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: test_util.rs
    path of file: ./repos/arrow-ballista/examples/src
    
    the code of the file until where you have to start completion: // Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at
//
//   http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under 
    
fn get_data_dir(udf_env: &str, submodule_data: &str) -> Result<PathBuf, Box<dyn Error>> {
    // Try user defined env.
    if let Ok(dir) = env::var(udf_env) {
        let trimmed = dir.trim().to_string();
        if !trimmed.is_empty() {
            let pb = PathBuf::from(trimmed);
            if pb.is_dir() {
                return Ok(pb);
            } else {
                return Err(format!(
                    "the data dir `{}` defined by env {udf_env} not found",
                    pb.display()
                )
                .into());
            }
        }
    }

    // The env is undefined or its value is trimmed to empty, let's try default dir.

    // env "CARGO_MANIFEST_DIR" is "the directory containing the manifest of your package",
    // set by `cargo run` or `cargo test`, see:
    // https://doc.rust-lang.org/cargo/reference/environment-variables.html
    let dir = env!("CARGO_MANIFEST_DIR");

    let pb = PathBuf::from(dir).join(submodule_data);
    if pb.is_dir() {
        Ok(pb)
    } else {
        Err(format!(
            "env `{udf_env}` is undefined or has empty value, and the pre-defined data dir `{}` not found\n\
             HINT: try running `git submodule update --init`",
            pb.display(),
        ).into())
    }
}