interpreter: Move validation of BuildTarget(extra_files) to Interpreter

This gets us to the point that the build layer can assume it's getting
valid inputs. This does two checks, an initial cheap check for generated
sources, and then an I/O check via `source_strings_to_files`.

mesonbuild/build.py

@@ -1294,17 +1294,7 @@ class BuildTarget(Target):
                                         (str, bool))
         self.install_mode = kwargs.get('install_mode', None)
         self.install_tag = stringlistify(kwargs.get('install_tag', [None]))
-        extra_files = kwargs.get('extra_files', [])
-        for i in extra_files:
-            # TODO: use an OrderedSet instead of a list?
-            if i in self.extra_files:
-                continue
-            # TODO: this prevents built `File` objects from being used as
-            # extra_files.
-            trial = os.path.join(self.environment.get_source_dir(), i.subdir, i.fname)
-            if not os.path.isfile(trial):
-                raise InvalidArguments(f'Tried to add non-existing extra file {i}.')
-            self.extra_files.append(i)
+        self.extra_files = kwargs.get('extra_files', [])
         self.install_rpath: str = kwargs.get('install_rpath', '')
         self.build_rpath = kwargs.get('build_rpath', '')
         self.resources = kwargs.get('resources', [])

mesonbuild/interpreter/interpreter.py

@@ -3445,7 +3445,8 @@ class Interpreter(InterpreterBase, HoldableObject):
         sources = self.source_strings_to_files(sources)
         objs = kwargs['objects']
         kwargs['dependencies'] = extract_as_list(kwargs, 'dependencies')
-        kwargs['extra_files'] = self.source_strings_to_files(kwargs['extra_files'])
+        # TODO: When we can do strings -> Files in the typed_kwargs validator, do this there too
+        kwargs['extra_files'] = mesonlib.unique_list(self.source_strings_to_files(kwargs['extra_files']))
         self.check_sources_exist(os.path.join(self.source_root, self.subdir), sources)
         self.__process_language_args(kwargs)
         if targetclass is build.StaticLibrary:

mesonbuild/interpreter/type_checking.py

@@ -588,11 +588,24 @@ def _target_install_convertor(val: object) -> bool:
     return bool(val)
 
 
+def _extra_files_validator(args: T.List[T.Union[File, str]]) -> T.Optional[str]:
+    generated = [a for a in args if isinstance(a, File) and a.is_built]
+    if generated:
+        return 'extra_files contains generated files: {}'.format(', '.join(f"{f.fname}" for f in generated))
+    return None
+
+
 # Applies to all build_target like classes
 _ALL_TARGET_KWS: T.List[KwargInfo] = [
     OVERRIDE_OPTIONS_KW,
     KwargInfo('build_by_default', bool, default=True, since='0.38.0'),
-    KwargInfo('extra_files', ContainerTypeInfo(list, (str, File)), default=[], listify=True),
+    KwargInfo(
+        'extra_files',
+        ContainerTypeInfo(list, (str, File)),
+        default=[],
+        listify=True,
+        validator=_extra_files_validator,
+    ),
     KwargInfo(
         'install',
         object,