Over in #44930, @folkertdev proposed:

We need c_variadics on stable for zlib-rs, so I'm going to try and move this forward. A draft for a modified API is at

https://gist.github.com/folkertdev/47c79e2f5b03f1138db9d53be5e51ed1

Some highlights

• VaListImpl is private!
• VaList just has a single lifetime parameter now, which makes a lot more sense
• The VaList::with_copy function and a new va_copy! macro can be used to duplicate a VaList

This all relies on a different desugaring of ..., roughly

fn foo(placeholder: i32, mut args: ...) {
    // stuff
}

// --->

fn foo(placeholder: i32) {
    let tag = core::mem::MaybeUninit::<VaListImpl>::uninit();
    unsafe { core::instrinsics::va_start(tag.as_mut_ptr()) }
    let mut args = VaList(tag.assume_init_mut())

    // stuff
}

It seems possible to make that work internally.

---

I think this new API is nicer, because it leaks fewer implementation details. But before we start implementation work, does anyone see issues with this direction?

pinging specifically the c2rust folks @thedataking @ahomescu, you've probably worked with the existing c_variadics most.

Thoughts?

Tracking:

• Tracking issue for RFC 2137: Support defining C-compatible variadic functions in Rust (c_variadic) #44930

cc @rust-lang/lang