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/sentry-cli
    
    the code of the file until where you have to start completion: use std::env;
use std::fs::File;
use std::io::Write;
use std::path::Path;

fn main() {
    let out_dir = env::var("OUT_DIR").unwrap();
    let dest_path = Path::new(&out_dir).join("constants.gen.rs");
    let mut f = File::create(dest_path).unwrap();

    let target = env::var("TARGET").unwrap();
    let mut target_bits = target.split('-');

    // https://rust-lang.github.io/rfcs/0131-target-specification.html#detailed-design
    let mut arch = target_bits.next().unwrap();
    let _vendor = target_bits.next();
    let platform = target_bits.next().unwrap();

    if platform == "darwin" && arch == "aarch64" {
        arch = "arm64"; // enforce Darwin naming conventions
    }

    writeln!(f, "/// The platform identifier")
    
fn main() {
    let out_dir = env::var("OUT_DIR").unwrap();
    let dest_path = Path::new(&out_dir).join("constants.gen.rs");
    let mut f = File::create(dest_path).unwrap();

    let target = env::var("TARGET").unwrap();
    let mut target_bits = target.split('-');

    // https://rust-lang.github.io/rfcs/0131-target-specification.html#detailed-design
    let mut arch = target_bits.next().unwrap();
    let _vendor = target_bits.next();
    let platform = target_bits.next().unwrap();

    if platform == "darwin" && arch == "aarch64" {
        arch = "arm64"; // enforce Darwin naming conventions
    }

    writeln!(f, "/// The platform identifier").ok();
    writeln!(f, "pub const PLATFORM: &str = \"{platform}\";").ok();
    writeln!(f, "/// The CPU architecture identifier").ok();
    writeln!(f, "pub const ARCH: &str = \"{arch}\";").ok();
    writeln!(f, "/// The user agent for sentry events").ok();
    writeln!(f, "pub const USER_AGENT: &str = \"sentry-cli/{arch}\";").ok();
    println!("cargo:rerun-if-changed=build.rs\n");
}