[3.0.x] Fixed #30776 -- Restored max length validation on AuthenticationForm.UsernameField.

Regression in 5ceaf14686ce626404afb6a5fbd3d8286410bf13.

Thanks gopackgo90 for the report and Mariusz Felisiak for tests.

Backport of 6c9778a58e4f680db180d4cc9dc5639d2ec1b40c from master

2 files changed, 5 insertions, 1 deletions

diff --git a/django/contrib/auth/forms.py b/django/contrib/auth/forms.py
index a0cfed0995..b4fb55c1b4 100644
--- a/django/contrib/auth/forms.py
+++ b/django/contrib/auth/forms.py
@@ -191,7 +191,9 @@ class AuthenticationForm(forms.Form):
 
         # Set the max length and label for the "username" field.
         self.username_field = UserModel._meta.get_field(UserModel.USERNAME_FIELD)
-        self.fields['username'].max_length = self.username_field.max_length or 254
+        username_max_length = self.username_field.max_length or 254
+        self.fields['username'].max_length = username_max_length
+        self.fields['username'].widget.attrs['maxlength'] = username_max_length
         if self.fields['username'].label is None:
             self.fields['username'].label = capfirst(self.username_field.verbose_name)
 
diff --git a/tests/auth_tests/test_forms.py b/tests/auth_tests/test_forms.py
index 7085dcfa11..40e3050144 100644
--- a/tests/auth_tests/test_forms.py
+++ b/tests/auth_tests/test_forms.py
@@ -423,6 +423,7 @@ class AuthenticationFormTest(TestDataMixin, TestCase):
         CustomEmailField.objects.create_user(**data)
         form = AuthenticationForm(None, data)
         self.assertEqual(form.fields['username'].max_length, 255)
+        self.assertEqual(form.fields['username'].widget.attrs.get('maxlength'), 255)
         self.assertEqual(form.errors, {})
 
     @override_settings(AUTH_USER_MODEL='auth_tests.IntegerUsernameUser')
@@ -435,6 +436,7 @@ class AuthenticationFormTest(TestDataMixin, TestCase):
         IntegerUsernameUser.objects.create_user(**data)
         form = AuthenticationForm(None, data)
         self.assertEqual(form.fields['username'].max_length, 254)
+        self.assertEqual(form.fields['username'].widget.attrs.get('maxlength'), 254)
         self.assertEqual(form.errors, {})
 
     def test_username_field_label(self):