allow RunTarget to skip wrapping due to env

Forcing serialization on when writing out the build rule makes very
little sense. It was always "forced" on because we mandated a couple of
environment variables due to legacy reasons.

Add an attribute to RunTarget to say that a given target doesn't *need*
those environment variables, and let ninja optimize them away and run
the command directly if set.

mesonbuild/backend/backends.py

@@ -1445,6 +1445,7 @@ class Backend:
 
     def get_run_target_env(self, target: build.RunTarget) -> build.EnvironmentVariables:
         env = target.env if target.env else build.EnvironmentVariables()
+        if target.default_env:
             introspect_cmd = join_args(self.environment.get_build_command() + ['introspect'])
             env.set('MESON_SOURCE_ROOT', [self.environment.get_source_dir()])
             env.set('MESON_BUILD_ROOT', [self.environment.get_build_dir()])

mesonbuild/backend/ninjabackend.py

@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 from collections import OrderedDict
 from enum import Enum, unique
@@ -1034,7 +1035,7 @@ class NinjaBackend(backends.Backend):
             subproject_prefix = ''
         return f'{subproject_prefix}{target.name}'
 
-    def generate_run_target(self, target):
+    def generate_run_target(self, target: build.RunTarget):
         target_name = self.build_run_target_name(target)
         if not target.command:
             # This is an alias target, it has no command, it just depends on
@@ -1044,9 +1045,9 @@ class NinjaBackend(backends.Backend):
             target_env = self.get_run_target_env(target)
             _, _, cmd = self.eval_custom_target_command(target)
             meson_exe_cmd, reason = self.as_meson_exe_cmdline(target.command[0], cmd[1:],
-                                                              force_serialize=True, env=target_env,
+                                                              env=target_env,
                                                               verbose=True)
-            cmd_type = f' (wrapped by meson {reason})'
+            cmd_type = f' (wrapped by meson {reason})' if reason else ''
             internal_target_name = f'meson-{target_name}'
             elem = NinjaBuildElement(self.all_outputs, internal_target_name, 'CUSTOM_COMMAND', [])
             elem.add_item('COMMAND', meson_exe_cmd)

mesonbuild/build.py

@@ -2596,7 +2596,8 @@ class RunTarget(Target, CommandBase):
                  subdir: str,
                  subproject: str,
                  environment: environment.Environment,
-                 env: T.Optional['EnvironmentVariables'] = None):
+                 env: T.Optional['EnvironmentVariables'] = None,
+                 default_env: bool = True):
         self.typename = 'run'
         # These don't produce output artifacts
         super().__init__(name, subdir, subproject, False, MachineChoice.BUILD, environment)
@@ -2605,6 +2606,7 @@ class RunTarget(Target, CommandBase):
         self.command = self.flatten_command(command)
         self.absolute_paths = False
         self.env = env
+        self.default_env = default_env
 
     def __repr__(self) -> str:
         repr_str = "<{0} {1}: {2}>"