What is Clap and how do you use it for CLI argument parsing?

Clap (Command Line Argument Parser) is the most popular Rust crate for building CLI applications with rich argument parsing, help text generation, and validation. The derive API (recommended) lets you define your CLI structure as a Rust struct: annotate it with #[derive(Parser)], and each field becomes a CLI argument. Use #[arg(short, long)] for flags, #[arg(value_name = "FILE")] for positional arguments, and #[command(author, version, about)] for app metadata read from Cargo.toml. Parse args with let cli = Cli::parse(); — Clap automatically generates --help and --version flags. Subcommands are modeled as enums with #[derive(Subcommand)]. Clap validates arguments at startup and exits with a helpful error message if inputs are invalid.