[generator] Add support for emitting [UnsupportedOSPlatform] (#1307)

Fixes: #1309

Context: dotnet/android@944d306

Sometimes, Android *removes* APIs.  For example, API-23 *removed* the
`android.Manifest.permission.CLEAR_APP_USER_DATA` field.

We still list this field, though:
[`Android.Manifest.Permission.ClearAppUserData` field][0].

>     [Android.Runtime.Register("CLEAR_APP_USER_DATA")]
>     public const string ClearAppUserData;

In dotnet/android@944d3061 we updated `api-merge` to emit
`//*/@#removed-since`.  Consider this snippet from
[`android/src/Mono.Android/Profiles/api-35.xml`][1]:

	<field
	    deprecated="not deprecated" final="true"
	    name="CLEAR_APP_USER_DATA" jni-signature="Ljava/lang/String;"
	    static="true" transient="false" type="java.lang.String" type-generic-aware="java.lang.String"
	    value="&quot;android.permission.CLEAR_APP_USER_DATA&quot;"
	    visibility="public" volatile="false"
	    removed-since="23"
	/>

Modify `generator` to consume this information and emit
`[UnsupportedOSPlatformAttribute ("androidXX.0")]` as needed for API
that has been removed from `Mono.Android`.  This would update
`Android.Manifest.Permission.ClearAppUserData` to be:

	[Android.Runtime.Register("CLEAR_APP_USER_DATA")]
	[System.Runtime.Versioning.UnsupportedOSPlatformAttribute ("android23.0")]
	public const string ClearAppUserData;

[0]: https://learn.microsoft.com/en-us/dotnet/api/android.manifest.permission.clearappuserdata?view=net-android-34.0
[1]: https://github.com/dotnet/android/blob/d012fb811b4bcebe260400f5fc7a0287108336d7/src/Mono.Android/Profiles/api-35.xml

Author: jpobst

tests/generator-Tests/Unit-Tests/CodeGeneratorTests.cs

@@ -1406,6 +1406,36 @@ public void SupportedOSPlatformConstFields ()
 			StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
 		}
 
+		[Test]
+		public void UnsupportedOSPlatform ()
+		{
+			var klass = SupportTypeBuilder.CreateClass ("java.code.MyClass", options);
+			klass.ApiRemovedSince = 30;
+
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
+
+			StringAssert.Contains ("[global::System.Runtime.Versioning.UnsupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain UnsupportedOSPlatform!");
+		}
+
+		[Test]
+		public void UnsupportedOSPlatformConstFields ()
+		{
+			var klass = new TestClass ("java.lang.Object", "com.mypackage.foo");
+			var field = new TestField ("java.lang.String", "bar").SetConstant ("MY_VALUE");
+
+			field.ApiRemovedSince = 30;
+
+			klass.Fields.Add (field);
+
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
+
+			StringAssert.Contains ("[global::System.Runtime.Versioning.UnsupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain UnsupportedOSPlatform!");
+		}
+
 		[Test]
 		public void StringPropertyOverride ([Values ("true", "false")] string final)
 		{

tests/generator-Tests/Unit-Tests/XmlApiImporterTests.cs

@@ -51,6 +51,15 @@ public void CreateClass_CorrectApiSinceOverridePackage ()
 			Assert.AreEqual (9, klass.ApiAvailableSince);
 		}
 
+		[Test]
+		public void CreateClass_CorrectApiRemoved ()
+		{
+			var xml = XDocument.Parse ("<package name='com.example.test' jni-name='com/example/test'><class name='myclass' removed-since='7' /></package>");
+			var klass = XmlApiImporter.CreateClass (xml.Root, xml.Root.Element ("class"), opt);
+
+			Assert.AreEqual (7, klass.ApiRemovedSince);
+		}
+
 		[Test]
 		public void CreateCtor_EnsureValidName ()
 		{

tools/generator/ApiVersionsSupport.cs

@@ -18,6 +18,7 @@ public static class ApiVersionsSupport
 		public interface IApiAvailability
 		{
 			int ApiAvailableSince { get; set; }
+			int ApiRemovedSince { get; set; }
 		}
 
 		static IEnumerable<GenBase> FlattenGens (IEnumerable<GenBase> gens)

tools/generator/Java.Interop.Tools.Generator.Importers/XmlApiImporter.cs

@@ -159,6 +159,7 @@ public static Ctor CreateCtor (GenBase declaringType, XElement elem, CodeGenerat
 			var ctor = new Ctor (declaringType) {
 				AnnotatedVisibility = elem.XGetAttribute ("annotated-visibility"),
 				ApiAvailableSince = declaringType.ApiAvailableSince,
+				ApiRemovedSince = declaringType.ApiRemovedSince,
 				CustomAttributes = elem.XGetAttribute ("customAttributes"),
 				Deprecated = elem.Deprecated (),
 				DeprecatedSince = elem.XGetAttributeAsIntOrNull ("deprecated-since"),
@@ -211,6 +212,7 @@ public static Field CreateField (GenBase declaringType, XElement elem, CodeGener
 			var field = new Field {
 				AnnotatedVisibility = elem.XGetAttribute ("annotated-visibility"),
 				ApiAvailableSince = declaringType.ApiAvailableSince,
+				ApiRemovedSince = declaringType.ApiRemovedSince,
 				DeprecatedComment = elem.XGetAttribute ("deprecated"),
 				DeprecatedSince = elem.XGetAttributeAsIntOrNull ("deprecated-since"),
 				IsAcw = true,
@@ -369,6 +371,7 @@ public static Method CreateMethod (GenBase declaringType, XElement elem, CodeGen
 			var method = new Method (declaringType) {
 				AnnotatedVisibility = elem.XGetAttribute ("annotated-visibility"),
 				ApiAvailableSince = declaringType.ApiAvailableSince,
+				ApiRemovedSince = declaringType.ApiRemovedSince,
 				ArgsType = elem.Attribute ("argsType")?.Value,
 				CustomAttributes = elem.XGetAttribute ("customAttributes"),
 				Deprecated = elem.Deprecated (),
@@ -515,9 +518,12 @@ static XElement GetPreviousClass (XNode n, string nameValue)
 		// Elements need to be passed in the above order. (package, class, member)
 		static void FillApiSince (ApiVersionsSupport.IApiAvailability model, params XElement[] elems)
 		{
-			foreach (var elem in elems)
+			foreach (var elem in elems) {
 				if (int.TryParse (elem.XGetAttribute ("api-since"), out var result))
 					model.ApiAvailableSince = result;
+				if (int.TryParse (elem.XGetAttribute ("removed-since"), out var removed))
+					model.ApiRemovedSince = removed;
+			}
 		}
 
