on X86 SysV the ABI of an empty struct is different from Clang and GCC

With Clang and GCC

https://godbolt.org/z/rWvcYYajx

typedef struct { } EmptyStruct;
EmptyStruct val_out(void) { EmptyStruct o = {}; return o; }

val_out:
        mov     eax, dword ptr [esp + 4]
        ret     4

But rustc

https://godbolt.org/z/E99cdz69n

#[repr(C)] pub struct EmptyStruct {}

#[unsafe(no_mangle)]
extern "C" fn val_out() -> EmptyStruct { 
    EmptyStruct {}
}

emits only

val_out:
        ret

zero-sized structs are only a GNU extension, but in practice rustc is compatible for most targets.

Reproduce the difference using the two linked Godbolt examples and compare rustc's x86 SysV ABI handling with the Clang and GCC output for val_out. Determine the expected calling convention for the empty #[repr(C)] struct and define completion as rustc's behavior being resolved or the incompatibility being explicitly justified and covered by a regression test.