# What is rustc?

Welcome to "The rustc book"! `rustc` is the compiler for the Rust programming language, provided by the project itself. Compilers take your source code and produce binary code, either as a library or executable.

Most Rust programmers don't invoke `rustc` directly, but instead do it through Cargo. It's all in service of `rustc` though! If you want to see how Cargo calls `rustc`, you can

    $ cargo build --verbose

And it will print out each `rustc` invocation. This book can help you understand what each of these options does. Additionally, while most Rustaceans use Cargo, not all do: sometimes they integrate `rustc` into other build systems. This book should provide a guide to all of the options you'd need to do so.

## Basic usage

Let's say you've got a little hello world program in a file `hello.rs`:

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

To turn this source code into an executable, you can use `rustc`:

    $ rustc hello.rs
    $ ./hello # on a *NIX
    $ .\hello.exe # on Windows

Note that we only ever pass `rustc` the crate root, not every file we wish to compile. For example, if we had a `main.rs` that looked like this:

    mod foo;

    fn main() {
        foo::hello();
    }

And a `foo.rs` that had this:

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

To compile this, we'd run this command:

    $ rustc main.rs

No need to tell `rustc` about `foo.rs`; the `mod` statements give it everything that it needs. This is different than how you would use a C compiler, where you invoke the compiler on each file, and then link everything together. In other words, the crate is a translation unit, not a particular module.