The std::io::Error type is Send but it's not Sync. This is because of the use of Box<Error+Send>, as that value is not Sync, and so therefore io::Error is not Sync.

This is a problem for me as I want to stuff an io::Error into a sync::Arc and have the resulting value be Send, but Arc is only Send if its element type is Send + Sync.

/cc @alexcrichton