Build generated mappers on netstandard2.0, and stand aside below C# 9

Mapsicle.sln

@@ -81,6 +81,10 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Mapsicle.SourceGen.Tests",
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Mapsicle.SourceGen.OptIn.Tests", "tests\Mapsicle.SourceGen.OptIn.Tests\Mapsicle.SourceGen.OptIn.Tests.csproj", "{7D3DC0FB-7864-436B-B4A3-C164A98289BA}"
 EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Mapsicle.SourceGen.NetStandard.Fixture", "tests\Mapsicle.SourceGen.NetStandard.Fixture\Mapsicle.SourceGen.NetStandard.Fixture.csproj", "{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Mapsicle.SourceGen.OldLanguage.Fixture", "tests\Mapsicle.SourceGen.OldLanguage.Fixture\Mapsicle.SourceGen.OldLanguage.Fixture.csproj", "{F91CD029-D893-43C2-AB7D-8A0B07E0C755}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -523,6 +527,30 @@ Global
 		{7D3DC0FB-7864-436B-B4A3-C164A98289BA}.Release|x64.Build.0 = Release|Any CPU
 		{7D3DC0FB-7864-436B-B4A3-C164A98289BA}.Release|x86.ActiveCfg = Release|Any CPU
 		{7D3DC0FB-7864-436B-B4A3-C164A98289BA}.Release|x86.Build.0 = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|x64.Build.0 = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Debug|x86.Build.0 = Debug|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|Any CPU.Build.0 = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|x64.ActiveCfg = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|x64.Build.0 = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|x86.ActiveCfg = Release|Any CPU
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B}.Release|x86.Build.0 = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|x64.Build.0 = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Debug|x86.Build.0 = Debug|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|Any CPU.Build.0 = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|x64.ActiveCfg = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|x64.Build.0 = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|x86.ActiveCfg = Release|Any CPU
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -564,5 +592,7 @@ Global
 		{C613DDA4-8E7C-4D39-A8EB-94B5836D04F4} = {6B6174EC-A782-4FDE-A29C-B16A5A827845}
 		{9B1FDC73-02B6-454C-8957-AF70DDFE6D62} = {3A0525C5-7A1E-4E6E-A617-442A73AC1522}
 		{7D3DC0FB-7864-436B-B4A3-C164A98289BA} = {3A0525C5-7A1E-4E6E-A617-442A73AC1522}
+		{567DDBA6-E35F-4B16-B1B9-A6D68F52BD3B} = {3A0525C5-7A1E-4E6E-A617-442A73AC1522}
+		{F91CD029-D893-43C2-AB7D-8A0B07E0C755} = {3A0525C5-7A1E-4E6E-A617-442A73AC1522}
 	EndGlobalSection
 EndGlobal

README.md

@@ -635,6 +635,20 @@ the fallback than fix the pair, exempt it:
 Nothing is broken when you see it. It is the difference between 1.00x and 1.80x on that pair, and it
 names the member responsible.
 
+**On netstandard2.0 or .NET Framework, set the language version.** Generated code needs C# 9, and
+those targets default to 7.3. Below C# 9 the generator emits nothing and reports `MSG003` once, and
+every pair maps through the engine:
+
+```xml
+<PropertyGroup>
+  <LangVersion>9.0</LangVersion>
+</PropertyGroup>
+```
+
+With that set, a netstandard2.0 project generates like any other. The target has no
+`ModuleInitializerAttribute`, so the generated file declares an internal one, unless your project
+already has its own.
+
 **To check it worked**, turn on the emitted files and read them:
 
 ```xml

changelog.d/sourcegen-netstandard.fixed.md

@@ -0,0 +1,6 @@
+- **A netstandard2.0 or .NET Framework project with `Mapsicle.SourceGen` installed builds again.**
+  The generated file referenced `ModuleInitializerAttribute`, which those targets do not have, so the
+  build failed with CS0234. The generator now declares an internal copy when the target has none.
+  Below C# 9, which is where those targets start, the generated code failed with CS8370 and CS8627.
+  The generator now emits nothing there, reports the new `MSG003` warning once, and every pair maps
+  through the engine. Set `<LangVersion>9.0</LangVersion>` or later to generate.

src/Mapsicle.SourceGen/AnalyzerReleases.Unshipped.md

@@ -7,3 +7,4 @@ Rule ID | Category | Severity | Notes
 --------|----------|----------|-------
 MSG001 | Mapsicle | Warning | A declared pair could not be generated and falls back to the runtime engine.
