Add AsyncVoidReturningRelayCommandMethodCodeFixer tests

tests/CommunityToolkit.Mvvm.SourceGenerators.Roslyn401.UnitTests/Test_AsyncVoidReturningRelayCommandMethodCodeFixer.cs

+// 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.
+
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.Input;
+using Microsoft.CodeAnalysis.Testing;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using CSharpCodeFixTest = Microsoft.CodeAnalysis.CSharp.Testing.CSharpCodeFixTest<
+    CommunityToolkit.Mvvm.SourceGenerators.AsyncVoidReturningRelayCommandMethodAnalyzer,
+    CommunityToolkit.Mvvm.CodeFixers.AsyncVoidReturningRelayCommandMethodCodeFixer,
+    Microsoft.CodeAnalysis.Testing.Verifiers.MSTestVerifier>;
+using CSharpCodeFixVerifier = Microsoft.CodeAnalysis.CSharp.Testing.CSharpCodeFixVerifier<
+    CommunityToolkit.Mvvm.SourceGenerators.AsyncVoidReturningRelayCommandMethodAnalyzer,
+    CommunityToolkit.Mvvm.CodeFixers.AsyncVoidReturningRelayCommandMethodCodeFixer,
+    Microsoft.CodeAnalysis.Testing.Verifiers.MSTestVerifier>;
+
+namespace CommunityToolkit.Mvvm.SourceGenerators.Roslyn401.UnitTests;
+
+[TestClass]
+public class Test_AsyncVoidReturningRelayCommandMethodCodeFixer
+{
+    [TestMethod]
+    public async Task AsyncVoidMethod_FileContainsSystemThreadingTasksUsingDirective()
+    {
+        string original = """
+            using System.Threading.Tasks;
+            using CommunityToolkit.Mvvm.Input;
+
+            partial class C
+            {
+                [RelayCommand]
+                private async void Foo()
+                {
+                }
+            }
+            """;
+
+        string @fixed = """
+            using System.Threading.Tasks;
+            using CommunityToolkit.Mvvm.Input;
+
+            partial class C
+            {
+                [RelayCommand]
+                private async Task Foo()
+                {
+                }
+            }
+            """;
+
+        CSharpCodeFixTest test = new()
+        {
+            TestCode = original,
+            FixedCode = @fixed,
+            ReferenceAssemblies = ReferenceAssemblies.Net.Net60
+        };
+
+        test.TestState.AdditionalReferences.Add(typeof(RelayCommand).Assembly);
+        test.ExpectedDiagnostics.AddRange(new[]
+        {
+            // /0/Test0.cs(7,24): error MVVMTK0039: The method C.Foo() annotated with [RelayCommand] is async void (make sure to return a Task type instead)
+            CSharpCodeFixVerifier.Diagnostic().WithSpan(7, 24, 7, 27).WithArguments("C.Foo()")
+        });
+
+        await test.RunAsync();
+    }
+
+    [TestMethod]
+    public async Task AsyncVoidMethod_FileDoesNotContainSystemThreadingTasksUsingDirective()
+    {
+        string original = """
+            using CommunityToolkit.Mvvm.Input;
+
+            partial class C
+            {
+                [RelayCommand]
+                private async void Foo()
+                {
+                }
+            }
+            """;
+
+        string @fixed = """
+            using System.Threading.Tasks;
+            using CommunityToolkit.Mvvm.Input;
+
+            partial class C
+            {
+                [RelayCommand]
+                private async Task Foo()
+                {
+                }
+            }
+            """;
+
+        CSharpCodeFixTest test = new()
+        {
+            TestCode = original,
+            FixedCode = @fixed,
+            ReferenceAssemblies = ReferenceAssemblies.Net.Net60
+        };
+
+        test.TestState.AdditionalReferences.Add(typeof(RelayCommand).Assembly);
+        test.ExpectedDiagnostics.AddRange(new[]
+        {
+            // /0/Test0.cs(7,24): error MVVMTK0039: The method C.Foo() annotated with [RelayCommand] is async void (make sure to return a Task type instead)
+            CSharpCodeFixVerifier.Diagnostic().WithSpan(6, 24, 6, 27).WithArguments("C.Foo()")
+        });
+
+        await test.RunAsync();
+    }
+}