Refactor DownlevelLibraryImportGenerator to inject types via RegisterPostInitializationOutput

eng/generators.targets

@@ -48,12 +48,6 @@
                                  " />
   </ItemGroup>
 
-  <ItemGroup Condition="'@(EnabledGenerators)' != '' and
-                        @(EnabledGenerators->AnyHaveMetadataValue('Identity', 'DownlevelLibraryImportGenerator'))">
-    <Compile Include="$(CoreLibSharedDir)System\Runtime\InteropServices\LibraryImportAttribute.cs" />
-    <Compile Include="$(CoreLibSharedDir)System\Runtime\InteropServices\StringMarshalling.cs" />
-  </ItemGroup>
-
   <!-- Use this complex item list based filtering to add the ProjectReference to make sure dotnet/runtime stays compatible with NuGet Static Graph Restore.
        That is required as the EnabledGenerators condition checks on the ProjectReference items and hence can't be a property condition. -->
   <ItemGroup Condition="'@(EnabledGenerators)' != ''">

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/LibraryImportAttribute.cs

@@ -14,16 +14,7 @@ namespace System.Runtime.InteropServices
     /// applied to static, partial, non-generic methods.
     /// </remarks>
     [AttributeUsage(AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
-#if SYSTEM_PRIVATE_CORELIB
-    public
-#else
-#pragma warning disable CS0436 // Type conflicts with imported type
-    // Some assemblies that target downlevel have InternalsVisibleTo to their test assemblies.
-    // As this is only used in this repo and isn't a problem in shipping code,
-    // just disable the duplicate type warning.
-    internal
-#endif
-    sealed class LibraryImportAttribute : Attribute
+    public sealed class LibraryImportAttribute : Attribute
     {
         /// <summary>
         /// Initializes a new instance of the <see cref="LibraryImportAttribute"/>.

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/StringMarshalling.cs

@@ -13,12 +13,7 @@ namespace System.Runtime.InteropServices
     /// <summary>
     /// Specifies how strings should be marshalled for generated p/invokes
     /// </summary>
-#if SYSTEM_PRIVATE_CORELIB || MICROSOFT_INTEROP_SOURCEGENERATION
-    public
-#else
-    internal
-#endif
-    enum StringMarshalling
+    public enum StringMarshalling
     {
         /// <summary>
         /// Indicates the user is supplying a specific marshaller in <see cref="LibraryImportAttribute.StringMarshallingCustomType"/>.

src/libraries/System.Runtime.InteropServices/gen/DownlevelLibraryImportGenerator/DownlevelLibraryImportGenerator.cs

@@ -35,8 +35,42 @@ public static class StepNames
             public const string GenerateSingleStub = nameof(GenerateSingleStub);
         }
 
+        // Internal definitions of LibraryImportAttribute and StringMarshalling that are injected
+        // into user projects targeting downlevel frameworks that don't have these types in the BCL.
+        // These definitions only expose the properties and enum values supported downlevel.
+        private const string GeneratedInteropTypes = """
+            // <auto-generated/>
+            #nullable enable
+            namespace System.Runtime.InteropServices
+            {
+                [global::System.AttributeUsage(global::System.AttributeTargets.Method, AllowMultiple = false, Inherited = false)]
+                [global::Microsoft.CodeAnalysis.Embedded]
+                internal sealed partial class LibraryImportAttribute : global::System.Attribute
+                {
+                    public LibraryImportAttribute(string libraryName) { }
+                    public string? EntryPoint { get; set; }
+                    public StringMarshalling StringMarshalling { get; set; }
+                    public bool SetLastError { get; set; }
+                }
+
+                [global::Microsoft.CodeAnalysis.Embedded]
+                internal enum StringMarshalling
+                {
+                    Utf16 = 2,
+                }
+            }
+            """;
+
         public void Initialize(IncrementalGeneratorInitializationContext context)
         {
+            // Inject internal definitions of LibraryImportAttribute and StringMarshalling into the compilation
+            // so that users targeting downlevel frameworks can apply [LibraryImport] to their methods.
+            context.RegisterPostInitializationOutput(static ctx =>
+            {
+                ctx.AddEmbeddedAttributeDefinition();
+                ctx.AddSource("LibraryImportInteropTypes.g.cs", GeneratedInteropTypes);
+            });
+
             // Collect all methods adorned with LibraryImportAttribute and filter out invalid ones
             // (diagnostics for invalid methods are reported by the analyzer)
             var methodsToGenerate = context.SyntaxProvider

