Added a Godot TraceListener, which is automatically installed on startup. Fixes that Debug/Trace Assertions are simply swallowed by Godot.

modules/mono/glue/Managed/Files/DebuggingUtils.cs

@@ -19,6 +19,12 @@ namespace Godot
             sb.Append(" ");
         }
 
+        public static void InstallTraceListener()
+        {
+            Trace.Listeners.Clear();
+            Trace.Listeners.Add(new GodotTraceListener());
+        }
+
         public static void GetStackFrameInfo(StackFrame frame, out string fileName, out int fileLineNumber, out string methodDecl)
         {
             fileName = frame.GetFileName();

modules/mono/glue/Managed/Files/GodotTraceListener.cs

@@ -0,0 +1,37 @@
+using System;
+using System.Diagnostics;
+
+namespace Godot
+{
+    internal class GodotTraceListener : TraceListener
+    {
+        public override void Write(string message)
+        {
+            GD.PrintRaw(message);
+        }
+
+        public override void WriteLine(string message)
+        {
+            GD.Print(message);
+        }
+
+        public override void Fail(string message, string detailMessage)
+        {
+            GD.PrintErr("Assertion failed: ", message);
+            if (detailMessage != null)
+            {
+                GD.PrintErr("  Details: ", detailMessage);
+            }
+
+            try
+            {
+                var stackTrace = new StackTrace(true).ToString();
+                GD.PrintErr(stackTrace);
+            }
+            catch
+            {
+                // ignored
+            }
+        }
+    }
+}

modules/mono/mono_gd/gd_mono.cpp

@@ -587,6 +587,8 @@ bool GDMono::_load_core_api_assembly() {
 										CS_GLUE_VERSION != api_assembly_ver.cs_glue_version;
 		if (!core_api_assembly_out_of_sync) {
 			GDMonoUtils::update_godot_api_cache();
+
+			_install_trace_listener();
 		}
 #else
 		GDMonoUtils::update_godot_api_cache();
@@ -690,6 +692,23 @@ bool GDMono::_load_api_assemblies() {
 	return true;
 }
 
+void GDMono::_install_trace_listener() {
+
+#ifdef DEBUG_ENABLED
+	// Install the trace listener now before the project assembly is loaded
+	typedef void (*DebuggingUtils_InstallTraceListener)(MonoObject **);
+	MonoException *exc = NULL;
+	GDMonoClass *debug_utils = core_api_assembly->get_class(BINDINGS_NAMESPACE, "DebuggingUtils");
+	DebuggingUtils_InstallTraceListener install_func =
+			(DebuggingUtils_InstallTraceListener)debug_utils->get_method_thunk("InstallTraceListener");
+	install_func((MonoObject **)&exc);
+	if (exc) {
+		ERR_PRINT("Failed to install System.Diagnostics.Trace listener");
+		GDMonoUtils::debug_print_unhandled_exception(exc);
+	}
+#endif
+}
+
 #ifdef TOOLS_ENABLED
 String GDMono::_get_api_assembly_metadata_path() {
 

modules/mono/mono_gd/gd_mono.h

@@ -125,6 +125,8 @@ class GDMono {
 	String _get_api_assembly_metadata_path();
 #endif
 
+	void _install_trace_listener();
+
 	void _register_internal_calls();
 
 	Error _load_scripts_domain();