Add option to keep/remove COM interfaces

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/UnusedComInterfaceIsRemoved.cs

@@ -0,0 +1,38 @@
+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 ();
+		}
+
+		interface IBar
+		{
+			void Bar ();
+		}
+
+		[Kept]
+		[KeptMember (".ctor()")]
+		class A : IBar
+		{
+			[Kept]
+			public void Foo ()
+			{
+			}
+
+			public void Bar ()
+			{
+			}
+		}
+	}
+}