Fix some strict concurrency warnings (apple#310)

# Motivation

There were a few new strict concurrency warnings that this PR fixes.

Package.swift

@@ -14,7 +14,7 @@ let package = Package(
     .library(name: "AsyncAlgorithms", targets: ["AsyncAlgorithms"]),
   ],
   dependencies: [
-    .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.4"),
+    .package(url: "https://github.com/apple/swift-collections.git", from: "1.1.0"),
     .package(url: "https://github.com/apple/swift-docc-plugin", from: "1.0.0"),
   ],
   targets: [

Sources/AsyncAlgorithms/Buffer/BoundedBufferStateMachine.swift

@@ -16,16 +16,19 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
   typealias SuspendedProducer = UnsafeContinuation<Void, Never>
   typealias SuspendedConsumer = UnsafeContinuation<Result<Base.Element, Error>?, Never>
 
+  // We are using UnsafeTransfer here since we have to get the elements from the task
+  // into the consumer task. This is a transfer but we cannot prove this to the compiler at this point
+  // since next is not marked as transferring the return value.
   fileprivate enum State {
     case initial(base: Base)
     case buffering(
       task: Task<Void, Never>,
-      buffer: Deque<Result<Element, Error>>,
+      buffer: Deque<Result<UnsafeTransfer<Element>, Error>>,
       suspendedProducer: SuspendedProducer?,
       suspendedConsumer: SuspendedConsumer?
     )
     case modifying
-    case finished(buffer: Deque<Result<Element, Error>>)
+    case finished(buffer: Deque<Result<UnsafeTransfer<Element>, Error>>)
   }
 
   private var state: State
@@ -139,7 +142,7 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
         // we have to stack the new element or suspend the producer if the buffer is full
         precondition(buffer.count < limit, "Invalid state. The buffer should be available for stacking a new element.")
         self.state = .modifying
-        buffer.append(.success(element))
+        buffer.append(.success(.init(element)))
         self.state = .buffering(task: task, buffer: buffer, suspendedProducer: nil, suspendedConsumer: nil)
         return .none
 
@@ -218,7 +221,7 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .buffering(task: task, buffer: buffer, suspendedProducer: nil, suspendedConsumer: nil)
-        return .returnResult(producerContinuation: suspendedProducer, result: result)
+        return .returnResult(producerContinuation: suspendedProducer, result: result.map { $0.wrapped })
 
       case .buffering(_, _, _, .some):
         preconditionFailure("Invalid states. There is already a suspended consumer.")
@@ -233,7 +236,7 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .finished(buffer: buffer)
-        return .returnResult(producerContinuation: nil, result: result)
+        return .returnResult(producerContinuation: nil, result: result.map { $0.wrapped })
     }
   }
 
@@ -257,7 +260,7 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .buffering(task: task, buffer: buffer, suspendedProducer: nil, suspendedConsumer: nil)
-        return .returnResult(producerContinuation: suspendedProducer, result: result)
+        return .returnResult(producerContinuation: suspendedProducer, result: result.map { $0.wrapped })
 
       case .buffering(_, _, _, .some):
         preconditionFailure("Invalid states. There is already a suspended consumer.")
@@ -272,7 +275,7 @@ struct BoundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .finished(buffer: buffer)
-        return .returnResult(producerContinuation: nil, result: result)
+        return .returnResult(producerContinuation: nil, result: result.map { $0.wrapped })
     }
   }
 

Sources/AsyncAlgorithms/Buffer/UnboundedBufferStateMachine.swift

@@ -21,15 +21,18 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
     case bufferingOldest(Int)
   }
 
+  // We are using UnsafeTransfer here since we have to get the elements from the task
+  // into the consumer task. This is a transfer but we cannot prove this to the compiler at this point
+  // since next is not marked as transferring the return value.
   fileprivate enum State {
     case initial(base: Base)
     case buffering(
       task: Task<Void, Never>,
-      buffer: Deque<Result<Element, Error>>,
+      buffer: Deque<Result<UnsafeTransfer<Element>, Error>>,
       suspendedConsumer: SuspendedConsumer?
     )
     case modifying
-    case finished(buffer: Deque<Result<Element, Error>>)
+    case finished(buffer: Deque<Result<UnsafeTransfer<Element>, Error>>)
   }
 
   private var state: State
@@ -84,15 +87,15 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         switch self.policy {
           case .unlimited:
-            buffer.append(.success(element))
+            buffer.append(.success(.init(element)))
           case .bufferingNewest(let limit):
             if buffer.count >= limit {
               _ = buffer.popFirst()
             }
-            buffer.append(.success(element))
+            buffer.append(.success(.init(element)))
           case .bufferingOldest(let limit):
             if buffer.count < limit {
-              buffer.append(.success(element))
+              buffer.append(.success(.init(element)))
             }
         }
         self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
@@ -170,7 +173,7 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
-        return .returnResult(result)
+        return .returnResult(result.map { $0.wrapped })
 
       case .modifying:
         preconditionFailure("Invalid state.")
@@ -182,7 +185,7 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .finished(buffer: buffer)
-        return .returnResult(result)
+        return .returnResult(result.map { $0.wrapped })
     }
   }
 
@@ -208,7 +211,7 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .buffering(task: task, buffer: buffer, suspendedConsumer: nil)
-        return .resumeConsumer(result)
+        return .resumeConsumer(result.map { $0.wrapped })
 
       case .modifying:
         preconditionFailure("Invalid state.")
@@ -220,7 +223,7 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
         self.state = .modifying
         let result = buffer.popFirst()!
         self.state = .finished(buffer: buffer)
-        return .resumeConsumer(result)
+        return .resumeConsumer(result.map { $0.wrapped })
     }
   }
 
@@ -251,3 +254,5 @@ struct UnboundedBufferStateMachine<Base: AsyncSequence> {
 
 extension UnboundedBufferStateMachine: Sendable where Base: Sendable { }
 extension UnboundedBufferStateMachine.State: Sendable where Base: Sendable { }
+
+

Sources/AsyncAlgorithms/Channels/ChannelStateMachine.swift

@@ -10,9 +10,6 @@
 //===----------------------------------------------------------------------===//
 import OrderedCollections
 
-// NOTE: this is only marked as unchecked since the swift-collections tag is before auditing for Sendable
-extension OrderedSet: @unchecked Sendable where Element: Sendable { }
-
 struct ChannelStateMachine<Element: Sendable, Failure: Error>: Sendable {
   private struct SuspendedProducer: Hashable, Sendable {
     let id: UInt64

Sources/AsyncAlgorithms/UnsafeTransfer.swift

@@ -0,0 +1,19 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Async Algorithms open source project
+//
+// Copyright (c) 2024 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
+//
+//===----------------------------------------------------------------------===//
+
+/// A wrapper struct to unconditionally to transfer an non-Sendable value.
+struct UnsafeTransfer<Element>: @unchecked Sendable {
+    let wrapped: Element
+
+    init(_ wrapped: Element) {
+        self.wrapped = wrapped
+    }
+}