std::io::Error is Send but not Sync

[lilyball]

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

[alexcrichton]

This is currently an intentional decision as by using a trait object it will always be opting out of some marker traits. The only real fundamental piece of functionality we wanted to add to io::Error was the ability to send it across threads, hence Send. That said we can always tweak this, however.

[alexcrichton]

I'm starting to think that putting an error in an Arc is a pretty important use case, in which case I would be fine going ahead and adding the Sync bound to io::Error

[lilyball]

I'm still in favor of just making io::Error use Arc directly instead of Box (which still requires the Sync bound), because then I could just go back to calling .clone() on io::Error instead of using a custom Error type that pretends to be the underlying shared io::Error.

[lilyball]

It occurs to me that, AFAIK, we can't use smart pointers with trait objects yet. So we can't actually say Arc<error::Error+Send+Sync>. io::Error currently has a Box-in-a-Box so we could use Arc on the outer Box, although I don't see any good reason for it to have two Boxes right now at all (AFAICT all it's doing is saving one word from the size of io::Errors initialized with OS error codes).

[lilyball]

#24133 makes io::Error implement Sync. I've filed #24135 for the proposal to make io::Error use Arc internally so it can conform to Clone.