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

@@ -4,12 +4,6 @@ import PackageDescription
 
 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"]),
   ],

Package@swift-5.7.swift

@@ -4,12 +4,6 @@ import PackageDescription
 
 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"]),
     .library(name: "AsyncSequenceValidation", targets: ["AsyncSequenceValidation"]),

Sources/AsyncAlgorithms/AsyncAdjacentPairsSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncSequence {
   /// An `AsyncSequence` that iterates over the adjacent pairs of the original
   /// original `AsyncSequence`.
@@ -35,6 +36,7 @@ extension AsyncSequence {
 /// An `AsyncSequence` that iterates over the adjacent pairs of the original
 /// `AsyncSequence`.
 @frozen
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncAdjacentPairsSequence<Base: AsyncSequence>: AsyncSequence {
   public typealias Element = (Base.Element, Base.Element)
 
@@ -83,6 +85,7 @@ public struct AsyncAdjacentPairsSequence<Base: AsyncSequence>: AsyncSequence {
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncAdjacentPairsSequence: Sendable where Base: Sendable, Base.Element: Sendable { }
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncBufferedByteIterator.swift

@@ -39,6 +39,7 @@
 ///     }
 ///
 ///
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncBufferedByteIterator: AsyncIteratorProtocol {
   public typealias Element = UInt8
   @usableFromInline var buffer: _AsyncBytesBuffer
@@ -68,6 +69,7 @@ public struct AsyncBufferedByteIterator: AsyncIteratorProtocol {
 extension AsyncBufferedByteIterator: Sendable { }
 
 @frozen @usableFromInline
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 internal struct _AsyncBytesBuffer {
   @usableFromInline
   final class Storage {

Sources/AsyncAlgorithms/AsyncChain2Sequence.swift

@@ -18,12 +18,14 @@
 /// - Returns: An asynchronous sequence that iterates first over the elements of `s1`, and
 ///   then over the elements of `s2`.
 @inlinable
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public func chain<Base1: AsyncSequence, Base2: AsyncSequence>(_ s1: Base1, _ s2: Base2) -> AsyncChain2Sequence<Base1, Base2> where Base1.Element == Base2.Element {
   AsyncChain2Sequence(s1, s2)
 }
 
 /// A concatenation of two asynchronous sequences with the same element type.
 @frozen
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChain2Sequence<Base1: AsyncSequence, Base2: AsyncSequence> where Base1.Element == Base2.Element {
   @usableFromInline
   let base1: Base1
@@ -38,6 +40,7 @@ public struct AsyncChain2Sequence<Base1: AsyncSequence, Base2: AsyncSequence> wh
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChain2Sequence: AsyncSequence {
   public typealias Element = Base1.Element
 
@@ -79,6 +82,7 @@ extension AsyncChain2Sequence: AsyncSequence {
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChain2Sequence: Sendable where Base1: Sendable, Base2: Sendable { }
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChain3Sequence.swift

@@ -19,12 +19,14 @@
 /// - 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`
 @inlinable
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public func chain<Base1: AsyncSequence, Base2: AsyncSequence, Base3: AsyncSequence>(_ s1: Base1, _ s2: Base2, _ s3: Base3) -> AsyncChain3Sequence<Base1, Base2, Base3> {
   AsyncChain3Sequence(s1, s2, s3)
 }
 
 /// A concatenation of three asynchronous sequences with the same element type.
 @frozen
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChain3Sequence<Base1: AsyncSequence, Base2: AsyncSequence, Base3: AsyncSequence> where Base1.Element == Base2.Element, Base1.Element == Base3.Element {
   @usableFromInline
   let base1: Base1
@@ -43,6 +45,7 @@ public struct AsyncChain3Sequence<Base1: AsyncSequence, Base2: AsyncSequence, Ba
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChain3Sequence: AsyncSequence {
   public typealias Element = Base1.Element
 
