diff --git a/src/libcore/iter/traits/iterator.rs b/src/libcore/iter/traits/iterator.rs
index 30923c7414504..b9a98236f18a7 100644
--- a/src/libcore/iter/traits/iterator.rs
+++ b/src/libcore/iter/traits/iterator.rs
@@ -2572,13 +2572,13 @@ pub trait Iterator {
     /// ```
     #[inline]
     #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
-    fn is_sorted_by_key<F, K>(self, mut f: F) -> bool
+    fn is_sorted_by_key<F, K>(self, f: F) -> bool
     where
         Self: Sized,
-        F: FnMut(&Self::Item) -> K,
+        F: FnMut(Self::Item) -> K,
         K: PartialOrd
     {
-        self.is_sorted_by(|a, b| f(a).partial_cmp(&f(b)))
+        self.map(f).is_sorted()
     }
 }
 
diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index fe48e2458cd16..fdf3cc8e00627 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -2459,12 +2459,12 @@ impl<T> [T] {
     /// ```
     #[inline]
     #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
-    pub fn is_sorted_by_key<F, K>(&self, mut f: F) -> bool
+    pub fn is_sorted_by_key<F, K>(&self, f: F) -> bool
     where
         F: FnMut(&T) -> K,
         K: PartialOrd
     {
-        self.is_sorted_by(|a, b| f(a).partial_cmp(&f(b)))
+        self.iter().is_sorted_by_key(f)
     }
 }