Auto merge of #62473 - timvermeulen:is_sorted_by_key, r=scottmcm

Only call the closure parameter of Iterator::is_sorted_by_key once per item

See https://github.com/rust-lang/rust/issues/53485#issuecomment-472314004.

This changes `Iterator::is_sorted_by_key` to only call the given closure once for each item, which allows us to pass the items to the closure by value instead of by reference.

**Important**: `is_sorted_by_key` for slices and slice iterators is now no longer implemented in terms of the custom `slice::Iter::is_sorted_by` implementation. It's a trade-off: we could forward `slice::Iter::is_sorted_by_key` to it directly for potential SIMD benefits, but that would mean that the closure is potentially called twice for (almost) every element of the slice.

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()
     }
 }
 

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)
     }
 }