Understanding Rust Error E0023: Incorrect Number of Fields in a Pattern

E0023 Error: A Pattern Extraction Mismatch

In Rust, error E0023 occurs when a pattern attempts to extract an incorrect number of fields from a variant. It is important to provide a sub-pattern for each field of the enum variant while matching.

E0023 Error Example

Consider this erroneous code example:

enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

match x {
    Fruit::Apple(a) => {}, // error!
    _ => {}
}

Here, a pattern is used to match the Apple variant, consisting of two fields. However, the match statement only attempts to extract one field, resulting in error E0023.

E0023 Error Solution

To resolve this error, ensure that the pattern matches the declared number of fields in the enum variant. The corrected code will look like this:

enum Fruit {
    Apple(String, String),
    Pear(u32),
}

let x = Fruit::Apple(String::new(), String::new());

match x {
    Fruit::Apple(a, b) => {},
    _ => {}
}

Now the match statement correctly extracts both fields of the Apple variant, and the error is resolved.

Preventing E0023 Error with Incorrect Match Patterns

Matching with the wrong number of fields leads to confusion, and doesn't have a clear interpretation. The following incorrect patterns will still trigger the error:

match x {
    Fruit::Apple(a) => {},
    Fruit::Apple(a, b, c) => {},
}

Review and match the patterns with the exact number of declared fields in the enum to avoid error E0023.