Early-breaking from Rust's match

Question

I want to switch through many possible cases for x and there's one case (here x == 0) where I want to check the result of some additional code to determine what to do next. One possibility is to return early from the match.

I'd use break to do this early-returning in C, but this isn't allowed in Rust. return returns from the parent function (in this case main()) and not from the match only (i.e. the println! at the end isn't run!).

I could just negate the sub-condition (here y == 0) and indent the whole lot of following code -- but I find this ugly and unreadable.

Putting the sub-condition into a match-guard is no option for me since it's simply too big.

Is this possible in Rust or is there a better alternative (except creating another subfunction or other work-arounds)?

Minimal example:

fn main() {
    let x = 1;

    match x {
        1 => {
            let y = 0;
            /*
             * do ev1l stuff to y that I don't want to put into the match-guard
             * as it's simply too much.
             */

            /* break early ... */
            if y == 0 {break;} // > error: `break` outside of loop [E0268]

            assert!(y != 0, "y was 0!");
            /* do other stuff in here. */
        }
        _ => {}
    }

    println!("done matching");
}

I found Mixing matching, mutation, and moves in Rust — is it wrong?

match embraces both imperative and functional styles of programming: you can continue using break statements, assignments, et cetera, rather than being forced to adopt an expression-oriented mindset.

Answer 1

Something else you could do is make a "self-executing" closure and use a return statement inside. I don't know whether there are any weird performance characteristics of this but syntactically it's pretty clean.

fn main() {
    let x = 1;

    // This closure is just used to catch the "return" statement.
    (|| {
        match x {
            1 => {
                let y = 0;
                /*
                 * do ev1l stuff to y that I don't want to put into the match-guard
                 * as it's simply too much.
                 */

                /* break early ... */
                if y == 0 { return; } // Ok!

                assert!(y != 0, "y was 0!");
                /* do other stuff in here. */
            }
            _ => {}
        }
    })();

    println!("done matching");
}

Here's a playground link showing it working.

Answer 2

You could create a macro like

macro_rules! block {
    ($xs:block) => {
        loop { break $xs }
    };
}

and do

match x {
    1 => block!({
        ...
        if y == 0 { break; }
        ...
    })
    _ => {}
}

It's not an amazing solution, but it is semantically meaningful.

Answer 3

You can wrap the match into a loop that only runs once and break out of the loop

fn main() {
    let x = 1;

    loop { match x {
        1 => {
            let y = 0;
            /*
             * do ev1l stuff to y that I don't want to put into the match-guard
             * as it's simply too much.
             */

            /* break early ... */
            if y == 0 { break; }

            assert!(y != 0, "y was 0!");
            /* do other stuff in here. */
        }
        _ => {}
    } break; }

    println!("done matching");
}

Answer 4

The answer is: you can't.

The fact that you are trying to do this is a sign that you are not writing idiomatic Rust.

If you absolutely need to break out of a match, consider putting your match in a function that returns a value... this way you can have good control over borrowing, compared to a simple block of code. Then pass the variable to match to the function that will match inside of it. i.e.

fn match_it(val: i8) -> i8 {
    let mut a: i8 = 0;
    match random_or(0, 1) {
        1 => {
                a = 5;
                // other ev1l stuff
                if a == 6 { return 1 }
             },
        0 => a = -5,
        _ => (),
    };
    
    a //return a
}

Based on your code, though, you could just flip two statements and you would achieve your desired control flow:

        if y == 0 {break;} // > error: `break` outside of loop [E0268]

        assert!(y != 0, "y was 0!");
        /* do other stuff in here. */

turns into:

        if y != 0 {
             assert!(y != 0, "y was 0!");
             /* do other stuff in here. */
        } else { 
             break; 
        }

        // if we reach here, by definition one of these conditions is true: y == 0, or y != 0 and code above has run and completed.