Lints to suggest deriving more traits

[clarfonthey]

A few suggestions:

1. If a type derives Eq, it should also derive Hash.
2. If a type is trivial (e.g. a unit struct or an enum of only unit variants) we should suggest to implement Copy, Eq, Default, Hash, and their dependencies.
3. If a type derives PartialEq but could also derive Eq, suggest deriving Eq.
4. Similar to 3 but with Ord.

[oli-obk]

Copy is special, because it changes the semantics of the type instead of just making it more usable.

[clarfonthey]

My thought process is that if you can already essentially "copy" a type by trivially constructing new values, it makes sense to just implement Copy.

Also adding to this list: implementing Ord for unit structs.

[oli-obk]

@clarcharr I misunderstood, I thought you meant struct Foo(());, too.

[nsunderland1]

| 3. If a type derives PartialEq but could also derive Eq, suggest deriving Eq.

I'm interested in taking this one on (and maybe Ord too, as far as I can tell from playing around it should be pretty much the same code), Can open a dedicated issue if you think it's needed.

[xFrednet]

Hey @nsunderland1, I don't think it's necessary to create a new issue. You can also assign yourself to this one with @rustbot claim and later release the assignment 🙃

[nsunderland1]

Sounds good :)

@rustbot claim

[nsunderland1]

@rustbot release

#8796 implemented the Eq lint. Ord should be very similar, but happy to let someone else take that (or any of the other suggestions from OP) up.

[nsunderland1]

@rustbot release-assignment