Elaborate on deriving vs implementing Copy by chancancode · Pull Request #129938 · rust-lang/rust (original) (raw)

I was reading this documentation and this wasn't immediately clear to me.

In my mind, it seemed obvious that a type can only claim to be Copy if the bits it is storing can be Copy, and in the case of a generic struct that can only be the case if T: Copy. So the bound added by the derive seemed necessary at all times, and I thought what the documentation was trying to say is that the custom implementation allows you to add additional bounds.

Of course what it was actually trying to point out is that just because you have a generic parameter T, it doesn't necessarily mean you are storing the bits of T. And if you aren't, it may be the case that your own bits can be copied regardless of whether the bits of T can be safely copied.

Thought it may be worth elaborating to make that a bit more clear. Haven't tested/didn't try to figure out how to render this locally. Mainly not sure if the PhantomData back link is going to just work or need some extra stuff, but I figured someone else probably could just tell.