Is there a way to "return" from a block expression?

Question

You can return from a function before reaching the last statement can be done using return:

fn example() -> i32 {
    if true {
        return 1;
    }

    0 // this line is never reached
}

Is it possible to do something similar with block expressions?

let foo = {
    if true {
        // somehow "return" so that foo is 1
    }

    0 // this line is never reached
};

In this example I could have used an if-else, but I'm asking about block expressions in general.

Phore

@AlexeyLarionov because using "return 1" inside a block expression returns from the entire function the expression is in, not just the expression. If you're asking "why not use a function", readability – Phore

Answer 1

From Rust 1.65.0, it is possible to name the block and break from it:

fn main() {
    let foo = 'bar: {
        if true {
            break 'bar 1;
        }

        0 // this line is never reached
    };
}

---

Before Rust 1.65.0, there was a well known "loop hack":

fn main() {
    let foo = loop {
        if true {
            break 1;
        }

        break 0;
    };
}

Library

Can the loop hack be used to break the hacky outer loop from within a legit inner loop? Something like break 'outer value;? – Farceuse

@Farceuse no, you'd need labels for that. – Library

FYI: cargo clippy complains about the loop hack; it says "this loop never actually loops". – Taciturn

You can now break from a labeled block since 1.65.0 blog.rust-lang.org/2022/11/03/Rust-1.65.0.html – Lillia

Answer 2

You could also use an immediately invoked closure.

fn main() {
    let foo = (|| {
        if true {
            return 1;
        }
        
        0
    })();
}

My bet would be that LLVM is smart enough to optimize the function call out in release builds.

Kall

This is a good hack. Might it have issues with needing to move variables into the closure though? – Farceuse

Answer 3

Edit: This feature has been stabilized in Rust 1.65.0.

---

This feature has been proposed: RFC 2046 - break label value. It uses the break-label syntax (used in loops) and is available as an unstable feature.

So this will work on the nightly compiler:

#![feature(label_break_value)]

let foo = 'a: {
    if true {
        break 'a 1;
    }
    0
};

Soelch

Answer 4

Edit: It's now possible to break from labeled blocks

---

Original answer:

I don't think it's currently possible but I found this tracking issue: https://github.com/rust-lang/rust/issues/48594

Kandis