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.