backends: Make object_filename_from_source() backend-agnostic

Some backends may need to use its own target directories instead of
our default one. For this, introduce an optional argument "targetdir"
that will still use our default one when not specified.

mesonbuild/backend/backends.py

@@ -834,7 +834,7 @@ class Backend:
             fname = fname.replace(ch, '_')
         return hashed + fname
 
-    def object_filename_from_source(self, target: build.BuildTarget, source: 'FileOrString') -> str:
+    def object_filename_from_source(self, target: build.BuildTarget, source: 'FileOrString', targetdir: T.Optional[str] = None) -> str:
         assert isinstance(source, mesonlib.File)
         if isinstance(target, build.CompileTarget):
             return target.sources_map[source]
@@ -855,9 +855,8 @@ class Backend:
         elif source.is_built:
             if os.path.isabs(rel_src):
                 rel_src = rel_src[len(build_dir) + 1:]
-            targetdir = self.get_target_private_dir(target)
             # A meson- prefixed directory is reserved; hopefully no-one creates a file name with such a weird prefix.
-            gen_source = 'meson-generated_' + os.path.relpath(rel_src, targetdir)
+            gen_source = 'meson-generated_' + os.path.relpath(rel_src, self.get_target_private_dir(target))
         else:
             if os.path.isabs(rel_src):
                 # Use the absolute path directly to avoid file name conflicts
@@ -866,7 +865,10 @@ class Backend:
                 gen_source = os.path.relpath(os.path.join(build_dir, rel_src),
                                              os.path.join(self.environment.get_source_dir(), target.get_subdir()))
         machine = self.environment.machines[target.for_machine]
-        return self.canonicalize_filename(gen_source) + '.' + machine.get_object_suffix()
+        ret = self.canonicalize_filename(gen_source) + '.' + machine.get_object_suffix()
+        if targetdir is not None:
+            return os.path.join(targetdir, ret)
+        return ret
 
     def _determine_ext_objs(self, extobj: 'build.ExtractedObjects', proj_dir_to_build_root: str) -> T.List[str]:
         result: T.List[str] = []
@@ -920,8 +922,8 @@ class Backend:
                     sources.append(_src)
 
         for osrc in sources:
-            objname = self.object_filename_from_source(extobj.target, osrc)
-            objpath = os.path.join(proj_dir_to_build_root, targetdir, objname)
+            objname = self.object_filename_from_source(extobj.target, osrc, targetdir)
+            objpath = os.path.join(proj_dir_to_build_root, objname)
             result.append(objpath)
 
         return result

mesonbuild/backend/vs2010backend.py

@@ -1448,12 +1448,10 @@ class Vs2010Backend(backends.Backend):
                                 extra_link_args.append(path[:-len(gen_src_ext)] + '.obj')
 
                     for src in l.srclist:
-                        obj_basename = None
                         if self.environment.is_source(src):
-                            obj_basename = self.object_filename_from_source(t, src)
                             target_private_dir = self.relpath(self.get_target_private_dir(t),
                                                               self.get_target_dir(t))
-                            rel_obj = os.path.join(target_private_dir, obj_basename)
+                            rel_obj = self.object_filename_from_source(t, src, target_private_dir)
                             extra_link_args.append(rel_obj)
 
                     extra_link_args.extend(self.flatten_object_list(t))

mesonbuild/backend/xcodebackend.py

@@ -230,7 +230,7 @@ class XCodeBackend(backends.Backend):
         os.makedirs(os.path.join(self.environment.get_build_dir(), dirname), exist_ok=True)
         return dirname
 
-    def object_filename_from_source(self, target: build.BuildTarget, source: mesonlib.FileOrString) -> str:
+    def object_filename_from_source(self, target: build.BuildTarget, source: mesonlib.FileOrString, targetdir: T.Optional[str] = None) -> str:
         # Xcode has the following naming scheme:
         # projectname.build/debug/prog@exe.build/Objects-normal/x86_64/func.o
         project = self.build.project_name