Move platform requirements to availability annotations

Adding or raising the deployment platforms in the package manifest is a
SemVer major breaking change as consumers must also add or raise their
deployment platforms. This is a known limitation of SwiftPM.

Unforunately this means that it's very difficult for non-leaf packages
to adopt packages which declare their platforms in the manifest. Doing
so puts the brakes on adoption and ecosystem growth. For 'core' packages
like this one availability constraints should be expressed on
declarations rather than in the manifest.

This patch adds equivalent availability annotations to declarations
across the package and removes platforms from the package manifest.

Author: glbrntt

Package.swift

@@ -1,15 +1,39 @@
 // swift-tools-version: 5.8
 
 import PackageDescription
+import CompilerPluginSupport
+
+// Availability Macros
+let availabilityTags = [_Availability("AsyncAlgorithms")]
+let versionNumbers = ["1.0"]
+
+// Availability Macro Utilities
+enum _OSAvailability: String {
+  // This should match the package's deployment target
+  case alwaysAvailable = "macOS 10.15, iOS 13.0, tvOS 13.0, watchOS 6.0"
+  // Use 10000 for future availability to avoid compiler magic around
+  // the 9999 version number but ensure it is greater than 9999
+  case future = "macOS 10000, iOS 10000, tvOS 10000, watchOS 10000"
+}
+
+struct _Availability {
+  let name: String
+  let osAvailability: _OSAvailability
+
+  init(_ name: String, availability: _OSAvailability = .alwaysAvailable) {
+    self.name = name
+    self.osAvailability = availability
+  }
+}
+
+let availabilityMacros: [SwiftSetting] = versionNumbers.flatMap { version in
+  availabilityTags.map {
+    .enableExperimentalFeature("AvailabilityMacro=\($0.name) \(version):\($0.osAvailability.rawValue)")
+  }
+}
 
 let package = Package(
   name: "swift-async-algorithms",
-  platforms: [
-    .macOS("10.15"),
-    .iOS("13.0"),
-    .tvOS("13.0"),
-    .watchOS("6.0"),
-  ],
   products: [
     .library(name: "AsyncAlgorithms", targets: ["AsyncAlgorithms"])
   ],
@@ -20,29 +44,29 @@ let package = Package(
         .product(name: "OrderedCollections", package: "swift-collections"),
         .product(name: "DequeModule", package: "swift-collections"),
       ],
-      swiftSettings: [
+      swiftSettings: availabilityMacros + [
         .enableExperimentalFeature("StrictConcurrency=complete")
       ]
     ),
     .target(
       name: "AsyncSequenceValidation",
       dependencies: ["_CAsyncSequenceValidationSupport", "AsyncAlgorithms"],
-      swiftSettings: [
+      swiftSettings: availabilityMacros + [
         .enableExperimentalFeature("StrictConcurrency=complete")
       ]
     ),
     .systemLibrary(name: "_CAsyncSequenceValidationSupport"),
     .target(
       name: "AsyncAlgorithms_XCTest",
       dependencies: ["AsyncAlgorithms", "AsyncSequenceValidation"],
-      swiftSettings: [
+      swiftSettings: availabilityMacros + [
         .enableExperimentalFeature("StrictConcurrency=complete")
       ]
     ),
     .testTarget(
       name: "AsyncAlgorithmsTests",
       dependencies: ["AsyncAlgorithms", "AsyncSequenceValidation", "AsyncAlgorithms_XCTest"],
-      swiftSettings: [
+      swiftSettings: availabilityMacros + [
         .enableExperimentalFeature("StrictConcurrency=complete")
       ]
     ),

Sources/AsyncAlgorithms/AsyncAdjacentPairsSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// An `AsyncSequence` that iterates over the adjacent pairs of the original
   /// original `AsyncSequence`.
@@ -26,6 +27,7 @@ extension AsyncSequence {
   ///
   /// - Returns: An `AsyncSequence` where the element is a tuple of two adjacent elements
   ///   or the original `AsyncSequence`.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func adjacentPairs() -> AsyncAdjacentPairsSequence<Self> {
     AsyncAdjacentPairsSequence(self)
@@ -34,6 +36,7 @@ extension AsyncSequence {
 
 /// An `AsyncSequence` that iterates over the adjacent pairs of the original
 /// `AsyncSequence`.
+@available(AsyncAlgorithms 1.0, *)
 @frozen
 public struct AsyncAdjacentPairsSequence<Base: AsyncSequence>: AsyncSequence {
   public typealias Element = (Base.Element, Base.Element)
@@ -83,6 +86,7 @@ public struct AsyncAdjacentPairsSequence<Base: AsyncSequence>: AsyncSequence {
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncAdjacentPairsSequence: Sendable where Base: Sendable, Base.Element: Sendable {}
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncBufferedByteIterator.swift

