C#: Move marshaling logic and generated glue to C#

We will be progressively moving most code to C#.
The plan is to only use Mono's embedding APIs to set things at launch.
This will make it much easier to later support CoreCLR too which
doesn't have rich embedding APIs.

Additionally the code in C# is more maintainable and makes it easier
to implement new features, e.g.: runtime codegen which we could use to
avoid using reflection for marshaling everytime a field, property or
method is accessed.

SOME NOTES ON INTEROP

We make the same assumptions as GDNative about the size of the Godot
structures we use. We take it a bit further by also assuming the layout
of fields in some cases, which is riskier but let's us squeeze out some
performance by avoiding unnecessary managed to native calls.

Code that deals with native structs is less safe than before as there's
no RAII and copy constructors in C#. It's like using the GDNative C API
directly. One has to take special care to free values they own.
Perhaps we could use roslyn analyzers to check this, but I don't know
any that uses attributes to determine what's owned or borrowed.

As to why we maily use pointers for native structs instead of ref/out:
- AFAIK (and confirmed with a benchmark) ref/out are pinned
  during P/Invoke calls and that has a cost.
- Native struct fields can't be ref/out in the first place.
- A `using` local can't be passed as ref/out, only `in`. Calling a
  method or property on an `in` value makes a silent copy, so we want
  to avoid `in`.

REGARDING THE BUILD SYSTEM

There's no longer a `mono_glue=yes/no` SCons options. We no longer
need to build with `mono_glue=no`, generate the glue and then build
again with `mono_glue=yes`. We build only once and generate the glue
(which is in C# now).
However, SCons no longer builds the C# projects for us. Instead one
must run `build_assemblies.py`, e.g.:
```sh
%godot_src_root%/modules/mono/build_scripts/build_assemblies.py \
        --godot-output-dir=%godot_src_root%/bin \
        --godot-target=release_debug`
```
We could turn this into a custom build target, but I don't know how
to do that with SCons (it's possible with Meson).

OTHER NOTES

Most of the moved code doesn't follow the C# naming convention and
still has the word Mono in the names despite no longer dealing with
Mono's embedding APIs. This is just temporary while transitioning,
to make it easier to understand what was moved where.

1 files changed, 40 insertions, 114 deletions

diff --git a/modules/mono/glue/collections_glue.cpp b/modules/mono/glue/collections_glue.cpp
index 8a9f30459c..30adea60cc 100644
--- a/modules/mono/glue/collections_glue.cpp
+++ b/modules/mono/glue/collections_glue.cpp
@@ -5,8 +5,8 @@
 /*                           GODOT ENGINE                                */
 /*                      https://godotengine.org                          */
 /*************************************************************************/
-/* Copyright (c) 2007-2022 Juan Linietsky, Ariel Manzur.                 */
-/* Copyright (c) 2014-2022 Godot Engine contributors (cf. AUTHORS.md).   */
+/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2021 Godot Engine contributors (cf. AUTHORS.md).   */
 /*                                                                       */
 /* Permission is hereby granted, free of charge, to any person obtaining */
 /* a copy of this software and associated documentation files (the       */
@@ -28,8 +28,6 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
-#ifdef MONO_GLUE_ENABLED
-
 #include <mono/metadata/exception.h>
 
 #include "core/variant/array.h"
@@ -39,28 +37,17 @@
 #include "../mono_gd/gd_mono_marshal.h"
 #include "../mono_gd/gd_mono_utils.h"
 
-Array *godot_icall_Array_Ctor() {
-	return memnew(Array);
-}
-
-void godot_icall_Array_Dtor(Array *ptr) {
-	memdelete(ptr);
-}
-
-MonoObject *godot_icall_Array_At(Array *ptr, int32_t index) {
-	if (index < 0 || index >= ptr->size()) {
-		GDMonoUtils::set_pending_exception(mono_get_exception_index_out_of_range());
-		return nullptr;
-	}
-	return GDMonoMarshal::variant_to_mono_object(ptr->operator[](index));
+void godot_icall_Array_Ctor(Array *r_dest) {
+	memnew_placement(r_dest, Array);
 }
 
-MonoObject *godot_icall_Array_At_Generic(Array *ptr, int32_t index, uint32_t type_encoding, GDMonoClass *type_class) {
+void godot_icall_Array_At(Array *ptr, int32_t index, Variant *r_elem) {
 	if (index < 0 || index >= ptr->size()) {
 		GDMonoUtils::set_pending_exception(mono_get_exception_index_out_of_range());
-		return nullptr;
+		*r_elem = Variant();
+		return;
 	}
-	return GDMonoMarshal::variant_to_mono_object(ptr->operator[](index), ManagedType(type_encoding, type_class));
+	*r_elem = ptr->operator[](index);
 }
 
 void godot_icall_Array_SetAt(Array *ptr, int32_t index, MonoObject *value) {
@@ -104,29 +91,27 @@ void godot_icall_Array_CopyTo(Array *ptr, MonoArray *array, int32_t array_index)
 	}
 }
 
