pure asm blocks must terminate

[RalfJung]

@Amanieu am I correct in assuming that all other asm blocks (i.e, not marked pure), may loop infinitely?

[Amanieu]

Since non-pure asm blocks can do anything C FFI can do, yes, it's valid.

However I'm not sure that pure should be prevented from looping indefinitely. How is that different from it just taking a very long time to run?

[RalfJung]

If they loop infinitely they can cannot be optimized away.

asm!("<infinite loop>", pure);
unreachable_unchecked();

It is my understanding that the compiler can remove this asm block, so this program must have UB. Our current docs do not explain where that UB comes from.

[RalfJung]

@ehuss we got an expert review; is anything else needed to land this?

[ehuss]

Thanks!

I'm going to go ahead and approve, but I'm a little uncomfortable with the process we've been using for defining assembly and its behavior. It feels very ad-hoc, without any oversight other than Amanieu. It's not that I don't trust @Amanieu (I trust him more than anyone else, and am very appreciative of his help), but that I think that having a very large surface area of Rust be defined by essentially one person without any discussion or broad feedback can lead to mistakes or disagreements or misunderstandings.

I don't know how else to handle this since I have difficulty getting the lang team to participate.