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/AdditionalAttributesOnStub.cs

@@ -128,7 +128,6 @@ public async Task SkipLocalsInitOnDownlevelTargetFrameworks(TestTargetFramework
         {
             string source = $$"""
                 using System.Runtime.InteropServices;
-                {{CodeSnippets.LibraryImportAttributeDeclaration}}
                 partial class C
                 {
                     [LibraryImportAttribute("DoesNotExist")]

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Compiles.cs

@@ -500,64 +500,65 @@ protected override ParseOptions CreateParseOptions()
         public static IEnumerable<object[]> CodeSnippetsToValidateFallbackForwarder()
         {
             // Confirm that all unsupported target frameworks can be generated.
+            // LibraryImportAttribute and StringMarshalling are injected by the generator via RegisterPostInitializationOutput.
             {
-                string code = CodeSnippets.BasicParametersAndModifiers<byte>(CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicParametersAndModifiers<byte>();
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, false };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, false };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, false };
             }
 
             // Confirm that all unsupported target frameworks fall back to a forwarder.
             {
-                string code = CodeSnippets.BasicParametersAndModifiers<byte[]>(CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicParametersAndModifiers<byte[]>();
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
 
             // Confirm that all unsupported target frameworks fall back to a forwarder.
             {
-                string code = CodeSnippets.BasicParametersAndModifiersWithStringMarshalling<string>(StringMarshalling.Utf16, CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicParametersAndModifiersWithStringMarshalling<string>(StringMarshalling.Utf16);
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
 
             // Confirm that if support is missing for a type with an ITypeBasedMarshallingInfoProvider (like arrays and SafeHandles), we fall back to a forwarder even if other types are supported.
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "System.Runtime.InteropServices.SafeHandle", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "System.Runtime.InteropServices.SafeHandle");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("System.Runtime.InteropServices.SafeHandle", "int", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("System.Runtime.InteropServices.SafeHandle", "int");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "int[]", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "int[]");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("int", "int[]", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("int", "int[]");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
 
             // Confirm that if support is missing for a type without an ITypeBasedMarshallingInfoProvider (like StringBuilder), we fall back to a forwarder even if other types are supported.
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "System.Text.StringBuilder", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("void", "System.Text.StringBuilder");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
             }
             {
-                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("int", "System.Text.StringBuilder", CodeSnippets.LibraryImportAttributeDeclaration);
+                string code = CodeSnippets.BasicReturnAndParameterWithAlwaysSupportedParameter("int", "System.Text.StringBuilder");
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_0, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Standard2_1, true };
                 yield return new object[] { ID(), code, TestTargetFramework.Framework, true };
@@ -761,14 +762,17 @@ public class Basic { }
         [InlineData(TestTargetFramework.Standard2_0)]
         [InlineData(TestTargetFramework.Framework)]
         [Theory]
-        public async Task ValidateNoGeneratedOutputForNoImportDownlevel(TestTargetFramework framework)
+        public async Task ValidateNoStubOutputForNoImportDownlevel(TestTargetFramework framework)
         {
+            // The DownlevelLibraryImportGenerator always injects type definitions for LibraryImportAttribute
+            // and StringMarshalling via RegisterPostInitializationOutput. When there are no [LibraryImport]
+            // usages, no method stubs should be generated beyond those type definitions.
             string source = """
                 using System.Runtime.InteropServices;
                 public class Basic { }
                 """;
 
-            var test = new NoChangeTest<Microsoft.Interop.DownlevelLibraryImportGenerator, Microsoft.CodeAnalysis.Testing.EmptyDiagnosticAnalyzer>(framework)
+            var test = new OnlyTypeDefinitionsOutputTest(framework)
             {
                 TestCode = source,
                 TestBehaviors = TestBehaviors.SkipGeneratedSourcesCheck
@@ -777,6 +781,28 @@ public class Basic { }
             await test.RunAsync();
         }
 
