Eliminate more excessive null-checks from slice iterators

This adds the assume() calls back that got lost when rebasing #21886.

Author: dotdash

src/libcollections/vec.rs

@@ -433,8 +433,10 @@ impl<T> Vec<T> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_mut_slice(&mut self) -> &mut [T] {
         unsafe {
+            let ptr = *self.ptr;
+            assume(!ptr.is_null());
             mem::transmute(RawSlice {
-                data: *self.ptr,
+                data: ptr,
                 len: self.len,
             })
         }
@@ -458,6 +460,7 @@ impl<T> Vec<T> {
     pub fn into_iter(self) -> IntoIter<T> {
         unsafe {
             let ptr = *self.ptr;
+            assume(!ptr.is_null());
             let cap = self.cap;
             let begin = ptr as *const T;
             let end = if mem::size_of::<T>() == 0 {

src/libcore/slice.rs

@@ -40,6 +40,7 @@ use cmp::{Ordering, PartialEq, PartialOrd, Eq, Ord};
 use cmp::Ordering::{Less, Equal, Greater};
 use cmp;
 use default::Default;
+use intrinsics::assume;
 use iter::*;
 use ops::{FnMut, self, Index};
 use ops::RangeFull;
@@ -137,6 +138,7 @@ impl<T> SliceExt for [T] {
     fn iter<'a>(&'a self) -> Iter<'a, T> {
         unsafe {
             let p = self.as_ptr();
+            assume(!p.is_null());
             if mem::size_of::<T>() == 0 {
                 Iter {ptr: p,
                       end: (p as usize + self.len()) as *const T,
@@ -276,6 +278,7 @@ impl<T> SliceExt for [T] {
     fn iter_mut<'a>(&'a mut self) -> IterMut<'a, T> {
         unsafe {
             let p = self.as_mut_ptr();
+            assume(!p.is_null());
             if mem::size_of::<T>() == 0 {
                 IterMut {ptr: p,
                          end: (p as usize + self.len()) as *mut T,