Creating a Ruby Gem with Rust

Posted on May 22, 2020

In this post you will learn how to create a Rust library and access it with a Rubygem.

I’m a big fan of the Ruby programming language, it’s been a one of my primary programming languages for the last 12 years. It’s an extremely expressive language that really lends itself to writing elegant and easy to read code. One drawback however, is it’s performance. In fact it’s probably the biggest criticism of the language that I hear. If you are writing code where performance is critical, then Ruby might not be the right tool for the job. This doesn’t necessarily mean you have to switch technologies entirely, or create another microservice, you can in fact write a native library and interface it with Ruby.

A language that I have grown to love over the last few years is Rust. It’s a systems programming language that’s focused on performance and memory safety, without the garbage collection. Ideal for creating libraries and tools where performance is important.

In this tutorial we will create a very simple Rust library that will print Hello World from Rust!. We will wrap this in a Rubygem and have a Ruby interface to the Rust code.

Create a new gem with bundler

Lets go ahead and create a new Ruby gem template using bundler.

bundle gem helloworld
cd helloworld

NOTE: There are a few fields in the created helloworld.gemspec with TODO tags. You will need to edit those first before the gem can build.

Create a new Rust library with Cargo

Rust has cargo, which you can think of as the equivalent to bundler with Ruby. Using cargo we can create crates, which are just like gems.

To create a new crate with cargo is extremely easy.

cargo init --lib

Initialising the new Rust library creates a Cargo.toml file, this is much like the Ruby Gemfile and contains information about the library and it’s dependencies.

Add the following line to the Cargo.toml file to tell Rust that this is a dynamic system library and we will be using it from another language.


The Cargo.toml file should now look like the following.

name = "helloworld"
version = "0.1.0"
authors = ["Your Name <>"]
edition = "2018"



With our basic gem and crate templates in place, we can start connecting them so that we can run some Rust code from Ruby.

Writing some Rust code

This code is going to simply return "Hello World from Rust!". Open the src/ file remove the boiler plate code, replacing it with our hello world function.

pub extern fn hello_world() {
    println!("Hello World from Rust!");

The above function, is public (pub) and accessible externally (extern). It has no return type, but simply uses println! to write a string to our output.

Let’s build the Rust library

We are going to build the Rust library and make it’s shared object (.so) file accessible to our Ruby gem.

task :rust_build do
  `cargo rustc --release`
  `mv -f ./target/release/ ./lib/helloworld/`

task :build => :rust_build
task :default => :build

Our default rake task is set to build a release version of our Rust crate and move it into the lib directory so that we can use it in our gem.

Running rake now should yield the following output.

$ rake
Finished release [optimized] target(s) in 0.01s
helloworld 0.1.0 built to pkg/helloworld-0.1.0.gem.

Foreign Function Interface (FFI)

The Ruby FFI library is going to allow us to dynamically link to our Rust crate and bind the functions we want to expose, in this case it will be the hello_world() function.

Open the helloworld.gemspec file and add the FFI dependency.

spec.add_runtime_dependency "ffi"

Then run bundle to install. Now that we have our FFI dependency in place we can write some ruby code to interface with our Rust crate!

Create a new file lib/helloworld/ffi.rb and add the following lines.

require 'ffi'

module Helloworld
  class FFI
    extend ::FFI::Library
    lib_name = "helloworld.#{::FFI::Platform::LIBSUFFIX}"
    ffi_lib File.expand_path(lib_name, __dir__)
    attach_function :hello_world, [], :void

In the above code, we extend the FFI library, create a lib_name variable that points to our file and then load it. Finally we attach our Rust crate function hello_world, with no arguments ([]), and a return type of void. Remember that Rust is a statically and strongly typed language, as opposed to Ruby.

The final step!

In our lib/helloworld.rb file add the following code

require "helloworld/ffi"
require "helloworld/version"

module Helloworld
  class Error < StandardError; end

  def self.say_hello

We require our ffi file and then call the Rust crates hello_world function. It’s time to test the output! Run the bin/console and call the Ruby method we created Helloworld.say_hello and you should see the output from the Rust crate function!

$ bin/console
irb(main):001:0> Helloworld.say_hello
Hello World from Rust!
=> nil

Although this is an extremely basic example, it should set a good foundation from which to explore Rust and Ruby further. There are many instances where you might need extra performance, that you just don’t get from Ruby and Rust is an extremely good candidate for that.

You can view the full source code on GitHub

Read more on: ruby  rust