Downstream crates may implement `Copy` for `Box<_>`

[scottmcm]

@dhm

explicit negative impls

That's where I think things would go too. There are a few other silly errors than can pop up right now, like "you need to implement Clone for a mutable reference", and having the explicit negative impl there seems both good for improving the error message and as a place to write about why that's definitely not ok in the docs. Previous conversation:

Pondering: Negative trait promises

I was looking at this error message in the QotW thread: note: downstream crates may implement trait `std::clone::Clone` for type `&mut _` It made me wonder if impl !Trait would make sense just from a diagnostics and documentation perspective: errors like “the core crate has promised that &mut _ will never be Clone”, and a place in rustdoc to, say, explain why Option doesn’t have a Display impl. I would expect it to be semver-compatible to add a new negative impl for a trait you didn’t impl be…