Reduce struct copies in ValueTask.GetAwaiter

src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/ConfiguredValueTaskAwaitable.cs

@@ -28,7 +28,15 @@ public readonly struct ConfiguredValueTaskAwaitable
 
         /// <summary>Returns an awaiter for this <see cref="ConfiguredValueTaskAwaitable"/> instance.</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ConfiguredValueTaskAwaiter GetAwaiter() => new ConfiguredValueTaskAwaiter(_value);
+        public ConfiguredValueTaskAwaiter GetAwaiter()
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ConfiguredValueTaskAwaiter and ConfiguredValueTaskAwaitable are structs, 
+            // exactly the same data layout and ConfiguredValueTaskAwaitable.ctor does no real initalization.
+
+            return Unsafe.As<ConfiguredValueTaskAwaitable, ConfiguredValueTaskAwaiter>(ref Unsafe.AsRef(in this));
+        }
 
         /// <summary>Provides an awaiter for a <see cref="ConfiguredValueTaskAwaitable"/>.</summary>
         [StructLayout(LayoutKind.Auto)]
@@ -134,7 +142,15 @@ public readonly struct ConfiguredValueTaskAwaitable<TResult>
 
         /// <summary>Returns an awaiter for this <see cref="ConfiguredValueTaskAwaitable{TResult}"/> instance.</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ConfiguredValueTaskAwaiter GetAwaiter() => new ConfiguredValueTaskAwaiter(_value);
+        public ConfiguredValueTaskAwaiter GetAwaiter()
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ConfiguredValueTaskAwaiter and ConfiguredValueTaskAwaitable are structs, 
+            // exactly the same data layout and ConfiguredValueTaskAwaitable.ctor does no real initalization.
+
+            return Unsafe.As<ConfiguredValueTaskAwaitable<TResult>, ConfiguredValueTaskAwaiter>(ref Unsafe.AsRef(in this));
+        }
 
         /// <summary>Provides an awaiter for a <see cref="ConfiguredValueTaskAwaitable{TResult}"/>.</summary>
         [StructLayout(LayoutKind.Auto)]

src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/ValueTaskAwaiter.cs

@@ -30,10 +30,18 @@ namespace System.Runtime.CompilerServices
         /// <summary>The value being awaited.</summary>
         private readonly ValueTask _value;
 
+        // NOTE: If any extra fields are added ValueTask.GetAwaiter 
+        // must be updated to call the constructor.
+
         /// <summary>Initializes the awaiter.</summary>
         /// <param name="value">The value to be awaited.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        internal ValueTaskAwaiter(ValueTask value) => _value = value;
+        internal ValueTaskAwaiter(ValueTask value)
+        {
+            // NOTE: If this method is ever updated to perform more initialization,
+            //       ValueTask<TResult>.GetAwaiter must be updated to call this.
+            _value = value;
+        }
 
         /// <summary>Gets whether the <see cref="ValueTask"/> has completed.</summary>
         public bool IsCompleted
@@ -113,10 +121,18 @@ void IStateMachineBoxAwareAwaiter.AwaitUnsafeOnCompleted(IAsyncStateMachineBox b
         /// <summary>The value being awaited.</summary>
         private readonly ValueTask<TResult> _value;
 
+        // NOTE: If any extra fields are added ValueTask<TResult>.GetAwaiter 
+        // must be updated to call the constructor.
+
         /// <summary>Initializes the awaiter.</summary>
         /// <param name="value">The value to be awaited.</param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        internal ValueTaskAwaiter(ValueTask<TResult> value) => _value = value;
+        internal ValueTaskAwaiter(ValueTask<TResult> value)
+        {
+            // NOTE: If this method is ever updated to perform more initialization,
+            //       ValueTask<TResult>.GetAwaiter must be updated to call this.
+            _value = value;
+        }
 
