Deriving `Error`

[Centril]

@withoutboats

Error has a non-deriveable supertrait. Currently this is not true of any built-in deriveable traits; are we okay with this change to the consistency of the std derives?

This is a good point, especially in lieu of https://github.com/rust-lang/rfcs/pull/2385, since it sorta implies we would never do that RFC, and perhaps we shouldn't.

@withoutboats

If the solution to #2 requires attributes (as it does in many ecosystem derives), error would be the first std derive to have attributes. Again, are we okay with using this feature in std derives?

I would personally like to see e.g. #[default] on enum variants used to make #[derive(Default)] work on enums. That seems quite useful to me.

@josh

I'd love to have a built-in derive(Display) , too.

What would it print?