setMwGlobals( [ 'wgExternalDiffEngine' => null ] ); $differenceEngine = new CustomDifferenceEngine(); $slotDiffRenderer = new DifferenceEngineSlotDiffRenderer( $differenceEngine ); $extraCacheKeys = $slotDiffRenderer->getExtraCacheKeys(); $this->assertSame( [ 'foo' ], $extraCacheKeys ); } }