Understanding Rust Error E0084: Unsupported Representation for Zero-variant Enum

The Rust Error E0084

Rust Error E0084 occurs when an unsupported representation is attempted on a zero-variant enum. Simply put, it means that you are trying to assign an integer type to an enum with no variants.

Erroneous Code Example

#![allow(unused)]
fn main() {
   #[repr(i32)]
   enum NightsWatch {} // error: unsupported representation for zero-variant enum
}

Explanation

In the code example above, the compiler throws an error because it is not possible to define an integer type for a zero-variant enum. This is due to the fact that there are no available values for this enum.

How to Fix Error E0084

There are two ways to fix this error:

1. Add Variants to the Enum

You can add one or more variants to the enum so that it has values that can be represented with the desired integer type.

Example with Variants

#![allow(unused)]
fn main() {
   #[repr(i32)]
   enum NightsWatch {
      JonSnow,
      Commander,
   }
}

2. Remove the Integer Representation

If the enum does not need to be represented as an integer, you can simply remove the integer representation attribute.

Example without Integer Representation

#![allow(unused)]
fn main() {
   enum NightsWatch {}
}

Conclusion

Rust Error E0084 occurs when trying to assign an integer representation to a zero-variant enum. To fix this error, either add variants to the enum, or remove the integer representation attribute. Understanding this error and its possible solutions will help you write cleaner, more effective Rust code.