[generator] Do not generate `BindingImplOptions.Optimizable` when code snippets are provided (#12165)

unless the snippet-based attribute also set the `Optimizable` property.

IOW this is now required to _opt-in_ (to optimize) instead of default,
with no ~easy~ way to turn it off.

Re-enabled each of the `[Dispose]` cases after protecting the required
extra calls they make.

Fix https://github.com/xamarin/xamarin-macios/issues/12150

* Add test for [Dispose] and SnippetAttribute subclasses

* Opt-in all [*Snippet] to be optimizable

This is exactly what we have been shipping for years (no changes).

Unlike [Dispose] there is no change in the generated code or the
optimizations.

Future snippets should come with tests - which is _normal_ for any
manual code (which they are) added to the SDK

src/CoreAnimation/CALayer.cs

@@ -103,6 +103,8 @@ namespace CoreAnimation {
 			}
 		}
 
+		// Note: preserving this member allows us to re-enable the `Optimizable` binding flag
+		[Preserve (Conditional = true)]
 		void OnDispose ()
 		{
 			if (calayerdelegate != null) {

src/UIKit/UIGestureRecognizer.cs

@@ -34,6 +34,8 @@ namespace UIKit {
 		// Called by the Dispose() method, because this can run from a finalizer, we need to
 		// (a) reference the handle, that we will release later, and (b) to remove the targets on the
 		// UI thread.
+		// Note: preserving this member allows us to re-enable the `Optimizable` binding flag
+		[Preserve (Conditional = true)]
 		void OnDispose ()
 		{
 			var copyOfRecognizers = recognizers;

src/appkit.cs

@@ -15103,16 +15103,16 @@ namespace AppKit {
 		void WillRemoveSubview ([NullAllowed] NSView subview);
 
 		[Export ("removeFromSuperview")]
-		[PreSnippet ("var mySuper = Superview;")]
+		[PreSnippet ("var mySuper = Superview;", Optimizable = true)]
-		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}")]
+		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}", Optimizable = true)]
 		void RemoveFromSuperview ();
 
 		[Export ("replaceSubview:with:")][PostGet ("Subviews")]
 		void ReplaceSubviewWith (NSView oldView, NSView newView);
 
 		[Export ("removeFromSuperviewWithoutNeedingDisplay")]
-		[PreSnippet ("var mySuper = Superview;")]
+		[PreSnippet ("var mySuper = Superview;", Optimizable = true)]
-		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}")]
+		[PostSnippet ("if (mySuper != null) {\n\t#pragma warning disable 168\n\tvar flush = mySuper.Subviews;\n#pragma warning restore 168\n\t}", Optimizable = true)]
 		void RemoveFromSuperviewWithoutNeedingDisplay ();
 
 		[Export ("resizeSubviewsWithOldSize:")]
@@ -19670,16 +19670,16 @@ namespace AppKit {
 		CGRect ContentRectFor (CGRect frameRect);
 
 		[Export ("init")]
-		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }")]
+		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }", Optimizable = true)]
 		IntPtr Constructor ();
 
 		[DesignatedInitializer]
 		[Export ("initWithContentRect:styleMask:backing:defer:")]
-		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }")]
+		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }", Optimizable = true)]
 		IntPtr Constructor (CGRect contentRect, NSWindowStyle aStyle, NSBackingStore bufferingType, bool deferCreation);
 	
 		[Export ("initWithContentRect:styleMask:backing:defer:screen:")]
-		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }")]
+		[PostSnippet ("if (!DisableReleasedWhenClosedInConstructor) { ReleasedWhenClosed = false; }", Optimizable = true)]
 		IntPtr Constructor (CGRect contentRect, NSWindowStyle aStyle, NSBackingStore bufferingType, bool deferCreation, NSScreen  screen);
 	
 		[Export ("title")]

src/avfoundation.cs

@@ -12412,7 +12412,7 @@ namespace AVFoundation {
 		IntPtr Constructor (AVQueuePlayer player, AVPlayerItem itemToLoop, CMTimeRange loopRange);
 
