remove some of the include dir dark magic by standardizing options

specifically remove the hardcoded extra include paths in the xcode clang wrapper

src/python/pants/backend/native/subsystems/native_toolchain.py

@@ -141,10 +141,12 @@ class GCCInstallLocationForLLVM(datatype(['toolchain_dir'])):
   """This class is convertible into a list of command line arguments for clang and clang++.
 
   This is only used on Linux. The option --gcc-toolchain stops clang from searching for another gcc
-  on the host system. The option appears to only exist on Linux clang and clang++."""
+  on the host system. The option appears to only exist on Linux clang and clang++.
+  """
 
   @property
   def as_clang_argv(self):
+    # TODO(#6143): describe exactly what this argument does to the clang/clang++ invocation!
     return ['--gcc-toolchain={}'.format(self.toolchain_dir)]
 
 
@@ -161,6 +163,7 @@ def select_llvm_c_toolchain(platform, native_toolchain):
   llvm_c_compiler_args = [
     '-x', 'c', '-std=c11',
     '-nobuiltininc',
+    '-nostdinc',
   ]
 
   if platform.normalized_os_name == 'darwin':
@@ -203,6 +206,7 @@ def select_llvm_cpp_toolchain(platform, native_toolchain):
     # implementation, or any from the host system. Instead, we use include dirs from the
     # XCodeCLITools or GCC.
     '-nobuiltininc',
+    '-nostdinc',
     '-nostdinc++',
   ]
 
@@ -226,7 +230,7 @@ def select_llvm_cpp_toolchain(platform, native_toolchain):
       # NB: we use g++'s headers on Linux, and therefore their C++ standard library.
       include_dirs=provided_gpp.include_dirs,
       extra_args=(llvm_cpp_compiler_args + provided_clangpp.extra_args + gcc_install.as_clang_argv))
-    # TODO: why are these necessary? this is very mysterious.
+    # TODO(#6855): why are these necessary? this is very mysterious.
     extra_linking_library_dirs = provided_gpp.library_dirs + provided_clangpp.library_dirs
     # Ensure we use libstdc++, provided by g++, during the linking stage.
     linker_extra_args=['-stdlib=libstdc++']
@@ -253,12 +257,15 @@ def select_gcc_c_toolchain(platform, native_toolchain):
   # GCC needs an assembler, so we provide that (platform-specific) tool here.
   assembler = yield Get(Assembler, NativeToolchain, native_toolchain)
 
+  gcc_c_compiler_args = [
+    '-x', 'c', '-std=c11',
+    '-nostdinc',
+  ]
+
   if platform.normalized_os_name == 'darwin':
     # GCC needs access to some headers that are only provided by the XCode toolchain
     # currently (e.g. "_stdio.h"). These headers are unlikely to change across versions, so this is
     # probably safe.
-    # TODO: we should be providing all of these (so we can eventually phase out XCodeCLITools
-    # entirely).
     xcode_clang = yield Get(CCompiler, XCodeCLITools, native_toolchain._xcode_cli_tools)
     new_include_dirs = provided_gcc.include_dirs + xcode_clang.include_dirs
   else:
@@ -267,7 +274,7 @@ def select_gcc_c_toolchain(platform, native_toolchain):
   working_c_compiler = provided_gcc.copy(
     path_entries=(provided_gcc.path_entries + assembler.path_entries),
     include_dirs=new_include_dirs,
-    extra_args=['-x', 'c', '-std=c11'])
+    extra_args=gcc_c_compiler_args)
 
   gcc_linker_wrapper = yield Get(GCCLinker, NativeToolchain, native_toolchain)
   gcc_linker = gcc_linker_wrapper.linker
@@ -290,6 +297,7 @@ def select_gcc_cpp_toolchain(platform, native_toolchain):
 
   gcc_cpp_compiler_args = [
     '-x', 'c++', '-std=c++11',
+    '-nostdinc',
     '-nostdinc++',
   ]
 

src/python/pants/backend/native/subsystems/xcode_cli_tools.py

@@ -84,7 +84,9 @@ def _all_existing_install_prefixes(self):
   # NB: We use @memoized_method in this file for methods which may raise.
   @memoized_method
   def _get_existing_subdirs(self, subdir_name):
-    # TODO: consume ArchiveFileMapper instead of doing all that logic over again in this file.
+    # TODO(#6143): We should attempt to use ParseSearchDirs here or find some documentation on which
+    # directories we should be adding to the include path, and why. If we do need to manually
+    # specify paths, use ArchiveFileMapper instead of doing all that logic over again in this file.
     maybe_subdirs = [os.path.join(pfx, subdir_name) for pfx in self._all_existing_install_prefixes]
     existing_dirs = [existing_dir for existing_dir in maybe_subdirs if is_readable_dir(existing_dir)]
 
@@ -125,22 +127,7 @@ def lib_dirs(self):
 
   @memoized_method
   def include_dirs(self, include_cpp_inc=False):
-    base_inc_dirs = self._get_existing_subdirs('include')
-
-    all_inc_dirs = base_inc_dirs
-    for d in base_inc_dirs:
-      # TODO: figure out what this directory does and why it's not already found by this compiler.
-      secure_inc_dir = os.path.join(d, 'secure')
-      if is_readable_dir(secure_inc_dir):
-        all_inc_dirs.append(secure_inc_dir)
-
-      # TODO: figure out why we need to specify this directory separately.
-      if include_cpp_inc:
-        cpp_inc_dir = os.path.join(d, 'c++/v1')
-        if is_readable_dir(cpp_inc_dir):
-          all_inc_dirs.append(cpp_inc_dir)
-
-    return all_inc_dirs
+    return self._get_existing_subdirs('include')
 
   @memoized_method
   def assembler(self):