Panics

Let's go back to the speed function you wrote for the "Variables" section. It probably looked something like this:

fn speed(start: u32, end: u32, time_elapsed: u32) -> u32 {
    let distance = end - start;
    distance / time_elapsed
}

If you have a keen eye, you might have spotted one issue1: what happens if time_elapsed is zero?

You can try it out on the Rust playground!
The program will exit with the following error message:

thread 'main' panicked at src/main.rs:3:5:
attempt to divide by zero

This is known as a panic.
A panic is Rust's way to signal that something went so wrong that the program can't continue executing, it's an unrecoverable error2. Division by zero classifies as such an error.

The panic! macro

You can intentionally trigger a panic by calling the panic! macro3:

fn main() {
    panic!("This is a panic!");
    // The line below will never be executed
    let x = 1 + 2;
}

There are other mechanisms to work with recoverable errors in Rust, which we'll cover later. For the time being we'll stick with panics as a brutal but simple stopgap solution.

Further reading

1

There's another issue with speed that we'll address soon enough. Can you spot it?

2

You can try to catch a panic, but it should be a last resort attempt reserved for very specific circumstances.

3

If it's followed by a !, it's a macro invocation. Think of macros as spicy functions for now. We'll cover them in more detail later in the course.

Exercise

The exercise for this section is located in 02_basic_calculator/04_panics