ABI of ZST arguments?

[joshlf]

Also posted to URLO.

Is it guaranteed that a ZST argument has no effect on a function's ABI? For example, are the following two functions guaranteed to have the same ABI?

extern "C" {
    fn foo(a: u8, b: u16);
    fn bar(a: u8, x: (), b: u16);
}

If this is in practice true but not yet guaranteed, would there be support for adding such a guarantee?

[RalfJung]

This is definitely not guaranteed at the moment.

And in fact, it's just not true for all ABIs at the moment. On windows-msvc, we pass ZST by-ptr for extern "C". We've done this since ~forever and the exact reasons are lost to time; I think that is just a bad work-around for rust-lang/rust#81996, and we should fix that issue instead. But we probably can't remove the ABI hack until we fixed the type layout.

Even once we fix that, there's a question of whether we truly can promise this for all future ABIs. Who knows what people come up with in the ABI space.

[RalfJung]

Oh, and furthermore we probably don't want to make any more ABI compatibility guarantees anyway, we want people to use something like rust-lang/rust#140803 instead.

[joshlf]

Okay thanks!