[XABT] Avoid non-blittable types in native callback methods

[jpobst]

Context: dotnet/java-interop#1296
Context: dotnet/java-interop#1027

Changing callback methods to blittable types (bool -> sbyte and char -> ushort) causes warnings when the marshal method classifier verifies that the marshal types match the native types:

Xamarin.Android.Common.targets(1502,3): Method 'System.SByte Java.Lang.Object::n_Equals_Ljava_lang_Object_(System.IntPtr,System.IntPtr,System.IntPtr)' doesn't match native callback signature (invalid return type: expected 'System.Boolean', found 'System.SByte')
Xamarin.Android.Common.targets(1502,3): Unable to find native callback method 'n_Equals_Ljava_lang_Object_' in type 'Java.Lang.Object', matching the 'System.Boolean Java.Lang.Object::Equals(Java.Lang.Object)' signature (jniName: 'equals') [Arch: X86_64; Assembly: obj/Release/android-x64/linked/Mono.Android.dll]
Xamarin.Android.Common.targets(1502,3): Method 'System.Boolean Java.Lang.Object::Equals(Java.Lang.Object)' will be registered dynamically [Arch: X86_64; Assembly: obj/Release/android-x64/linked/Mono.Android.dll]
Xamarin.Android.Common.targets(1502,3): Method 'System.SByte Java.Lang.Object::n_Equals_Ljava_lang_Object_(System.IntPtr,System.IntPtr,System.IntPtr)' doesn't match native callback signature (invalid return type: expected 'System.Boolean', found 'System.SByte')
Xamarin.Android.Common.targets(1502,3): Unable to find native callback method 'n_Equals_Ljava_lang_Object_' in type 'Java.Lang.Object', matching the 'System.Boolean Java.Lang.Object::Equals(Java.Lang.Object)' signature (jniName: 'equals') [Arch: Arm64; Assembly: obj/Release/android-arm64/linked/Mono.Android.dll]
Xamarin.Android.Common.targets(1502,3): Method 'System.Boolean Java.Lang.Object::Equals(Java.Lang.Object)' will be registered dynamically [Arch: Arm64; Assembly: obj/Release/android-arm64/linked/Mono.Android.dll]
Xamarin.Android.Common.targets(1502,3): Type 'Android.Runtime.JavaObject, Mono.Android, Version=0.0.0.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065' will register some of its Java override methods dynamically. This may adversely affect runtime performance. See preceding warnings for names of dynamically registered methods.
Xamarin.Android.Common.targets(1502,3): [Arm64] Number of methods in the project that will be registered dynamically: 1
Xamarin.Android.Common.targets(1502,3): [X86_64] Number of methods in the project that will be registered dynamically: 1

Fix this by treating our blittable types as equivalent to their native types in the marshal method classifier.

Additionally, Mono.Android has "built-in delegates" that marshal bool types (eg: _JniMarshal_PP_Z). Because generator no longer creates these delegates (it will now create _JniMarshal_PP_B instead), we need to add them manually.

[jonpryor]

I would prefer the use of "named ValueTuple<…>":

static readonly (string Source, string Replacement)[] equivalent_types = new[]{
    (Source: "System.Boolean", Replacement: "System.SByte"),
    (Source: "System.Char", Replacement: "System.UInt16"),
};

Then TypeMatches() can use .Source instead of .Item1, etc.

[jonpryor]

The need for this entire method confuses me.

From the PR message:

Changing callback methods to blittable types (bool -> sbyte and char -> ushort) causes warnings when the marshal method classifier verifies that the marshal types match the native types:

Xamarin.Android.Common.targets(1502,3): Method 'System.SByte Java.Lang.Object::n_Equals_Ljava_lang_Object_(System.IntPtr,System.IntPtr,System.IntPtr)' doesn't match native callback signature (invalid return type: expected 'System.Boolean', found 'System.SByte')

but why? The invalid return type: expected message is from here:

android/src/Xamarin.Android.Build.Tasks/Utilities/MarshalMethodsClassifier.cs

Lines 211 to 214 in dbda951

	if (String.Compare (returnType, method.ReturnType.FullName, StringComparison.Ordinal) != 0) {
	log.LogWarning ($"Method '{method.FullName}' doesn't match native callback signature (invalid return type: expected '{returnType}', found '{method.ReturnType.FullName}')");
	return false;
	}

returnType is MapType(target.ReturnType), which uses the verbatimTypes set, and thus should be System.SByte (which is in verbatimTypes), but apparently isn't?

Why doesn't the existing code work? Why is this method needed?

[jpobst]

If I'm understanding what this code does, it is trying to match bound methods to their native callback methods.

Thus, given this bound method:

[Register ("markSupported", "()Z", "GetMarkSupportedHandler")]
public virtual unsafe bool MarkSupported () { ... }

It is trying to find this callback with a matching signature:

static bool n_MarkSupported (IntPtr jnienv, IntPtr native__this) { ... }

However, because the native callback is now sbyte instead of bool the return types no longer match and thus the "new" callback is rejected because bool != sbyte:

static sbyte n_MarkSupported (IntPtr jnienv, IntPtr native__this) { ... }

The method I added simply says that for this purpose bool and sbyte are the same.