ninja backend: don't hide all compiler warnings for transpiled languages

This was originally added for vala only, with the rationale that vala
generates bad code that has warnings. Unfortunately, the rationale was
fatally flawed. The compiler warns about a number of things, which the
user can control depending on their code (or their code generator's
code), but some of those things are absolutely critical to warn about.

In particular, GCC 14 and clang 17 are updating their defaults to warn
-- and error by default for -- invalid C code that breaks the standard,
but has been silently accepted for over 20 years "because lots of people
do it". The code in question is UB, and compilers will generate faulty
machine code that behaves erroneously and probably has a mass of CVEs
waiting to happen.

Compiler warnings are NOT safe to just... universally turn off. Compiler
warnings could be either:

- coding style lints

- threatening statements that the code is factually and behaviorally wrong

There is no magic bullet to ignore the former while respecting the
latter. And the very last thing we should ever do is pass `-w`, since
that causes ALL warnings to be disabled, even the manually added
`-Werror=XXX`.

If vala generated code creates warnings, then the vala compiler can
decrease the log level by generating better code, or by adding warning
suppression pragmas for *specific* issues, such as unused functions.

mesonbuild/backend/backends.py

@@ -986,7 +986,7 @@ def get_no_stdlib_args(self, target: 'build.BuildTarget', compiler: 'Compiler')
             return compiler.get_no_stdinc_args()
         return []
 
-    def generate_basic_compiler_args(self, target: build.BuildTarget, compiler: 'Compiler', no_warn_args: bool = False) -> 'CompilerArgs':
+    def generate_basic_compiler_args(self, target: build.BuildTarget, compiler: 'Compiler') -> 'CompilerArgs':
         # Create an empty commands list, and start adding arguments from
         # various sources in the order in which they must override each other
         # starting from hard-coded defaults followed by build options and so on.
@@ -999,17 +999,12 @@ def generate_basic_compiler_args(self, target: build.BuildTarget, compiler: 'Com
         commands += self.get_no_stdlib_args(target, compiler)
         # Add things like /NOLOGO or -pipe; usually can't be overridden
         commands += compiler.get_always_args()
-        # Only add warning-flags by default if the buildtype enables it, and if
-        # we weren't explicitly asked to not emit warnings (for Vala, f.ex)
-        if no_warn_args:
-            commands += compiler.get_no_warn_args()
-        else:
-            # warning_level is a string, but mypy can't determine that
-            commands += compiler.get_warn_args(T.cast('str', target.get_option(OptionKey('warning_level'))))
+        # warning_level is a string, but mypy can't determine that
+        commands += compiler.get_warn_args(T.cast('str', target.get_option(OptionKey('warning_level'))))
         # Add -Werror if werror=true is set in the build options set on the
         # command-line or default_options inside project(). This only sets the
         # action to be done for warnings if/when they are emitted, so it's ok
-        # to set it after get_no_warn_args() or get_warn_args().
+        # to set it after or get_warn_args().
         if target.get_option(OptionKey('werror')):
             commands += compiler.get_werror_args()
         # Add compile args for c_* or cpp_* build options set on the

mesonbuild/backend/ninjabackend.py

@@ -1939,7 +1939,7 @@ def generate_rust_target(self, target: build.BuildTarget) -> None:
         if cratetype in {'bin', 'dylib'}:
             args.extend(rustc.get_linker_always_args())
 
-        args += self.generate_basic_compiler_args(target, rustc, False)
+        args += self.generate_basic_compiler_args(target, rustc)
         # Rustc replaces - with _. spaces or dots are not allowed, so we replace them with underscores
         args += ['--crate-name', target.name.replace('-', '_').replace(' ', '_').replace('.', '_')]
         depfile = os.path.join(target.subdir, target.name + '.d')
@@ -2804,10 +2804,9 @@ def generate_inc_dir(self, compiler: 'Compiler', d: str, basedir: str, is_system
             bargs = []
         return (sargs, bargs)
 
-    def _generate_single_compile(self, target: build.BuildTarget, compiler: 'Compiler',
-                                 is_generated: bool = False) -> 'CompilerArgs':
+    def _generate_single_compile(self, target: build.BuildTarget, compiler: Compiler) -> CompilerArgs:
         commands = self._generate_single_compile_base_args(target, compiler)
-        commands += self._generate_single_compile_target_args(target, compiler, is_generated)
+        commands += self._generate_single_compile_target_args(target, compiler)
         return commands
 
     def _generate_single_compile_base_args(self, target: build.BuildTarget, compiler: 'Compiler') -> 'CompilerArgs':
@@ -2825,14 +2824,10 @@ def _generate_single_compile_base_args(self, target: build.BuildTarget, compiler
         return commands
 
