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/mullvadvpn-app/mullvad-version
    
    the code of the file until where you have to start completion: use std::{
    env, fs,
    path::{Path, PathBuf},
    process::Command,
};

/// How many characters of the git commit that should be added to the version name
/// in dev builds.
const GIT_HASH_DEV_SUFFIX_LEN: usize = 6;

const ANDROID_VERSION_FILE_PATH: &str = "../dist-assets/android-product-version.txt";
const DESKTOP_VERSION_FILE_PATH: &str = "../dist-assets/desktop-product-version.txt";
    
fn git_rev_parse_commit_hash(git_ref: &str) -> Option<String> {
    let git_dir = Path::new("..").join(".git");
    // If we build our output on information about HEAD we need to re-run if HEAD moves
    if git_ref == "HEAD" {
        let head_path = git_dir.join("HEAD");
        if head_path.exists() {
            println!("cargo:rerun-if-changed={}", head_path.display());
        }
    }
    // If we build our output on information about a git reference, we need to re-run
    // if it moves. Instead of trying to be smart, just re-run if any git reference moves.
    let git_refs_dir = git_dir.join("refs");
    if git_refs_dir.exists() {
        println!("cargo:rerun-if-changed={}", git_refs_dir.display());
    }

    let output = Command::new("git")
        .arg("rev-parse")
        .arg(format!("{git_ref}^{{commit}}"))
        .output()
        .ok()?;
    if !output.status.success() {
        return None;
    }
    Some(String::from_utf8(output.stdout).unwrap().trim().to_owned())
}