make assert_stderr_contains print its contents on panic

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

@@ -163,7 +163,16 @@ impl CompletedProcess {
     /// Checks that trimmed `stdout` matches trimmed `content`.
     #[track_caller]
     pub fn assert_stdout_equals<S: AsRef<str>>(&self, content: S) -> &Self {
-        assert_eq!(self.stdout_utf8().trim(), content.as_ref().trim());
+        assert_eq!(
+            self.stdout_utf8().trim(),
+            content.as_ref().trim(),
+            "{}",
+            format!(
+                "The stdout \"{}\" did not equal the string \"{}\"",
+                self.stdout_utf8().as_str(),
+                content.as_ref().trim()
+            )
+        );
         self
     }
 
@@ -176,18 +185,28 @@ impl CompletedProcess {
     /// Checks that trimmed `stderr` matches trimmed `content`.
     #[track_caller]
     pub fn assert_stderr_equals<S: AsRef<str>>(&self, content: S) -> &Self {
-        assert_eq!(self.stderr_utf8().trim(), content.as_ref().trim());
+        assert_eq!(
+            self.stderr_utf8().trim(),
+            content.as_ref().trim(),
+            "{}",
+            format!(
+                "The stderr \"{}\" did not equal the string \"{}\"",
+                self.stderr_utf8().as_str(),
+                content.as_ref().trim()
+            )
+        );
         self
     }
 
     #[track_caller]
     pub fn assert_stderr_contains<S: AsRef<str>>(&self, needle: S) -> &Self {
         assert!(
             self.stderr_utf8().contains(needle.as_ref()),
+            "{}",
             format!(
-                "The stderr {} did not contain the string {}",
+                "The stderr \"{}\" did not contain the string \"{}\"",
                 self.stderr_utf8().as_str(),
-                needle.as_ref(),
+                needle.as_ref()
             )
         );
         self