Add Rust to C example

.gitignore

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+Cargo.lock
+target

rust-to-c/Cargo.toml

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+[package]
+name = "rust-to-c"
+version = "0.1.0"
+authors = ["Alex Crichton <alex@alexcrichton.com>"]
+build = "build.rs"
+
+[dependencies]
+libc = "0.1"
+
+[build-dependencies]
+gcc = "0.3"

rust-to-c/build.rs

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+extern crate gcc;
+
+fn main() {
+    gcc::Config::new().file("src/double.c").compile("libdouble.a");
+}

rust-to-c/src/double.c

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+int double_input(int input) {
+    return input * 2;
+}

rust-to-c/src/main.rs

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+extern crate libc;
+
+extern {
+    fn double_input(input: libc::c_int) -> libc::c_int;
+}
+
+fn main() {
+    let input = 4;
+    let output = unsafe { double_input(input) };
+    println!("{} * 2 = {}", input, output);
+}