Turn OStringLiteral into a consteval'ed, static-refcound rtl_String

...from which an OString can cheaply be instantiated.

The one downside is that OStringLiteral 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 containers 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::string_view, without loss of efficiency
compared to the original OStringLiteral, and without loss of expressivity (esp.
with the newly introduced OString(std::string_view) ctor).

The new OStringLiteral 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 OStringLiteral is in all cases where an
object that shall itself not be an OString (e.g., because it shall be a
global static variable for which the OString ctor/dtor would be detrimental at
library load/unload) must be converted to an OString instance in at least one
place.  Other string literal abstractions could use std::string_view (or just
plain char const[N]), but interestingly OStringLiteral might be more efficient
than constexpr std::string_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.

sal/qa/rtl/strings/test_ostring_concat.cxx documents some workarounds for GCC
bug <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96878> "Failed class template
argument deduction in unevaluated, parenthesized context".  Those places, as
well as uses of OStringLiteral in incodemaker/source/javamaker/javaoptions.cxx
and i18npool/source/breakiterator/breakiterator_unicode.cxx, which have been
replaced with OString::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).

This change also revealed a bug in at least recent Clang 12 trunk
CastExpr::getSubExprAsWritten (still to be reported to LLVM), triggered at least
in some calls from loplugin code (for which it can be fixed for now in the
existing compat::getSubStringAsWritten).

A similar commit for OUStringLiteral is planned, too.

Change-Id: Ib192f4ed4c44769512a16364cb55c25627bae6f4
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/101814
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

4 files changed, 27 insertions, 4 deletions

diff --git a/compilerplugins/clang/compat.hxx b/compilerplugins/clang/compat.hxx
index fb8791f978b4..70b462e54e4b 100644
--- a/compilerplugins/clang/compat.hxx
+++ b/compilerplugins/clang/compat.hxx
@@ -215,6 +215,28 @@ inline clang::Expr * getSubExpr(clang::MaterializeTemporaryExpr const * expr) {
 //        `-MemberExpr 0x2b74e8e656a0 '<bound member function type>' .operator ResString 0x2b74e8dc1f00
 //          `-DeclRefExpr 0x2b74e8e65648 'struct ErrorResource_Impl' lvalue Var 0x2b74e8e653b0 'aEr' 'struct ErrorResource_Impl'
 //
+// Also work around CastExpr::getSubExprAsWritten firing
+//
+//   include/llvm/Support/Casting.h:269: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*)
+//   [with X = clang::CXXConstructExpr; Y = clang::Expr;
+//   typename llvm::cast_retty<X, Y*>::ret_type = clang::CXXConstructExpr*]: Assertion
+//   `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
+//
+// for CastExprs involving ConstantExpr (introduced with
+// <https://github.com/llvm/llvm-project/commit/7c44da279e399d302a685c500e7f802f8adf9762> "Create
+// ConstantExpr class" towards LLVM 8) like
+//
+//   CXXFunctionalCastExpr 0xc01c4e8 'class rtl::OStringLiteral<9>':'class rtl::OStringLiteral<9>' functional cast to OStringLiteral <ConstructorConversion>
+//   `-ConstantExpr 0xc01c380 'class rtl::OStringLiteral<9>':'class rtl::OStringLiteral<9>'
+//     |-value: Struct
+//     | |-fields: Int 1073741824, Int 8
+//     | `-field: Array size=9
+//     |   |-elements: Int 46, Int 111, Int 115, Int 108
+//     |   |-elements: Int 45, Int 116, Int 109, Int 112
+//     |   `-element: Int 0
+//     `-CXXConstructExpr 0xc01c350 'class rtl::OStringLiteral<9>':'class rtl::OStringLiteral<9>' 'void (const char (&)[9])'
+//       `-StringLiteral 0xc019ad8 'const char [9]' lvalue ".osl-tmp"
+//
 // Copies code from Clang's lib/AST/Expr.cpp:
 namespace detail {
   inline clang::Expr *skipImplicitTemporary(clang::Expr *expr) {
@@ -240,7 +262,7 @@ inline clang::Expr *getSubExprAsWritten(clang::CastExpr *This) {
     // subexpression describing the call; strip it off.
     if (E->getCastKind() == clang::CK_ConstructorConversion)
       SubExpr =
-        detail::skipImplicitTemporary(clang::cast<clang::CXXConstructExpr>(SubExpr)->getArg(0));
+        detail::skipImplicitTemporary(clang::cast<clang::CXXConstructExpr>(SubExpr->IgnoreImplicit())->getArg(0));
     else if (E->getCastKind() == clang::CK_UserDefinedConversion) {
       assert((clang::isa<clang::CXXMemberCallExpr>(SubExpr) ||
               clang::isa<clang::BlockExpr>(SubExpr)) &&
diff --git a/compilerplugins/clang/stringconcatliterals.cxx b/compilerplugins/clang/stringconcatliterals.cxx
index 25872ff6ee32..509a10363c01 100644
--- a/compilerplugins/clang/stringconcatliterals.cxx
+++ b/compilerplugins/clang/stringconcatliterals.cxx
@@ -27,7 +27,7 @@ Expr const * stripCtor(Expr const * expr) {
         return expr;
     }
     auto qt = loplugin::DeclCheck(e2->getConstructor());
-    if (qt.MemberFunction().Struct("OStringLiteral").Namespace("rtl").GlobalNamespace()) {
+    if (qt.MemberFunction().Class("OStringLiteral").Namespace("rtl").GlobalNamespace()) {
         if (e2->getNumArgs() == 1) {
             return e2->getArg(0)->IgnoreParenImpCasts();
         }
diff --git a/compilerplugins/clang/test/stringconcatliterals.cxx b/compilerplugins/clang/test/stringconcatliterals.cxx
index 76b82797f77e..348440f0ec4c 100644
--- a/compilerplugins/clang/test/stringconcatliterals.cxx
+++ b/compilerplugins/clang/test/stringconcatliterals.cxx
@@ -19,7 +19,7 @@
 
 void f(std::ostream& s1)
 {
-    static char const foo[] = "foo";
+    static constexpr char foo[] = "foo";
     static char16_t const foou[] = u"foo";
     s1 << "foo"
        << "foo";
diff --git a/compilerplugins/clang/unusedmember.cxx b/compilerplugins/clang/unusedmember.cxx
index bcd9a8593a68..a32fd341c665 100644
--- a/compilerplugins/clang/unusedmember.cxx
+++ b/compilerplugins/clang/unusedmember.cxx
@@ -21,6 +21,7 @@
 #include "config_clang.h"
 
 #include "check.hxx"
+#include "compat.hxx"
 #include "plugin.hxx"
 
 namespace
@@ -289,7 +290,7 @@ public:
             return true;
         }
         auto const t1 = expr->getType();
-        auto const t2 = expr->getSubExprAsWritten()->getType();
+        auto const t2 = compat::getSubExprAsWritten(expr)->getType();
         if (loplugin::TypeCheck(t1).Pointer().Void())
         {
             recordCastedRecordDecl(t2);