Turn OUStringLiteral into a consteval'ed, static-refcound rtl_uString

...from which an OUString can cheaply be instantiated.  This is the OUString
equivalent of 4b9e440c51be3e40326bc90c33ae69885bfb51e4 "Turn OStringLiteral into
a consteval'ed, static-refcound rtl_String".  Most remarks about that commit
apply here too (this commit is just substantially bigger and a bit more
complicated because there were so much more uses of OUStringLiteral than of
OStringLiteral):

The one downside is that OUStringLiteral now needs to be a template abstracting
over the string length.  But any uses for which that is a problem (e.g., as the
element type of a container that would no longer be homogeneous, or in the
signature of a function that shall not be turned into a template for one reason
or another) can be replaced with std::u16string_view, without loss of efficiency
compared to the original OUStringLiteral, and without loss of expressivity.

The new OUStringLiteral ctor code would probably not be very efficient if it
were ever executed at runtime, but it is intended to be only executed at compile
time.  Where available, C++20 "consteval" is used to statically ensure that.

The intended use of the new OUStringLiteral is in all cases where an
object that shall itself not be an OUString (e.g., because it shall be a
global static variable for which the OUString ctor/dtor would be detrimental at
library load/unload) must be converted to an OUString instance in at least one
place.  Other string literal abstractions could use std::u16string_view (or just
plain char16_t const[N]), but interestingly OUStringLiteral might be more
efficient than constexpr std::u16string_view even for such cases, as it should
not need any relocations at library load time.  For now, no existing uses of
OUStringLiteral have been changed to some other abstraction (unless technically
necessary as discussed above), and no additional places that would benefit from
OUStringLiteral have been changed to use it.

Global constexpr OUStringLiteral variables defined in an included file would be
somewhat suboptimal, as each translation unit that uses them would create its
own, unshared instance.  The envisioned solution is to turn them into static
data members of some class (and there may be a loplugin coming to find and fix
affected places).  Another approach that has been taken here in a few cases
where such variables were only used in one .cxx anyway is to move their
definitions from the .hxx into that one .cxx (in turn causing some files to
become empty and get removed completely)---which also silenced some GCC
-Werror=unused-variable if a variable from a .hxx was not used in some .cxx
including it.

To keep individual commits reasonably manageable, some consumers of
OUStringLiteral in rtl/ustrbuf.hxx and rtl/ustring.hxx are left in a somewhat
odd state for now, where they don't take advantage of OUStringLiteral's
equivalence to rtl_uString, but just keep extracting its contents and copy it
elsewhere.  In follow-up commits, those consumers should be changed
appropriately, making them treat OUStringLiteral like an rtl_uString or
dropping the OUStringLiteral overload in favor of an existing (and cheap to use
now) OUString overload, etc.

In a similar vein, comparison operators between OUString and std::u16string_view
have been added to the existing plethora of comparison operator overloads.  It
would be nice to eventually consolidate them, esp. with the overloads taking
OUStringLiteral and/or char16_t const[N] string literals, but that appears
tricky to get right without introducing new ambiguities.  Also, a handful of
places across the code base use comparisons between OUString and OUStringNumber,
which are now ambiguous (converting the OUStringNumber to either OUString or
std::u16string_view).  For simplicity, those few places have manually been fixed
for now by adding explicit conversion to std::u16string_view.

Also some compilerplugins code needed to be adapted, and some of the
compilerplugins/test cases have become irrelevant (and have been removed), as
the tested code would no longer compile in the first place.

sal/qa/rtl/strings/test_oustring_concat.cxx documents a workaround for GCC bug
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96878> "Failed class template
argument deduction in unevaluated, parenthesized context".  That place, as well
as uses of OUStringLiteral in extensions/source/abpilot/fieldmappingimpl.cxx and
i18npool/source/localedata/localedata.cxx, which have been replaced with
OUString::Concat (and which is arguably a better choice, anyway), also caused
failures with at least Clang 5.0.2 (but would not have caused failures with at
least recent Clang 12 trunk, so appear to be bugs in Clang that have meanwhile
been fixed).

Change-Id: I34174462a28f2000cfeb2d219ffd533a767920b8
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/102222
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 4 insertions, 10 deletions

diff --git a/svgio/inc/svgtools.hxx b/svgio/inc/svgtools.hxx
index d5df009f8ab6..a78c3210cd7f 100644
--- a/svgio/inc/svgtools.hxx
+++ b/svgio/inc/svgtools.hxx
@@ -36,10 +36,10 @@ namespace svgio::svgreader
         // common non-token strings
         struct commonStrings
         {
-            static const OUStringLiteral aStrUserSpaceOnUse;
-            static const OUStringLiteral aStrObjectBoundingBox;
-            static const OUStringLiteral aStrNonzero;
-            static const OUStringLiteral aStrEvenOdd;
+            static constexpr OUStringLiteral aStrUserSpaceOnUse = u"userSpaceOnUse";
+            static constexpr OUStringLiteral aStrObjectBoundingBox = u"objectBoundingBox";
+            static constexpr OUStringLiteral aStrNonzero = u"nonzero";
+            static constexpr OUStringLiteral aStrEvenOdd = u"evenodd";
         };
 
         enum SvgUnits
diff --git a/svgio/source/svgreader/svgtools.cxx b/svgio/source/svgreader/svgtools.cxx
index 4be1514f69a4..867c3b1b0bea 100644
--- a/svgio/source/svgreader/svgtools.cxx
+++ b/svgio/source/svgreader/svgtools.cxx
@@ -29,12 +29,6 @@
 
 namespace svgio::svgreader
 {
-        // common non-token strings
-        const OUStringLiteral commonStrings::aStrUserSpaceOnUse(u"userSpaceOnUse");
-        const OUStringLiteral commonStrings::aStrObjectBoundingBox(u"objectBoundingBox");
-        const OUStringLiteral commonStrings::aStrNonzero(u"nonzero");
-        const OUStringLiteral commonStrings::aStrEvenOdd(u"evenodd");
-
         basegfx::B2DHomMatrix SvgAspectRatio::createLinearMapping(const basegfx::B2DRange& rTarget, const basegfx::B2DRange& rSource)
         {
             basegfx::B2DHomMatrix aRetval;