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>

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

src/corelib/io/qsettings.cpp

@@ -2914,9 +2914,12 @@ void QSettings::setAtomicSyncRequired(bool enable)
     Call endGroup() to reset the current group to what it was before
     the corresponding beginGroup() call. Groups can be nested.
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa endGroup(), group()
 */
-void QSettings::beginGroup(const QString &prefix)
+void QSettings::beginGroup(QAnyStringView prefix)
 {
     Q_D(QSettings);
     d->beginGroupOrArray(QSettingsGroup(d->normalizedKey(prefix)));
@@ -2970,9 +2973,12 @@ QString QSettings::group() const
 
     Use beginWriteArray() to write the array in the first place.
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa beginWriteArray(), endArray(), setArrayIndex()
 */
-int QSettings::beginReadArray(const QString &prefix)
+int QSettings::beginReadArray(QAnyStringView prefix)
 {
     Q_D(QSettings);
     d->beginGroupOrArray(QSettingsGroup(d->normalizedKey(prefix), false));
@@ -3006,9 +3012,12 @@ int QSettings::beginReadArray(const QString &prefix)
 
     To read back an array, use beginReadArray().
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa beginReadArray(), endArray(), setArrayIndex()
 */
-void QSettings::beginWriteArray(const QString &prefix, int size)
+void QSettings::beginWriteArray(QAnyStringView prefix, int size)
 {
     Q_D(QSettings);
     d->beginGroupOrArray(QSettingsGroup(d->normalizedKey(prefix), size < 0));
@@ -3169,9 +3178,12 @@ bool QSettings::isWritable() const
 
   \snippet code/src_corelib_io_qsettings.cpp 23
 
+  \note In Qt versions prior to 6.4, this function took QString, not
+  QAnyStringView.
+
   \sa value(), remove(), contains()
 */
-void QSettings::setValue(const QString &key, const QVariant &value)
+void QSettings::setValue(QAnyStringView key, const QVariant &value)
 {
     Q_D(QSettings);
     if (key.isEmpty()) {
@@ -3203,9 +3215,12 @@ void QSettings::setValue(const QString &key, const QVariant &value)
     case-sensitive keys. To avoid portability problems, see the
     \l{Section and Key Syntax} rules.
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa setValue(), value(), contains()
 */
-void QSettings::remove(const QString &key)
+void QSettings::remove(QAnyStringView key)
 {
     Q_D(QSettings);
     /*
@@ -3238,9 +3253,12 @@ void QSettings::remove(const QString &key)
     case-sensitive keys. To avoid portability problems, see the
     \l{Section and Key Syntax} rules.
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa value(), setValue()
 */
-bool QSettings::contains(const QString &key) const
+bool QSettings::contains(QAnyStringView key) const
 {
     Q_D(const QSettings);
     return d->get(d->actualKey(key)) != std::nullopt;
@@ -3288,8 +3306,8 @@ bool QSettings::event(QEvent *event)
 #endif
 
 /*!
-    \fn QSettings::value(const QString &key) const
+    \fn QSettings::value(QAnyStringView key) const
-    \fn QSettings::value(const QString &key, const QVariant &defaultValue) const
+    \fn QSettings::value(QAnyStringView key, const QVariant &defaultValue) const
 
     Returns the value for setting \a key. If the setting doesn't
     exist, returns \a defaultValue.
@@ -3306,15 +3324,18 @@ bool QSettings::event(QEvent *event)
 
     \snippet code/src_corelib_io_qsettings.cpp 26
 
+    \note In Qt versions prior to 6.4, this function took QString, not
+    QAnyStringView.
+
     \sa setValue(), contains(), remove()
 */
-QVariant QSettings::value(const QString &key) const
+QVariant QSettings::value(QAnyStringView key) const
 {
     Q_D(const QSettings);
     return d->value(key, nullptr);
 }
 
-QVariant QSettings::value(const QString &key, const QVariant &defaultValue) const
+QVariant QSettings::value(QAnyStringView key, const QVariant &defaultValue) const
 {
     Q_D(const QSettings);
     return d->value(key, &defaultValue);

src/corelib/io/qsettings.h

@@ -148,12 +148,19 @@ public:
     bool isAtomicSyncRequired() const;
     void setAtomicSyncRequired(bool enable);
 
+#if QT_CORE_REMOVED_SINCE(6, 4)
     void beginGroup(const QString &prefix);
+#endif
+    void beginGroup(QAnyStringView prefix);
     void endGroup();
     QString group() const;
 
+#if QT_CORE_REMOVED_SINCE(6, 4)
     int beginReadArray(const QString &prefix);
     void beginWriteArray(const QString &prefix, int size = -1);
+#endif
+    int beginReadArray(QAnyStringView prefix);
+    void beginWriteArray(QAnyStringView prefix, int size = -1);
     void endArray();
     void setArrayIndex(int i);
 
@@ -162,12 +169,21 @@ public:
     QStringList childGroups() const;
     bool isWritable() const;
 
+#if QT_CORE_REMOVED_SINCE(6, 4)
     void setValue(const QString &key, const QVariant &value);
     QVariant value(const QString &key, const QVariant &defaultValue) const;
     QVariant value(const QString &key) const;
+#endif
+    void setValue(QAnyStringView key, const QVariant &value);
+    QVariant value(QAnyStringView key, const QVariant &defaultValue) const;
+    QVariant value(QAnyStringView key) const;
 
+#if QT_CORE_REMOVED_SINCE(6, 4)
     void remove(const QString &key);
     bool contains(const QString &key) const;
+#endif
+    void remove(QAnyStringView key);
+    bool contains(QAnyStringView key) const;
 
     void setFallbacksEnabled(bool b);
     bool fallbacksEnabled() const;