From 78b7646bdb91285873ac26bca060591e06c45afe Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Sat, 17 Jan 2026 14:19:15 +0900
Subject: [Bug #21842] Let `rb_interned_str` return US-ASCII if possible

---
 string.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

(limited to 'string.c')

diff --git a/string.c b/string.c
index 6f4ea03fb3..2d74c46a36 100644
--- a/string.c
+++ b/string.c
@@ -12709,7 +12709,15 @@ VALUE
 rb_interned_str(const char *ptr, long len)
 {
     struct RString fake_str = {RBASIC_INIT};
-    return register_fstring(setup_fake_str(&fake_str, ptr, len, ENCINDEX_US_ASCII), true, false);
+    int encidx = ENCINDEX_US_ASCII;
+    int coderange = ENC_CODERANGE_7BIT;
+    if (len > 0 && search_nonascii(ptr, ptr + len)) {
+        encidx = ENCINDEX_ASCII_8BIT;
+        coderange = ENC_CODERANGE_VALID;
+    }
+    VALUE str = setup_fake_str(&fake_str, ptr, len, encidx);
+    ENC_CODERANGE_SET(str, coderange);
+    return register_fstring(str, true, false);
 }
 
 VALUE
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