IL Trim attributes with a feature flag in all assemblies

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

@@ -12,7 +12,7 @@
     </type>
   </assembly>
   <!-- The following attributes are only necessary when debugging is supported -->
-  <assembly fullname="System.Private.CoreLib" feature="System.Diagnostics.Debugger.IsSupported" featurevalue="false">
+  <assembly fullname="*" feature="System.Diagnostics.Debugger.IsSupported" featurevalue="false">
     <type fullname="System.Diagnostics.DebuggableAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
@@ -42,13 +42,13 @@
     </type>
   </assembly>
 
-  <assembly fullname="System.Private.CoreLib" feature="System.Reflection.Metadata.MetadataUpdater.IsSupported" featurevalue="false">
+  <assembly fullname="*" feature="System.Reflection.Metadata.MetadataUpdater.IsSupported" featurevalue="false">
     <type fullname="System.Reflection.Metadata.MetadataUpdateHandlerAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
   </assembly>
 
-  <assembly fullname="System.Private.CoreLib" feature="System.Diagnostics.Tracing.EventSource.IsSupported" featurevalue="false">
+  <assembly fullname="*" feature="System.Diagnostics.Tracing.EventSource.IsSupported" featurevalue="false">
     <type fullname="System.Diagnostics.Tracing.EventSourceAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
@@ -70,7 +70,7 @@
   </assembly>
 
   <!-- The following attributes are only necessary when COM is supported -->
-  <assembly fullname="System.Private.CoreLib" feature="System.Runtime.InteropServices.BuiltInComInterop.IsSupported" featurevalue="false">
+  <assembly fullname="*" feature="System.Runtime.InteropServices.BuiltInComInterop.IsSupported" featurevalue="false">
     <type fullname="System.Runtime.InteropServices.ClassInterfaceAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>

src/libraries/System.Runtime/tests/System.Runtime.Tests/TrimmingTests/MetadataUpdateHandlerTrimmed.cs

@@ -0,0 +1,65 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Linq;
+using System.Reflection;
+using System.Reflection.Metadata;
+
+/// <summary>
+/// Tests that when MetadataUpdater.IsSupported is false, the MetadataUpdateHandlerAttribute
+/// instances and the handler types they reference are trimmed from all assemblies.
+/// </summary>
+class Program
+{
+    static int Main(string[] args)
+    {
+        // Hard references to ensure these assemblies are loaded and not entirely removed by the trimmer.
+        // We use types that the trimmer cannot remove (public API surface).
+        _ = typeof(System.ComponentModel.TypeConverter);
+        _ = typeof(System.Text.Json.JsonSerializer);
+
+        // MetadataUpdater.IsSupported should be false
+        if (MetadataUpdater.IsSupported)
+        {
+            Console.WriteLine("Failed: MetadataUpdater.IsSupported should be false");
+            return -1;
+        }
+
+        // Verify MetadataUpdateHandlerAttribute is not present on any assembly
+        foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
+        {
+            foreach (var attr in assembly.GetCustomAttributesData())
+            {
+                if (attr.AttributeType.Name == "MetadataUpdateHandlerAttribute")
+                {
+                    Console.WriteLine($"Failed: {assembly.GetName().Name} still has MetadataUpdateHandlerAttribute");
+                    return -1;
+                }
+            }
+        }
+
+        // Verify the handler types themselves are trimmed
+        string[] handlerTypeNames = new[]
+        {
+            "System.Reflection.Metadata.RuntimeTypeMetadataUpdateHandler",
+            "System.ComponentModel.ReflectionCachesUpdateHandler",
+            "System.Text.Json.JsonSerializerOptionsUpdateHandler",
+        };
+
+        foreach (var assembly in AppDomain.CurrentDomain.GetAssemblies())
+        {
+            foreach (var typeName in handlerTypeNames)
+            {
+                var type = assembly.GetType(typeName);
+                if (type != null)
+                {
+                    Console.WriteLine($"Failed: {assembly.GetName().Name} still contains {typeName}");
+                    return -1;
+                }
+            }
+        }
+
+        return 100;
+    }
+}

src/libraries/System.Runtime/tests/System.Runtime.Tests/TrimmingTests/System.Runtime.TrimmingTests.proj

@@ -70,6 +70,9 @@
       <!-- Reflection.Emit doesn't work with native AOT -->
       <NativeAotIncompatible>true</NativeAotIncompatible>
     </TestConsoleAppSourceFiles>
+    <TestConsoleAppSourceFiles Include="MetadataUpdateHandlerTrimmed.cs">
+      <DisabledFeatureSwitches>System.Reflection.Metadata.MetadataUpdater.IsSupported</DisabledFeatureSwitches>
+    </TestConsoleAppSourceFiles>
   </ItemGroup>
   <ItemGroup Condition="'$(TargetsWindows)' == 'true'">
     <TestConsoleAppSourceFiles Include="UseWindowsThreadPoolFalse.cs">