It's not just about using C++ libraries from Rust, but also the other way around.

You have 15 million lines of C++ code, and you want to start writing some components in Rust. At the interface, you need a bridge.

I have a pretty cool use case where I successfully interfaced between C++ and Rust code.

This was for a project consisting of a sculpture with a bunch of LED strips on it, and my part was to do the programming, electronics and wiring for it.

To control the LED strips I use Teensy 3.2 microcontroller boards, with the OctoWS2811 adaptor board [0], along with an industrial singleboard computer.

The Teensy 3.2 boards I programmed using the Arduino libraries and the OctoWS2811 library [1]. The code I wrote for this part was C++. (But I don’t use the Arduino IDE, I use JetBrains CLion.)

Compiling and flashing the firmware for the Teensy, and disconnecting and reconnecting cables etc was making the development a bit annoying.

So in Rust I wrote a LED simulator program that renders the “pixels” of the LED strips on my screen, and I compiled my C++ code that I’ve written for the microcontrollers on the host, with some shims that I wrote to provide some functions that my firmware uses, and I link this with my Rust program.

The end result is that I have a fully functioning LED simulator that I use while developing, that runs on my computer. It runs on both macOS and Linux. And then the same C++ code of mine gets included and built without modification when I build my firmware for the microcontrollers.

It is work that I am very satisfied about :)

Should also note that I use the bindgen crate, not cxx. https://crates.io/crates/bindgen

[0]: https://www.pjrc.com/store/octo28_adaptor.html

[1]: https://www.pjrc.com/teensy/td_libs_OctoWS2811.html

There is a lot of code written in C++. For a lot of large companies, one of the impediments to migrating from C++ to Rust is the investment required to expose the existing C++ interfaces in Rust. There are also many open source libraries written in C++ that you can more easily call from Rust using this bridge library.

Rewriting a few million lines of existing C++ is not cheap nor quickly done.

If you have existing resources written in C++, external libs, maybe you've created a 'plugin' which is loaded by an application but the plugin is written in Rust and the app is written in C++ and it needs to invoke back into the main applications code. Just examples of stuff where I've had to interop Rust with other things outside of my control :)

It takes a lot less time than rewriting [component] in Rust

I guess if you already have C++ that you'd prefer not to re-write. Also things like graphics programming, while making great strides in rust, are definitely still superior in C++ in certain aspects so you might want to write, say, an OpenGL interface in C++.

Plenty of ecosystems where C++ rules and no one is going to rewrite any of those libraries.

There's lots of useful C++ libraries that don't have (good) rust bindings.