From 99de3728f95d1a3fa6063aa4ef2974a45cf35f4c Mon Sep 17 00:00:00 2001
From: Amanieu d'Antras <amanieu@gmail.com>
Date: Wed, 22 Apr 2020 22:22:48 +0100
Subject: [PATCH] Only use read_unaligned in transmute_copy if necessary

---
 src/libcore/mem/mod.rs | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/libcore/mem/mod.rs b/src/libcore/mem/mod.rs
index 07f7d28bb7546..e42b73ddd23a9 100644
--- a/src/libcore/mem/mod.rs
+++ b/src/libcore/mem/mod.rs
@@ -923,7 +923,12 @@ pub fn drop<T>(_x: T) {}
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
-    ptr::read_unaligned(src as *const T as *const U)
+    // If U has a higher alignment requirement, src may not be suitably aligned.
+    if align_of::<U>() > align_of::<T>() {
+        ptr::read_unaligned(src as *const T as *const U)
+    } else {
+        ptr::read(src as *const T as *const U)
+    }
 }
 
 /// Opaque type representing the discriminant of an enum.