 MSG002 | Mapsicle | Info | MapperGenerator
+MSG003 | Mapsicle | Warning | The project compiles with a C# version older than 9, so nothing is generated and the engine maps every pair.

src/Mapsicle.SourceGen/MapperGenerator.cs

@@ -70,6 +70,24 @@ public sealed class MapperGenerator : IIncrementalGenerator
                          "keeps working through the engine, and is reported as information because nothing asked " +
                          "for it by name.");
 
+        /// <summary>Reported once when the project's language version is too old to emit for.</summary>
+        /// <remarks>
+        /// Generated code needs a module initializer and an unconstrained <c>TDest?</c>, and both
+        /// arrived in C# 9. A netstandard2.0 project defaults to 7.3, and emitting there failed the
+        /// build with CS8370 and CS8627 inside the generated files.
+        /// </remarks>
+        private static readonly DiagnosticDescriptor LanguageTooOld = new(
+            id: "MSG003",
+            title: "Mapsicle needs C# 9 to generate mappers",
+            messageFormat: "Nothing was generated because this project compiles with C# {0} and generated mappers need C# 9 or later. Every pair still maps through the runtime engine. Set <LangVersion>9.0</LangVersion> or later to generate.",
+            category: "Mapsicle",
+            defaultSeverity: DiagnosticSeverity.Warning,
+            isEnabledByDefault: true,
+            description: "A project on netstandard2.0 or .NET Framework defaults to C# 7.3. The generator stands aside " +
+                         "there and the runtime engine maps every pair, so the build and the call sites are unaffected.");
+
+        private const string ModuleInitializerName = "System.Runtime.CompilerServices.ModuleInitializerAttribute";
+
         /// <summary>Wires the generator to the assembly's declared pairs.</summary>
         /// <param name="context">Supplied by Roslyn.</param>
         public void Initialize(IncrementalGeneratorInitializationContext context)
@@ -97,7 +115,9 @@ public void Initialize(IncrementalGeneratorInitializationContext context)
 
                 if (requested.Plans.Count == 0) return;
 
-                spc.AddSource("MapsicleGenerated.g.cs", SourceText.From(Emit(requested.Plans), Encoding.UTF8));
+                spc.AddSource(
+                    "MapsicleGenerated.g.cs",
+                    SourceText.From(Emit(requested.Plans, requested.NeedsModuleInitializer), Encoding.UTF8));
                 spc.AddSource("MapsicleGeneratedExtensions.g.cs", SourceText.From(EmitExtensions(requested.Plans), Encoding.UTF8));
             });
         }
@@ -175,14 +195,18 @@ private static (INamedTypeSymbol Source, INamedTypeSymbol Destination)? ResolveC
 
         private readonly struct Requested
         {
-            internal Requested(List<MapPlan> plans, List<Diagnostic> diagnostics)
+            internal Requested(List<MapPlan> plans, List<Diagnostic> diagnostics, bool needsModuleInitializer = false)
             {
                 Plans = plans;
                 Diagnostics = diagnostics;
+                NeedsModuleInitializer = needsModuleInitializer;
             }
 
             internal List<MapPlan> Plans { get; }
             internal List<Diagnostic> Diagnostics { get; }
+
+            /// <summary>Set when the target has no <c>ModuleInitializerAttribute</c> this assembly can use.</summary>
+            internal bool NeedsModuleInitializer { get; }
         }
 
         private static Requested ReadPairs(
@@ -195,6 +219,28 @@ private static Requested ReadPairs(
             var marker = compilation.GetTypeByMetadataName(AttributeName);
             if (marker is null) return new Requested(plans, diagnostics);
 
+            if (compilation is CSharpCompilation { LanguageVersion: < LanguageVersion.CSharp9 } old)
+            {
+                // Only when the assembly asked for something. The package can arrive through a
+                // reference the author never looked at, and a warning about pairs nobody declared
+                // is noise.
+                if (AsksForGeneration(compilation, marker))
+                {
+                    diagnostics.Add(Diagnostic.Create(
+                        LanguageTooOld, Location.None, old.LanguageVersion.ToDisplayString()));
+                }
+
+                return new Requested(plans, diagnostics);
+            }
+
+            // A copy already in this assembly's source is used as it stands, since a second one
+            // would be a duplicate definition. A non-public copy in a referenced assembly is not
+            // relied on, whether or not it happens to be visible from here.
