Understanding Rust Error E0528

Overview of Rust Error E0528

Rust Error E0528 occurs when an array or slice pattern requires more elements than are present in the matched array. In this case, the pattern cannot be fully matched due to insufficient elements in the array.

Error E0528 Example

Consider the following erroneous code:

#![allow(unused)]
fn main() {
    let r = &[1, 2];
    match r {
        &[a, b, c, rest @ ..] => { // error: pattern requires at least 3
                                  //        elements but array has 2
            println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
        }
    }
}

The code creates an array `r` with two elements and tries to match it with a pattern containing at least three elements. Because the array has only two elements, Rust produces Error E0528.

Fixing Error E0528

To fix Error E0528, ensure that the matched array has at least the same number of elements as the pattern requires. You can use the '..' syntax to match an arbitrary number of remaining elements if the exact size of the array is unknown. Here's a corrected version of the previous example:

#![allow(unused)]
fn main() {
    let r = &[1, 2, 3, 4, 5];
    match r {
        &[a, b, c, rest @ ..] => { // ok!
            // prints `a=1, b=2, c=3 rest=[4, 5]`
            println!("a={}, b={}, c={} rest={:?}", a, b, c, rest);
        }
    }
}

This example works because the array `r` now contains at least three elements. The '..' syntax allows for matching any remaining elements.

Conclusion

To avoid Rust Error E0528, ensure the matched array contains enough elements to satisfy the pattern. Use the '..' syntax to match arbitrarily many remaining elements in the array if their count is unknown. Always keep in mind that an array must have the required elements for a successful match.