-Array *godot_icall_Array_Ctor_MonoArray(MonoArray *mono_array) {
-	Array *godot_array = memnew(Array);
+void godot_icall_Array_Ctor_MonoArray(MonoArray *mono_array, Array *r_dest) {
+	memnew_placement(r_dest, Array);
 	unsigned int count = mono_array_length(mono_array);
-	godot_array->resize(count);
+	r_dest->resize(count);
 	for (unsigned int i = 0; i < count; i++) {
 		MonoObject *item = mono_array_get(mono_array, MonoObject *, i);
-		godot_icall_Array_SetAt(godot_array, i, item);
+		godot_icall_Array_SetAt(r_dest, i, item);
 	}
-	return godot_array;
 }
 
-Array *godot_icall_Array_Duplicate(Array *ptr, MonoBoolean deep) {
-	return memnew(Array(ptr->duplicate(deep)));
+void godot_icall_Array_Duplicate(Array *ptr, MonoBoolean deep, Array *r_dest) {
+	memnew_placement(r_dest, Array(ptr->duplicate(deep)));
 }
 
-Array *godot_icall_Array_Concatenate(Array *left, Array *right) {
+void godot_icall_Array_Concatenate(Array *left, Array *right, Array *r_dest) {
 	int count = left->size() + right->size();
-	Array *new_array = memnew(Array(left->duplicate(false)));
-	new_array->resize(count);
+	memnew_placement(r_dest, Array(left->duplicate(false)));
+	r_dest->resize(count);
 	for (unsigned int i = 0; i < (unsigned int)right->size(); i++) {
-		new_array->operator[](i + left->size()) = right->operator[](i);
+		r_dest->operator[](i + left->size()) = right->operator[](i);
 	}
-	return new_array;
 }
 
 int32_t godot_icall_Array_IndexOf(Array *ptr, MonoObject *item) {
@@ -166,41 +151,15 @@ void godot_icall_Array_Shuffle(Array *ptr) {
 	ptr->shuffle();
 }
 
-void godot_icall_Array_Generic_GetElementTypeInfo(MonoReflectionType *refltype, uint32_t *type_encoding, GDMonoClass **type_class) {
-	MonoType *elem_type = mono_reflection_type_get_type(refltype);
-
-	*type_encoding = mono_type_get_type(elem_type);
-	MonoClass *type_class_raw = mono_class_from_mono_type(elem_type);
-	*type_class = GDMono::get_singleton()->get_class(type_class_raw);
-}
-
 MonoString *godot_icall_Array_ToString(Array *ptr) {
 	return GDMonoMarshal::mono_string_from_godot(Variant(*ptr).operator String());
 }
 
-Dictionary *godot_icall_Dictionary_Ctor() {
-	return memnew(Dictionary);
-}
-
-void godot_icall_Dictionary_Dtor(Dictionary *ptr) {
-	memdelete(ptr);
-}
-
-MonoObject *godot_icall_Dictionary_GetValue(Dictionary *ptr, MonoObject *key) {
-	Variant *ret = ptr->getptr(GDMonoMarshal::mono_object_to_variant(key));
-	if (ret == nullptr) {
-		MonoObject *exc = mono_object_new(mono_domain_get(), CACHED_CLASS(KeyNotFoundException)->get_mono_ptr());
-#ifdef DEBUG_ENABLED
-		CRASH_COND(!exc);
-#endif
-		GDMonoUtils::runtime_object_init(exc, CACHED_CLASS(KeyNotFoundException));
-		GDMonoUtils::set_pending_exception((MonoException *)exc);
-		return nullptr;
-	}
-	return GDMonoMarshal::variant_to_mono_object(ret);
+void godot_icall_Dictionary_Ctor(Dictionary *r_dest) {
+	memnew_placement(r_dest, Dictionary);
 }
 
