[1.10.x] Fixed #27238 -- Disabled check_pattern_startswith_slash if settings.APPEND_SLASH=False.

Thanks strycore for the report and timgraham for suggesting the
solution.

Backport of 911d9f4ed1 from master

django/core/checks/urls.py

@@ -95,6 +95,10 @@ def check_pattern_startswith_slash(pattern):
     Check that the pattern does not begin with a forward slash.
     """
     regex_pattern = pattern.regex.pattern
+    if not settings.APPEND_SLASH:
+        # Skip check as it can be useful to start a URL pattern with a slash
+        # when APPEND_SLASH=False.
+        return []
     if regex_pattern.startswith('/') or regex_pattern.startswith('^/'):
         warning = Warning(
             "Your URL pattern {} has a regex beginning with a '/'. Remove this "

docs/releases/1.10.2.txt

@@ -14,3 +14,6 @@ Bugfixes
 
 * Allowed combining ``contrib.postgres.search.SearchQuery`` with more than one
   ``&`` or ``|`` operators (:ticket:`27143`).
+
+* Disabled system check for URL patterns beginning with a '/' when
+  ``APPEND_SLASH=False`` (:ticket:`27238`).

tests/check_framework/test_urls.py

@@ -55,6 +55,16 @@ class CheckUrlsTest(SimpleTestCase):
 
         self.assertIn(expected_msg, warning.msg)
 
+    @override_settings(
+        ROOT_URLCONF='check_framework.urls.beginning_with_slash',
+        APPEND_SLASH=False,
+    )
+    def test_beginning_with_slash_append_slash(self):
+        # It can be useful to start a URL pattern with a slash when
+        # APPEND_SLASH=False (#27238).
+        result = check_url_config(None)
+        self.assertEqual(result, [])
+
     @override_settings(ROOT_URLCONF='check_framework.urls.name_with_colon')
     def test_name_with_colon(self):
         result = check_url_config(None)