From 2e3ee230220cc81b159d99d833cba46c667e3321 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Beno=C3=AEt=20du=20Garreau?= <bdgdlm@outlook.com>
Date: Wed, 10 Apr 2024 19:16:31 +0200
Subject: [PATCH] Avoid panicking branch in `append_to_string`

---
 library/std/src/io/mod.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
index 98973a43e1ded..450fa43ebea95 100644
--- a/library/std/src/io/mod.rs
+++ b/library/std/src/io/mod.rs
@@ -384,7 +384,10 @@ where
 {
     let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
     let ret = f(g.buf);
-    if str::from_utf8(&g.buf[g.len..]).is_err() {
+
+    // SAFETY: the caller promises to only append data to `buf`
+    let appended = g.buf.get_unchecked(g.len..);
+    if str::from_utf8(appended).is_err() {
         ret.and_then(|_| Err(Error::INVALID_UTF8))
     } else {
         g.len = g.buf.len();