Understanding Rust Error E0081: Duplicate Enum Discriminant

Introduction to Enum Discriminants in Rust

In Rust, enums are a versatile way to represent a value that could be one of several variants. Each variant within an enum can optionally possess an associated value called a discriminant. The discriminant is an integral value used to differentiate enum variants stored in memory. When no discriminant is explicitly assigned to an enum variant, the compiler automatically assigns it a value, beginning with 0 for the first variant and incrementing by 1 for each subsequent variant.

Understanding Rust Error E0081

Rust Error E0081 occurs when two or more enum variants are assigned the same discriminant value. This causes a problem because the discriminant is used to distinguish the enum variants in memory. If two variants share the same discriminant value, the compiler cannot differentiate between them. To fix this error, you need to ensure that each variant has a unique discriminant value.

Example of Erroneous Code

#![allow(unused)]
fn main() {
    enum Enum {
        P = 3,
        X = 3, // error!
        Y = 5,
    }
}

In this example, the Enum variant P and X are both assigned the discriminant value 3, which leads to Error E0081. To fix this error, assign a unique value to each variant.

Example of Fixed Code

#![allow(unused)]
fn main() {
    enum Enum {
        P = 1,
        X = 3, // ok!
        Y = 5,
    }
}

In this updated version of the code, Enum variant P is now assigned the discriminant value 1, ensuring that each variant has a unique value.

Possible Pitfalls with Auto-Assigned Discriminants

Even when you don't manually specify discriminant values for enum variants, it's possible to encounter Rust Error E0081 due to conflicts with automatically assigned values. For example:

#![allow(unused)]
fn main() {
    enum Bad {
        X,
        Y = 0, // error!
    }
}

In this example, the first variant X is automatically assigned the discriminant value 0. When Y is assigned the same value, a conflict arises, resulting in Error E0081. To fix such conflicts, pay close attention to the discriminant values assigned by the compiler and adjust your code accordingly.