[generator] Add [SupportedOSPlatform] in assemblies using ApiSince (#773)

Context: dotnet/android#5338

.NET 5 provides a new
[`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.

`SupportedOSPlatformAttribute` is fundamentally the same as our
existing `RegisterAttribute.ApiSince` property, except tooling has
actually been built to consume the information.

As such, we need `generator` support to put this information into
`Mono.Android.dll`:

	partial class Activity {
	    // Metadata.xml XPath method reference: path="/api/package[@name='android.app']/class[@name='Activity']/method[@name='dismissKeyboardShortcutsHelper' and count(parameter)=0]"
	    [global::System.Runtime.Versioning.SupportedOSPlatformAttribute ("android24.0")]
	    [Register ("dismissKeyboardShortcutsHelper", "()V", "", ApiSince = 24)]
	    public unsafe void DismissKeyboardShortcutsHelper () { … }
	}


Some interesting notes:

  - `SupportedOSPlatformAttribute` is only available in .NET 5+, so
    we include a local version for earlier frameworks so we can
    compile without needing to `#ifdef` every attribute use.

  - The local version is marked as `[Conditional ("NEVER")]` so the
    attributes will not actually get compiled into the resulting
    pre-NET5.0 assembly.

  - The attribute cannot be placed on interfaces or fields, so we
    aren't able to annotate them.

  - Our minimum supported API for .NET 6 is 21, so we only write
    attributes for API added in versions newer than 21, as API added
    earlier are always available.

[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

@@ -215,12 +215,39 @@ public void WriteDuplicateInterfaceEventArgs ()
 
 			Assert.AreEqual (GetExpected (nameof (WriteDuplicateInterfaceEventArgs)), writer.ToString ().NormalizeLineEndings ());
 		}
+
+		[Test]
+		public void SupportedOSPlatform ()
+		{
+			// We do not write [SupportedOSPlatform] for JavaInterop, only XAJavaInterop
+			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 ();
+
+			StringAssert.DoesNotContain ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
+		}
 	}
 
 	[TestFixture]
 	class XAJavaInteropCodeGeneratorTests : CodeGeneratorTests
 	{
 		protected override CodeGenerationTarget Target => CodeGenerationTarget.XAJavaInterop1;
+
+		[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 ();
+
+			StringAssert.Contains ("[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android30.0\")]", builder.ToString (), "Should contain SupportedOSPlatform!");
+		}
 	}
 
 	[TestFixture]

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

@@ -31,6 +31,21 @@ public void Generate (CodeGenerationOptions opt, GenerationInfo gen_info)
 				// delegate bool _JniMarshal_PPL_Z (IntPtr jnienv, IntPtr klass, IntPtr a);
 				foreach (var jni in opt.GetJniMarshalDelegates ())
 					sw.WriteLine ($"delegate {FromJniType (jni[jni.Length - 1])} {jni} (IntPtr jnienv, IntPtr klass{GetDelegateParameters (jni)});");
+
+				// [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 !NET5_0_OR_GREATER");
+					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

@@ -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 SupportedOSPlatformAttr : AttributeWriter
+	{
+		public int Version { get; }
+
+		public SupportedOSPlatformAttr (int version) => Version = version;
+
+		public override void WriteAttribute (CodeWriter writer)
+		{
+			writer.WriteLine ($"[global::System.Runtime.Versioning.SupportedOSPlatformAttribute (\"android{Version}.0\")]");
+		}
+	}
+}

tools/generator/SourceWriters/BoundAbstractProperty.cs

@@ -41,6 +41,8 @@ public BoundAbstractProperty (GenBase gen, Property property, CodeGenerationOpti
 			if (property.Getter.IsReturnEnumified)
 				GetterAttributes.Add (new GeneratedEnumAttr (true));
 
