Understanding Rust Error E0416: Duplicate Identifier in Pattern

What is Rust Error E0416?

Rust Error E0416 occurs when an identifier is bound more than once in a pattern. When working with pattern matching in Rust, each identifier should represent a unique value, and binding the same identifier multiple times within the same pattern is not allowed.

Erroneous Code Example

#![allow(unused)]
fn main() {
    match (1, 2) {
        (x, x) => {} // error: identifier `x` is bound more than once in the
                     //        same pattern
    }
}

How to Fix Rust Error E0416

Ensure that you have not misspelled any identifier names, and each identifier in the pattern represents a unique value. Correcting the aforementioned code example, we can bind the tuple to unique identifiers. This will resolve the error.

Corrected Code Example

#![allow(unused)]
fn main() {
    match (1, 2) {
        (x, y) => {} // ok!
    }
}

Using a Guard for Conditional Matching

If you want to match patterns conditionally, consider using a guard (an if clause after the pattern). This allows you to add conditions for your pattern matches, enabling different behaviors based on the values of the matched elements.

Guard Example

#![allow(unused)]
fn main() {
    let (A, B, C) = (1, 2, 3);
    match (A, B, C) {
        (x, x2, see) if x == x2 => { /* A and B are equal, do one thing */ }
        (y, z, see) => { /* A and B not equal; do another thing */ }
    }
}

Recommended Reading

• Rust Patterns