From 85f48d300a1161338e14b9579007b003c48dc4ba Mon Sep 17 00:00:00 2001
From: Amit Aryeh Levy <amit@amitlevy.com>
Date: Sat, 19 Sep 2015 15:33:34 -0400
Subject: [PATCH 1/2] Make FixedSizeArray an unsafe trait

---
 src/libcore/array.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 8c785b109236d..976689beccd11 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -35,14 +35,14 @@ use slice::{Iter, IterMut, SliceExt};
 ///
 /// This trait can be used to implement other traits on fixed-size arrays
 /// without causing much metadata bloat.
-pub trait FixedSizeArray<T> {
+pub unsafe trait FixedSizeArray<T> {
     /// Converts the array to immutable slice
     fn as_slice(&self) -> &[T];
     /// Converts the array to mutable slice
     fn as_mut_slice(&mut self) -> &mut [T];
 }
 
-impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
+unsafe impl<T, A: Unsize<[T]>> FixedSizeArray<T> for A {
     #[inline]
     fn as_slice(&self) -> &[T] {
         self

From b30d8969e86fa2c9dd3b8e2e28ddda2202331f0f Mon Sep 17 00:00:00 2001
From: Amit Aryeh Levy <amit@amitlevy.com>
Date: Wed, 23 Sep 2015 11:38:01 -0400
Subject: [PATCH 2/2] Explain in comment why FixedSizeArray is unsafe

---
 src/libcore/array.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 976689beccd11..c986914440de0 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -35,6 +35,15 @@ use slice::{Iter, IterMut, SliceExt};
 ///
 /// This trait can be used to implement other traits on fixed-size arrays
 /// without causing much metadata bloat.
+///
+/// The trait is marked unsafe in order to restrict implementors to fixed-size
+/// arrays. User of this trait can assume that implementors have the exact
+/// layout in memory of a fixed size array (for example, for unsafe
+/// initialization).
+///
+/// Note that the traits AsRef and AsMut provide similar methods for types that
+/// may not be fixed-size arrays. Implementors should prefer those traits
+/// instead.
 pub unsafe trait FixedSizeArray<T> {
     /// Converts the array to immutable slice
     fn as_slice(&self) -> &[T];