[NativeAOT] Add support for conditional [Preserve] attributes (#18803)

This is a follow-up to #18666

The idea is to transform `[Preserve(Conditional = true)]` into
`[DynamicDependency(signature, type)]` on the static constructor of the
given type:

```c#
class MyClass {
    [Preserve (Conditional = true)]
    public void MyMethod () { }

    // transformed into ...

    [DynamicDependency ("MyMethod", typeof (MyClass))]
    static MyClass() { /* either the existing cctor or a new empty cctor */ }

    public void MyMethod () { }
}
```

tools/dotnet-linker/ApplyPreserveAttributeBase.cs

@@ -24,6 +24,7 @@ namespace Mono.Tuner {
 	public abstract class ApplyPreserveAttributeBase : ConfigurationAwareSubStep {
 
 		AppBundleRewriter? abr;
+		Queue<Action> deferredActions = new ();
 
 		protected override string Name { get => "Apply Preserve Attribute"; }
 
@@ -32,16 +33,7 @@ public abstract class ApplyPreserveAttributeBase : ConfigurationAwareSubStep {
 		// set 'removeAttribute' to true if you want the preserved attribute to be removed from the final assembly
 		protected abstract bool IsPreservedAttribute (ICustomAttributeProvider provider, CustomAttribute attribute, out bool removeAttribute);
 
-		public override SubStepTargets Targets {
-			get {
-				return SubStepTargets.Type
-					| SubStepTargets.Field
-					| SubStepTargets.Method
-					| SubStepTargets.Property
-					| SubStepTargets.Event
-					| SubStepTargets.Assembly;
-			}
-		}
+		public override SubStepTargets Targets => SubStepTargets.Assembly;
 
 		public override void Initialize (LinkContext context)
 		{
@@ -51,6 +43,51 @@ public override void Initialize (LinkContext context)
 				abr = Configuration.AppBundleRewriter;
 		}
 
+		protected override void Process (AssemblyDefinition assembly)
+		{
+			BrowseTypes (assembly.MainModule.Types);
+			ProcessDeferredActions ();
+		}
+
+		void BrowseTypes (IEnumerable<TypeDefinition> types)
+		{
+			foreach (TypeDefinition type in types) {
+				ProcessType (type);
+
+				if (type.HasFields) {
+					foreach (FieldDefinition field in type.Fields)
+						ProcessField (field);
+				}
+
+				if (type.HasMethods) {
+					foreach (MethodDefinition method in type.Methods)
+						ProcessMethod (method);
+				}
+
+				if (type.HasProperties) {
+					foreach (PropertyDefinition property in type.Properties)
+						ProcessProperty (property);
+				}
+
+				if (type.HasEvents) {
+					foreach (EventDefinition @event in type.Events)
+						ProcessEvent (@event);
+				}
+
+				if (type.HasNestedTypes) {
+					BrowseTypes (type.NestedTypes);
+				}
+			}
+		}
+
+		void ProcessDeferredActions ()
+		{
+			while (deferredActions.Count > 0) {
+				var action = deferredActions.Dequeue ();
+				action.Invoke ();
+			}
+		}
+
 		public override bool IsActiveFor (AssemblyDefinition assembly)
 		{
 			return Annotations.GetAction (assembly) == AssemblyAction.Link;
@@ -205,13 +242,7 @@ protected void PreserveType (TypeDefinition type, bool allMembers)
 		MethodDefinition GetOrCreateModuleConstructor (ModuleDefinition @module)
 		{
 			var moduleType = @module.GetModuleType ();
-			var moduleConstructor = moduleType.GetTypeConstructor ();
-			if (moduleConstructor is null) {
-				moduleConstructor = moduleType.AddMethod (".cctor", MethodAttributes.Private | MethodAttributes.HideBySig | MethodAttributes.RTSpecialName | MethodAttributes.SpecialName | MethodAttributes.Static, abr!.System_Void);
-				moduleConstructor.CreateBody (out var il);
-				il.Emit (OpCodes.Ret);
-			}
-			return moduleConstructor;
+			return GetOrCreateStaticConstructor (moduleType);
 		}
 
 		void AddDynamicDependencyAttribute (TypeDefinition type, bool allMembers)
@@ -234,8 +265,7 @@ void AddConditionalDynamicDependencyAttribute (TypeDefinition onType, MethodDefi
 			if (abr is null)
 				return;
 
-			// I haven't found a way to express a conditional Preserve attribute using DynamicDependencyAttribute :/
-			ErrorHelper.Warning (2112, Errors.MX2112 /* Unable to apply the conditional [Preserve] attribute on the member {0} */, forMethod.FullName);
+			deferredActions.Enqueue (() => AddDynamicDependencyAttributeToStaticConstructor (onType, forMethod));
 		}
 
 		void AddDynamicDependencyAttribute (IMetadataTokenProvider provider)
@@ -258,5 +288,34 @@ void AddDynamicDependencyAttribute (IMetadataTokenProvider provider)
 
 			abr.ClearCurrentAssembly ();
 		}
+
+		void AddDynamicDependencyAttributeToStaticConstructor (TypeDefinition onType, MethodDefinition forMethod)
+		{
+			if (abr is null)
+				return;
+
+			abr.ClearCurrentAssembly ();
+			abr.SetCurrentAssembly (onType.Module.Assembly);
+
+			var cctor = GetOrCreateStaticConstructor (onType);
+			var signature = DocumentationComments.GetSignature (forMethod);
+			var attrib = abr.CreateDynamicDependencyAttribute (signature, onType);
+			cctor.CustomAttributes.Add (attrib);
+			Annotations.AddPreservedMethod (onType, cctor);
+
+			abr.ClearCurrentAssembly ();
+		}
+
+		MethodDefinition GetOrCreateStaticConstructor (TypeDefinition type)
+		{
+			var staticCtor = type.GetTypeConstructor ();
+			if (staticCtor is null) {
+				staticCtor = type.AddMethod (".cctor", MethodAttributes.Private | MethodAttributes.HideBySig | MethodAttributes.RTSpecialName | MethodAttributes.SpecialName | MethodAttributes.Static, abr!.System_Void);
+				staticCtor.CreateBody (out var il);
+				il.Emit (OpCodes.Ret);
+			}
+
+			return staticCtor;
+		}
 	}
 }

tools/mtouch/Errors.resx

@@ -1346,11 +1346,6 @@
 		</value>
 	</data>
 
-	<data name="MX2112" xml:space="preserve">
-		<!-- This is a warning -->
-		<value>Unable to apply the conditional [Preserve] attribute on the member {0}.</value>
-	</data>
-
 	<!-- 2200 -> 2299 is used by/reserved for ExceptionalSubStep subclasses in the linker -->
 	<!-- 220x: PreserveSmartEnumConversionsSubStep -->
 	<!-- 221x: RemoveBitcodeIncompatibleCodeStep -->