diff --git a/src/librustdoc/clean/auto_trait.rs b/src/librustdoc/clean/auto_trait.rs
index a87e1df5efc2..322527839e02 100644
--- a/src/librustdoc/clean/auto_trait.rs
+++ b/src/librustdoc/clean/auto_trait.rs
@@ -1435,9 +1435,7 @@ impl<'a, 'tcx, 'rcx> AutoTraitFinder<'a, 'tcx, 'rcx> {
     // involved (impls rarely have more than a few bounds) means that it
     // shouldn't matter in practice.
     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
-        vec.sort_unstable_by(|first, second| {
-            format!("{:?}", first).cmp(&format!("{:?}", second))
-        });
+        vec.sort_by_cached_key(|x| format!("{:?}", x))
     }
 
     fn is_fn_ty(&self, tcx: &TyCtxt, ty: &Type) -> bool {
diff --git a/src/librustdoc/lib.rs b/src/librustdoc/lib.rs
index 42e87f88fd40..e34cf6222cf8 100644
--- a/src/librustdoc/lib.rs
+++ b/src/librustdoc/lib.rs
@@ -20,6 +20,7 @@
 #![feature(box_syntax)]
 #![feature(fs_read_write)]
 #![feature(set_stdio)]
+#![feature(slice_sort_by_cached_key)]
 #![feature(test)]
 #![feature(unicode)]
 #![feature(vec_remove_item)]