 #if !XAMCORE_4_0 // This API got introduced in Xcode 8.0 binding but is not currently present nor in Xcode 8.3 or Xcode 9.0 needs research
-		[PostSnippet ("loopingEnabled = false;")]
+		[PostSnippet ("loopingEnabled = false;", Optimizable = true)]
 #endif
 		[Export ("disableLooping")]
 		void DisableLooping ();

src/coreanimation.cs

@@ -186,7 +186,7 @@ namespace CoreAnimation {
 	}
 
 	[BaseType (typeof (NSObject))]
-	[Dispose ("OnDispose ();")]
+	[Dispose ("OnDispose ();", Optimizable = true)]
 	interface CALayer : CAMediaTiming, NSSecureCoding {
 		[Export ("layer")][Static]
 		CALayer Create ();
@@ -432,7 +432,7 @@ namespace CoreAnimation {
 		string Name { get; set; }
 
 		[Export ("delegate", ArgumentSemantic.Weak)][NullAllowed]
-		NSObject WeakDelegate { get; [PostSnippet (@"SetCALayerDelegate (value as CALayerDelegate);")] set; }
+		NSObject WeakDelegate { get; [PostSnippet (@"SetCALayerDelegate (value as CALayerDelegate);", Optimizable = true)] set; }
 
 		[Wrap ("WeakDelegate")]
 		[Protocolize]

src/foundation.cs

@@ -473,7 +473,7 @@ namespace Foundation
 		[Mac (10,15), iOS (13,0)]
 		[Static]
 		[Export ("loadFromHTMLWithRequest:options:completionHandler:")]
-		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory")]
+		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory", Optimizable = true)]
 		[Async (ResultTypeName = "NSLoadFromHtmlResult")]
 		[EditorBrowsable (EditorBrowsableState.Advanced)]
 		void LoadFromHtml (NSUrlRequest request, NSDictionary options, NSAttributedStringCompletionHandler completionHandler);
@@ -489,7 +489,7 @@ namespace Foundation
 		[Mac (10,15), iOS (13,0)]
 		[Static]
 		[Export ("loadFromHTMLWithFileURL:options:completionHandler:")]
-		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory")]
+		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory", Optimizable = true)]
 		[Async (ResultTypeName = "NSLoadFromHtmlResult")]
 		[EditorBrowsable (EditorBrowsableState.Advanced)]
 		void LoadFromHtml (NSUrl fileUrl, NSDictionary options, NSAttributedStringCompletionHandler completionHandler);
@@ -505,7 +505,7 @@ namespace Foundation
 		[Mac (10,15), iOS (13,0)]
 		[Static]
 		[Export ("loadFromHTMLWithString:options:completionHandler:")]
-		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory")]
+		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory", Optimizable = true)]
 		[Async (ResultTypeName = "NSLoadFromHtmlResult")]
 		[EditorBrowsable (EditorBrowsableState.Advanced)]
 		void LoadFromHtml (string @string, NSDictionary options, NSAttributedStringCompletionHandler completionHandler);
@@ -521,7 +521,7 @@ namespace Foundation
 		[Mac (10,15), iOS (13,0)]
 		[Static]
 		[Export ("loadFromHTMLWithData:options:completionHandler:")]
-		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory")]
+		[PreSnippet ("GC.KeepAlive (WebKit.WKContentMode.Recommended); // no-op to ensure WebKit.framework is loaded into memory", Optimizable = true)]
 		[Async (ResultTypeName = "NSLoadFromHtmlResult")]
 		[EditorBrowsable (EditorBrowsableState.Advanced)]
 		void LoadFromHtml (NSData data, NSDictionary options, NSAttributedStringCompletionHandler completionHandler);
@@ -3634,11 +3634,11 @@ namespace Foundation
 	[BaseType (typeof (NSData))]
 	interface NSMutableData {
 		[Static, Export ("dataWithCapacity:")] [Autorelease]
-		[PreSnippet ("if (capacity < 0 || capacity > nint.MaxValue) throw new ArgumentOutOfRangeException ();")]
+		[PreSnippet ("if (capacity < 0 || capacity > nint.MaxValue) throw new ArgumentOutOfRangeException ();", Optimizable = true)]
 		NSMutableData FromCapacity (nint capacity);
 
