Everywhere: Switch from (void) to [[maybe_unused]] (#4473)

Problem:
- `(void)` simply casts the expression to void. This is understood to
  indicate that it is ignored, but this is really a compiler trick to
  get the compiler to not generate a warning.

Solution:
- Use the `[[maybe_unused]]` attribute to indicate the value is unused.

Note:
- Functions taking a `(void)` argument list have also been changed to
  `()` because this is not needed and shows up in the same grep
  command.

Applications/Browser/DownloadWidget.cpp

@@ -156,11 +156,8 @@ void DownloadWidget::did_progress(Optional<u32> total_size, u32 downloaded_size)
     }
 }
 
-void DownloadWidget::did_finish(bool success, ReadonlyBytes payload, RefPtr<SharedBuffer> payload_storage, const HashMap<String, String, CaseInsensitiveStringTraits>& response_headers)
+void DownloadWidget::did_finish(bool success, [[maybe_unused]] ReadonlyBytes payload, [[maybe_unused]] RefPtr<SharedBuffer> payload_storage, [[maybe_unused]] const HashMap<String, String, CaseInsensitiveStringTraits>& response_headers)
 {
-    (void)payload;
-    (void)payload_storage;
-    (void)response_headers;
     dbg() << "did_finish, success=" << success;
 
     m_close_button->set_enabled(true);

Applications/Browser/Tab.cpp

@@ -82,7 +82,7 @@ static void start_download(const URL& url)
     window->set_resizable(false);
     window->set_main_widget<DownloadWidget>(url);
     window->show();
-    (void)window.leak_ref();
+    [[maybe_unused]] auto& unused = window.leak_ref();
 }
 
 Tab::Tab(Type type)
@@ -308,7 +308,7 @@ Tab::Tab(Type type)
                 window->set_title(url);
                 window->set_icon(Gfx::Bitmap::load_from_file("/res/icons/16x16/filetype-text.png"));
                 window->show();
-                (void)window.leak_ref();
+                [[maybe_unused]] auto& unused = window.leak_ref();
             } else {
                 TODO();
             }

Applications/FileManager/main.cpp

@@ -183,13 +183,12 @@ void show_properties(const String& container_dir_path, const String& path, const
     properties->exec();
 }
 
-int run_in_desktop_mode(RefPtr<Core::ConfigFile> config)
+int run_in_desktop_mode([[maybe_unused]] RefPtr<Core::ConfigFile> config)
 {
     static constexpr const char* process_name = "FileManager (Desktop)";
     set_process_name(process_name, strlen(process_name));
     pthread_setname_np(pthread_self(), process_name);
 
-    (void)config;
     auto window = GUI::Window::construct();
     window->set_title("Desktop Manager");
     window->set_window_type(GUI::WindowType::Desktop);
@@ -198,8 +197,7 @@ int run_in_desktop_mode(RefPtr<Core::ConfigFile> config)
     auto& desktop_widget = window->set_main_widget<FileManager::DesktopWidget>();
     desktop_widget.set_layout<GUI::VerticalBoxLayout>();
 
-    auto& directory_view = desktop_widget.add<DirectoryView>(DirectoryView::Mode::Desktop);
-    (void)directory_view;
+    [[maybe_unused]] auto& directory_view = desktop_widget.add<DirectoryView>(DirectoryView::Mode::Desktop);
 
     auto copy_action = GUI::CommonActions::make_copy_action(
         [&](auto&) {

Applications/IRCClient/IRCClient.cpp

@@ -773,9 +773,8 @@ void IRCClient::handle_rpl_whoisuser(const Message& msg)
     auto& nick = msg.arguments[1];
     auto& username = msg.arguments[2];
     auto& host = msg.arguments[3];
-    auto& asterisk = msg.arguments[4];
+    [[maybe_unused]] auto& asterisk = msg.arguments[4];
     auto& realname = msg.arguments[5];
-    (void)asterisk;
     add_server_message(String::formatted("* {} is {}@{}, real name: {}", nick, username, host, realname));
 }
 

Applications/Piano/RollWidget.cpp

@@ -204,9 +204,8 @@ void RollWidget::mousemove_event(GUI::MouseEvent& event)
     update();
 }
 
-void RollWidget::mouseup_event(GUI::MouseEvent& event)
+void RollWidget::mouseup_event([[maybe_unused]] GUI::MouseEvent& event)
 {
-    (void)event;
     m_note_drag_start = {};
     m_note_drag_location = {};
 }

Applications/QuickShow/QSWidget.cpp

@@ -200,10 +200,7 @@ void QSWidget::mousedown_event(GUI::MouseEvent& event)
     m_saved_pan_origin = m_pan_origin;
 }
 
-void QSWidget::mouseup_event(GUI::MouseEvent& event)
-{
-    UNUSED_PARAM(event);
-}
+void QSWidget::mouseup_event([[maybe_unused]] GUI::MouseEvent& event) { }
 
 void QSWidget::mousemove_event(GUI::MouseEvent& event)
 {

Applications/SystemMonitor/MemoryStatsWidget.cpp

@@ -105,8 +105,7 @@ void MemoryStatsWidget::refresh()
     ASSERT(json_result.has_value());
     auto json = json_result.value().as_object();
 
-    unsigned kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
-    (void)kmalloc_eternal_allocated;
+    [[maybe_unused]] unsigned kmalloc_eternal_allocated = json.get("kmalloc_eternal_allocated").to_u32();
     unsigned kmalloc_allocated = json.get("kmalloc_allocated").to_u32();
     unsigned kmalloc_available = json.get("kmalloc_available").to_u32();
     unsigned user_physical_allocated = json.get("user_physical_allocated").to_u32();

Applications/SystemMonitor/main.cpp

@@ -278,8 +278,7 @@ int main(int argc, char** argv)
     process_context_menu->add_separator();
     process_context_menu->add_action(profile_action);
     process_context_menu->add_action(inspect_action);
-    process_table_view.on_context_menu_request = [&](const GUI::ModelIndex& index, const GUI::ContextMenuEvent& event) {
-        (void)index;
+    process_table_view.on_context_menu_request = [&]([[maybe_unused]] const GUI::ModelIndex& index, const GUI::ContextMenuEvent& event) {
         process_context_menu->popup(event.screen_position());
     };