How to correctly use rust library from C++?

[NoeMurr]

I’ve developed a Rust library for some core computations, and I want to expose it so it can be used from a C++ project.

Example

To explore how this integration works, I’m using the following minimal demo.

Rust: src/lib.rs

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn print_message_from_cpp(msg: &str);
    }
}

fn print_message_from_cpp(msg: &str) {
    println!("Received from C++: {}", msg);
}

C++: src/main.cc

#include "cxx.h"
#include "lib.rs.h"
#include <string>

int main() {
    std::string cpp_msg = "Hello from C++!";
    rust::String rust_compatible(cpp_msg.c_str());
    print_message_from_cpp(rust_compatible);
    return 0;
}

Build script: build.rs

fn main() {
    cxx_build::bridge("src/lib.rs").compile("rust_cpp_demo");
    println!("cargo:rerun-if-changed=src/lib.rs");
}

Compilation

I'm trying to compile the C++ code with:

g++ -std=c++17 src/main.cc -o cpp_runner \
    -I target/cxxbridge/cxx_demo/src/ \
    -I target/cxxbridge/rust/ \
    -L target/debug/build/cxx_demo-<hash>/out \
    -L target/debug/build/cxx-<hash>/out \
    -lcxxbridge1 -lrust_cpp_demo -lpthread -ldl

(Replacing with the correct build hashes.)

However, this leads to numerous undefined reference errors for symbols defined by cxx, like:

undefined reference to `cxxbridge1$rust_vec$str$set_len`

The Problem

I'm unclear on the correct steps for compiling and packaging the Rust library so that its C++ bindings can be linked and used from an external project. Specifically, how should I build and expose the cxx and Rust-generated symbols correctly?