perf: emit source-generated test types after user code (Defender scan 5s → 0.2s at 10k tests)

eng/TestLibrary.props

@@ -1,6 +1,23 @@
 <Project>
     <Import Project="$(MSBuildThisFileDirectory)..\src\TUnit.Core\TUnit.Core.props" />
 
+    <!--
+        TUnit.Core.props adds TUnit.Core.GeneratedNamespace.cs so the compiler emits the source-generated
+        TUnit.Generated types after the project's own types (see that file). From a NuGet package the props is
+        imported before the SDK's default Compile glob, so the file is already first. Repo projects import this
+        file from their project body, after the glob, so the file lands last and has no effect. Move it to the
+        front just before compiling. This only reorders the Compile items passed to Csc, so evaluation (and
+        Visual Studio's view of globbed items) is unchanged.
+    -->
+    <Target Name="_TUnitCompileGeneratedNamespaceFirst" BeforeTargets="CoreCompile" Condition="'$(Language)' == 'C#'">
+        <ItemGroup>
+            <_TUnitGeneratedNamespaceCompile Include="@(Compile)" Condition="'%(Filename)%(Extension)' == 'TUnit.Core.GeneratedNamespace.cs'" />
+            <_TUnitOtherCompile Include="@(Compile)" Exclude="@(_TUnitGeneratedNamespaceCompile)" Condition="'@(_TUnitGeneratedNamespaceCompile)' != ''" />
+            <Compile Remove="@(Compile)" Condition="'@(_TUnitGeneratedNamespaceCompile)' != ''" />
+            <Compile Include="@(_TUnitGeneratedNamespaceCompile);@(_TUnitOtherCompile)" Condition="'@(_TUnitGeneratedNamespaceCompile)' != ''" />
+        </ItemGroup>
+    </Target>
+
     <PropertyGroup>
         <OutputType>Library</OutputType>
 

src/TUnit.Core/TUnit.Core.GeneratedNamespace.cs

@@ -0,0 +1,13 @@
+// <auto-generated/>
+// Added to test projects by TUnit.Core.props so it compiles ahead of the project's own files.
+//
+// The C# compiler writes namespaces to the assembly in reverse order of their first declaration.
+// Source-generated trees are always added after the project's files, so without this declaration
+// the TUnit.Generated types (large static constructors registering every test) are written at the
+// very start of the assembly. Antivirus scanners such as Microsoft Defender analyse code at the
+// start of an assembly far more expensively, which added seconds to every build and first test run
+// of large test projects on Windows. Declaring the namespace first moves the generated types to the
+// end of the assembly instead.
+namespace TUnit.Generated
+{
+}

src/TUnit.Core/TUnit.Core.csproj

@@ -35,6 +35,18 @@
             <Pack>true</Pack>
             <PackagePath>$(BuildPath)</PackagePath>
         </None>
+
+        <!-- Compiled into consuming test projects by TUnit.Core.props, never into TUnit.Core itself. -->
+        <Compile Remove="TUnit.Core.GeneratedNamespace.cs" />
+        <TfmSpecificPackageFile Include="TUnit.Core.GeneratedNamespace.cs">
+            <Pack>true</Pack>
+            <PackagePath>$(BuildTransitivePath)</PackagePath>
+        </TfmSpecificPackageFile>
+
+        <None Include="TUnit.Core.GeneratedNamespace.cs">
+            <Pack>true</Pack>
+            <PackagePath>$(BuildPath)</PackagePath>
+        </None>
     </ItemGroup>
     <ItemGroup>
         <None

src/TUnit.Core/TUnit.Core.props

@@ -33,4 +33,18 @@
         <CompilerVisibleProperty Include="EnableTUnitSourceGeneration" />
     </ItemGroup>
 
+    <!--
+        Declares the TUnit.Generated namespace before the project's own files (items added here precede the SDK's
+        default Compile glob) so the compiler writes the source-generated types at the end of the assembly rather
+        than the start, where Microsoft Defender scans them far more slowly. See TUnit.Core.GeneratedNamespace.cs.
+
+        This must stay in the .props: items added from a .targets land after the default glob. The condition is safe
+        here even though Language is set later by the C# targets and EnableTUnitSourceGeneration may be set in the
+        project body, because MSBuild evaluates every property before any item, so item conditions see final values.
+        Guarded by TUnit.NugetTester's GeneratedTypeOrderTests.
+    -->
+    <ItemGroup Condition="'$(Language)' == 'C#' and '$(EnableTUnitSourceGeneration)' != 'false'">
+        <Compile Include="$(MSBuildThisFileDirectory)TUnit.Core.GeneratedNamespace.cs" Visible="false" />
+    </ItemGroup>
+
 </Project>

tools/tunit-nuget-tester/TUnit.NugetTester/TUnit.NugetTester/GeneratedTypeOrderTests.cs

@@ -0,0 +1,32 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+
+namespace TUnit.NugetTester;
+
+/// <summary>
+/// TUnit.Core.props adds TUnit.Core.GeneratedNamespace.cs ahead of the project's own files so the compiler
+/// emits the source-generated TUnit.Generated types after the user's types. Emitting them first made
+/// Microsoft Defender scan large test assemblies for seconds on every build (see PR #6908).
+/// This project consumes the packed packages, so it guards the real NuGet import path.
+/// </summary>
+public class GeneratedTypeOrderTests
+{
+    [Test]
+    [UnconditionalSuppressMessage("Trimming", "IL2026", Justification = "Only inspects this assembly's own type definitions.")]
+    public async Task Generated_Types_Are_Emitted_After_User_Types()
+    {
+        var types = typeof(GeneratedTypeOrderTests).Assembly.GetTypes();
+
+        var generatedTypes = types.Where(t => t.Namespace == "TUnit.Generated").ToArray();
+        var userTypes = types.Where(t => !t.IsNested && t.Namespace == typeof(GeneratedTypeOrderTests).Namespace).ToArray();
+
+        await Assert.That(generatedTypes).IsNotEmpty();
+        await Assert.That(userTypes).IsNotEmpty();
+
+        // TypeDef metadata tokens follow the order the compiler emitted the types in.
+        var firstGenerated = generatedTypes.Min(t => t.MetadataToken);
+        var lastUser = userTypes.Max(t => t.MetadataToken);
+
+        await Assert.That(firstGenerated).IsGreaterThan(lastUser);
+    }
+}