Any plans for clonable errors?

[c410-f3r]

Related: 121, 134 and 24135.

[withoutboats]

First, nothing stops you from implementing Clone for a type that implements Fail. Errors in general can be Clone just like any other type, because unlike error-chain we don't inject a trait object into your error type.

However, the Error type provided by failure itself is not Clone, because it is essentially a trait object. It has occurred to me that we could make it into an Arc instead of a Box, but we can't do that backwards compatibly because we exposed the downcast_mut method on `Error. I don't feel great about this because I think mutating an error is an anti-pattern, whereas cloning it is not. But I don't know what to do.

[Celti]

If the issue is "we can't make breaking changes to Error &c. because then things using old-failure won't be using the same types as things using new-failure" ...I am probably completely ignorant of how this sort of thing actually works, and if so I will apologise and continue to watch and learn things — but could you instead of the dtolnay trick, do something like making new-failure import the type from old-failure and impl From<old-failure::Error> for Error?

[withoutboats]

I think coherence won't let us provide that impl. That would solve the most annoying problems but not all of them, e.g. if Error is a part of the signature of a trait method and its being implemented by another crate using the other Error type.

Currently grepping the source of all reverse dependencies to find out if anyone actually uses downcast_mut ever.

[withoutboats]

Actually, nevermind. This would also invalidate downcast (by value). I don't want to give up the ability to turn an Error into a concrete value again.

In theory, Clone will some day be object-safe using alloca. Unfortunately, we can't then add Clone as a supertrait on Fail without breaking everything. So I think Error will never be Clone.

Fortunately, there are several work arounds:

• Rc<Error> and Arc<Error>.
• Using your own type that implements Clone and Fail.

[c410-f3r]

Thanks @withoutboats for the great explanation! The Arc and Rc approach is a nice workaround for this problem.