Add a `NotStarted` state to `PollState` about futures-concurrency

[michaelwoerister]

This code in the compiler calls it "Unresumed":
https://cs.github.com/rust-lang/rust/blob/0aaad9e757207657064d70bf9e6c1e6eb327bf15/compiler/rustc_middle/src/ty/sty.rs#L487

[michaelwoerister]

NotStarted sounds like a better name to me though.

[michaelwoerister]

Do we know for sure that a future has never been polled yet?

[yoshuawuyts]

Do we know for sure that a future has never been polled yet?

We can't guarantee that, no. Futures would individually have to track whether they've been started or not. Unpin futures could theoretically be polled once, moved out of their Pin container, moved into a concurrency operator, and then polled. There's nothing preventing that from happening.

But I think NotStarted should be viewed from the perspective of the concurrency future: it has not yet started polling it, and in order to guarantee it starts it has to, at least once, loop over each future and hand it a waker.