     @lru_cache(maxsize=None)
-    def _generate_single_compile_target_args(self, target: build.BuildTarget, compiler: 'Compiler',
-                                             is_generated: bool = False) -> 'ImmutableListProtocol[str]':
-        # The code generated by valac is usually crap and has tons of unused
-        # variables and such, so disable warnings for Vala C sources.
-        no_warn_args = is_generated == 'vala'
+    def _generate_single_compile_target_args(self, target: build.BuildTarget, compiler: Compiler) -> ImmutableListProtocol[str]:
         # Add compiler args and include paths from several sources; defaults,
         # build options, external dependencies, etc.
-        commands = self.generate_basic_compiler_args(target, compiler, no_warn_args)
+        commands = self.generate_basic_compiler_args(target, compiler)
         # Add custom target dirs as includes automatically, but before
         # target-specific include directories.
         if target.implicit_include_directories:
@@ -2901,7 +2896,7 @@ def generate_common_compile_args_per_src_type(self, target: build.BuildTarget) -
             if use_pch and 'mw' not in compiler.id:
                 commands += self.get_pch_include_args(compiler, target)
 
-            commands += self._generate_single_compile_target_args(target, compiler, is_generated=False)
+            commands += self._generate_single_compile_target_args(target, compiler)
 
             # Metrowerks compilers require PCH include args to come after intraprocedural analysis args
             if use_pch and 'mw' in compiler.id:
@@ -2935,7 +2930,7 @@ def generate_single_compile(self, target: build.BuildTarget, src,
         if use_pch and 'mw' not in compiler.id:
             commands += self.get_pch_include_args(compiler, target)
 
-        commands += self._generate_single_compile_target_args(target, compiler, is_generated)
+        commands += self._generate_single_compile_target_args(target, compiler)
 
         # Metrowerks compilers require PCH include args to come after intraprocedural analysis args
         if use_pch and 'mw' in compiler.id:

test cases/vala/5 target glib/meson.build

@@ -1,9 +1,5 @@
 project('valatest', 'vala', 'c')
 
-if not meson.is_unity()
-  add_global_arguments('-Werror', language : 'c')
-endif
-
 valadeps = [dependency('glib-2.0', version : '>=2.32'), dependency('gobject-2.0')]
 
 e = executable('valaprog', 'GLib.Thread.vala', 'retcode.c', dependencies : valadeps)

unittests/linuxliketests.py

@@ -298,43 +298,6 @@ def test_symlink_builddir(self) -> None:
         self.build()
         self._run(self.mtest_command)
 
-    def test_vala_c_warnings(self):
-        '''
-        Test that no warnings are emitted for C code generated by Vala. This
-        can't be an ordinary test case because we need to inspect the compiler
-        database.
-        https://github.com/mesonbuild/meson/issues/864
-        '''
-        if not shutil.which('valac'):
-            raise SkipTest('valac not installed.')
-        testdir = os.path.join(self.vala_test_dir, '5 target glib')
-        self.init(testdir)
-        compdb = self.get_compdb()
-        vala_command = None
-        c_command = None
-        for each in compdb:
-            if each['file'].endswith('GLib.Thread.c'):
-                vala_command = each['command']
-            elif each['file'].endswith('GLib.Thread.vala'):
-                continue
-            elif each['file'].endswith('retcode.c'):
-                c_command = each['command']
-            else:
-                m = 'Unknown file {!r} in vala_c_warnings test'.format(each['file'])
-                raise AssertionError(m)
-        self.assertIsNotNone(vala_command)
-        self.assertIsNotNone(c_command)
-        # -w suppresses all warnings, should be there in Vala but not in C
-        self.assertIn(" -w ", vala_command)
-        self.assertNotIn(" -w ", c_command)
-        # -Wall enables all warnings, should be there in C but not in Vala
-        self.assertNotIn(" -Wall ", vala_command)
-        self.assertIn(" -Wall ", c_command)
-        # -Werror converts warnings to errors, should always be there since it's
-        # injected by an unrelated piece of code and the project has werror=true
-        self.assertIn(" -Werror ", vala_command)
-        self.assertIn(" -Werror ", c_command)
-
     @skipIfNoPkgconfig
     def test_qtdependency_pkgconfig_detection(self):
         '''