef<str>>(self, needle: S) -> Self {
+        let needle_str = needle.as_ref();
+        let stderr_str = self.stderr_utf8().as_str();
+
         assert!(
-            self.stderr_utf8().contains(needle.as_ref()),
-            format!(
-                "The stderr {} did not contain the string {}",
-                self.stderr_utf8().as_str(),
-                needle.as_ref(),
-            )
+            self.stderr_utf8().contains(needle_str),
+            format!("The stderr \"{}\" did not contain the string \"{}\"", stderr_str, needle_str)
         );
+
         self
     }