Add RuntimeAsyncMethodGenerationAttribute

src/Compilers/CSharp/Portable/Compilation/CSharpCompilation.cs

@@ -316,9 +316,7 @@ internal bool IsNullableAnalysisEnabledAlways
         /// Returns true if this method should be processed with runtime async handling instead
         /// of compiler async state machine generation.
         /// </summary>
-#pragma warning disable IDE0060 // Remove unused parameter
         internal bool IsRuntimeAsyncEnabledIn(MethodSymbol method)
-#pragma warning restore IDE0060 // Remove unused parameter
         {
             // PROTOTYPE: EE tests fail this assert, handle and test
             //Debug.Assert(ReferenceEquals(method.ContainingAssembly, Assembly));
@@ -327,8 +325,12 @@ internal bool IsRuntimeAsyncEnabledIn(MethodSymbol method)
                 return false;
             }
 
-            // PROTOTYPE: Check for attributes that turn on/off the feature member-by-member
-            return true;
+            return method switch
+            {
+                SourceMethodSymbol { IsRuntimeAsyncEnabledInMethod: ThreeState.True } => true,
+                SourceMethodSymbol { IsRuntimeAsyncEnabledInMethod: ThreeState.False } => false,
+                _ => Feature("runtime-async") == "on"
+            };
         }
 
         /// <summary>

src/Compilers/CSharp/Portable/Symbols/Attributes/WellKnownAttributeData/MethodWellKnownAttributeData.cs

@@ -149,5 +149,21 @@ public UnmanagedCallersOnlyAttributeData? UnmanagedCallersOnlyAttributeData
                 SetDataStored();
             }
         }
+
+        private ThreeState _runtimeAsyncMethodGenerationSetting;
+        public ThreeState RuntimeAsyncMethodGenerationSetting
+        {
+            get
+            {
+                VerifySealed(expected: true);
+                return _runtimeAsyncMethodGenerationSetting;
+            }
+            set
+            {
+                VerifySealed(expected: false);
+                _runtimeAsyncMethodGenerationSetting = value;
+                SetDataStored();
+            }
+        }
     }
 }

src/Compilers/CSharp/Portable/Symbols/Source/SourceMethodSymbolWithAttributes.cs

@@ -651,6 +651,14 @@ private void DecodeWellKnownAttributeAppliedToMethod(ref DecodeWellKnownAttribut
                         arguments.AttributeSyntaxOpt);
                 }
             }
+            else if (attribute.IsTargetAttribute(AttributeDescription.RuntimeAsyncMethodGenerationAttribute))
+            {
+                // PROTOTYPE: Validate langversion? Validate previewness of the runtime feature flag?
+                arguments.GetOrCreateData<MethodWellKnownAttributeData>().RuntimeAsyncMethodGenerationSetting =
+                    attribute.CommonConstructorArguments[0].DecodeValue<bool>(SpecialType.System_Boolean)
+                        ? ThreeState.True
+                        : ThreeState.False;
+            }
             else
             {
                 var compilation = this.DeclaringCompilation;
@@ -661,6 +669,9 @@ private void DecodeWellKnownAttributeAppliedToMethod(ref DecodeWellKnownAttribut
             }
         }
 
+        internal ThreeState IsRuntimeAsyncEnabledInMethod
+            => GetDecodedWellKnownAttributeData()?.RuntimeAsyncMethodGenerationSetting ?? ThreeState.Unknown;
+
         internal override ImmutableArray<string> NotNullMembers =>
             GetDecodedWellKnownAttributeData()?.NotNullMembers ?? ImmutableArray<string>.Empty;