Tracking Issue for `extern_system_varargs`

This is a tracking issue for enabling using an extern "system" function with varargs.
The feature gate for the issue is #![feature(extern_system_varargs)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

• Implementation
• Adjust documentation (see instructions on rustc-dev-guide)
• Formatting for new syntax has been added to the Style Guide (nightly-style-procedure)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• should extern "system" work with varargs at all? it translates to extern "stdcall" on x86, which does not? while MSVC sometimes translates __stdcall to be identical to the C ABI, we have previously rejected implementing MSVC's oddities:
  • https://github.com/rust-lang/rust/issues/87678

Implementation history

• https://github.com/rust-lang/rust/pull/119587
• https://github.com/rust-lang/rust/pull/136948

Start with the unchecked documentation, Style Guide formatting, and stabilization items in this tracking issue, following the linked rustc-dev-guide and nightly-style-procedure instructions. Review the unresolved question about whether extern "system" should support varargs and the linked implementation history before proceeding. Done means the open checklist items are addressed and the stabilization decision is recorded.