Skip additional cast from common EC.Run calls (#21789)

* Skip additional cast from common EC.Run calls

* Unsafe.As

2 files changed, 13 insertions, 2 deletions

diff --git a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
index df0e141519..cfd45f6286 100644
--- a/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
+++ b/src/System.Private.CoreLib/src/System/Runtime/CompilerServices/AsyncMethodBuilder.cs
@@ -527,7 +527,12 @@ namespace System.Runtime.CompilerServices
             where TStateMachine : IAsyncStateMachine
         {
             /// <summary>Delegate used to invoke on an ExecutionContext when passed an instance of this box type.</summary>
-            private static readonly ContextCallback s_callback = s => ((AsyncStateMachineBox<TStateMachine>)s).StateMachine.MoveNext();
+            private static readonly ContextCallback s_callback = s =>
+            {
+                Debug.Assert(s is AsyncStateMachineBox<TStateMachine>);
+                // Only used privately to pass directly to EC.Run
+                Unsafe.As<AsyncStateMachineBox<TStateMachine>>(s).StateMachine.MoveNext();
+            };
 
             /// <summary>A delegate to the <see cref="MoveNext()"/> method.</summary>
             private Action _moveNextAction;
diff --git a/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs b/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
index b9b60dd7a2..c3a4af3034 100644
--- a/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
+++ b/src/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
@@ -17,6 +17,7 @@ using System.Diagnostics.Tracing;
 using System.Runtime.CompilerServices;
 using System.Runtime.ExceptionServices;
 using Internal.Runtime.Augments;
+using Internal.Runtime.CompilerServices;
 
 // Disable the "reference to volatile field not treated as volatile" error.
 #pragma warning disable 0420
@@ -2485,7 +2486,12 @@ namespace System.Threading.Tasks
             }
         }
 
-        private static readonly ContextCallback s_ecCallback = obj => ((Task)obj).InnerInvoke();
+        private static readonly ContextCallback s_ecCallback = obj =>
+        {
+            Debug.Assert(obj is Task);
+            // Only used privately to pass directly to EC.Run
+            Unsafe.As<Task>(obj).InnerInvoke();
+        };
 
         /// <summary>
         /// The actual code which invokes the body of the task. This can be overridden in derived types.