No `main` function was found in a binary crate.

To fix this error, add a `main` function:

```
fn main() {
    // Your program will start here.
    println!("Hello world!");
}
```

If you don't know the basics of Rust, you can look at the
[Rust Book][rust-book] to get started.

[rust-book]: https://doc.rust-lang.org/book/