Add annotations for classification types

src/Workspaces/CSharp/Portable/Classification/ClassificationHelpers.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System.Threading;
 using Microsoft.CodeAnalysis.Classification;
 using Microsoft.CodeAnalysis.CSharp.Extensions;
@@ -24,7 +26,7 @@ internal static class ClassificationHelpers
         /// </summary>
         /// <param name="token">The token.</param>
         /// <returns>The correct syntactic classification for the token.</returns>
-        public static string GetClassification(SyntaxToken token)
+        public static string? GetClassification(SyntaxToken token)
         {
             if (IsControlKeyword(token))
             {
@@ -183,7 +185,7 @@ private static bool IsVerbatimStringToken(SyntaxToken token)
             return false;
         }
 
-        private static string GetClassificationForIdentifier(SyntaxToken token)
+        private static string? GetClassificationForIdentifier(SyntaxToken token)
         {
             if (token.Parent is BaseTypeDeclarationSyntax typeDeclaration && typeDeclaration.Identifier == token)
             {
@@ -232,7 +234,7 @@ private static string GetClassificationForIdentifier(SyntaxToken token)
             else if (token.Parent is VariableDeclaratorSyntax variableDeclarator && variableDeclarator.Identifier == token)
             {
                 var varDecl = variableDeclarator.Parent as VariableDeclarationSyntax;
-                return varDecl.Parent switch
+                return varDecl?.Parent switch
                 {
                     FieldDeclarationSyntax fieldDeclaration => fieldDeclaration.Modifiers.Any(SyntaxKind.ConstKeyword) ? ClassificationTypeNames.ConstantName : ClassificationTypeNames.FieldName,
                     LocalDeclarationStatementSyntax localDeclarationStatement => localDeclarationStatement.IsConst ? ClassificationTypeNames.ConstantName : ClassificationTypeNames.LocalName,
@@ -343,7 +345,7 @@ private static bool IsExtensionMethod(MethodDeclarationSyntax methodDeclaration)
             return methodDeclaration.ParameterList.Parameters.FirstOrDefault()?.Modifiers.Any(SyntaxKind.ThisKeyword) == true;
         }
 
-        private static string GetClassificationForTypeDeclarationIdentifier(SyntaxToken identifier)
+        private static string? GetClassificationForTypeDeclarationIdentifier(SyntaxToken identifier)
             => identifier.Parent.Kind() switch
             {
                 SyntaxKind.ClassDeclaration => ClassificationTypeNames.ClassName,

src/Workspaces/CSharp/Portable/Classification/SyntaxClassification/NameSyntaxClassifier.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using System.Threading;
 using Microsoft.CodeAnalysis.Classification;
@@ -128,7 +131,7 @@ protected override bool IsParentAnAttribute(SyntaxNode node)
 
         private bool TryClassifySymbol(
             NameSyntax name,
-            ISymbol symbol,
+            [NotNullWhen(returnValue: true)] ISymbol? symbol,
             SemanticModel semanticModel,
             CancellationToken cancellationToken,
             out ClassifiedSpan classifiedSpan)
@@ -315,7 +318,9 @@ private bool IsInVarContext(NameSyntax name)
             var identifierName = name as IdentifierNameSyntax;
             if (symbolInfo.Symbol.IsImplicitValueParameter())
             {
+#nullable disable // Can 'identifierName' be null here?
                 result.Add(new ClassifiedSpan(identifierName.Identifier.Span, ClassificationTypeNames.Keyword));
+#nullable enable
                 return true;
             }
 
@@ -337,7 +342,7 @@ private bool IsInVarContext(NameSyntax name)
             return false;
         }
 
-        private bool IsSymbolWithName(ISymbol symbol, string name)
+        private bool IsSymbolWithName([NotNullWhen(returnValue: true)] ISymbol? symbol, string name)
         {
             if (symbol is null || symbol.Name != name)
             {

src/Workspaces/CSharp/Portable/Classification/SyntaxClassification/SyntaxTokenClassifier.cs

@@ -44,7 +44,9 @@ internal class SyntaxTokenClassifier : AbstractSyntaxClassifier
                     var types = semanticModel.LookupTypeRegardlessOfArity(identifier, cancellationToken);
                     if (types.Any(s_shouldInclude))
                     {
+#nullable disable // Can 'GetClassificationForType(types.First()' be null here?
                         result.Add(new ClassifiedSpan(identifier.Span, GetClassificationForType(types.First())));
+#nullable enable
                     }
                 }
             }

src/Workspaces/CSharp/Portable/Classification/Worker_DocumentationComments.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using Microsoft.CodeAnalysis.Classification;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
 using Microsoft.CodeAnalysis.Text;
@@ -60,7 +62,7 @@ private void ClassifyXmlNode(XmlNodeSyntax node)
             }
         }
 
-        private void ClassifyXmlTrivia(SyntaxTriviaList triviaList, string whitespaceClassificationType = null)
+        private void ClassifyXmlTrivia(SyntaxTriviaList triviaList, string? whitespaceClassificationType = null)
         {
             foreach (var t in triviaList)
             {

src/Workspaces/CSharp/Portable/Classification/Worker_Preprocesser.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using Microsoft.CodeAnalysis.Classification;
 using Microsoft.CodeAnalysis.CSharp.Syntax;
@@ -104,7 +106,7 @@ private void ClassifyPreprocessorTrivia(SyntaxTrivia trivia, bool allowComments)
             }
         }
 
