Compare diff against HEAD if diff_head is passed

When group/project/merge_requests/iid/diffs?diff_head=true
is visited, then the diff which compared against HEAD
will be shown

app/controllers/projects/merge_requests/diffs_controller.rb

@@ -111,6 +111,11 @@ def find_merge_request_diff_compare
       end
     end
+    if Gitlab::Utils.to_boolean(params[:diff_head]) && @merge_request.diffable_merge_ref?
+      return CompareService.new(@project, @merge_request.merge_ref_head.sha)
+        .execute(@project, @merge_request.target_branch)
+    end
+
     if @start_sha
       @merge_request_diff.compare_with(@start_sha)
     else

app/models/merge_request.rb

@@ -840,6 +840,10 @@ def check_mergeability(async: false)
   end
   # rubocop: enable CodeReuse/ServiceClass
+  def diffable_merge_ref?
+    Feature.enabled?(:diff_compare_with_head, target_project) && can_be_merged? && merge_ref_head.present?
+  end
+
   # Returns boolean indicating the merge_status should be rechecked in order to
   # switch to either can_be_merged or cannot_be_merged.
   def recheck_merge_status?

app/serializers/merge_request_diff_entity.rb

@@ -34,6 +34,14 @@ class MergeRequestDiffEntity < Grape::Entity
     merge_request_version_path(project, merge_request, merge_request_diff)
   end
+  expose :head_version_path do |merge_request_diff|
+    project = merge_request.target_project
+
+    next unless project && merge_request.diffable_merge_ref?
+
+    diffs_project_merge_request_path(project, merge_request, diff_head: true)
+  end
+
   expose :version_path do |merge_request_diff|
     start_sha = options[:start_sha]
     project = merge_request.target_project

spec/controllers/projects/merge_requests/diffs_controller_spec.rb

@@ -203,6 +203,41 @@ def go(extra_params = {})
       end
     end
+    context 'with diff_head param passed' do
+      before do
+        allow(merge_request).to receive(:diffable_merge_ref?)
+          .and_return(diffable_merge_ref)
+      end
+
+      context 'the merge request can be compared with head' do
+        let(:diffable_merge_ref) { true }
+
+        it 'compares diffs with the head' do
+          MergeRequests::MergeToRefService.new(project, merge_request.author).execute(merge_request)
+
+          expect(CompareService).to receive(:new).with(
+            project, merge_request.merge_ref_head.sha
+          ).and_call_original
+
+          go(diff_head: true)
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
+
+      context 'the merge request cannot be compared with head' do
+        let(:diffable_merge_ref) { false }
+
+        it 'compares diffs with the base' do
+          expect(CompareService).not_to receive(:new)
+
+          go(diff_head: true)
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
+    end
+
     context 'with MR regular diff params' do
       it 'returns success' do
         go

spec/models/merge_request_spec.rb

@@ -3576,4 +3576,44 @@ def create_build(pipeline, coverage, name)
       expect(merge_request.recent_visible_deployments.count).to eq(10)
     end
   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
+        end
+      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
+    end
+
+    context 'diff_compare_with_head disabled' do
+      before do
+        stub_feature_flags(diff_compare_with_head: { enabled: false, thing: subject.target_project })
+      end
+
+      it 'returns false' do
+        expect(subject.diffable_merge_ref?).to eq(false)
+      end
+    end
+  end
 end

spec/serializers/merge_request_diff_entity_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 describe MergeRequestDiffEntity do
-  let(:project) { create(:project, :repository) }
+  let_it_be(:project) { create(:project, :repository) }
   let(:request) { EntityRequest.new(project: project) }
   let(:merge_request) { create(:merge_request_with_diffs, target_project: project, source_project: project) }
   let(:merge_request_diffs) { merge_request.merge_request_diffs }
@@ -36,4 +36,29 @@
       expect(subject[:short_commit_sha]).to eq(nil)
     end
   end
+
+  describe '#head_version_path' do
+    before do
+      allow(merge_request).to receive(:diffable_merge_ref?)
+        .and_return(diffable_merge_ref)
+    end
+
+    context 'merge request can be merged' do
+      let(:diffable_merge_ref) { true }
+
+      it 'returns diff path with diff_head param set' do
+        expect(subject[:head_version_path]).to eq(
+          "/#{project.full_path}/-/merge_requests/#{merge_request.iid}/diffs?diff_head=true"
+        )
+      end
+    end
+
+    context 'merge request cannot be merged' do
+      let(:diffable_merge_ref) { false }
+
+      it 'returns diff path with diff_head param set' do
+        expect(subject[:head_version_path]).to be_nil
+      end
+    end
+  end
 end