Tweak illegal `Copy` impl message The phrase "may not" can both mean "is not able to" and "possibly does not". Disambiguate this by just using "cannot". ``@Lokathor`` expressed being annoyed by this [here](https://twitter.com/Lokathor/status/1633200313544089602?s=20). Also drive-by fix for this extremely noisy message: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6a37275bc810f7846bfe191845b7d11d. r? diagnostics