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/ckb/script
    
    the code of the file until where you have to start completion: //! Build script for crate `ckb-script`.
use std::env;

fn main() {
    let target_family = e
    
fn main() {
    let target_family = env::var("CARGO_CFG_TARGET_FAMILY").unwrap_or_default();
    let target_arch = env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
    let is_windows = target_family == "windows";
    let is_unix = target_family == "unix";
    let is_x86_64 = target_arch == "x86_64";
    let is_aarch64 = target_arch == "aarch64";
    let x64_asm = is_x86_64 && (is_windows || is_unix);
    let aarch64_asm = is_aarch64 && is_unix;
    let can_enable_asm = x64_asm || aarch64_asm;

    if cfg!(feature = "asm") && (!can_enable_asm) {
        panic!("ASM feature is not available for target {target_arch} on {target_family}!");
    }

    if cfg!(any(feature = "asm", feature = "detect-asm")) && can_enable_asm {
        println!("cargo:rustc-cfg=has_asm");
    }
}