Make unwinding task-independent

[brson]

For the sake of embedding applications it would be nice if Rust's unwinding implementation was not directly coupled to the task abstraction.

Right now unwinding is mostly just try/catch, but it also sets a flag on the task to indicate that the task is failing. It would be better if std could fulfill its unwinding obligations without involving the task.

[alexcrichton]

There are a two questions I can think of that the unwinding code needs to have answers to:

• Am I currently unwinding? -- abort on double failure
• Am I currently in a try/catch block? -- abort on try/catch patterns

Perhaps these can be solved with thread-local globals? Right now we only have to worry about one thread local global which is quite convenient.

Additionally, exposing a safe interface around this is going to be tough. I think that we can get by with:

fn try(f: ||) -> bool;

I remember there being issues around stack closures leaving captured things in an unusual state, but this doesn't seem much different to me than normal failure, so I may be forgetting why stack closures on try boundaries is a bad idea.

[alexcrichton]

Note that we now have a try function, but it's unsafe and sill relies on Task for safety (see the comments on that function).

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#632