Use ICU from Windows SDK for codec support (QStringConverter)

Windows has shipped a (limited) subset of ICU for quite a while:

  https://learn.microsoft.com/en-us/windows/win32/intl/international-components-for-unicode--icu-

The C API for this subset uses non-standard names for headers and lib
though, and is therefore not picked up by the ICU configure logic in
CMake.

This patch adds a separate configure test that enables the use of this
ICU API, specifically for text codec support in QStringConverter.
If a 'proper' ICU is enabled by the user, this still takes precedence.
Also, note that MinGW-w64 misses the respective Windows API, and will
therefore still use the old fallback.

[ChangeLog][QtCore][Text] Qt on Windows now uses the ICU API of the
Windows SDK for text codec support (if available and no 'full' ICU is
configured). This greatly enhances the list of supported text codecs
available.

Fixes: QTBUG-132056
Change-Id: Id02ae9f8cf56dee8c34795c0e60d2113f319bfd9
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/CMakeLists.txt

@@ -871,6 +871,11 @@ qt_internal_extend_target(Core CONDITION WIN32
         text/qlocale_win.cpp
 )
 
+qt_internal_extend_target(Core CONDITION QT_FEATURE_winsdkicu
+    LIBRARIES
+        icu
+)
+
 qt_internal_extend_target(Core CONDITION WASM
     SOURCES
         text/qlocale_wasm.cpp

src/corelib/configure.cmake

@@ -515,6 +515,21 @@ renameat2(AT_FDCWD, argv[1], AT_FDCWD, argv[2], RENAME_NOREPLACE | RENAME_WHITEO
 }
 ")
 
+qt_config_compile_test(winsdkicu
+    LABEL "Windows SDK: ICU"
+    LIBRARIES icu
+    CODE
+"#include <icu.h>
+
+int main(void)
+{
+    /* BEGIN TEST: */
+    /* END TEST: */
+    return 0;
+}
+"
+)
+
 # cpp_winrt
 qt_config_compile_test(cpp_winrt
     LABEL "cpp/winrt"
@@ -666,6 +681,12 @@ qt_feature("icu" PRIVATE
     AUTODETECT NOT WIN32
     CONDITION ICU_FOUND
 )
+qt_feature("winsdkicu" PRIVATE
+    LABEL "ICU (Windows SDK)"
+    AUTODETECT WIN32
+    CONDITION TEST_winsdkicu
+    DISABLE QT_FEATURE_icu
+)
 qt_feature("inotify" PUBLIC PRIVATE
     LABEL "inotify"
     CONDITION TEST_inotify OR TEST_fsnotify

src/corelib/global/qconfig-bootstrapped.h

@@ -94,5 +94,6 @@
 #define QT_NO_TRANSLATION
 #define QT_FEATURE_translation -1
 #define QT_NO_VARIANT -1
+#define QT_FEATURE_winsdkicu -1
 
 #endif // QT_BOOTSTRAPPED

src/corelib/text/qstringconverter.cpp

@@ -19,7 +19,11 @@
 #include <unicode/ucnv_cb.h>
 #include <unicode/ucnv_err.h>
 #include <unicode/ustring.h>
-#endif
+#define QT_USE_ICU_CODECS
+#elif QT_CONFIG(winsdkicu)
+#include <icu.h>
+#define QT_USE_ICU_CODECS
+#endif // QT_CONFIG(icu) || QT_CONFIG(winsdkicu)
 
 #ifdef Q_OS_WIN
 #include <qt_windows.h>
@@ -1836,7 +1840,7 @@ void QStringConverter::State::clear() noexcept
 void QStringConverter::State::reset() noexcept
 {
     if (flags & Flag::UsesIcu) {
-#if QT_CONFIG(icu)
+#if defined(QT_USE_ICU_CODECS)
         UConverter *converter = static_cast<UConverter *>(d[0]);
         if (converter)
             ucnv_reset(converter);
@@ -2152,7 +2156,7 @@ static bool nameMatch(const char *a, QAnyStringView b)
 */
 
 
-#if QT_CONFIG(icu)
+#if defined(QT_USE_ICU_CODECS)
 // only derives from QStringConverter to get access to protected types
 struct QStringConverterICU : QStringConverter
 {
@@ -2418,7 +2422,7 @@ QStringConverter::QStringConverter(QAnyStringView name, Flags f)
     auto e = encodingForName(name);
     if (e)
         iface = encodingInterfaces + int(*e);
-#if QT_CONFIG(icu)
+#if defined(QT_USE_ICU_CODECS)
     else
         iface = QStringConverterICU::make_icu_converter(&state, name);
 #endif
@@ -2430,7 +2434,7 @@ const char *QStringConverter::name() const noexcept
     if (!iface)
         return nullptr;
     if (state.flags & QStringConverter::Flag::UsesIcu) {
-#if QT_CONFIG(icu)
+#if defined(QT_USE_ICU_CODECS)
         return static_cast<const char*>(state.d[1]);
 #else
         return nullptr;
@@ -2610,7 +2614,7 @@ std::optional<QStringConverter::Encoding> QStringConverter::encodingForHtml(QByt
 
 static qsizetype availableCodecCount()
 {
-#if !QT_CONFIG(icu)
+#if !defined(QT_USE_ICU_CODECS)
     return QStringConverter::Encoding::LastEncoding;
 #else
     /* icu contains also the names of what Qt provides
@@ -2637,7 +2641,7 @@ QStringList QStringConverter::availableCodecs()
 {
     auto availableCodec = [](qsizetype index) -> QString
     {
-    #if !QT_CONFIG(icu)
+    #if !defined(QT_USE_ICU_CODECS)
         return QString::fromLatin1(encodingInterfaces[index].name);
     #else
         if (index == 0) // "Locale", not provided by icu

tests/auto/corelib/text/qstringconverter/tst_qstringconverter.cpp

@@ -151,7 +151,7 @@ private slots:
 
     void convertL1U16();
 
-#if QT_CONFIG(icu)
+#if QT_CONFIG(icu) || QT_CONFIG(winsdkicu)
     void roundtripIcu_data();
     void roundtripIcu();
     void icuInvalidCharacter_data();
@@ -501,7 +501,7 @@ void tst_QStringConverter::convertL1U8()
     }
 }
 
-#if QT_CONFIG(icu)
+#if QT_CONFIG(icu) || QT_CONFIG(winsdkicu)
 
 void tst_QStringConverter::roundtripIcu_data()
 {