Command-Line Argument Parser with structopt

This challenge focuses on using the structopt crate in Rust to create a robust and user-friendly command-line interface (CLI) for a simple application. structopt simplifies the process of parsing command-line arguments by deriving parsing logic directly from struct fields, reducing boilerplate and improving code readability. Your task is to build a CLI tool that takes input file paths, an optional output file path, and a verbosity level, then prints a message indicating the operation.

Problem Description

You need to implement a Rust program that uses structopt to parse command-line arguments and perform a simple action based on those arguments. The program should accept the following arguments:

• --input <file> (required): The path to the input file.
• --output <file> (optional): The path to the output file. If not provided, the program should print a message indicating that no output file was specified.
• --verbose <level> (optional): A verbosity level (0, 1, or 2). Higher levels produce more detailed output. If not provided, the default verbosity level is 0.

The program should then print a message to the console that includes the input file path, the output file path (if provided), and the verbosity level.

Key Requirements:

• Use the structopt crate for argument parsing.
• The --input argument is mandatory.
• The --output argument is optional.
• The --verbose argument is optional and must be one of 0, 1, or 2.
• Handle invalid verbosity levels gracefully (e.g., print an error message and exit).
• Provide helpful usage information if the program is called with incorrect arguments.

Expected Behavior:

The program should parse the command-line arguments and print a message in the following format:

Processing input file: <input_file_path>
Output file: <output_file_path> (or "Not specified")
Verbosity level: <verbosity_level>

If the --output argument is not provided, the "Output file" line should read "Output file: Not specified". If an invalid verbosity level is provided, the program should print an error message to stderr and exit with a non-zero exit code.

Examples

Example 1:

Input: my_program --input input.txt --output output.txt --verbose 1
Output:
Processing input file: input.txt
Output file: output.txt
Verbosity level: 1

Example 2:

Input: my_program --input input.txt --verbose 2
Output:
Processing input file: input.txt
Output file: Not specified
Verbosity level: 2

Example 3:

Input: my_program --input input.txt --verbose 3
Output:
Error: Invalid verbosity level. Must be 0, 1, or 2.

Example 4:

Input: my_program --output output.txt --verbose 1
Output:
Error: Missing required argument: --input <file>
USAGE:
my_program --input <file> [OPTIONS]
OPTIONS:
    -i, --input <file>      The path to the input file
    -o, --output <file>     The path to the output file
    -v, --verbose <level>   Verbosity level [default: 0]

Constraints

• The program must compile and run without errors.
• The program must correctly parse all valid command-line arguments.
• The program must handle invalid verbosity levels and missing required arguments gracefully.
• The program should use structopt effectively to minimize boilerplate code.
• The program should exit with a non-zero exit code if an error occurs.

Notes

• You'll need to add structopt as a dependency in your Cargo.toml file.
• Consider using Result to handle potential errors during argument parsing.
• The clap crate (which structopt builds upon) provides extensive documentation and examples. Refer to it if you need more information.
• Focus on clear and concise code that is easy to understand and maintain. Error handling is important.
• The specific action performed on the input file is not important for this challenge; the focus is on argument parsing.