I’ve falling in love with Rust over the last year and a half. I like it for a lot of reasons, a few being the compiler, seperation of noun/verb, and it’s deeply integrated testing. However, recently been thinking through ways to introduce it to the team and the technology portfolio at Fooda.

One idea we had, was move some of our computational intensive tasks to Rust given that is one of it’s “sweet spots” as a technology. However, this left us wondering how exactly would we mix it in with Ruby. Specifically, how do we call Rust from within Ruby.

As it turns out, it’s actually fairly easy to call Rust from within Ruby so long as you understand a few simple things, namely

  • Rust
  • C and ABI
  • FFI

With those, and a few lines of code to begin calling Rust from within your Ruby code.

Preparing Rust to be “callable” from Ruby

Ultimately, you don’t need to do a whole lot to make your Rust code available to C.
In fact, here’s a simple hello world example.

With the code snippet above, you’ll see a couple things different from your typical Rust function. So lets break it down …

  • #[no_mangle] This tells the Rust compiler to expose this a global function and don’t mangle it with an internal name
  • extern "C" This exposes the function as if it was a C function via the C ABI
  • *const libc::c_char Our function returns a C “string”

Outside that, our function is a regular Rust function that does it’s thing.

Compile in a slightly different way

Now that your Rust code is exposed prepared properly, we need to ensure we compile in a manner Ruby can accept, specifically a dynamic library. You can do this two ways, the command line or a Cargo toml file. I’m doing it through Cargo and a toml file below

name = "library"
version = "0.1.0"
authors = ["Anthony Broad-Crawford <anthony@anthonybroadcrawford.com>"]

crate-type = ["dylib"]

libc = "0.1.10"

When you execute Cargo build you will now have a dynamic library .dylib in your targets folder.

Including your Rust library in Ruby

If there’s an icky part to this whole process, this is it right here. You’ll need a one-to-one mapping of the functionality you expose in your Rust dynamic library to your Ruby application.

Here’s an example of how this is done leveraging ffi

require 'ffi'

module Rust
  extend FFI::Library
  ffi_lib '../rust-library/target/debug/liblibrary.dylib'

  attach_function :hello_world, [], :string

Please note, the path reference above in the ffi_lib method call is specific to a sample project I’ve included below.

Test drive in IRB

At this point you are able to take it for a test drive in irb.

ABC-194:rust-library anthony$ Cargo build
   Compiling libc v0.1.10
   Compiling library v0.1.0 (file:///Users/anthony/example_rust_called_from_ruby/rust-library)
ABC-194:rust-library anthony$ cd ..
ABC-194:example_rust_called_from_ruby anthony$ ls
.            ..           .git         ruby-library rust-library
ABC-194:example_rust_called_from_ruby anthony$ cd ruby-library/
ABC-194:ruby-library anthony$ ls
.              ..             ruby-client.rb
ABC-194:ruby-library anthony$ irb
2.0.0-p247 :001 > require "./ruby-client"
 => true
2.0.0-p247 :002 > Rust::hello_world
 => "hello world"
2.0.0-p247 :003 >

That’s it. I was incredibly surprised at how easy it was. The only thing I don’t like, is maintaining that one-to-one mapping in via ffi in Ruby. A small price to pay, I guess.

Complete working example

I’ve put together a complete working example that will allow you from irb to call into a Rust module. It’s a hello world but it works.

It’s found here