Hello, World and cargo

cargo new, run, build, test

cargo is Rust's build tool and package manager. cargo new app creates a project with Cargo.toml and src/main.rs; cargo run compiles and runs; cargo build --release produces an optimised binary; cargo test runs tests; cargo add adds dependencies. println! is a macro (note the !) that formats text, with {name} inline arguments and format specifiers for width, alignment and precision.

A first program, run

I ran this with Rust 1.99.0 (cargo run, edition 2024, standard library only). println! inserts variables by name and formats width, alignment and decimal places.

fn main() {
    let name = "Ferris";
    let crates = 3;
    println!("Hello, {name}! You have {crates} crates to review.");
    println!("{:>8}|{:<6}|{:.2}", "right", "left", 3.14159);
}

Output:

Hello, Ferris! You have 3 crates to review.
   right|left  |3.14

Use --release for benchmarks

Debug builds are much slower; always measure performance with cargo build --release or cargo run --release.

Quick check: Why does println! end with an exclamation mark?

• It is a macro, not a function
• It prints in bold
• It panics after printing
• It runs at compile time only