Add support for ExperimentalAttribute

src/Compilers/CSharp/Portable/Symbols/ObsoleteAttributeHelpers.cs

@@ -100,6 +100,7 @@ internal static ObsoleteDiagnosticKind GetObsoleteDiagnosticKind(Symbol symbol,
                 case ObsoleteAttributeKind.None:
                     return ObsoleteDiagnosticKind.NotObsolete;
                 case ObsoleteAttributeKind.Experimental:
+                case ObsoleteAttributeKind.NewExperimental:
                     return ObsoleteDiagnosticKind.Diagnostic;
                 case ObsoleteAttributeKind.Uninitialized:
                     // If we haven't cracked attributes on the symbol at all or we haven't
@@ -162,6 +163,16 @@ static DiagnosticInfo createObsoleteDiagnostic(Symbol symbol, BinderFlags locati
                     return new CSDiagnosticInfo(ErrorCode.WRN_Experimental, new FormattedSymbol(symbol, SymbolDisplayFormat.CSharpErrorMessageFormat));
                 }
 
+                if (data.Kind == ObsoleteAttributeKind.NewExperimental)
+                {
+                    Debug.Assert(data.Message is null);
+                    Debug.Assert(!data.IsError);
+
+                    // Provide an explicit format for fully-qualified type names.
+                    return new CustomObsoleteDiagnosticInfo(MessageProvider.Instance, (int)ErrorCode.WRN_Experimental,
+                        data, new FormattedSymbol(symbol, SymbolDisplayFormat.CSharpErrorMessageFormat));
+                }
+
                 // Issue a specialized diagnostic for add methods of collection initializers
                 var isColInit = location.Includes(BinderFlags.CollectionInitializerAddMethod);
                 Debug.Assert(!isColInit || symbol.Name == WellKnownMemberNames.CollectionInitializerAddMethodName);

src/Compilers/CSharp/Portable/Symbols/Symbol.cs

@@ -1321,6 +1321,7 @@ internal ThreeState ObsoleteState
                 {
                     case ObsoleteAttributeKind.None:
                     case ObsoleteAttributeKind.Experimental:
+                    case ObsoleteAttributeKind.NewExperimental:
                         return ThreeState.False;
                     case ObsoleteAttributeKind.Uninitialized:
                         return ThreeState.Unknown;

src/Compilers/CSharp/Portable/Symbols/Symbol_Attributes.cs

@@ -166,6 +166,10 @@ internal static bool EarlyDecodeDeprecatedOrExperimentalOrObsoleteAttribute(
             {
                 kind = ObsoleteAttributeKind.Experimental;
             }
+            else if (CSharpAttributeData.IsTargetEarlyAttribute(type, syntax, AttributeDescription.NewExperimentalAttribute))
+            {
+                kind = ObsoleteAttributeKind.NewExperimental;
+            }
             else
             {
                 obsoleteData = null;