Tracking Issue for `extern_system_varargs`

[workingjubilee]

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:
  • Tracking issue for future-incompatibility lint unsupported_calling_conventions #87678

Implementation history

• Varargs support for system ABI #119587
• Split out the extern_system_varargs feature #136948

[RalfJung]

we have previously rejected implementing MSVC's oddities:

You keep saying this and I keep correcting you.^^
We have implemented their oddities on Windows targets: stdcall works on x86_64-pc-windows-msvc and even aarch64-pc-windows-msvc. There was some discussion about this here and here.

[workingjubilee]

Yes yes, I copied some text around, so you get to copy some text around too.

If extern "stdcall" is downgraded to extern "C" anyways when variable arguments are involved on MSVC targets, why doesn't that apply to extern "stdcall" for all cases where the ABI is supported?

[workingjubilee]

At least if we did that, it would be... well, absurd, I think, but equally absurd in all cases?

[RalfJung]

I don't understand the question / proposal. This does apply in all cases, no?

[ChrisDenton]

stdcall works on x86_64-pc-windows-msvc and even aarch64-pc-windows-msvc.

That is not true for the raw-dylib feature, which is stricter.

[workingjubilee]

I don't understand the question / proposal. This does apply in all cases, no?

Currently rustc_hir_analysis will reject an attempt to use a variable argument function with extern "stdcall", such as extern "stdcall" fn(i32, ...), without further consideration.

[RalfJung]

Currently rustc_hir_analysis will reject an attempt to use a variable argument function with extern "stdcall", such as extern "stdcall" fn(i32, ...), without further consideration.

I guess the argument is that MSVC accepts varagrs for system but not for stdcall? Or is that not correct?

That is not true for the raw-dylib feature, which is stricter.

Oh, that is an odd inconsistency... I filed a separate issue for that: #137018.

[workingjubilee]

I guess the argument is that MSVC accepts varagrs for system but not for stdcall? Or is that not correct?

extern "system" is a Rust invention, as far as I am aware.