Worked on the facebook/buck2-prelude repository, focusing on refining error handling within Starlark rules for build system configuration. Addressed a nuanced edge case by updating assertion messaging in common.bzl to clarify scenarios where no modifier constraint is specified, thereby reducing developer confusion and improving the debugging process. The technical approach preserved all existing behavior, isolating changes to error messages for greater clarity without introducing new features. Utilized Bzl and Starlark to ensure assertion logic accurately reflected possible constraint states. This targeted bug fix enhanced the developer experience by making assertion failures more informative, while maintaining overall system stability and reliability.