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diff --git a/include/rtl/stringconcat.hxx b/include/rtl/stringconcat.hxx
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--- /dev/null
+++ b/include/rtl/stringconcat.hxx
@@ -0,0 +1,588 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/*
+ * This file is part of the LibreOffice project.
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+#ifndef INCLUDED_RTL_STRINGCONCAT_HXX
+#define INCLUDED_RTL_STRINGCONCAT_HXX
+
+// This file is only included from LIBO_INTERNAL_ONLY
+
+#include "rtl/stringutils.hxx"
+#include "rtl/string.h"
+#include "rtl/ustring.h"
+
+#include <cassert>
+#include <cstddef>
+#include <memory>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <utility>
+
+#include <string.h>
+
+#if defined RTL_STRING_UNITTEST_CONCAT
+extern bool rtl_string_unittest_invalid_concat;
+#endif
+
+#ifdef RTL_STRING_UNITTEST
+#define rtl rtlunittest
+#endif
+namespace rtl
+{
+#ifdef RTL_STRING_UNITTEST
+#undef rtl
+#endif
+
+/*
+Implementation of efficient string concatenation.
+
+The whole system is built around two basic template classes:
+- ToStringHelper< T > - for each T it can give the length of the resulting string representation and can write
+    this string representation to a buffer
+- O(U)StringConcat< T1, T2 > - operator+ now, instead of creating O(U)String object, returns only this helper object,
+    that keeps a reference to both operator+ operands; only when converted to O(U)String it will actually create
+    the resulting string object using ToStringHelper, creating directly the resulting object without any string
+    intermediate objects
+As all the code is inline methods, it allows for extensive optimization and will usually result in very effective code
+(even surpassing strlen/strcat and equalling handwritten), while allowing for very easy and intuitive syntax.
+*/
+
+/**
+@internal
+
+Helper class for converting a given type to a string representation.
+*/
+template< typename T >
+struct ToStringHelper
+    {
+    /// Return length of the string representation of the given object.
+    static std::size_t length( const T& );
+    /// Add 8-bit representation of the given object to the given buffer and return position right after the added data.
+    static char* addData( char* buffer, const T& ) SAL_RETURNS_NONNULL;
+    /// Add Unicode representation of the given object to the given buffer and return position right after the added data.
+    static sal_Unicode* addData( sal_Unicode* buffer, const T& ) SAL_RETURNS_NONNULL;
+    /// If true, T can be used in concatenation resulting in OString.
+    static const bool allowOStringConcat = false;
+    /// If true, T can be used in concatenation resulting in OUString.
+    static const bool allowOUStringConcat = false;
+    };
+
+inline
+char* addDataHelper( char* buffer, const char* data, std::size_t length )
+    {
+    if (length != 0) {
+        memcpy( buffer, data, length );
+    }
+    return buffer + length;
+    }
+
+inline
+sal_Unicode* addDataHelper( sal_Unicode* buffer, const sal_Unicode* data, std::size_t length )
+    {
+    if (length != 0) {
+        memcpy( buffer, data, length * sizeof( sal_Unicode ));
+    }
+    return buffer + length;
+    }
+
+inline
+sal_Unicode* addDataLiteral( sal_Unicode* buffer, const char* data, std::size_t length )
+    {
+    for( std::size_t i = 0; i != length; ++i )
+        *buffer++ = *data++;
+    return buffer;
+    }
+
+inline
+char* addDataCString( char* buffer, const char* str )
+    {
+    while( *str != '\0' )
+        *buffer++ = *str++;
+    return buffer;
+    }
+
+inline
+sal_Unicode* addDataUString( sal_Unicode* buffer, const sal_Unicode* str )
+    {
+    while( *str != '\0' )
+        *buffer++ = *str++;
+    return buffer;
+    }
+
+template<>
+struct ToStringHelper< const char* >
+    {
+    static std::size_t length( const char* str ) {
+        return str ? strlen( str ) : 0;
+    }
+    static char* addData( char* buffer, const char* str ) {
+        return str ? addDataCString( buffer, str ) : buffer;
+    }
+    static const bool allowOStringConcat = true;
+    static const bool allowOUStringConcat = false;
+    };
+
+template<>
+struct ToStringHelper< char* > : public ToStringHelper< const char* > {};
+
+template< std::size_t N >
+struct ToStringHelper< char[ N ] >
+    {
+    static std::size_t length( const char str[ N ] ) {
+        return strlen( str );
