From 323a27967abe75da79e44132e449fb36cefd240b Mon Sep 17 00:00:00 2001
From: Matthew Jasper <mjjasper1@gmail.com>
Date: Fri, 25 Sep 2020 19:46:06 +0100
Subject: [PATCH] Improve <vec::IntoIter>::get_unchecked` safety comment

---
 library/alloc/src/vec.rs | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/library/alloc/src/vec.rs b/library/alloc/src/vec.rs
index c54b3aef95ed4..e973c3287ede0 100644
--- a/library/alloc/src/vec.rs
+++ b/library/alloc/src/vec.rs
@@ -2985,8 +2985,14 @@ impl<T> Iterator for IntoIter<T> {
     where
         Self: TrustedRandomAccess,
     {
-        // SAFETY: the caller must uphold the contract for
-        // `Iterator::get_unchecked`.
+        // SAFETY: the caller must guarantee that `i` is in bounds of the
+        // `Vec<T>`, so `i` cannot overflow an `isize`, and the `self.ptr.add(i)`
+        // is guaranteed to pointer to an element of the `Vec<T>` and
+        // thus guaranteed to be valid to dereference.
+        //
+        // Also note the implementation of `Self: TrustedRandomAccess` requires
+        // that `T: Copy` so reading elements from the buffer doesn't invalidate
+        // them for `Drop`.
         unsafe {
             if mem::size_of::<T>() == 0 { mem::zeroed() } else { ptr::read(self.ptr.add(i)) }
         }