Fix a condition and just call rb_fstring [Feature #15251]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65374 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -1564,25 +1564,11 @@ hash_aset(st_data_t *key, st_data_t *val, struct update_arg *arg, int existing)
     return ST_CONTINUE;
 }
 
-static VALUE
-fstring_key_str(VALUE str)
-{
-    st_data_t fstr;
-    st_table *tbl = rb_vm_fstring_table();
-
-    if (st_lookup(tbl, str, &fstr) && !rb_objspace_garbage_object_p(fstr)) {
-        return (VALUE)fstr;
-    }
-    else {
-        return rb_fstring(str);
-    }
-}
-
 VALUE
 rb_hash_key_str(VALUE key)
 {
-    if (!FL_ANY_RAW(key, FL_TAINT|FL_SINGLETON)) {
-        return fstring_key_str(key);
+    if (!RB_FL_ANY_RAW(key, FL_TAINT|FL_EXIVAR) && RBASIC_CLASS(key) == rb_cString) {
+        return rb_fstring(key);
     }
     else {
 	return rb_str_new_frozen(key);