Adapt C++ std::string dataformatter for D125496

[aprantl]

Summary

https://reviews.llvm.org/D125496 changed the layout of std::string without updating the LLDB dataformatter. This patch adds code to recognize the now format.

How do we usually add tests for STL data structures?

[aprantl]

I'm going to land this quickly to get the bots going again, but a thorough review would still be very much appreciated!

[aprantl]

That seemed to have done the trick.

[JDevlieghere]

Thanks!

[aprantl]

How do we usually add tests for STL data structures?

I guess that matrix bot sort of covers this.
https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake-matrix/

[labath]

How do we usually add tests for STL data structures?

I have https://reviews.llvm.org/D124155, which would allow testing of different string layouts in one go. It remains to be seen how maintainable will that test be when it starts supporting many string layouts. OTOH, one could say it will be as maintainable as a data formatter which supports all those string layouts...

lldb/source/Plugins/Language/CPlusPlus/LibCxx.cpp

if (valobj.IsPointerType()) {
     if (!value)
       return false;
     stream.Printf("0x%016" PRIx64 " ", value);
   }
   return FormatEntity::FormatStringRef("size=${svar%#}", stream, nullptr,
                                        nullptr, nullptr, &valobj, false, false);
 }
 
-// the field layout in a libc++ string (cap, side, data or data, size, cap)
+/// The field layout in a libc++ string (cap, side, data or data, size, cap).
 enum LibcxxStringLayoutMode {
   eLibcxxStringLayoutModeCSD = 0,
   eLibcxxStringLayoutModeDSC = 1,
   eLibcxxStringLayoutModeInvalid = 0xffff
 };
 
 /// Determine the size in bytes of \p valobj (a libc++ std::string object) and
 /// extract its data payload. Return the size + payload pair.
ExtractLibcxxStringInfo(ValueObject &valobj) {
   }
 
   if (short_mode) {
     ValueObjectSP short_sp(dataval_sp->GetChildAtIndex(1, true));
     if (!short_sp)
       return {};
     ValueObjectSP location_sp = short_sp->GetChildAtIndex(
         (layout == eLibcxxStringLayoutModeDSC) ? 0 : 1, true);
+    // After D125496, there is a flat layout.
+    if (location_sp->GetName() == g_size_name)
+      location_sp = short_sp->GetChildAtIndex(3, true);
     if (using_bitmasks)
       size = (layout == eLibcxxStringLayoutModeDSC)
                  ? size_mode_value
                  : ((size_mode_value >> 1) % 256);
 
     // When the small-string optimization takes place, the data must fit in the
     // inline string buffer (23 bytes on x86_64/Darwin). If it doesn't, it's
     // likely that the string isn't initialized and we're reading garbage.