QSettings: port API from QString to QAnyStringView keys

With the public interface ported to QAnyStringView, we can now
internally optimize memory allocations _in a central place_ (e.g. by
returning std::u16string or QVarLengthArray<QChar> from normalizeKey()
instead of QString). But first we needed to get rid of all the
unwarranted allocations in user code.

Effects on Linux AMD64 stripped C++20 release builds:

GCC 11.2 libstdc++ (TEXT -= 6.5%):

     text    data     bss     dec     hex filename
   635148   10992    2824  648964   9e704 tst_qsettings-01-baseline
   593691   10992    2824  607507   94513 tst_qsettings-02-qanystringview

Clang 10.0.0 libc++ (TEXT -= 11.6%(!)):

     text    data     bss     dec     hex filename
   790336   10640    2832  803808   c43e0 tst_qsettings-01-baseline
   698572   10640    2832  712044   add6c tst_qsettings-02-qanystringview

That's the beauty of QAnyStringView: transparently reducing temporary
QString creation; and the simplest code is also the most efficient.

[ChangeLog][QtCore][QSettings] Keys can now be passed as
QAnyStringView (was: QString). The most efficient way to pass literal
keys is now "key"_L1, the backwards-compatible way is
QStringLiteral("key").

Fixes: QTBUG-101390
Change-Id: I510fb4ce17ef109dac7c9cdc5d90ede0d1a9db5f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

1 files changed, 43 insertions, 0 deletions

diff --git a/src/corelib/compat/removed_api.cpp b/src/corelib/compat/removed_api.cpp
index 64c3e9ccff..51482d1e60 100644
--- a/src/corelib/compat/removed_api.cpp
+++ b/src/corelib/compat/removed_api.cpp
@@ -214,6 +214,49 @@ void QObject::setObjectName(const QString &name)
 }
 
 
+#include "qsettings.h"
+
+void QSettings::beginGroup(const QString &prefix)
+{
+    return beginGroup(qToAnyStringViewIgnoringNull(prefix));
+}
+
+int QSettings::beginReadArray(const QString &prefix)
+{
+    return beginReadArray(qToAnyStringViewIgnoringNull(prefix));
+}
+
+void QSettings::beginWriteArray(const QString &prefix, int size)
+{
+    beginWriteArray(qToAnyStringViewIgnoringNull(prefix), size);
+}
+
+void QSettings::setValue(const QString &key, const QVariant &value)
+{
+    setValue(qToAnyStringViewIgnoringNull(key), value);
+}
+
+void QSettings::remove(const QString &key)
+{
+    remove(qToAnyStringViewIgnoringNull(key));
+}
+
+bool QSettings::contains(const QString &key) const
+{
+    return contains(qToAnyStringViewIgnoringNull(key));
+}
+
+QVariant QSettings::value(const QString &key, const QVariant &defaultValue) const
+{
+    return value(qToAnyStringViewIgnoringNull(key), defaultValue);
+}
+
+QVariant QSettings::value(const QString &key) const
+{
+    return value(qToAnyStringViewIgnoringNull(key));
+}
+
+
 #include "qversionnumber.h"
 
 QT_WARNING_PUSH