From e83666f45e3d93439775daefda7800b2ab193d30 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Wed, 28 Oct 2020 18:52:45 +0100
Subject: [PATCH 1/2] Prevent String::retain from creating non-utf8 strings
 when abusing panic

---
 library/alloc/src/string.rs | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 72ed036637d..ce216e5336e 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -1235,6 +1235,10 @@ impl String {
         let mut del_bytes = 0;
         let mut idx = 0;
 
+        unsafe {
+            self.vec.set_len(0);
+        }
+
         while idx < len {
             let ch = unsafe { self.get_unchecked(idx..len).chars().next().unwrap() };
             let ch_len = ch.len_utf8();
@@ -1255,10 +1259,8 @@ impl String {
             idx += ch_len;
         }
 
-        if del_bytes > 0 {
-            unsafe {
-                self.vec.set_len(len - del_bytes);
-            }
+        unsafe {
+            self.vec.set_len(len - del_bytes);
         }
     }
 

From 1f6f917f73a4372f098e9b19560b5945be145dc3 Mon Sep 17 00:00:00 2001
From: Giacomo Stevanato <giaco.stevanato@gmail.com>
Date: Thu, 29 Oct 2020 11:48:56 +0100
Subject: [PATCH 2/2] Added test for issue #78498

---
 library/alloc/tests/string.rs | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/library/alloc/tests/string.rs b/library/alloc/tests/string.rs
index a6e41b21b61..b28694186b6 100644
--- a/library/alloc/tests/string.rs
+++ b/library/alloc/tests/string.rs
@@ -1,6 +1,7 @@
 use std::borrow::Cow;
 use std::collections::TryReserveError::*;
 use std::ops::Bound::*;
+use std::panic;
 
 pub trait IntoCow<'a, B: ?Sized>
 where
@@ -378,6 +379,20 @@ fn test_retain() {
 
     s.retain(|_| false);
     assert_eq!(s, "");
+
+    let mut s = String::from("0è0");
+    let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
+        let mut count = 0;
+        s.retain(|_| {
+            count += 1;
+            match count {
+                1 => false,
+                2 => true,
+                _ => panic!(),
+            }
+        });
+    }));
+    assert!(std::str::from_utf8(s.as_bytes()).is_ok());
 }
 
 #[test]