[Java.Interop.Tools.TypeNameMappings] fix trimmer warnings (#1194)

Context: https://github.com/xamarin/xamarin-android/blob/e987ac458536e59a8329a06d5c5d5f4d4ea2c6b6/src/Mono.Android/Mono.Android.csproj#L69-L71

In xamarin/xamarin-android's `Mono.Android.dll`, we [import][0]
`Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs`, and
unfortunately that causes some trimmer warnings:

	external\Java.Interop\src\Java.Interop.Tools.TypeNameMappings\Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs(182,9):
	error IL2070: 'this' argument does not satisfy 'DynamicallyAccessedMemberTypes.Interfaces' in call to 'System.Type.GetInterfaces()'.
	  The parameter 'type' of method 'Java.Interop.Tools.TypeNameMappings.JavaNativeTypeManager.ToJniName(Type, ExportParameterKind)'
	  does not have matching annotations.
	  The source value must declare at least the same requirements as those declared on the target location it is assigned to.

From the code:

	if (!type.GetInterfaces ().Any (t => t.FullName == "Android.Runtime.IJavaObject"))

It appears we can instead look for `IJavaPeerable` and use trim-safe
behavior instead:

	if (Type.GetType ("Java.Interop.IJavaPeerable, Java.Interop", throwOnError: true)
	        .IsAssignableFrom (type))

I also cache the `Type.GetType()` call with `Lazy<T>`.

However, in some cases we run this code under an MSBuild context.
In this case, we don't have `Java.Interop.dll` to load, so we should
instead use a variation on the previous code:

	type.GetInterfaces ().Any (t => t.FullName == "Java.Interop.IJavaPeerable");

To catch warnings in this project going forward:

  * Multi-target to `netstandard2.0` and `net8.0` using
    `$(DotNetTargetFramework)`.
    
    `netstandard2.0` is used for MSBuild-task assemblies inside VS.

  * Enable trimmer warnings for `net8.0`.

Additionally, as I targeted `net8.0`, various null-reference-type
warnings appeared, which I also fixed.

[0]: https://github.com/xamarin/xamarin-android/blob/e199d62210bfb666595d95ca60579c5c766be1d6/src/Mono.Android/Mono.Android.csproj#L69-L71

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.Adapters/CecilImporter.cs

@@ -547,13 +547,13 @@ static IEnumerable<CallableWrapperTypeAnnotation> CreateAnnotations (ICustomAttr
 	static string ManagedValueToJavaSource (object value)
 	{
 		if (value is string)
-			return "\"" + value.ToString ().Replace ("\"", "\"\"") + '"';
+			return "\"" + value.ToString ()?.Replace ("\"", "\"\"") + '"';
 		else if (value.GetType ().FullName == "Java.Lang.Class")
 			return value.ToString () + ".class";
 		else if (value is bool v)
 			return v ? "true" : "false";
 		else
-			return value.ToString ();
+			return value.ToString () ?? "";
 	}
 
 	static string GetJavaAccess (MethodAttributes access)

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.CallableWrapperMembers/CallableWrapperType.cs

@@ -315,7 +315,7 @@ public void Generate (string outputPath, CallableWrapperWriterOptions options)
 	StreamWriter OpenStream (string outputPath)
 	{
 		var destination = GetDestinationPath (outputPath);
-		Directory.CreateDirectory (Path.GetDirectoryName (destination));
+		Directory.CreateDirectory (Path.GetDirectoryName (destination)!);
 
 		return new StreamWriter (new FileStream (destination, FileMode.Create, FileAccess.Write));
 	}

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers.csproj

@@ -1,12 +1,18 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <XAConfigPath>..\..\bin\Build$(Configuration)\XAConfig.props</XAConfigPath>
+  </PropertyGroup>
+  <Import Condition="Exists ('$(XAConfigPath)')" Project="$(XAConfigPath)" />
+
+  <PropertyGroup>
+    <TargetFrameworks>netstandard2.0;$(DotNetTargetFramework)</TargetFrameworks>
     <AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
     <LangVersion>11.0</LangVersion>
     <Nullable>enable</Nullable>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <SignAssembly>true</SignAssembly>
+    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
     <DefineConstants>$(DefineConstants);JCW_ONLY_TYPE_NAMES;HAVE_CECIL</DefineConstants>
   </PropertyGroup>
@@ -17,13 +23,19 @@
     <OutputPath>$(ToolOutputFullPath)</OutputPath>
   </PropertyGroup>
 
