Remove diff_compare_with_head feature flag

Always display minimal merge-ref-diff option
which does not include commenting support
but will allow users to compare against master (HEAD) and master (base)

app/assets/javascripts/diffs/store/getters_versions_dropdowns.js

@@ -40,10 +40,7 @@ export const diffCompareDropdownTargetVersions = (state, getters) => {
     };
   };
 
-  if (gon.features?.diffCompareWithHead) {
-    return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
-  }
-  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion];
+  return [...state.mergeRequestDiffs.slice(1).map(formatVersion), baseVersion, headVersion];
 };
 
 export const diffCompareDropdownSourceVersions = (state, getters) => {

app/controllers/projects/merge_requests_controller.rb

@@ -26,7 +26,6 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
     push_frontend_feature_flag(:code_navigation, @project)
     push_frontend_feature_flag(:widget_visibility_polling, @project, default_enabled: true)
     push_frontend_feature_flag(:merge_ref_head_comments, @project)
-    push_frontend_feature_flag(:diff_compare_with_head, @project)
     push_frontend_feature_flag(:accessibility_merge_request_widget, @project)
   end
 

app/models/merge_request.rb

@@ -875,7 +875,7 @@ class MergeRequest < ApplicationRecord
   # rubocop: enable CodeReuse/ServiceClass
 
   def diffable_merge_ref?
-    Feature.enabled?(:diff_compare_with_head, target_project) && can_be_merged? && merge_ref_head.present?
+    can_be_merged? && merge_ref_head.present?
   end
 
   # Returns boolean indicating the merge_status should be rechecked in order to

changelogs/unreleased/id-remove-diff-compare-with-head-feature-flag.yml

+---
+title: Allow showing merge request diffs compared to current version of target branch
+merge_request: 31325
+author:
+type: added

doc/user/project/merge_requests/versions.md

@@ -57,7 +57,7 @@ source and target branch can be shown mixed together making it hard to
 understand which changes are being added and which already exist in the
 target branch.
 
-In GitLab 12.10, we added an **experimental** comparison mode, which
+In GitLab 12.10, we added a comparison mode, which
 shows a diff calculated by simulating how it would look like once merged - a more accurate
 representation of the changes rather than using the base of the two
 branches. The new mode is available from the comparison target drop down
@@ -67,26 +67,6 @@ current default comparison.
 
 ![Merge request versions compare HEAD](img/versions_compare_head_v12_10.png)
 
-### Enable or disable `HEAD` comparison mode **(CORE ONLY)**
-
-`HEAD` comparison mode is under development and not ready for production use. It is
-deployed behind a feature flag that is **disabled by default**.
-[GitLab administrators with access to the GitLab Rails console](../../../administration/troubleshooting/navigating_gitlab_via_rails_console.md#starting-a-rails-console-session)
-can enable it for your instance. You're welcome to test it, but use it at your
-own risk.
-
-To enable it:
-
-```ruby
-Feature.enable(:diff_compare_with_head)
-```
-
-To disable it:
-
-```ruby
-Feature.disable(:diff_compare_with_head)
-```
-
 <!-- ## Troubleshooting
 
 Include any troubleshooting steps that you can foresee. If you know beforehand what issues

spec/frontend/diffs/store/getters_versions_dropdowns_spec.js

@@ -18,7 +18,6 @@ describe('Compare diff version dropdowns', () => {
     };
     localState.targetBranchName = 'baseVersion';
     localState.mergeRequestDiffs = diffsMockData;
-    gon.features = { diffCompareWithHead: true };
   });
 
   describe('selectedTargetIndex', () => {
@@ -129,14 +128,6 @@ describe('Compare diff version dropdowns', () => {
       });
       assertVersions(targetVersions);
     });
-
-    it('does not list head version if feature flag is not enabled', () => {
-      gon.features = { diffCompareWithHead: false };
-      setupTest();
-      const targetVersions = getters.diffCompareDropdownTargetVersions(localState, getters);
-
-      expect(targetVersions.find(version => version.isHead)).toBeUndefined();
-    });
   });
 
   it('diffCompareDropdownSourceVersions', () => {

spec/models/merge_request_spec.rb

@@ -3836,40 +3836,28 @@ describe MergeRequest do
   end
 
   describe '#diffable_merge_ref?' do
-    context 'diff_compare_with_head enabled' do
-      context 'merge request can be merged' do
-        context 'merge_to_ref is not calculated' do
-          it 'returns true' do
-            expect(subject.diffable_merge_ref?).to eq(false)
-          end
-        end
-
-        context 'merge_to_ref is calculated' do
-          before do
-            MergeRequests::MergeToRefService.new(subject.project, subject.author).execute(subject)
-          end
-
-          it 'returns true' do
-            expect(subject.diffable_merge_ref?).to eq(true)
-          end
+    context 'merge request can be merged' do
+      context 'merge_to_ref is not calculated' do
+        it 'returns true' do
+          expect(subject.diffable_merge_ref?).to eq(false)
         end
       end
 
-      context 'merge request cannot be merged' do
-        it 'returns false' do
-          subject.mark_as_unchecked!
+      context 'merge_to_ref is calculated' do
+        before do
+          MergeRequests::MergeToRefService.new(subject.project, subject.author).execute(subject)
+        end
 
-          expect(subject.diffable_merge_ref?).to eq(false)
+        it 'returns true' do
+          expect(subject.diffable_merge_ref?).to eq(true)
         end
       end
     end
 
-    context 'diff_compare_with_head disabled' do
-      before do
-        stub_feature_flags(diff_compare_with_head: { enabled: false, thing: subject.target_project })
-      end
-
+    context 'merge request cannot be merged' do
       it 'returns false' do
+        subject.mark_as_unchecked!
+
         expect(subject.diffable_merge_ref?).to eq(false)
       end
     end