From 98b54fbd7afd55f40531bdb350a5adb870958111 Mon Sep 17 00:00:00 2001
From: Tim Vermeulen <tvermeulen@me.com>
Date: Sun, 7 Jul 2019 20:46:38 +0200
Subject: [PATCH] Only call the closure parameter of Iterator::is_sorted_by_key
 once per item

---
 src/libcore/iter/traits/iterator.rs | 6 +++---
 src/libcore/slice/mod.rs            | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

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 c6d44324ef5ee..0cd725d9b3628 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)
     }
 }