@@ -39,6 +39,7 @@
 ///     }
 ///
 ///
+@available(AsyncAlgorithms 1.0, *)
 public struct AsyncBufferedByteIterator: AsyncIteratorProtocol {
   public typealias Element = UInt8
   @usableFromInline var buffer: _AsyncBytesBuffer
@@ -67,6 +68,7 @@ public struct AsyncBufferedByteIterator: AsyncIteratorProtocol {
 @available(*, unavailable)
 extension AsyncBufferedByteIterator: Sendable {}
 
+@available(AsyncAlgorithms 1.0, *)
 @frozen @usableFromInline
 internal struct _AsyncBytesBuffer {
   @usableFromInline

Sources/AsyncAlgorithms/AsyncChain2Sequence.swift

@@ -17,6 +17,7 @@
 ///   - s2: The second asynchronous sequence.
 /// - Returns: An asynchronous sequence that iterates first over the elements of `s1`, and
 ///   then over the elements of `s2`.
+@available(AsyncAlgorithms 1.0, *)
 @inlinable
 public func chain<Base1: AsyncSequence, Base2: AsyncSequence>(
   _ s1: Base1,
@@ -26,6 +27,7 @@ public func chain<Base1: AsyncSequence, Base2: AsyncSequence>(
 }
 
 /// A concatenation of two asynchronous sequences with the same element type.
+@available(AsyncAlgorithms 1.0, *)
 @frozen
 public struct AsyncChain2Sequence<Base1: AsyncSequence, Base2: AsyncSequence> where Base1.Element == Base2.Element {
   @usableFromInline
@@ -41,10 +43,12 @@ public struct AsyncChain2Sequence<Base1: AsyncSequence, Base2: AsyncSequence> wh
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChain2Sequence: AsyncSequence {
   public typealias Element = Base1.Element
 
   /// The iterator for a `AsyncChain2Sequence` instance.
+  @available(AsyncAlgorithms 1.0, *)
   @frozen
   public struct Iterator: AsyncIteratorProtocol {
     @usableFromInline
@@ -76,12 +80,14 @@ extension AsyncChain2Sequence: AsyncSequence {
     }
   }
 
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func makeAsyncIterator() -> Iterator {
     Iterator(base1.makeAsyncIterator(), base2.makeAsyncIterator())
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChain2Sequence: Sendable where Base1: Sendable, Base2: Sendable {}
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChain3Sequence.swift

@@ -18,6 +18,7 @@
 ///   - s3: The third asynchronous sequence.
 /// - Returns: An asynchronous sequence that iterates first over the elements of `s1`, and
 ///   then over the elements of `s2`, and then over the elements of `s3`
+@available(AsyncAlgorithms 1.0, *)
 @inlinable
 public func chain<Base1: AsyncSequence, Base2: AsyncSequence, Base3: AsyncSequence>(
   _ s1: Base1,
@@ -28,6 +29,7 @@ public func chain<Base1: AsyncSequence, Base2: AsyncSequence, Base3: AsyncSequen
 }
 
 /// A concatenation of three asynchronous sequences with the same element type.
+@available(AsyncAlgorithms 1.0, *)
 @frozen
 public struct AsyncChain3Sequence<Base1: AsyncSequence, Base2: AsyncSequence, Base3: AsyncSequence>
 where Base1.Element == Base2.Element, Base1.Element == Base3.Element {
@@ -48,10 +50,12 @@ where Base1.Element == Base2.Element, Base1.Element == Base3.Element {
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChain3Sequence: AsyncSequence {
   public typealias Element = Base1.Element
 
   /// The iterator for a `AsyncChain3Sequence` instance.
+  @available(AsyncAlgorithms 1.0, *)
   @frozen
   public struct Iterator: AsyncIteratorProtocol {
     @usableFromInline
@@ -93,12 +97,14 @@ extension AsyncChain3Sequence: AsyncSequence {
     }
   }
 
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func makeAsyncIterator() -> Iterator {
     Iterator(base1.makeAsyncIterator(), base2.makeAsyncIterator(), base3.makeAsyncIterator())
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChain3Sequence: Sendable where Base1: Sendable, Base2: Sendable, Base3: Sendable {}
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunkedByGroupSequence.swift

@@ -9,9 +9,11 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection`
   /// type by testing if elements belong in the same group.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunked<Collected: RangeReplaceableCollection>(
     into: Collected.Type,
@@ -21,6 +23,7 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks by testing if elements belong in the same group.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunked(
     by belongInSameGroup: @escaping @Sendable (Element, Element) -> Bool
@@ -51,6 +54,7 @@ extension AsyncSequence {
 ///      // [10, 20, 30]
 ///      // [10, 40, 40]
 ///      // [10, 20]
+@available(AsyncAlgorithms 1.0, *)
 public struct AsyncChunkedByGroupSequence<Base: AsyncSequence, Collected: RangeReplaceableCollection>: AsyncSequence
 where Collected.Element == Base.Element {
   public typealias Element = Collected
@@ -121,6 +125,7 @@ where Collected.Element == Base.Element {
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChunkedByGroupSequence: Sendable where Base: Sendable, Base.Element: Sendable {}
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunkedOnProjectionSequence.swift