+    }
+    static char* addData( char* buffer, const char str[ N ] ) { return addDataCString( buffer, str ); }
+    static const bool allowOStringConcat = true;
+    static const bool allowOUStringConcat = false;
+    };
+
+template< std::size_t N >
+struct ToStringHelper< const char[ N ] >
+    {
+    static std::size_t length( const char str[ N ] ) { (void)str; assert( strlen( str ) == N - 1 ); return N - 1; }
+    static char* addData( char* buffer, const char str[ N ] ) { return addDataHelper( buffer, str, N - 1 ); }
+    static sal_Unicode* addData( sal_Unicode* buffer, const char str[ N ] ) { return addDataLiteral( buffer, str, N - 1 ); }
+    static const bool allowOStringConcat = true;
+    static const bool allowOUStringConcat = true;
+    };
+
+template<>
+struct ToStringHelper<OStringChar>
+    {
+    static std::size_t length(OStringChar) { return 1; }
+    static char* addData(char* buffer, OStringChar data)
+    { return addDataHelper(buffer, &data.c, 1); }
+    static bool const allowOStringConcat = true;
+    static bool const allowOUStringConcat = false;
+    };
+
+template<>
+struct ToStringHelper< const sal_Unicode* >
+    {
+    static std::size_t length( const sal_Unicode* str ) {
+        return str ? std::char_traits<char16_t>::length( str ) : 0;
+    }
+    static sal_Unicode* addData( sal_Unicode* buffer, const sal_Unicode* str ) {
+        return str ? addDataUString( buffer, str ) : buffer;
+    }
+    static const bool allowOStringConcat = false;
+    static const bool allowOUStringConcat = true;
+    };
+
+template<>
+struct ToStringHelper< sal_Unicode* > : public ToStringHelper< const sal_Unicode* > {};
+
+template<std::size_t N>
+struct ToStringHelper<sal_Unicode[ N ]>
+    {
+    static std::size_t length( const sal_Unicode str[ N ] ) {
+        return std::char_traits<char16_t>::length( str );
+    }
+    static sal_Unicode * addData(sal_Unicode * buffer, sal_Unicode const str[N])
+    { return addDataHelper(buffer, str, N - 1); }
+    static bool const allowOStringConcat = false;
+    static bool const allowOUStringConcat = true;
+    };
+
+template<std::size_t N>
+struct ToStringHelper<sal_Unicode const[N]>
+    {
+    static std::size_t length( const sal_Unicode str[ N ] ) { (void)str; assert( std::char_traits<char16_t>::length( str ) == N - 1 ); return N - 1; }
+    static sal_Unicode * addData(sal_Unicode * buffer, sal_Unicode const str[N])
+    { return addDataHelper(buffer, str, N - 1); }
+    static bool const allowOStringConcat = false;
+    static bool const allowOUStringConcat = true;
+    };
+
+template<>
+struct ToStringHelper<OUStringChar_>
+    {
+    static std::size_t length(OUStringChar_) { return 1; }
+    static sal_Unicode * addData(sal_Unicode * buffer, OUStringChar_ literal)
+    { return addDataHelper(buffer, &literal.c, 1); }
+    static bool const allowOStringConcat = false;
+    static bool const allowOUStringConcat = true;
+    };
+
+/**
+@internal
+
+Objects returned by operator+, instead of OString. These objects (possibly recursively) keep a representation of the whole
+concatenation operation.
+
+If you get a build error related to this class, you most probably need to explicitly convert the result of a string
+concatenation to OString.
+*/
+template< typename T1, typename T2 >
+struct OStringConcat
+    {
+    public:
+        OStringConcat( const T1& left_, const T2& right_ ) : left( left_ ), right( right_ ) {}
+        std::size_t length() const { return ToStringHelper< T1 >::length( left ) + ToStringHelper< T2 >::length( right ); }
+        char* addData( char* buffer ) const SAL_RETURNS_NONNULL { return ToStringHelper< T2 >::addData( ToStringHelper< T1 >::addData( buffer, left ), right ); }
+        // NOTE here could be functions that would forward to the "real" temporary OString. Note however that e.g. getStr()
+        // is not so simple, as the OString temporary must live long enough (i.e. can't be created here in a function, a wrapper
+        // temporary object containing it must be returned instead).
+    private:
+        const T1& left;
+        const T2& right;
+    };
+
+/**
+@internal
+
+Objects returned by operator+, instead of OUString. These objects (possibly recursively) keep a representation of the whole
+concatenation operation.
+
+If you get a build error related to this class, you most probably need to explicitly convert the result of a string
+concatenation to OUString.
