Allow ref enumerators in async/iterator methods

src/Compilers/CSharp/Portable/Binder/ForEachLoopBinder.cs

@@ -527,7 +527,7 @@ private BoundForEachStatement BindForEachPartsWorker(BindingDiagnosticBag diagno
 
             if (builder.InlineArraySpanType == WellKnownType.Unknown && getEnumeratorType.IsRestrictedType() && (IsDirectlyInIterator || IsInAsyncMethod()))
             {
-                diagnostics.Add(ErrorCode.ERR_BadSpecialByRefIterator, foreachKeyword.GetLocation(), getEnumeratorType);
+                CheckFeatureAvailability(foreachKeyword, MessageID.IDS_FeatureRefUnsafeInIteratorAsync, diagnostics);
             }
 
             diagnostics.Add(_syntax.ForEachKeyword, useSiteInfo);

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -3845,9 +3845,6 @@ Give the compiler some way to differentiate the methods. For example, you can gi
   <data name="ERR_BadAsyncLacksBody" xml:space="preserve">
     <value>The 'async' modifier can only be used in methods that have a body.</value>
   </data>
-  <data name="ERR_BadSpecialByRefIterator" xml:space="preserve">
-    <value>foreach statement cannot operate on enumerators of type '{0}' in async or iterator methods because '{0}' is a ref struct or a type parameter that allows ref struct.</value>
-  </data>
   <data name="ERR_BadSpecialByRefParameter" xml:space="preserve">
     <value>Parameters of type '{0}' cannot be declared in async methods or async lambda expressions.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1529,7 +1529,7 @@ internal enum ErrorCode
         ERR_AutoPropsInRoStruct = 8341,
         ERR_FieldlikeEventsInRoStruct = 8342,
         // ERR_RefStructInterfaceImpl = 8343,
-        ERR_BadSpecialByRefIterator = 8344,
+        // ERR_BadSpecialByRefIterator = 8344,
         ERR_FieldAutoPropCantBeByRefLike = 8345,
         ERR_StackAllocConversionNotPossible = 8346,
 

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -1837,7 +1837,6 @@ or ErrorCode.ERR_InExtensionMustBeValueType
                 or ErrorCode.ERR_FieldsInRoStruct
                 or ErrorCode.ERR_AutoPropsInRoStruct
                 or ErrorCode.ERR_FieldlikeEventsInRoStruct
-                or ErrorCode.ERR_BadSpecialByRefIterator
                 or ErrorCode.ERR_FieldAutoPropCantBeByRefLike
                 or ErrorCode.ERR_StackAllocConversionNotPossible
                 or ErrorCode.ERR_EscapeCall