support wrapping error_chain Error's ? / document that Send+Sync are required on top of std::error::Error about anyhow

[dtolnay]

I would prefer not to make a change for this; my best recommendation would be to not use error-chain. Non-sync errors are extremely unusual and it's a bug in error-chain that their errors are not sync. They decided more than 3 years ago to fix it and make sync errors but the library is not maintained enough to actually make that happen.

[rbtcollins]

So the issue is how to deal with it in two cases; for migrating a big-patch approach can be done, which is painful but fine. But for using anyhow with some other library that does use error-chain this then becomes a transitive problem. Similar bug was filed on failure rust-lang-deprecated/failure#284 :/. I agree with you that not using error-chain is ideal, but providing some migration path off of error-chain is desirable no? Right now if there are two crates A and B that both use error chain, and one uses the other, neither of them can convert to anyhow, because error-chain cannot convert from anyhow, and anyhow cannot convert from error-chain.

... or maybe using failure.SyncFailure is the thing to do?

[dtolnay]

For migrating, they should patch in the PR from 3 years ago that makes the error-chain errors Sync. ;)

[rbtcollins]

OH! rust-lang-deprecated/error-chain#241 ta. Now to figure out the process to make that happen :).