Add experimental Iterator::contains

library/core/src/iter/traits/iterator.rs

@@ -2770,6 +2770,39 @@ pub trait Iterator {
         self.try_fold((), check(f)) == ControlFlow::Break(())
     }
 
+    /// Returns `true` if the iterator contains a value.
+    ///
+    /// `contains()` is short-circuiting; in other words, it will stop processing
+    /// as soon as the function finds the item in the `Iterator`.
+    ///
+    /// This method checks the whole iterator, which is O(n). If the iterator is a sorted
+    /// slice, [`binary_search`](slice::binary_search) may be faster. If this is an iterator
+    /// on collections that have a `.contains()` or `.contains_key()` method (such as
+    /// `HashMap` or `BtreeSet`, using those methods directly will be faster.
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// #![feature(iter_contains)]
+    /// assert_eq!(true, [1, 2, 3].iter().contains(2));
+    /// assert_eq!(false, [1, 2, 3].iter().contains(5));
+    /// ```
+    /// [`Iterator::contains`] can be used where [`slice::contains`] cannot be used:
+    /// ```
+    /// #![feature(iter_contains)]
+    /// let s = [String::from("a"), String::from("b"), String::from("c")];
+    /// assert_eq!(s.iter().contains("b"), s.iter().any(|e| e == "b"));
+    /// ```
+    #[inline]
+    #[unstable(feature = "iter_contains", reason = "new API", issue = "127494")]
+    fn contains<Q: ?Sized>(&mut self, item: Q) -> bool
+    where
+        Q: PartialEq<Self::Item>,
+        Self: Sized,
+    {
+        self.any(|elem| item == elem)
+    }
+
     /// Searches for an element of an iterator that satisfies a predicate.
     ///
     /// `find()` takes a closure that returns `true` or `false`. It applies

library/core/src/lib.rs

@@ -118,6 +118,7 @@
 #![feature(internal_impls_macro)]
 #![feature(ip)]
 #![feature(is_ascii_octdigit)]
+#![feature(iter_contains)]
 #![feature(lazy_get)]
 #![feature(link_cfg)]
 #![feature(non_null_from_ref)]

library/core/src/slice/iter/macros.rs

@@ -289,6 +289,16 @@ macro_rules! iterator {
                 false
             }
 
+            fn contains<Q: ?Sized>(&mut self, item: Q) -> bool
+            where
+                Q: PartialEq<Self::Item>,
+                Self: Sized,
+            {
+               // specialize
+               self.any(|elem|item == elem)
+
+            }
+
             // We override the default implementation, which uses `try_fold`,
             // because this simple implementation generates less LLVM IR and is
             // faster to compile.

library/core/src/str/iter.rs

@@ -331,6 +331,13 @@ impl Iterator for Bytes<'_> {
         self.0.any(f)
     }
 
+    fn contains<Q: ?Sized>(&mut self, item: Q) -> bool
+    where
+        Q: PartialEq<Self::Item>,
+    {
+        self.0.contains(item)
+    }
+
     #[inline]
     fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
     where

library/core/tests/iter/traits/iterator.rs

@@ -271,6 +271,16 @@ fn test_any() {
     assert!(!v.iter().any(|&x| x > 100));
     assert!(!v[..0].iter().any(|_| panic!()));
 }
+#[test]
+fn test_iterator_contains() {
+    let v: Box<[isize]> = Box::new([1, 2, 3, 4, 5]);
+    assert_eq!(true, v.iter().contains(3));
+    assert_eq!(v.iter().contains(3), v.iter().any(|&x| x == 3));
+    assert_eq!(false, v.iter().contains(10));
+    assert_eq!(v.iter().contains(10), v.iter().any(|&x| x == 10));
+    assert_eq!(true, Iterator::contains(&mut (1..=5), 3));
+    assert_eq!(false, Iterator::contains(&mut (1..=5), 10));
+}
 
 #[test]
 fn test_find() {