Support for Optional arguments

Input C/C++ Header

Note: In the real world, priority is an enum, but I wanted to keep this header minimal, as the outcome is the same.

extern "C" {
    void op_log(const char *message, const int priority = 1, const int line = -1, const char *function = "", const char *file = "");
}

Bindgen Invocation

This is probably irrelevant, but for completeness:

let bindings = bindgen::Builder::default()
        .clang_arg("include/rust_wrapper/wrapper.hpp")
        .clang_arg("-I./openpose/include")
        .clang_arg("-xc++")
        .clang_arg("-std=c++14")
        .parse_callbacks(Box::new(bindgen::CargoCallbacks))
        .generate()
        .expect("Unable to generate bindings");

Actual Results

extern "C" {
    pub fn op_log(
        message: *const ::std::os::raw::c_char,
        priority: op_Priority,
        line: ::std::os::raw::c_int,
        function: *const ::std::os::raw::c_char,
        file: *const ::std::os::raw::c_char,
    );
}

Expected Results

extern "C" {
    pub fn op_log(
        message: *const ::std::os::raw::c_char,
        priority: ::std::Option<op_Priority>,
        line: ::std::Option<::std::os::raw::c_int>,
        function:  ::std::Option<*const ::std::os::raw::c_char>,
        file:  ::std::Option<*const ::std::os::raw::c_char>,
    );
}

Now I am uncertain whether extern "C" supports that, as it might change the ABI, but if that's the case, one could keep op_log private (maybe _op_log) and expose a small op_log wrapper, which only passes the parameters when Some().

Alternatively, one could extract the default values and pass them using said wrapper.

Maybe you have a different idea as well :)

Start with the minimal C++ header and the shown bindgen Builder invocation, then inspect how bindgen represents C++ default arguments in generated Rust declarations. Check whether Optional parameters are ABI-safe for extern "C" functions; done should be a settled implementation or documented decision covering the expected generated bindings and default values.