This is a tracking issue for the RFC "Support defining C-compatible variadic functions in Rust" (rust-lang/rfcs#2137).

Steps:

• Implement the RFC (PR Support defining C compatible variadic functions #57760)
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

Unresolved questions:

• "When implementing this feature, we will need to determine whether the compiler
  can provide an appropriate lifetime that prevents a VaList from outliving its
  corresponding variadic function."
• Continuing bikeshed on the ... syntax.
• Ensure that even when this gets stabilized for regular functions, it is still rejected on const fn.
• What even is the semantics of this? What exactly is allowed and disallowed? Would be good to have a Miri implementation.
• borrowck problems: Poor interaction between NLL-borrowck, async, and c_variadic's ... desugaring (VaListImpl<'_>) #125431
• What exactly are the ABI requirements around variadic calls? What are the ABI compatibility rules? Do we have to be concerned about LLVM target features affecting variadic ABI?