Fix issues with nullable annotations found during review

src/Workspaces/Core/Portable/AddImports/AbstractAddImportsService.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.Generic;
 using System.Collections.Immutable;
@@ -35,14 +37,14 @@ protected AbstractAddImportsService()
 
         public bool HasExistingImport(
             Compilation compilation, SyntaxNode root,
-            SyntaxNode contextLocation, SyntaxNode import)
+            SyntaxNode? contextLocation, SyntaxNode import)
         {
             var globalImports = GetGlobalImports(compilation);
             var containers = GetAllContainers(root, contextLocation);
             return HasExistingImport(import, containers, globalImports);
         }
 
-        private static ImmutableArray<SyntaxNode> GetAllContainers(SyntaxNode root, SyntaxNode contextLocation)
+        private static ImmutableArray<SyntaxNode> GetAllContainers(SyntaxNode root, SyntaxNode? contextLocation)
         {
             contextLocation ??= root;
 
@@ -77,7 +79,7 @@ private static ImmutableArray<SyntaxNode> GetAllContainers(SyntaxNode root, Synt
             return false;
         }
 
-        public SyntaxNode GetImportContainer(SyntaxNode root, SyntaxNode contextLocation, SyntaxNode import)
+        public SyntaxNode GetImportContainer(SyntaxNode root, SyntaxNode? contextLocation, SyntaxNode import)
         {
             contextLocation ??= root;
             GetContainers(root, contextLocation,
@@ -106,7 +108,7 @@ public SyntaxNode GetImportContainer(SyntaxNode root, SyntaxNode contextLocation
         public SyntaxNode AddImports(
             Compilation compilation,
             SyntaxNode root,
-            SyntaxNode contextLocation,
+            SyntaxNode? contextLocation,
             IEnumerable<SyntaxNode> newImports,
             bool placeSystemNamespaceFirst)
         {

src/Workspaces/Core/Portable/AddImports/IAddImportsService.cs

@@ -15,16 +15,16 @@ internal interface IAddImportsService : ILanguageService
         /// <paramref name="import"/> in scope at <paramref name="contextLocation"/>.  This includes
         /// global imports for VB.
         /// </summary>
-        bool HasExistingImport(Compilation compilation, SyntaxNode root, SyntaxNode contextLocation, SyntaxNode import);
+        bool HasExistingImport(Compilation compilation, SyntaxNode root, SyntaxNode? contextLocation, SyntaxNode import);
 
         /// <summary>
         /// Given a context location in a provided syntax tree, returns the appropriate container
         /// that <paramref name="import"/> should be added to.
         /// </summary>
-        SyntaxNode GetImportContainer(SyntaxNode root, SyntaxNode contextLocation, SyntaxNode import);
+        SyntaxNode GetImportContainer(SyntaxNode root, SyntaxNode? contextLocation, SyntaxNode import);
 
         SyntaxNode AddImports(
-            Compilation compilation, SyntaxNode root, SyntaxNode contextLocation,
+            Compilation compilation, SyntaxNode root, SyntaxNode? contextLocation,
             IEnumerable<SyntaxNode> newImports, bool placeSystemNamespaceFirst);
     }
 

src/Workspaces/Core/Portable/Shared/Extensions/IPropertySymbolExtensions.cs

@@ -46,7 +46,7 @@ public static IPropertySymbol RenameParameters(this IPropertySymbol property, IL
             }
 
             bool shouldRemoveAttribute(AttributeData a) =>
-                attributesToRemove.Where(attr => attr != null).Any(attr => attr!.Equals(a.AttributeClass));
+                attributesToRemove.Any(attr => attr?.Equals(a.AttributeClass) ?? false);
 
             var someParameterHasAttribute = property.Parameters
                 .Any(p => p.GetAttributes().Any(shouldRemoveAttribute));

src/Workspaces/Core/Portable/Shared/Extensions/ITypeSymbolExtensions.cs

@@ -81,7 +81,7 @@ public static bool IsAnonymousType([NotNullWhen(returnValue: true)] this INamedT
         }
 
         [return: NotNullIfNotNull(parameterName: "symbol")]
-        public static ITypeSymbol? RemoveNullableIfPresent([NotNullWhen(returnValue: true)] this ITypeSymbol? symbol)
+        public static ITypeSymbol? RemoveNullableIfPresent(this ITypeSymbol? symbol)
         {
             if (symbol.IsNullable())
             {
@@ -255,7 +255,7 @@ public static bool IsAnonymousType([NotNullWhen(returnValue: true)] this INamedT
             return null;
         }
 
-        private static ISymbol FindImplementations<TSymbol>(
+        private static ISymbol? FindImplementations<TSymbol>(
             this ITypeSymbol typeSymbol,
             TSymbol constructedInterfaceMember,
             Workspace workspace) where TSymbol : class, ISymbol
@@ -436,6 +436,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
                 && symbol.ContainingNamespace.ContainingNamespace?.IsGlobalNamespace == true;
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? RemoveUnavailableTypeParameters(
             this ITypeSymbol? type,
             Compilation compilation,
@@ -444,6 +445,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return type?.RemoveUnavailableTypeParameters(compilation, availableTypeParameters.Select(t => t.Name).ToSet());
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         private static ITypeSymbol? RemoveUnavailableTypeParameters(
             this ITypeSymbol? type,
             Compilation compilation,
@@ -452,6 +454,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return type?.Accept(new UnavailableTypeParameterRemover(compilation, availableTypeParameterNames));
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? RemoveAnonymousTypes(
             this ITypeSymbol? type,
             Compilation compilation)
@@ -459,6 +462,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return type?.Accept(new AnonymousTypeRemover(compilation));
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? ReplaceTypeParametersBasedOnTypeConstraints(
             this ITypeSymbol? type,
             Compilation compilation,
@@ -469,6 +473,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return type?.Accept(new ReplaceTypeParameterBasedOnTypeConstraintVisitor(compilation, availableTypeParameters.Select(t => t.Name).ToSet(), solution, cancellationToken));
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? RemoveUnnamedErrorTypes(
             this ITypeSymbol? type,
             Compilation compilation)
@@ -492,6 +497,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return result;
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? SubstituteTypes<TType1, TType2>(
             this ITypeSymbol? type,
             IDictionary<TType1, TType2> mapping,
@@ -502,6 +508,7 @@ public static bool IsFormattableString([NotNullWhen(returnValue: true)] this ITy
             return type.SubstituteTypes(mapping, new CompilationTypeGenerator(compilation));
         }
 
+        [return: NotNullIfNotNull(parameterName: "type")]
         public static ITypeSymbol? SubstituteTypes<TType1, TType2>(
             this ITypeSymbol? type,
             IDictionary<TType1, TType2> mapping,