SL-10153: Introduce ll_convert, windows_message() templates.

Add ll_convert<TO, FROM> template, used as (e.g.):
ll_convert<std::string>(value_of_some_other_string_type);
There is no generic template implementation -- the template exists solely to
provide generic aliases for a bewildering family of llstring.h string-
conversion functions with highly-specific names. There's a generic
implementation, though, for the degenerate case where FROM and TO are
identical.

Add ll_convert<> specialization aliases for most of the string-conversion
functions declared in llstring.h, including the Windows-specific ones
involving llutf16string and std::wstring.

Add a mini-lecture in llstring.h about appropriate use of string types on
Windows.

Add LL_WCHAR_T_NATIVE llpreprocessor.h macro so we can detect whether to
provide separate conversions for llutf16string and std::wstring, or whether
those would collide because the types are identical.

Add inline ll_convert_wide_to_string(const std::wstring&) overloads so caller
isn't required to call arg.c_str(), which naturally permits an ll_convert
alias.

Add ll_convert_wide_to_wstring(), ll_convert_wstring_to_wide() as placeholders
for converting between Windows std::wstring and Linden LLWString, with
corresponding ll_convert aliases. We don't yet have library code to perform
such conversions officially; for now, just copy characters.

Add LLStringUtil::getenv(key) and getoptenv(key) functions. The latter returns
boost::optional<string_type> in case the caller needs to detect absence of a
given environment variable rather than simply accepting a default value.
Naturally getenv(), which accepts a default, is implemented using getoptenv().
getoptenv(), in turn, is implemented using an underlying llstring_getoptenv().

On Windows, llstring_getoptenv() returns boost::optional<std::wstring> (based
on GetEnvironmentVariableW()), whereas elsewhere, llstring_getoptenv() returns
boost::optional<std::string> (based on classic Posix getenv()).

The beauty of generic ll_convert is that the portable LLStringUtilBase<T>::
getoptenv() template can call the platform-specific llstring_getoptenv() and
transparently perform whatever conversion is necessary to return the desired
string_type.

Add windows_message<T>(error) template, with an overload that implicitly calls
GetLastError(). We provide a single concrete windows_message<std::wstring>()
implementation because that's what we get from Windows FormatMessageW() --
everything else is a generic conversion to the desired target string type.

This obviates llprocess.cpp's previous WindowsErrorString() implementation --
reimplement using windows_message<std::string>().

1 files changed, 21 insertions, 0 deletions

diff --git a/indra/llcommon/llpreprocessor.h b/indra/llcommon/llpreprocessor.h
index ef015fdce4..e8f9981437 100644
--- a/indra/llcommon/llpreprocessor.h
+++ b/indra/llcommon/llpreprocessor.h
@@ -101,6 +101,9 @@
 
 #endif
 
+// Although thread_local is now a standard storage class, we can't just
+// #define LL_THREAD_LOCAL as thread_local because the *usage* is different.
+// We'll have to take the time to change LL_THREAD_LOCAL declarations by hand.
 #if LL_WINDOWS
 # define LL_THREAD_LOCAL __declspec(thread)
 #else
@@ -177,6 +180,24 @@
 #define LL_DLLIMPORT
 #endif // LL_WINDOWS
 
+#if ! defined(LL_WINDOWS)
+#define LL_WCHAR_T_NATIVE 1
+#else  // LL_WINDOWS
+// https://docs.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+// _WCHAR_T_DEFINED is defined if wchar_t is provided at all.
+// Specifically, it has value 1 if wchar_t is an intrinsic type, else empty.
+// _NATIVE_WCHAR_T_DEFINED has value 1 if wchar_t is intrinsic, else undefined.
+// For years we have compiled with /Zc:wchar_t-, meaning that wchar_t is a
+// typedef for unsigned short (in stddef.h). Lore has it that one of our
+// proprietary binary-only libraries has traditionally been built that way and
+// therefore EVERYTHING ELSE requires it. Therefore, in a typical Linden
+// Windows build, _WCHAR_T_DEFINED is defined but empty, while
+// _NATIVE_WCHAR_T_DEFINED is undefined.
+# if defined(_NATIVE_WCHAR_T_DEFINED)
+#  define LL_WCHAR_T_NATIVE 1
+# endif // _NATIVE_WCHAR_T_DEFINED
+#endif // LL_WINDOWS
+
 #if LL_COMMON_LINK_SHARED
 // CMake automagically defines llcommon_EXPORTS only when building llcommon
 // sources, and only when llcommon is a shared library (i.e. when