Understanding Rust Error E0091

Overview

Rust Error E0091 occurs when an unnecessary type or const parameter is provided in a type alias. This error indicates that one or more parameters are not used in the definition of the type alias, causing a syntactic issue. In this article, we will explain the E0091 error, its causes, and how to fix it.

Causes of E0091

The main cause of Rust Error E0091 is the presence of unused type or const parameters in a type alias. In other words, you have provided a parameter that is not used in the definition, rendering it useless. Let's explore erroneous code examples that generate this error:

#![allow(unused)]
fn main() {
    type Foo<T> = u32; // error: type parameter `T` is unused
    // or:
    type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
}

In the first code example, the type parameter `T` is not used in the definition of the type Foo. Similarly, in the second one, the type parameter `B` is not used while defining Foo. Both of these cases cause Rust Error E0091.

Fixing E0091

To fix Rust Error E0091, you need to remove the unused type or const parameters from the type alias, ensuring that each provided parameter plays a role in the definition. The corrected code examples are as follows:

#![allow(unused)]
fn main() {
    type Foo = u32; // ok!
    type Foo2<A> = Box<A>; // ok!
}

In these corrected versions, the first example has no type parameter since it is not relevant for the u32 type alias. The second example now uses only the required type parameter `A` for the Box type alias. This way, the code becomes free of error E0091.

Conclusion

By understanding Rust Error E0091 and its causes, you can avoid unused type or const parameters in type alias definitions. Ensure to remove any unnecessary parameters to keep your code clean and syntactically correct.