Understanding Rust Error E0730: Pattern Matching on Arrays without a Fixed Length

Introduction to Rust Error E0730

Rust Error E0730 occurs when an array without a fixed length is pattern-matched. In this article, we'll explore the root cause of this error and discuss how to fix it in your Rust code.

Example of Erroneous Code

Here is an example of Rust code that triggers Error E0730:

#![allow(unused)]
fn main() {
fn is_123<const N: usize>(x: [u32; N]) -> bool {
    match x {
        [1, 2, ..] => true, // error: cannot pattern-match on an
                            //        array without a fixed length
        _ => false
    }
}
}

In this example, the is_123 function takes an array argument with a generic length N and tries to pattern-match it. However, the Rust compiler doesn't support pattern matching on an array without a fixed length.

Solutions to Rust Error E0730

To fix this error, you have two solutions: 1. Use an array with a fixed length. 2. Use a slice.

Solution 1: Using an Array with a Fixed Length

One way to resolve the error is to use an array with a fixed length when pattern matching. Here's an example of how you can fix the erroneous code:

#![allow(unused)]
fn main() {
fn is_123(x: [u32; 3]) -> bool { // We use an array with a fixed size
    match x {
        [1, 2, ..] => true, // ok!
        _ => false
    }
}
}

In this example, the is_123 function now takes an array of length 3. The Rust compiler will successfully compile this code without any errors.

Solution 2: Using a Slice

An alternative solution is to use a slice instead of an array. A slice is a reference to a part or all of an array, and it always has a fixed size. Here's an example of how to fix the erroneous code with a slice:

#![allow(unused)]
fn main() {
fn is_123(x: &[u32]) -> bool { // We use a slice
    match x {
        [1, 2, ..] => true, // ok!
        _ => false
    }
}
}

In this example, the is_123 function takes a slice parameter instead of an array with a generic length. The Rust compiler will successfully compile this code without any errors.

Conclusion

When encountering Rust Error E0730, ensure to either use an array with a fixed length or a slice in your pattern matching to avoid the error. By making these adjustments, you'll ensure your Rust code compiles successfully.