Give the env to custom descitem parse functions.

sphinx/directives.py

@@ -321,7 +321,7 @@ def desc_directive(desctype, arguments, options, content, lineno,
                 # another registered generic x-ref directive
                 rolename, indextext, parse_node = additional_xref_types[desctype]
                 if parse_node:
-                    fullname = parse_node(sig, signode)
+                    fullname = parse_node(env, sig, signode)
                 else:
                     signode.clear()
                     signode += addnodes.desc_name(sig, sig)