backends/vs: Do not emit dummy command for alias_command().

Alias commands did not work with the vs backend, due to trying to access
target.command[0] with an empty command. Fix this by just not emitting a
CustomBuild node for alias targets - the project references are enough to
trigger the necessary actions.

Fixes: #9247

mesonbuild/backend/vs2010backend.py

@@ -27,7 +27,7 @@ from .. import mlog
 from .. import compilers
 from ..interpreter import Interpreter
 from ..mesonlib import (
-    File, MesonException, python_command, replace_if_different, OptionKey, version_compare, MachineChoice
+    File, MesonException, replace_if_different, OptionKey, version_compare, MachineChoice
 )
 from ..environment import Environment, build_filename
 
@@ -550,19 +550,27 @@ class Vs2010Backend(backends.Backend):
 
     def gen_run_target_vcxproj(self, target, ofname, guid):
         root = self.create_basic_crap(target, guid)
+        depend_files = self.get_custom_target_depend_files(target)
+
         if not target.command:
-            # FIXME: This is an alias target that doesn't run any command, there
-            # is probably a better way than running a this dummy command.
-            cmd_raw = python_command + ['-c', 'exit']
+            # This is an alias target and thus doesn't run any command. It's
+            # enough to emit the references to the other projects for them to
+            # be built/run/..., if necessary.
+            assert isinstance(target, build.AliasTarget)
+            assert len(depend_files) == 0
         else:
-            _, _, cmd_raw = self.eval_custom_target_command(target)
-        depend_files = self.get_custom_target_depend_files(target)
+            assert not isinstance(target, build.AliasTarget)
+
             target_env = self.get_run_target_env(target)
+            _, _, cmd_raw = self.eval_custom_target_command(target)
             wrapper_cmd, _ = self.as_meson_exe_cmdline(target.command[0], cmd_raw[1:],
                                                        force_serialize=True, env=target_env,
                                                        verbose=True)
             self.add_custom_build(root, 'run_target', ' '.join(self.quote_arguments(wrapper_cmd)),
                                   deps=depend_files)
+
+        # The import is needed even for alias targets, otherwise the build
+        # target isn't defined
         ET.SubElement(root, 'Import', Project=r'$(VCTargetsPath)\Microsoft.Cpp.targets')
         self.add_regen_dependency(root)
         self.add_target_deps(root, target)

unittests/allplatformstests.py

@@ -3066,9 +3066,6 @@ class AllPlatformTests(BasePlatformTests):
         self.init(testdir, extra_args=['-Dcmake_prefix_path=' + os.path.join(testdir, 'prefix')])
 
     def test_alias_target(self):
-        if self.backend is Backend.vs:
-            # FIXME: This unit test is broken with vs backend, needs investigation
-            raise SkipTest(f'Skipping alias_target test with {self.backend.name} backend')
         testdir = os.path.join(self.unit_test_dir, '65 alias target')
         self.init(testdir)
         self.build()