Understanding Rust Error E0426: Undeclared Label

Introduction to Rust Error E0426

Error E0426 occurs when you try to use a label that has not been declared in your Rust code. Labels are used with loop constructs to help control the flow of execution and allow breaking or continuing a specific loop, even when nested loops are present. In this article, we'll dive deeper into this error, provide code examples, and explain how to fix it.

Illustrating Error E0426

Consider this erroneous piece of code:

#![allow(unused)]
fn main() {
loop {
    break 'a; // error: use of undeclared label `'a`
}
}

In this case, the break statement tries to use the label 'a, which has not been declared earlier in the code. This results in Rust Error E0426.

Rectifying Error E0426

To fix this error, you need to ensure that a given label is only used in statements after it has been declared, as shown in this code example:

#![allow(unused)]
fn main() {
'a: loop {
    break 'a; // ok!
}
}

Here, the label 'a is properly declared before being used in the break statement. The code compiles successfully without raising Error E0426.

Utilizing Labels with Nested Loops

In cases with nested loops, using labels is helpful to specify which loop to break or continue. Take a look at this example:

#![allow(unused)]
fn main() {
    'outer: for i in 1..6 {
        'inner: for j in 1..6 {
            if i == j {
                continue 'inner; // jump to the next iteration of the 'inner loop
            }
            if i % 2 == 0 {
                break 'outer; // break the 'outer loop
            }
            println!("i: {}, j: {}", i, j);
        }
    }
}

In this example, we have two nested loops labeled 'outer and 'inner. The continue 'inner statement makes the inner loop jump to the next iteration, while the break 'outer statement takes the execution out of the outer loop.