[bgen] Don't emit/use [Preserve] attributes anymore. Fixes #19524. (#22860)

Use [DynamicDependency] instead!

Fixes #19524.

Author: rolfbjarne

src/Foundation/NSTimer.cs

@@ -20,8 +20,10 @@
 //
 // Copyright 2011-2014 Xamarin Inc.
 //
+
 using System.Reflection;
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 
 namespace Foundation {
 
@@ -120,5 +122,18 @@ public NSTimer (NSDate date, TimeSpan when, Action<NSTimer> action, System.Boole
 			: this (date, when.TotalSeconds, new NSTimerActionDispatcher (action), NSTimerActionDispatcher.Selector, null, repeats)
 		{
 		}
+
+		// The dependency attribute is required because:
+		// * We inject a call to Invalidate in the generated Dispose method
+		// * We optimize Dispose methods to not touch fields that aren't otherwise used, which we do by
+		//   removing the contents of the Dispose method before the linker runs, then after the linker runs
+		//   we determine which fields the Dispose method touches have been linked away and remove any such code.
+		//   We won't remove the call to Invalidate, but the linker may have trimmed away the Invalidate method
+		///  itself (because we temporarly removed the call to it). So make sure the linker doesn't remove it.
+		[DynamicDependency ("Invalidate()")]
+		static NSTimer ()
+		{
+			GC.KeepAlive (null);
+		}
 	}
 }

src/Resources.Designer.cs

@@ -637,4 +637,7 @@
 		<value>The type {0} has a [Protocol] and a [BaseType] attribute, but no [Model] attribute. This is likely incorrect; either remove the [BaseType] attribute, or add a [Model] attribute.</value>
 	</data>
 
+	<data name="BI1124" xml:space="preserve">
+		<value>Found a [Preserve] attribute on {0}: [Preserve] is deprecated; use [DynamicDependency] instead.</value>
+	</data>
 </root>

src/Resources.resx

@@ -1595,11 +1595,6 @@ void GenerateTrampolinesForQueue (TrampolineInfo [] queue)
 			print ("//\n// This class bridges native block invocations that call into C#\n//");
 			PrintExperimentalAttribute (ti.Type);
 			print ("static internal class {0} {{", ti.StaticName); indent++;
-			// it can't be conditional without fixing https://github.com/mono/linker/issues/516
-			// but we have a workaround in place because we can't fix old, binary bindings so...
-			// print ("[Preserve (Conditional=true)]");
-			// For .NET we fix it using the DynamicDependency attribute below
-			print ("[Preserve (Conditional = true)]");
 			print ("[UnmanagedCallersOnly]");
 			print ("[UserDelegateType (typeof ({0}))]", ti.UserDelegate);
 			print ("internal static unsafe {0} Invoke ({1}) {{", ti.ReturnType, ti.Parameters);
@@ -1672,7 +1667,12 @@ void GenerateTrampolinesForQueue (TrampolineInfo [] queue)
 			print ("invoker = block->GetDelegateForBlock<{0}> ();", ti.DelegateName);
 			indent--; print ("}");
 			print ("");
-			print ("[Preserve (Conditional=true)]");
+			print ("[DynamicDependency (nameof (Create))]");
+			print ($"static {ti.NativeInvokerName} ()");
+			print ("{");
+			print ("\tGC.KeepAlive (null);"); // need to do _something_ (doesn't seem to matter what), otherwise the static cctor (and the DynamicDependency attributes) are trimmed away.
+			print ("}");
+			print ("");
 			print_generated_code ();
 			print ("public unsafe static {0}? Create (IntPtr block)\n{{", ti.UserDelegate); indent++;
 			print ("if (block == IntPtr.Zero)"); indent++;
@@ -1900,13 +1900,11 @@ void GenerateStrongDictionaryTypes ()
 				if (BindingTouch.SupportsXmlDocumentation) {
 					print ($"/// <summary>Creates a new <see cref=\"{typeName}\" /> with default (empty) values.</summary>");
 				}
-				print ("[Preserve (Conditional = true)]");
 				print ("public {0} () : base (new NSMutableDictionary ()) {{}}\n", typeName);
 				if (BindingTouch.SupportsXmlDocumentation) {
 					print ($"/// <summary>Creates a new <see cref=\"{typeName}\" /> from the values that are specified in <paramref name=\"dictionary\" />.</summary>");
 					print ($"/// <param name=\"dictionary\">The dictionary to use to populate the properties of this type.</param>");
 				}
-				print ("[Preserve (Conditional = true)]");
 				print ("public {0} (NSDictionary? dictionary) : base (dictionary) {{}}\n", typeName);
 
 				foreach (var pi in dictType.GatherProperties (this)) {
@@ -5107,6 +5105,7 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
 			}
 		}
 
