Override Iterator::is_sorted_by in slice::Iter impl

Additionally, the root implementation was changed a bit: it now uses
`all` instead of coding that logic manually.

To avoid duplicate code, the inherent `[T]::is_sorted_by` method now
calls `self.iter().is_sorted_by(...)`. This should always be inlined
and not result in overhead.

Author: LukasKalbertodt

Diff for: src/libcore/slice/mod.rs

@@ -2293,16 +2293,7 @@ impl<T> [T] {
     where
         F: FnMut(&T, &T) -> Option<Ordering>
     {
-        for pair in self.windows(2) {
-            if compare(&pair[0], &pair[1])
-                .map(|o| o == Ordering::Greater)
-                .unwrap_or(true)
-            {
-                return false;
-            }
-        }
-
-        true
+        self.iter().is_sorted_by(|a, b| compare(*a, *b))
     }
 
     /// Checks if the elements of this slice are sorted using the given key extraction function.
@@ -2853,7 +2844,13 @@ macro_rules! len {
 
 // The shared definition of the `Iter` and `IterMut` iterators
 macro_rules! iterator {
-    (struct $name:ident -> $ptr:ty, $elem:ty, $raw_mut:tt, $( $mut_:tt )*) => {
+    (
+        struct $name:ident -> $ptr:ty,
+        $elem:ty,
+        $raw_mut:tt,
+        {$( $mut_:tt )*},
+        {$($extra:tt)*}
+    ) => {
         impl<'a, T> $name<'a, T> {
             // Helper function for creating a slice from the iterator.
             #[inline(always)]
@@ -3030,6 +3027,8 @@ macro_rules! iterator {
                         i
                     })
             }
+
+            $($extra)*
         }
 
         #[stable(feature = "rust1", since = "1.0.0")]
@@ -3167,7 +3166,17 @@ impl<'a, T> Iter<'a, T> {
     }
 }
 
-iterator!{struct Iter -> *const T, &'a T, const, /* no mut */}
+iterator!{struct Iter -> *const T, &'a T, const, {/* no mut */}, {
+    fn is_sorted_by<F>(self, mut compare: F) -> bool
+    where
+        Self: Sized,
+        F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
+    {
+        self.as_slice().windows(2).all(|w| {
+            compare(&&w[0], &&w[1]).map(|o| o != Ordering::Greater).unwrap_or(false)
+        })
+    }
+}}
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Clone for Iter<'_, T> {
@@ -3268,7 +3277,7 @@ impl<'a, T> IterMut<'a, T> {
     }
 }
 
-iterator!{struct IterMut -> *mut T, &'a mut T, mut, mut}
+iterator!{struct IterMut -> *mut T, &'a mut T, mut, {mut}, {}}
 
 /// An internal abstraction over the splitting iterators, so that
 /// splitn, splitn_mut etc can be implemented once.

Diff for: src/test/ui/feature-gates/feature-gate-is_sorted.rs

@@ -1,13 +1,3 @@
-// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution and at
-// http://rust-lang.org/COPYRIGHT.
-//
-// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
-// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
-// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
-// option. This file may not be copied, modified, or distributed
-// except according to those terms.
-
 fn main() {
     // Assert `Iterator` methods are feature gated
     assert!([1, 2, 2, 9].iter().is_sorted());

Diff for: src/test/ui/feature-gates/feature-gate-is_sorted.stderr

@@ -1,29 +1,29 @@
 error[E0658]: use of unstable library feature 'is_sorted': new API (see issue #53485)
-  --> $DIR/feature-gate-is_sorted.rs:13:33
+  --> $DIR/feature-gate-is_sorted.rs:3:33
    |
 LL |     assert!([1, 2, 2, 9].iter().is_sorted());
    |                                 ^^^^^^^^^
    |
    = help: add #![feature(is_sorted)] to the crate attributes to enable
 
 error[E0658]: use of unstable library feature 'is_sorted': new API (see issue #53485)
-  --> $DIR/feature-gate-is_sorted.rs:15:39
+  --> $DIR/feature-gate-is_sorted.rs:5:39
    |
 LL |     assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
    |                                       ^^^^^^^^^^^^^^^^
    |
    = help: add #![feature(is_sorted)] to the crate attributes to enable
 
 error[E0658]: use of unstable library feature 'is_sorted': new API (see issue #53485)
-  --> $DIR/feature-gate-is_sorted.rs:19:26
+  --> $DIR/feature-gate-is_sorted.rs:9:26
    |
 LL |     assert!([1, 2, 2, 9].is_sorted());
    |                          ^^^^^^^^^
    |
    = help: add #![feature(is_sorted)] to the crate attributes to enable
 
 error[E0658]: use of unstable library feature 'is_sorted': new API (see issue #53485)
-  --> $DIR/feature-gate-is_sorted.rs:21:32
+  --> $DIR/feature-gate-is_sorted.rs:11:32
    |
 LL |     assert!(![-2i32, -1, 0, 3].is_sorted_by_key(|n| n.abs()));
    |                                ^^^^^^^^^^^^^^^^