From 37718f949fe5bef4bbfa4ee393c2abf022771ccb Mon Sep 17 00:00:00 2001
From: Joshua Wong <joshuawong@anticentri.st>
Date: Tue, 19 Mar 2024 22:47:35 -0500
Subject: [PATCH] fix OOB pointer formed in Vec::index

Move the length check to before using `index` with `ptr::add` to prevent
an out of bounds pointer from being formed.

Fixes #122760
---
 library/alloc/src/vec/mod.rs | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index db56b8d07c71f..94bed825bb2f6 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -1541,6 +1541,9 @@ impl<T, A: Allocator> Vec<T, A> {
         }
 
         let len = self.len();
+        if index > len {
+            assert_failed(index, len);
+        }
 
         // space for the new element
         if len == self.buf.capacity() {
@@ -1556,10 +1559,6 @@ impl<T, A: Allocator> Vec<T, A> {
                     // Shift everything over to make space. (Duplicating the
                     // `index`th element into two consecutive places.)
                     ptr::copy(p, p.add(1), len - index);
-                } else if index == len {
-                    // No elements need shifting.
-                } else {
-                    assert_failed(index, len);
                 }
                 // Write it in, overwriting the first copy of the `index`th
                 // element.