Add nullability support to use local function

Fixes #30322

Author: jasonmalinowski

src/EditorFeatures/CSharpTest/UseLocalFunction/UseLocalFunctionTests.cs

@@ -3,6 +3,7 @@
 using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.CodeFixes;
 using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Test.Utilities;
 using Microsoft.CodeAnalysis.CSharp.UseLocalFunction;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Diagnostics;
@@ -3660,5 +3661,33 @@ void M()
     }
 }", parseOptions: CSharp8ParseOptions);
         }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsUseLocalFunction)]
+        public async Task TestWithNullableParameterAndReturn()
+        {
+            await TestInRegularAndScriptAsync(
+@"#nullable enable
+
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        Func<string?, string?> [||]f = s => s;
+    }
+}",
+@"#nullable enable
+
+using System;
+
+class Program
+{
+    static void Main(string[] args)
+    {
+        static string? f(string? s) => s;
+    }
+}", parseOptions: TestOptions.Regular8WithNullableAnalysis);
+        }
     }
 }

src/Features/CSharp/Portable/UseLocalFunction/CSharpUseLocalFunctionCodeFixProvider.cs

@@ -245,7 +245,7 @@ ParameterSyntax PromoteParameter(ParameterSyntax parameterNode, IParameterSymbol
 
                 if (parameterNode.Type == null)
                 {
-                    parameterNode = parameterNode.WithType(delegateParameter?.Type.GenerateTypeSyntax() ?? s_objectType);
+                    parameterNode = parameterNode.WithType(delegateParameter?.Type.WithNullability(delegateParameter.NullableAnnotation).GenerateTypeSyntax() ?? s_objectType);
                 }
 
                 if (delegateParameter?.HasExplicitDefaultValue == true)

src/Workspaces/CSharp/Portable/Extensions/TypeSyntaxExtensions.cs

@@ -93,17 +93,19 @@ public static bool IsTypeInferred(this TypeSyntax typeSyntax, SemanticModel sema
 
         public static TypeSyntax GenerateReturnTypeSyntax(this IMethodSymbol method)
         {
+            var returnType = method.ReturnType.WithNullability(method.ReturnNullableAnnotation);
+
             if (method.ReturnsByRef)
             {
-                return method.ReturnType.GenerateRefTypeSyntax();
+                return returnType.GenerateRefTypeSyntax();
             }
             else if (method.ReturnsByRefReadonly)
             {
-                return method.ReturnType.GenerateRefReadOnlyTypeSyntax();
+                return returnType.GenerateRefReadOnlyTypeSyntax();
             }
             else
             {
-                return method.ReturnType.GenerateTypeSyntax();
+                return returnType.GenerateTypeSyntax();
             }
         }
 

src/Workspaces/Core/Portable/CodeGeneration/Symbols/CodeGenerationAbstractMethodSymbol.cs

@@ -48,9 +48,9 @@ protected CodeGenerationAbstractMethodSymbol(
         public abstract IMethodSymbol PartialDefinitionPart { get; }
         public abstract IMethodSymbol PartialImplementationPart { get; }
 
-        public NullableAnnotation ReceiverNullableAnnotation => throw new NotImplementedException();
-        public NullableAnnotation ReturnNullableAnnotation => throw new NotImplementedException();
-        public ImmutableArray<NullableAnnotation> TypeArgumentsNullableAnnotations => throw new NotImplementedException();
+        public NullableAnnotation ReceiverNullableAnnotation => ReceiverType.GetNullability();
+        public NullableAnnotation ReturnNullableAnnotation => ReturnType.GetNullability();
+        public ImmutableArray<NullableAnnotation> TypeArgumentsNullableAnnotations => TypeArguments.SelectAsArray(a => a.GetNullability());
 
         public virtual ITypeSymbol ReceiverType
         {