Understanding Rust Error E0001

Introduction to Rust Error E0001

Error E0001 occurs in Rust when a pattern in a match expression is unreachable because it is too specific or one of the preceding patterns is too general. Specifically, it means that all possible values of the expression being matched have already been covered by the previous patterns, making the current pattern unnecessary.

Code example of Error E0001

Consider the following match block that has too many arms:

fn main() {
  match Some(0) {
    Some(bar) => { /* ... */ }
    x => { /* ... */ } // This handles the `None` case
    _ => { /* ... */ } // All possible cases have already been handled
  }
}

Why Error E0001 Occurs

The error in the above code example occurs because match patterns are evaluated in order. The wildcard (_) pattern in the last arm will never be reached as all other possible cases have already been handled by the previous patterns. The error may suggest reordering or removing some of the patterns, but in this case, it is correct to remove the last arm, as the second arm already covers the 'None' case.

How to Fix Error E0001

To fix Error E0001, ensure the ordering of patterns in the match expression is correct, and remove any unnecessary or unreachable patterns. In the provided example, the corrected code would be as follows:

fn main() {
  match Some(0) {
    Some(bar) => { /* ... */ }
    x => { /* ... */ } // This handles the `None` case
  }
}

Conclusion