Need not to call mrb_obj_as_string() is sep is nil.

matz · matz · commit 6db38c376880 · 2017-03-19T00:28:52.000+09:00
diff --git a/src/array.c b/src/array.c
@@ -1071,7 +1071,9 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)
 MRB_API mrb_value
 mrb_ary_join(mrb_state *mrb, mrb_value ary, mrb_value sep)
 {
-  sep = mrb_obj_as_string(mrb, sep);
+  if (!mrb_nil_p(sep)) {
+    sep = mrb_obj_as_string(mrb, sep);
+  }
   return join_ary(mrb, ary, sep, mrb_ary_new(mrb));
 }
 

Original file line number	Diff line number	Diff line change
`@@ -1071,7 +1071,9 @@ join_ary(mrb_state *mrb, mrb_value ary, mrb_value sep, mrb_value list)`
`1071`	`1071`	`MRB_API mrb_value`
`1072`	`1072`	`mrb_ary_join(mrb_state *mrb, mrb_value ary, mrb_value sep)`
`1073`	`1073`	`{`
`1074`		`- sep = mrb_obj_as_string(mrb, sep);`
	`1074`	`+ if (!mrb_nil_p(sep)) {`
	`1075`	`+ sep = mrb_obj_as_string(mrb, sep);`
	`1076`	`+ }`
`1075`	`1077`	`return join_ary(mrb, ary, sep, mrb_ary_new(mrb));`
`1076`	`1078`	`}`
`1077`	`1079`