+  <PropertyGroup Condition=" '$(TargetFramework)' != 'netstandard2.0' ">
+    <IsTrimmable>true</IsTrimmable>
+    <EnableSingleFileAnalyzer>true</EnableSingleFileAnalyzer>
+    <EnableAotAnalyzer>true</EnableAotAnalyzer>
+  </PropertyGroup>
+
   <Import Project="..\..\build-tools\scripts\cecil.projitems" />
 
   <ItemGroup>
     <Compile Include="..\Java.Interop.Tools.TypeNameMappings\Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs">
       <Link>JavaNativeTypeManager.cs</Link>
     </Compile>
-    <Compile Include="..\utils\NullableAttributes.cs">
+    <Compile Include="..\utils\NullableAttributes.cs" Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
       <Link>NullableAttributes.cs</Link>
     </Compile>
   </ItemGroup>

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/CecilExtensions.cs

@@ -22,7 +22,7 @@ public static IEnumerable<ExportFieldAttribute> GetExportFieldAttributes (Mono.C
 	public static IEnumerable<TAttribute> GetAttributes<TAttribute> (Mono.Cecil.ICustomAttributeProvider p, Func<CustomAttribute, TAttribute?> selector)
 			where TAttribute : class
 	{
-		return GetAttributes (p, typeof (TAttribute).FullName, selector);
+		return GetAttributes (p, typeof (TAttribute).FullName!, selector);
 	}
 
 	public static IEnumerable<TAttribute> GetAttributes<TAttribute> (Mono.Cecil.ICustomAttributeProvider p, string attributeName, Func<CustomAttribute, TAttribute?> selector)

src/Java.Interop.Tools.JavaCallableWrappers/Java.Interop.Tools.JavaCallableWrappers/TypeNameMapGenerator.cs

@@ -179,7 +179,7 @@ Dictionary<string, string> GetTypeMapping (Func<TypeDefinition, bool> skipType,
 
 				var k = key (type, Cache);
 
-				TypeDefinition e;
+				TypeDefinition? e;
 				if (!typeMap.TryGetValue (k, out e)) {
 					typeMap.Add (k, type);
 				} else if (type.IsAbstract || type.IsInterface || e.IsAbstract || e.IsInterface) {
@@ -190,7 +190,7 @@ Dictionary<string, string> GetTypeMapping (Func<TypeDefinition, bool> skipType,
 						b = type;
 					typeMap [k] = b;
 				} else {
-					List<string> a;
+					List<string>? a;
 					if (!aliases.TryGetValue (k, out a)) {
 						aliases.Add (k, a = new List<string> ());
 						a.Add (value (e, Cache));
@@ -266,7 +266,7 @@ public void WriteManagedToJava (Stream output)
 
 	class ArrayComparer<T> : IComparer<T[]> {
 
-		public int Compare (T[] x, T[] y)
+		public int Compare (T []? x, T []? y)
 		{
 			if (x == null && y == null)
 				return 0;

src/Java.Interop.Tools.TypeNameMappings/Java.Interop.Tools.TypeNameMappings/JavaNativeTypeManager.cs

@@ -178,15 +178,29 @@ public static string ToJniName (Type type)
 			if (type == typeof (string))
 				return "java/lang/String";
 
-
-			if (!type.GetInterfaces ().Any (t => t.FullName == "Android.Runtime.IJavaObject"))
+			if (ShouldCheckSpecialExportJniType (type))
 				return GetSpecialExportJniType (type.FullName!, exportKind);
 
 			return ToJniName (type, t => t.DeclaringType!, t => t.Name, GetPackageName, t => {
 				return ToJniNameFromAttributes (t);
 			}, _ => false);
 		}
 
+#if !NETSTANDARD2_0
+		static readonly Lazy<Type> IJavaPeerableType = new Lazy<Type> (() =>
+			Type.GetType ("Java.Interop.IJavaPeerable, Java.Interop", throwOnError: true)!
+		);
+#endif
+
+		// NOTE: NETSTANDARD2_0 could be running in an MSBuild context where Java.Interop.dll is not available.
+		// Trimming warnings are not enabled for netstandard2.0 in this project.
+		static bool ShouldCheckSpecialExportJniType (Type type) =>
+#if NETSTANDARD2_0
+			type.GetInterfaces ().Any (t => t.FullName == "Java.Interop.IJavaPeerable");
+#else
+			IJavaPeerableType.Value.IsAssignableFrom (type);
+#endif
+
 		public static string ToJniName (string jniType, int rank)
 		{
 			if (rank == 0)