There is a feature in mdBook that let's you hide code lines by prepending them with a #.

# fn main() {
    let x = 5;
    let y = 6;

    println!("{}", x + y);
# }

Will render as

# fn main() {
    let x = 5;
    let y = 7;

    println!("{}", x + y);
# }

With the following syntax, you can insert runnable Rust files into your book:

{{#playpen file.rs}}

The path to the Rust file has to be relative from the current source file.

When play is clicked, the code snippet will be send to the Rust Playpen to be compiled and run. The result is send back and displayed directly underneath the code.

Here is what a rendered code snippet looks like:

fn main() {
    println!("Hello World!");
#
#    // You can even hide lines! :D
#   println!("I am hidden! Expand the code snippet to see me");
}