@@ -94,6 +97,7 @@ extension AsyncChain3Sequence: AsyncSequence {
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChain3Sequence: Sendable where Base1: Sendable, Base2: Sendable, Base3: Sendable { }
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunkedByGroupSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection`
   /// type by testing if elements belong in the same group.
@@ -46,6 +47,7 @@ extension AsyncSequence {
 ///      // [10, 20, 30]
 ///      // [10, 40, 40]
 ///      // [10, 20]
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChunkedByGroupSequence<Base: AsyncSequence, Collected: RangeReplaceableCollection>: AsyncSequence where Collected.Element == Base.Element {
   public typealias Element = Collected
 
@@ -116,6 +118,7 @@ public struct AsyncChunkedByGroupSequence<Base: AsyncSequence, Collected: RangeR
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChunkedByGroupSequence : Sendable where Base : Sendable, Base.Element : Sendable { }
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunkedOnProjectionSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` type on the uniqueness of a given subject.
   @inlinable
@@ -24,6 +25,7 @@ extension AsyncSequence {
 }
 
 /// An `AsyncSequence` that chunks on a subject when it differs from the last element.
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChunkedOnProjectionSequence<Base: AsyncSequence, Subject: Equatable, Collected: RangeReplaceableCollection>: AsyncSequence where Collected.Element == Base.Element {
   public typealias Element = (Subject, Collected)
 
@@ -96,6 +98,7 @@ public struct AsyncChunkedOnProjectionSequence<Base: AsyncSequence, Subject: Equ
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChunkedOnProjectionSequence : Sendable where Base : Sendable, Base.Element : Sendable { }
 
 @available(*, unavailable)

Sources/AsyncAlgorithms/AsyncChunksOfCountOrSignalSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 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.
   public func chunks<Signal, Collected: RangeReplaceableCollection>(ofCount count: Int, or signal: Signal, into: Collected.Type) -> AsyncChunksOfCountOrSignalSequence<Self, Collected, Signal> where Collected.Element == Element {
@@ -56,6 +57,7 @@ extension AsyncSequence {
 }
 
 /// An `AsyncSequence` that chunks elements into collected `RangeReplaceableCollection` instances by either count or a signal from another `AsyncSequence`.
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChunksOfCountOrSignalSequence<Base: AsyncSequence, Collected: RangeReplaceableCollection, Signal: AsyncSequence>: AsyncSequence, Sendable where Collected.Element == Base.Element, Base: Sendable, Signal: Sendable, Base.Element: Sendable, Signal.Element: Sendable {
 
   public typealias Element = Collected

Sources/AsyncAlgorithms/AsyncChunksOfCountSequence.swift

@@ -9,6 +9,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncSequence {
   /// Creates an asynchronous sequence that creates chunks of a given `RangeReplaceableCollection` of a given count.
   @inlinable
@@ -24,6 +25,7 @@ extension AsyncSequence {
 }
 
 /// An `AsyncSequence` that chunks elements into `RangeReplaceableCollection` instances of at least a given count.
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 public struct AsyncChunksOfCountSequence<Base: AsyncSequence, Collected: RangeReplaceableCollection>: AsyncSequence where Collected.Element == Base.Element {
   public typealias Element = Collected
 
@@ -85,8 +87,14 @@ public struct AsyncChunksOfCountSequence<Base: AsyncSequence, Collected: RangeRe
   }
 }
 
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChunksOfCountSequence : Sendable where Base : Sendable, Base.Element : Sendable { }
+@available(macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0, *)
 extension AsyncChunksOfCountSequence.Iterator : Sendable where Base.AsyncIterator : Sendable, Base.Element : Sendable { }
 
-@available(*, unavailable)
-extension AsyncChunksOfCountSequence.Iterator: Sendable { }
+// The following conflicts with the above conformance. The compiler is okay with this
+// when 'platforms' are specified in the package manifest but not when the @available
+// attribute is used instead.
+//
+// @available(*, unavailable)
+// extension AsyncChunksOfCountSequence.Iterator: Sendable { }