compilers: cpp: wire up debugstl for Clang

For Clang, we now pass -D_GLIBCXX_DEBUG=1 if debugstl is enabled, and
we also pass -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG.

Per https://discourse.llvm.org/t/building-a-program-with-d-libcpp-debug-1-against-a-libc-that-is-not-itself-built-with-that-define/59176/3,
we can't use _LIBCPP_DEBUG for older Clang versions as it's unreliable unless
libc++ was built with it.

We choose MODE_DEBUG for stldebug while building with assertions will do
MODE_EXTENSIVE.

Signed-off-by: Sam James <sam@gentoo.org>

docs/markdown/snippets/cpp-stldebug.md

@@ -0,0 +1,4 @@
+## `stldebug` gains Clang support
+
+For Clang, we now pass `-D_GLIBCXX_DEBUG=1` if `debugstl` is enabled, and
+we also pass `-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG`.

mesonbuild/compilers/cpp.py

@@ -242,6 +242,10 @@ def get_options(self) -> 'MutableKeyedOptionDictType':
         opts = CPPCompiler.get_options(self)
         key = OptionKey('key', machine=self.for_machine, lang=self.language)
         opts.update({
+            key.evolve('debugstl'): coredata.UserBooleanOption(
+                'STL debug mode',
+                False,
+            ),
             key.evolve('eh'): coredata.UserComboOption(
                 'C++ exception handling type.',
                 ['none', 'default', 'a', 's', 'sc'],
@@ -277,6 +281,9 @@ def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]
 
         non_msvc_eh_options(options[key.evolve('eh')].value, args)
 
+        if options[key.evolve('debugstl')].value:
+            args.append('-D_GLIBCXX_DEBUG=1')
+
         if not options[key.evolve('rtti')].value:
             args.append('-fno-rtti')
 
@@ -650,6 +657,13 @@ def get_option_compile_args(self, options: 'KeyedOptionDictType') -> T.List[str]
 
         if options[key.evolve('debugstl')].value:
             args.append('-D_GLIBCXX_DEBUG=1')
+
+            # We can't do _LIBCPP_DEBUG because it's unreliable unless libc++ was built with it too:
+            # https://discourse.llvm.org/t/building-a-program-with-d-libcpp-debug-1-against-a-libc-that-is-not-itself-built-with-that-define/59176/3
+            # Note that unlike _GLIBCXX_DEBUG, _MODE_DEBUG doesn't break ABI. It's just slow.
+            if version_compare(self.version, '>=18'):
+                args.append('-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG')
+
         return args
 
     def get_option_link_args(self, options: 'KeyedOptionDictType') -> T.List[str]: