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, 0 insertions, 80 deletions

diff --git a/modules/mono/build_scripts/api_solution_build.py b/modules/mono/build_scripts/api_solution_build.py
deleted file mode 100644
index 9abac22df6..0000000000
--- a/modules/mono/build_scripts/api_solution_build.py
+++ /dev/null
@@ -1,80 +0,0 @@
-# Build the Godot API solution
-
-import os
-
-from SCons.Script import Dir
-
-
-def build_api_solution(source, target, env):
-    # source and target elements are of type SCons.Node.FS.File, hence why we convert them to str
-
-    module_dir = env["module_dir"]
-
-    solution_path = os.path.join(module_dir, "glue/GodotSharp/GodotSharp.sln")
-
-    build_config = env["solution_build_config"]
-
-    extra_msbuild_args = ["/p:NoWarn=1591"]  # Ignore missing documentation warnings
-
-    from .solution_builder import build_solution
-
-    build_solution(env, solution_path, build_config, extra_msbuild_args=extra_msbuild_args)
-
-    # Copy targets
-
-    core_src_dir = os.path.abspath(os.path.join(solution_path, os.pardir, "GodotSharp", "bin", build_config))
-    editor_src_dir = os.path.abspath(os.path.join(solution_path, os.pardir, "GodotSharpEditor", "bin", build_config))
-
-    dst_dir = os.path.abspath(os.path.join(str(target[0]), os.pardir))
-
-    if not os.path.isdir(dst_dir):
-        assert not os.path.isfile(dst_dir)
-        os.makedirs(dst_dir)
-
-    def copy_target(target_path):
-        from shutil import copy
-
-        filename = os.path.basename(target_path)
-
-        src_path = os.path.join(core_src_dir, filename)
-        if not os.path.isfile(src_path):
-            src_path = os.path.join(editor_src_dir, filename)
-
-        copy(src_path, target_path)
-
-    for scons_target in target:
-        copy_target(str(scons_target))
-
-
-def build(env_mono):
-    assert env_mono["tools"]
-
-    target_filenames = [
-        "GodotSharp.dll",
-        "GodotSharp.pdb",
-        "GodotSharp.xml",
-        "GodotSharpEditor.dll",
-        "GodotSharpEditor.pdb",
-        "GodotSharpEditor.xml",
-    ]
-
-    depend_cmd = []
-
-    for build_config in ["Debug", "Release"]:
-        output_dir = Dir("#bin").abspath
-        editor_api_dir = os.path.join(output_dir, "GodotSharp", "Api", build_config)
-
-        targets = [os.path.join(editor_api_dir, filename) for filename in target_filenames]
-
-        cmd = env_mono.CommandNoCache(
-            targets, depend_cmd, build_api_solution, module_dir=os.getcwd(), solution_build_config=build_config
-        )
-        env_mono.AlwaysBuild(cmd)
-
-        # Make the Release build of the API solution depend on the Debug build.
-        # We do this in order to prevent SCons from building them in parallel,
-        # which can freak out MSBuild. In many cases, one of the builds would
-        # hang indefinitely requiring a key to be pressed for it to continue.
-        depend_cmd = cmd
-
-    return depend_cmd