Add a check to make sure users don't request custom __builtins__

We don't support that, and hopefully we don't have to.  CPython seems
pretty inconsistent about it and PyPy doesn't support it.

src/codegen/irgen/hooks.cpp

@@ -561,8 +561,14 @@ static void pickGlobalsAndLocals(Box*& globals, Box*& locals) {
         Box* globals_dict = globals;
         if (globals->cls == module_cls)
             globals_dict = globals->getAttrWrapper();
-        if (PyDict_GetItemString(globals_dict, "__builtins__") == NULL)
+
+        auto requested_builtins = PyDict_GetItemString(globals_dict, "__builtins__");
+        if (requested_builtins == NULL)
             PyDict_SetItemString(globals_dict, "__builtins__", builtins_module);
+        else
+            RELEASE_ASSERT(requested_builtins == builtins_module
+                               || requested_builtins == builtins_module->getAttrWrapper(),
+                           "we don't support overriding __builtins__");
     }
 }
 

test/tests/builtins.py

@@ -129,3 +129,11 @@ print format(5.011111111111, '+.6')
 print format("abc", '')
 
 print '{n}'.format(n=None)
+
+# Thankfully, setting __builtins__ has no effect:
+__builtins__ = {'zzz': 2}
+try:
+    print zzz
+    assert 0
+except NameError as e:
+    print "caught NameError"