+*/
+template< typename T1, typename T2 >
+struct OUStringConcat
+    {
+    public:
+        OUStringConcat( const T1& left_, const T2& right_ ) : left( left_ ), right( right_ ) {}
+        std::size_t length() const { return ToStringHelper< T1 >::length( left ) + ToStringHelper< T2 >::length( right ); }
+        sal_Unicode* addData( sal_Unicode* buffer ) const SAL_RETURNS_NONNULL { return ToStringHelper< T2 >::addData( ToStringHelper< T1 >::addData( buffer, left ), right ); }
+    private:
+        const T1& left;
+        const T2& right;
+    };
+
+template< typename T1, typename T2 >
+struct ToStringHelper< OStringConcat< T1, T2 > >
+    {
+    static std::size_t length( const OStringConcat< T1, T2 >& c ) { return c.length(); }
+    static char* addData( char* buffer, const OStringConcat< T1, T2 >& c ) SAL_RETURNS_NONNULL { return c.addData( buffer ); }
+    static const bool allowOStringConcat = ToStringHelper< T1 >::allowOStringConcat && ToStringHelper< T2 >::allowOStringConcat;
+    static const bool allowOUStringConcat = false;
+    };
+
+template< typename T1, typename T2 >
+struct ToStringHelper< OUStringConcat< T1, T2 > >
+    {
+    static std::size_t length( const OUStringConcat< T1, T2 >& c ) { return c.length(); }
+    static sal_Unicode* addData( sal_Unicode* buffer, const OUStringConcat< T1, T2 >& c ) SAL_RETURNS_NONNULL { return c.addData( buffer ); }
+    static const bool allowOStringConcat = false;
+    static const bool allowOUStringConcat = ToStringHelper< T1 >::allowOUStringConcat && ToStringHelper< T2 >::allowOUStringConcat;
+    };
+
+template< typename T1, typename T2 >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T1 >::allowOStringConcat && ToStringHelper< T2 >::allowOStringConcat, OStringConcat< T1, T2 > > operator+( const T1& left, const T2& right )
+    {
+    return OStringConcat< T1, T2 >( left, right );
+    }
+
+// char[N] and const char[N] need to be done explicitly, otherwise the compiler likes to treat them the same way for some reason
+template< typename T, std::size_t N >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T >::allowOStringConcat, OStringConcat< T, const char[ N ] > > operator+( const T& left, const char (&right)[ N ] )
+    {
+    return OStringConcat< T, const char[ N ] >( left, right );
+    }
+
+template< typename T, std::size_t N >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T >::allowOStringConcat, OStringConcat< const char[ N ], T > > operator+( const char (&left)[ N ], const T& right )
+    {
+    return OStringConcat< const char[ N ], T >( left, right );
+    }
+
+template< typename T, std::size_t N >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T >::allowOStringConcat, OStringConcat< T, char[ N ] > > operator+( const T& left, char (&right)[ N ] )
+    {
+    return OStringConcat< T, char[ N ] >( left, right );
+    }
+
+template< typename T, std::size_t N >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T >::allowOStringConcat, OStringConcat< char[ N ], T > > operator+( char (&left)[ N ], const T& right )
+    {
+    return OStringConcat< char[ N ], T >( left, right );
+    }
+
+template< typename T1, typename T2 >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T1 >::allowOUStringConcat && ToStringHelper< T2 >::allowOUStringConcat, OUStringConcat< T1, T2 > > operator+( const T1& left, const T2& right )
+    {
+    return OUStringConcat< T1, T2 >( left, right );
+    }
+
+template< typename T1, typename T2 >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T1 >::allowOUStringConcat && ToStringHelper< T2 >::allowOUStringConcat && libreoffice_internal::ConstCharArrayDetector< T1, void >::ok, OUStringConcat< T1, T2 > > operator+( T1& left, const T2& right )
+    {
+    return OUStringConcat< T1, T2 >( left, right );
+    }
+
+template< typename T1, typename T2 >
+[[nodiscard]]
+inline
+typename std::enable_if_t< ToStringHelper< T1 >::allowOUStringConcat && ToStringHelper< T2 >::allowOUStringConcat && libreoffice_internal::ConstCharArrayDetector< T2, void >::ok, OUStringConcat< T1, T2 > > operator+( const T1& left, T2& right )
+    {
+    return OUStringConcat< T1, T2 >( left, right );
+    }
+
+#ifdef RTL_STRING_UNITTEST_CONCAT
+// Special overload to catch the remaining invalid combinations. The helper struct must
+// be used to make this operator+ overload a worse choice than all the existing overloads above.
