Async Iterators

Features:

• "Async iterator item": function-like item that let's us
• "Async iterator trait": AsyncIterator (the trait formerly known as Stream)
• "Async iterator expression": for await x in foo {}

These three features are independent of each other and can be advanced on their own. All three are needed to consider "async iteration" a "solved problem".

Milestones

• Update wg-async-foundations repo to link to async-iteration repo - estebank
• Author evaluation doc for async iteration item - estebank
• RFC for async iteration trait - nell DONE
• Coordinate with stake-holders about current state of trait AsyncIterator
• Author evaluation doc for async iteration expression 💤
• Merge basic async iteration item support in rustc💤
• Resolve questions around size_hint for async iteration item 💤
• Update RFC for async iterator item to reflect the current state 💤
• Feature complete for async iterator item 💤
• Feature complete for async iterator trait 💤
• Feature complete for async iterator expression 💤

Async iterator item

Impact

Able to write custom sync and async iterators and combinators without creating a new struct and impl, and reducing the need to understand Pin.

async generator zip<A, B>(
    mut a: impl AsyncIterator<Item = A>,
    mut b: impl AsyncIterator<Item = B>,
) yields (A, B) {
    while let (Some(a), Some(b)) = (a.next().await, b.next().await) {
        yield (a, b);
    }
}

Status

• Draft RFCs written: syntax, semantics
• Proposed to T-lang
  • Need to update to the current "iniciative process"
• Syntax: bikeshedding needed, but it's not a blocker for exploration of semantics
  • Need to write a draft with some alternatives and open a conversation (independent of semantics) with people in the WG (to limit bikeshedding)
• Semantics:

Async iterator trait

Impact

Having a single AsyncIterator trait in std:: that can be used by the entire async ecosystem.

Status

Tracking issue #79024.

Async iterator expression

Impact

Allowing easy consumption of AsyncIterators by providing a new expression syntax for these.

async fn foo<I: Display>(x: impl AsyncIterator<Item = I> {
    for await i in x {
        println!("{}", i);
    }
}

Potentially, enable syntactic sugar for "rayon-like" parallel iteration.

async fn foo<I: Display>(x: impl AsyncIterator<Item = I> {
    for await (i, j, k) in x.chunks(3) {
        println!("{} {} {}", i, j, k);
    }
}

Status

Not started.