What is Cargo?

In PHP

Composer starts a project, reads composer.json, writes composer.lock, and downloads packages from Packagist. You still run PHP yourself. Cargo is that, and it is also the thing that compiles, tests, formats, and lints. One tool, the way you wish php and composer were sometimes the same command.

In Rust

cargo new creates a directory, a git repo, Cargo.toml, and src/main.rs. cargo init does the same in the current directory. Libraries (cargo new --lib) are in the modules chapter.

cargo new hello-cargo
cd hello-cargo

The generated program is small.

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

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

edition = "2024" is the language edition. It is not the compiler version. Leave it on 2024 for this course.

cargo run
cargo fmt
cargo clippy
cargo build
cargo build --release

• cargo run compiles and runs src/main.rs.
• cargo fmt formats the crate. Run it before you commit.
• cargo clippy is the linter. Default lints are enough for this course. In a crate where you want stricter advice, you can add #![warn(clippy::pedantic)] at the top of src/main.rs; this course does not require a clean pedantic run.
• cargo build writes target/debug/hello-cargo. cargo build --release writes target/release/hello-cargo, optimised. Release is faster; debug keeps more information for backtraces. Debug builds also panic on integer overflow; release builds wrap. That difference is why the calculator later uses checked_* instead of plain + and /.

Keep warnings as warnings while you work locally. In CI, treat them as errors with RUSTFLAGS="-D warnings" before you build, or run cargo clippy -- -D warnings. An unused Result then fails the pipeline (unused_must_use) instead of scrolling past. This course points at that CI setup from here on. Do not put warnings = "deny" under [lints.rust] in Cargo.toml for day-to-day work: every unused variable, field, or import becomes a hard error, which fights incomplete code.

cargo check type-checks without writing the executable. It is the fast loop while you are fixing errors.

Watch for: hand-editing a version into Cargo.toml for every crate. From the next lesson, cargo add does that.

Idiomatic Rust: commit the source, not target/. Cargo recreates target/ when you build.

Side by side

	PHP	Rust
New project	composer init (or composer create-project from a template)	cargo new
Build and run	You run php yourself	cargo build and cargo run
Format and lint	Extra tools	cargo fmt and cargo clippy

Try it

cargo new a project, change the string in println!, and cargo run. Then cargo fmt and cargo clippy. Clippy should print nothing.

Key takeaways

• cargo new makes a directory. cargo init uses the one you are in.
• cargo run, cargo fmt, and cargo clippy are the everyday commands.
• cargo check is the fast loop while fixing errors.

What's next: Managing dependencies with Cargo.toml