Speculative fix for pre 6.1 compiler crashes by removing the sending keyword, and correct sendable metatype shim availability

Author: phausler

Sources/AsyncAlgorithms/AsyncShareSequence.swift

@@ -338,15 +338,22 @@ where Base.Element: Sendable, Base: AsyncSequenceSendableMetatype, Base.AsyncIte
         failure = error
       }
     }
-
+    #if compiler(>=6.1)
     let state: Mutex<State>
+    #else
+    let state: ManagedCriticalState<State>
+    #endif
     let limit: Int?
 
     init(
       _ iteratorFactory: @escaping @Sendable () -> sending Base.AsyncIterator,
       bufferingPolicy: AsyncBufferSequencePolicy
     ) {
+      #if compiler(>=6.1)
       state = Mutex(State(iteratorFactory, bufferingPolicy: bufferingPolicy))
+      #else
+      state = ManagedCriticalState(State(iteratorFactory, bufferingPolicy: bufferingPolicy))
+      #endif
       switch bufferingPolicy.policy {
       case .bounded(let limit):
         self.limit = limit

Sources/AsyncAlgorithms/Locking.swift

@@ -153,6 +153,10 @@ struct ManagedCriticalState<State> {
       return try critical(&header.pointee)
     }
   }
+
+  func withLock<R>(_ critical: (inout State) throws -> R) rethrows -> R {
+    return try withCriticalRegion(critical)
+  }
 }
 
 extension ManagedCriticalState: @unchecked Sendable where State: Sendable {}

Sources/AsyncAlgorithms/SendableMetatypes.swift

@@ -10,9 +10,13 @@
 //===----------------------------------------------------------------------===//
 
 #if compiler(>=6.2)
+@available(AsyncAlgorithms 1.1, *)
 public typealias AsyncSequenceSendableMetatype = SendableMetatype & AsyncSequence
+@available(AsyncAlgorithms 1.1, *)
 public typealias AsyncIteratorSendableMetatype = SendableMetatype & AsyncIteratorProtocol
 #else
+@available(AsyncAlgorithms 1.1, *)
 public typealias AsyncSequenceSendableMetatype = AsyncSequence
+@available(AsyncAlgorithms 1.1, *)
 public typealias AsyncIteratorSendableMetatype = AsyncIteratorProtocol
 #endif