`extern "C" fn` with zero-sized argument?

[madsmtm]

C and C++ have a hard time with zero-sized types, they're basically not possible without special compiler extensions as far as I can tell? However, the following Rust code compiles fine without any warnings:

#[repr(C)]
pub struct X([u8; 0]);

extern "C" {
    fn my_fn1(_x: X);
}

pub extern "C" fn my_fn2(_x: X) {}

fn main() {
    my_fn1(X([]));
    my_fn2(X([]));
}

How do we currently handle this (as far as I can tell, they're just ignored)? Do we want to make any guarantees about how they work? Are there any calling conventions that reserve "space" for zero-sized arguments?

(I have a little bit of a use-case for this: In objc2, it would make the API slightly nicer to be able to "act" as-if a function took a specific zero-sized marker type MainThreadMarker).

If this is UB, I think we should at least extend the improper_c_types lint to warn on zero-sized arguments.

[bjorn3]

Are there any calling conventions that reserve "space" for zero-sized arguments?

Unfortunately there are:

https://github.com/rust-lang/rust/blob/263d8682d6e01bb02727b15b1c72ffabc0e7396b/compiler/rustc_middle/src/ty/layout.rs#L3306-L3314

[CAD97]

Also note that the specific case of a #[repr(C)] struct containing a single zero-length array is already thorny for other reasons.