+			SourceWriterExtensions.AddSupportedOSPlatform (GetterAttributes, property.Getter, opt);
+
 			GetterAttributes.Add (new RegisterAttr (property.Getter.JavaName, property.Getter.JniSignature, property.Getter.GetConnectorNameFull (opt), additionalProperties: property.Getter.AdditionalAttributeString ()));
 
 			SourceWriterExtensions.AddMethodCustomAttributes (GetterAttributes, property.Getter);
@@ -51,6 +53,8 @@ public BoundAbstractProperty (GenBase gen, Property property, CodeGenerationOpti
 				if (gen.IsGeneratable)
 					SetterComments.Add ($"// Metadata.xml XPath method reference: path=\"{gen.MetadataXPathReference}/method[@name='{property.Setter.JavaName}'{property.Setter.Parameters.GetMethodXPathPredicate ()}]\"");
 
+				SourceWriterExtensions.AddSupportedOSPlatform (SetterAttributes, property.Setter, opt);
+
 				SourceWriterExtensions.AddMethodCustomAttributes (SetterAttributes, property.Setter);
 				SetterAttributes.Add (new RegisterAttr (property.Setter.JavaName, property.Setter.JniSignature, property.Setter.GetConnectorNameFull (opt), additionalProperties: property.Setter.AdditionalAttributeString ()));
 			}

tools/generator/SourceWriters/BoundClass.cs

@@ -45,6 +45,8 @@ public BoundClass (ClassGen klass, CodeGenerationOptions opt, CodeGeneratorConte
 			if (klass.IsDeprecated)
 				Attributes.Add (new ObsoleteAttr (klass.DeprecatedComment) { WriteAttributeSuffix = true });
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, klass, opt);
+
 			Attributes.Add (new RegisterAttr (klass.RawJniName, null, null, true, klass.AdditionalAttributeString ()) { UseGlobal = true, UseShortForm = true });
 
 			if (klass.TypeParameters != null && klass.TypeParameters.Any ())

tools/generator/SourceWriters/BoundConstructor.cs

@@ -27,6 +27,8 @@ public BoundConstructor (ClassGen klass, Ctor constructor, bool useBase, CodeGen
 			constructor.JavadocInfo?.AddJavadocs (Comments);
 			Comments.Add (string.Format ("// Metadata.xml XPath constructor reference: path=\"{0}/constructor[@name='{1}'{2}]\"", klass.MetadataXPathReference, klass.JavaSimpleName, constructor.Parameters.GetMethodXPathPredicate ()));
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, constructor, opt);
+
 			Attributes.Add (new RegisterAttr (".ctor", constructor.JniSignature, string.Empty, additionalProperties: constructor.AdditionalAttributeString ()));
 
 			if (constructor.Deprecated != null)

tools/generator/SourceWriters/BoundFieldAsProperty.cs

@@ -31,6 +31,8 @@ public BoundFieldAsProperty (GenBase type, Field field, CodeGenerationOptions op
 			if (field.IsEnumified)
 				Attributes.Add (new GeneratedEnumAttr ());
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, field, opt);
+
 			Attributes.Add (new RegisterAttr (field.JavaName, additionalProperties: field.AdditionalAttributeString ()));
 
 			if (field.IsDeprecated)

tools/generator/SourceWriters/BoundInterfaceMethodDeclaration.cs

@@ -31,6 +31,8 @@ public BoundInterfaceMethodDeclaration (Method method, string adapter, CodeGener
 			if (method.IsInterfaceDefaultMethod)
 				Attributes.Add (new CustomAttr ("[global::Java.Interop.JavaInterfaceDefaultMethod]"));
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, method, opt);
+
 			Attributes.Add (new RegisterAttr (method.JavaName, method.JniSignature, method.ConnectorName + ":" + method.GetAdapterName (opt, adapter), additionalProperties: method.AdditionalAttributeString ()));
 
 			method.JavadocInfo?.AddJavadocs (Comments);

tools/generator/SourceWriters/BoundInterfacePropertyDeclaration.cs

@@ -25,6 +25,8 @@ public BoundInterfacePropertyDeclaration (GenBase gen, Property property, string
 				if (property.Getter.GenericArguments?.Any () == true)
 					GetterAttributes.Add (new CustomAttr (property.Getter.GenericArguments.ToGeneratedAttributeString ()));
 
