QStringView: stop instantiating std::char_traits<QChar> and <uchar>

It's deprecated and will be removed with LLVM 19.

Amends b1ee49b46533d39f7fabda68d0bd08a1ab130a27.

Change-Id: I5dd50a1a7ca5424d9e7afffd17ad07b3ab3fc18a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit dc2ae08e02730ab795445bc047221aa56914f723)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qstringalgorithms.h

@@ -4,15 +4,17 @@
 #ifndef QSTRINGALGORITHMS_H
 #define QSTRINGALGORITHMS_H
 
+#include <QtCore/qbytearrayalgorithms.h>
+#include <QtCore/qcontainerfwd.h>
 #include <QtCore/qnamespace.h>
 #include <QtCore/qstringfwd.h>
-#include <QtCore/qcontainerfwd.h>
 #if 0
 #pragma qt_class(QStringAlgorithms)
 #endif
 
 #include <algorithm>        // std::find
-#include <string>           // std::char_traits
+
+#include <string.h>         // for memchr
 
 QT_BEGIN_NAMESPACE
 
@@ -121,6 +123,25 @@ namespace QtPrivate {
 [[nodiscard]] Q_CORE_EXPORT Q_DECL_PURE_FUNCTION bool isLatin1(QStringView   s) noexcept;
 [[nodiscard]] Q_CORE_EXPORT Q_DECL_PURE_FUNCTION bool isValidUtf16(QStringView s) noexcept;
 
+template <typename Char, size_t N> [[nodiscard]] constexpr Q_ALWAYS_INLINE
+qsizetype lengthHelperContainerLoop(const Char (&str)[N])
+{
+#if defined(__cpp_lib_constexpr_algorithms) && defined(Q_CC_GNU_ONLY)
+    // libstdc++'s std::find / std::find_if manages to execute more steps
+    // than the loop below
+    const auto it = std::find(str, str + N, Char(0));
+    return it - str;
+#else
+    // std::char_traits<C> is deprecated for C not one of the standard char
+    // types, so we have to roll out our own loop.
+    for (size_t i = 0; i < N; ++i) {
+        if (str[i] == Char(0))
+            return qsizetype(i);
+    }
+    return qsizetype(N);
+#endif
+}
+
 template <typename Char, size_t N> [[nodiscard]] constexpr Q_ALWAYS_INLINE
 std::enable_if_t<sizeof(Char) == sizeof(char16_t), qsizetype>
 lengthHelperContainer(const Char (&str)[N])
@@ -129,9 +150,12 @@ lengthHelperContainer(const Char (&str)[N])
     // at which the compiler gives up pre-calculating the std::find() below and
     // instead inserts code to be executed at runtime.
     constexpr size_t RuntimeThreshold =
-#if defined(Q_CC_CLANG) // tested through Clang 16.0.0
+#if defined(Q_CC_CLANG)
-            100
+            // tested on Clang 15, 16 & 17
-#elif defined(Q_CC_GNU) // tested through GCC 13.1 at -O3 compilation level
+            1023
+#elif defined(Q_CC_GNU)
+            // tested through GCC 13.1 at -O3 compilation level
+            // note: at -O2, GCC always generates a loop!
             __cplusplus >= 202002L ? 39 : 17
 #else
             0
@@ -146,24 +170,18 @@ lengthHelperContainer(const Char (&str)[N])
 #endif
     }
 
-    // libstdc++'s std::find_if yields a higher threshold than
+    return lengthHelperContainerLoop(str);
-    // std::char_traits::find
-
-#if __cplusplus >= 202002 && defined(__cpp_lib_constexpr_algorithms)
-    const auto it = std::find(str, str + N, Char(0));
-    return it - str;
-#else
-    const auto it = std::char_traits<Char>::find(str, N, Char(0));
-    return it ? std::distance(str, it) : ptrdiff_t(N);
-#endif
 }
 
 template <typename Char, size_t N> [[nodiscard]] constexpr inline
 std::enable_if_t<sizeof(Char) == 1, qsizetype> lengthHelperContainer(const Char (&str)[N])
 {
-    // std::char_traits::find will call memchr or __builtin_memchr for us
+#ifdef QT_SUPPORTS_IS_CONSTANT_EVALUATED
-    const auto it = std::char_traits<Char>::find(str, N, Char(0));
+    if (!qIsConstantEvaluated())
-    return it ? std::distance(str, it) : ptrdiff_t(N);
+        return qstrnlen(reinterpret_cast<const char *>(str), N);
+#endif
+
+    return lengthHelperContainerLoop(str);
 }
 
 template <typename Container>