Optionally bounds-check primops
It would be awfully nice if there were a way to optionally get bounds-checked versions of array primitives. The idea would be that if GHC is compiled with a bounds-checking option, then the code generator would use primops that verify indices are in bounds. Based on a question by Henning Thielemann, I suspect such a feature would get some use.
The biggest challenge:
CallStack, etc., all live in an entirely different universe than the primops, so it's not exactly obvious how to get started. I don't know if it would be possible to inject references to such things into a lower level. Sounds hard, but maybe it's possible.
An alternative, probably much simpler, approach would be to crash hard with an error message instead of trying to throw a proper exception. Ideally, the error message would include the name of the primop and (for modules compiled with profiling) some call stack info.