[linker] Do not mark NSObject subclasses from Xamarin.Forms.Platform.iOS.dll assembly (#7473)

Turn older #7165 prototype into an experimental feature. It can be
enabled by adding `--optimize=experimental-xforms-product-type` to the
**Additional mtouch arguments** of the project.

ref: https://github.com/xamarin/xamarin-macios/pull/7165

docs/website/optimizations.md

@@ -755,3 +755,17 @@ disabling the managed linker.
 
 The default behavior can be overridden by passing
 `--optimize=[+|-]custom-attributes-removal` to `mtouch` or `mmp`.
+
+## Experimental Xamarin.Forms.Platform.iOS ProductType inclusion
+
+This optimization requires the linker to be enabled and is only applied
+on `Xamarin.Forms.Platform.iOS.dll`.
+
+This is **experimental** and might be removed or replaced in future 
+versions of Xamarin.iOS.
+
+This optimization consider the assembly `Xamarin.Forms.Platform.iOS` as
+a product assembly and does not automagically mark all `NSObject` 
+subclasses. This allows additional removes and optimizations to be 
+applied to the assembly, including the ability to remove `UIWebView` if
+nothing else in the application requires it.

tests/mmptest/src/MMPTest.cs

@@ -622,7 +622,7 @@ namespace Xamarin.MMP.Tests
 						"<LinkMode>Full</LinkMode>",
 				};
 				var rv = TI.TestUnifiedExecutable (test, shouldFail: false);
-				rv.Messages.AssertWarning (132, $"Unknown optimization: '{opt}'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, trim-architectures, inline-is-arm64-calling-convention, cctor-beforefieldinit, custom-attributes-removal.");
+				rv.Messages.AssertWarning (132, $"Unknown optimization: '{opt}'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, trim-architectures, inline-is-arm64-calling-convention, cctor-beforefieldinit, custom-attributes-removal, experimental-xforms-product-type.");
 				rv.Messages.AssertErrorCount (0);
 			});
 		}

tests/mtouch/MTouch.cs

@@ -1773,7 +1773,7 @@ public class TestApp {
 				mtouch.Linker = MTouchLinker.LinkSdk;
 				mtouch.Optimize = new string [] { "foo" };
 				mtouch.AssertExecute (MTouchAction.BuildSim, "build");
-				mtouch.AssertWarning (132, "Unknown optimization: 'foo'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, inline-runtime-arch, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, remove-dynamic-registrar, remove-unsupported-il-for-bitcode, inline-is-arm64-calling-convention, seal-and-devirtualize, cctor-beforefieldinit, custom-attributes-removal.");
+				mtouch.AssertWarning (132, "Unknown optimization: 'foo'. Valid optimizations are: remove-uithread-checks, dead-code-elimination, inline-isdirectbinding, inline-intptr-size, inline-runtime-arch, blockliteral-setupblock, register-protocols, inline-dynamic-registration-supported, static-block-to-delegate-lookup, remove-dynamic-registrar, remove-unsupported-il-for-bitcode, inline-is-arm64-calling-convention, seal-and-devirtualize, cctor-beforefieldinit, custom-attributes-removal, experimental-xforms-product-type.");
 			}
 		}
 
@@ -3696,7 +3696,8 @@ public partial class NotificationService : UNNotificationServiceExtension
 				mtouch.AssertWarning (2003, "Option '--optimize=seal-and-devirtualize' will be ignored since linking is disabled");
 				mtouch.AssertWarning (2003, "Option '--optimize=cctor-beforefieldinit' will be ignored since linking is disabled");
 				mtouch.AssertWarning (2003, "Option '--optimize=custom-attributes-removal' will be ignored since linking is disabled");
-				mtouch.AssertWarningCount (14);
+				mtouch.AssertWarning (2003, "Option '--optimize=experimental-xforms-product-type' will be ignored since linking is disabled");
+				mtouch.AssertWarningCount (15);
 			}
 
 			using (var mtouch = new MTouchTool ()) {

tools/common/Optimizations.cs

@@ -42,6 +42,7 @@ namespace Xamarin.Bundler
 #endif
 			"cctor-beforefieldinit",
 			"custom-attributes-removal",
+			"experimental-xforms-product-type",
 		};
 
 		enum Opt
@@ -62,6 +63,7 @@ namespace Xamarin.Bundler
 			SealAndDevirtualize,
 			StaticConstructorBeforeFieldInit,
 			CustomAttributesRemoval,
+			ExperimentalFormsProductType,
 		}
 
 		bool? all;
@@ -145,6 +147,11 @@ namespace Xamarin.Bundler
 			set { values [(int) Opt.CustomAttributesRemoval] = value; }
 		}
 
+		public bool? ExperimentalFormsProductType {
+			get { return values [(int) Opt.ExperimentalFormsProductType]; }
+			set { values [(int) Opt.ExperimentalFormsProductType] = value; }
+		}
+
 		public Optimizations ()
 		{
 			values = new bool? [opt_names.Length];

tools/linker/MarkNSObjects.cs

@@ -33,6 +33,7 @@ using System;
 using Mono.Cecil;
 using Mono.Linker;
 using Mono.Tuner;
+using Xamarin.Bundler;
 
 namespace Xamarin.Linker.Steps {
 
@@ -151,9 +152,15 @@ namespace Xamarin.Linker.Steps {
 			return (method.DeclaringType.Module.Assembly.Name.Name == ProductAssembly);
 		}
 
-		static bool IsProductType (TypeDefinition type)
+		bool IsProductType (TypeDefinition type)
 		{
-			return type.Module.Assembly.Name.Name == ProductAssembly;
+			var name = type.Module.Assembly.Name.Name;
+			switch (name) {
+			case "Xamarin.Forms.Platform.iOS":
+				return LinkContext.App.Optimizations.ExperimentalFormsProductType == true;
+			default:
+				return name == ProductAssembly;
+			}
 		}
 	}
 }