Calling rust from ruby ffi

Cargo.toml

[package]
name = "double"
version = "0.1.0"
authors = ["Jason Yeo <jasonyeo88@gmail.com>"]

[lib]
name = "double"
crate-type = ["dylib"]

double.rb

require 'ffi'

module MyFabulousLib
  extend FFI::Library
  ffi_lib "./target/release/libdouble.so"
  attach_function :double, [:int], :int
end

puts MyFabulousLib.double(5)
puts MyFabulousLib.double(8)

lib.rs

#[no_mangle]
pub extern fn double(x: i32) -> i32 {
    println!("Hello from rust!");
    x * 2
}

#[test]
fn it_works() {
    assert_eq!(double(0), 0);
    assert_eq!(double(2), 4);
    assert_eq!(double(-1), -2);
}