Support C++ exceptions without undefined behaviour

Bindgen currently does not (cannot) support C++ exceptions, because C++ won't be able to unwind correctly into the rust code.

An idea to support this would be to emit a C++ wrapper for the C++ functions (see https://github.com/rust-lang-nursery/rust-bindgen/issues/1172#issuecomment-354831001 for the design idea). Emitting a C++ wrapper for functions would also allow to (by making it extern "C") remove all of bindgen's code for mangling C++ functions and make supporting new C++ compilers much easier.

A C++ wrapper function could look like this (syntax untested esp. the union one, and there are tricks with non-copyable return types, I can never remember the correct syntax for moving c++ stuff):

// Original function
ret_t fun(arg_t arg) {
    // ...
}

// Wrapper function
struct XXX_bindgen_result_ret_t {
    enum { OK, ERROR, UNKNOWN_ERROR } tag;
    union {
        ret_t ok;
        char * error_msg;
    } value;

    static XXX_bindgen_result_ret_t make_ok(ret_t r) {
        XXX_bindgen_result_ret_t ret;
        ret.tag = OK;
        ret.value.ok = r;
        return ret;
    }
    static XXX_bindgen_result_ret_t make_err(char * s) {
        XXX_bindgen_result_ret_t ret;
        ret.tag = ERROR;
        ret.value.error_msg = s;
        return ret;
    }
    static XXX_bindgen_result_ret_t make_unknown_error() {
        XXX_bindgen_result_ret_t ret;
        ret.tag = UNKNOWN_ERROR;
        return ret;
    }
};

extern "C"
XXX_bindgen_result_ret_t XXX_bindgen_function_fun(arg_t arg) {
    try {
        return XXX_bindgen_result_ret_t::make_ok(original_cpp_function(arg));
    } catch (exception e) {
        return XXX_bindgen_result_ret_t::make_err(e.message());
    } catch (...) {
        return XXX_bindgen_result_ret_t::make_unknown_error();
    }
}

Basically, with this C++ wrapper, bindgen takes control of the way FFI is performed, and can almost ignore the fact that it's a C++ compiler on which it's running.

The drawbacks are:

• A (single) additional function call at each FFI call
• Requirement to build an additional C++ file in addition to building rust code
• This requirement is a backward-incompatible change if it's not hidden behind a flag (and I guess if exceptions are supported it should be by opt-out and not opt-in, unless not opting in also means not allowing C++, given it's such an easy way to trigger UB and exceptions are way too rarely documented)

Start by reading the proposed C++ wrapper design in this issue and the linked rust-bindgen issue 1172. Work out how wrapper generation, exception-to-result handling, and the additional C++ build requirement would fit together; done means C++ exceptions cannot unwind into Rust and the design's compatibility trade-offs are resolved.