Adopt `Sendable` in `Codec.swift` and `SingleStepByteToMessageDecoder.swift` (#2149)

Co-authored-by: Cory Benfield <lukasa@apple.com>

Sources/NIOCore/Codec.swift

@@ -14,7 +14,7 @@
 
 
 /// State of the current decoding process.
-public enum DecodingState {
+public enum DecodingState: NIOSendable {
     /// Continue decoding.
     case `continue`
 
@@ -485,6 +485,11 @@ public final class ByteToMessageHandler<Decoder: ByteToMessageDecoder> {
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension ByteToMessageHandler: Sendable {}
+#endif
+
 // MARK: ByteToMessageHandler: Test Helpers
 extension ByteToMessageHandler {
     internal var cumulationBuffer: ByteBuffer? {
@@ -771,6 +776,11 @@ public final class MessageToByteHandler<Encoder: MessageToByteEncoder>: ChannelO
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension MessageToByteHandler: Sendable {}
+#endif
+
 extension MessageToByteHandler {
     public func handlerAdded(context: ChannelHandlerContext) {
         precondition(self.state.readyToBeAddedToChannel,

Sources/NIOCore/SingleStepByteToMessageDecoder.swift

@@ -274,6 +274,10 @@ public final class NIOSingleStepByteToMessageProcessor<Decoder: NIOSingleStepByt
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension NIOSingleStepByteToMessageProcessor: Sendable {}
+#endif
 
 // MARK: NIOSingleStepByteToMessageProcessor Public API
 extension NIOSingleStepByteToMessageProcessor {