Understanding Rust Error E0767: Unreachable Label

Introduction to Rust Error E0767

Rust Error E0767 occurs when you attempt to use a label that is not reachable. This error is raised when the Rust compiler detects that you're trying to reference a label outside its scope. Labels are generally used as a way to control the flow within loops, but their scope is limited to the block they are in and not visible through functions, closures, async blocks, or modules.

Erroneous code example

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

In the given erroneous code example, a closure is attempting to access the label 'a. However, this is not allowed, and the Rust compiler will produce Error E0767.

Resolving Rust Error E0767

To resolve Rust Error E0767, you need to ensure that the label is within the right scope. You can do this by refactoring your code so that the label is only utilized inside the same block where it's defined.

Corrected code example

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

In this corrected code example, the label 'a is properly referenced in the same block where it's defined. This code would now compile and run without throwing Error E0767.

Additional Information