tauri/cli/core/build.rs

use std::env;
use std::env::current_dir;
use std::error::Error;
use std::fs::{read_to_string, File};
use std::io::{BufWriter, Write};
use std::path::Path;

pub fn main() -> Result<(), Box<dyn Error>> {
  let out_dir = env::var("OUT_DIR")?;

  let dest_bundle_umd_path = Path::new(&out_dir).join("tauri.bundle.umd.js");
  let mut bundle_umd_file = BufWriter::new(File::create(&dest_bundle_umd_path)?);

  let bundle_umd_path = current_dir()?.join("../../api/dist/tauri.bundle.umd.js");
  println!("cargo:rerun-if-changed={:?}", bundle_umd_path);
  if let Ok(bundle_umd_js) = read_to_string(bundle_umd_path) {
    write!(bundle_umd_file, "{}", bundle_umd_js)?;
  } else {
    write!(
      bundle_umd_file,
      r#"throw new Error("you are trying to use the global Tauri script but the @tauri-apps/api package wasn't compiled; run `yarn build` first")"#
    )?;
  }

  Ok(())
}