Add Ruby to Rust example

ruby-to-rust/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "ruby-to-rust"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+
+[lib]
+name = "double_input"
+crate-type = ["dylib"]

ruby-to-rust/Makefile

@@ -0,0 +1,9 @@
+all: target/libdouble_input.so
+	ruby src/main.rb
+
+target/libdouble_input.so: src/lib.rs Cargo.toml
+	cargo build
+	(cd target && ln -nsf debug/libdouble_input-* libdouble_input.so)
+
+clean:
+	rm -rf target

ruby-to-rust/src/lib.rs

@@ -0,0 +1,4 @@
+#[no_mangle]
+pub extern fn double_input(input: i32) -> i32 {
+    input * 2
+}

ruby-to-rust/src/main.rb

@@ -0,0 +1,11 @@
+require 'ffi'
+
+module Hello
+  extend FFI::Library
+  ffi_lib 'target/libdouble_input.so'
+  attach_function :double_input, [ :int ], :int
+end
+
+input = 4
+output = Hello.double_input(input)
+puts "#{input} * 2 = #{output}"