--- a/tests/run-tests.py	Sun Jul 11 15:41:18 2021 +0200
+++ b/tests/run-tests.py	Sun Jul 11 16:09:38 2021 +0200
@@ -3990,6 +3990,15 @@
 
 
 if __name__ == '__main__':
+    if WINDOWS and not os.getenv('MSYSTEM'):
+        print('cannot run test on Windows without MSYSTEM', file=sys.stderr)
+        print(
+            '(if you need to do so contact the mercurial devs: '
+            'mercurial@mercurial-scm.org)',
+            file=sys.stderr,
+        )
+        sys.exit(255)
+
     runner = TestRunner()
 
     try: