use String.sub intead of unwrap_uppercase_exotic

Author: cometkim

jscomp/syntax/src/res_printer.ml

@@ -434,12 +434,10 @@ let printPolyVarIdent txt =
   else
     match classifyIdentContent ~allowUident:true txt with
     | UppercaseExoticIdent ->
-      Doc.concat
-        [
-          Doc.text "\"";
-          Doc.text (Ext_ident.unwrap_uppercase_exotic txt);
-          Doc.text "\"";
-        ]
+      let len = String.length txt in
+      (* UppercaseExoticIdent follows the \"..." format,
+         so removing the leading backslash is enough to transform it into polyvar style *)
+      Doc.text ((String.sub [@doesNotRaise]) txt 1 (len - 1))
     | ExoticIdent -> Doc.concat [Doc.text "\""; Doc.text txt; Doc.text "\""]
     | NormalIdent -> (
       match txt with