Fixed #22294 -- Prevented converting length filter output to string

Thanks Steve Pike for the report.

django/template/defaultfilters.py

@@ -572,7 +572,7 @@ def last(value):
         return ''
 
 
-@register.filter(is_safe=True)
+@register.filter(is_safe=False)
 def length(value):
     """Returns the length of the value - useful for lists."""
     try:

docs/ref/templates/builtins.txt

@@ -1680,7 +1680,8 @@ For example::
 
     {{ value|length }}
 
-If ``value`` is ``['a', 'b', 'c', 'd']``, the output will be ``4``.
+If ``value`` is ``['a', 'b', 'c', 'd']`` or ``"abcd"``, the output will be
+``4``.
 
 .. templatefilter:: length_is
 
@@ -1693,7 +1694,8 @@ For example::
 
     {{ value|length_is:"4" }}
 
-If ``value`` is ``['a', 'b', 'c', 'd']``, the output will be ``True``.
+If ``value`` is ``['a', 'b', 'c', 'd']`` or ``"abcd"``, the output will be
+``True``.
 
 .. templatefilter:: linebreaks
 

tests/defaultfilters/tests.py

@@ -20,7 +20,7 @@ from django.test import TestCase
 from django.utils import six
 from django.utils import translation
 from django.utils.encoding import python_2_unicode_compatible
-from django.utils.safestring import SafeData
+from django.utils.safestring import mark_safe, SafeData
 
 
 class DefaultFiltersTests(TestCase):
@@ -495,6 +495,7 @@ class DefaultFiltersTests(TestCase):
 
     def test_length(self):
         self.assertEqual(length('1234'), 4)
+        self.assertEqual(length(mark_safe('1234')), 4)
         self.assertEqual(length([1, 2, 3, 4]), 4)
         self.assertEqual(length_is([], 0), True)
         self.assertEqual(length_is([], 1), False)

tests/template_tests/filters.py

@@ -320,9 +320,10 @@ def get_filter_tests():
         'length02': ('{{ list|length }}', {'list': []}, '0'),
         'length03': ('{{ string|length }}', {'string': ''}, '0'),
         'length04': ('{{ string|length }}', {'string': 'django'}, '6'),
+        'length05': ('{% if string|length == 6 %}Pass{% endif %}', {'string': mark_safe('django')}, 'Pass'),
         # Invalid uses that should fail silently.
-        'length05': ('{{ int|length }}', {'int': 7}, ''),
-        'length06': ('{{ None|length }}', {'None': None}, ''),
+        'length06': ('{{ int|length }}', {'int': 7}, ''),
+        'length07': ('{{ None|length }}', {'None': None}, ''),
 
         # length_is filter.
         'length_is01': ('{% if some_list|length_is:"4" %}Four{% endif %}', {'some_list': ['4', None, True, {}]}, 'Four'),