[Concurrency] Add availability to Concurrency APIs.

This allows programs to target older OSes while using Concurrency behind an availability check. When targeting older OSes, the symbols are weak-linked and the compiler will require the use of Concurrency features to be guarded by an availability check.

rdar://75850003

Author: mikeash

include/swift/AST/ASTContext.h

@@ -738,6 +738,10 @@ class ASTContext final {
   /// compiler for the target platform.
   AvailabilityContext getSwift54Availability();
 
+  /// Get the runtime availability of features introduced in the Swift 5.5
+  /// compiler for the target platform.
+  AvailabilityContext getSwift55Availability();
+
   /// Get the runtime availability of features that have been introduced in the
   /// Swift compiler for future versions of the target platform.
   AvailabilityContext getSwiftFutureAvailability();

lib/AST/Availability.cpp

@@ -324,7 +324,7 @@ ASTContext::getIntermodulePrespecializedGenericMetadataAvailability() {
 }
 
 AvailabilityContext ASTContext::getConcurrencyAvailability() {
-  return getSwiftFutureAvailability();
+  return getSwift55Availability();
 }
 
 AvailabilityContext ASTContext::getDifferentiationAvailability() {
@@ -409,6 +409,11 @@ AvailabilityContext ASTContext::getSwift54Availability() {
   }
 }
 
+AvailabilityContext ASTContext::getSwift55Availability() {
+  return getSwiftFutureAvailability();
+}
+
+
 AvailabilityContext ASTContext::getSwiftFutureAvailability() {
   auto target = LangOpts.Target;
 

stdlib/public/Concurrency/Actor.swift

@@ -17,25 +17,30 @@ import Swift
 ///
 /// The \c Actor protocol generalizes over all actor types. Actor types
 /// implicitly conform to this protocol.
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public protocol Actor: AnyObject, Sendable {
 }
 
 /// Called to initialize the default actor instance in an actor.
 /// The implementation will call this within the actor's initializer.
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @_silgen_name("swift_defaultActor_initialize")
 public func _defaultActorInitialize(_ actor: AnyObject)
 
 /// Called to destroy the default actor instance in an actor.
 /// The implementation will call this within the actor's deinit.
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @_silgen_name("swift_defaultActor_destroy")
 public func _defaultActorDestroy(_ actor: AnyObject)
 
 /// FIXME: only exists for the quick-and-dirty MainActor implementation.
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @_silgen_name("swift_MainActor_register")
 fileprivate func _registerMainActor(actor: AnyObject)
 
 /// A singleton actor whose executor is equivalent to 
 /// \c DispatchQueue.main, which is the main dispatch queue.
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @globalActor public actor MainActor {
   public static let shared = MainActor()
 
@@ -44,6 +49,7 @@ fileprivate func _registerMainActor(actor: AnyObject)
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension MainActor {
   /// Execute the given body closure on the main actor.
   public static func run<T>(

stdlib/public/Concurrency/AsyncCompactMapSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func compactMap<ElementOfResult>(
@@ -21,6 +22,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncCompactMapSequence<Base: AsyncSequence, ElementOfResult> {
   @usableFromInline
   let base: Base
@@ -38,6 +40,7 @@ public struct AsyncCompactMapSequence<Base: AsyncSequence, ElementOfResult> {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncCompactMapSequence: AsyncSequence {
   public typealias Element = ElementOfResult
   public typealias AsyncIterator = Iterator

stdlib/public/Concurrency/AsyncDropFirstSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func dropFirst(
@@ -23,6 +24,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncDropFirstSequence<Base: AsyncSequence> {
   @usableFromInline
   let base: Base
@@ -37,6 +39,7 @@ public struct AsyncDropFirstSequence<Base: AsyncSequence> {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncDropFirstSequence: AsyncSequence {
   public typealias Element = Base.Element
   public typealias AsyncIterator = Iterator
@@ -75,6 +78,7 @@ extension AsyncDropFirstSequence: AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncDropFirstSequence {
   @inlinable
   public __consuming func dropFirst(

stdlib/public/Concurrency/AsyncDropWhileSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func drop(
@@ -21,6 +22,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncDropWhileSequence<Base: AsyncSequence> {
   @usableFromInline
   let base: Base
@@ -38,6 +40,7 @@ public struct AsyncDropWhileSequence<Base: AsyncSequence> {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncDropWhileSequence: AsyncSequence {
   public typealias Element = Base.Element
   public typealias AsyncIterator = Iterator

stdlib/public/Concurrency/AsyncFilterSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func filter(
@@ -21,6 +22,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncFilterSequence<Base: AsyncSequence> {
   @usableFromInline
   let base: Base
@@ -38,6 +40,7 @@ public struct AsyncFilterSequence<Base: AsyncSequence> {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncFilterSequence: AsyncSequence {
   public typealias Element = Base.Element
   public typealias AsyncIterator = Iterator

stdlib/public/Concurrency/AsyncFlatMapSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func flatMap<SegmentOfResult: AsyncSequence>(
@@ -21,6 +22,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncFlatMapSequence<Base: AsyncSequence, SegmentOfResult: AsyncSequence> {
   @usableFromInline
   let base: Base
@@ -38,6 +40,7 @@ public struct AsyncFlatMapSequence<Base: AsyncSequence, SegmentOfResult: AsyncSe
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncFlatMapSequence: AsyncSequence {
   public typealias Element = SegmentOfResult.Element
   public typealias AsyncIterator = Iterator

stdlib/public/Concurrency/AsyncIteratorProtocol.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 @rethrows
 public protocol AsyncIteratorProtocol {
   associatedtype Element

stdlib/public/Concurrency/AsyncMapSequence.swift

@@ -12,6 +12,7 @@
 
 import Swift
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncSequence {
   @inlinable
   public __consuming func map<Transformed>(
@@ -21,6 +22,7 @@ extension AsyncSequence {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 public struct AsyncMapSequence<Base: AsyncSequence, Transformed> {
   @usableFromInline
   let base: Base
@@ -38,6 +40,7 @@ public struct AsyncMapSequence<Base: AsyncSequence, Transformed> {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
 extension AsyncMapSequence: AsyncSequence {
   public typealias Element = Transformed
   public typealias AsyncIterator = Iterator