- multi-body functions should not exist (once monomorphic)
- extraction code first determines all instances and then converts to stable, should invert that
- unevaluated constants should not exist in monomorphic code
- binders should not exist
- possibly more... (complete this as we go)
Generally, first calling the conversion to stable MIR should remove these cases.
If internal MIR is required, we should note the case and report it.