[generator][dotnet] Add support for [Uns|S]upportedOSPlatformAttribute

This moves our current/legacy attributes to the ones added in dotnet 5 [1].

Short Forms (only in bindings)

| Old                                   | New                                 |
|---------------------------------------|-------------------------------------|
| [iOS (7,0)]                           | [SupportedOSPlatform ("ios7.0")]    |
| [NoIOS]                               | [UnsupportedOSPlatform ("ios")]     |

Long Forms

| Old                                   | New                                 |
|---------------------------------------|-------------------------------------|
| [Introduced (PlatformName.iOS, 7,0)]  | [SupportedOSPlatform ("ios7.0")]    |
| [Obsoleted (PlatformName.iOS, 12,1)]  | [Obsolete (...)]                    |
| [Deprecated (PlatformName.iOS, 14,3)] | [UnsupportedOSPlatform ("ios14.3")] |
| [Unavailable (PlatformName.iOS)]      | [UnsupportedOSPlatform ("ios")]     |

Other changes

* `[SupportedOSPlatform]` and `[UnsupportedOSPlatform]` are not allowed on `interface` [2] which means they cannot be used for protocols. This is currently handled by inlining the existing attributes on all members.
* `[ObsoletedInOSPlatform]` was removed in net5 RC. This PR is now mapping the existing attributes to `[Obsolote]`, however multiple ones cannot be added so they need to be platform specific.

Still Missing

* [ ] Generator deduplication of type/members attributes for interfaces (due to [2])
* [ ] Manual bindings
* [ ] Introspection tests updates

References

* [1] xamarin#10170
* [2] dotnet/runtime#47599
* [3] dotnet/runtime#47601

src/ObjCRuntime/PlatformAvailability2.cs

@@ -92,6 +92,76 @@ internal AvailabilityBaseAttribute (
 		public override string ToString ()
 		{
 			var builder = new StringBuilder ();
+#if NET
+			switch (AvailabilityKind) {
+			case AvailabilityKind.Introduced:
+				builder.Append ("[SupportedOSPlatform (\"");
+				break;
+			case AvailabilityKind.Obsoleted:
+				switch (Platform) {
+				case PlatformName.iOS:
+					builder.AppendLine ("#if __IOS__");
+					break;
+				case PlatformName.TvOS:
+					builder.AppendLine ("#if __TVOS__");
+					break;
+				case PlatformName.WatchOS:
+					builder.AppendLine ("#if __WATCHOS__");
+					break;
+				case PlatformName.MacOSX:
+					builder.AppendLine ("#if __MACOS__");
+					break;
+				case PlatformName.MacCatalyst:
+					builder.AppendLine ("#if __MACCATALYST__");
+					break;
+				}
+				builder.Append ("[Obsolete (\"Starting with ");
+				break;
+			case AvailabilityKind.Deprecated:
+			case AvailabilityKind.Unavailable:
+				builder.Append ("[UnsupportedOSPlatform (\"");
+				break;
+			}
+
+			switch (Platform) {
+			case PlatformName.iOS:
+				builder.Append ("ios");
+				break;
+			case PlatformName.TvOS:
+				builder.Append ("tvos");
+				break;
+			case PlatformName.WatchOS:
+				builder.Append ("watchos");
+				break;
+			case PlatformName.MacOSX:
+				builder.Append ("macos"); // no 'x'
+				break;
+			case PlatformName.MacCatalyst:
+				builder.Append ("maccatalyst");
+				break;
+			}
+
+			if (Version != null) {
+				builder.Append (Version.Major).Append ('.').Append (Version.Minor);
+				if (Version.Build >= 0)
+					builder.Append ('.').Append (Version.Build);
+			}
+
+			switch (AvailabilityKind) {
+			case AvailabilityKind.Obsoleted:
+				if (!String.IsNullOrEmpty (Message))
+					builder.Append (' ').Append (Message);
+				// TODO add a URL (wiki?) and DiagnosticId (one per platform?) for documentation
+				builder.AppendLine ("\", DiagnosticId = \"BI1234\", UrlFormat = \"https://github.com/xamarin/xamarin-macios/wiki/Obsolete\")]");
+				builder.Append ("#endif");
+				break;
+			case AvailabilityKind.Introduced:
+			case AvailabilityKind.Deprecated:
+			case AvailabilityKind.Unavailable:
+				builder.Append ("\")]");
+				break;
+			}
+#else
 			builder.AppendFormat ("[{0} ({1}.{2}", AvailabilityKind, nameof (PlatformName), Platform);
 
 			if (Version != null) {
@@ -107,6 +177,7 @@ public override string ToString ()
 				builder.AppendFormat (", message: \"{0}\"", Message.Replace ("\"", "\"\""));
 
 			builder.Append (")]");
+#endif
 			return builder.ToString ();
 		}
 	}

src/generator.cs

@@ -728,6 +728,9 @@ public NamespaceManager (BindingTouch binding_touch, string customObjCRuntimeNS,
 		ImplicitNamespaces.Add ("System.Diagnostics");
 		ImplicitNamespaces.Add ("System.Diagnostics.CodeAnalysis");
 		ImplicitNamespaces.Add ("System.ComponentModel");
+#if NET
+		ImplicitNamespaces.Add ("System.Runtime.Versioning");
+#endif
 		ImplicitNamespaces.Add ("System.Threading.Tasks");
 		ImplicitNamespaces.Add ("CoreFoundation");
 		ImplicitNamespaces.Add ("Foundation");
@@ -5682,7 +5685,13 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
 		var optionalInstanceProperties = allProtocolProperties.Where ((v) => !IsRequired (v) && !AttributeManager.HasAttribute<StaticAttribute> (v));
 		var requiredInstanceAsyncMethods = requiredInstanceMethods.Where (m => AttributeManager.HasAttribute<AsyncAttribute> (m)).ToList ();
 
+#if NET
+		// [SupportedOSPlatform] cannot be used on interface - https://github.com/dotnet/runtime/issues/47599
+		// instead we'll generate them on the members
+		PrintAttributes (type, platform:false, preserve:true, advice:true);
+#else
 		PrintAttributes (type, platform:true, preserve:true, advice:true);
+#endif
 		print ("[Protocol (Name = \"{1}\", WrapperType = typeof ({0}Wrapper){2}{3})]", 
 		       TypeName, 
 		       protocol_name, 
@@ -5822,6 +5831,10 @@ void GenerateProtocolTypes (Type type, string class_visibility, string TypeName,
 			var mod = string.Empty;
 
 			PrintMethodAttributes (minfo);
+#if NET
+			// TODO - generate them only if they do not exists on the member
+			PrintPlatformAttributes (type);
+#endif
 			PrintPlatformAttributes (mi);
 			print_generated_code ();
 			PrintDelegateProxy (minfo);