diff --git a/src/libcore/array.rs b/src/libcore/array.rs
index 8c785b109236d..c986914440de0 100644
--- a/src/libcore/array.rs
+++ b/src/libcore/array.rs
@@ -35,14 +35,23 @@ 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> {
+///
+/// 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];
     /// 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