# Hello, World and cargo — Rust

Source: https://www.skillbyai.com/en/rust/i-cargo

> One tool to build, run, test and publish.

## 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.

```rust
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.

**Quiz:** Why does println! end with an exclamation mark?

- [x] It is a macro, not a function
- [ ] It prints in bold
- [ ] It panics after printing
- [ ] It runs at compile time only

*Answer:* It is a macro, not a function. Macros generate code.