 		[Static, Export ("dataWithLength:")] [Autorelease]
-		[PreSnippet ("if (length < 0 || length > nint.MaxValue) throw new ArgumentOutOfRangeException ();")]
+		[PreSnippet ("if (length < 0 || length > nint.MaxValue) throw new ArgumentOutOfRangeException ();", Optimizable = true)]
 		NSMutableData FromLength (nint length);
 		
 		[Static, Export ("data")] [Autorelease]
@@ -3648,7 +3648,7 @@ namespace Foundation
 		IntPtr MutableBytes { get; }
 
 		[Export ("initWithCapacity:")]
-		[PreSnippet ("if (capacity > (ulong) nint.MaxValue) throw new ArgumentOutOfRangeException ();")]
+		[PreSnippet ("if (capacity > (ulong) nint.MaxValue) throw new ArgumentOutOfRangeException ();", Optimizable = true)]
 		IntPtr Constructor (nuint capacity);
 
 		[Export ("appendData:")]
@@ -5050,7 +5050,7 @@ namespace Foundation
 	}
 	
 	[BaseType (typeof(NSObject))]
-	[Dispose ("if (disposing) { Invalidate (); } ")]
+	[Dispose ("if (disposing) { Invalidate (); } ", Optimizable = true)]
 	// init returns NIL
 	[DisableDefaultCtor]
 	interface NSTimer {
@@ -5086,6 +5086,8 @@ namespace Foundation
 		[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 ();
 
@@ -5402,9 +5404,9 @@ namespace Foundation
 			// in turns, means that the Intents.framework is loaded into memory and this makes the
 			// selectors (getter and setter) work at runtime. Other selectors do not need it.
 			// reference: https://github.com/xamarin/xamarin-macios/issues/4894
-			[PreSnippet ("GC.KeepAlive (Intents.INCallCapabilityOptions.AudioCall); // no-op to ensure Intents.framework is loaded into memory")]
+			[PreSnippet ("GC.KeepAlive (Intents.INCallCapabilityOptions.AudioCall); // no-op to ensure Intents.framework is loaded into memory", Optimizable = true)]
 			get;
-			[PreSnippet ("GC.KeepAlive (Intents.INCallCapabilityOptions.AudioCall); // no-op to ensure Intents.framework is loaded into memory")]
+			[PreSnippet ("GC.KeepAlive (Intents.INCallCapabilityOptions.AudioCall); // no-op to ensure Intents.framework is loaded into memory", Optimizable = true)]
 			set;
 		}
 
@@ -8481,11 +8483,11 @@ namespace Foundation
 		[Export ("initWithCapacity:")]
 		IntPtr Constructor (nint capacity);
 
-		[PreSnippet ("Check (index);")]
+		[PreSnippet ("Check (index);", Optimizable = true)]
 		[Export ("insertString:atIndex:")]
 		void Insert (NSString str, nint index);
 
-		[PreSnippet ("Check (range);")]
+		[PreSnippet ("Check (range);", Optimizable = true)]
 		[Export ("deleteCharactersInRange:")]
 		void DeleteCharacters (NSRange range);
 
@@ -8495,7 +8497,7 @@ namespace Foundation
 		[Export ("setString:")]
 		void SetString (NSString str);
 
-		[PreSnippet ("Check (range);")]
+		[PreSnippet ("Check (range);", Optimizable = true)]
 		[Export ("replaceOccurrencesOfString:withString:options:range:")]
 		nuint ReplaceOcurrences (NSString target, NSString replacement, NSStringCompareOptions options, NSRange range);
 
@@ -8840,12 +8842,12 @@ namespace Foundation
 		[Export ("methodForSelector:")]
 		IntPtr GetMethodForSelector (Selector sel);
 
-		[PreSnippet ("if (!(this is INSCopying)) throw new InvalidOperationException (\"Type does not conform to NSCopying\");")]
+		[PreSnippet ("if (!(this is INSCopying)) throw new InvalidOperationException (\"Type does not conform to NSCopying\");", Optimizable = true)]
 		[Export ("copy")]
 		[return: Release ()]
 		NSObject Copy ();
 
