Command-Line Argument Parsing with structopt

This challenge focuses on leveraging the structopt crate in Rust to elegantly parse command-line arguments. You will create a Rust program that accepts various command-line options and subcommands, demonstrating how structopt simplifies the process of mapping these arguments to a structured representation. This is a fundamental skill for building robust and user-friendly command-line applications in Rust.

Problem Description

Your task is to build a simple command-line utility that performs different operations based on user-provided arguments. You will use the structopt crate to define the structure of your command-line interface (CLI) and automatically handle the parsing of arguments.

Specifically, your program should:

1. Define a main CLI structure: This structure will hold global options and subcommands.
2. Implement at least two subcommands: Each subcommand should have its own set of arguments.
3. Support different argument types: Include string, integer, and boolean arguments.
4. Handle optional arguments: Demonstrate how to make arguments optional.
5. Parse and display the arguments: Upon execution, the program should print a nicely formatted representation of the parsed arguments, showing which subcommand was invoked and its associated parameters.

Consider the following when implementing:

• Argument Naming: structopt uses Rust's field names and attributes to infer command-line argument names.
• Subcommand Structure: structopt allows you to define subcommands using an enum.
• Default Values: Implement default values for certain optional arguments.
• Help Messages: Ensure that meaningful help messages are generated by structopt for your arguments and subcommands.

Examples

Example 1: add subcommand

Input:

./my_cli add --name "Alice" --age 30 --verbose

Output:

Operation: Add
Name: Alice
Age: 30
Verbose: true

Explanation: The program parses the add subcommand with the provided name, age, and verbose flags.

Example 2: greet subcommand with optional argument

Input:

./my_cli greet --person Bob

Output:

Operation: Greet
Person: Bob
Greeting: Hello

Explanation: The greet subcommand is invoked. The person argument is provided, and since the greeting argument is optional and not provided, its default value is used.

Example 3: greet subcommand with optional argument and custom greeting

Input:

./my_cli greet --person Charlie --greeting "Good morning"

Output:

Operation: Greet
Person: Charlie
Greeting: Good morning

Explanation: The greet subcommand is invoked with a custom greeting.

Example 4: Help message

Input:

./my_cli --help

Output:

Usage: my_cli [OPTIONS] <SUBCOMMAND>

Options:
  -h, --help     Print help information

Subcommands:
  add     Add a new entry
  greet   Greet a person

Explanation: structopt automatically generates help messages based on your struct and enum definitions.

Constraints

• The program must be written in Rust.
• The structopt crate must be used for argument parsing.
• The program must compile and run without errors.
• The output for each example must match the provided output format exactly.
• The add subcommand should have a required --name (String), a required --age (u32), and an optional --verbose (bool) flag.
• The greet subcommand should have a required --person (String) and an optional --greeting (String) with a default value of "Hello".

Notes

• You'll need to add structopt as a dependency in your Cargo.toml.
• Consider using #[derive(StructOpt)] for your main struct and #[derive(StructOpt)] for your subcommand enum.
• Use #[structopt(about = "...")] and #[structopt(long_help = "...")] for better help messages.
• Pay attention to how you define argument names, types, and attributes (required, default_value, short, long).