only allow 0-offset on integer pointers

[RalfJung]

Triggered by rust-lang/rust#54857

[oli-obk]

Integer pointers to non-zsts are normal on embedded. This would make most embedded code UB

[RalfJung]

That's platform-specific. On embedded, it might also be okay to do (0x400usize as *mut u32) = 14, and yet miri will always flag that as UB.

Currently we can only say whether your code is okay on all platforms (of the given target). We cannot test code that makes specific extra assumptions about e.g. allocations existing at fixed locations.

[oli-obk]

I don't quite agree, since offsetting integer pointers

1. is something you can easily replicate by casting to an int, adding and casting back
2. something that you can do without emulating the hardware (writing/reading to/from integer addresses needs such emulation)

That said, there's probably many things that go wrong when looking at embedded. I'll rather address this once I have a basic emulation setup and a bunch of tests

[RalfJung]

is something you can easily replicate by casting to an int, adding and casting back

Supporting that is tracked in #224.

something that you can do without emulating the hardware (writing/reading to/from integer addresses needs such emulation)

We are not talking about hardware though, but about an abstract machine where whether an allocation exists somewhere makes a difference. In that abstract machine, the offset can not be emulated without further knowledge.

[RalfJung]

That said, there's probably many things that go wrong when looking at embedded. I'll rather address this once I have a basic emulation setup and a bunch of tests

Wow, you have plans there?
But yes, an emulation setup that can fake the presence of non-ZST allocations at fixed addresses (including read/write accesses) should fix this properly.