Error format

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

@@ -159,15 +159,19 @@ static DiagnosticInfo createObsoleteDiagnostic(Symbol symbol, BinderFlags locati
                 {
                     Debug.Assert(data.Message == null);
                     Debug.Assert(!data.IsError);
-                    return new CSDiagnosticInfo(ErrorCode.WRN_Experimental, symbol);
+                    // Provide an explicit format for fully-qualified type names.
+                    return new CSDiagnosticInfo(ErrorCode.WRN_Experimental,
+                        new FormattedSymbol(symbol, SymbolDisplayFormat.CSharpErrorMessageFormat));
                 }
 
                 if (data.Kind == ObsoleteAttributeKind.Experimental)
                 {
                     Debug.Assert(data.Message is null);
                     Debug.Assert(!data.IsError);
 
-                    return new CustomObsoleteDiagnosticInfo(MessageProvider.Instance, (int)ErrorCode.WRN_Experimental, data, symbol);
+                    // 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

src/Compilers/CSharp/Test/Emit2/Semantics/ExperimentalAttributeTests.cs

@@ -893,7 +893,7 @@ public static void M() { }
 """;
         var comp = CreateCompilation(new[] { src, experimentalAttributeSrc });
         comp.VerifyDiagnostics(
-            // 0.cs(1,1): warning DiagID1: 'C' is for evaluation purposes only and is subject to change or removal in future updates.
+            // 0.cs(1,1): warning DiagID1: 'N.C' is for evaluation purposes only and is subject to change or removal in future updates.
             // N.C.M();
             Diagnostic("DiagID1", "N.C").WithArguments("N.C").WithLocation(1, 1)
             );