Split environment variable and command line cflags

They are supposed to have different behavior. The environment variables
apply to both the compiler and linker when the compiler acts as a
linker, but the command line ones do not.

Fixes #8345

mesonbuild/compilers/compilers.py

@@ -1234,18 +1234,21 @@ def get_global_options(lang: str,
     description = f'Extra arguments passed to the {lang}'
     argkey = OptionKey('args', lang=lang, machine=for_machine)
     largkey = argkey.evolve('link_args')
+    envkey = argkey.evolve('env_args')
 
     cargs = coredata.UserArrayOption(
         description + ' compiler',
         env.options.get(argkey, []), split_args=True, user_input=True, allow_dups=True)
+    # the compiler args always gets the environemtn variable arguments
+    cargs.extend_value(env.options.get(envkey, []))
+
     largs = coredata.UserArrayOption(
         description + ' linker',
         env.options.get(largkey, []), split_args=True, user_input=True, allow_dups=True)
-
-    # This needs to be done here, so that if we have string values in the env
-    # options that we can safely combine them *after* they've been split
+    # The linker gets the compiler environment variable only if the comiler
+    # acts as the linker
     if comp.INVOKES_LINKER:
-        largs.set_value(largs.value + cargs.value)
+        largs.extend_value(env.options.get(envkey, []))
 
     opts: 'KeyedOptionDictType' = {argkey: cargs, largkey: largs}
 

mesonbuild/coredata.py

@@ -243,6 +243,11 @@ class UserArrayOption(UserOption[T.List[str]]):
                     ', '.join(bad), ', '.join(self.choices)))
         return newvalue
 
+    def extend_value(self, value: T.Union[str, T.List[str]]) -> None:
+        """Extend the value with an additional value."""
+        new = self.validate_value(value)
+        self.set_value(self.value + new)
+
 
 class UserFeatureOption(UserComboOption):
     static_choices = ['enabled', 'disabled', 'auto']
@@ -776,8 +781,10 @@ class CoreData:
                       for_machine: MachineChoice, env: 'Environment') -> None:
         """Add global language arguments that are needed before compiler/linker detection."""
         from .compilers import compilers
-        options = compilers.get_global_options(lang, comp, for_machine, env)
-        self.add_compiler_options(options, lang, for_machine, env)
+        # These options are all new at this point, because the compiler is
+        # responsible for adding its own options, thus calling
+        # `self.options.update()`` is perfectly safe.
+        self.options.update(compilers.get_global_options(lang, comp, for_machine, env))
 
     def process_new_compiler(self, lang: str, comp: 'Compiler', env: 'Environment') -> None:
         from . import compilers

mesonbuild/environment.py

@@ -798,7 +798,11 @@ class Environment:
 
         env_opts: T.DefaultDict[OptionKey, T.List[str]] = collections.defaultdict(list)
 
-        for (evar, keyname), for_machine in itertools.product(opts, MachineChoice):
+        if self.is_cross_build():
+            for_machine = MachineChoice.BUILD
+        else:
+            for_machine = MachineChoice.HOST
+        for evar, keyname in opts:
             p_env = _get_env_var(for_machine, self.is_cross_build(), evar)
             if p_env is not None:
                 # these may contain duplicates, which must be removed, else
@@ -834,6 +838,23 @@ class Environment:
                             env_opts[key].extend(p_list)
                     else:
                         key = OptionKey.from_string(keyname).evolve(machine=for_machine)
+                        if evar in compilers.compilers.CFLAGS_MAPPING.values():
+                            # If this is an environment variable, we have to
+                            # store it separately until the compiler is
+                            # instantiated, as we don't know whether the
+                            # compiler will want to use these arguments at link
+                            # time and compile time (instead of just at compile
+                            # time) until we're instantiating that `Compiler`
+                            # object. This is required so that passing
+                            # `-Dc_args=` on the command line and `$CFLAGS`
+                            # have subtely differen behavior. `$CFLAGS` will be
+                            # added to the linker command line if the compiler
+                            # acts as a linker driver, `-Dc_args` will not.
+                            #
+                            # We stil use the original key as the base here, as
+                            # we want to inhert the machine and the compiler
+                            # language
+                            key = key.evolve('env_args')
                         env_opts[key].extend(p_list)
 
         # Only store options that are not already in self.options,

run_unittests.py

@@ -5634,6 +5634,28 @@ class AllPlatformTests(BasePlatformTests):
                 matches += 1
         self.assertEqual(matches, 1)
 
+    def test_env_flags_to_linker(self) -> None:
+        # Compilers that act as drivers should add their compiler flags to the
+        # linker, those that do not shouldn't
+        with mock.patch.dict(os.environ, {'CFLAGS': '-DCFLAG', 'LDFLAGS': '-flto'}):
+            env = get_fake_env()
+
+            # Get the compiler so we know which compiler class to mock.
+            cc = env.detect_compiler_for('c', MachineChoice.HOST)
+            cc_type = type(cc)
+
+            # Test a compiler that acts as a linker
+            with mock.patch.object(cc_type, 'INVOKES_LINKER', True):
+                cc = env.detect_compiler_for('c', MachineChoice.HOST)
+                link_args = env.coredata.get_external_link_args(cc.for_machine, cc.language)
+                self.assertEqual(sorted(link_args), sorted(['-DCFLAG', '-flto']))
+
+            # And one that doesn't
+            with mock.patch.object(cc_type, 'INVOKES_LINKER', False):
+                cc = env.detect_compiler_for('c', MachineChoice.HOST)
+                link_args = env.coredata.get_external_link_args(cc.for_machine, cc.language)
+                self.assertEqual(sorted(link_args), sorted(['-flto']))
+
 class FailureTests(BasePlatformTests):
     '''
     Tests that test failure conditions. Build files here should be dynamically