-MonoObject *godot_icall_Dictionary_GetValue_Generic(Dictionary *ptr, MonoObject *key, uint32_t type_encoding, GDMonoClass *type_class) {
+void godot_icall_Dictionary_GetValue(Dictionary *ptr, MonoObject *key, Variant *r_value) {
 	Variant *ret = ptr->getptr(GDMonoMarshal::mono_object_to_variant(key));
 	if (ret == nullptr) {
 		MonoObject *exc = mono_object_new(mono_domain_get(), CACHED_CLASS(KeyNotFoundException)->get_mono_ptr());
@@ -209,42 +168,37 @@ MonoObject *godot_icall_Dictionary_GetValue_Generic(Dictionary *ptr, MonoObject
 #endif
 		GDMonoUtils::runtime_object_init(exc, CACHED_CLASS(KeyNotFoundException));
 		GDMonoUtils::set_pending_exception((MonoException *)exc);
-		return nullptr;
+		*r_value = Variant();
+		return;
 	}
-	return GDMonoMarshal::variant_to_mono_object(ret, ManagedType(type_encoding, type_class));
+	*r_value = *ret;
 }
 
 void godot_icall_Dictionary_SetValue(Dictionary *ptr, MonoObject *key, MonoObject *value) {
 	ptr->operator[](GDMonoMarshal::mono_object_to_variant(key)) = GDMonoMarshal::mono_object_to_variant(value);
 }
 
-Array *godot_icall_Dictionary_Keys(Dictionary *ptr) {
-	return memnew(Array(ptr->keys()));
+void godot_icall_Dictionary_Keys(Dictionary *ptr, Array *r_dest) {
+	memnew_placement(r_dest, Array(ptr->keys()));
 }
 
-Array *godot_icall_Dictionary_Values(Dictionary *ptr) {
-	return memnew(Array(ptr->values()));
+void godot_icall_Dictionary_Values(Dictionary *ptr, Array *r_dest) {
+	memnew_placement(r_dest, Array(ptr->values()));
 }
 
 int32_t godot_icall_Dictionary_Count(Dictionary *ptr) {
 	return ptr->size();
 }
 
-int32_t godot_icall_Dictionary_KeyValuePairs(Dictionary *ptr, Array **keys, Array **values) {
-	*keys = godot_icall_Dictionary_Keys(ptr);
-	*values = godot_icall_Dictionary_Values(ptr);
-	return godot_icall_Dictionary_Count(ptr);
-}
-
-void godot_icall_Dictionary_KeyValuePairAt(Dictionary *ptr, int index, MonoObject **key, MonoObject **value) {
-	*key = GDMonoMarshal::variant_to_mono_object(ptr->get_key_at_index(index));
-	*value = GDMonoMarshal::variant_to_mono_object(ptr->get_value_at_index(index));
+int32_t godot_icall_Dictionary_KeyValuePairs(Dictionary *ptr, Array *keys, Array *values) {
+	memnew_placement(keys, Array(ptr->keys()));
+	memnew_placement(values, Array(ptr->values()));
+	return ptr->size();
 }
 
-void godot_icall_Dictionary_KeyValuePairAt_Generic(Dictionary *ptr, int index, MonoObject **key, MonoObject **value, uint32_t value_type_encoding, GDMonoClass *value_type_class) {
-	ManagedType type(value_type_encoding, value_type_class);
-	*key = GDMonoMarshal::variant_to_mono_object(ptr->get_key_at_index(index));
-	*value = GDMonoMarshal::variant_to_mono_object(ptr->get_value_at_index(index), type);
+void godot_icall_Dictionary_KeyValuePairAt(Dictionary *ptr, int index, Variant *r_key, Variant *r_value) {
+	memnew_placement(r_key, Variant(ptr->get_key_at_index(index)));
+	memnew_placement(r_value, Variant(ptr->get_value_at_index(index)));
 }
 
 void godot_icall_Dictionary_Add(Dictionary *ptr, MonoObject *key, MonoObject *value) {
@@ -271,8 +225,8 @@ MonoBoolean godot_icall_Dictionary_ContainsKey(Dictionary *ptr, MonoObject *key)
 	return ptr->has(GDMonoMarshal::mono_object_to_variant(key));
 }
 
-Dictionary *godot_icall_Dictionary_Duplicate(Dictionary *ptr, MonoBoolean deep) {
-	return memnew(Dictionary(ptr->duplicate(deep)));
+void godot_icall_Dictionary_Duplicate(Dictionary *ptr, MonoBoolean deep, Dictionary *r_dest) {
+	memnew_placement(r_dest, Dictionary(ptr->duplicate(deep)));
 }
 
 MonoBoolean godot_icall_Dictionary_RemoveKey(Dictionary *ptr, MonoObject *key) {
@@ -292,34 +246,16 @@ MonoBoolean godot_icall_Dictionary_Remove(Dictionary *ptr, MonoObject *key, Mono
 	return false;
 }
 
