Clonable errors

[jugglerchris]

I have an application where it would be handy to be able to clone the Error type declared byerror_chain!(). Is there (or could there be) a way to add a #[derive(Clone)]?

[Yamakaky]

Hum, we have the same thing for Sync, Eq... Maybe allow the user to declare the exact types he wants? What do you think?

[jugglerchris]

Yes, that sounds useful.

[Yamakaky]

Idea about the syntax changes? Maybe adding a new block type like foreign_errors or links, with a default to Sync + Send so that it's backward compatible.

[jugglerchris]

My first thought would be allowing an actual #[derive(Clone,Sync)] before the errors { ... } block; I haven't looked into how easy (if at all) that would be to implement in the macro. A new block type would fit in with other error-chain syntax though, so I'm not really fussed either way personally. :-)

[Yamakaky]

I starting looking about how to make the change. There is two troubles:

1. State is crate-level, so we can't modify it for each error. We could make it generic but
2. Rust doesn't allow us to do things like Box<Error + Debug>, Sync and Send being special cases.
   I think we have to generate an empty trait per Error, add the desired traits as dependencies and implement it for each struct that implements all the dependencies. Something like

trait Trait: Debug + Sync + Clone {}
impl<T: Debug + Sync + Clone> Trait for T {}

What do you think?

[Yamakaky]

Apparently, Clone is not object safe, so we can't add it as a dependency like above. The rest works though: https://play.rust-lang.org/?gist=7c05d66e1e017248fb61162b4c9aea92&version=stable&backtrace=1

[jugglerchris]

I discovered the Clone problem when trying to implement it manually as a workaround. I guess I'll have to think again!

[Yamakaky]

Closing for #134