Fixed #32824 -- Improved performance of NodeList.render().

This avoids the following:
 - checking that each item in the nodelist is a subclass of Node,
 - calling str() on the render_annotated() output, because it's
   documented that Node.render() must return a string,
 - calling mark_safe() on the output, when the value to be wrapped is
   definitively known to be a string because the result of ''.join()
   is always of that type,
 - using an intermediate list to store each individual string.

django/template/base.py

@@ -59,7 +59,7 @@ from django.template.context import BaseContext
 from django.utils.formats import localize
 from django.utils.html import conditional_escape, escape
 from django.utils.regex_helper import _lazy_re_compile
-from django.utils.safestring import SafeData, mark_safe
+from django.utils.safestring import SafeData, SafeString, mark_safe
 from django.utils.text import (
     get_text_list, smart_split, unescape_string_literal,
 )
@@ -954,14 +954,9 @@ class NodeList(list):
     contains_nontext = False
 
     def render(self, context):
-        bits = []
-        for node in self:
-            if isinstance(node, Node):
-                bit = node.render_annotated(context)
-            else:
-                bit = node
-            bits.append(str(bit))
-        return mark_safe(''.join(bits))
+        return SafeString(''.join([
+            node.render_annotated(context) for node in self
+        ]))
 
     def get_nodes_by_type(self, nodetype):
         "Return a list of all nodes of the given type"

docs/releases/4.0.txt

@@ -463,6 +463,10 @@ Miscellaneous
   :class:`~django.utils.feedgenerator.Atom1Feed`, and their subclasses now
   emit elements with no content as self-closing tags.
 
+* ``NodeList.render()`` no longer casts the output of ``render()`` method for
+  individual nodes to a string. ``Node.render()`` should always return a string
+  as documented.
+
 .. _deprecated-features-4.0:
 
 Features deprecated in 4.0

tests/template_tests/templatetags/custom.py

@@ -198,4 +198,4 @@ class CounterNode(template.Node):
     def render(self, context):
         count = self.count
         self.count = count + 1
-        return count
+        return str(count)