diff -r 6a372f943e49 -r 4277e20cfec4 mercurial/__init__.py
--- a/mercurial/__init__.py	Tue Dec 18 00:54:32 2018 -0500
+++ b/mercurial/__init__.py	Mon Dec 17 15:05:52 2018 +0100
@@ -40,6 +40,10 @@
             # zstd is already dual-version clean, don't try and mangle it
             if fullname.startswith('mercurial.zstd'):
                 return None
+            # rustext is built for the right python version,
+            # don't try and mangle it
+            if fullname.startswith('mercurial.rustext'):
+                return None
             # pywatchman is already dual-version clean, don't try and mangle it
             if fullname.startswith('hgext.fsmonitor.pywatchman'):
                 return None