Safe and convenient resource destruction

[kvark]

Would be nice to have our resources self-destructed, though I'm not sure we can really do that with our architecture and potential other device back-ends.

The path seems to be mostly cleared now (at last!). Here are the steps we will take:

• wrap resources inside handles with Arc<>
• make Factory to store the references to all created resources
• implement a sweep() pass for the device/factory. The device may be able to enumerate all the factories and call sweep() on them.
• add a trait that is implemented by both Rc and Arc. Examples.
• generically parametrize all the handle types by this trait
• make sure using the Graphics enforces Rc, while the general way goes with Arc
• proceed with @csherratt's mutability protection. It's big, so needs its own issue.

PR #633 covers the basics.
Issue #636 will cover Rc usage.

[kvark]

Actually, screw RAII. We really just need linear types for the resources.

Edit: that was a silly idea! Linear types are great, but not for our case.

[bvssvni]

Does Rust support linear types?

[kvark]

AFAIK, not yet, unfortunately. I hope someone will post the status of linear types support here.

[brendanzab]

Could we cc the linear type RFC? Would be good to show them a real use case.

[kvark]

@bjz what RFC is that? I only found pre-RFC some time ago and already commented on that.

[brendanzab]

Ohh, I rembered it, but forgot it wasn't up on Github yet.

[kvark]

@bjz I find it weird that linear types are still in pre-RFC stage. For our purpose, it would suffice to just be able to disable going out-of-scope for some type: generate a compile error whenever a variable of this type goes out of scope. The only way to destruct it would be to deconstruct it, and since the type is provided by our library, we can prevent user from deconstructing it by adding any private fields.