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.

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:

pub 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.