Add initial impl of is_sorted to Iterator

src/libcore/iter/iterator.rs

@@ -2605,6 +2605,90 @@ pub trait Iterator {
             }
         }
     }
+
+    /// Checks if the elements of this iterator are sorted.
+    ///
+    /// That is, for each element `a` and its following element `b`, `a <= b`
+    /// must hold. If the iterator yields exactly zero or one element, `true`
+    /// is returned.
+    ///
+    /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above
+    /// definition implies that this function returns `false` if any two
+    /// consecutive items are not comparable.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(is_sorted)]
+    /// 
+    /// assert!([1, 2, 2, 9].iter().is_sorted());
+    /// assert!(![1, 3, 2, 4].iter().is_sorted());
+    /// assert!([0].iter().is_sorted());
+    /// assert!(std::iter::empty::<i32>().is_sorted());
+    /// assert!(![0.0, 1.0, std::f32::NAN].iter().is_sorted());
+    /// ```
+    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
+    fn is_sorted(self) -> bool
+    where
+        Self: Sized,
+        Self::Item: PartialOrd,
+    {
+        self.is_sorted_by(|a, b| a.partial_cmp(b))
+    }
+
+    /// Checks if the elements of this iterator are sorted using the given
+    /// comparator function.
+    ///
+    /// Instead of using `PartialOrd::partial_cmp`, this function uses the given
+    /// `compare` function to determine the ordering of two elements. Apart from
+    /// that, it's equivalent to `is_sorted`; see its documentation for more
+    /// information.
+    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
+    fn is_sorted_by<F>(mut self, mut compare: F) -> bool
+    where
+        Self: Sized,
+        F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>
+    {
+        let mut last = match self.next() {
+            Some(e) => e,
+            None => return true,
+        };
+
+        while let Some(curr) = self.next() {
+            if compare(&last, &curr).map(|o| o == Ordering::Greater).unwrap_or(true) {
+                return false;
+            }
+            last = curr;
+        }
+
+        true
+    }
+
+    /// Checks if the elements of this iterator are sorted using the given
+    /// key extraction function.
+    ///
+    /// Instead of comparing the iterator's elements directly, this function
+    /// compares the keys of the elements, as determined by `f`. Apart from
+    /// that, it's equivalent to `is_sorted`; see its documentation for more
+    /// information.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(is_sorted)]
+    /// 
+    /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
+    /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
+    /// ```
+    #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
+    fn is_sorted_by_key<F, K>(self, mut f: F) -> bool
+    where
+        Self: Sized,
+        F: FnMut(&Self::Item) -> K,
+        K: PartialOrd
+    {
+        self.is_sorted_by(|a, b| f(a).partial_cmp(&f(b)))
+    }
 }
 
 /// Select an element from an iterator based on the given "projection"

src/libcore/lib.rs

@@ -79,6 +79,7 @@
 #![feature(extern_types)]
 #![feature(fundamental)]
 #![feature(intrinsics)]
+#![feature(is_sorted)]
 #![feature(iter_once_with)]
 #![feature(lang_items)]
 #![feature(link_llvm_intrinsics)]

src/libcore/tests/iter.rs

@@ -2235,3 +2235,16 @@ fn test_monad_laws_associativity() {
     assert_eq!((0..10).flat_map(f).flat_map(g).sum::<usize>(),
                 (0..10).flat_map(|x| f(x).flat_map(g)).sum::<usize>());
 }
+
+#[test]
+fn test_is_sorted() {
+    assert!([1, 2, 2, 9].iter().is_sorted());
+    assert!(![1, 3, 2].iter().is_sorted());
+    assert!([0].iter().is_sorted());
+    assert!(std::iter::empty::<i32>().is_sorted());
+    assert!(![0.0, 1.0, std::f32::NAN].iter().is_sorted());
+    assert!([-2, -1, 0, 3].iter().is_sorted());
+    assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
+    assert!(!["c", "bb", "aaa"].iter().is_sorted());
+    assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
+}

src/libcore/tests/lib.rs

@@ -10,6 +10,7 @@
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
 #![feature(hashmap_internals)]
+#![feature(is_sorted)]
 #![feature(iter_copied)]
 #![feature(iter_nth_back)]
 #![feature(iter_once_with)]

src/test/ui/feature-gates/feature-gate-is_sorted.rs

@@ -0,0 +1,16 @@
+// 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!([1, 2, 2, 9].iter().is_sorted());
+    //^ ERROR: use of unstable library feature 'is_sorted'
+    assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
+    //^ ERROR: use of unstable library feature 'is_sorted'
+}

src/test/ui/feature-gates/feature-gate-is_sorted.stderr

@@ -0,0 +1,19 @@
+error[E0658]: use of unstable library feature 'is_sorted': new API (see issue #53485)
+  --> $DIR/feature-gate-is_sorted.rs:12:33
+   |
+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:14:39
+   |
+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: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.