Prevent rebase from being run in quick action when there are conflicts

When there are merge conflicts, a rebase will always fail. Since the
`cannot_be_merged` state is cached, we can use that to tell the user
that the rebase will not go through until the conflicts are resolved.

changelogs/unreleased/sh-disable-rebase-on-conflicts.yml

+---
+title: Prevent rebase from being run in quick action when there are conflicts
+merge_request: 51243
+author:
+type: fixed

lib/gitlab/quick_actions/merge_request_actions.rb

@@ -57,6 +57,11 @@ module Gitlab
           access_check.can_push_to_branch?(merge_request.source_branch)
         end
         command :rebase do
+          if quick_action_target.cannot_be_merged?
+            @execution_message[:rebase] = _('This merge request cannot be rebased while there are conflicts.')
+            next
+          end
+
           if quick_action_target.rebase_in_progress?
             @execution_message[:rebase] = _('A rebase is already in progress.')
             next

locale/gitlab.pot

@@ -28776,6 +28776,9 @@ msgstr ""
 msgid "This means you can not push code until you create an empty repository or import existing one."
 msgstr ""
 
+msgid "This merge request cannot be rebased while there are conflicts."
+msgstr ""
+
 msgid "This merge request does not have accessibility reports"
 msgstr ""
 

spec/support/shared_examples/quick_actions/merge_request/rebase_quick_action_shared_examples.rb

@@ -63,6 +63,16 @@ RSpec.shared_examples 'rebase quick action' do
           expect(page).not_to have_content 'Scheduled a rebase'
         end
       end
+
+      context 'when there are conflicts in the merge request' do
+        let!(:merge_request) { create(:merge_request, source_project: project, target_project: project, source_branch: 'conflict-missing-side', target_branch: 'conflict-start', merge_status: :cannot_be_merged) }
+
+        it 'does not rebase the MR' do
+          add_note("/rebase")
+
+          expect(page).to have_content 'This merge request cannot be rebased while there are conflicts.'
+        end
+      end
     end
 
     context 'when the current user cannot rebase the MR' do