Auto merge of #19586 - tigercosmos:x1, r=jdm

let mutation testing display diff if compilation fails

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let mutation testing display diff if compilation fails

r? @jdm

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #19566 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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python/servo/mutation/test.py

@@ -42,8 +42,11 @@ def mutation_test(file_name, tests):
             if mutated_line != -1:
                 logging.info("Mutated {0} at line {1}".format(file_name, mutated_line))
                 logging.info("compiling mutant {0}:{1}".format(file_name, mutated_line))
-                if subprocess.call('python mach build --release', shell=True, stdout=DEVNULL):
+                test_command = "python mach build --release"
+                if subprocess.call(test_command, shell=True, stdout=DEVNULL):
                     logging.error("Compilation Failed: Unexpected error")
+                    logging.error("Failed: while running `{0}`".format(test_command))
+                    subprocess.call('git --no-pager diff {0}'.format(file_name), shell=True)
                     status = Status.UNEXPECTED
                 else:
                     for test in tests: