Fix Regexp#to_s for GC compaction

The test fails when RGENGC_CHECK_MODE is turned on:

    TestRegexp#test_to_s_under_gc_compact_stress = 13.46 s
    1) Failure:
    TestRegexp#test_to_s_under_gc_compact_stress [test/ruby/test_regexp.rb:81]:
    <"(?-mix:abcd\u3042)"> expected but was
    <"(?-mix:\u5C78\u3030\u5C78\u3030\u5C78\u3030\u5C78\u3030\u5C78\u3030)">.

1 files changed, 5 insertions, 4 deletions

diff --git a/re.c b/re.c
index f3be9a1fce..e51b782cf6 100644
--- a/re.c
+++ b/re.c
@@ -565,8 +565,6 @@ rb_reg_str_with_term(VALUE re, int term)
 {
     int options, opt;
     const int embeddable = ONIG_OPTION_MULTILINE|ONIG_OPTION_IGNORECASE|ONIG_OPTION_EXTEND;
-    long len;
-    const UChar* ptr;
     VALUE str = rb_str_buf_new2("(?");
     char optbuf[OPTBUF_SIZE + 1]; /* for '-' */
     rb_encoding *enc = rb_enc_get(re);
@@ -575,8 +573,9 @@ rb_reg_str_with_term(VALUE re, int term)
 
     rb_enc_copy(str, re);
     options = RREGEXP_PTR(re)->options;
-    ptr = (UChar*)RREGEXP_SRC_PTR(re);
-    len = RREGEXP_SRC_LEN(re);
+    VALUE src_str = RREGEXP_SRC(re);
+    const UChar *ptr = (UChar *)RSTRING_PTR(src_str);
+    long len = RSTRING_LEN(src_str);
   again:
     if (len >= 4 && ptr[0] == '(' && ptr[1] == '?') {
         int err = 1;
@@ -666,6 +665,8 @@ rb_reg_str_with_term(VALUE re, int term)
     }
     rb_enc_copy(str, re);
 
+    RB_GC_GUARD(src_str);
+
     return str;
 }