`struct`s in `Akka.Streams` with exclusively read-only members were made `readonly` (#6941)

* `struct`s with exclusively read-only members were made `readonly`

* added API approvals

---------

Co-authored-by: Lehonti Ramos <lehonti@ramos>
Co-authored-by: Gregorius Soedharmo <arkatufus@yahoo.com>
Co-authored-by: Aaron Stannard <aaron@petabridge.com>

src/core/Akka.API.Tests/verify/CoreAPISpec.ApproveStreams.DotNet.verified.txt

@@ -784,6 +784,7 @@ namespace Akka.Streams
         public static Akka.Streams.IGraph<Akka.Streams.FlowShape<T, T>, Akka.Streams.UniqueKillSwitch> Single<T>() { }
         public static Akka.Streams.IGraph<Akka.Streams.BidiShape<T1, T1, T2, T2>, Akka.Streams.UniqueKillSwitch> SingleBidi<T1, T2>() { }
     }
+    [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
     public struct MaterializationContext
     {
         public readonly Akka.Streams.Attributes EffectiveAttributes;
@@ -2361,6 +2362,7 @@ namespace Akka.Streams.Dsl
     {
         public Tcp() { }
         public override Akka.Streams.Dsl.TcpExt CreateExtension(Akka.Actor.ExtendedActorSystem system) { }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct IncomingConnection
         {
             public readonly Akka.Streams.Dsl.Flow<Akka.IO.ByteString, Akka.IO.ByteString, Akka.NotUsed> Flow;
@@ -2369,12 +2371,14 @@ namespace Akka.Streams.Dsl
             public IncomingConnection(System.Net.EndPoint localAddress, System.Net.EndPoint remoteAddress, Akka.Streams.Dsl.Flow<Akka.IO.ByteString, Akka.IO.ByteString, Akka.NotUsed> flow) { }
             public TMat HandleWith<TMat>(Akka.Streams.Dsl.Flow<Akka.IO.ByteString, Akka.IO.ByteString, TMat> handler, Akka.Streams.IMaterializer materializer) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OutgoingConnection
         {
             public readonly System.Net.EndPoint LocalAddress;
             public readonly System.Net.EndPoint RemoteAddress;
             public OutgoingConnection(System.Net.EndPoint remoteAddress, System.Net.EndPoint localAddress) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct ServerBinding
         {
             public readonly System.Net.EndPoint LocalAddress;
@@ -2753,6 +2757,7 @@ namespace Akka.Streams.IO
         public AbruptIOTerminationException(Akka.Streams.IO.IOResult ioResult, System.Exception cause) { }
         public Akka.Streams.IO.IOResult IoResult { get; }
     }
+    [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
     public struct IOResult
     {
         public readonly long Count;
@@ -3034,23 +3039,27 @@ namespace Akka.Streams.Implementation
         public const byte Depleted = 4;
         public const byte Marked = 1;
         public const byte Pending = 2;
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnComplete : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly int Id;
             public OnComplete(int id) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnError : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly System.Exception Cause;
             public readonly int Id;
             public OnError(int id, System.Exception cause) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnNext : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly object Element;
             public readonly int Id;
             public OnNext(int id, object element) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnSubscribe : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly int Id;
@@ -3080,6 +3089,7 @@ namespace Akka.Streams.Implementation
         public void PumpFinished() { }
         protected override bool Receive(object message) { }
         public void WaitForUpstream(int waitForUpstream) { }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct SubInput<T> : Reactive.Streams.ISubscriber<T>
         {
             public SubInput(Akka.Actor.IActorRef impl, int id) { }
@@ -3092,22 +3102,26 @@ namespace Akka.Streams.Implementation
     [Akka.Annotations.InternalApiAttribute()]
     public class static FanOut
     {
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct ExposedPublishers<T> : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly System.Collections.Immutable.ImmutableList<Akka.Streams.Implementation.ActorPublisher<T>> Publishers;
             public ExposedPublishers(System.Collections.Immutable.ImmutableList<Akka.Streams.Implementation.ActorPublisher<T>> publishers) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct SubstreamCancel : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly int Id;
             public SubstreamCancel(int id) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct SubstreamRequestMore : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly long Demand;
             public readonly int Id;
             public SubstreamRequestMore(int id, long demand) { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct SubstreamSubscribePending : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             public readonly int Id;
@@ -3820,6 +3834,7 @@ namespace Akka.Streams.Implementation
         public const string GraphStageLogicTimer = "GraphStageLogicTimer";
         public static System.TimeSpan IdleTimeoutCheckInterval(System.TimeSpan timeout) { }
     }
+    [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
     public struct TransferPhase
     {
         public readonly System.Action Action;
@@ -3910,19 +3925,19 @@ namespace Akka.Streams.Implementation.Fusing
