You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: build.rs
    path of file: ./repos/weld/weld
    
    the code of the file until where you have to start completion: use std::env;
use std::process::Command;

/// Write the build ID into an environment variable.
fn register_build_id() {
    // Set the build ID, which is either unknown or the git hash
    let unknown_build = String::from("unknown");
    let build_id = ma
    
fn register_build_id() {
    // Set the build ID, which is either unknown or the git hash
    let unknown_build = String::from("unknown");
    let build_id = match Command::new("git")
        .arg("rev-parse")
        .arg("--short")
        .arg("HEAD")
        .output()
    {
        Ok(output) => String::from_utf8(output.stdout).unwrap_or(unknown_build),
        Err(_) => unknown_build,
    };
    println!("cargo:rustc-env=BUILD_ID={}", build_id);
}