QStringView: introduce a user-defined literal operator

Although char16_t string literals implicitly convert to QStringView,
there are corner-cases where one may want to explicitly create a
QStringView out of them.

A couple of examples I've found is where these string literals decay
into pointers:

  // range is a std::initializer_list<const char16_t *>
  for (QStringView v : { u"foo", u"bar" }) { ... }

  // ternary will decay arguments
  void print(QStringView);
  print(check ? u"hi" : u"there");

When this happens the resulting code gets pessimized and polluted by
runtime calls to qustrlen in order to build the QStringView objects [1].

We can restore optimal codegen by directly dealing with QStringView
objects instead. Adding explicit conversions may make the code
cumbersome to read, so I'm introducing a UDL for QStringView, matching
the one for QString (and std::string_view).

[1] for instance: https://gcc.godbolt.org/z/eY7xvEje3

Apply the new operator to a couple of places.

[ChangeLog][QtCore][QStringView] Is it now possible to create
QStringView objects by using the u""_sv user-defined literal.

Fixes: QTBUG-123851
Change-Id: I8af7d2e211b356d284de160a222eab9e91d09500
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/ipc/qtipccommon.cpp

@@ -200,7 +200,7 @@ QNativeIpcKey QtIpcCommon::platformSafeKey(const QString &key, QtIpcCommon::IpcT
             QStringView prefix;
             QStringView payload = key;
             // see https://learn.microsoft.com/en-us/windows/win32/termserv/kernel-object-namespaces
-            for (QStringView candidate : { u"Local\\", u"Global\\" }) {
+            for (QStringView candidate : { u"Local\\"_sv, u"Global\\"_sv }) {
                 if (!key.startsWith(candidate))
                     continue;
                 prefix = candidate;

src/corelib/text/qstringview.cpp

@@ -1496,4 +1496,44 @@ or the character \a ch
     Returns maxSize().
 */
 
+/*!
+    \fn Qt::Literals::StringLiterals::operator""_sv(const char16_t *str, size_t size)
+
+    \relates QStringView
+    \since 6.10
+
+    Literal operator that creates a QStringView out of the first
+    \a size characters in the char16_t string literal \a str.
+
+    There is rarely need to explicitly construct a QStringView from a
+    char16_t string literal, as QStringView is implicitly constructible
+    from one:
+
+    \code
+    QStringView greeting = u"hello"; // OK even without _sv
+
+    void print(QStringView s);
+    print(u"world"); // OK even without _sv
+    \endcode
+
+    To use this operator, you need to be using the corresponding
+    namespace(s):
+
+    \code
+    using namespace Qt::Literals::StringLiterals;
+    auto sv = u"peace"_sv;
+    \endcode
+
+    Note that the returned QStringView will span over any NUL embedded
+    in the string literal. This is different from passing the string
+    literal to QStringView's constructor (explicitly or implicitly):
+
+    \code
+    QStringView sv1 = u"abc\0def";    // sv1 == "abc"
+    QStringView sv2 = u"abc\0def"_sv; // sv2 == "abc\0def"
+    \endcode
+
+    \sa Qt::Literals::StringLiterals
+*/
+
 QT_END_NAMESPACE

src/corelib/text/qstringview.h

@@ -512,6 +512,17 @@ qsizetype QtPrivate::findString(QStringView str, qsizetype from, QChar ch, Qt::C
     return -1;
 }
 
+namespace Qt {
+inline namespace Literals {
+inline namespace StringLiterals {
+constexpr QStringView operator""_sv(const char16_t *str, size_t size) noexcept
+{
+    return QStringView(str, qsizetype(size));
+}
+} // StringLiterals
+} // Literals
+} // Qt
+
 QT_END_NAMESPACE
 
 #endif /* QSTRINGVIEW_H */

src/widgets/styles/qcommonstyle.cpp

@@ -6071,7 +6071,7 @@ QIcon QCommonStylePrivate::iconFromResourceTheme(QCommonStyle::StandardPixmap st
         addIconFiles(u"normalizedockup-", dockTitleIconSizes, icon);
         break;
     case QStyle::SP_ToolBarHorizontalExtensionButton:
-        addIconFiles(rtl(option) ? u"toolbar-ext-h-rtl-" : u"toolbar-ext-h-", toolBarExtHSizes, icon);
+        addIconFiles(rtl(option) ? u"toolbar-ext-h-rtl-"_sv : u"toolbar-ext-h-"_sv, toolBarExtHSizes, icon);
         break;
     case QStyle::SP_ToolBarVerticalExtensionButton:
         addIconFiles(u"toolbar-ext-v-", toolBarExtVSizes, icon);

tests/auto/corelib/text/qstringview/tst_qstringview.cpp

@@ -287,6 +287,7 @@ private Q_SLOTS:
     void tokenize() const;
 
     void std_stringview_conversion();
+    void userDefinedLiterals();
 
 private:
     template <typename String>
@@ -974,5 +975,27 @@ void tst_QStringView::std_stringview_conversion()
     QCOMPARE(sv, std::u16string_view(u"Hello\0world\0", 12));
 }
 
+void tst_QStringView::userDefinedLiterals()
+{
+    using namespace Qt::StringLiterals;
+    auto sv = u"test"_sv;
+    static_assert(std::is_same_v<decltype(sv), QStringView>);
+
+    QCOMPARE(sv.size(), 4);
+    QCOMPARE(sv, "test");
+
+    sv = u""_sv;
+    QCOMPARE(sv.size(), 0);
+    QCOMPARE(sv, "");
+
+    sv = u"embedded\0nul"_sv;
+    QCOMPARE(sv.size(), 12);
+    QCOMPARE(sv, QStringView(u"embedded\0nul", 12));
+
+    constexpr auto csv = u"constexpr test"_sv;
+    static_assert(csv.size() == 14);
+    QCOMPARE(csv, "constexpr test");
+}
+
 QTEST_APPLESS_MAIN(tst_QStringView)
 #include "tst_qstringview.moc"