qt5, qt6: use versioned QML import paths in wrappers

Qt is smart enough to figure out the target Qt version for native plugins.
However, Qt is _not_ smart enough to figure out the target Qt version for
QML imports, which causes all kinds of funny breakage when you start running
Qt 5 applications from Qt 6 ones and vice versa.

So, do some minimally invasive surgery to make different Qt versions pick up
different QML import path variables, so they don't mess with each other.

This is kind of very cursed, but what can you do.

3 files changed, 36 insertions, 1 deletions

diff --git a/pkgs/development/libraries/qt-5/5.15/default.nix b/pkgs/development/libraries/qt-5/5.15/default.nix
index 7a5ff60c9acbb..b0e92290fe5b7 100644
--- a/pkgs/development/libraries/qt-5/5.15/default.nix
+++ b/pkgs/development/libraries/qt-5/5.15/default.nix
@@ -51,6 +51,8 @@ let
       ./qtdeclarative.patch
       # prevent headaches from stale qmlcache data
       ./qtdeclarative-default-disable-qmlcache.patch
+      # add version specific QML import path
+      ./qtdeclarative-qml-paths.patch
     ];
     qtlocation = lib.optionals stdenv.cc.isClang [
       # Fix build with Clang 16
diff --git a/pkgs/development/libraries/qt-5/5.15/qtdeclarative-qml-paths.patch b/pkgs/development/libraries/qt-5/5.15/qtdeclarative-qml-paths.patch
new file mode 100644
index 0000000000000..2ea3554df7d2a
--- /dev/null
+++ b/pkgs/development/libraries/qt-5/5.15/qtdeclarative-qml-paths.patch
@@ -0,0 +1,33 @@
+diff --git a/src/qml/qml/qqmlimport.cpp b/src/qml/qml/qqmlimport.cpp
+index 289f11d006..80c422403c 100644
+--- a/src/qml/qml/qqmlimport.cpp
++++ b/src/qml/qml/qqmlimport.cpp
+@@ -1897,17 +1897,22 @@ QQmlImportDatabase::QQmlImportDatabase(QQmlEngine *e)
+     addImportPath(installImportsPath);
+ 
+     // env import paths
+-    if (Q_UNLIKELY(!qEnvironmentVariableIsEmpty("QML2_IMPORT_PATH"))) {
+-        const QString envImportPath = qEnvironmentVariable("QML2_IMPORT_PATH");
++    auto addEnvImportPath = [this](const char *var) {
+ #if defined(Q_OS_WIN)
+         QLatin1Char pathSep(';');
+ #else
+         QLatin1Char pathSep(':');
+ #endif
+-        QStringList paths = envImportPath.split(pathSep, Qt::SkipEmptyParts);
+-        for (int ii = paths.count() - 1; ii >= 0; --ii)
+-            addImportPath(paths.at(ii));
+-    }
++        if (Q_UNLIKELY(!qEnvironmentVariableIsEmpty(var))) {
++            const QString envImportPath = qEnvironmentVariable(var);
++            QStringList paths = envImportPath.split(pathSep, Qt::SkipEmptyParts);
++            for (int ii = paths.count() - 1; ii >= 0; --ii)
++                addImportPath(paths.at(ii));
++        }
++    };
++
++    addEnvImportPath("NIXPKGS_QT5_QML_IMPORT_PATH");
++    addEnvImportPath("QML2_IMPORT_PATH");
+ 
+     addImportPath(QStringLiteral("qrc:/qt-project.org/imports"));
+     addImportPath(QCoreApplication::applicationDirPath());
diff --git a/pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh b/pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh
index 2127be5e0a2a0..94df0a1a8e661 100644
--- a/pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh
+++ b/pkgs/development/libraries/qt-5/hooks/wrap-qt-apps-hook.sh
@@ -31,7 +31,7 @@ qtHostPathHook() {
     local qmlDir="$1/${qtQmlPrefix:?}"
     if [ -d "$qmlDir" ]
     then
-        qtWrapperArgs+=(--prefix QML2_IMPORT_PATH : "$qmlDir")
+        qtWrapperArgs+=(--prefix NIXPKGS_QT5_QML_IMPORT_PATH : "$qmlDir")
     fi
 }
 addEnvHooks "$targetOffset" qtHostPathHook