Understanding Rust Error E0579

Introduction to Rust Error E0579

Rust Error E0579 occurs when an exclusive range pattern has a start point that is not less than its end point, resulting in an empty range. In this article, we will delve into the details of this error, explore reasons for its occurrence, and discuss how to fix it.

Understanding Exclusive Range Patterns

Exclusive range patterns in Rust are denoted by two periods `..`, and they include the start point but not the end point. This means that in order for the range to be non-empty, the start point must be less than the end point. A simple example of an exclusive range pattern in Rust is as follows:

1..5 // includes 1, 2, 3, and 4

Identifying the Error

Consider the following erroneous code example provided in the Rust documentation:

#![feature(exclusive_range_pattern)]

fn main() {
    match 5u32 {
        1..2 => {},
        5..5 => {} // error!
    }
}

In this example, the first range pattern (1..2) is valid as it includes the number 1. However, the second range pattern (5..5) is empty because it contains no numbers. The start and end points are equal, causing Error E0579 to be thrown by the Rust compiler.

Fixing the Error

To fix Rust Error E0579, ensure that the start point of the exclusive range pattern is less than the end point. In the example above, modifying the second range pattern to have a start point less than the end point fixes the error:

#![feature(exclusive_range_pattern)]

fn main() {
    match 5u32 {
        1..2 => {},
        4..5 => {}
    }
}

Now, the second range pattern (4..5) is valid, as it contains the number 4, and the Rust compiler no longer throws Error E0579.

Conclusion

By understanding exclusive range patterns in Rust and ensuring that the start point of the range is less than the end point, developers can avoid Rust Error E0579. Always verify that the range patterns in your Rust code are non-empty to prevent this error from occurring.