+            var initializer = compilation.GetTypeByMetadataName(ModuleInitializerName);
+            var needsModuleInitializer = initializer is null
+                || (initializer.DeclaredAccessibility != Accessibility.Public
+                    && !SymbolEqualityComparer.Default.Equals(initializer.ContainingAssembly, compilation.Assembly));
+
             var index = 0;
             var seen = new HashSet<string>(StringComparer.Ordinal);
 
@@ -233,7 +279,7 @@ private static Requested ReadPairs(
                 && compilation.Assembly.GetAttributes().Any(
                     a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, allMarker));
 
-            if (!scanAll) return new Requested(plans, diagnostics);
+            if (!scanAll) return new Requested(plans, diagnostics, needsModuleInitializer);
 
             foreach (var candidate in scanned
                          .Where(c => c is not null)
@@ -261,7 +307,17 @@ private static Requested ReadPairs(
                 index++;
             }
 
-            return new Requested(plans, diagnostics);
+            return new Requested(plans, diagnostics, needsModuleInitializer);
+        }
+
+        /// <summary>Whether the assembly carries either attribute that asks for generated mappers.</summary>
+        private static bool AsksForGeneration(Compilation compilation, INamedTypeSymbol marker)
+        {
+            var allMarker = compilation.GetTypeByMetadataName(GenerateAllAttributeName);
+
+            return compilation.Assembly.GetAttributes().Any(
+                a => SymbolEqualityComparer.Default.Equals(a.AttributeClass, marker)
+                     || (allMarker is not null && SymbolEqualityComparer.Default.Equals(a.AttributeClass, allMarker)));
         }
 
         /// <summary>A stable key for a pair, so the two doors cannot emit it twice.</summary>
@@ -1402,11 +1458,29 @@ private static void Preamble(StringBuilder sb)
         /// write and cannot edit. Prefixing unconditionally is valid for ordinary identifiers too,
         /// so there is no keyword table to keep current.
         /// </remarks>
-        private static string Emit(List<MapPlan> plans)
+        private static string Emit(List<MapPlan> plans, bool needsModuleInitializer)
         {
             var sb = new StringBuilder();
             Preamble(sb);
 
+            // netstandard2.0 and .NET Framework have no ModuleInitializerAttribute, and the file
+            // referenced it anyway, so a project on either failed with CS0234. The compiler only
+            // looks for the name, so declaring it here is enough, and every runtime runs a module
+            // initializer. CS0436 is for a referenced assembly that exposes its own copy.
+            if (needsModuleInitializer)
+            {
+                sb.AppendLine("#pragma warning disable CS0436");
+                sb.AppendLine();
+                sb.AppendLine("namespace System.Runtime.CompilerServices");
+                sb.AppendLine("{");
+                sb.AppendLine("    [global::System.AttributeUsage(global::System.AttributeTargets.Method, Inherited = false)]");
+                sb.AppendLine("    internal sealed class ModuleInitializerAttribute : global::System.Attribute");
+                sb.AppendLine("    {");
+                sb.AppendLine("    }");
+                sb.AppendLine("}");
+                sb.AppendLine();
+            }
+
             sb.AppendLine("namespace Mapsicle.Generated");
             sb.AppendLine("{");
             sb.AppendLine("    /// <summary>Compile-time mappers, registered into the engine at startup.</summary>");

tests/Mapsicle.NetStandard.Tests/Mapsicle.NetStandard.Tests.csproj

@@ -18,6 +18,8 @@
   -->
   <ItemGroup>
     <ProjectReference Include="../../src/Mapsicle/Mapsicle.csproj" SetTargetFramework="TargetFramework=netstandard2.0" />
+    <ProjectReference Include="../Mapsicle.SourceGen.NetStandard.Fixture/Mapsicle.SourceGen.NetStandard.Fixture.csproj" />
+    <ProjectReference Include="../Mapsicle.SourceGen.OldLanguage.Fixture/Mapsicle.SourceGen.OldLanguage.Fixture.csproj" />
   </ItemGroup>
 
   <ItemGroup>

tests/Mapsicle.NetStandard.Tests/NetStandardGeneratedTests.cs

@@ -0,0 +1,104 @@
+using System;
+using System.Collections;
+using System.Linq;
+using System.Reflection;
+using Mapsicle.SourceGen.NetStandard.Fixture;
+using Mapsicle.SourceGen.OldLanguage.Fixture;
+using Xunit;
+
+namespace Mapsicle.NetStandard.Tests
+{
+    /// <summary>
+    /// Generated mappers in a consumer that targets netstandard2.0.
+    /// </summary>
+    /// <remarks>
+    /// The two fixture projects are the test as much as the assertions are. Each one is a
+    /// netstandard2.0 library with the generator installed, and neither used to build: the target
+    /// has no <c>ModuleInitializerAttribute</c>, and its default language version has no nullable
+    /// annotations.
