build.rs

use std::env;
use std::fs;
use std::io;
use std::path::{Path, PathBuf};

fn main() -> io::Result<()> {
    println!("cargo:rerun-if-changed=assets");

    let manifest_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR"));
    let source_assets = manifest_dir.join("assets");
    if !source_assets.is_dir() {
        return Ok(());
    }

    let out_dir = PathBuf::from(env::var("OUT_DIR").expect("OUT_DIR"));
    let profile_dir = out_dir
        .ancestors()
        .nth(3)
        .expect("OUT_DIR should have target/<profile>/build/.../out layout");
    let target_assets = profile_dir.join("assets");

    copy_dir_all(&source_assets, &target_assets)
}

fn copy_dir_all(src: &Path, dst: &Path) -> io::Result<()> {
    fs::create_dir_all(dst)?;

    for entry in fs::read_dir(src)? {
        let entry = entry?;
        let entry_path = entry.path();
        let target_path = dst.join(entry.file_name());
        let file_type = entry.file_type()?;

        if file_type.is_dir() {
            copy_dir_all(&entry_path, &target_path)?;
        } else if file_type.is_file() {
            fs::copy(&entry_path, &target_path)?;
        }
    }

    Ok(())
}