Understanding Rust Error E0590

Introduction to Rust Error E0590

Rust Error E0590 occurs when the break or continue keywords are used in the condition of a while loop without a proper label. In this article, we will examine the cause of this error and learn how to fix it with appropriate code examples.

Illustrating the Error

Consider the following erroneous code:

#![allow(unused)]
fn main() {
    while break {}
}

This code will produce Rust Error E0590 because the break keyword is used in the condition of the while loop without an associated label.

Fixing the Error

To resolve Rust Error E0590, you must add a label to the while loop that specifies which loop the break or continue keyword is referring to. Here's an example of corrected code:

#![allow(unused)]
fn main() {
    'foo: while break 'foo {}
}

In this example, we've added the label 'foo to the while loop. The break keyword now includes the label 'foo, specifying which loop is being exited. This code will no longer produce Rust Error E0590.

Additional Examples

Let's explore more examples to better understand the use of break and continue with labels in Rust loops.

Example with Nested Loops:

fn main() {
    'outer: for i in 1..=3 {
        for j in 1..=3 {
            if i == 2 {
                continue 'outer;
            }
            println!("i: {}, j: {}", i, j);
        }
    }
}

In this example, the continue 'outer; statement skips the remaining part of the 'outer loop when i is equal to 2. This prevents the inner loop from executing when i is 2.

Conclusion

By properly using labels with break and continue keywords in Rust while loops, you can avoid encountering Rust Error E0590. Remember to always include a label when using break or continue in the condition of a while loop to clearly specify which loop is being affected.