From 4607447e66024ed5e163057893f74f2832cad033 Mon Sep 17 00:00:00 2001
From: Peter Todd <pete@petertodd.org>
Date: Mon, 6 May 2019 17:39:39 -0400
Subject: [PATCH 1/2] Document layout guarantee of MaybeUninit

---
 src/libcore/mem.rs | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index 733c5985c33ed..b378ed89bad00 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -1110,6 +1110,28 @@ impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
 /// to its fields.
 ///
 /// [ub]: ../../reference/behavior-considered-undefined.html
+///
+/// # Layout
+///
+/// `MaybeUninit<T>` is guaranteed to have the same size and alignment as `T`:
+///
+/// ```rust
+/// use std::mem::{MaybeUninit, size_of, align_of};
+/// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
+/// assert_eq!(align_of::<MaybeUninit<u64>>(), size_of::<u64>());
+/// ```
+///
+/// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same
+/// layout; Rust does not in general guarantee that the fields of a `Foo<T>` have the same order as
+/// a `Foo<U>` even if `T` and `U` have the same size and alignment. Furthermore because any bit
+/// value is valid for a `MaybeUninit<T>` the compiler can't apply non-zero/niche-filling
+/// optimizations, potentially resulting in a larger size:
+///
+/// ```rust
+/// # use std::mem::{MaybeUninit, size_of, align_of};
+/// assert_eq!(size_of::<Option<bool>>(), 1);
+/// assert_eq!(size_of::<Option<MaybeUninit<bool>>>(), 2);
+/// ```
 #[allow(missing_debug_implementations)]
 #[stable(feature = "maybe_uninit", since = "1.36.0")]
 #[derive(Copy)]

From b992528568352571019a55b71778cb7f7e3bbb3c Mon Sep 17 00:00:00 2001
From: Peter Todd <pete@petertodd.org>
Date: Wed, 8 May 2019 13:22:32 -0400
Subject: [PATCH 2/2] Fix typo

---
 src/libcore/mem.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index b378ed89bad00..ae24cc7eaf10f 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -1118,7 +1118,7 @@ impl<T: ?Sized> DerefMut for ManuallyDrop<T> {
 /// ```rust
 /// use std::mem::{MaybeUninit, size_of, align_of};
 /// assert_eq!(size_of::<MaybeUninit<u64>>(), size_of::<u64>());
-/// assert_eq!(align_of::<MaybeUninit<u64>>(), size_of::<u64>());
+/// assert_eq!(align_of::<MaybeUninit<u64>>(), align_of::<u64>());
 /// ```
 ///
 /// However remember that a type *containing* a `MaybeUninit<T>` is not necessarily the same