+struct StringConcatInvalid
+    {
+    template< typename T >
+    StringConcatInvalid( const T& ) {}
+    };
+template< typename T >
+inline
+int operator+( const StringConcatInvalid&, const T& )
+    {
+    rtl_string_unittest_invalid_concat = true;
+    return 0; // doesn't matter
+    }
+#endif
+
+// Lightweight alternative to OString when a (temporary) object is needed to hold an OStringConcat
+// result that can then be used as a std::string_view:
+class OStringConcatenation {
+public:
+    template<typename T1, typename T2>
+    explicit OStringConcatenation(OStringConcat<T1, T2> const & c):
+        length_(c.length()),
+        buffer_(new char[length_])
+    {
+        auto const end = c.addData(buffer_.get());
+        assert(end == buffer_.get() + length_); (void)end;
+    }
+
+    operator std::string_view() const { return {buffer_.get(), length_}; }
+
+private:
+    std::size_t length_;
+    std::unique_ptr<char[]> buffer_;
+};
+
+// Lightweight alternative to OUString when a (temporary) object is needed to hold an
+// OUStringConcat result that can then be used as a std::u16string_view:
+class OUStringConcatenation {
+public:
+    template<typename T1, typename T2>
+    explicit OUStringConcatenation(OUStringConcat<T1, T2> const & c):
+        length_(c.length()),
+        buffer_(new char16_t[length_])
+    {
+        auto const end = c.addData(buffer_.get());
+        assert(end == buffer_.get() + length_); (void)end;
+    }
+
+    operator std::u16string_view() const { return {buffer_.get(), length_}; }
+
+private:
+    std::size_t length_;
+    std::unique_ptr<char16_t[]> buffer_;
+};
+
+/**
+ @internal
+
+Objects returned by OString::number(), instead of OString. These objects keep a representation of the number() operation.
+
+If you get a build error related to this class, you most probably need to explicitly convert the result of calling
+OString::number() to OString.
+*/
+template< typename T >
+struct OStringNumber;
+
+template <typename Number, std::size_t nBufSize> struct OStringNumberBase
+{
+    using number_t = Number;
+    // OString::number(value).getStr() is very common (writing xml code, ...),
+    // so implement that one also here, to avoid having to explicitly to convert
+    // to OString in all such places
+    const char * getStr() const SAL_RETURNS_NONNULL { return buf; }
+    OStringNumber<number_t>&& toAsciiUpperCase()
+    {
+        rtl_str_toAsciiUpperCase_WithLength(buf, length);
+        return std::move(*static_cast<OStringNumber<number_t>*>(this));
+    }
+    operator std::string_view() const { return std::string_view(buf, length); }
+    char buf[nBufSize];
+    sal_Int32 length;
+};
+
+template<>
+struct OStringNumber< int >
+    : public OStringNumberBase<int, RTL_STR_MAX_VALUEOFINT32>
+{
+    OStringNumber(number_t i, sal_Int16 radix) { length = rtl_str_valueOfInt32(buf, i, radix); }
+};
+
+template<>
+struct OStringNumber< long long >
+    : public OStringNumberBase<long long, RTL_STR_MAX_VALUEOFINT64>
+{
+    OStringNumber(number_t i, sal_Int16 radix) { length = rtl_str_valueOfInt64(buf, i, radix); }
+};
+
+template<>
+struct OStringNumber< unsigned long long >
+    : public OStringNumberBase<unsigned long long, RTL_STR_MAX_VALUEOFUINT64>
+{
+    OStringNumber(number_t i, sal_Int16 radix) { length = rtl_str_valueOfUInt64(buf, i, radix); }
+};
+
+template<>
+struct OStringNumber< float >
+    : public OStringNumberBase<float, RTL_STR_MAX_VALUEOFFLOAT>
+{
+    OStringNumber(number_t f) { length = rtl_str_valueOfFloat(buf, f); }
+};
+
+template<>
+struct OStringNumber< double >
+    : public OStringNumberBase<double, RTL_STR_MAX_VALUEOFDOUBLE>
+{
+    OStringNumber(number_t d) { length = rtl_str_valueOfDouble(buf, d); }
+};
+
+template< typename T >
+struct ToStringHelper< OStringNumber< T > >
+    {
+    static std::size_t length( const OStringNumber< T >& n ) { return n.length; }
+    static char* addData( char* buffer, const OStringNumber< T >& n ) SAL_RETURNS_NONNULL { return addDataHelper( buffer, n.buf, n.length ); }
+    static const bool allowOStringConcat = true;
+    static const bool allowOUStringConcat = false;
+    };
+
+
+/**
+ @internal
+
+Objects returned by OUString::number(), instead of OUString. These objects keep a representation of the number() operation.
