Disallow memory views as struct members since structs do not allow safe buffer...

Disallow memory views as struct members since structs do not allow safe buffer reference management.

Cython/Compiler/ExprNodes.py

@@ -9170,6 +9170,7 @@ class PyCFunctionNode(ExprNode, ModuleNameMixin):
                 if not arg.default.is_literal:
                     arg.is_dynamic = True
                     if arg.type.is_pyobject:
+                        # FIXME: should we include memory views here?
                         nonliteral_objects.append(arg)
                     else:
                         nonliteral_other.append(arg)

Cython/Compiler/Symtab.py

@@ -1874,11 +1874,12 @@ class StructOrUnionScope(Scope):
         entry.is_variable = 1
         self.var_entries.append(entry)
         if type.is_pyobject and not allow_pyobject:
-            error(pos,
-                  "C struct/union member cannot be a Python object")
+            error(pos, "C struct/union member cannot be a Python object")
+        elif type.is_memoryviewslice and not allow_pyobject:
+            # Memory views wrap their buffer owner as a Python object.
+            error(pos, "C struct/union member cannot be a memory view")
         if visibility != 'private':
-            error(pos,
-                  "C struct/union member cannot be declared %s" % visibility)
+            error(pos, "C struct/union member cannot be declared %s" % visibility)
         return entry
 
     def declare_cfunction(self, name, type, pos,