Revert "[generator] Disable [SupportedOSPlatform] until .NET 5/6. (#781)" (#841)

Context: dotnet/android#5338

This reverts commit 00862ad.

Now that we are properly building with `$(TargetFramework)`=net6.0
(4d0cba6), we can re-enable the `[SupportedOSPlatform]` support,
as originally introduced in da12df4.

Emit the [`System.Runtime.Versioning.SupportedOSPlatformAttribute`][0]
custom attribute which is used to specify on which platforms and
platform versions an API is available.  This is used to build analyzers
to give users warnings if they are trying to use an API when it will
not be available on their target platform.

[0]: https://docs.microsoft.com/en-us/dotnet/api/system.runtime.versioning.supportedosplatformattribute?view=net-5.0

Author: jpobst

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

@@ -276,20 +276,18 @@ class XAJavaInteropCodeGeneratorTests : CodeGeneratorTests
 	{
 		protected override CodeGenerationTarget Target => CodeGenerationTarget.XAJavaInterop1;
 
+		[Test]
+		public void SupportedOSPlatform ()
+		{
+			var klass = SupportTypeBuilder.CreateClass ("java.code.MyClass", options);
+			klass.ApiAvailableSince = 30;
 
-		// Disabled until we can properly build .NET 5/6 assemblies in our XA tree.
-		//[Test]
-		//public void SupportedOSPlatform ()
-		//{
-		//	var klass = SupportTypeBuilder.CreateClass ("java.code.MyClass", options);
-		//	klass.ApiAvailableSince = 30;
-
-		//	generator.Context.ContextTypes.Push (klass);
-		//	generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
-		//	generator.Context.ContextTypes.Pop ();
+			generator.Context.ContextTypes.Push (klass);
+			generator.WriteType (klass, string.Empty, new GenerationInfo ("", "", "MyAssembly"));
+			generator.Context.ContextTypes.Pop ();
 
-		//	StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
-		//}
+			StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
+		}
 	}
 
 	[TestFixture]

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

@@ -32,21 +32,20 @@ public void Generate (CodeGenerationOptions opt, GenerationInfo gen_info)
 				foreach (var jni in opt.GetJniMarshalDelegates ())
 					sw.WriteLine ($"delegate {FromJniType (jni[jni.Length - 1])} {jni} (IntPtr jnienv, IntPtr klass{GetDelegateParameters (jni)});");
 
-				// Disabled until we can properly build .NET 5/6 assemblies in our XA tree.
 				// [SupportedOSPlatform] only exists in .NET 5.0+, so we need to generate a
 				// dummy one so earlier frameworks can compile.
-				//if (opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1) {
-				//	sw.WriteLine ("#if !NET");
-				//	sw.WriteLine ("namespace System.Runtime.Versioning {");
-				//	sw.WriteLine ("    [System.Diagnostics.Conditional(\"NEVER\")]");
-				//	sw.WriteLine ("    [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Event | AttributeTargets.Method | AttributeTargets.Module | AttributeTargets.Property | AttributeTargets.Struct, AllowMultiple = true, Inherited = false)]");
-				//	sw.WriteLine ("    internal sealed class SupportedOSPlatformAttribute : Attribute {");
-				//	sw.WriteLine ("        public SupportedOSPlatformAttribute (string platformName) { }");
-				//	sw.WriteLine ("    }");
-				//	sw.WriteLine ("}");
-				//	sw.WriteLine ("#endif");
-				//	sw.WriteLine ("");
-				//}
+				if (opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1) {
+					sw.WriteLine ("#if !NET");
+					sw.WriteLine ("namespace System.Runtime.Versioning {");
+					sw.WriteLine ("    [System.Diagnostics.Conditional(\"NEVER\")]");
+					sw.WriteLine ("    [AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Constructor | AttributeTargets.Event | AttributeTargets.Method | AttributeTargets.Module | AttributeTargets.Property | AttributeTargets.Struct, AllowMultiple = true, Inherited = false)]");
+					sw.WriteLine ("    internal sealed class SupportedOSPlatformAttribute : Attribute {");
+					sw.WriteLine ("        public SupportedOSPlatformAttribute (string platformName) { }");
+					sw.WriteLine ("    }");
+					sw.WriteLine ("}");
+					sw.WriteLine ("#endif");
+					sw.WriteLine ("");
+				}
 			}
 		}
 

tools/generator/SourceWriters/Attributes/SupportedOSPlatformAttr.cs

@@ -15,8 +15,7 @@ public class SupportedOSPlatformAttr : AttributeWriter
 
 		public override void WriteAttribute (CodeWriter writer)
 		{
-			// Disabled until we can properly build .NET 5/6 assemblies in our XA tree.
-			//writer.WriteLine ($"[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android{Version}.0\")]");
+			writer.WriteLine ($"[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android{Version}.0\")]");
 		}
 	}
 }