[2025H1] Propose a null and alignment checks goal

[1c3t3a]

Rendered

cc: @tmandry, @RalfJung, @rcvalle

[RalfJung]

This mostly already exists? @saethlin added alignment checks in debug builds.

[1c3t3a]

Oh I didn't see that! I could pivot this goal to something different in the area? E.g. around pointer arithmetics or checking enum discriminants?

[saethlin]

Aye, we already have checks for reads/writes through misaligned pointers and those have been on stable for a long time. The MIR transform that implements that could be extended to catch reads/writes to null but I suspect that will be very low-yield because on most hosted systems, that operation will fault anyway.

In addition, the standard library has runtime checks for invalid pointer offsets starting in 1.83 which is stable today.

I have prototyped something like invalid enum discriminants here: rust-lang/rust#121174. I'm sure the PR needs work to just get it to build. I was going for a more general approach of checking all niches not just enums, and that mostly emits a lot of checks for references being null, so maybe targeting enums only is the way.

This goals PR seems well-intentioned, but I'm surprised to see it so disconnected from our current state. I would be happy to collaborate on sketching out a coherent story for our hodgepodge of runtime UB checks, but I'm a bit busy this week and next.

[1c3t3a]

I would be happy to collaborate on sketching out a coherent story for our hodgepodge of runtime UB checks, but I'm a bit busy this week and next.

I would like to collaborate on this! I send you a message on Zulip to discuss this further.

For this goal: I'd love to pivot it to something helpful regarding runtime checks, so probably update it after having synced with @saethlin.

[nikomatsakis]

@1c3t3a ok, I'll hold off on merging until you and @saethlin align on the right description.

[nikomatsakis]

I haven't heard anything here, I'm going to close this PR as the initial deadline for goals has passed and I don't think it's ready to merge -- please feel free to re-open once you've got a consensus plan in place (although time IS running out...).

[1c3t3a]

I actually pivoted the proposal here and just uploaded the new version. I was waiting for @saethlin's feedback but I think I can work that in later as well. The new PR is: #177. I hope I am not too late, sorry for the delay!