Add a schematic state machine implementing Future

[djmitche]

@fw-immunant generated something like this on-the-fly in teaching the course, and I thought it was great. I think having a schematic understanding of what's going on here helps students through some of the pitfalls. Particularly, it motivates Pin, which is where @fw-immunant did this derivation.

[fw-immunant]

I would clarify this by invoking the analogy with recursive enums: they need an indirection to avoid being infinite-sized types. Prior to Rust 1.77, recursion in async fn was forbidden entirely and code had to forgo the async fn transform in favor of explicitly returning a Box<impl Future<Output=...>> type, but now only "bare" recursion (without an indirection) is forbidden.

[fw-immunant]

I think it makes more sense for explanation to first trace the control flow so we can enumerate states and get in our heads something like "we'll have three possible execution states for this async fn, so it makes sense that its future type would be (morally) an enum with three variants". Then we can look at variable liveness at each of the awaits to determine the payload of each variant. This lets us get to "futures store live locals" without having to introduce the notion of liveness explicitly like a compilers course would--these are just the variables whose values we'll need to run the rest of the function.

I think it's better for an example of the state machine transform to use an async fn that doesn't have async in a loop, so that it's easy for readers to enumerate the entire set of states in their head.

So I might suggest an example more like this:

async fn send(s: String) -> usize {
    println!("{}", s);
    s.len()
}

async fn example(x: i32) -> usize {
    let double_x = x*2;
    let mut bytes_written = send(format!("x = {x}")).await;
    bytes_written += send(format!("double_x = {double_x}")).await;
    bytes_written
}

This gives us three states:

• an initial state holding the argument x
• a state holding double_x until we return from our first await
• a state holding bytes_written until we return from our second await

This is, I think, less confusing than a state we return to across iterations of our loop on 1..=count.

[djmitche]

Implicit in this suggestion is to use enum variants to represent the liveness of variables. That's great, but does require a bit more mucking about with pins than we want to present at this point. In fact, it uses pin_project, which we don't even talk about in the Pin section. Should I maybe revert this to a flat struct with a fut field, just to avoid this?

[fw-immunant]

Some comments inline.