clarify that unchecked indexing is UB even if the reference is never used

src/libcore/slice/mod.rs

@@ -292,10 +292,13 @@ impl<T> [T] {
     /// Returns a reference to an element or subslice, without doing bounds
     /// checking.
     ///
-    /// This is generally not recommended, use with caution! For a safe
-    /// alternative see [`get`].
+    /// This is generally not recommended, use with caution!
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// For a safe alternative see [`get`].
     ///
     /// [`get`]: #method.get
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///
@@ -317,10 +320,13 @@ impl<T> [T] {
     /// Returns a mutable reference to an element or subslice, without doing
     /// bounds checking.
     ///
-    /// This is generally not recommended, use with caution! For a safe
-    /// alternative see [`get_mut`].
+    /// This is generally not recommended, use with caution!
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// For a safe alternative see [`get_mut`].
     ///
     /// [`get_mut`]: #method.get_mut
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
     /// # Examples
     ///
@@ -2629,11 +2635,17 @@ pub trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 
     /// Returns a shared reference to the output at this location, without
     /// performing any bounds checking.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "0")]
     unsafe fn get_unchecked(self, slice: &T) -> &Self::Output;
 
     /// Returns a mutable reference to the output at this location, without
     /// performing any bounds checking.
+    /// Calling this method with an out-of-bounds index is *[undefined behavior]*
+    /// even if the resulting reference is not used.
+    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "0")]
     unsafe fn get_unchecked_mut(self, slice: &mut T) -> &mut Self::Output;