         public static Akka.Actor.Props Props(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell) { }
         protected override bool Receive(object message) { }
         public Akka.Actor.IActorRef RegisterShell(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell) { }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct Abort : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public Abort(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct AsyncInput : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly object Event;
             public readonly System.Action<object> Handler;
             public readonly Akka.Streams.Stage.GraphStageLogic Logic;
             public AsyncInput(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, Akka.Streams.Stage.GraphStageLogic logic, object @event, System.Action<object> handler) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
         public class BatchingActorInputBoundary : Akka.Streams.Implementation.Fusing.GraphInterpreter.UpstreamBoundaryStageLogic
@@ -3958,77 +3973,76 @@ namespace Akka.Streams.Implementation.Fusing
             public void Request(long elements) { }
             public override string ToString() { }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct Cancel : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly int Id;
             public Cancel(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, System.Exception cause) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public System.Exception Cause { get; }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct ExposedPublisher : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly int Id;
             public readonly Akka.Streams.Implementation.IActorPublisher Publisher;
             public ExposedPublisher(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, Akka.Streams.Implementation.IActorPublisher publisher) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
         public interface IBoundaryEvent : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression
         {
             Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnComplete : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly int Id;
             public OnComplete(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnError : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly System.Exception Cause;
             public readonly int Id;
             public OnError(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, System.Exception cause) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnNext : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly object Event;
             public readonly int Id;
             public OnNext(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, object @event) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct OnSubscribe : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly int Id;
             public readonly Reactive.Streams.ISubscription Subscription;
             public OnSubscribe(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, Reactive.Streams.ISubscription subscription) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct RequestMore : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly long Demand;
             public readonly int Id;
             public RequestMore(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id, long demand) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct Resume : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public Resume(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
+        [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public struct SubscribePending : Akka.Actor.INoSerializationVerificationNeeded, Akka.Event.IDeadLetterSuppression, Akka.Streams.Implementation.Fusing.ActorGraphInterpreter.IBoundaryEvent
         {
             public readonly int Id;
             public SubscribePending(Akka.Streams.Implementation.Fusing.GraphInterpreterShell shell, int id) { }
-            [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
             public Akka.Streams.Implementation.Fusing.GraphInterpreterShell Shell { get; }
         }
     }
@@ -5074,12 +5088,11 @@ namespace Akka.Streams.Stage
         public virtual void OnUpstreamFailure(System.Exception e) { }
         public virtual void OnUpstreamFinish() { }
     }
+    [System.Runtime.CompilerServices.IsReadOnlyAttribute()]
     public struct LogicAndMaterializedValue<TMaterialized> : Akka.Streams.Stage.ILogicAndMaterializedValue<TMaterialized>
     {
         public LogicAndMaterializedValue(Akka.Streams.Stage.GraphStageLogic logic, TMaterialized materializedValue) { }
-        [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public Akka.Streams.Stage.GraphStageLogic Logic { get; }
-        [get: System.Runtime.CompilerServices.IsReadOnlyAttribute()]
         public TMaterialized MaterializedValue { get; }
     }
     public abstract class OutGraphStageLogic : Akka.Streams.Stage.GraphStageLogic, Akka.Streams.Stage.IOutHandler

src/core/Akka.Streams/Dsl/Tcp.cs

@@ -33,7 +33,7 @@ namespace Akka.Streams.Dsl
         /// <summary>
         /// Represents a successful TCP server binding.
         /// </summary>
-        public struct ServerBinding
+        public readonly struct ServerBinding
         {
             private readonly Func<Task> _unbindAction;
 