-		[PreSnippet ("if (!(this is INSMutableCopying)) throw new InvalidOperationException (\"Type does not conform to NSMutableCopying\");")]
+		[PreSnippet ("if (!(this is INSMutableCopying)) throw new InvalidOperationException (\"Type does not conform to NSMutableCopying\");", Optimizable = true)]
 		[Export ("mutableCopy")]
 		[return: Release ()]
 		NSObject MutableCopy ();
@@ -10637,7 +10639,7 @@ namespace Foundation
 		NSNotificationCenter DefaultCenter { get; }
 	
 		[Export ("addObserver:selector:name:object:")]
-		[PostSnippet ("AddObserverToList (observer, aName, anObject);")]
+		[PostSnippet ("AddObserverToList (observer, aName, anObject);", Optimizable = true)]
 		void AddObserver (NSObject observer, Selector aSelector, [NullAllowed] NSString aName, [NullAllowed] NSObject anObject);
 	
 		[Export ("postNotification:")]
@@ -10650,11 +10652,11 @@ namespace Foundation
 		void PostNotificationName (string aName, [NullAllowed] NSObject anObject, [NullAllowed] NSDictionary aUserInfo);
 	
 		[Export ("removeObserver:")]
-		[PostSnippet ("RemoveObserversFromList (observer, null, null);")]
+		[PostSnippet ("RemoveObserversFromList (observer, null, null);", Optimizable = true)]
 		void RemoveObserver (NSObject observer);
 	
 		[Export ("removeObserver:name:object:")]
-		[PostSnippet ("RemoveObserversFromList (observer, aName, anObject);")]
+		[PostSnippet ("RemoveObserversFromList (observer, aName, anObject);", Optimizable = true)]
 		void RemoveObserver (NSObject observer, [NullAllowed] string aName, [NullAllowed] NSObject anObject);
 
 		[Export ("addObserverForName:object:queue:usingBlock:")]

src/generator-attributes.cs

@@ -583,10 +583,13 @@ public class SnippetAttribute : Attribute {
 		Code = s;
 	}
 	public string Code { get; set; }
+
+	public bool Optimizable { get; set; }
 }
 
 //
 // PreSnippet code is inserted after the parameters have been validated/marshalled
+// Adding this attribute will, by default, make the method non-optimizable by the SDK tools
 // 
 public class PreSnippetAttribute : SnippetAttribute {
 	public PreSnippetAttribute (string s) : base (s) {}
@@ -594,6 +597,7 @@ public class PreSnippetAttribute : SnippetAttribute {
 
 //
 // PrologueSnippet code is inserted before any code is generated
+// Adding this attribute will, by default, make the method non-optimizable by the SDK tools
 // 
 public class PrologueSnippetAttribute : SnippetAttribute {
 	public PrologueSnippetAttribute (string s) : base (s) {}
@@ -601,13 +605,15 @@ public class PrologueSnippetAttribute : SnippetAttribute {
 
 //
 // PostSnippet code is inserted before returning, before paramters are disposed/released
+// Adding this attribute will, by default, make the method non-optimizable by the SDK tools
 // 
 public class PostSnippetAttribute : SnippetAttribute {
 	public PostSnippetAttribute (string s) : base (s) {}
 }
 
 //
-// Code to run from a generated Dispose method
+// Code to run from a generated Dispose method, before any generated code is executed
+// Adding this attribute will, by default, make the method non-optimizable by the SDK tools
 //
 [AttributeUsage(AttributeTargets.Interface, AllowMultiple=true)]
 public class DisposeAttribute : SnippetAttribute {

src/generator.cs

@@ -4034,7 +4034,18 @@ public partial class Generator : IMemberGatherer {
 			print (l);
 		}
 	}
-	
+
+	bool IsOptimizable (MemberInfo method)
+	{
+		var optimizable = true;
+		var snippets = AttributeManager.GetCustomAttributes<SnippetAttribute> (method);
+		if (snippets.Length > 0) {
+			foreach (SnippetAttribute snippet in snippets)
+				optimizable &= snippet.Optimizable;
+		}
+		return optimizable;
+	}
+
 	[Flags]
 	public enum BodyOption {
 		None = 0x0,
@@ -4956,7 +4967,7 @@ public partial class Generator : IMemberGatherer {
 			}
 		}
 
-		print_generated_code ();
+		print_generated_code (optimizable: IsOptimizable (pi));
 		PrintPropertyAttributes (pi, minfo.type);
 
