drop redundant specialisation code from for-in loop if we know it's not a list or tuple

Cython/Compiler/ExprNodes.py

@@ -1519,14 +1519,16 @@ class IteratorNode(ExprNode):
     def generate_result_code(self, code):
         is_builtin_sequence = self.sequence.type is list_type or \
                               self.sequence.type is tuple_type
+        may_be_a_sequence = is_builtin_sequence or not self.sequence.type.is_builtin_type
         if is_builtin_sequence:
             code.putln(
                 "if (likely(%s != Py_None)) {" % self.sequence.py_result())
-        else:
+        elif may_be_a_sequence:
             code.putln(
                 "if (PyList_CheckExact(%s) || PyTuple_CheckExact(%s)) {" % (
                     self.sequence.py_result(),
                     self.sequence.py_result()))
+        if may_be_a_sequence:
             code.putln(
                 "%s = 0; %s = %s; __Pyx_INCREF(%s);" % (
                     self.counter_cname,
@@ -1545,6 +1547,7 @@ class IteratorNode(ExprNode):
                     self.sequence.py_result(),
                     code.error_goto_if_null(self.result(), self.pos)))
             code.put_gotref(self.py_result())
+        if may_be_a_sequence:
             code.putln("}")
 
 
@@ -1564,12 +1567,15 @@ class NextNode(AtomicExprNode):
         self.is_temp = 1
     
     def generate_result_code(self, code):
-        if self.iterator.sequence.type is list_type:
+        sequence_type = self.iterator.sequence.type
+        if sequence_type is list_type:
             type_checks = [(list_type, "List")]
-        elif self.iterator.sequence.type is tuple_type:
+        elif sequence_type is tuple_type:
             type_checks = [(tuple_type, "Tuple")]
-        else:
+        elif not sequence_type.is_builtin_type:
             type_checks = [(list_type, "List"), (tuple_type, "Tuple")]
+        else:
+            type_checks = []
 
         for py_type, prefix in type_checks:
             if len(type_checks) > 1: