Reading Pointer bytes as Integers

[chorman0773]

This came up in rust-lang/reference#1664. I wanted to ask what T-opsem thinks about the behaviour of reading pointer bytes as integer types (or as char/bool/etc.).

As far as I can tell, there are two "sensible" behaviours, given that integers themselves do no carry provenance:

• The pointer fragment is ignored,
• Decoding error (thus undefined behaviour).

Given provenance monotonicity, which would be violated by the decoding error, it seems like the best option is that the fragments are ignored. Is there anything missed here? If not, can we get a formal sign off on this behaviour.

Note that I'm only considering the runtime behaviour, which can be a point against adopting the behaviour. Given that it's impossible to get the address of certain pointers in const-eval, it does need to be undefined behaviour (or otherwise an error) to read pointer bytes (to at least symbolic allocations) as integer types.

[saethlin]

Given that it's impossible to get the address of certain pointers

Which pointers?

[chorman0773]

I failed to clarify that. It was referring to the consteval AM, where allocations that exist outside of the particular constant evaluation (what I call symbolic pointers) can't be assigned an address.

[RalfJung]

Const-eval can't assign an address to any allocation, "inside" or "outside". (Not sure what you mean with that distinction.)

[RalfJung]

Anyway that sub-discussion seems off-topic here, please move it to Zulip. And please update the issue description to clarify that "certain pointers" refers to const-eval.