From c353ec0c9e8e941f49910a51f7dc10cf3d3cdd17 Mon Sep 17 00:00:00 2001
From: nobu <nobu@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date: Mon, 2 May 2016 03:58:28 +0000
Subject: [PATCH] string.c: shortcut

* string.c (rb_str_concat): shortcut concatenation to ASCII-8BIT
  as well as US-ASCII.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54882 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
---
 string.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/string.c b/string.c
index a22505f592..35e53f7306 100644
--- a/string.c
+++ b/string.c
@@ -2742,6 +2742,7 @@ rb_str_concat(VALUE str1, VALUE str2)
 {
     unsigned int code;
     rb_encoding *enc = STR_ENC_GET(str1);
+    int encidx;
 
     if (FIXNUM_P(str2) || RB_TYPE_P(str2, T_BIGNUM)) {
 	if (rb_num_to_uint(str2, &code) == 0) {
@@ -2757,7 +2758,8 @@ rb_str_concat(VALUE str1, VALUE str2)
 	return rb_str_append(str1, str2);
     }
 
-    if (enc == rb_usascii_encoding()) {
+    encidx = rb_enc_to_index(enc);
+    if (encidx == ENCINDEX_ASCII || encidx == ENCINDEX_US_ASCII) {
 	/* US-ASCII automatically extended to ASCII-8BIT */
 	char buf[1];
 	buf[0] = (char)code;
@@ -2765,8 +2767,8 @@ rb_str_concat(VALUE str1, VALUE str2)
 	    rb_raise(rb_eRangeError, "%u out of char range", code);
 	}
 	rb_str_cat(str1, buf, 1);
-	if (code > 127) {
-	    rb_enc_associate_index(str1, rb_ascii8bit_encindex());
+	if (encidx == ENCINDEX_US_ASCII && code > 127) {
+	    rb_enc_associate_index(str1, ENCINDEX_ASCII);
 	    ENC_CODERANGE_SET(str1, ENC_CODERANGE_VALID);
 	}
     }