E0091 - Error codes index
Description: Book listing all Rust error codes

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# Error code E0091

An unnecessary type parameter was given in a type alias.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    type Foo<T> = u32; // error: type parameter `T` is never used
    // or:
    type Foo<A, B> = Box<A>; // error: type parameter `B` is never used
    }

Please check you didn’t write too many parameters. Example:

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