         /// <summary>Gets whether the <see cref="ValueTask{TResult}"/> has completed.</summary>
         public bool IsCompleted

src/System.Private.CoreLib/shared/System/Threading/Tasks/ValueTask.cs

@@ -369,15 +369,35 @@ internal void ThrowIfCompletedUnsuccessfully()
         }
 
         /// <summary>Gets an awaiter for this <see cref="ValueTask"/>.</summary>
-        public ValueTaskAwaiter GetAwaiter() => new ValueTaskAwaiter(this);
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public ValueTaskAwaiter GetAwaiter()
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ValueTaskAwaiter and ValueTask are structs, 
+            // exactly the same data layout and ValueTaskAwaiter does no real initalization.
+
+            return Unsafe.As<ValueTask, ValueTaskAwaiter>(ref Unsafe.AsRef(in this));
+        }
 
         /// <summary>Configures an awaiter for this <see cref="ValueTask"/>.</summary>
         /// <param name="continueOnCapturedContext">
         /// true to attempt to marshal the continuation back to the captured context; otherwise, false.
         /// </param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public ConfiguredValueTaskAwaitable ConfigureAwait(bool continueOnCapturedContext) =>
-            new ConfiguredValueTaskAwaitable(new ValueTask(_obj, _token, continueOnCapturedContext));
+            ConfigureTask(new ValueTask(_obj, _token, continueOnCapturedContext));
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ref readonly ConfiguredValueTaskAwaitable ConfigureTask(in ValueTask vt)
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ConfiguredValueTaskAwaitable and ValueTask are structs, 
+            // exactly the same data layout and ConfiguredValueTaskAwaitable.ctor does no real initalization.
+
+            return ref Unsafe.As<ValueTask, ConfiguredValueTaskAwaitable>(ref Unsafe.AsRef(in vt));
+        }
     }
 
     /// <summary>Provides a value type that can represent a synchronously available value or a task object.</summary>
@@ -765,15 +785,34 @@ public TResult Result
 
         /// <summary>Gets an awaiter for this <see cref="ValueTask{TResult}"/>.</summary>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ValueTaskAwaiter<TResult> GetAwaiter() => new ValueTaskAwaiter<TResult>(this);
+        public ValueTaskAwaiter<TResult> GetAwaiter()
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ValueTaskAwaiter<TResult> and ValueTask<TResult> are structs, 
+            // exactly the same data layout and ValueTaskAwaiter.ctor does no real initalization.
+
+            return Unsafe.As<ValueTask<TResult>, ValueTaskAwaiter<TResult>>(ref Unsafe.AsRef(in this));
+        }
 
         /// <summary>Configures an awaiter for this <see cref="ValueTask{TResult}"/>.</summary>
         /// <param name="continueOnCapturedContext">
         /// true to attempt to marshal the continuation back to the captured context; otherwise, false.
         /// </param>
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
-        public ConfiguredValueTaskAwaitable<TResult> ConfigureAwait(bool continueOnCapturedContext) =>
-            new ConfiguredValueTaskAwaitable<TResult>(new ValueTask<TResult>(_obj, _result, _token, continueOnCapturedContext));
+        public ConfiguredValueTaskAwaitable<TResult> ConfigureAwait(bool continueOnCapturedContext)
+            => ConfigureTask(new ValueTask<TResult>(_obj, _result, _token, continueOnCapturedContext));
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static ref readonly ConfiguredValueTaskAwaitable<TResult> ConfigureTask(in ValueTask<TResult> vt)
+        {
+            // Unsafe casting to work around https://github.com/dotnet/coreclr/issues/18542
+
+            // NOTE: This only works because ConfiguredValueTaskAwaitable and ValueTask are structs, 
+            // exactly the same data layout and ConfiguredValueTaskAwaitable.ctor does no real initalization.
+
+            return ref Unsafe.As<ValueTask<TResult>, ConfiguredValueTaskAwaitable<TResult>>(ref Unsafe.AsRef(in vt));
+        }
 
         /// <summary>Gets a string-representation of this <see cref="ValueTask{TResult}"/>.</summary>
         public override string ToString()