Warn on non-derived Clone impl on Copy types?

[SimonSapin]

From #27750 (comment)

a Clone impl can diverge from the Copy one (because Clone is manually done)

This sounds bad, maybe?

Should we have a lint that warns in this case? Maybe even make it an error after a while?

Sort of related: #31085

[sfackler]

There are cases where you can't rely on the derived versions (the bounds may be overly conservative for example), but warning when one of the impls is derived an the other is not may be doable?

[alexcrichton]

I would not want a lint for something like this personally as there are types that are Copy but not Clone (unfortunately), such as:

• Large fixed-size arrays
• Many kinds of function pointers
• Large tuples I believe

I think this is a compiler bug kinda but it's unfortunately what we have today. This means that #[derive(Clone, Copy)] is not always possible if #[derive(Copy)] is present.

[SimonSapin]

@sfackler Is there a case like that where you also have Copy?

@alexcrichton If this is a lint, #[allow(…)] can be used in these case which will hopefully become more rare as we fix compiler bugs/limitations.

[sfackler]

struct Foo<T>(PhantomData<T>) is one case where you could want an unconditional impl of Copy and Clone for all T, not just T: Clone.

[alexcrichton]

@SimonSapin indeed! My personal opinions on lints is to be far more restrictive to the point that if I ever have to write #![allow] at all I would rather the lint not exist (e.g. false positives in my opinion kill the usefulness of the lint).

Either way, however, that's just my personal opinion, and I don't want to digress this issue to talking about lines, I mainly just wanted to point out that such a lint would indeed have false positives (including the cases that @sfackler is mentioning)

[SimonSapin]

Alright, that’s fair enough. Maybe this lint would belong in Clippy more than rustc? CC @Manishearth

[Manishearth]

Yes, this makes sense, we just got a PR for a similar lint for Hash/PartialEq

[Manishearth]

cc @mcarton

[steveklabnik]

Closing since this has been implemented in Clippy.