[Java.Interop.Tools.TypeNameMappings] fix trimmer warnings

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

In xamarin/xamarin-android, we import the source for
`Java.Interop.Tools.TypeNameMappings\JavaNativeTypeManager.cs`, and
unfortunately there are 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 cached the `Type.GetType()` call with `Lazy<T>`.

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`

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

Author: jonathanpeppers

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>8.0</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/JavaCallableWrapperGenerator.cs

@@ -203,7 +203,7 @@ void AddNestedTypes (TypeDefinition type)
 			};
 			foreach (var bt in type.GetBaseTypes (cache)) {
 				ctorTypes.Add (bt);
-				RegisterAttribute rattr = GetMethodRegistrationAttributes (bt).FirstOrDefault ();
+				RegisterAttribute? rattr = GetMethodRegistrationAttributes (bt).FirstOrDefault ();
 				if (rattr != null && rattr.DoNotGenerateAcw)
 					break;
 			}
@@ -296,7 +296,7 @@ void AddConstructor (MethodDefinition ctor, TypeDefinition type, string? outerTy
 					return;
 				}
 
-				ExportAttribute eattr = GetExportAttributes (ctor).FirstOrDefault ();
+				ExportAttribute? eattr = GetExportAttributes (ctor).FirstOrDefault ();
 				if (eattr != null) {
 					if (!string.IsNullOrEmpty (eattr.Name)) {
 						// Diagnostic.Warning (log, "Use of ExportAttribute.Name property is invalid on constructors");
@@ -306,7 +306,7 @@ void AddConstructor (MethodDefinition ctor, TypeDefinition type, string? outerTy
 					return;
 				}
 
-				RegisterAttribute rattr = GetMethodRegistrationAttributes (ctor).FirstOrDefault ();
+				RegisterAttribute? rattr = GetMethodRegistrationAttributes (ctor).FirstOrDefault ();
 				if (rattr != null) {
 					if (ctors.Any (c => c.JniSignature == rattr.Signature))
 						return;
@@ -513,7 +513,7 @@ static IEnumerable<ExportFieldAttribute> GetExportFieldAttributes (Mono.Cecil.IC
 		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);
 		}
 
 		static IEnumerable<TAttribute> GetAttributes<TAttribute> (Mono.Cecil.ICustomAttributeProvider p, string attributeName, Func<CustomAttribute, TAttribute?> selector)
@@ -702,13 +702,13 @@ static void WriteAnnotations (string indent, TextWriter sw, IEnumerable<CustomAt
 		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)
 				return ((bool) value) ? "true" : "false";
 			else
-				return value.ToString ();
+				return value.ToString () ?? "";
 		}
 
 		void GenerateHeader (TextWriter sw)
@@ -1160,7 +1160,7 @@ void WriteInstrumentationOnCreate (TextWriter sw, Action<TextWriter> extra)
 		StreamWriter OpenStream (string outputPath)
 		{
 			string 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/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

@@ -167,6 +167,10 @@ public static string ToJniName (Type type)
 				"java/lang/Object";
 		}
 
+		static readonly Lazy<Type> IJavaPeerableType = new Lazy<Type> (() =>
+			Type.GetType ("Java.Interop.IJavaPeerable, Java.Interop", throwOnError: true)!
+		);
+
 		static string? ToJniName (Type type, ExportParameterKind exportKind)
 		{
 			if (type == null)
@@ -178,8 +182,7 @@ 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 (IJavaPeerableType.Value.IsAssignableFrom (type))
 				return GetSpecialExportJniType (type.FullName!, exportKind);
 
 			return ToJniName (type, t => t.DeclaringType!, t => t.Name, GetPackageName, t => {