Don't collect preprocssor flags separately from compiler flags

I recall that @jpakkane never wanted this, but @nirbheek did, but then
@nirbheek changed his mind.

I am fine either way except for the cross inconsistency that exists
today: There is no `c_preproc_args` or similar one can put in the cross
file, so no way to replicate the effect of CPPFLAGS during cross
compilation.

mesonbuild/compilers/c.py

@@ -422,11 +422,8 @@ class CCompiler(Compiler):
             for_machine = MachineChoice.BUILD
         else:
             for_machine = MachineChoice.HOST
-        if mode == 'preprocess':
-            # Add CPPFLAGS from the env.
-            args += env.coredata.get_external_preprocess_args(for_machine, self.language)
-        elif mode == 'compile':
-            # Add CFLAGS/CXXFLAGS/OBJCFLAGS/OBJCXXFLAGS from the env
+        if mode in {'compile', 'preprocess'}:
+            # Add CFLAGS/CXXFLAGS/OBJCFLAGS/OBJCXXFLAGS and CPPFLAGS from the env
             sys_args = env.coredata.get_external_args(for_machine, self.language)
             # Apparently it is a thing to inject linker flags both
             # via CFLAGS _and_ LDFLAGS, even though the former are

mesonbuild/compilers/compilers.py

@@ -970,10 +970,11 @@ class Compiler:
         """
         return []
 
-    def get_preproc_flags(self):
-        if self.get_language() in ('c', 'cpp', 'objc', 'objcpp'):
-            return os.environ.get('CPPFLAGS', '')
-        return ''
+    def use_preproc_flags(self):
+        """
+        Whether the compiler (or processes it spawns) cares about CPPFLAGS
+        """
+        return self.get_language() in ('c', 'cpp', 'objc', 'objcpp')
 
     def get_args_from_envvars(self):
         """
@@ -1007,8 +1008,9 @@ class Compiler:
             # this when the linker is stand-alone such as with MSVC C/C++, etc.
             link_flags = compile_flags + link_flags
 
-        # Pre-processor flags (not for fortran or D)
-        preproc_flags = self.get_preproc_flags()
+        # Pre-processor flags for certain languages
+        if self.use_preproc_flags():
+            preproc_flags = os.environ.get('CPPFLAGS', '')
             log_var('CPPFLAGS', preproc_flags)
             preproc_flags = shlex.split(preproc_flags)
             compile_flags += preproc_flags

mesonbuild/coredata.py

@@ -264,7 +264,6 @@ class CoreData:
         self.user_options = {}
         self.compiler_options = PerMachine({}, {}, {})
         self.base_options = {}
-        self.external_preprocess_args = PerMachine({}, {}, {}) # CPPFLAGS only
         self.cross_files = self.__load_config_files(options.cross_file)
         self.compilers = OrderedDict()
         self.cross_compilers = OrderedDict()
@@ -446,9 +445,6 @@ class CoreData:
     def get_external_link_args(self, for_machine: MachineChoice, lang):
         return self.compiler_options[for_machine][lang + '_link_args'].value
 
-    def get_external_preprocess_args(self, for_machine: MachineChoice, lang):
-        return self.external_preprocess_args[for_machine][lang]
-
     def merge_user_options(self, options):
         for (name, value) in options.items():
             if name not in self.user_options:
@@ -532,21 +528,18 @@ class CoreData:
 
         # Native compiler always exist so always add its options.
         new_options_for_build = comp.get_and_default_options(env.properties.build)
-        preproc_flags_for_build = comp.get_preproc_flags()
         if cross_comp is not None:
             new_options_for_host = cross_comp.get_and_default_options(env.properties.host)
-            preproc_flags_for_host = cross_comp.get_preproc_flags()
         else:
             new_options_for_host = comp.get_and_default_options(env.properties.host)
-            preproc_flags_for_host = comp.get_preproc_flags()
 
         opts_machines_list = [
-            (new_options_for_build, preproc_flags_for_build, MachineChoice.BUILD),
-            (new_options_for_host, preproc_flags_for_host, MachineChoice.HOST),
+            (new_options_for_build, MachineChoice.BUILD),
+            (new_options_for_host, MachineChoice.HOST),
         ]
 
         optprefix = lang + '_'
-        for new_options, preproc_flags, for_machine in opts_machines_list:
+        for new_options, for_machine in opts_machines_list:
             for k, o in new_options.items():
                 if not k.startswith(optprefix):
                     raise MesonException('Internal error, %s has incorrect prefix.' % k)
@@ -556,17 +549,6 @@ class CoreData:
                     o.set_value(env.cmd_line_options[k])
                 self.compiler_options[for_machine].setdefault(k, o)
 
-            # Unlike compiler and linker flags, preprocessor flags are not in
-            # compiler_options because they are not visible to user.
-            preproc_flags = shlex.split(preproc_flags)
-            k = lang + '_args'
-            if lang in ('c', 'cpp', 'objc', 'objcpp') and k in env.properties[for_machine]:
-                # `c_args` in the cross file are used, like CPPFLAGS but *not*
-                # CFLAGS, for tests. this is weird, but how it was already
-                # implemented. Hopefully a new version of #3916 fixes it.
-                preproc_flags = stringlistify(env.properties[for_machine][k])
-            self.external_preprocess_args[for_machine].setdefault(lang, preproc_flags)
-
         enabled_opts = []
         for optname in comp.base_options:
             if optname in self.base_options:

mesonbuild/munstable_coredata.py

@@ -118,10 +118,6 @@ def run(options):
                 print('Cached dependencies:')
                 for dep_key, dep in cross:
                     print_dep(dep_key, dep)
-        elif k == 'external_preprocess_args':
-            for lang, opts in v.items():
-                if opts:
-                    print('Preprocessor args for ' + lang + ': ' + ' '.join(opts))
         else:
             print(k + ':')
             print(textwrap.indent(pprint.pformat(v), '  '))