What do you mean by "reasonable?" There are several crates that provide inline C++ macros on both nightly (rustcxx) and stable (rust-cpp). I haven't used the latter but the former works really well (albeit using a gcc specific feature). I usually break out the C++ code into a wrapper (unless its a few lines) to make it more idiomatic Rust but at the end of the day, there aren't that many hoops to jump through.