-MonoBoolean godot_icall_Dictionary_TryGetValue(Dictionary *ptr, MonoObject *key, MonoObject **value) {
-	Variant *ret = ptr->getptr(GDMonoMarshal::mono_object_to_variant(key));
-	if (ret == nullptr) {
-		*value = nullptr;
-		return false;
-	}
-	*value = GDMonoMarshal::variant_to_mono_object(ret);
-	return true;
-}
-
-MonoBoolean godot_icall_Dictionary_TryGetValue_Generic(Dictionary *ptr, MonoObject *key, MonoObject **value, uint32_t type_encoding, GDMonoClass *type_class) {
+MonoBoolean godot_icall_Dictionary_TryGetValue(Dictionary *ptr, MonoObject *key, Variant *value) {
 	Variant *ret = ptr->getptr(GDMonoMarshal::mono_object_to_variant(key));
 	if (ret == nullptr) {
-		*value = nullptr;
+		*value = Variant();
 		return false;
 	}
-	*value = GDMonoMarshal::variant_to_mono_object(ret, ManagedType(type_encoding, type_class));
+	*value = *ret;
 	return true;
 }
 
-void godot_icall_Dictionary_Generic_GetValueTypeInfo(MonoReflectionType *refltype, uint32_t *type_encoding, GDMonoClass **type_class) {
-	MonoType *value_type = mono_reflection_type_get_type(refltype);
-
-	*type_encoding = mono_type_get_type(value_type);
-	MonoClass *type_class_raw = mono_class_from_mono_type(value_type);
-	*type_class = GDMono::get_singleton()->get_class(type_class_raw);
-}
-
 MonoString *godot_icall_Dictionary_ToString(Dictionary *ptr) {
 	return GDMonoMarshal::mono_string_from_godot(Variant(*ptr).operator String());
 }
@@ -327,9 +263,7 @@ MonoString *godot_icall_Dictionary_ToString(Dictionary *ptr) {
 void godot_register_collections_icalls() {
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Ctor", godot_icall_Array_Ctor);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Ctor_MonoArray", godot_icall_Array_Ctor_MonoArray);
-	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Dtor", godot_icall_Array_Dtor);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_At", godot_icall_Array_At);
-	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_At_Generic", godot_icall_Array_At_Generic);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_SetAt", godot_icall_Array_SetAt);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Count", godot_icall_Array_Count);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Add", godot_icall_Array_Add);
@@ -344,20 +278,16 @@ void godot_register_collections_icalls() {
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_RemoveAt", godot_icall_Array_RemoveAt);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Resize", godot_icall_Array_Resize);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Shuffle", godot_icall_Array_Shuffle);
-	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_Generic_GetElementTypeInfo", godot_icall_Array_Generic_GetElementTypeInfo);
 	GDMonoUtils::add_internal_call("Godot.Collections.Array::godot_icall_Array_ToString", godot_icall_Array_ToString);
 
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Ctor", godot_icall_Dictionary_Ctor);
-	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Dtor", godot_icall_Dictionary_Dtor);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_GetValue", godot_icall_Dictionary_GetValue);
-	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_GetValue_Generic", godot_icall_Dictionary_GetValue_Generic);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_SetValue", godot_icall_Dictionary_SetValue);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Keys", godot_icall_Dictionary_Keys);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Values", godot_icall_Dictionary_Values);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Count", godot_icall_Dictionary_Count);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_KeyValuePairs", godot_icall_Dictionary_KeyValuePairs);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_KeyValuePairAt", godot_icall_Dictionary_KeyValuePairAt);
-	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_KeyValuePairAt_Generic", godot_icall_Dictionary_KeyValuePairAt_Generic);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Add", godot_icall_Dictionary_Add);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Clear", godot_icall_Dictionary_Clear);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Contains", godot_icall_Dictionary_Contains);
@@ -366,9 +296,5 @@ void godot_register_collections_icalls() {
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_RemoveKey", godot_icall_Dictionary_RemoveKey);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Remove", godot_icall_Dictionary_Remove);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_TryGetValue", godot_icall_Dictionary_TryGetValue);
-	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_TryGetValue_Generic", godot_icall_Dictionary_TryGetValue_Generic);
-	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_Generic_GetValueTypeInfo", godot_icall_Dictionary_Generic_GetValueTypeInfo);
 	GDMonoUtils::add_internal_call("Godot.Collections.Dictionary::godot_icall_Dictionary_ToString", godot_icall_Dictionary_ToString);
 }
-
-#endif // MONO_GLUE_ENABLED