Variadic functions don't allow "system" on non-x86 Windows

[kennykerr]

While testing variadic function support with raw-dylib I discovered that Rust requires such functions to use either "C" or "cdecl" ABIs but raw-dylib requires the "system" ABI on non-x86 Windows.

My undertstanding from discussing with @dpaoliello is that this is not an issue with raw-dylib itself but with Rust's insistence that such functions aren't allowed to use the "system" ABI. It's just that raw-dylib is very strict about calling convention so that's where the issue surfaced.

#[link(name = "library", modifiers = "+verbatim")]
extern "system" {
    pub fn DbgPrint(format: *const u8, ...) -> u32;
}

On x64 I should be able to compile this but Rust says no:

error[E0045]: C-variadic function must have a compatible calling convention, like `C` or `cdecl`
 --> src\lib.rs:3:5
  |
3 |     pub fn DbgPrint(format: *const u8, ...) -> u32;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ C-variadic function must have a compatible calling convention

This example works if I change the ABI to "cdecl" but that's not meant to be a valid ABI on x64 and so when I try to compile this with raw-dylib it fails:

#[link(name = "library", kind = "raw-dylib", modifiers = "+verbatim")]
extern "cdecl" {
    pub fn DbgPrint(format: *const u8, ...) -> u32;
}

error: ABI not supported by `#[link(kind = "raw-dylib")]` on this architecture
 --> src\lib.rs:3:5
  |
3 |     pub fn DbgPrint(format: *const u8, ...) -> u32;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The practical result is that it is impossible to link this function with raw-dylib on non-x86.

[compiler-errors]

cc #97971 which originally added variadics support to some other calling conventions (though not extern "system" it seems) behind the feature gate #![feature(extended_varargs_abi_support)].

[kennykerr]

A workaround is to use "C" on non-x86 as I implemented here: microsoft/windows-rs#2458

[dpaoliello]

@wesleywiser @nagisa Thoughts on permitting variadic parameters with the system calling convention for non-x86 Windows? If #87678 gets merged then we'll be pushing folks to use system so that they get the correct calling convention for Windows APIs on all architectures, but that breaks compatibility with variadic parameters.

[RalfJung]

A workaround is to use "C" on non-x86 as I implemented here: microsoft/windows-rs#2458

This work-around is causing pain. Would be great if there was a way that it could be avoided. :)

[RalfJung]

Thoughts on permitting variadic parameters with the system calling convention for non-x86 Windows?

There's a comment saying this can't work:

rust/compiler/rustc_target/src/spec/abi/mod.rs

Lines 70 to 78 in ef7ebaa

	// * C and Cdecl obviously support varargs.
	// * C can be based on Aapcs, SysV64 or Win64, so they must support varargs.
	// * EfiApi is based on Win64 or C, so it also supports it.
	//
	// * Stdcall does not, because it would be impossible for the callee to clean
	// up the arguments. (callee doesn't know how many arguments are there)
	// * Same for Fastcall, Vectorcall and Thiscall.
	// * System can become Stdcall, so is also a no-no.
	// * Other calling conventions are related to hardware or the compiler itself.

        // * Stdcall does not, because it would be impossible for the callee to clean
        //   up the arguments. (callee doesn't know how many arguments are there)
        // * System can become Stdcall, so is also a no-no.

I don't understand anything about these ABIs so I can't say how accurate this is.

Also I see nothing here special-casing x86, so why does the issue specifically mention non-x86?