move JoinedStrNode cleanup into ConstantFolding transform as that's essentially what it does

Cython/Compiler/Optimize.py

@@ -3,6 +3,7 @@ from __future__ import absolute_import
 import sys
 import copy
 import codecs
+import itertools
 
 from . import TypeSlots
 from .ExprNodes import not_a_constant
@@ -3951,6 +3952,35 @@ class ConstantFolding(Visitor.VisitorTransform, SkipDeclarations):
                 sequence_node.mult_factor = factor
         return sequence_node
 
+    def visit_JoinedStrNode(self, node):
+        """
+        Clean up after the parser by discarding empty Unicode strings and merging
+        substring sequences.  Empty or single-value join lists are not uncommon
+        because f-string format specs are always parsed into JoinedStrNodes.
+        """
+        self.visitchildren(node)
+        unicode_node = ExprNodes.UnicodeNode
+        values = []
+        for is_unode_group, substrings in itertools.groupby(node.values, lambda v: isinstance(v, unicode_node)):
+            if is_unode_group:
+                substrings = list(substrings)
+                unode = substrings[0]
+                if len(substrings) > 1:
+                    unode.value = EncodedString(u''.join(node.value for node in substrings))
+                # ignore empty Unicode strings
+                if unode.value:
+                    values.append(unode)
+            else:
+                values.extend(substrings)
+
+        if not values:
+            node = ExprNodes.UnicodeNode(node.pos, value=EncodedString(''))
+        elif len(values) == 1:
+            node = values[0]
+        else:
+            node.values = values
+        return node
+
     def visit_MergedDictNode(self, node):
         """Unpack **args in place if we can."""
         self.visitchildren(node)

Cython/Compiler/ParseTreeTransforms.py

@@ -7,7 +7,6 @@ cython.declare(PyrexTypes=object, Naming=object, ExprNodes=object, Nodes=object,
                error=object, warning=object, copy=object, _unicode=object)
 
 import copy
-import itertools
 
 from . import PyrexTypes
 from . import Naming
@@ -349,35 +348,6 @@ class PostParse(ScopeTrackingTransform):
         self.visitchildren(node)
         return node
 
-    def visit_JoinedStrNode(self, node):
-        """
-        Clean up after the parser by discarding empty Unicode strings and merging
-        substring sequences.  Empty or single-value join lists are not uncommon
-        because f-string format specs are always parsed into JoinedStrNodes.
-        """
-        self.visitchildren(node)
-        unicode_node = ExprNodes.UnicodeNode
-        values = []
-        for is_unode_group, substrings in itertools.groupby(node.values, lambda v: isinstance(v, unicode_node)):
-            if is_unode_group:
-                substrings = list(substrings)
-                unode = substrings[0]
-                if len(substrings) > 1:
-                    unode.value = EncodedString(u''.join(node.value for node in substrings))
-                # ignore empty Unicode strings
-                if unode.value:
-                    values.append(unode)
-            else:
-                values.extend(substrings)
-
-        if not values:
-            node = ExprNodes.UnicodeNode(node.pos, value=EncodedString(''))
-        elif len(values) == 1:
-            node = values[0]
-        else:
-            node.values = values
-        return node
-
 
 def eliminate_rhs_duplicates(expr_list_list, ref_node_sequence):
     """Replace rhs items by LetRefNodes if they appear more than once.