+    /// </remarks>
+    public class NetStandardGeneratedTests
+    {
+        private static NsGenOrder Order() => new NsGenOrder
+        {
+            Id = 7,
+            Name = "Ana",
+            Status = NsGenStatus.Paid,
+            Label = NsGenStatus.Paid,
+            PlacedAt = new DateTime(2026, 10, 2, 8, 0, 0, DateTimeKind.Utc),
+            Country = new NsGenCountry { Iso = "PH" },
+            Lines = { new NsGenLine { Sku = "a", Qty = 2 } },
+            Extras = new[] { new NsGenLine { Sku = "b", Qty = 3 } },
+        };
+
+        private static bool IsRegistered(Type source, Type destination)
+        {
+            var registry = typeof(Mapper)
+                .GetField("_generatedPairs", BindingFlags.NonPublic | BindingFlags.Static)!
+                .GetValue(null)!;
+
+            return ((IEnumerable)registry.GetType().GetProperty("Keys")!.GetValue(registry)!)
+                .Cast<object>()
+                .Select(k => k.ToString() ?? "")
+                .Any(k => k.Contains(source.Name) && k.Contains(destination.Name));
+        }
+
+        [Fact]
+        public void TheFixtureIsANetStandardBuild()
+        {
+            var target = typeof(NsGenCalls).Assembly.GetCustomAttribute<System.Runtime.Versioning.TargetFrameworkAttribute>();
+
+            Assert.Equal(".NETStandard,Version=v2.0", target!.FrameworkName);
+        }
+
+        [Fact]
+        public void ADeclaredPairRegistersFromTheModuleInitializer()
+        {
+            NsGenCalls.Typed(Order());
+
+            Assert.True(IsRegistered(typeof(NsGenOrder), typeof(NsGenOrderDto)));
+        }
+
+        [Fact]
+        public void ADeclaredPairMapsLikeTheEngine()
+        {
+            var typed = NsGenCalls.Typed(Order());
+            var untyped = NsGenCalls.Untyped(Order());
+
+            using var runtime = MapperFactory.Create();
+            var interpreted = runtime.MapTo<NsGenOrderDto>(Order())!;
+
+            foreach (var generated in new[] { typed, untyped })
+            {
+                Assert.Equal(7L, generated.Id);
+                Assert.Equal("Ana", generated.Name);
+                Assert.Equal(NsGenStatusDto.Paid, generated.Status);
+                Assert.Equal("Paid", generated.Label);
+                Assert.Equal(interpreted.PlacedAt, generated.PlacedAt);
+                Assert.Equal("PH", generated.Country.Iso);
+                Assert.Equal("PH", generated.CountryIso);
+                Assert.Equal("a", Assert.Single(generated.Lines).Sku);
+                Assert.Equal(2L, generated.Lines[0].Qty);
+                Assert.Equal("b", Assert.Single(generated.Extras).Sku);
+                Assert.Equal(3L, generated.Extras[0].Qty);
+            }
+
+            Assert.Equal(7L, interpreted.Id);
+            Assert.Equal(NsGenStatusDto.Paid, interpreted.Status);
+            Assert.Equal("Paid", interpreted.Label);
+            Assert.Equal("PH", interpreted.CountryIso);
+            Assert.Equal(2L, interpreted.Lines[0].Qty);
+            Assert.Equal(3L, interpreted.Extras[0].Qty);
+        }
+
+        [Fact]
+        public void ADeclaredPairUnderAnOlderLanguageVersionMapsThroughTheEngine()
+        {
+            var mapped = NsOldCalls.Typed(new NsOldBox { X = 42, Name = "Ana" });
+
+            Assert.Equal(42L, mapped.X);
+            Assert.Equal("Ana", mapped.Name);
+            Assert.False(IsRegistered(typeof(NsOldBox), typeof(NsOldBoxDto)));
+        }
+    }
+}

tests/Mapsicle.SourceGen.NetStandard.Fixture/Mapsicle.SourceGen.NetStandard.Fixture.csproj

@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <!--
+    A consumer on netstandard2.0 that has raised its language version, which is the oldest setup
+    the generator emits for. The target has no ModuleInitializerAttribute, and the generated file
+    used to reference it anyway, so this project did not build.
+  -->
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <LangVersion>9.0</LangVersion>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Mapsicle\Mapsicle.csproj" />
+    <ProjectReference Include="..\..\src\Mapsicle.SourceGen\Mapsicle.SourceGen.csproj"
+                      OutputItemType="Analyzer" ReferenceOutputAssembly="false" />
+  </ItemGroup>
+
+</Project>