@@ -63,7 +63,7 @@ namespace Akka.Streams.Dsl
         /// <summary>
         /// Represents an accepted incoming TCP connection.
         /// </summary>
-        public struct IncomingConnection
+        public readonly struct IncomingConnection
         {
             /// <summary>
             /// Initializes a new instance of the <see cref="IncomingConnection"/> class.
@@ -110,7 +110,7 @@ namespace Akka.Streams.Dsl
         /// <summary>
         /// Represents a prospective outgoing TCP connection.
         /// </summary>
-        public struct OutgoingConnection
+        public readonly struct OutgoingConnection
         {
             /// <summary>
             /// Initializes a new instance of the <see cref="OutgoingConnection"/> class.

src/core/Akka.Streams/IMaterializer.cs

@@ -187,7 +187,7 @@ namespace Akka.Streams
     /// <summary>
     /// Context parameter to the create methods of sources and sinks.
     /// </summary>
-    public struct MaterializationContext
+    public readonly struct MaterializationContext
     {
         /// <summary>
         /// TBD

src/core/Akka.Streams/IO/IOResult.cs

@@ -14,7 +14,7 @@ namespace Akka.Streams.IO
     /// Holds a result of an IO operation.
     /// </summary>
     // ReSharper disable once InconsistentNaming
-    public struct IOResult
+    public readonly struct IOResult
     {
         private readonly Result<NotUsed> _status;
 

src/core/Akka.Streams/Implementation/FanIn.cs

@@ -425,7 +425,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct OnError : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct OnError : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -452,7 +452,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct OnComplete : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct OnComplete : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -473,7 +473,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct OnNext : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct OnNext : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -500,7 +500,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct OnSubscribe : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct OnSubscribe : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -557,7 +557,7 @@ namespace Akka.Streams.Implementation
         /// <summary>
         /// TBD
         /// </summary>
-        public struct SubInput : ISubscriber<T>
+        public readonly struct SubInput : ISubscriber<T>
         {
             private readonly IActorRef _impl;
             private readonly int _id;

src/core/Akka.Streams/Implementation/FanOut.cs

@@ -415,7 +415,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct SubstreamRequestMore : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct SubstreamRequestMore : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -442,7 +442,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct SubstreamCancel : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct SubstreamCancel : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -463,7 +463,7 @@ namespace Akka.Streams.Implementation
         /// TBD
         /// </summary>
         [Serializable]
-        public struct SubstreamSubscribePending : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct SubstreamSubscribePending : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD
@@ -519,7 +519,7 @@ namespace Akka.Streams.Implementation
         /// </summary>
         /// <typeparam name="T">TBD</typeparam>
         [Serializable]
-        public struct ExposedPublishers<T> : INoSerializationVerificationNeeded, IDeadLetterSuppression
+        public readonly struct ExposedPublishers<T> : INoSerializationVerificationNeeded, IDeadLetterSuppression
         {
             /// <summary>
             /// TBD