+        class OnlyTypeDefinitionsOutputTest : Microsoft.Interop.UnitTests.Verifiers.CSharpSourceGeneratorVerifier<Microsoft.Interop.DownlevelLibraryImportGenerator, Microsoft.CodeAnalysis.Testing.EmptyDiagnosticAnalyzer>.Test
+        {
+            public OnlyTypeDefinitionsOutputTest(TestTargetFramework framework)
+                : base(framework)
+            {
+            }
+
+            protected override async Task<(Compilation compilation, ImmutableArray<Diagnostic> generatorDiagnostics)> GetProjectCompilationAsync(Project project, IVerifier verifier, CancellationToken cancellationToken)
+            {
+                var originalCompilation = await project.GetCompilationAsync(cancellationToken);
+                var (newCompilation, diagnostics) = await base.GetProjectCompilationAsync(project, verifier, cancellationToken);
+                // The DownlevelLibraryImportGenerator should inject type definitions (for example, via
+                // RegisterPostInitializationOutput) but, when there are no [LibraryImport] usages, it
+                // must not emit any stub output such as the generated "LibraryImports" stub tree.
+                var originalTrees = originalCompilation!.SyntaxTrees.ToImmutableArray();
+                var newTrees = newCompilation.SyntaxTrees.ToImmutableArray();
+                var addedTrees = newTrees.Except(originalTrees).ToImmutableArray();
+                Assert.DoesNotContain(addedTrees, tree => tree.FilePath.Contains("LibraryImports", StringComparison.Ordinal));
+                return (newCompilation, diagnostics);
+            }
+        }
+
         class NoChangeTest<TSourceGenerator, TAnalyzer> : Microsoft.Interop.UnitTests.Verifiers.CSharpSourceGeneratorVerifier<TSourceGenerator, TAnalyzer>.Test
             where TSourceGenerator : new()
             where TAnalyzer : DiagnosticAnalyzer, new()

src/libraries/System.Runtime.InteropServices/tests/LibraryImportGenerator.UnitTests/Diagnostics.cs

@@ -241,40 +241,26 @@ await VerifyCS.VerifyAnalyzerAsync(source,
         [OuterLoop("Uses the network for downlevel ref packs")]
         public async Task StringMarshallingForwardingNotSupported_ReportsDiagnostic()
         {
+            // The downlevel injected StringMarshalling enum only has Utf16 = 2.
+            // Use a numeric cast to exercise a non-Utf16 value that should trigger CannotForwardToDllImport.
             string source = """
-
                 using System.Runtime.InteropServices;
                 partial class Test
                 {
-                    [LibraryImport("DoesNotExist", StringMarshalling = StringMarshalling.Utf8)]
+                    [LibraryImport("DoesNotExist", StringMarshalling = (StringMarshalling)1)]
                     public static partial void {|#0:Method1|}(string s);
-
-                    [LibraryImport("DoesNotExist", StringMarshalling = StringMarshalling.Custom, StringMarshallingCustomType = typeof(Native))]
-                    public static partial void {|#1:Method2|}(string s);
-
-                    struct Native
-                    {
-                        public Native(string s) { }
-                        public string ToManaged() => default;
-                    }
                 }
-                """ + CodeSnippets.LibraryImportAttributeDeclaration;
-            DiagnosticResult[] expectedDiags =
-            [
-                VerifyCS.Diagnostic(GeneratorDiagnostics.CannotForwardToDllImport)
-                    .WithLocation(0)
-                    .WithArguments($"{nameof(TypeNames.LibraryImportAttribute)}{Type.Delimiter}{nameof(StringMarshalling)}={nameof(StringMarshalling)}{Type.Delimiter}{nameof(StringMarshalling.Utf8)}"),
-                VerifyCS.Diagnostic(GeneratorDiagnostics.CannotForwardToDllImport)
-                    .WithLocation(1)
-                    .WithArguments($"{nameof(TypeNames.LibraryImportAttribute)}{Type.Delimiter}{nameof(StringMarshalling)}={nameof(StringMarshalling)}{Type.Delimiter}{nameof(StringMarshalling.Custom)}")
-            ];
+                """;
 
             var test = new Microsoft.Interop.UnitTests.Verifiers.CSharpSourceGeneratorVerifier<DownlevelLibraryImportGenerator, Microsoft.Interop.Analyzers.DownlevelLibraryImportDiagnosticsAnalyzer>.Test(TestTargetFramework.Standard2_0)
             {
                 TestCode = source,
                 TestBehaviors = TestBehaviors.SkipGeneratedSourcesCheck
             };
-            test.ExpectedDiagnostics.AddRange(expectedDiags);
+            test.ExpectedDiagnostics.Add(
+                VerifyCS.Diagnostic(GeneratorDiagnostics.CannotForwardToDllImport)
+                    .WithLocation(0)
+                    .WithArguments($"{nameof(TypeNames.LibraryImportAttribute)}{Type.Delimiter}{nameof(StringMarshalling)}={nameof(StringMarshalling)}{Type.Delimiter}{nameof(StringMarshalling.Utf8)}"));
             await test.RunAsync();
         }