Auto merge of #33289 - birkenfeld:chain-find, r=bluss

Implement find() on Chain iterators

This results in a roughly 2x speedup compared to the default impl
"inherited" from Iterator.

Benchmark: https://gist.github.com/birkenfeld/aa9b92cb7d55666dd4821207527eaf5b

src/libcore/iter/mod.rs

@@ -541,6 +541,23 @@ impl<A, B> Iterator for Chain<A, B> where
         }
     }
 
+    #[inline]
+    fn find<P>(&mut self, mut predicate: P) -> Option<Self::Item> where
+        P: FnMut(&Self::Item) -> bool,
+    {
+        match self.state {
+            ChainState::Both => match self.a.find(&mut predicate) {
+                None => {
+                    self.state = ChainState::Back;
+                    self.b.find(predicate)
+                }
+                v => v
+            },
+            ChainState::Front => self.a.find(predicate),
+            ChainState::Back => self.b.find(predicate),
+        }
+    }
+
     #[inline]
     fn last(self) -> Option<A::Item> {
         match self.state {

src/libcoretest/iter.rs

@@ -133,6 +133,19 @@ fn test_iterator_chain_count() {
     assert_eq!(zs.iter().chain(&ys).count(), 4);
 }
 
+#[test]
+fn test_iterator_chain_find() {
+    let xs = [0, 1, 2, 3, 4, 5];
+    let ys = [30, 40, 50, 60];
+    let mut iter = xs.iter().chain(&ys);
+    assert_eq!(iter.find(|&&i| i == 4), Some(&4));
+    assert_eq!(iter.next(), Some(&5));
+    assert_eq!(iter.find(|&&i| i == 40), Some(&40));
+    assert_eq!(iter.next(), Some(&50));
+    assert_eq!(iter.find(|&&i| i == 100), None);
+    assert_eq!(iter.next(), None);
+}
+
 #[test]
 fn test_filter_map() {
     let it = (0..).step_by(1).take(10)