Add option to keep/remove COM interfaces (#101087)

Adds an illink option that can be used to remove COM
interfaces (the existing behavior is to preserve COM interfaces
on marked types). If a type implements a COM interface that is
also marked elsewhere, it will still be kept even with
`--keep-com-interfaces false`. This includes windows runtime
interfaces under the same option name.

This eliminates some trim warnings in winforms due to
trim-incompatible code in Control's implementations of COM
interfaces like IPersistPropertyBag. Experimentally, this still
produces a working app for the scenario we've been testing
(https://github.com/dotnet/winforms/tree/main/src/System.Windows.Forms/tests/IntegrationTests/TrimTest).
For now, this just introduces the option without setting it by
default anywhere (or exposing it in MSBuild) so that we can opt
in from the winforms scenario. We might consider setting it
wherever `BuiltInComInteropSupport` is false.

Author: sbomer

src/tools/illink/src/linker/CompatibilitySuppressions.xml

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <!-- https://learn.microsoft.com/en-us/dotnet/fundamentals/package-validation/diagnostic-ids -->
 <Suppressions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <Suppression>
@@ -1497,6 +1497,18 @@
     <DiagnosticId>CP0002</DiagnosticId>
     <Target>M:Mono.Linker.Steps.SubStepsDispatcher.Process(Mono.Linker.LinkContext)</Target>
   </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Mono.Linker.LinkContext.get_KeepComInterfaces</Target>
+    <Left>ref/net9.0/illink.dll</Left>
+    <Right>lib/net9.0/illink.dll</Right>
+  </Suppression>
+  <Suppression>
+    <DiagnosticId>CP0002</DiagnosticId>
+    <Target>M:Mono.Linker.LinkContext.set_KeepComInterfaces(System.Boolean)</Target>
+    <Left>ref/net9.0/illink.dll</Left>
+    <Right>lib/net9.0/illink.dll</Right>
+  </Suppression>
   <Suppression>
     <DiagnosticId>CP0008</DiagnosticId>
     <Target>T:Mono.Linker.LinkContext</Target>
@@ -1537,4 +1549,4 @@
     <DiagnosticId>CP0017</DiagnosticId>
     <Target>M:Mono.Linker.LinkContext.Resolve(Mono.Cecil.AssemblyNameReference)$0</Target>
   </Suppression>
-</Suppressions>
+</Suppressions>

src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

@@ -2438,7 +2438,7 @@ protected virtual bool ShouldMarkInterfaceImplementationList (TypeDefinition typ
 
 			// It's hard to know if a com or windows runtime interface will be needed from managed code alone,
 			// so as a precaution we will mark these interfaces once the type is instantiated
-			if (resolvedInterfaceType.IsImport || resolvedInterfaceType.IsWindowsRuntime)
+			if (Context.KeepComInterfaces && (resolvedInterfaceType.IsImport || resolvedInterfaceType.IsWindowsRuntime))
 				return true;
 
 			return IsFullyPreserved (type);

src/tools/illink/src/linker/Linker/Driver.cs

@@ -362,6 +362,12 @@ protected int SetupContext (ILogger? customLogger = null)
 						context.SetCustomData (values[0], values[1]);
 						continue;
 
+					case "--keep-com-interfaces":
+						if (!GetBoolParam (token, l => context.KeepComInterfaces = l))
+							return -1;
+
+						continue;
+
 					case "--keep-compilers-resources":
 						if (!GetBoolParam (token, l => keepCompilersResources = l))
 							return -1;
@@ -1284,6 +1290,7 @@ protected virtual LinkContext GetDefaultContext (Pipeline pipeline, ILogger? log
 			return new LinkContext (pipeline, logger ?? new ConsoleLogger (), "output") {
 				TrimAction = AssemblyAction.Link,
 				DefaultAction = AssemblyAction.Link,
+				KeepComInterfaces = true,
 			};
 		}
 
@@ -1369,6 +1376,7 @@ static void Usage ()
 			Console.WriteLine ("                               sealer: Any method or type which does not have override is marked as sealed");
 			Console.WriteLine ("  --explicit-reflection      Adds to members never used through reflection DisablePrivateReflection attribute. Defaults to false");
 			Console.WriteLine ("  --feature FEATURE VALUE    Apply any optimizations defined when this feature setting is a constant known at link time");
+			Console.WriteLine ("  --keep-com-interfaces      Keep COM interfaces implemented by kept types. Defaults to true");
 			Console.WriteLine ("  --keep-compilers-resources Keep assembly resources used for F# compilation resources. Defaults to false");
 			Console.WriteLine ("  --keep-dep-attributes      Keep attributes used for manual dependency tracking. Defaults to false");
 			Console.WriteLine ("  --keep-metadata NAME       Keep metadata which would otherwise be removed if not used");

src/tools/illink/src/linker/Linker/LinkContext.cs

@@ -108,6 +108,8 @@ public Pipeline Pipeline {
 
 		public bool LinkSymbols { get; set; }
 
+		public bool KeepComInterfaces { get; set; }
+
 		public bool KeepMembersForDebugger { get; set; } = true;
 
 		public bool IgnoreUnresolved { get; set; } = true;

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/UnusedComInterfaceIsKept.cs

@@ -21,6 +21,9 @@ public static void Main ()
 		[Guid ("D7BB1889-3AB7-4681-A115-60CA9158FECA")]
 		interface IBar
 		{
+			// Trimming may remove members from COM interfaces
+			// even when keeping the COM-related attributes.
+			// https://github.com/dotnet/runtime/issues/101128
 			void Bar ();
 		}
 

src/tools/illink/test/Mono.Linker.Tests.Cases/Inheritance.Interfaces/OnReferenceType/UnusedComInterfaceIsRemoved.cs

@@ -0,0 +1,40 @@
+using System.Runtime.InteropServices;
+using Mono.Linker.Tests.Cases.Expectations.Assertions;
+using Mono.Linker.Tests.Cases.Expectations.Metadata;
+
+namespace Mono.Linker.Tests.Cases.Inheritance.Interfaces.OnReferenceType
+{
+	/// <summary>
+	/// With --keep-com-interfaces false, we apply the unused interface  rules also to com interfaces.
+	/// </summary>
+	[SetupLinkerArgument ("--keep-com-interfaces", "false")]
+	public class UnusedComInterfaceIsRemoved
+	{
+		public static void Main ()
+		{
+			var i = new A ();
+			i.Foo ();
+		}
+
+		[ComImport]
+		[Guid ("D7BB1889-3AB7-4681-A115-60CA9158FECA")]
+		interface IBar
+		{
+			void Bar ();
+		}
+
+		[Kept]
+		[KeptMember (".ctor()")]
+		class A : IBar
+		{
+			[Kept]
+			public void Foo ()
+			{
+			}
+
+			public void Bar ()
+			{
+			}
+		}
+	}
+}