[linker/trimmer] Add opt-in support for not marking NSObjects in user assemblies. Fixes #15723.

dotnet/targets/Xamarin.Shared.Sdk.targets

@@ -507,6 +507,7 @@
 				SdkDevPath=$(_SdkDevPath)
 				SdkRootDirectory=$(_XamarinSdkRootDirectory)
 				SdkVersion=$(_SdkVersion)
+				SkipMarkingNSObjectsInUserAssemblies=$(_SkipMarkingNSObjectsInUserAssemblies)
 				TargetArchitectures=$(TargetArchitectures)
 				TargetFramework=$(_ComputedTargetFrameworkMoniker)
 				UseLlvm=$(MtouchUseLlvm)

msbuild/Xamarin.MacDev.Tasks/Tasks/ParseBundlerArgumentsTaskBase.cs

@@ -53,6 +53,9 @@ public abstract class ParseBundlerArgumentsTaskBase : XamarinTask {
 		[Output]
 		public string NoStrip { get; set; }
 
+		[Output]
+		public string SkipMarkingNSObjectsInUserAssemblies { get; set; }
+
 		[Output]
 		public int Verbosity { get; set; }
 
@@ -165,6 +168,9 @@ public override bool Execute ()
 						item.SetMetadata ("Value", value.Substring (colon + 1));
 						envVariables.Add (item);
 						break;
+					case "skip-marking-nsobjects-in-user-assemblies":
+						SkipMarkingNSObjectsInUserAssemblies = ParseBool (value) ? "false" : "true";
+						break;
 					case "xml":
 						if (xml == null)
 							xml = new List<string> ();

msbuild/Xamarin.Shared/Xamarin.Shared.targets

@@ -1851,6 +1851,7 @@ Copyright (C) 2018 Microsoft. All rights reserved.
 			<Output TaskParameter="PackageDebugSymbols" PropertyName="PackageDebugSymbols" />
 			<Output TaskParameter="Registrar" PropertyName="Registrar" />
 			<Output TaskParameter="RequirePInvokeWrappers" PropertyName="_RequirePInvokeWrappers" />
+			<Output TaskParameter="SkipMarkingNSObjectsInUserAssemblies" PropertyName="_SkipMarkingNSObjectsInUserAssemblies" />
 			<Output TaskParameter="Verbosity" PropertyName="_BundlerVerbosity" />
 			<Output TaskParameter="XmlDefinitions" ItemName="_BundlerXmlDefinitions" />
 			<Output TaskParameter="NoStrip" PropertyName="EnableAssemblyILStripping" />

tools/common/Application.cs

@@ -165,6 +165,8 @@ public bool IsDefaultMarshalManagedExceptionMode {
 		public bool EnableMarkerOnlyBitCode { get { return BitCodeMode == BitCodeMode.MarkerOnly; } }
 		public bool EnableBitCode { get { return BitCodeMode != BitCodeMode.None; } }
 
+		public bool SkipMarkingNSObjectsInUserAssemblies { get; set; }
+
 		// assembly_build_targets describes what kind of native code each assembly should be compiled into for mobile targets (iOS, tvOS, watchOS).
 		// An assembly can be compiled into: static object (.o), dynamic library (.dylib) or a framework (.framework).
 		// In the case of a framework, each framework may contain the native code for multiple assemblies.

tools/common/Driver.cs

@@ -254,6 +254,9 @@ static bool ParseOptions (Application app, Mono.Options.OptionSet options, strin
 			options.Add ("require-pinvoke-wrappers:", v => {
 				app.RequiresPInvokeWrappers = ParseBool (v, "--require-pinvoke-wrappers");
 			});
+			options.Add ("skip-marking-nsobjects-in-user-assemblies:", "Don't mark NSObject (and any subclass of NSObject) in user assemblies in the linker. This may break your app, use at own risk.", v => {
+				app.SkipMarkingNSObjectsInUserAssemblies = ParseBool (v, "--skip-marking-nsobjects-in-user-assemblies");
+			});
 
 
 			// Keep the ResponseFileSource option at the end.

tools/dotnet-linker/LinkerConfiguration.cs

@@ -257,6 +257,11 @@ public static LinkerConfiguration GetInstance (LinkContext context, bool createI
 						throw new InvalidOperationException ($"Unable to parse the {key} value: {value} in {linker_file}");
 					SdkVersion = sdk_version;
 					break;
+				case "SkipMarkingNSObjectsInUserAssemblies":
+					if (!TryParseOptionalBoolean (value, out var skip_marking_nsobjects_in_user_assemblies))
+						throw new InvalidOperationException ($"Unable to parse the {key} value: {value} in {linker_file}");
+					Application.SkipMarkingNSObjectsInUserAssemblies = skip_marking_nsobjects_in_user_assemblies;
+					break;
 				case "TargetArchitectures":
 					if (!Enum.TryParse<Abi> (value, out var arch))
 						throw new InvalidOperationException ($"Unknown target architectures: {value} in {linker_file}");

tools/linker/MarkNSObjects.cs

@@ -143,8 +143,11 @@ static bool IsProductMethod (MethodDefinition method)
 			return (method.DeclaringType.Module.Assembly.Name.Name == ProductAssembly);
 		}
 
-		static bool IsProductType (TypeDefinition type)
+		bool IsProductType (TypeDefinition type)
 		{
+			if (LinkContext.App.SkipMarkingNSObjectsInUserAssemblies)
+				return true;
+
 			var name = type.Module.Assembly.Name.Name;
 			switch (name) {
 			case "Xamarin.Forms.Platform.iOS":