Use numeric field Names ("0", "1" etc) for positional fields

src/libsyntax/parse/token.rs

@@ -542,7 +542,7 @@ declare_special_idents_and_keywords! {
         // outside of libsyntax
         (7,                          clownshoe_abi,          "__rust_abi");
         (8,                          opaque,                 "<opaque>");
-        (9,                          unnamed_field,          "<unnamed_field>");
+        (9,                          __unused1,              "<__unused1>");
         (super::SELF_TYPE_KEYWORD_NAME_NUM, type_self,       "Self");
         (11,                         prelude_import,         "prelude_import");
     }