 		PrintAttributes (pi, preserve:true, advice:true, bindAs:true);
@@ -5511,7 +5522,7 @@ public partial class Generator : IMemberGatherer {
 		var mod = minfo.GetVisibility ();
 
 		var is_abstract = minfo.is_abstract;
-		print_generated_code ();
+		print_generated_code (optimizable: IsOptimizable (minfo.mi));
 		print ("{0} {1}{2}{3}",
 		       mod,
 		       minfo.GetModifiers (),
@@ -7423,15 +7434,19 @@ public partial class Generator : IMemberGatherer {
 			// Do we need a dispose method?
 			//
 			if (!is_static_class){
-				var disposeAttr = AttributeManager.GetCustomAttributes<DisposeAttribute> (type);
+				var attrs = AttributeManager.GetCustomAttributes<DisposeAttribute> (type);
-				if (disposeAttr.Length > 0 || instance_fields_to_clear_on_dispose.Count > 0){
+				// historical note: unlike many attributes our `DisposeAttribute` has `AllowMultiple=true`
-					print_generated_code (optimizable: disposeAttr.Length == 0);
+				var has_dispose_attributes = attrs.Length > 0;
+				if (has_dispose_attributes || (instance_fields_to_clear_on_dispose.Count > 0)) {
+					// if there'a any [Dispose] attribute then they all must opt-in in order for the generated Dispose method to be optimizable
+					bool optimizable = !has_dispose_attributes || IsOptimizable (type);
+					print_generated_code (optimizable: optimizable);
 					print ("protected override void Dispose (bool disposing)");
 					print ("{");
 					indent++;
-					if (disposeAttr.Length > 0){
+					if (has_dispose_attributes) {
-						var snippet = disposeAttr [0];
+						foreach (var da in attrs)
-						Inject (snippet);
+							Inject (da);
 					}
 					
 					print ("base.Dispose (disposing);");

src/safariservices.cs

@@ -59,7 +59,7 @@ namespace SafariServices {
 
 		[Static, Export ("supportsURL:")]
 		// Apple says it's __nonnull so let's be safe and maintain compatibility with our current behaviour
-		[PreSnippet ("if (url is null) return false;")]
+		[PreSnippet ("if (url is null) return false;", Optimizable = true)]
 		bool SupportsUrl ([NullAllowed] NSUrl url);
 
 		[Export ("addReadingListItemWithURL:title:previewText:error:")]

src/uikit.cs

@@ -2035,7 +2035,7 @@ namespace UIKit {
 		void OpenUrl (NSUrl url, UIApplicationOpenUrlOptions options, [NullAllowed] Action<bool> completion);
 
 		[Export ("canOpenURL:")]
-		[PreSnippet ("if (url is null) return false;")] // null not really allowed (but it's a behaviour change with known bug reports)
+		[PreSnippet ("if (url is null) return false;", Optimizable = true)] // null not really allowed (but it's a behaviour change with known bug reports)
 		bool CanOpenUrl ([NullAllowed] NSUrl url);
 		
 		[Export ("sendEvent:")]
@@ -5562,7 +5562,7 @@ namespace UIKit {
 
 #if !WATCH
 	[BaseType (typeof(NSObject), Delegates=new string [] {"WeakDelegate"}, Events=new Type[] {typeof (UIGestureRecognizerDelegate)})]
-	[Dispose ("OnDispose ();")]
+	[Dispose ("OnDispose ();", Optimizable = true)]
 	interface UIGestureRecognizer {
 		[DesignatedInitializer]
 		[Export ("initWithTarget:action:")]
@@ -13243,7 +13243,7 @@ namespace UIKit {
 		[Export ("typingAttributes", ArgumentSemantic.Copy)]
 		NSDictionary TypingAttributes {
 			// this avoids a crash (see unit tests) and behave like UITextField does (return null)
-			[PreSnippet ("if (SelectedRange.Length == 0) return null;")]
+			[PreSnippet ("if (SelectedRange.Length == 0) return null;", Optimizable = true)]
 			get;
 			set;
 		}

tests/generator/BGenTests.cs

@@ -645,6 +645,85 @@ namespace GeneratorTests
 		[Test]
 		public void GHIssue9065_Sealed () => BuildFile (Profile.iOS, nowarnings: true, "ghissue9065.cs");
 