-        private void ClassifyPreprocessorExpression(ExpressionSyntax node)
+        private void ClassifyPreprocessorExpression(ExpressionSyntax? node)
         {
             if (node == null)
             {

src/Workspaces/Core/Portable/Classification/AbstractClassificationService.cs

@@ -42,7 +42,7 @@ public async Task AddSemanticClassificationsAsync(Document document, TextSpan te
                 return;
             }
 
-            var extensionManager = document.Project.Solution.Workspace.Services.GetService<IExtensionManager>();
+            var extensionManager = document.Project.Solution.Workspace.Services.GetRequiredService<IExtensionManager>();
             var classifiers = classificationService.GetDefaultSyntaxClassifiers();
 
             var getNodeClassifiers = extensionManager.CreateNodeExtensionGetter(classifiers, c => c.SyntaxNodeTypes);

src/Workspaces/Core/Portable/Classification/ClassificationExtensions.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 namespace Microsoft.CodeAnalysis.Classification
 {
     internal static class ClassificationExtensions
     {
-        public static string GetClassification(this ITypeSymbol type)
+        public static string? GetClassification(this ITypeSymbol type)
         {
             switch (type.TypeKind)
             {

src/Workspaces/Core/Portable/Classification/Classifier.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -30,11 +32,11 @@ public static class Classifier
             Workspace workspace,
             CancellationToken cancellationToken = default)
         {
-            var service = workspace.Services.GetLanguageServices(semanticModel.Language).GetService<ISyntaxClassificationService>();
+            var service = workspace.Services.GetLanguageServices(semanticModel.Language).GetRequiredService<ISyntaxClassificationService>();
 
             var syntaxClassifiers = service.GetDefaultSyntaxClassifiers();
 
-            var extensionManager = workspace.Services.GetService<IExtensionManager>();
+            var extensionManager = workspace.Services.GetRequiredService<IExtensionManager>();
             var getNodeClassifiers = extensionManager.CreateNodeExtensionGetter(syntaxClassifiers, c => c.SyntaxNodeTypes);
             var getTokenClassifiers = extensionManager.CreateTokenExtensionGetter(syntaxClassifiers, c => c.SyntaxTokenKinds);
 
@@ -61,19 +63,6 @@ public static class Classifier
             }
         }
 
-        internal static async Task<ImmutableArray<SymbolDisplayPart>> GetClassifiedSymbolDisplayPartsAsync(
-            Document document,
-            TextSpan textSpan,
-            CancellationToken cancellationToken = default)
-        {
-            var semanticModel = await document.GetSemanticModelAsync(cancellationToken).ConfigureAwait(false);
-
-            return await GetClassifiedSymbolDisplayPartsAsync(
-                semanticModel, textSpan,
-                document.Project.Solution.Workspace,
-                cancellationToken).ConfigureAwait(false);
-        }
-
         internal static async Task<ImmutableArray<SymbolDisplayPart>> GetClassifiedSymbolDisplayPartsAsync(
             SemanticModel semanticModel, TextSpan textSpan, Workspace workspace,
             CancellationToken cancellationToken = default)

src/Workspaces/Core/Portable/Classification/SyntaxClassification/AbstractNameSyntaxClassifier.cs

-using Microsoft.CodeAnalysis.PooledObjects;
+// Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
+
+#nullable enable
+
+using Microsoft.CodeAnalysis.PooledObjects;
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
 
@@ -9,7 +13,7 @@ internal abstract class AbstractNameSyntaxClassifier : AbstractSyntaxClassifier
         protected abstract int? GetRightmostNameArity(SyntaxNode node);
         protected abstract bool IsParentAnAttribute(SyntaxNode node);
 
-        protected ISymbol TryGetSymbol(SyntaxNode node, SymbolInfo symbolInfo, SemanticModel semanticModel)
+        protected ISymbol? TryGetSymbol(SyntaxNode node, SymbolInfo symbolInfo, SemanticModel semanticModel)
         {
             var symbol = symbolInfo.Symbol;
 
@@ -76,7 +80,7 @@ protected ISymbol TryGetSymbol(SyntaxNode node, SymbolInfo symbolInfo, SemanticM
         }
 
         protected void TryClassifyStaticSymbol(
-            ISymbol symbol,
+            ISymbol? symbol,
             TextSpan span,
             ArrayBuilder<ClassifiedSpan> result)
         {

src/Workspaces/Core/Portable/Classification/SyntaxClassification/AbstractSyntaxClassifier.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+#nullable enable
+
 using System;
 using System.Collections.Immutable;
 using System.Threading;
@@ -14,7 +16,7 @@ protected AbstractSyntaxClassifier()
         {
         }
 
-        protected string GetClassificationForType(ITypeSymbol type)
+        protected string? GetClassificationForType(ITypeSymbol type)
             => type.GetClassification();
 
         public virtual void AddClassifications(Workspace workspace, SyntaxNode syntax, SemanticModel semanticModel, ArrayBuilder<ClassifiedSpan> result, CancellationToken cancellationToken)