diff -r 8a137ef6e5da -r a04f5c651e52 mercurial/policy.py
--- a/mercurial/policy.py	Thu May 11 14:52:02 2017 -0700
+++ b/mercurial/policy.py	Wed Apr 26 23:02:43 2017 +0900
@@ -18,8 +18,10 @@
 #    cffi-allow - allow pure Python implementation if cffi version is missing
 #    py - only load pure Python modules
 #
-# By default, require the C extensions for performance reasons.
-policy = b'c'
+# By default, fall back to the pure modules so the in-place build can
+# run without recompiling the C extensions. This will be overridden by
+# __modulepolicy__ generated by setup.py.
+policy = b'allow'
 policynoc = (b'cffi', b'cffi-allow', b'py')
 policynocffi = (b'c', b'py')