E0193 - Error codes index
Description: Book listing all Rust error codes

# Error code E0193

#### Note: this error code is no longer emitted by the compiler.

`where` clauses must use generic type parameters: it does not make sense to use them otherwise. An example causing this error:

    #![allow(unused)]
    fn main() {
    trait Foo {
        fn bar(&self);
    }

    #[derive(Copy,Clone)]
    struct Wrapper<T> {
        Wrapped: T
    }

    impl Foo for Wrapper<u32> where Wrapper<u32>: Clone {
        fn bar(&self) { }
    }
    }

This use of a `where` clause is strange - a more common usage would look something like the following:

    #![allow(unused)]
    fn main() {
    trait Foo {
        fn bar(&self);
    }

    #[derive(Copy,Clone)]
    struct Wrapper<T> {
        Wrapped: T
    }
    impl <T> Foo for Wrapper<T> where Wrapper<T>: Clone {
        fn bar(&self) { }
    }
    }

Here, we're saying that the implementation exists on Wrapper only when the wrapped type `T` implements `Clone`. The `where` clause is important because some types will not implement `Clone`, and thus will not get this method.

In our erroneous example, however, we're referencing a single concrete type. Since we know for certain that `Wrapper<u32>` implements `Clone`, there's no reason to also specify it in a `where` clause.