A few more eradications of plain strings.

backends.py

@@ -165,10 +165,9 @@ class Backend():
         targetdir = self.get_target_private_dir(extobj.target)
         suffix = '.' + self.environment.get_object_suffix()
         for osrc in extobj.srclist:
+            osrc = osrc.fname
             if not self.source_suffix_in_objs:
                 osrc = '.'.join(osrc.split('.')[:-1])
-            if hasattr(osrc, 'fname'): # FIXME allow only strings.
-                osrc = osrc.fname
             objname = os.path.join(proj_dir_to_build_root,
                                    targetdir, os.path.basename(osrc) + suffix)
             result.append(objname)

interpreter.py

@@ -1490,7 +1490,7 @@ class Interpreter():
             if isinstance(s, File) or isinstance(s, GeneratedListHolder) or \
             isinstance(s, CustomTargetHolder):
                 pass
-            elif isinstance(s, str): # FIXME do not allow plain strings.
+            elif isinstance(s, str):
                 s = File.from_source_file(self.environment.source_dir, self.subdir, s)
             else:
                 raise RuntimeError("Unreachable code")

ninjabackend.py

@@ -1107,10 +1107,10 @@ rule FORTRAN_DEP_HACK
                 rel_src = os.path.join(self.get_target_private_dir(target), src)
                 abs_src = os.path.join(self.environment.get_source_dir(), rel_src)
         else:
-            if isinstance(src, File): # FIXME, accept only Files.
+            if isinstance(src, File):
                 rel_src = src.rel_to_builddir(self.build_to_src)
             else:
-                rel_src = os.path.join(self.build_to_src, target.get_source_subdir(), src)
+                raise build.InvaliArguments('Invalid source type.')
             abs_src = os.path.join(self.environment.get_build_dir(), rel_src)
         if isinstance(src, RawFilename):
             src_filename = src.fname