+		var dynamicDependencies = new List<string> ();
 		if (instanceMethods.Any () || instanceProperties.Any ()) {
 			// Tell the trimmer to not remove any instance method/property if the interface itself isn't trimmed away.
 			// These members are required for the registrar to determine if a particular implementing method
@@ -5117,10 +5116,14 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
 			var docIds = instanceMethods
 				.Select (mi => DocumentationManager.GetDocId (mi, includeDeclaringType: false, alwaysIncludeParenthesis: true))
 				.Concat (instanceProperties.Select (v => v.Name))
-				.OrderBy (name => name);
-			foreach (var docId in docIds) {
-				print ($"[DynamicDependencyAttribute (\"{docId}\")]");
-			}
+				.Select (v => $"\"{v}\"");
+			dynamicDependencies.AddRange (docIds);
+		}
+		// Tell the trimmer to not remove the wrapper type if the interface itself isn't trimmed away
+		dynamicDependencies.Add ($"DynamicallyAccessedMemberTypes.Interfaces | DynamicallyAccessedMemberTypes.PublicConstructors, typeof ({TypeName}Wrapper)");
+		if (dynamicDependencies.Count > 0) {
+			foreach (var dd in dynamicDependencies.OrderBy (v => v))
+				print ($"[DynamicDependencyAttribute ({dd})]");
 			print ("[BindingImpl (BindingImplOptions.GeneratedCode | BindingImplOptions.Optimizable)]");
 			print ($"static I{TypeName} ()");
 			print ("{");
@@ -5233,12 +5236,19 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
 		indent++;
 		// ctor (IntPtr, bool)
 		PrintExperimentalAttribute (type);
-		print ("[Preserve (Conditional = true)]");
 		print ("public {0}Wrapper ({1} handle, bool owns)", TypeName, NativeHandleType);
 		print ("\t: base (handle, owns)");
 		print ("{");
 		print ("}");
 		print ("");
+
+		print ($"[DynamicDependencyAttribute (DynamicallyAccessedMemberTypes.PublicConstructors, typeof ({TypeName}Wrapper))]");
+		print ($"static {TypeName}Wrapper ()");
+		print ("{");
+		print ("\tGC.KeepAlive (null);"); // need to do _something_ (doesn't seem to matter what), otherwise the static cctor (and the DynamicDependency attribute) is trimmed away.
+		print ("}");
+		print ("");
+
 		// Methods
 		// First find duplicates and select the best one. We use the selector to determine what's a duplicate.
 		var methodData = requiredInstanceMethods.Select ((v) => {
@@ -5451,6 +5461,9 @@ public void PrintPreserveAttribute (ICustomAttributeProvider mi)
 		if (p is null)
 			return;
 
+		if (!BindThirdPartyLibrary)
+			exceptions.Add (ErrorHelper.CreateError (1124 /* Found a [Preserve] attribute on {0}: [Preserve] is deprecated; use [DynamicDependency] instead. */, FormatProvider (mi)));
+
 		if (p.AllMembers)
 			print ("[Preserve (AllMembers = true)]");
 		else if (p.Conditional)
@@ -6743,7 +6756,6 @@ public void Generate (Type type)
 						} else
 							print ("internal {0}? {1};", Nomenclator.GetDelegateName (mi), miname);
 
-						print ("[Preserve (Conditional = true)]");
 						if (isProtocolEventBacked)
 							print ("[Export (\"{0}\")]", FindSelector (dtype, mi));
 
@@ -6850,7 +6862,6 @@ public void Generate (Type type)
 							selRespondsToSelector = "selRespondsToSelector";
 						}
 
-						print ("[Preserve (Conditional = true)]");
 						print ("public override bool RespondsToSelector (Selector? sel)");
 						print ("{");
 						++indent;

src/bgen/Generator.cs

@@ -7378,8 +7378,6 @@ interface NSTimer {
 		[Export ("fireDate", ArgumentSemantic.Copy)]
 		NSDate FireDate { get; set; }
 
-		// Note: preserving this member allows us to re-enable the `Optimizable` binding flag
-		[Preserve (Conditional = true)]
 		[Export ("invalidate")]
 		void Invalidate ();