Implement the %command% placeholder in the Main Run Args setting

core/config/project_settings.cpp

@@ -1104,6 +1104,8 @@ ProjectSettings::ProjectSettings() {
 	}
 	extensions.push_back("shader");
 
+	GLOBAL_DEF("editor/run/main_run_args", "");
+
 	GLOBAL_DEF("editor/script/search_in_file_extensions", extensions);
 	custom_prop_info["editor/script/search_in_file_extensions"] = PropertyInfo(Variant::PACKED_STRING_ARRAY, "editor/script/search_in_file_extensions");
 

doc/classes/OS.xml

@@ -143,7 +143,7 @@
 				Returns the command-line arguments passed to the engine.
 				Command-line arguments can be written in any form, including both [code]--key value[/code] and [code]--key=value[/code] forms so they can be properly parsed, as long as custom command-line arguments do not conflict with engine arguments.
 				You can also incorporate environment variables using the [method get_environment] method.
-				You can set [code]editor/main_run_args[/code] in the Project Settings to define command-line arguments to be passed by the editor when running the project.
+				You can set [member ProjectSettings.editor/run/main_run_args] to define command-line arguments to be passed by the editor when running the project.
 				Here's a minimal example on how to parse command-line arguments into a dictionary using the [code]--key=value[/code] form for arguments:
 				[codeblocks]
 				[gdscript]

doc/classes/ProjectSettings.xml

@@ -542,6 +542,14 @@
 		<member name="editor/node_naming/name_num_separator" type="int" setter="" getter="" default="0">
 			What to use to separate node name from number. This is mostly an editor setting.
 		</member>
+		<member name="editor/run/main_run_args" type="String" setter="" getter="" default="&quot;&quot;">
+			The command-line arguments to append to Godot's own command line when running the project. This doesn't affect the editor itself.
+			It is possible to make another executable run Godot by using the [code]%command%[/code] placeholder. The placeholder will be replaced with Godot's own command line. Program-specific arguments should be placed [i]before[/i] the placeholder, whereas Godot-specific arguments should be placed [i]after[/i] the placeholder.
+			For example, this can be used to force the project to run on the dedicated GPU in a NVIDIA Optimus system on Linux:
+			[codeblock]
+			prime-run %command%
+			[/codeblock]
+		</member>
 		<member name="editor/script/search_in_file_extensions" type="PackedStringArray" setter="" getter="" default="PackedStringArray( &quot;gd&quot;, &quot;shader&quot; )">
 			Text-based file extensions to include in the script editor's "Find in Files" feature. You can add e.g. [code]tscn[/code] if you wish to also parse your scene files, especially if you use built-in scripts which are serialized in the scene files.
 		</member>

editor/editor_node.cpp

@@ -5877,8 +5877,6 @@ EditorNode::EditorNode() {
 
 	register_exporters();
 
-	GLOBAL_DEF("editor/run/main_run_args", "");
-
 	ClassDB::set_class_enabled("RootMotionView", true);
 
 	//defs here, use EDITOR_GET in logic

editor/editor_run.cpp

@@ -183,15 +183,50 @@ Error EditorRun::run(const String &p_scene, const String &p_custom_args, const L
 		args.push_back(p_scene);
 	}
 
+	String exec = OS::get_singleton()->get_executable_path();
+
 	if (p_custom_args != "") {
-		Vector<String> cargs = p_custom_args.split(" ", false);
-		for (int i = 0; i < cargs.size(); i++) {
-			args.push_back(cargs[i].replace(" ", "%20"));
+		// Allow the user to specify a command to run, similar to Steam's launch options.
+		// In this case, Godot will no longer be run directly; it's up to the underlying command
+		// to run it. For instance, this can be used on Linux to force a running project
+		// to use Optimus using `prime-run` or similar.
+		// Example: `prime-run %command% --time-scale 0.5`
+		const int placeholder_pos = p_custom_args.find("%command%");
+
+		Vector<String> custom_args;
+
+		if (placeholder_pos != -1) {
+			// Prepend executable-specific custom arguments.
+			// If nothing is placed before `%command%`, behave as if no placeholder was specified.
+			Vector<String> exec_args = p_custom_args.substr(0, placeholder_pos).split(" ", false);
+			if (exec_args.size() >= 1) {
+				exec = exec_args[0];
+				exec_args.remove(0);
+
+				// Append the Godot executable name before we append executable arguments
+				// (since the order is reversed when using `push_front()`).
+				args.push_front(OS::get_singleton()->get_executable_path());
+			}
+
+			for (int i = exec_args.size() - 1; i >= 0; i--) {
+				// Iterate backwards as we're pushing items in the reverse order.
+				args.push_front(exec_args[i].replace(" ", "%20"));
+			}
+
+			// Append Godot-specific custom arguments.
+			custom_args = p_custom_args.substr(placeholder_pos + String("%command%").size()).split(" ", false);
+			for (int i = 0; i < custom_args.size(); i++) {
+				args.push_back(custom_args[i].replace(" ", "%20"));
+			}
+		} else {
+			// Append Godot-specific custom arguments.
+			custom_args = p_custom_args.split(" ", false);
+			for (int i = 0; i < custom_args.size(); i++) {
+				args.push_back(custom_args[i].replace(" ", "%20"));
+			}
 		}
 	}
 
-	String exec = OS::get_singleton()->get_executable_path();
-
 	printf("Running: %s", exec.utf8().get_data());
 	for (List<String>::Element *E = args.front(); E; E = E->next()) {
 		printf(" %s", E->get().utf8().get_data());