Isnaini

Posted on Nov 3 AI-assisted

The Tiny Semicolon That Broke My Loop

#rust #programming #beginners

I stared at this error for 30 minutes.

fn main() {
    loop {
        break;
    }
    println!("done");
}

Why does Rust want me to return? I am inside a loop {}. I just want to break.

Turns out, I forgot a semicolon.

In Rust, almost everything is an expression. And the rule is simple but easy to forget:

If the last line has NO semicolon, its value is returned.
If the last line HAS a semicolon, it becomes ().

That () is the unit type. Empty. Nothing.

Look at this:

let a = {
    5
}; // a = 5

let b = {
    5;
}; // b = ()

One character. Completely different value.

This is where I got hit.

1. Loop in the middle of a function needs ;

If the loop is not the last thing, you must close it.

fn main() {
    loop {
        // do something
        break;
    }; // <- this ; is mandatory

    println!("now we continue");
}

Without it, Rust sees two expressions back-to-back with no separator and gets confused.

2. Loop at the end of a function must NOT have ; if you want to return its value

This is the part the Rust Book says in one line and moves on.

fn get_number() -> i32 {
    loop {
        break 42;
    } // <- NO semicolon, so 42 is returned
}

If I add a semicolon there:

fn get_number() -> i32 {
    loop {
        break 42;
    }; // <- this is now ()
}

The function promises to return i32, but I just returned (). Compiler error.

So if you use ;, you discard the value. If you want to keep the value, the last row must NOT have ;.

3. break vs return inside loop

This also confused me.

loop {
    break 10; // exits only the loop, value 10
}

loop {
    return 10; // exits the whole function
}

If you just want to get out of the loop with a value, use break with a value and leave the last line without a semicolon.

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