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

[rbtcollins]

Anyhow claims to support wrapping any implementor of std::error::Error, but error_chain's impl seems incompatible:

error[E0277]: `(dyn std::error::Error + std::marker::Send + 'static)` cannot be shared between threads safely
  --> src/cli/main.rs:66:25
   |
66 |     utils::current_dir()?;
   |                         ^ `(dyn std::error::Error + std::marker::Send + 'static)` cannot be shared between threads safely
   |
   = help: the trait `std::marker::Sync` is not implemented for `(dyn std::error::Error + std::marker::Send + 'static)`
   = note: required because of the requirements on the impl of `std::marker::Sync` for `std::ptr::Unique<(dyn std::error::Error + std::marker::Send + 'static)>`
   = note: required because it appears within the type `std::boxed::Box<(dyn std::error::Error + std::marker::Send + 'static)>`
   = note: required because it appears within the type `std::option::Option<std::boxed::Box<(dyn std::error::Error + std::marker::Send + 'static)>>`
   = note: required because it appears within the type `error_chain::State`
   = note: required because it appears within the type `rustup::errors::Error`
   = note: required because of the requirements on the impl of `std::convert::From<rustup::errors::Error>` for `anyhow::Error`
   = note: required by `std::convert::From::from`

rust-lang-deprecated/error-chain#240 is presumably the underlying cause, but perhaps anyhow could do something on it's end via specialisation or using ?Send and ?Sync or something?

[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 :).