readonly readonly structs (#1908)

src/Common/ForceAsyncAwaiter.cs

@@ -26,7 +26,7 @@ namespace System.Threading.Tasks
         }
     }
 
-    internal struct ForceAsyncAwaiter : ICriticalNotifyCompletion
+    internal readonly struct ForceAsyncAwaiter : ICriticalNotifyCompletion
     {
         private readonly Task _task;
 
@@ -50,7 +50,7 @@ namespace System.Threading.Tasks
         }
     }
 
-    internal struct ForceAsyncAwaiter<T> : ICriticalNotifyCompletion
+    internal readonly struct ForceAsyncAwaiter<T> : ICriticalNotifyCompletion
     {
         private readonly Task<T> _task;
 

src/Microsoft.AspNetCore.Http.Connections.Client/Internal/TaskExtensions.cs

@@ -14,7 +14,7 @@ namespace System.Threading.Tasks
         }
     }
 
-    internal struct NoThrowAwaiter : ICriticalNotifyCompletion
+    internal readonly struct NoThrowAwaiter : ICriticalNotifyCompletion
     {
         private readonly Task _task;
         public NoThrowAwaiter(Task task) { _task = task; }

src/Microsoft.AspNetCore.Http.Connections/Internal/AwaitableThreadPool.cs

@@ -16,7 +16,7 @@ namespace Microsoft.AspNetCore.Http.Connections.Internal
             return new Awaitable();
         }
 
-        public struct Awaitable : ICriticalNotifyCompletion
+        public readonly struct Awaitable : ICriticalNotifyCompletion
         {
             public void GetResult()
             {

src/Microsoft.AspNetCore.SignalR.Client.Core/HubConnection.cs

@@ -807,7 +807,7 @@ namespace Microsoft.AspNetCore.SignalR.Client
             }
         }
 
-        private struct InvocationHandler
+        private readonly struct InvocationHandler
         {
             public Type[] ParameterTypes { get; }
             private readonly Func<object[], object, Task> _callback;