Add plugins of imported QtModules to the _qt_plugins property

This covers the following use case:
QtModuleX is pre-built and installed, it's imported. The plugin has
a PLUGIN_TYPE that is associated with QtModuleX and is built with
application that links QtModuleX. When deploying the application
it's expected that the plugin is deployed, as the one that belongs
to the linked QtModuleX.

This ensures that we udpate the internal _qt_plugins property that
is used in the __qt_internal_collect_plugin_targets_from_dependencies
function.

Pick-to: 6.6 6.5
Change-Id: I9824351ebab5a24509800da4db69f1e282a35884
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
(cherry picked from commit 75d83d5a6521a5cf5c7159f1ef09e8978e429919)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

cmake/QtPluginHelpers.cmake

@@ -211,6 +211,13 @@ function(qt_internal_add_plugin target)
             # This QT_PLUGINS assignment is only used by QtPostProcessHelpers to decide if a
             # QtModulePlugins.cmake file should be generated.
             set_property(TARGET "${qt_module_target}" APPEND PROPERTY QT_PLUGINS "${target}")
+        else()
+            # The _qt_plugins property is considered when collecting the plugins in
+            # deployment process. The usecase is following:
+            # QtModuleX is built separately and installed, so it's imported.
+            # The plugin is built in some application build tree and its PLUGIN_TYPE is associated
+            # with QtModuleX.
+            set_property(TARGET "${qt_module_target}" APPEND PROPERTY _qt_plugins "${target}")
         endif()
 
         set(plugin_target_versioned "${QT_CMAKE_EXPORT_NAMESPACE}::${target}")

tests/auto/cmake/CMakeLists.txt

@@ -392,3 +392,6 @@ endif()
 
 _qt_internal_test_expect_pass(test_config_expressions)
 _qt_internal_test_expect_pass(test_QTP0003)
+if(NOT NO_GUI)
+    _qt_internal_test_expect_pass(test_collecting_plugins)
+endif()

tests/auto/cmake/test_collecting_plugins/CMakeLists.txt

@@ -0,0 +1,31 @@
+# Copyright (C) 2024 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+cmake_minimum_required(VERSION 3.16)
+
+project(test_collecting_plugins
+    VERSION
+        "${CMAKE_Core_MODULE_MAJOR_VERSION}.${CMAKE_Core_MODULE_MINOR_VERSION}.${CMAKE_Core_MODULE_PATCH_VERSION}")
+
+find_package(Qt6 COMPONENTS Core Gui BuildInternals REQUIRED)
+
+qt_prepare_standalone_project()
+
+qt_internal_add_plugin(QTestImagePlugin
+    SHARED
+    PLUGIN_TYPE imageformats
+    SOURCES
+        plugin.cpp
+    LIBRARIES
+        Qt6::Gui
+    SKIP_INSTALL # Make sure that we do not package this plugin
+)
+
+qt_add_executable(TestExecutable main.cpp)
+target_link_libraries(TestExecutable PRIVATE Qt6::Gui)
+
+__qt_internal_collect_plugin_targets_from_dependencies(TestExecutable plugin_targets)
+
+if(NOT "QTestImagePlugin" IN_LIST plugin_targets)
+    message(FATAL_ERROR "QTestImagePlugin plugin is missing")
+endif()

tests/auto/cmake/test_collecting_plugins/main.cpp

@@ -0,0 +1,7 @@
+// Copyright (C) 2024 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
+
+int main(int, char *[])
+{
+    return 0;
+}

tests/auto/cmake/test_collecting_plugins/plugin.cpp

@@ -0,0 +1,18 @@
+// Copyright (C) 2024 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
+
+#include <qimageiohandler.h>
+
+class TestImagePlugin : public QImageIOPlugin
+{
+    Q_OBJECT
+    Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QImageIOHandlerFactoryInterface")
+public:
+    Capabilities capabilities(QIODevice *, const QByteArray &) const override { return {}; }
+    QImageIOHandler *create(QIODevice *, const QByteArray & = QByteArray()) const override
+    {
+        return nullptr;
+    }
+};
+
+#include "plugin.moc"