templates: fix --executable

* In 1.7.1, the behaviour of --executable was to just ignore it.
* After my recent 9104bb6, the behaviour
  was that we'd require, for --executable xyz, xyz.c to exist or we'd
  fail to generate.

Neither are good! Instead, create the sample source file w/ the project
name, but call the executable whatever the user passed with `--executable`.

Bug: #15286

Author: thesamesam

mesonbuild/templates/sampleimpl.py

@@ -133,7 +133,7 @@ class FileImpl(SampleImpl):
 
     def __init__(self, args: Arguments):
         super().__init__(args)
-        self.sources = args.srcfiles if args.srcfiles else [Path(f'{self.executable_name}.{self.source_ext}')]
+        self.sources = args.srcfiles if args.srcfiles else [Path(f'{self.name}.{self.source_ext}')]
 
     def create_executable(self) -> None:
         source_name = f'{self.lowercase_token}.{self.source_ext}'

unittests/allplatformstests.py

@@ -2541,6 +2541,17 @@ def _template_test_fresh(lang, target_type):
                 self._run(ninja,
                             workdir=os.path.join(tmpdir, 'builddir'))
 
+            # custom executable name
+            if target_type == 'executable':
+                with tempfile.TemporaryDirectory() as tmpdir:
+                    self._run(self.meson_command + ['init', '--language', lang, '--type', target_type,
+                                                    '--executable', 'foobar'], workdir=tmpdir)
+                    self._run(self.setup_command + ['--backend=ninja', 'builddir'],
+                                workdir=tmpdir)
+                    self._run(ninja,
+                                workdir=os.path.join(tmpdir, 'builddir'))
+
+
         def _template_test_dirty(lang, target_type):
             if is_windows() and lang == 'fortran' and target_type == 'library':
                 # non-Gfortran Windows Fortran compilers do not do shared libraries in a Fortran standard way