diff --git a/src/libcore/raw.rs b/src/libcore/raw.rs
index 81edfe7d6e26..bb0c22d3561d 100644
--- a/src/libcore/raw.rs
+++ b/src/libcore/raw.rs
@@ -154,7 +154,7 @@ pub struct TraitObject {
 
 /// This trait is meant to map equivalences between raw structs and their
 /// corresponding rust values.
-pub trait Repr<T> {
+pub unsafe trait Repr<T> {
     /// This function "unwraps" a rust value (without consuming it) into its raw
     /// struct representation. This can be used to read/write different values
     /// for the struct. This is a safe method because by default it does not
@@ -163,5 +163,5 @@ pub trait Repr<T> {
     fn repr(&self) -> T { unsafe { mem::transmute_copy(&self) } }
 }
 
-impl<T> Repr<Slice<T>> for [T] {}
-impl Repr<Slice<u8>> for str {}
+unsafe impl<T> Repr<Slice<T>> for [T] {}
+unsafe impl Repr<Slice<u8>> for str {}