+		// looking for [BindingImpl (BindingImplOptions.Optimizable)]
+		bool IsOptimizable (MethodDefinition method)
+		{
+			const int Optimizable = 0x2; // BindingImplOptions flag
+
+			if (!method.HasCustomAttributes)
+				return false;
+
+			foreach (var ca in method.CustomAttributes) {
+				if (ca.AttributeType.Name != "BindingImplAttribute")
+					continue;
+				foreach (var a in ca.ConstructorArguments)
+					return (((int) a.Value & Optimizable) == Optimizable);
+			}
+			return false;
+		}
+
+		[Test]
+		public void DisposeAttributeOptimizable ()
+		{
+			var bgen = BuildFile (Profile.iOS, "tests/dispose-attribute.cs");
+
+			// processing custom attributes (like its properties) will call Resolve so we must be able to find the platform assembly to run this test
+			var platform_dll = Path.Combine (Configuration.SdkRootXI, "lib/mono/Xamarin.iOS/Xamarin.iOS.dll");
+			var resolver = bgen.ApiAssembly.MainModule.AssemblyResolver as BaseAssemblyResolver;
+			resolver.AddSearchDirectory (Path.Combine (Configuration.SdkRootXI, "lib/mono/Xamarin.iOS/"));
+
+			// [Dispose] is, by default, not optimizable
+			var with_dispose = bgen.ApiAssembly.MainModule.GetType ("NS", "WithDispose").Methods.First ((v) => v.Name == "Dispose");
+			Assert.NotNull (with_dispose, "WithDispose");
+			Assert.That (IsOptimizable (with_dispose), Is.False, "WithDispose/Optimizable");
+
+			// [Dispose] can opt-in being optimizable
+			var with_dispose_optin = bgen.ApiAssembly.MainModule.GetType ("NS", "WithDisposeOptInOptimizable").Methods.First ((v) => v.Name == "Dispose");
+			Assert.NotNull (with_dispose_optin, "WithDisposeOptInOptimizable");
+			Assert.That (IsOptimizable (with_dispose_optin), Is.True, "WithDisposeOptInOptimizable/Optimizable");
+
+			// Without a [Dispose] attribute the generated method is optimizable
+			var without_dispose = bgen.ApiAssembly.MainModule.GetType ("NS", "WithoutDispose").Methods.First ((v) => v.Name == "Dispose");
+			Assert.NotNull (without_dispose, "WitoutDispose");
+			Assert.That (IsOptimizable (without_dispose), Is.True, "WitoutDispose/Optimizable");
+		}
+
+		[Test]
+		public void SnippetAttributesOptimizable ()
+		{
+			var bgen = BuildFile (Profile.iOS, "tests/snippet-attributes.cs");
+
+			// processing custom attributes (like its properties) will call Resolve so we must be able to find the platform assembly to run this test
+			var platform_dll = Path.Combine (Configuration.SdkRootXI, "lib/mono/Xamarin.iOS/Xamarin.iOS.dll");
+			var resolver = bgen.ApiAssembly.MainModule.AssemblyResolver as BaseAssemblyResolver;
+			resolver.AddSearchDirectory (Path.Combine (Configuration.SdkRootXI, "lib/mono/Xamarin.iOS/"));
+
+			// [SnippetAttribute] subclasses are, by default, not optimizable
+			var not_opt = bgen.ApiAssembly.MainModule.GetType ("NS", "NotOptimizable");
+			Assert.NotNull (not_opt, "NotOptimizable");
+			var pre_not_opt = not_opt.Methods.First ((v) => v.Name == "Pre");
+			Assert.That (IsOptimizable (pre_not_opt), Is.False, "NotOptimizable/Pre");
+			var prologue_not_opt = not_opt.Methods.First ((v) => v.Name == "Prologue");
+			Assert.That (IsOptimizable (prologue_not_opt), Is.False, "NotOptimizable/Prologue");
+			var post_not_opt = not_opt.Methods.First ((v) => v.Name == "Post");
+			Assert.That (IsOptimizable (post_not_opt), Is.False, "NotOptimizable/Post");
+
+			// [SnippetAttribute] subclasses can opt-in being optimizable
+			var optin_opt = bgen.ApiAssembly.MainModule.GetType ("NS", "OptInOptimizable");
+			Assert.NotNull (optin_opt, "OptInOptimizable");
+			var pre_optin_opt = optin_opt.Methods.First ((v) => v.Name == "Pre");
+			Assert.That (IsOptimizable (pre_optin_opt), Is.True, "OptInOptimizable/Pre");
+			var prologue_optin_opt = optin_opt.Methods.First ((v) => v.Name == "Prologue");
+			Assert.That (IsOptimizable (prologue_optin_opt), Is.True, "OptInOptimizable/Prologue");
+			var post_optin_opt = optin_opt.Methods.First ((v) => v.Name == "Post");
+			Assert.That (IsOptimizable (post_optin_opt), Is.True, "OptInOptimizable/Post");
+
+			// Without a [SnippetAttribute] subclass attribute the generated method is optimizable
+			var nothing = bgen.ApiAssembly.MainModule.GetType ("NS", "NoSnippet").Methods.First ((v) => v.Name == "Nothing");
+			Assert.NotNull (nothing, "NoSnippet");
+			Assert.That (IsOptimizable (nothing), Is.True, "Nothing/Optimizable");
+		}
+
 		BGenTool BuildFile (Profile profile, params string [] filenames)
 		{
 			return BuildFile (profile, true, false, filenames);

