Here are the compile time warnings from retro-gobject:
log/file-stream-log.vala:9.2-9.56: warning: the modifier `static' is not applicable to constants
	private static const string default_color = "\033[39m";
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
core-factory.vala:5.2-5.11: warning: [Deprecated] is deprecated. Use [Version (deprecated = true, deprecated_since = "", replacement = "")]
hardware-render.vala:9.2-9.58: warning: the modifier `static' is not applicable to constants
	public static const ulong FRAME_BUFFER_VALID = ulong.MAX;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
retro.vala:9.1-9.41: warning: the modifier `static' is not applicable to constants
public static const uint API_VERSION = 1;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
retro.vala:45.9-45.34: warning: Config.PROJECT_PLUGINS_DIR has been deprecated since 0.8. Use Config.RETRO_PLUGIN_PATH
retro-module-query.vala:64.34-64.57: warning: unhandled error `GLib.Error'
		var dis = new DataInputStream (module_info_file.read ());
		                               ^^^^^^^^^^^^^^^^^^^^^^^^
retro-module-query.vala:65.19-65.38: warning: unhandled error `GLib.IOError'
		for (var line = dis.read_line (null) ; line != null ; line = dis.read_line (null)) {
		                ^^^^^^^^^^^^^^^^^^^^
retro-module-query.vala:65.64-65.83: warning: unhandled error `GLib.IOError'
		for (var line = dis.read_line (null) ; line != null ; line = dis.read_line (null)) {
		                                                             ^^^^^^^^^^^^^^^^^^^^

And here are the ones from retro-gtk:
retro-gobject-0.6.vapi:67.3-67.12: warning: [Deprecated] is deprecated. Use [Version (deprecated = true, deprecated_since = "", replacement = "")]
input/gamepad-configuration-dialog.vala:17.2-35.3: warning: the modifier `static' is not applicable to constants
input/gamepad-view.vala:9.2-9.67: warning: the modifier `static' is not applicable to constants
	private static const StateFlags DEFAULT_STATE = StateFlags.NORMAL;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
input/gamepad-view.vala:10.2-10.67: warning: the modifier `static' is not applicable to constants
	private static const StateFlags HIGHLIGHT_STATE = StateFlags.LINK;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
input/gamepad-view.vala:14.2-14.47: warning: the modifier `static' is not applicable to constants
	private static const double TAU = Math.PI * 2;
	^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
video/clutter-display.vala:9.2-9.11: warning: [Deprecated] is deprecated. Use [Version (deprecated = true, deprecated_since = "", replacement = "")]
retro-gtk.vala:12.2-12.11: warning: [Deprecated] is deprecated. Use [Version (deprecated = true, deprecated_since = "", replacement = "")]
input/mouse.vala:212.16-212.51: warning: Gdk.Cursor.new has been deprecated since 3.16
input/mouse.vala:213.3-213.13: warning: Gdk.Device.grab has been deprecated since 3.20.
input/mouse.vala:219.5-219.17: warning: Gdk.Device.ungrab has been deprecated since 3.20.
video/clutter-display.vala:93.3-93.26: warning: Clutter.Texture.set_cogl_texture has been deprecated since 1.12
video/clutter-display.vala:37.13-37.26: warning: Clutter.Texture.new has been deprecated since 1.12
video/clutter-display.vala:39.3-39.17: warning: Clutter.Container.add_actor has been deprecated since 1.10
input/gamepad-configuration-dialog.vala:79.7-79.39: warning: local variable `button_name' declared but never used
		var button_name = button.to_string ();
		    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^