Add support for safe feature checking at compile time

Added a qtConfig(feature) function to qmake, and a QT_CONFIG(feature)
macro. These can safely check whether a certain compile time feature of
Qt is enabled or not.

For this to work the feature has to have a publicFeature or
privateFeature output in the configure.json file.

In pro files, please use the qtConfig(feature) test function
instead of checking contains(QT_CONFIG, feature), as the latter
will be unreliable with the upcoming modularization (it requires a
load(qt_module_config) before doing any such checks). Note that
feature names are now lowercase, and identical (except for hyphens
versus underscores currently) in the pro and c++ files.

This makes the logic easier to follow, as we avoid all double negations,
and most importantly, QT_CONFIG and qtConfig are implemented in a
way that you'll get a build error for a mistyped or non-existent
feature. This will also prevent accidental use of a widget feature
in gui in the future.

This gives us complete symmetry between the handling in pro and
c++ files.

Change-Id: I60404f97953724e639ffb6386cce2e8b1e4b735a
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

1 files changed, 11 insertions, 1 deletions

diff --git a/src/corelib/global/qglobal.h b/src/corelib/global/qglobal.h
index d9ffca3810..e6d65b0f99 100644
--- a/src/corelib/global/qglobal.h
+++ b/src/corelib/global/qglobal.h
@@ -62,13 +62,23 @@
 #endif
 
 // The QT_SUPPORTS macro is deprecated. Don't use it in new code.
-// Instead, use #ifdef/ndef QT_NO_feature.
+// Instead, use QT_CONFIG(feature)
 // ### Qt6: remove macro
 #ifdef _MSC_VER
 #  define QT_SUPPORTS(FEATURE) (!defined QT_NO_##FEATURE)
 #else
 #  define QT_SUPPORTS(FEATURE) (!defined(QT_NO_##FEATURE))
 #endif
+
+/*
+    The QT_CONFIG macro implements a safe compile time check for features of Qt.
+    Features can be in three states:
+        0 or undefined: This will lead to a compile error when testing for it
+        -1: The feature is not available
+        1: The feature is available
+*/
+#define QT_CONFIG(feature) (1/QT_FEATURE_##feature == 1)
+
 #if QT_VERSION >= QT_VERSION_CHECK(6,0,0)
 #  define QT_NO_UNSHARABLE_CONTAINERS
 #endif