+				SourceWriterExtensions.AddSupportedOSPlatform (GetterAttributes, property.Getter, opt);
+
 				GetterAttributes.Add (new RegisterAttr (property.Getter.JavaName, property.Getter.JniSignature, property.Getter.ConnectorName + ":" + property.Getter.GetAdapterName (opt, adapter), additionalProperties: property.Getter.AdditionalAttributeString ()));
 			}
 
@@ -36,6 +38,8 @@ public BoundInterfacePropertyDeclaration (GenBase gen, Property property, string
 				if (property.Setter.GenericArguments?.Any () == true)
 					SetterAttributes.Add (new CustomAttr (property.Setter.GenericArguments.ToGeneratedAttributeString ()));
 
+				SourceWriterExtensions.AddSupportedOSPlatform (SetterAttributes, property.Setter, opt);
+
 				SetterAttributes.Add (new RegisterAttr (property.Setter.JavaName, property.Setter.JniSignature, property.Setter.ConnectorName + ":" + property.Setter.GetAdapterName (opt, adapter), additionalProperties: property.Setter.AdditionalAttributeString ()));
 			}
 		}

tools/generator/SourceWriters/BoundMethod.cs

@@ -70,6 +70,8 @@ public BoundMethod (GenBase type, Method method, CodeGenerationOptions opt, bool
 			if (method.IsReturnEnumified)
 				Attributes.Add (new GeneratedEnumAttr (true));
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, method, opt);
+
 			Attributes.Add (new RegisterAttr (method.JavaName, method.JniSignature, method.IsVirtual ? method.GetConnectorNameFull (opt) : string.Empty, additionalProperties: method.AdditionalAttributeString ()));
 
 			SourceWriterExtensions.AddMethodCustomAttributes (Attributes, method);

tools/generator/SourceWriters/BoundMethodAbstractDeclaration.cs

@@ -46,6 +46,8 @@ public BoundMethodAbstractDeclaration (GenBase gen, Method method, CodeGeneratio
 			if (method.DeclaringType.IsGeneratable)
 				Comments.Add ($"// Metadata.xml XPath method reference: path=\"{method.GetMetadataXPathReference (method.DeclaringType)}\"");
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, method, opt);
+
 			Attributes.Add (new RegisterAttr (method.JavaName, method.JniSignature, method.ConnectorName, additionalProperties: method.AdditionalAttributeString ()));
 
 			SourceWriterExtensions.AddMethodCustomAttributes (Attributes, method);

tools/generator/SourceWriters/BoundProperty.cs

@@ -74,6 +74,8 @@ public BoundProperty (GenBase gen, Property property, CodeGenerationOptions opt,
 			if (gen.IsGeneratable)
 				GetterComments.Add ($"// Metadata.xml XPath method reference: path=\"{gen.MetadataXPathReference}/method[@name='{property.Getter.JavaName}'{property.Getter.Parameters.GetMethodXPathPredicate ()}]\"");
 
+			SourceWriterExtensions.AddSupportedOSPlatform (GetterAttributes, property.Getter, opt);
+
 			GetterAttributes.Add (new RegisterAttr (property.Getter.JavaName, property.Getter.JniSignature, property.Getter.IsVirtual ? property.Getter.GetConnectorNameFull (opt) : string.Empty, additionalProperties: property.Getter.AdditionalAttributeString ()));
 
