Create a File

We create a file in our crate's root. Cargo will pick up on the existence of this file, then compile and execute it before the rest of the crate is built. This can be used to generate code at compile time. And of course in our case, we will be generating Rust FFI bindings to bzip2 at compile time. The resulting bindings will be written to $OUT_DIR/ where $OUT_DIR is chosen by cargo and is something like ./target/debug/build/bindgen-tutorial-bzip2-sys-afc7747d7eafd720/out/.

extern crate bindgen;

use std::env;
use std::path::PathBuf;

fn main() {
    // Tell cargo to tell rustc to link the system bzip2
    // shared library.

    // The bindgen::Builder is the main entry point
    // to bindgen, and lets you build up options for
    // the resulting bindings.
    let bindings = bindgen::Builder::default()
        // The input header we would like to generate
        // bindings for.
        // Finish the builder and generate the bindings.
        // Unwrap the Result and panic on failure.
        .expect("Unable to generate bindings");

    // Write the bindings to the $OUT_DIR/ file.
    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
        .expect("Couldn't write bindings!");

Now, when we run cargo build, our bindings to bzip2 are generated on the fly!

There's more info about files in the documentation.