These are all great points, and I will update the blog post to reflect them in the morning.

I believe this approach can work while retaining the most apparently-idiomatic mapping. What do you guys think?

impl IRibbonExtensibility_Impl for Addin_Impl {

    unsafe fn GetCustomUI(&self, _ribbon_id: BSTR, out: *mut BSTR) -> HRESULT {

        log("GetCustomUI called()");

        std::mem::forget(_ribbon_id);

        if out.is_null() {

            return windows::Win32::Foundation::E_POINTER;
        }

        unsafe {

            std::ptr::write(out, BSTR::from(RIBBON_XML));
        }

        S_OK
    }

}