fstrings: implement concatenation using PyUnicode_Join

Using _PyUnicode_Writer might be faster but seems more complicated to implement.

Cython/Compiler/ExprNodes.py

@@ -2886,16 +2886,34 @@ class JoinedStrNode(ExprNode):
         self.values = [v.analyse_types(env) for v in self.values]
         self.values = [v.coerce_to_pyobject(env) for v in self.values]
         self.is_temp = 1
-        # TODO make this actually work
-        return self.values[1] if len(self.values) > 1 else self.values[0]
+        if len(self.values) == 1:
+            # this is not uncommon because f-string format specs are parsed into JoinedStrNodes
+            return self.values[0]
+        else:
             return self
 
     def generate_result_code(self, code):
-        # TODO this just returns the first value and the refnanny doesn't like it
-        val = self.values[1] if len(self.values) > 1 else self.values[0]
-        code.putln('%s = %s;' % (self.result(), val.py_result()))
-        code.putln('__Pyx_INCREF(%s);' % self.result())
+        list_var = Naming.quick_temp_cname
+        num_items = len(self.values)
+
+        code.putln('{')
+        code.putln('PyObject *%s = PyList_New(%s); %s' % (
+            list_var,
+            num_items,
+            code.error_goto_if_null(list_var, self.pos)))
+        code.put_gotref(list_var)
+        for i, value in enumerate(self.values):
+            code.put_incref(value.result(), value.ctype())
+            code.put_giveref(value.py_result())
+            code.putln('PyList_SET_ITEM(%s, %s, %s);' % (list_var, i, value.py_result()))
+        code.putln('%s = PyUnicode_Join(%s, %s); __Pyx_DECREF(%s); %s' % (
+            self.result(), 
+            Naming.empty_unicode, 
+            list_var,
+            list_var,
+            code.error_goto_if_null(list_var, self.pos)))
         code.put_gotref(self.py_result())
+        code.putln('}')
 
 
 class FormattedValueNode(ExprNode):

Cython/Compiler/ModuleNode.py

@@ -675,6 +675,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
         code.putln('static PyObject *%s;' % Naming.builtins_cname)
         code.putln('static PyObject *%s;' % Naming.empty_tuple)
         code.putln('static PyObject *%s;' % Naming.empty_bytes)
+        code.putln('static PyObject *%s;' % Naming.empty_unicode)
         if Options.pre_import is not None:
             code.putln('static PyObject *%s;' % Naming.preimport_cname)
         code.putln('static int %s;' % Naming.lineno_cname)
@@ -2103,6 +2104,8 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             Naming.empty_tuple, code.error_goto_if_null(Naming.empty_tuple, self.pos)))
         code.putln("%s = PyBytes_FromStringAndSize(\"\", 0); %s" % (
             Naming.empty_bytes, code.error_goto_if_null(Naming.empty_bytes, self.pos)))
+        code.putln("%s = PyUnicode_FromStringAndSize(\"\", 0); %s" % (
+            Naming.empty_unicode, code.error_goto_if_null(Naming.empty_unicode, self.pos)))
 
         for ext_type in ('CyFunction', 'FusedFunction', 'Coroutine', 'Generator', 'StopAsyncIteration'):
             code.putln("#ifdef __Pyx_%s_USED" % ext_type)

Cython/Compiler/Naming.py

@@ -95,6 +95,7 @@ gilstate_cname   = pyrex_prefix + "state"
 skip_dispatch_cname = pyrex_prefix + "skip_dispatch"
 empty_tuple      = pyrex_prefix + "empty_tuple"
 empty_bytes      = pyrex_prefix + "empty_bytes"
+empty_unicode	 = pyrex_prefix + "empty_unicode"
 print_function   = pyrex_prefix + "print"
 print_function_kwargs   = pyrex_prefix + "print_kwargs"
 cleanup_cname    = pyrex_prefix + "module_cleanup"