build: Add dummy base class for CustomTarget and CustomTargetIndex

CustomTarget and CustomTargetIndex often have to be special cased
because they are not subclass of BuildTarget. It's easier to introduce a
dummy base class.

mesonbuild/build.py

@@ -1276,21 +1276,21 @@ def get_extra_args(self, language: str) -> T.List[str]:
         return self.extra_args[language]
 
     @lru_cache(maxsize=None)
-    def get_dependencies(self) -> OrderedSet[Target]:
+    def get_dependencies(self) -> OrderedSet[BuildTargetTypes]:
         # Get all targets needed for linking. This includes all link_with and
         # link_whole targets, and also all dependencies of static libraries
         # recursively. The algorithm here is closely related to what we do in
         # get_internal_static_libraries(): Installed static libraries include
         # objects from all their dependencies already.
-        result: OrderedSet[Target] = OrderedSet()
+        result: OrderedSet[BuildTargetTypes] = OrderedSet()
         for t in itertools.chain(self.link_targets, self.link_whole_targets):
             if t not in result:
                 result.add(t)
                 if isinstance(t, StaticLibrary):
                     t.get_dependencies_recurse(result)
         return result
 
-    def get_dependencies_recurse(self, result: OrderedSet[Target], include_internals: bool = True) -> None:
+    def get_dependencies_recurse(self, result: OrderedSet[BuildTargetTypes], include_internals: bool = True) -> None:
         # self is always a static library because we don't need to pull dependencies
         # of shared libraries. If self is installed (not internal) it already
         # include objects extracted from all its internal dependencies so we can
@@ -1299,7 +1299,7 @@ def get_dependencies_recurse(self, result: OrderedSet[Target], include_internals
         for t in self.link_targets:
             if t in result:
                 continue
-            if isinstance(t, SharedLibrary) and t.rust_crate_type == 'proc-macro':
+            if t.rust_crate_type == 'proc-macro':
                 continue
             if include_internals or not t.is_internal():
                 result.add(t)
@@ -1394,7 +1394,7 @@ def get_external_deps(self) -> T.List[dependencies.Dependency]:
     def is_internal(self) -> bool:
         return False
 
-    def link(self, targets):
+    def link(self, targets: T.List[BuildTargetTypes]):
         for t in targets:
             if not isinstance(t, (Target, CustomTargetIndex)):
                 if isinstance(t, dependencies.ExternalLibrary):
@@ -1420,7 +1420,7 @@ def link(self, targets):
             self.check_can_link_together(t)
             self.link_targets.append(t)
 
-    def link_whole(self, targets, promoted: bool = False):
+    def link_whole(self, targets: T.List[BuildTargetTypes], promoted: bool = False):
         for t in targets:
             if isinstance(t, (CustomTarget, CustomTargetIndex)):
                 if not t.is_linkable_target():
@@ -2527,7 +2527,26 @@ def flatten_command(self, cmd: T.Sequence[T.Union[str, File, programs.ExternalPr
                 raise InvalidArguments(f'Argument {c!r} in "command" is invalid')
         return final_cmd
 
-class CustomTarget(Target, CommandBase):
+class CustomTargetBase:
+    ''' Base class for CustomTarget and CustomTargetIndex
+
+    This base class can be used to provide a dummy implementation of some
+    private methods to avoid repeating `isinstance(t, BuildTarget)` when dealing
+    with custom targets.
+    '''
+
+    rust_crate_type = ''
+
+    def get_dependencies_recurse(self, result: OrderedSet[BuildTargetTypes], include_internals: bool = True) -> None:
+        pass
+
+    def get_internal_static_libraries(self) -> OrderedSet[BuildTargetTypes]:
+        return OrderedSet()
+
+    def get_internal_static_libraries_recurse(self, result: OrderedSet[BuildTargetTypes]) -> None:
+        pass
+
+class CustomTarget(Target, CustomTargetBase, CommandBase):
 
     typename = 'custom'
 
@@ -2904,7 +2923,7 @@ def get_default_install_dir(self) -> T.Tuple[str, str]:
         return self.environment.get_jar_dir(), '{jardir}'
 
 @dataclass(eq=False)
-class CustomTargetIndex(HoldableObject):
+class CustomTargetIndex(CustomTargetBase, HoldableObject):
 
     """A special opaque object returned by indexing a CustomTarget. This object
     exists in Meson, but acts as a proxy in the backends, making targets depend