Make DllImportGenerator emit a GeneratedCodeAttribute on our stubs

src/libraries/System.Private.CoreLib/src/ILLink/ILLink.LinkAttributes.Shared.xml

@@ -268,6 +268,9 @@
     <type fullname="System.Runtime.InteropServices.GeneratedDllImportAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
+    <type fullname="System.CodeDom.Compiler.GeneratedCodeAttribute">
+      <attribute internal="RemoveAttributeInstances" />
+    </type>
 
     <!-- The following attributes are generated by the compiler, so they could be in any assembly -->
 

src/libraries/System.Runtime.InteropServices/gen/DllImportGenerator/DllImportStubContext.cs

@@ -42,6 +42,10 @@ public static bool AreCompilationSettingsEqual(StubEnvironment env1, StubEnviron
 
     internal sealed class DllImportStubContext : IEquatable<DllImportStubContext>
     {
+        private static readonly string GeneratorName = typeof(DllImportGenerator).Assembly.GetName().Name;
+
+        private static readonly string GeneratorVersion = typeof(DllImportGenerator).Assembly.GetName().Version.ToString();
+
         // We don't need the warnings around not setting the various
         // non-nullable fields/properties on this type in the constructor
         // since we always use a property initializer.
@@ -123,18 +127,31 @@ public static DllImportStubContext Create(
 
             ImmutableArray<AttributeListSyntax>.Builder additionalAttrs = ImmutableArray.CreateBuilder<AttributeListSyntax>();
 
-            // Define additional attributes for the stub definition.
+            if (env.TargetFramework != TargetFramework.Unknown)
+            {
+                // Define additional attributes for the stub definition.
+                additionalAttrs.Add(
+                    AttributeList(
+                        SingletonSeparatedList(
+                            Attribute(ParseName(TypeNames.System_CodeDom_Compiler_GeneratedCodeAttribute),
+                                AttributeArgumentList(
+                                    SeparatedList(
+                                        new[]
+                                        {
+                                            AttributeArgument(LiteralExpression(SyntaxKind.StringLiteralExpression, Literal(GeneratorName))),
+                                            AttributeArgument(LiteralExpression(SyntaxKind.StringLiteralExpression, Literal(GeneratorVersion)))
+                                        }))))));
+            }
+
             if (env.TargetFrameworkVersion >= new Version(5, 0) && !MethodIsSkipLocalsInit(env, method))
             {
                 additionalAttrs.Add(
                     AttributeList(
-                        SeparatedList(new[]
-                        {
+                        SingletonSeparatedList(
                             // Adding the skip locals init indiscriminately since the source generator is
                             // targeted at non-blittable method signatures which typically will contain locals
                             // in the generated code.
-                            Attribute(ParseName(TypeNames.System_Runtime_CompilerServices_SkipLocalsInitAttribute))
-                        })));
+                            Attribute(ParseName(TypeNames.System_Runtime_CompilerServices_SkipLocalsInitAttribute)))));
             }
 
             return new DllImportStubContext()

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/TypeNames.cs

@@ -67,5 +67,7 @@ public static string MarshalEx(InteropGenerationOptions options)
         public const string DefaultDllImportSearchPathsAttribute = "System.Runtime.InteropServices.DefaultDllImportSearchPathsAttribute";
 
         public const string DllImportSearchPath = "System.Runtime.InteropServices.DllImportSearchPath";
+
+        public const string System_CodeDom_Compiler_GeneratedCodeAttribute = "System.CodeDom.Compiler.GeneratedCodeAttribute";
     }
 }

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

@@ -64,6 +64,57 @@ partial class C
             Assert.DoesNotContain(stubMethod.GetAttributes(), attr => attr.AttributeClass!.ToDisplayString() == typeof(SkipLocalsInitAttribute).FullName);
         }
 
+        [ConditionalFact]
+        public async Task GeneratedCodeAdded()
+        {
+            string source = @"
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+[assembly:DisableRuntimeMarshalling]
+partial class C
+{
+    [GeneratedDllImportAttribute(""DoesNotExist"")]
+    public static partial S Method();
+}
+
+[NativeMarshalling(typeof(Native))]
+struct S
+{
+}
+
+struct Native
+{
+    public Native(S s) { }
+    public S ToManaged() { return default; }
+}";
+            Compilation comp = await TestUtils.CreateCompilation(source);
+
+            Compilation newComp = TestUtils.RunGenerators(comp, out _, new Microsoft.Interop.DllImportGenerator());
+
+            ITypeSymbol c = newComp.GetTypeByMetadataName("C")!;
+            IMethodSymbol stubMethod = c.GetMembers().OfType<IMethodSymbol>().Single(m => m.Name == "Method");
+            Assert.Contains(stubMethod.GetAttributes(), attr => attr.AttributeClass!.ToDisplayString() == typeof(System.CodeDom.Compiler.GeneratedCodeAttribute).FullName);
+        }
+
+        [ConditionalFact]
+        public async Task GeneratedCodeNotAddedOnForwardingStub()
+        {
+            string source = @"
+using System.Runtime.InteropServices;
+partial class C
+{
+    [GeneratedDllImportAttribute(""DoesNotExist"")]
+    public static partial void Method();
+}";
+            Compilation comp = await TestUtils.CreateCompilation(source);
+
+            Compilation newComp = TestUtils.RunGenerators(comp, out _, new Microsoft.Interop.DllImportGenerator());
+
+            ITypeSymbol c = newComp.GetTypeByMetadataName("C")!;
+            IMethodSymbol stubMethod = c.GetMembers().OfType<IMethodSymbol>().Single(m => m.Name == "Method");
+            Assert.DoesNotContain(stubMethod.GetAttributes(), attr => attr.AttributeClass!.ToDisplayString() == typeof(System.CodeDom.Compiler.GeneratedCodeAttribute).FullName);
+        }
+
         public static IEnumerable<object[]> GetDownlevelTargetFrameworks()
         {
             yield return new object[] { TestTargetFramework.Net, true };