Mark ISourceGenerator as obsolete for implementation

Analyzer that will flag implementation is dotnet/roslyn-analyzers#7419. We don't just mark `ISourceGenerator` as obsolete directly because we do use it in public APIs like `AnalyzerFileReference` and `GeneratorDriver`, and we can't remove it from there.

docs/features/incremental-generators.md

@@ -2,8 +2,8 @@
 
 ## Summary
 
-Incremental generators are a new API that exists alongside
-[source generators](source-generators.md) to allow users to specify generation
+Incremental generators are a new API that replaces
+[v1 source generators](source-generators.md) to allow users to specify generation
 strategies that can be applied in a high performance way by the hosting layer.
 
 ### High Level Design Goals

src/Compilers/Core/Portable/ImplementationIsObsoleteAttribute.cs

@@ -0,0 +1,11 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Runtime.CompilerServices;
+
+[AttributeUsage(AttributeTargets.Interface, AllowMultiple = false)]
+internal sealed class ImplementationObsoleteAttribute(string url) : Attribute
+{
+    public string Url { get; } = url;
+}

src/Compilers/Core/Portable/SourceGeneration/ISourceGenerator.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Immutable;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.PooledObjects;
@@ -19,13 +20,15 @@ namespace Microsoft.CodeAnalysis
     /// is no guarantee that the same instance will be used on a 
     /// subsequent generation pass.
     /// </remarks>
+    [ImplementationObsolete(url: "https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.md")]
     public interface ISourceGenerator
     {
         /// <summary>
         /// Called before generation occurs. A generator can use the <paramref name="context"/>
         /// to register callbacks required to perform generation.
         /// </summary>
         /// <param name="context">The <see cref="GeneratorInitializationContext"/> to register callbacks on</param>
+        [Obsolete("ISourceGenerator is deprecated and should not be implemented. Please implement IIncrementalGenerator instead. See https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.md.")]
         void Initialize(GeneratorInitializationContext context);
 
         /// <summary>
@@ -42,6 +45,7 @@ public interface ISourceGenerator
         /// discover information about the users compilation and make decisions on what source to 
         /// provide. 
         /// </remarks>
+        [Obsolete("ISourceGenerator is deprecated and should not be implemented. Please implement IIncrementalGenerator instead. See https://github.com/dotnet/roslyn/blob/main/docs/features/incremental-generators.md.")]
         void Execute(GeneratorExecutionContext context);
     }
 }

src/Compilers/VisualBasic/Test/Semantic/SourceGeneration/GeneratorDriverTests.vb

@@ -749,11 +749,13 @@ End Class
 
         Public _receiver As Receiver = New Receiver()
 
+#Disable Warning BC40000
         Public Sub Initialize(context As GeneratorInitializationContext) Implements ISourceGenerator.Initialize
             context.RegisterForSyntaxNotifications(Function() _receiver)
         End Sub
 
         Public Sub Execute(context As GeneratorExecutionContext) Implements ISourceGenerator.Execute
+#Enable Warning BC40000
             context.AddSource("source.vb", "
 Public Class D
 End Class
@@ -801,13 +803,15 @@ End Class
             _initialized = True
         End Sub
 
+#Disable Warning BC40000
         Public Sub Initialize(context As GeneratorInitializationContext) Implements ISourceGenerator.Initialize
             _sourceInitialized = True
         End Sub
 
         Public Sub Execute(context As GeneratorExecutionContext) Implements ISourceGenerator.Execute
             _sourceExecuted = True
         End Sub
+#Enable Warning BC40000
 
     End Class
 

src/EditorFeatures/Test2/Rename/CSharp/SourceGeneratorTests.vb

@@ -78,10 +78,12 @@ public partial class GeneratedClass : IInterface { }
         Private Class GeneratorThatImplementsInterfaceMethod
             Implements ISourceGenerator
 
+#Disable Warning BC40000
             Public Sub Initialize(context As GeneratorInitializationContext) Implements ISourceGenerator.Initialize
             End Sub
 
             Public Sub Execute(context As GeneratorExecutionContext) Implements ISourceGenerator.Execute
+#Enable Warning BC40000
                 Dim [interface] = context.Compilation.GetTypeByMetadataName("IInterface")
                 Dim memberName = [interface].MemberNames.Single()