Your first program and cargo

Why cargo?

You could compile a single file with rustc main.rs, but real programs have many files and use libraries written by others. cargo handles all of that: it creates the project, downloads libraries, compiles everything in the right order and runs your tests. Almost every Rust project uses cargo.

cargo new hello
cd hello
cargo run

What cargo new creates

hello/
├── Cargo.toml      <- project name, version, edition, dependencies
├── .gitignore      <- tells git to ignore the target folder
└── src/
    └── main.rs     <- your code starts here

[package]
name = "hello"
version = "0.1.0"
edition = "2024"

[dependencies]

Cargo.toml is written in TOML, a simple settings format. The [dependencies] section lists libraries your project uses. In Rust, a library you can download is called a crate. After your first build, cargo also writes Cargo.lock (the exact versions it used) and a target folder (the compiled output).

Reading main.rs line by line

fn main() {
    println!("Hello, world!");
}

• fn main() defines a function named main. Every Rust program starts running here.
• The curly braces { } hold the body of the function.
• println! prints text and then a new line. The ! means println! is a macro, not a normal function. A macro is code that writes more code for you at compile time; println! uses this to check your placeholders.
• The line ends with a semicolon ;.

Printing values with placeholders

fn main() {
    let city = "Pune";
    let temp = 31;
    println!("{} is {} degrees today", city, temp);
    println!("{city} is {temp} degrees today");
    println!("{:?}", (city, temp));
}

{} uses the Display format, meant for users. {:?} uses the Debug format, meant for programmers; it works for things like lists and tuples that have no Display format. If you pass the wrong number of values, the program does not compile; the println! macro checks this for you.

The cargo commands you will use every day

Common mistakes

• Writing println("Hi") without the !. The error says println is a macro, so add the !.
• Using single quotes for text: 'Hello'. In Rust, single quotes are for one character (a char). Text uses double quotes.
• Forgetting a placeholder: println!("Total", total) does not compile. Write println!("Total {}", total).
• Committing the target folder to git. cargo's .gitignore already excludes it; keep it that way.