pub unsafe extern "C" fn weld_runst_assert(run: WeldRuntimeContextRef, cond: u8) -> u8 {
    let run = &mut *run;
    if cond == 0 {
        // Doesn't return.
        run.set_errno(WeldRuntimeErrno::AssertionError);
        unreachable!()
    } else {
        1
    }
}
Check whether cond is 0 (assertion fails).
weld/src/runtime/ffi.rs