perf: add Moq1208 benchmarks for issue #777 completion

src/CodeFixes/ReturnsDelegateShouldReturnTaskFixer.cs

@@ -62,6 +62,10 @@ private static Task<Document> ReplaceReturnsWithReturnsAsync(
     {
         SimpleNameSyntax oldName = memberAccess.Name;
         SimpleNameSyntax newName;
+
+        // Moq's IReturns<TMock, TResult> interface defines method-level generic overloads
+        // like Returns<T1>(Func<T1, TResult>). When a user writes .Returns<string>(s => 42),
+        // the syntax is GenericNameSyntax. Preserve type arguments to maintain developer intent.
         if (oldName is GenericNameSyntax genericName)
         {
             newName = SyntaxFactory.GenericName(

tests/Moq.Analyzers.Benchmarks/Moq1208ReturnsDelegateBenchmarks.cs

@@ -0,0 +1,97 @@
+using System.Collections.Immutable;
+using System.Diagnostics.CodeAnalysis;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.Diagnostics;
+using Moq.Analyzers.Benchmarks.Helpers;
+
+namespace Moq.Analyzers.Benchmarks;
+
+[InProcess]
+[MemoryDiagnoser]
+[BenchmarkCategory("Moq1208")]
+public class Moq1208ReturnsDelegateBenchmarks
+{
+#pragma warning disable ECS0900
+    [Params(1, 1_000)]
+#pragma warning restore ECS0900
+    public int FileCount { get; set; }
+
+    [Params("Net80WithOldMoq", "Net80WithNewMoq")]
+    public string MoqKey { get; set; } = "Net80WithOldMoq";
+
+    private CompilationWithAnalyzers? BaselineCompilation { get; set; }
+
+    private CompilationWithAnalyzers? TestCompilation { get; set; }
+
+    [IterationSetup]
+    [SuppressMessage("Usage", "VSTHRD002:Avoid problematic synchronous waits", Justification = "Async setup not supported in BenchmarkDotNet.See https://github.com/dotnet/BenchmarkDotNet/issues/2442.")]
+    public void SetupCompilation()
+    {
+        List<(string Name, string Content)> sources = [];
+        for (int index = 0; index < FileCount; index++)
+        {
+            string name = "TypeName" + index;
+            sources.Add((name, @$"
+using System;
+using System.Threading.Tasks;
+using Moq;
+
+public class AsyncClient{index}
+{{
+    public virtual Task<int> GetValueAsync() => Task.FromResult(0);
+    public virtual Task<string> GetNameAsync() => Task.FromResult(string.Empty);
+    public virtual ValueTask<int> GetValueTaskAsync() => ValueTask.FromResult(0);
+}}
+
+internal class {name}
+{{
+    private void Test()
+    {{
+        new Mock<AsyncClient{index}>().Setup(c => c.GetValueAsync()).Returns(() => 42);
+        _ = ""sample test""; // Add an expression that looks similar but does not match
+    }}
+}}
+"));
+        }
+
+        Microsoft.CodeAnalysis.Testing.ReferenceAssemblies referenceAssemblies = CompilationCreator.GetReferenceAssemblies(MoqKey);
+        (BaselineCompilation, TestCompilation) =
+            BenchmarkCSharpCompilationFactory
+            .CreateAsync<ReturnsDelegateShouldReturnTaskAnalyzer>(sources.ToArray(), referenceAssemblies)
+            .GetAwaiter()
+            .GetResult();
+    }
+
+    [Benchmark]
+    public async Task Moq1208WithDiagnostics()
+    {
+        ImmutableArray<Diagnostic> diagnostics =
+            (await TestCompilation!
+            .GetAnalysisResultAsync(CancellationToken.None)
+            .ConfigureAwait(false))
+            .AssertValidAnalysisResult()
+            .GetAllDiagnostics();
+
+        // Each file has one Returns(() => 42) on a Task<int> method
+        if (diagnostics.Length != FileCount)
+        {
+            throw new InvalidOperationException($"Expected '{FileCount:N0}' analyzer diagnostics but found '{diagnostics.Length}'");
+        }
+    }
+
+    [Benchmark(Baseline = true)]
+    public async Task Moq1208Baseline()
+    {
+        ImmutableArray<Diagnostic> diagnostics =
+            (await BaselineCompilation!
+            .GetAnalysisResultAsync(CancellationToken.None)
+            .ConfigureAwait(false))
+            .AssertValidAnalysisResult()
+            .GetAllDiagnostics();
+
+        if (diagnostics.Length != 0)
+        {
+            throw new InvalidOperationException($"Expected no analyzer diagnostics but found '{diagnostics.Length}'");
+        }
+    }
+}

tests/Moq.Analyzers.Test/ReturnsDelegateShouldReturnTaskAnalyzerTests.cs

@@ -160,6 +160,12 @@ public static IEnumerable<object[]> ValidWithCompilerSuppression()
 
             // Invocation as value, not delegate: GetInt() is a call, not a method group (GH PR #942 review thread)
             ["""new Mock<AsyncService>().Setup(c => c.GetValueAsync()).Returns(GetInt());"""],
+
+            // Generic Returns<T> with sync lambda: target-type inference masks the mismatch (analyzer gap, see PR #1089)
+            ["""new Mock<AsyncService>().Setup(c => c.ProcessAsync(It.IsAny<string>())).Returns<string>(s => s.Length);"""],
+
+            // Generic Returns<T> with async lambda (no mismatch)
+            ["""new Mock<AsyncService>().Setup(c => c.ProcessAsync(It.IsAny<string>())).Returns<string>(async s => s.Length);"""],
         };
 
         return data.WithNamespaces().WithMoqReferenceAssemblyGroups();