Add the Sync bound to io::Error by requiring that any wrapped custom errors also conform to Sync in addition to error::Error + Send.


Adding the Sync bound to io::Error has 3 primary benefits:

  • Values that contain io::Errors will be able to be Sync
  • Perhaps more importantly, io::Error will be able to be stored in an Arc
  • By using the above, a cloneable wrapper can be created that shares an io::Error using an Arc in order to simulate the old behavior of being able to clone an io::Error.

Detailed design

The only thing keeping io::Error from being Sync today is the wrapped custom error type Box<error::Error+Send>. Changing this to Box<error::Error+Send+Sync> and adding the Sync bound to io::Error::new() is sufficient to make io::Error be Sync. In addition, the relevant convert::From impls that convert to Box<error::Error+Send> will be updated to convert to Box<error::Error+Send+Sync> instead.


The only downside to this change is it means any types that conform to error::Error and are Send but not Sync will no longer be able to be wrapped in an io::Error. It’s unclear if there’s any types in the standard library that will be impacted by this. Looking through the list of implementors for error::Error, here’s all of the types that may be affected:

  • io::IntoInnerError: This type is only Sync if the underlying buffered writer instance is Sync. I can’t be sure, but I don’t believe we have any writers that are Send but not Sync. In addition, this type has a From impl that converts it to io::Error even if the writer is not Send.
  • sync::mpsc::SendError: This type is only Sync if the wrapped value T is Sync. This is of course also true for Send. I’m not sure if anyone is relying on the ability to wrap a SendError in an io::Error.
  • sync::mpsc::TrySendError: Same situation as SendError.
  • sync::PoisonError: This type is already not compatible with io::Error because it wraps mutex guards (such as sync::MutexGuard) which are not Send.
  • sync::TryLockError: Same situation as PoisonError.

So the only real question is about sync::mpsc::SendError. If anyone is relying on the ability to convert that into an io::Error a From impl could be added that returns an io::Error that is indistinguishable from a wrapped SendError.


Don’t do this. Not adding the Sync bound to io::Error means io::Errors cannot be stored in an Arc and types that contain an io::Error cannot be Sync.

We should also consider whether we should go a step further and change io::Error to use Arc instead of Box internally. This would let us restore the Clone impl for io::Error.

Unresolved questions

Should we add the From impl for SendError? There is no code in the rust project that relies on SendError being converted to io::Error, and I’m inclined to think it’s unlikely for anyone to be relying on that, but I don’t know if there are any third-party crates that will be affected.