slice: Implement .rfind() for slice iterators Iter and IterMut

Just like the forward case find, implement rfind explicitly

src/libcore/slice/mod.rs

@@ -1190,6 +1190,19 @@ fn next_back(&mut self) -> Option<$elem> {
                     }
                 }
             }
+
+            fn rfind<F>(&mut self, mut predicate: F) -> Option<Self::Item>
+                where F: FnMut(&Self::Item) -> bool,
+            {
+                self.rsearch_while(None, move |elt| {
+                    if predicate(&elt) {
+                        SearchWhile::Done(Some(elt))
+                    } else {
+                        SearchWhile::Continue
+                    }
+                })
+            }
+
         }
 
         // search_while is a generalization of the internal iteration methods.

src/libcore/tests/lib.rs

@@ -20,6 +20,7 @@
 #![feature(fixed_size_array)]
 #![feature(flt2dec)]
 #![feature(fmt_internals)]
+#![feature(iter_rfind)]
 #![feature(libc)]
 #![feature(nonzero)]
 #![feature(rand)]

src/libcore/tests/slice.rs

@@ -225,6 +225,19 @@ fn get_unchecked_mut_range() {
     }
 }
 
+#[test]
+fn test_find_rfind() {
+    let v = [0, 1, 2, 3, 4, 5];
+    let mut iter = v.iter();
+    let mut i = v.len();
+    while let Some(&elt) = iter.rfind(|_| true) {
+        i -= 1;
+        assert_eq!(elt, v[i]);
+    }
+    assert_eq!(i, 0);
+    assert_eq!(v.iter().rfind(|&&x| x <= 3), Some(&3));
+}
+
 #[test]
 fn sort_unstable() {
     let mut v = [0; 600];