Replace MaybeDone/Fuse with separate input/output/state fields

[yoshuawuyts]

Right now we're using MaybeDone internally to track the state of each future. It can either be: "pending", "containing data", or "done". This is nice from an implementation perspective, but bad for performance. Namely:

1. It creates unnecessary copies, and sometimes also allocations. Once to store the futures in the MaybeDone structure. And once to copy the data to the output structure.
2. It creates unnecessarily large memory needs. At any point we may be using up to 2x the amount of memory needed, since we're doing redundant copies.

The solution is to move the container of futures in-line. Create the container of outputs containing MaybeUninit. And finally create a separate container to track the state. Once all futures have completed, we can mark the output container as "initialized" and immediately yield that.

Tasks

Priority

• Join for Vec Remove MaybeDone from impl Join for Vec #29
• Join for Array Remove MaybeDone from impl Join for array #72
• Join for tuple Remove MaybeDone from tuple::join #74
• Race for Vec (not needed)
• Race for Array (not needed)
• Race for tuple (not needed)

Secondary

• Merge for Vec inline poll_states and remove Fuse for vec::merge #79
• Merge for Array Remove Fuse from impl Merge for array #70
• Merge for tuple

Eventually

• TryJoin for Vec
• TryJoin for Array
• RaceOk for Vec
• RaceOk for Array

[poliorcetics]

So @yoshuawuyts talked about this on mastodon, I proposed in response to associate index and relevant generic in the macros like so:

macro_rules! associated {
    (/* other elements */; $( ($Fi: literal, $Ft: ident) ),+; /* other elements */) => {
        #[test]
        fn test_indexes_are_sorted() {
            let indexes = [$($Fi),+];
            assert!(indexes.is_sorted(), "Indexes are not sorted in call to macro `associated!()`");
        }
        // With the test running in CI you can use the following to get the max index easily:
        const LEN: usize = {
            let indexes = [$($Fi),+];
            let min = match indexes {
                 [min, ..] => min,
            };
            let max = match indexes {
                 [.., max] => max,
            };
            max - min + 1
        } ;
    };
}

[poliorcetics]

If you are having names conflicts for testing or such, what you can do is add a $test_mod_name: ident member to the macro and put them in

#[cfg(test)
mod $test_mod_name {
    use super::*;
    /* put tests here */
}

[yoshuawuyts]

#74 is probably the furthest along in setting up the machinery required for the tuples. It just needs one last change to ensure that tuple outputs are written in-place, rather than copied.

[poliorcetics]

It can probably be done without using "paste" but I'm being picky 😄 if I find the time to make a PR before the other is merged I'll make a comparison