policy: relax the default for in-place build

We're going to make the 'c' policy more strict, where no missing attribute
will be allowed. Since we want to run 'hg bisect' without rebuilding the C
extension modules, we'll need a looser policy for development environment.

The default for system installation isn't changed.

Note that the current 'c' policy is practically 'allow'-ish as we have lots
of adhoc fallbacks to pure functions.

mercurial/policy.py

@@ -18,8 +18,10 import sys
 #    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')
 

setup.py

@@ -372,6 +372,9 class hgbuildpy(build_py):
     def run(self):
         if self.distribution.pure:
             modulepolicy = 'py'
+        elif self.build_lib == '.':
+            # in-place build should run without rebuilding C extensions
+            modulepolicy = 'allow'
         else:
             modulepolicy = 'c'
         with open("mercurial/__modulepolicy__.py", "w") as f: