zip: remove Sendable constraint + limit task creation

Author: twittemb

Sources/AsyncAlgorithms/AsyncZip2Sequence.swift

@@ -0,0 +1,54 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Async Algorithms open source project
+//
+// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+//
+//===----------------------------------------------------------------------===//
+
+/// Creates an asynchronous sequence that concurrently awaits values from two `AsyncSequence` types
+/// and emits a tuple of the values.
+public func zip<Base1: AsyncSequence, Base2: AsyncSequence>(
+  _ base1: Base1,
+  _ base2: Base2
+) -> AsyncZip2Sequence<Base1, Base2> {
+  AsyncZip2Sequence(base1, base2)
+}
+
+/// An asynchronous sequence that concurrently awaits values from two `AsyncSequence` types
+/// and emits a tuple of the values.
+public struct AsyncZip2Sequence<Base1: AsyncSequence, Base2: AsyncSequence>: AsyncSequence
+where Base1: Sendable, Base1.Element: Sendable, Base2: Sendable, Base2.Element: Sendable {
+  public typealias Element = (Base1.Element, Base2.Element)
+  public typealias AsyncIterator = Iterator
+
+  let base1: Base1
+  let base2: Base2
+
+  init(_ base1: Base1, _ base2: Base2) {
+    self.base1 = base1
+    self.base2 = base2
+  }
+
+  public func makeAsyncIterator() -> AsyncIterator {
+    Iterator(
+      base1,
+      base2
+    )
+  }
+
+  public struct Iterator: AsyncIteratorProtocol {
+    let runtime: Zip2Runtime<Base1, Base2>
+
+    init(_ base1: Base1, _ base2: Base2) {
+      self.runtime = Zip2Runtime(base1, base2)
+    }
+
+    public mutating func next() async rethrows -> Element? {
+      try await self.runtime.next()
+    }
+  }
+}