Hint slice.{r}position is within slice bounds

src/libcore/slice/mod.rs

@@ -1199,8 +1199,10 @@ macro_rules! iterator {
             fn position<F>(&mut self, mut predicate: F) -> Option<usize>
                 where F: FnMut(Self::Item) -> bool,
             {
+                let len = self.len();
                 let mut index = 0;
                 self.search_while(None, move |elt| {
+                    unsafe { assume(index < len); }
                     if predicate(elt) {
                         SearchWhile::Done(Some(index))
                     } else {
@@ -1213,9 +1215,11 @@ macro_rules! iterator {
             fn rposition<F>(&mut self, mut predicate: F) -> Option<usize>
                 where F: FnMut(Self::Item) -> bool,
             {
+                let len = self.len();
                 let mut index = self.len();
                 self.rsearch_while(None, move |elt| {
                     index -= 1;
+                    unsafe { assume(index < len); }
                     if predicate(elt) {
                         SearchWhile::Done(Some(index))
                     } else {

src/libcore/tests/slice.rs

@@ -13,6 +13,25 @@ use core::slice::heapsort;
 use core::result::Result::{Ok, Err};
 use rand::{Rng, XorShiftRng};
 
+
+#[test]
+fn test_position() {
+    let b = [1, 2, 3, 5, 5];
+    assert!(b.iter().position(|&v| v == 9) == None);
+    assert!(b.iter().position(|&v| v == 5) == Some(3));
+    assert!(b.iter().position(|&v| v == 3) == Some(2));
+    assert!(b.iter().position(|&v| v == 0) == None);
+}
+
+#[test]
+fn test_rposition() {
+    let b = [1, 2, 3, 5, 5];
+    assert!(b.iter().rposition(|&v| v == 9) == None);
+    assert!(b.iter().rposition(|&v| v == 5) == Some(4));
+    assert!(b.iter().rposition(|&v| v == 3) == Some(2));
+    assert!(b.iter().rposition(|&v| v == 0) == None);
+}
+
 #[test]
 fn test_binary_search() {
     let b = [1, 2, 4, 6, 8, 9];