Use different cache key when feature flag is enabled

lib/gitlab/diff/highlight_cache.rb

@@ -69,7 +69,12 @@ module Gitlab
 
       def key
         strong_memoize(:redis_key) do
-          ['highlighted-diff-files', diffable.cache_key, VERSION, diff_options].join(":")
+          [
+            'highlighted-diff-files',
+            diffable.cache_key, VERSION,
+            diff_options,
+            Feature.enabled?(:introduce_marker_ranges, diffable.project, default_enabled: :yaml)
+          ].join(":")
         end
       end
 

spec/lib/gitlab/diff/highlight_cache_spec.rb

@@ -238,7 +238,17 @@ RSpec.describe Gitlab::Diff::HighlightCache, :clean_gitlab_redis_cache do
     subject { cache.key }
 
     it 'returns cache key' do
-      is_expected.to start_with("highlighted-diff-files:#{cache.diffable.cache_key}:2")
+      is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true")
+    end
+
+    context 'when feature flag is disabled' do
+      before do
+        stub_feature_flags(introduce_marker_ranges: false)
+      end
+
+      it 'returns the original version of the cache' do
+        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:false")
+      end
     end
   end
 end