Function Pointer cast

Surely the solution to that problem is to fix miri so it corresponds to what actual machines do, rather than try to "fix" code that works fine on real machines but doesn't work on miri? The point of an abstract machine is to abstract away the irrelevant differences in the details between different concrete machines. But all actual targets for Rust have a single address space, at least for code (and most have a single address space for code and data). So there is no actual reason why this should be undefined.

Then again I would say the same thing for the "undefined behaviour" that is dereferencing an invalid pointer. It's possible to define it perfectly well: you get the result of dereferencing it. And if that means that your system faults, then it faults. If it means you read garbage, you get garbage. But it doesn't give the compiler permission to compile out your later pointer validity checks.