All really awesome, non controversial and ergonomic things.

Currently, Rust has scoped threads: Threads that are guaranteed to terminate before the function that spawned them returns. This is powerful because it allows you to pass references to data that lives on your own stack to threads that you spawn, without any bookkeeping or synchronization mechanism - just the normal borrow checker rules.

For example, you can allocate a large array, then split it into multiple non-overlapping slices, and then have a group of threads populate each slice, all in safe Rust code.

But the same isn't true for async tasks in Rust, because futures are just objects representing a state machine, and they don't get any special treatment. In particular, they carry no guarantee that the state machine will actually run to completion, which is fundamentally different from how functions run (stack frames are guaranteed to unwind in some way, either by returning or panicking, unless the entire program has terminated).

To make the situation worse, there are many cases where Rust futures are much more prone to cancellation than synchronous code, because that is also one of the big benefits of using async in the first place - for example, you may be running multiple futures in parallel, pick the result from the one that finishes first, and then cancel the rest.

Getting this stuff under control is why people say that "async cancellation" is a difficult problem to solve, and that is true in all languages that have async. These traits will hopefully make it much easier to work with in Rust.

(There are also many other interesting things you could do with this, unrelated to async. Immovable and unforgettable are both interesting properties of an object that could be used to design many cool APIs in general.)

But there's an easy solution for that: you make the reference-counted smart pointers require their pointee type to be Forget. It will be like how Arc<T> doesn't implement Send unless <T: Sync>.