Pointer to `va_list` is translated wrong

[Rua]

When the va_list type appears by value in a function parameter, c2rust translates it as a Rust VaList. That's correct. But when a function takes va_list * by pointer, then that gets translated as Rust *mut VaList, which is not correct. To see why, consider what such a pointer actually points to on architectures that use a dedicated struct type for va_list, such as x86-64 or Linux aarch64:

• In C, va_list is a struct or array containing a struct. A pointer va_list * is thus a pointer to this struct or array. Dereferencing the pointer gives you the struct.
• In Rust, VaList does not contain the struct, which is in VaListImpl. VaList only contains a reference to the struct. Thus, dereferencing *mut VaList gives you the VaList, which is not the struct but that reference. You need to dereference VaList again to get the struct inside VaListImpl.

Clearly, these two are not equivalent and therefore not ABI compatible. The correct translation would be to use VaListImpl normally, but VaList if and only if C passes va_list by value. va_list * by pointer should always be translated to *mut VaListImpl.

[ahomescu]

There were some recent changes in libcore to these types, to improve ABI compatibility. I remember seeing somewhere (don't remember where) that the next step would be to update C2Rust. I'm not sure if we need to do it or @folkertdev was planning to, adding him to this discussion.

[Rua]

I have been following this discussion here:
rust-lang/rust#141980

But since c2rust targets Rust 1.65 nightly, that's probably not relevant until c2rust updates to a newer version.

[folkertdev]

Sure, but getting the output to be correct with the 1.65 implementation of c_variadics will be a pain I suspect, and c2rust does want to (as far as I know) update the version of rust it targets.

We're seeing some movement on c_variadics again, I'm hoping we can get the changes from rust-lang/rust#141980 merged this year, at which point the story for c2rust becomes a lot simpler if it did upgrade.