 			SourceWriterExtensions.AddMethodBody (GetBody, property.Getter, opt);
@@ -84,6 +86,8 @@ public BoundProperty (GenBase gen, Property property, CodeGenerationOptions opt,
 				if (gen.IsGeneratable)
 					SetterComments.Add ($"// Metadata.xml XPath method reference: path=\"{gen.MetadataXPathReference}/method[@name='{property.Setter.JavaName}'{property.Setter.Parameters.GetMethodXPathPredicate ()}]\"");
 
+				SourceWriterExtensions.AddSupportedOSPlatform (SetterAttributes, property.Setter, opt);
+
 				SourceWriterExtensions.AddMethodCustomAttributes (SetterAttributes, property.Setter);
 				SetterAttributes.Add (new RegisterAttr (property.Setter.JavaName, property.Setter.JniSignature, property.Setter.IsVirtual ? property.Setter.GetConnectorNameFull (opt) : string.Empty, additionalProperties: property.Setter.AdditionalAttributeString ()));
 

tools/generator/SourceWriters/Extensions/SourceWriterExtensions.cs

@@ -285,6 +285,15 @@ public static void AddParameterListCallArgs (List<string> body, ParameterList pa
 			}
 		}
 
+		public static void AddSupportedOSPlatform (List<AttributeWriter> attributes, ApiVersionsSupport.IApiAvailability member, 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 (member.ApiAvailableSince > 21 && opt.CodeGenerationTarget == Xamarin.Android.Binder.CodeGenerationTarget.XAJavaInterop1)
+				attributes.Add (new SupportedOSPlatformAttr (member.ApiAvailableSince));
+
+		}
+
 		public static void WriteMethodInvokerBody (CodeWriter writer, Method method, CodeGenerationOptions opt, string contextThis)
 		{
 			writer.WriteLine ($"if ({method.EscapedIdName} == IntPtr.Zero)");

tools/generator/SourceWriters/GenericExplicitInterfaceImplementationProperty.cs

@@ -27,6 +27,8 @@ public GenericExplicitInterfaceImplementationProperty (Property property, Generi
 				if (property.Getter.GenericArguments != null && property.Getter.GenericArguments.Any ())
 					GetterAttributes.Add (new CustomAttr (property.Getter.GenericArguments.ToGeneratedAttributeString ()));
 
+				SourceWriterExtensions.AddSupportedOSPlatform (GetterAttributes, property.Getter, opt);
+
 				GetterAttributes.Add (new RegisterAttr (property.Getter.JavaName, property.Getter.JniSignature, property.Getter.ConnectorName + ":" + property.Getter.GetAdapterName (opt, adapter), additionalProperties: property.Getter.AdditionalAttributeString ()));
 
 				GetBody.Add ($"return {property.Name};");
@@ -40,6 +42,8 @@ public GenericExplicitInterfaceImplementationProperty (Property property, Generi
 				if (property.Setter.GenericArguments != null && property.Setter.GenericArguments.Any ())
 					SetterAttributes.Add (new CustomAttr (property.Setter.GenericArguments.ToGeneratedAttributeString ()));
 
+				SourceWriterExtensions.AddSupportedOSPlatform (SetterAttributes, property.Setter, opt);
+
 				SetterAttributes.Add (new RegisterAttr (property.Setter.JavaName, property.Setter.JniSignature, property.Setter.ConnectorName + ":" + property.Setter.GetAdapterName (opt, adapter), additionalProperties: property.Setter.AdditionalAttributeString ()));
 
 				// Temporarily rename the parameter to "value"

tools/generator/SourceWriters/InterfaceMemberAlternativeClass.cs

@@ -36,6 +36,8 @@ public InterfaceMemberAlternativeClass (InterfaceGen iface, CodeGenerationOption
 
 			UsePriorityOrder = true;
 
+			SourceWriterExtensions.AddSupportedOSPlatform (Attributes, iface, opt);
+
 			Attributes.Add (new RegisterAttr (iface.RawJniName, noAcw: true, additionalProperties: iface.AdditionalAttributeString ()) { AcwLast = true });
 
 			if (should_obsolete)