@@ -9,8 +9,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` type on the uniqueness of a given subject.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunked<Subject: Equatable, Collected: RangeReplaceableCollection>(
     into: Collected.Type,
@@ -20,6 +22,7 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks on the uniqueness of a given subject.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunked<Subject: Equatable>(
     on projection: @escaping @Sendable (Element) -> Subject
@@ -29,6 +32,7 @@ extension AsyncSequence {
 }
 
 /// An `AsyncSequence` that chunks on a subject when it differs from the last element.
+@available(AsyncAlgorithms 1.0, *)
 public struct AsyncChunkedOnProjectionSequence<
   Base: AsyncSequence,
   Subject: Equatable,
@@ -104,6 +108,7 @@ public struct AsyncChunkedOnProjectionSequence<
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChunkedOnProjectionSequence: Sendable where Base: Sendable, Base.Element: Sendable {}
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunksOfCountOrSignalSequence.swift

@@ -9,8 +9,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` type of a given count or when a signal `AsyncSequence` produces an element.
+  @available(AsyncAlgorithms 1.0, *)
   public func chunks<Signal, Collected: RangeReplaceableCollection>(
     ofCount count: Int,
     or signal: Signal,
@@ -20,6 +22,7 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks of a given count or when a signal `AsyncSequence` produces an element.
+  @available(AsyncAlgorithms 1.0, *)
   public func chunks<Signal>(
     ofCount count: Int,
     or signal: Signal
@@ -28,6 +31,7 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` type when a signal `AsyncSequence` produces an element.
+  @available(AsyncAlgorithms 1.0, *)
   public func chunked<Signal, Collected: RangeReplaceableCollection>(
     by signal: Signal,
     into: Collected.Type
@@ -36,6 +40,7 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks when a signal `AsyncSequence` produces an element.
+  @available(AsyncAlgorithms 1.0, *)
   public func chunked<Signal>(by signal: Signal) -> AsyncChunksOfCountOrSignalSequence<Self, [Element], Signal> {
     chunked(by: signal, into: [Element].self)
   }
@@ -78,6 +83,7 @@ extension AsyncSequence {
 }
 
 /// An `AsyncSequence` that chunks elements into collected `RangeReplaceableCollection` instances by either count or a signal from another `AsyncSequence`.
+@available(AsyncAlgorithms 1.0, *)
 public struct AsyncChunksOfCountOrSignalSequence<
   Base: AsyncSequence,
   Collected: RangeReplaceableCollection,

Sources/AsyncAlgorithms/AsyncChunksOfCountSequence.swift

@@ -9,8 +9,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` of a given count.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunks<Collected: RangeReplaceableCollection>(
     ofCount count: Int,
@@ -20,13 +22,15 @@ extension AsyncSequence {
   }
 
   /// Creates an asynchronous sequence that creates chunks of a given count.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func chunks(ofCount count: Int) -> AsyncChunksOfCountSequence<Self, [Element]> {
     chunks(ofCount: count, into: [Element].self)
   }
 }
 
 /// An `AsyncSequence` that chunks elements into `RangeReplaceableCollection` instances of at least a given count.
+@available(AsyncAlgorithms 1.0, *)
 public struct AsyncChunksOfCountSequence<Base: AsyncSequence, Collected: RangeReplaceableCollection>: AsyncSequence
 where Collected.Element == Base.Element {
   public typealias Element = Collected
@@ -89,8 +93,7 @@ where Collected.Element == Base.Element {
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChunksOfCountSequence: Sendable where Base: Sendable, Base.Element: Sendable {}
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncChunksOfCountSequence.Iterator: Sendable where Base.AsyncIterator: Sendable, Base.Element: Sendable {}
-
-@available(*, unavailable)
-extension AsyncChunksOfCountSequence.Iterator: Sendable {}

Sources/AsyncAlgorithms/AsyncCompactedSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncSequence {
   /// Returns a new `AsyncSequence` that iterates over every non-nil element from the
   /// original `AsyncSequence`.
@@ -18,6 +19,7 @@ extension AsyncSequence {
   /// - Returns: An `AsyncSequence` where the element is the unwrapped original
   ///   element and iterates over every non-nil element from the original
   ///   `AsyncSequence`.
+  @available(AsyncAlgorithms 1.0, *)
   @inlinable
   public func compacted<Unwrapped>() -> AsyncCompactedSequence<Self, Unwrapped>
   where Element == Unwrapped? {
@@ -27,6 +29,7 @@ extension AsyncSequence {
 
 /// An `AsyncSequence` that iterates over every non-nil element from the original
 /// `AsyncSequence`.
+@available(AsyncAlgorithms 1.0, *)
 @frozen
 public struct AsyncCompactedSequence<Base: AsyncSequence, Element>: AsyncSequence
 where Base.Element == Element? {
@@ -66,6 +69,7 @@ where Base.Element == Element? {
   }
 }
 
+@available(AsyncAlgorithms 1.0, *)
 extension AsyncCompactedSequence: Sendable where Base: Sendable, Base.Element: Sendable {}
 
 @available(*, unavailable)