Function va_arg 

pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaListImpl<'_>) -> T

🔬This is a nightly-only experimental API. (core_intrinsics)

Available on non-crate feature ferrocene_certified only.

Loads an argument of type T from the va_list ap and increment the argument ap points to.

FIXME: document safety requirements