Optional function argument is not exported correctly

I stumbled upon a weird case... it's unlikely to appear in real code but rust supports it, so cbindgen should support it too.

example.rs

#[allow(unexpected_cfgs)]
#[no_mangle]
pub extern "C" fn foobar(a1: u32, #[cfg(feature = "baz")] a2: u32) -> u32 {
    #[cfg(feature = "baz")]
    {
        a1 + a2
    }
    #[cfg(not(feature = "baz"))]
    {
        a1
    }
}

cbindgen.toml

language = "C"
[defines]
"feature = baz" = "FEATURE_BAZ"

BAD example.h (generated)

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

uint32_t foobar(uint32_t a1, uint32_t a2);

What would be the preferred way to handle this case?

• generate multiple function declarations, each surrounded by an appropriate ifdef
• surround each optional argument with an appropriate ifdef
• apply a macro call to the optional argument text
• something else

Start by reproducing the issue with the provided example.rs and cbindgen.toml, then inspect the generated example.h and the existing handling of cfg-gated function arguments. Done means reaching an agreed representation for the optional argument and adding coverage that verifies the generated C declaration for both feature configurations.