Add alternate_viewer for renamed viewer

We'd like to be able to change the underlying viewer when the current
viewer is renamed so that we can display the full content of the file in
line.

app/serializers/diff_file_base_entity.rb

@@ -88,6 +88,7 @@ class DiffFileBaseEntity < Grape::Entity
   expose :b_mode
 
   expose :viewer, using: DiffViewerEntity
+  expose :alternate_viewer, using: DiffViewerEntity
 
   expose :old_size do |diff_file|
     diff_file.old_blob&.raw_size

lib/gitlab/diff/file.rb

@@ -314,6 +314,10 @@ module Gitlab
         @rich_viewer = rich_viewer_class&.new(self)
       end
 
+      def alternate_viewer
+        alternate_viewer_class&.new(self)
+      end
+
       def rendered_as_text?(ignore_errors: true)
         simple_viewer.is_a?(DiffViewer::Text) && (ignore_errors || simple_viewer.render_error.nil?)
       end
@@ -419,6 +423,17 @@ module Gitlab
         return if collapsed?
         return unless diffable?
         return unless modified_file?
+
+        find_renderable_viewer_class(classes)
+      end
+
+      def alternate_viewer_class
+        return unless viewer.class == DiffViewer::Renamed
+
+        find_renderable_viewer_class(RICH_VIEWERS) || (DiffViewer::Text if text?)
+      end
+
+      def find_renderable_viewer_class(classes)
         return if different_type? || external_storage_error?
 
         verify_binary = !stored_externally?

spec/lib/gitlab/diff/file_spec.rb

@@ -567,6 +567,61 @@ describe Gitlab::Diff::File do
     end
   end
 
+  describe '#alternate_viewer' do
+    subject { diff_file.alternate_viewer }
+
+    where(:viewer_class) do
+      [
+        DiffViewer::Image,
+        DiffViewer::Collapsed,
+        DiffViewer::NotDiffable,
+        DiffViewer::Text,
+        DiffViewer::NoPreview,
+        DiffViewer::Added,
+        DiffViewer::Deleted,
+        DiffViewer::ModeChanged,
+        DiffViewer::ModeChanged,
+        DiffViewer::NoPreview
+      ]
+    end
+
+    with_them do
+      let(:viewer) { viewer_class.new(diff_file) }
+
+      before do
+        allow(diff_file).to receive(:viewer).and_return(viewer)
+      end
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'when viewer is DiffViewer::Renamed' do
+      let(:viewer) { DiffViewer::Renamed.new(diff_file) }
+
+      before do
+        allow(diff_file).to receive(:viewer).and_return(viewer)
+      end
+
+      context 'when it can be rendered as text' do
+        it { is_expected.to be_a(DiffViewer::Text) }
+      end
+
+      context 'when it can be rendered as image' do
+        let(:commit) { project.commit('2f63565e7aac07bcdadb654e253078b727143ec4') }
+        let(:diff_file) { commit.diffs.diff_file_with_new_path('files/images/6049019_460s.jpg') }
+
+        it { is_expected.to be_a(DiffViewer::Image) }
+      end
+
+      context 'when it is something else' do
+        let(:commit) { project.commit('ae73cb07c9eeaf35924a10f713b364d32b2dd34f') }
+        let(:diff_file) { commit.diffs.diff_file_with_new_path('Gemfile.zip') }
+
+        it { is_expected.to be_nil }
+      end
+    end
+  end
+
   describe '#rendered_as_text?' do
     context 'when the simple viewer is text-based' do
       let(:commit) { project.commit('570e7b2abdd848b95f2f578043fc23bd6f6fd24d') }