Using temporary files on disk is a common practice in our lit tests. Indeed, in some (many/all?) cases, it is actually useful for debugging purposes, because it is possible to inspect the intermediate files if a test fails, to see what the problem is, without needing to modify and rerun the test. As a result, I see supporting this syntax as low priority personally. Do you have an idea of how many tests use the $() syntax and where those tests are? It’s important we get feedback from those who maintain the tests that use that syntax, to ensure we are meeting their needs.