src/core/Akka.Streams/Implementation/Fusing/ActorGraphInterpreter.cs

@@ -454,7 +454,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct OnError : IBoundaryEvent
+        public readonly struct OnError : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -486,7 +486,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct OnComplete : IBoundaryEvent
+        public readonly struct OnComplete : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -512,7 +512,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct OnNext : IBoundaryEvent
+        public readonly struct OnNext : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -544,7 +544,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct OnSubscribe : IBoundaryEvent
+        public readonly struct OnSubscribe : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -576,7 +576,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct RequestMore : IBoundaryEvent
+        public readonly struct RequestMore : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -608,7 +608,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct Cancel : IBoundaryEvent
+        public readonly struct Cancel : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -639,7 +639,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct SubscribePending : IBoundaryEvent
+        public readonly struct SubscribePending : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -665,7 +665,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct ExposedPublisher : IBoundaryEvent
+        public readonly struct ExposedPublisher : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -697,7 +697,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct AsyncInput : IBoundaryEvent
+        public readonly struct AsyncInput : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -736,7 +736,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct Resume : IBoundaryEvent
+        public readonly struct Resume : IBoundaryEvent
         {
             /// <summary>
             /// TBD
@@ -753,7 +753,7 @@ namespace Akka.Streams.Implementation.Fusing
         /// <summary>
         /// TBD
         /// </summary>
-        public struct Abort : IBoundaryEvent
+        public readonly struct Abort : IBoundaryEvent
         {
             /// <summary>
             /// TBD

src/core/Akka.Streams/Implementation/IO/FilePublisher.cs

@@ -61,7 +61,7 @@ namespace Akka.Streams.Implementation.IO
                 .WithDeploy(Deploy.Local);
         }
 
-        private struct Continue : IDeadLetterSuppression
+        private readonly struct Continue : IDeadLetterSuppression
         {
             public static readonly Continue Instance = new();
         }

src/core/Akka.Streams/Implementation/IO/InputStreamPublisher.cs

@@ -39,7 +39,7 @@ namespace Akka.Streams.Implementation.IO
             return Actor.Props.Create<InputStreamPublisher>(inputstream, completionSource, chunkSize).WithDeploy(Deploy.Local);
         }
 
-        private struct Continue : IDeadLetterSuppression
+        private readonly struct Continue : IDeadLetterSuppression
         {
             public static Continue Instance { get; } = new();
         }

src/core/Akka.Streams/Implementation/IO/InputStreamSinkStage.cs

@@ -72,7 +72,7 @@ namespace Akka.Streams.Implementation.IO
         /// <summary>
         /// TBD
         /// </summary>
-        internal struct Data : IStreamToAdapterMessage
+        internal readonly struct Data : IStreamToAdapterMessage
         {
             /// <summary>
             /// TBD
@@ -124,7 +124,7 @@ namespace Akka.Streams.Implementation.IO
         /// <summary>
         /// TBD
         /// </summary>
-        internal struct Failed : IStreamToAdapterMessage
+        internal readonly struct Failed : IStreamToAdapterMessage
         {
             /// <summary>
             /// TBD

src/core/Akka.Streams/Implementation/IO/TcpStages.cs

@@ -335,7 +335,7 @@ namespace Akka.Streams.Implementation.IO
         /// <summary>
         /// TBD
         /// </summary>
-        internal struct Outbound : ITcpRole
+        internal readonly struct Outbound : ITcpRole
         {
             /// <summary>
             /// TBD
@@ -376,7 +376,7 @@ namespace Akka.Streams.Implementation.IO
         /// <summary>
         /// TBD
         /// </summary>
-        internal struct Inbound : ITcpRole
+        internal readonly struct Inbound : ITcpRole
         {
             /// <summary>
             /// TBD

src/core/Akka.Streams/Implementation/Transfer.cs

@@ -362,7 +362,7 @@ namespace Akka.Streams.Implementation
     /// <summary>
     /// TBD
     /// </summary>
-    public struct TransferPhase
+    public readonly struct TransferPhase
     {
         /// <summary>
         /// TBD

src/core/Akka.Streams/Stage/GraphStage.cs

@@ -46,7 +46,7 @@ namespace Akka.Streams.Stage
     /// TBD
     /// </summary>
     /// <typeparam name="TMaterialized">TBD</typeparam>
-    public struct LogicAndMaterializedValue<TMaterialized> : ILogicAndMaterializedValue<TMaterialized>
+    public readonly struct LogicAndMaterializedValue<TMaterialized> : ILogicAndMaterializedValue<TMaterialized>
     {
         /// <summary>
         /// TBD