Designer: Add -designer-qt-pluginpath argument also for Qt < 6.7

It doesn't have the public API yet, but hack it by temporarily changing
the application's library path, which is used by designer for the
default plugin paths.

Change-Id: I658e41d1228cb89cb6e72d51f3e70b17de32127c
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

3 files changed, 41 insertions, 6 deletions

diff --git a/src/plugins/designer/CMakeLists.txt b/src/plugins/designer/CMakeLists.txt
index eea16798a5d..581f7ec2a7e 100644
--- a/src/plugins/designer/CMakeLists.txt
+++ b/src/plugins/designer/CMakeLists.txt
@@ -14,7 +14,14 @@ if(Qt6_VERSION VERSION_GREATER_EQUAL 6.7.0)
     }
     ],")
 else()
-  set(DESIGNER_PLUGIN_ARGUMENTS)
+  set(DESIGNER_PLUGIN_ARGUMENTS
+  "\"Arguments\" : [
+    {
+        \"Name\" : \"-designer-qt-pluginpath\",
+        \"Parameter\" : \"path\",
+        \"Description\" : \"Override the default search path for Qt Designer plugins\"
+    }
+    ],")
 endif()
 
 add_qtc_plugin(Designer
diff --git a/src/plugins/designer/formeditor.cpp b/src/plugins/designer/formeditor.cpp
index 9629f905266..93603e1a28e 100644
--- a/src/plugins/designer/formeditor.cpp
+++ b/src/plugins/designer/formeditor.cpp
@@ -28,6 +28,7 @@
 
 #include <utils/infobar.h>
 #include <utils/qtcassert.h>
+#include <utils/stringutils.h>
 #include <utils/theme/theme.h>
 
 #include <QDesignerFormEditorPluginInterface>
@@ -224,7 +225,18 @@ FormEditorData::FormEditorData()
     m_formeditor = QDesignerComponents::createFormEditorWithPluginPaths(designerPluginPaths(),
                                                                         nullptr);
 #else
+    // Qt < 6.7.0 doesn't have API for changing the plugin path yet.
+    // Work around it by temporarily changing the application's library paths,
+    // which are used for Designer's plugin paths.
+    // This must be done before creating the FormEditor, and with it QDesignerPluginManager.
+    const QStringList restoreLibraryPaths = sQtPluginPath->isEmpty()
+                                                ? QStringList()
+                                                : QCoreApplication::libraryPaths();
+    if (!sQtPluginPath->isEmpty())
+        QCoreApplication::setLibraryPaths(QStringList(*sQtPluginPath));
     m_formeditor = QDesignerComponents::createFormEditor(nullptr);
+    if (!sQtPluginPath->isEmpty())
+        QCoreApplication::setLibraryPaths(restoreLibraryPaths);
 #endif
     if (Designer::Constants::Internal::debug)
         qDebug() << Q_FUNC_INFO;
@@ -894,7 +906,23 @@ void FormEditorData::print()
 void setQtPluginPath(const QString &qtPluginPath)
 {
     QTC_CHECK(!d);
-    *sQtPluginPath = qtPluginPath;
+    *sQtPluginPath = QDir::fromNativeSeparators(qtPluginPath);
+#if QT_VERSION < QT_VERSION_CHECK(6, 7, 0)
+    // Cut a "/designer" postfix off, if present.
+    // For Qt < 6.7.0 we hack the plugin path by temporarily setting the application library paths
+    // and Designer adds "/designer" to these.
+    static const QString postfix = "/designer";
+    *sQtPluginPath = Utils::trimBack(*sQtPluginPath, '/');
+    if (sQtPluginPath->endsWith(postfix))
+        sQtPluginPath->chop(postfix.size());
+    if (!QFile::exists(*sQtPluginPath + postfix)) {
+        qWarning() << qPrintable(
+            QLatin1String(
+                "Warning: The path \"%1\" passed to -designer-qt-pluginpath does not exist. "
+                "Note that \"%2\" at the end is enforced.")
+                .arg(*sQtPluginPath + postfix, postfix));
+    }
+#endif
 }
 
 void addPluginPath(const QString &pluginPath)
diff --git a/src/plugins/designer/formeditorplugin.cpp b/src/plugins/designer/formeditorplugin.cpp
index 37d0f7ce5df..e839ce2b3a2 100644
--- a/src/plugins/designer/formeditorplugin.cpp
+++ b/src/plugins/designer/formeditorplugin.cpp
@@ -60,7 +60,6 @@ FormEditorPlugin::~FormEditorPlugin()
     delete d;
 }
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 7, 0)
 static void parseArguments(const QStringList &arguments)
 {
     const auto doWithNext = [arguments](auto it, const std::function<void(QString)> &fun) {
@@ -71,11 +70,14 @@ static void parseArguments(const QStringList &arguments)
     for (auto it = arguments.cbegin(); it != arguments.cend(); ++it) {
         if (*it == "-designer-qt-pluginpath")
             doWithNext(it, [](const QString &path) { setQtPluginPath(path); });
+#if QT_VERSION >= QT_VERSION_CHECK(6, 7, 0)
+        // -designer-pluginpath is only supported when building with Qt >= 6.7.0, which added the
+        // required API
         else if (*it == "-designer-pluginpath")
             doWithNext(it, [](const QString &path) { addPluginPath(path); });
+#endif
     }
 }
-#endif
 
 bool FormEditorPlugin::initialize([[maybe_unused]] const QStringList &arguments,
                                   [[maybe_unused]] QString *errorString)
@@ -110,9 +112,7 @@ bool FormEditorPlugin::initialize([[maybe_unused]] const QStringList &arguments,
             QCoreApplication::installTranslator(qtr);
     }
 
-#if QT_VERSION >= QT_VERSION_CHECK(6, 7, 0)
     parseArguments(arguments);
-#endif
     return true;
 }