Fix sphinx-doc#8959: using UNIX path separator confuses Sphinx on Win…

…dows

CHANGES

@@ -120,6 +120,8 @@ Deprecated
 Features added
 --------------
 
+* #8959: using UNIX path separator in image directive confuses Sphinx on Windows
+
 Bugs fixed
 ----------
 

sphinx/environment/__init__.py

@@ -34,6 +34,7 @@
 from sphinx.util.docutils import LoggingReporter
 from sphinx.util.i18n import CatalogRepository, docname_to_domain
 from sphinx.util.nodes import is_translatable
+from sphinx.util.osutil import canon_path, os_path
 
 if TYPE_CHECKING:
     from sphinx.application import Sphinx
@@ -335,14 +336,15 @@ def relfn2path(self, filename: str, docname: str = None) -> Tuple[str, str]:
         source dir, while relative filenames are relative to the dir of the
         containing document.
         """
+        filename = os_path(filename)
         if filename.startswith('/') or filename.startswith(os.sep):
             rel_fn = filename[1:]
         else:
             docdir = path.dirname(self.doc2path(docname or self.docname,
                                                 base=None))
             rel_fn = path.join(docdir, filename)
 
-        return (posixpath.normpath(rel_fn),
+        return (canon_path(path.normpath(rel_fn)),
                 path.normpath(path.join(self.srcdir, rel_fn)))
 
     @property