Creating a New Package - The Cargo Book

## Creating a New Package

To start a new package with Cargo, use `cargo new`:

    $ cargo new hello_world --bin

We’re passing `--bin` because we’re making a binary program: if we were making a library, we’d pass `--lib`. This also initializes a new `git` repository by default. If you don't want it to do that, pass `--vcs none`.

Let’s check out what Cargo has generated for us:

    $ cd hello_world
    $ tree .
    .
    ├── Cargo.toml
    └── src
        └── main.rs

    1 directory, 2 files

Let’s take a closer look at `Cargo.toml`:

    [package]
    name = "hello_world"
    version = "0.1.0"
    authors = ["Your Name <you@example.com>"]
    edition = "2018"

    [dependencies]

This is called a manifest, and it contains all of the metadata that Cargo needs to compile your package.

Here’s what’s in `src/main.rs`:

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

Cargo generated a “hello world” for us. Let’s compile it:

    $ cargo build
       Compiling hello_world v0.1.0 (file:///path/to/package/hello_world)

And then run it:

    $ ./target/debug/hello_world
    Hello, world!

We can also use `cargo run` to compile and then run it, all in one step (You won't see the `Compiling` line if you have not made any changes since you last compiled):

    $ cargo run
       Compiling hello_world v0.1.0 (file:///path/to/package/hello_world)
         Running `target/debug/hello_world`
    Hello, world!

You’ll now notice a new file, `Cargo.lock`. It contains information about our dependencies. Since we don’t have any yet, it’s not very interesting.

Once you’re ready for release, you can use `cargo build --release` to compile your files with optimizations turned on:

    $ cargo build --release
       Compiling hello_world v0.1.0 (file:///path/to/package/hello_world)

`cargo build --release` puts the resulting binary in `target/release` instead of `target/debug`.

Compiling in debug mode is the default for development. Compilation time is shorter since the compiler doesn't do optimizations, but the code will run slower. Release mode takes longer to compile, but the code will run faster.