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: standard_lib.rs
    path of file: ./repos/risc0/risc0/zkvm/methods/std/src/bin
    
    the code of the file until where you have to start completion: // Copyright 2024 RISC Zero, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the L
    
fn main() {
    let test_mode = std::env::var("TEST_MODE").unwrap();

    match test_mode.as_str() {
        "STDIO" => {
            // Test stdin, stdout, stderr
            eprintln!("Hello world on stderr!");
            println!("Hello world on stdout!");
            let mut from_stdin = Vec::new();
            stdin().read_to_end(&mut from_stdin).unwrap();
            stdout().write_all(&from_stdin).unwrap();
        }
        "ENV_VARS" => {
            // Read environment variables from standard input and print them to the journal.
            for var_name in stdin().lines() {
                let var_name = var_name.unwrap();
                let msg = match std::env::var(&var_name) {
                    Err(_) => format!("!{var_name}\n"),
                    Ok(val) => format!("{var_name}={val}\n"),
                };
                risc0_zkvm::guest::env::commit_slice(msg.as_bytes());
            }
        }
        "ARGS" => {
            // Collect args into a vector and commit them to the journal.
            let args: Vec<String> = std::env::args().collect();
            risc0_zkvm::guest::env::commit(&args);
        }
        _ => {
            panic!("Unknown test mode {test_mode}");
        }
    }
}