That's awesome, darwin failures that only happened on nixpkgs-unstable no longer happen when I rebuilt lix today. Meanwhile, there's a new failure, this time libcmd. There's this small snippet:

    # Use a temporary home directory for the unit tests.
    # Otherwise, /homeless-shelter is created in the single-user sandbox, and functional tests will fail.
    # TODO(alois31): handle TMPDIR properly (meson can't, and setting HOME in the test is too late)…
    'HOME': '/tmp/nix-test/libcmd-unit-tests',

which you'd think makes everything work just fine, right? WRONG. The code does this:

        auto homeDir = getEnv("HOME");
        if (homeDir) {
            // Only use `$HOME` if it exists and is owned by the current user.
            struct stat st;
            int result = stat(homeDir->c_str(), &st);

and obviously HOME does not exist so it falls back to getpwuid_r. Which just so happens to work on Linux because dynamic sandbox user gets put in the user database, but OOPS it doesn't work on darwin!

Fair but also I'd argue this is an improvement in the meson file (also addresses the TODO which claimed respecting TMPDIR is impossible with meson)

    # Use a temporary home directory for the unit tests.
    # Otherwise, /homeless-shelter is created in the single-user sandbox, and functional tests will fail.
    'HOME': run_command('mktemp', '-d', 'libcmd-unit-tests.XXXXXX', check: true).stdout().strip()

and it addresses the failure (which, while a band-aid, is a fix nonetheless, and lix not building with auto-allocate-uids is a regression)