+
+If you get a build error related to this class, you most probably need to explicitly convert the result of calling
+OUString::number() to OUString.
+*/
+template< typename T >
+struct OUStringNumber;
+
+template <typename Number, std::size_t nBufSize> struct OUStringNumberBase
+{
+    using number_t = Number;
+    OUStringNumber<number_t>&& toAsciiUpperCase()
+    {
+        rtl_ustr_toAsciiUpperCase_WithLength(buf, length);
+        return std::move(*static_cast<OUStringNumber<number_t>*>(this));
+    }
+    operator std::u16string_view() const { return std::u16string_view(buf, length); }
+    sal_Unicode buf[nBufSize];
+    sal_Int32 length;
+};
+
+template<>
+struct OUStringNumber< int >
+    : public OUStringNumberBase<int, RTL_USTR_MAX_VALUEOFINT32>
+{
+    OUStringNumber(number_t i, sal_Int16 radix) { length = rtl_ustr_valueOfInt32(buf, i, radix); }
+};
+
+template<>
+struct OUStringNumber< long long >
+    : public OUStringNumberBase<long long, RTL_USTR_MAX_VALUEOFINT64>
+{
+    OUStringNumber(number_t i, sal_Int16 radix) { length = rtl_ustr_valueOfInt64(buf, i, radix); }
+};
+
+template<>
+struct OUStringNumber< unsigned long long >
+    : public OUStringNumberBase<unsigned long long, RTL_USTR_MAX_VALUEOFUINT64>
+{
+    OUStringNumber(number_t i, sal_Int16 radix) { length = rtl_ustr_valueOfUInt64(buf, i, radix); }
+};
+
+template<>
+struct OUStringNumber< float >
+    : public OUStringNumberBase<float, RTL_USTR_MAX_VALUEOFFLOAT>
+{
+    OUStringNumber(number_t f) { length = rtl_ustr_valueOfFloat(buf, f); }
+};
+
+template<>
+struct OUStringNumber< double >
+    : public OUStringNumberBase<double, RTL_USTR_MAX_VALUEOFDOUBLE>
+{
+    OUStringNumber(number_t d) { length = rtl_ustr_valueOfDouble(buf, d); }
+};
+
+template< typename T >
+struct ToStringHelper< OUStringNumber< T > >
+    {
+    static std::size_t length( const OUStringNumber< T >& n ) { return n.length; }
+    static sal_Unicode* addData( sal_Unicode* buffer, const OUStringNumber< T >& n ) SAL_RETURNS_NONNULL { return addDataHelper( buffer, n.buf, n.length ); }
+    static const bool allowOStringConcat = false;
+    static const bool allowOUStringConcat = true;
+    };
+
+template<> struct ToStringHelper<std::string_view> {
+    static constexpr std::size_t length(std::string_view s) { return s.size(); }
+
+    static char * addData(char * buffer, std::string_view s) SAL_RETURNS_NONNULL
+    { return addDataHelper(buffer, s.data(), s.size()); }
+
+    static constexpr bool allowOStringConcat = true;
+    static constexpr bool allowOUStringConcat = false;
+};
+
+template<> struct ToStringHelper<std::u16string_view> {
+    static constexpr std::size_t length(std::u16string_view s) { return s.size(); }
+
+    static sal_Unicode * addData(sal_Unicode * buffer, std::u16string_view s) SAL_RETURNS_NONNULL
+    { return addDataHelper(buffer, s.data(), s.size()); }
+
+    static constexpr bool allowOStringConcat = false;
+    static constexpr bool allowOUStringConcat = true;
+};
+
+// An internal marker class used by OString::Concat:
+struct OStringConcatMarker {};
+
+template<> struct ToStringHelper<OStringConcatMarker> {
+    static constexpr std::size_t length(OStringConcatMarker) { return 0; }
+
+    static constexpr char * addData(char * buffer, OStringConcatMarker) SAL_RETURNS_NONNULL
+    { return buffer; }
+
+    static constexpr bool allowOStringConcat = true;
+    static constexpr bool allowOUStringConcat = false;
+};
+
+// An internal marker class used by OUString::Concat:
+struct OUStringConcatMarker {};
+
+template<> struct ToStringHelper<OUStringConcatMarker> {
+    static constexpr std::size_t length(OUStringConcatMarker) { return 0; }
+
+    static constexpr sal_Unicode * addData(sal_Unicode * buffer, OUStringConcatMarker)
+        SAL_RETURNS_NONNULL
+    { return buffer; }
+
+    static constexpr bool allowOStringConcat = false;
+    static constexpr bool allowOUStringConcat = true;
+};
+
+} // namespace
+
+#endif