compilers: Only insert -flto-jobs in clang's link arguments

Clang has a hand `-Wunused-command-line-argument` switch, which when
turned to an error, gets very grump about `-flto-jobs=0` being set in
the compiler arguments (although `-flto=` belongs there). We'll refactor
a bit to put that only in the link arguments.

GCC doesn't have this probably because, a) it doesn't have an equivalent
warning, and b) it uses `-flto=<$numthreads.

Fixes: mesonbuild#8347

mesonbuild/compilers/compilers.py

@@ -375,7 +375,9 @@ def get_base_link_args(options: 'KeyedOptionDictType', linker: 'Compiler',
     args = []  # type: T.List[str]
     try:
         if options[OptionKey('b_lto')].value:
-            args.extend(linker.get_lto_link_args())
+            args.extend(linker.get_lto_link_args(
+                threads=get_option_value(options, OptionKey('b_lto_threads'), 0),
+                mode=get_option_value(options, OptionKey('b_lto_mode'), 'default')))
     except KeyError:
         pass
     try:
@@ -950,7 +952,7 @@ def remove_linkerlike_args(self, args: T.List[str]) -> T.List[str]:
     def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         return []
 
-    def get_lto_link_args(self) -> T.List[str]:
+    def get_lto_link_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         return self.linker.get_lto_args()
 
     def sanitizer_compile_args(self, value: str) -> T.List[str]:

mesonbuild/compilers/mixins/clang.py

@@ -148,6 +148,10 @@ def get_lto_compile_args(self, *, threads: int = 0, mode: str = 'default') -> T.
         else:
             assert mode == 'default', 'someone forgot to wire something up'
             args.extend(super().get_lto_compile_args(threads=threads))
+        return args
+
+    def get_lto_link_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
+        args = self.get_lto_compile_args(threads=threads, mode=mode)
         # In clang -flto=0 means auto
         if threads >= 0:
             args.append(f'-flto-jobs={threads}')

mesonbuild/compilers/mixins/islinker.py

@@ -48,7 +48,7 @@ class BasicLinkerIsCompilerMixin(Compiler):
     def sanitizer_link_args(self, value: str) -> T.List[str]:
         return []
 
-    def get_lto_link_args(self) -> T.List[str]:
+    def get_lto_link_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         return []
 
     def can_linker_accept_rsp(self) -> bool:

run_unittests.py

@@ -3062,10 +3062,14 @@ def test_lto_threads(self):
 
         env = get_fake_env(testdir, self.builddir, self.prefix)
         cc = env.detect_c_compiler(MachineChoice.HOST)
-        if cc.get_id() == 'clang' and is_windows():
-            raise unittest.SkipTest('LTO not (yet) supported by windows clang')
+        extra_args: T.List[str] = []
+        if cc.get_id() == 'clang':
+            if is_windows():
+                raise unittest.SkipTest('LTO not (yet) supported by windows clang')
+            else:
+                extra_args.append('-D_cargs=-Werror=unused-command-line-argument')
 
-        self.init(testdir, extra_args=['-Db_lto=true', '-Db_lto_threads=8'])
+        self.init(testdir, extra_args=['-Db_lto=true', '-Db_lto_threads=8'] + extra_args)
         self.build()
         self.run_tests()
 
@@ -3091,7 +3095,7 @@ def test_lto_mode(self):
         elif is_windows():
             raise unittest.SkipTest('LTO not (yet) supported by windows clang')
 
-        self.init(testdir, extra_args=['-Db_lto=true', '-Db_lto_mode=thin', '-Db_lto_threads=8'])
+        self.init(testdir, extra_args=['-Db_lto=true', '-Db_lto_mode=thin', '-Db_lto_threads=8', '-Dc_args=-Werror=unused-command-line-argument'])
         self.build()
         self.run_tests()
 
@@ -3100,7 +3104,7 @@ def test_lto_mode(self):
         # This assumes all of the targets support lto
         for t in targets:
             for s in t['target_sources']:
-                assert expected.issubset(set(s['parameters'])), f'Incorrect values for {t["name"]}'
+                self.assertTrue(expected.issubset(set(s['parameters'])), f'Incorrect values for {t["name"]}')
 
     def test_dist_git(self):
         if not shutil.which('git'):