error[E0392]: parameter `T` is never used --> $DIR/issue-17904-2.rs:4:12 | LL | struct Foo<T> where T: Copy; | ^ unused parameter | = help: consider removing `T`, referring to it in a field, or using a marker such as `PhantomData` error: aborting due to previous error For more information about this error, try `rustc --explain E0392`.