Deprecate API and add NFW telemetry for it

src/Compilers/CSharp/Portable/Syntax/CSharpSyntaxNode.cs

@@ -9,6 +9,7 @@
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.ErrorReporting;
 using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
 
@@ -182,6 +183,7 @@ public new SyntaxTriviaList GetTrailingTrivia()
         /// <summary>
         /// Deserialize a syntax node from the byte stream.
         /// </summary>
+        [Obsolete("Syntax serialization support is deprecated and will be removed in a future version of this API", error: false)]
         public static SyntaxNode DeserializeFrom(Stream stream, CancellationToken cancellationToken = default)
         {
             if (stream == null)
@@ -194,6 +196,8 @@ public static SyntaxNode DeserializeFrom(Stream stream, CancellationToken cancel
                 throw new InvalidOperationException(CodeAnalysisResources.TheStreamCannotBeReadFrom);
             }
 
+            // Report NFW to see if this is being used in the wild.
+            FatalError.ReportAndPropagate(new SerializationDeprecationException());
             using var reader = ObjectReader.TryGetReader(stream, leaveOpen: true, cancellationToken);
 
             if (reader == null)

src/Compilers/CSharp/Test/Syntax/Syntax/SerializationTests.cs

@@ -13,6 +13,8 @@
 using Roslyn.Test.Utilities;
 using Xunit;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace Microsoft.CodeAnalysis.CSharp.UnitTests
 {
     public class SerializationTests
@@ -305,3 +307,5 @@ public class C
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete

src/Compilers/Core/Portable/Syntax/SyntaxNode.cs

@@ -10,6 +10,7 @@
 using System.Linq;
 using System.Text;
 using System.Threading;
+using Microsoft.CodeAnalysis.ErrorReporting;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
 
@@ -1391,6 +1392,7 @@ public bool IsEquivalentTo(SyntaxNode node, bool topLevel = false)
         /// Serializes the node to the given <paramref name="stream"/>.
         /// Leaves the <paramref name="stream"/> open for further writes.
         /// </summary>
+        [Obsolete("Syntax serialization support is deprecated and will be removed in a future version of this API", error: false)]
         public virtual void SerializeTo(Stream stream, CancellationToken cancellationToken = default)
         {
             if (stream == null)
@@ -1403,10 +1405,24 @@ public virtual void SerializeTo(Stream stream, CancellationToken cancellationTok
                 throw new InvalidOperationException(CodeAnalysisResources.TheStreamCannotBeWrittenTo);
             }
 
+            // Report NFW to see if this is being used in the wild.
+            FatalError.ReportAndPropagate(new SerializationDeprecationException());
             using var writer = new ObjectWriter(stream, leaveOpen: true, cancellationToken);
             writer.WriteValue(Green);
         }
 
+        /// <summary>
+        /// Specialized exception subtype to make it easier to search telemetry streams for this specific case.
+        /// </summary>
+        private protected sealed class SerializationDeprecationException : Exception
+        {
+            public SerializationDeprecationException()
+                : base("Syntax serialization support is deprecated and will be removed in a future version of this API")
+            {
+
+            }
+        }
+
         #region Core Methods
 
         /// <summary>

src/Compilers/Test/Utilities/CSharp/CSharpTestSource.cs

@@ -45,13 +45,15 @@ private CSharpTestSource(object value)
 
         private static void CheckSerializable(SyntaxTree tree)
         {
+#pragma warning disable CS0618 // Type or member is obsolete
             using var stream = new MemoryStream();
             var root = tree.GetRoot();
             root.SerializeTo(stream);
             stream.Position = 0;
 
             // verify absence of exception:
             _ = CSharpSyntaxNode.DeserializeFrom(stream);
+#pragma warning restore CS0618 // Type or member is obsolete
         }
 
         public SyntaxTree[] GetSyntaxTrees(CSharpParseOptions parseOptions, string sourceFileName = "")

src/Compilers/VisualBasic/Portable/Syntax/VisualBasicSyntaxNode.vb

@@ -7,6 +7,7 @@ Imports System.Collections.ObjectModel
 Imports System.ComponentModel
 Imports System.Reflection
 Imports System.Threading
+Imports Microsoft.CodeAnalysis.ErrorReporting
 Imports Microsoft.CodeAnalysis.PooledObjects
 Imports Microsoft.CodeAnalysis.Text
 Imports Microsoft.CodeAnalysis.VisualBasic.Symbols
@@ -125,6 +126,7 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
         ''' <summary>
         ''' Deserialize a syntax node from a byte stream.
         ''' </summary>
+        <Obsolete("Syntax serialization support is deprecated and will be removed in a future version of this API", False)>
         Public Shared Function DeserializeFrom(stream As IO.Stream, Optional cancellationToken As CancellationToken = Nothing) As SyntaxNode
             If stream Is Nothing Then
                 Throw New ArgumentNullException(NameOf(stream))
@@ -134,6 +136,9 @@ Namespace Microsoft.CodeAnalysis.VisualBasic
                 Throw New InvalidOperationException(CodeAnalysisResources.TheStreamCannotBeReadFrom)
             End If
 
+            ' Report NFW to see if this Is being used in the wild.
+            FatalError.ReportAndPropagate(New SerializationDeprecationException())
+
             Using reader = ObjectReader.TryGetReader(stream, leaveOpen:=True, cancellationToken:=cancellationToken)
                 If reader Is Nothing Then
                     Throw New ArgumentException(CodeAnalysisResources.Stream_contains_invalid_data, NameOf(stream))

src/Compilers/VisualBasic/Test/Syntax/Syntax/SerializationTests.vb

@@ -11,8 +11,9 @@ Imports Microsoft.CodeAnalysis.VisualBasic.Syntax
 Imports Roslyn.Test.Utilities
 Imports Xunit
 
-Namespace Microsoft.CodeAnalysis.VisualBasic.UnitTests
+#Disable Warning BC40000 ' Type or member is obsolete
 
+Namespace Microsoft.CodeAnalysis.VisualBasic.UnitTests
     Public Class SerializationTests
 
         Private Sub RoundTrip(text As String, Optional expectRecursive As Boolean = True)
@@ -461,3 +462,5 @@ End Module
         End Sub
     End Class
 End Namespace
+
+#Enable Warning BC40000 ' Type or member is obsolete

src/Tools/IdeCoreBenchmarks/SerializationBenchmarks.cs

@@ -13,6 +13,8 @@
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Text;
 
+#pragma warning disable CS0618 // Type or member is obsolete
+
 namespace IdeCoreBenchmarks
 {
     [MemoryDiagnoser]
@@ -96,3 +98,5 @@ public void DeserializeSyntaxNode()
         }
     }
 }
+
+#pragma warning restore CS0618 // Type or member is obsolete