Rollup merge of rust-lang#41028 - bluss:rev-rfind, r=alexcrichton

Let .rev()'s find use the underlying rfind and vice versa

- Connect the plumbing in an obvious way from Rev's find → underlying rfind and vice versa
- A style change in the provided implementation for Iterator::rfind, using simple next_back when it is enough

src/libcore/iter/mod.rs

@@ -358,12 +358,24 @@ impl<I> Iterator for Rev<I> where I: DoubleEndedIterator {
     fn next(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next_back() }
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
+
+    fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
+        where P: FnMut(&Self::Item) -> bool
+    {
+        self.iter.rfind(predicate)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> DoubleEndedIterator for Rev<I> where I: DoubleEndedIterator {
     #[inline]
     fn next_back(&mut self) -> Option<<I as Iterator>::Item> { self.iter.next() }
+
+    fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
+        where P: FnMut(&Self::Item) -> bool
+    {
+        self.iter.find(predicate)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]

src/libcore/iter/traits.rs

@@ -467,7 +467,7 @@ pub trait DoubleEndedIterator: Iterator {
         Self: Sized,
         P: FnMut(&Self::Item) -> bool
     {
-        for x in self.by_ref().rev() {
+        while let Some(x) = self.next_back() {
             if predicate(&x) { return Some(x) }
         }
         None