Use the slice length to hint the optimizer

Using the len of the iterator doesn't give the same result.
That's also why we can't generalize it to all TrustedLen iterators.

src/libcore/slice/mod.rs

@@ -1223,7 +1223,8 @@ macro_rules! iterator {
                 P: FnMut(Self::Item) -> bool,
             {
                 // The addition might panic on overflow
-                let n = self.len();
+                // Use the len of the slice to hint optimizer to remove result index bounds check.
+                let n = make_slice!(self.ptr, self.end).len();
                 self.try_fold(0, move |i, x| {
                     if predicate(x) { Err(i) }
                     else { Ok(i + 1) }
@@ -1241,7 +1242,8 @@ macro_rules! iterator {
             {
                 // No need for an overflow check here, because `ExactSizeIterator`
                 // implies that the number of elements fits into a `usize`.
-                let n = self.len();
+                // Use the len of the slice to hint optimizer to remove result index bounds check.
+                let n = make_slice!(self.ptr, self.end).len();
                 self.try_rfold(n, move |i, x| {
                     let i = i - 1;
                     if predicate(x) { Err(i) }