Allow trimming FeatureGuard and FeatureSwitchDefinition attributes

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

@@ -250,6 +250,12 @@
     <type fullname="System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>
+    <type fullname="System.Diagnostics.CodeAnalysis.FeatureGuardAttribute">
+      <attribute internal="RemoveAttributeInstances" />
+    </type>
+    <type fullname="System.Diagnostics.CodeAnalysis.FeatureSwitchDefinitionAttribute">
+      <attribute internal="RemoveAttributeInstances" />
+    </type>
     <type fullname="System.Diagnostics.CodeAnalysis.RequiresAssemblyFilesAttribute">
       <attribute internal="RemoveAttributeInstances" />
     </type>

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

@@ -0,0 +1,80 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+#nullable enable
+
+using System;
+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+using System.Reflection;
+
+/// <summary>
+/// Ensures setting _AggressiveAttributeTrimming = true causes various attributes to be trimmed
+/// </summary>
+class Program
+{
+    [UnconditionalSuppressMessage ("ReflectionAnalysis", "IL2111", Justification = "Expected trim warning for reflection over annotated members.")]
+    [UnconditionalSuppressMessage ("ReflectionAnalysis", "IL2026", Justification = "Expected trim warning for reflection over annotated members.")]
+    static int Main(string[] args)
+    {
+        // Reference to IsDynamicCodeSupported (which has FeatureGuard(typeof(RequiresDynamicCodeAttribute)))
+        // should not produce a warning because both RequiresDynamicCodeAttribute and FeatureGuardAttribute are removed.
+        if (RuntimeFeature.IsDynamicCodeSupported)
+        {
+            UseDynamicCode();
+        }
+
+        // Check that a few attribute instances are indeed removed
+        CheckRemovedAttributes(typeof(MembersWithRemovedAttributes));
+
+        return 100;
+    }
+
+    [RequiresDynamicCode(nameof(UseDynamicCode))]
+    static void UseDynamicCode() { }
+
+    class MembersWithRemovedAttributes
+    {
+        static void DynamicallyAccessedMembers([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.PublicMethods)] Type t) { }
+
+        [FeatureGuard(typeof(RequiresUnreferencedCodeAttribute))]
+        static bool FeatureGuard => throw null!;
+
+        [FeatureSwitchDefinition("Program.MembersWithRemovedAttributes.FeatureSwitchDefinition")]
+        static bool FeatureSwitchDefinition => throw null!;
+
+        [RequiresDynamicCode(nameof(RequiresDynamicCode))]
+        static void RequiresDynamicCode() { }
+
+        [RequiresUnreferencedCode(nameof(RequiresUnreferencedCode))]
+        static void RequiresUnreferencedCode() { }
+    }
+
+    static void CheckRemovedAttributes([DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.All)] Type type)
+    {
+        Console.WriteLine($"Validating {type}");
+        foreach (var member in type.GetMembers(BindingFlags.Static | BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic | BindingFlags.DeclaredOnly))
+        {
+            CheckRemovedAttributes(member);
+
+            if (member is MethodInfo method)
+            {
+                foreach (var parameter in method.GetParameters())
+                {
+                    CheckRemovedAttributes(parameter);
+                }
+            }
+        }
+    }
+
+    static void CheckRemovedAttributes(ICustomAttributeProvider provider)
+    {
+        foreach (var attribute in provider.GetCustomAttributes(false))
+        {
+            if (attribute is NullableContextAttribute)
+                continue;
+
+            throw new Exception($"Unexpected attribute {attribute.GetType()} on {provider}");
+        }
+    }
+}

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

@@ -2,6 +2,10 @@
   <Import Project="$([MSBuild]::GetPathOfFileAbove(Directory.Build.props))" />
 
   <ItemGroup>
+    <TestConsoleAppSourceFiles Include="AggressiveAttributeTrimmingTest.cs">
+      <EnabledProperties>_AggressiveAttributeTrimming</EnabledProperties>
+      <DisabledProperties>SuppressTrimAnalysisWarnings;TrimmerSingleWarn</DisabledProperties>
+    </TestConsoleAppSourceFiles>
     <TestConsoleAppSourceFiles Include="AppDomainGetThreadGenericPrincipalTest.cs" />
     <TestConsoleAppSourceFiles Include="AppDomainGetThreadWindowsPrincipalTest.cs">
       <SkipOnTestRuntimes>osx-x64;linux-x64;browser-wasm</SkipOnTestRuntimes>