tests/generator/tests/dispose-attribute.cs

@@ -0,0 +1,36 @@
+using System;
+using Foundation;
+using ObjCRuntime;
+
+namespace NS {
+
+    // injecting custom code makes the Dispose method not-optimizable by default
+    [Dispose ("Console.WriteLine (\"Disposing!\");")]
+    [BaseType (typeof (NSObject))]
+	interface WithDispose {
+
+		[Export ("delegate", ArgumentSemantic.Weak)]
+        [NullAllowed]
+		NSObject WeakDelegate { get; set; }
+	}
+
+    // but we can opt-in to make it optimizable
+    [Dispose ("// just a comment, that's safe to optimize, if not very useful otherwise", Optimizable = true)]
+    [BaseType (typeof (NSObject))]
+	interface WithDisposeOptInOptimizable {
+
+		[Export ("delegate", ArgumentSemantic.Weak)]
+        [NullAllowed]
+		NSObject WeakDelegate { get; set; }
+	}
+
+    // if nothing is injected then we know we generate code that our tools can optimize
+    [BaseType (typeof (NSObject))]
+	interface WithoutDispose {
+
+        // this ensure we have a Dispose method generated for the type
+		[Export ("delegate", ArgumentSemantic.Weak)]
+        [NullAllowed]
+		NSObject WeakDelegate { get; set; }
+	}
+}

tests/generator/tests/snippet-attributes.cs

@@ -0,0 +1,48 @@
+using System;
+using Foundation;
+using ObjCRuntime;
+
+namespace NS {
+
+	// injecting custom code makes the method method not-optimizable by default
+	[BaseType (typeof (NSObject))]
+	interface NotOptimizable {
+
+		[PreSnippet ("Console.WriteLine (\"Pre!\");")]
+		[Export ("pre")]
+		void Pre ();
+
+		[PrologueSnippet ("Console.WriteLine (\"Prologue!\");")]
+		[Export ("prologue")]
+		void Prologue ();
+
+		[PostSnippet ("Console.WriteLine (\"Post!\");")]
+		[Export ("post")]
+		void Post ();
+	}
+
+	// but we can opt-in to make it optimizable
+	[BaseType (typeof (NSObject))]
+	interface OptInOptimizable {
+
+		[PreSnippet ("Console.WriteLine (\"Pre!\");", Optimizable = true)]
+		[Export ("pre")]
+		void Pre ();
+
+		[PrologueSnippet ("Console.WriteLine (\"Prologue!\");", Optimizable = true)]
+		[Export ("prologue")]
+		void Prologue ();
+
+		[PostSnippet ("Console.WriteLine (\"Post!\");", Optimizable = true)]
+		[Export ("post")]
+		void Post ();
+	}
+
+	// if nothing is injected then we know we generate code that our tools can optimize
+	[BaseType (typeof (NSObject))]
+	interface NoSnippet {
+
+		[Export ("nothing")]
+		void Nothing ();
+	}
+}