Feature gate for object_safe_for_dispatch not suggested

[Centril]

Code:

struct S(Box<dyn Copy>);

errors with:

error[E0038]: the trait `std::marker::Copy` cannot be made into an object
 --> f.rs:1:10
  |
1 | struct S(Box<dyn Copy>);
  |          ^^^^^^^^^^^^^ the trait `std::marker::Copy` cannot be made into an object
  |
  = note: the trait cannot require that `Self : Sized`

But it does not suggest adding the feature gate.

[Centril]

cc @nikomatsakis