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.

Rust is expression-oriented

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.

Where it hurts: loop

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.

The rule I will remember

  • Need the value? Last row, no semicolon.
  • Just running a statement? Add semicolon, it becomes ().