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