Day 4 – Rust Enums and Cargo

Today's Focus

Deepen Rust knowledge with enums, pattern matching, and Cargo dependency management.

Tasks

• Refactor your Rust CLI to use clap for argument parsing: cargo add clap --features derive. Use the derive macro to define a struct with #[derive(Parser)] and subcommands for count and top. Read the generated help text with --help.
• Implement a custom error type using an enum MyError { IoError(std::io::Error), ParseError(String) } and implement std::fmt::Display for it. Replace Box<dyn Error> with MyError in your function signatures.
• Use match exhaustively on your MyError enum in main() to print a different message for each variant. Add a new variant and confirm the compiler forces you to handle it everywhere.
• Explore Rust's Option<T>: rewrite a function that previously returned a sentinel value (e.g. "" for "not found") to return Option<&str>. Call it with .unwrap_or("default"), .map(|s| s.to_uppercase()), and if let Some(v) = result { ... }.
• Inspect Cargo.toml and Cargo.lock: add a dependency with a feature flag (e.g. serde with features = ["derive"]) and one marked optional = true. Understand the difference between Cargo.lock (committed in binaries) and when to omit it (libraries).
• Run cargo audit (install with cargo install cargo-audit) to check your dependencies. Investigate any advisory reported.

Reading / Reference

• The Rust Book — Chapters 10 (Generics), 6 (Enums and Pattern Matching), 9 (Error Handling).
• Cargo Book — Specifying Dependencies and Features sections.
• clap documentation — Derive tutorial.