 		static bool IsObfuscatedName (int threshold, string name)

tools/generator/Java.Interop.Tools.Generator.ObjectModel/Field.cs

@@ -9,6 +9,7 @@ public class Field : ApiVersionsSupport.IApiAvailability, ISourceLineInfo
 		public string AnnotatedVisibility { get; set; }
 		public string Annotation { get; set; }
 		public int ApiAvailableSince { get; set; }
+		public int ApiRemovedSince { get; set; }
 		public string DeprecatedComment { get; set; }
 		public int? DeprecatedSince { get; set; }
 		public bool IsAcw { get; set; }

tools/generator/Java.Interop.Tools.Generator.ObjectModel/GenBase.cs

@@ -188,6 +188,8 @@ IEnumerable<GenBase> Ancestors ()
 
 		public int ApiAvailableSince { get; set; }
 
+		public int ApiRemovedSince { get; set; }
+
 		public virtual ClassGen BaseGen => null;
 
 		public GenBase BaseSymbol =>

tools/generator/Java.Interop.Tools.Generator.ObjectModel/MethodBase.cs

@@ -16,6 +16,7 @@ protected MethodBase (GenBase declaringType)
 		public string AnnotatedVisibility { get; set; }
 		public string Annotation { get; internal set; }
 		public int ApiAvailableSince { get; set; }
+		public int ApiRemovedSince { get; set; }
 		public string AssemblyName { get; set; }
 		public GenBase DeclaringType { get; }
 		public string Deprecated { get; set; }

tools/generator/SourceWriters/Attributes/UnsupportedOSPlatformAttr.cs

@@ -0,0 +1,21 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.SourceWriter;
+
+namespace generator.SourceWriters
+{
+	public class UnsupportedOSPlatformAttr : AttributeWriter
+	{
+		public int Version { get; }
+
+		public UnsupportedOSPlatformAttr (int version) => Version = version;
+
+		public override void WriteAttribute (CodeWriter writer)
+		{
+			writer.WriteLine ($"[global::System.Runtime.Versioning.UnsupportedOSPlatformAttribute (\"android{Version}.0\")]");
+		}
+	}
+}

tools/generator/SourceWriters/Extensions/SourceWriterExtensions.cs

@@ -12,6 +12,8 @@ namespace generator.SourceWriters
 {
 	public static class SourceWriterExtensions
 	{
+		const int MINIMUM_API_LEVEL = 21;
+
 		public static void AddField (TypeWriter tw, GenBase type, Field field, CodeGenerationOptions opt)
 		{
 			if (field.NeedsProperty)
@@ -306,16 +308,26 @@ public static void AddParameterListCallArgs (List<string> body, ParameterList pa
 		}
 
 		public static void AddSupportedOSPlatform (List<AttributeWriter> attributes, ApiVersionsSupport.IApiAvailability member, CodeGenerationOptions opt)
-			=> AddSupportedOSPlatform (attributes, member.ApiAvailableSince, opt);
+		{
+			AddSupportedOSPlatform (attributes, member.ApiAvailableSince, opt);
+			AddUnsupportedOSPlatform (attributes, member.ApiRemovedSince, opt);
+		}
 
 		public static void AddSupportedOSPlatform (List<AttributeWriter> attributes, int since, CodeGenerationOptions opt)
 		{
 			// There's no sense in writing say 'android15' because we do not support older APIs,
 			// so those APIs will be available in all of our versions.
-			if (since > 21 && opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1)
+			if (since > MINIMUM_API_LEVEL && opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1)
 				attributes.Add (new SupportedOSPlatformAttr (since));
 		}
 
+		public static void AddUnsupportedOSPlatform (List<AttributeWriter> attributes, int since, CodeGenerationOptions opt)
+		{
+			// Here it makes sense to still write 'android15' because it will be missing in later versions like `android35`.
+			if (since > 0 && opt.CodeGenerationTarget == CodeGenerationTarget.XAJavaInterop1)
+				attributes.Add (new UnsupportedOSPlatformAttr (since));
+		}
+
 		public static void AddObsolete (List<AttributeWriter> attributes, string message, CodeGenerationOptions opt, bool forceDeprecate = false, bool isError = false, int? deprecatedSince = null)
 		{
 			// Bail if we're not obsolete
@@ -336,7 +348,7 @@ static bool AddObsoletedOSPlatformAttribute (List<AttributeWriter> attributes, s
 				return false;
 
 			// If it was obsoleted in a version earlier than we support (like 15), use a regular [Obsolete] instead
-			if (!deprecatedSince.HasValue || deprecatedSince <= 21)
+			if (!deprecatedSince.HasValue || deprecatedSince <= MINIMUM_API_LEVEL)
 				return false;
 
 			// This is the default Android message, but it isn't useful so remove it